Helke Mocke

Martin Pickford

Brigitte Senut

Dominique Gommery

A note on Plio-Pleistocene insect cocoons from Prospekteerkop, Rietfontein, Northern Namibia

Keywords: Plio-Pleistocene, Otavi Mountainland, fossilised cocoons

Cocoons of invertebrates are rarely preserved as fossils, only a few reports existing in the literature. Yet many insect species produce cocoons as part of their life histories. We here report the discovery of what appear to be fossilised cocoons from Plio-Pleistocene cave breccia at Prospekteerkop, on farm Rietfontein in the Otavi Mountainland and try to identify them by comparing them to modern analogues among Lepidoptera (butterflies and moths) and Coleoptera (beetles). The specimens are most likely to have been made by a species of beetle.

INTRODUCTION

The Otavi Mountainland (OML) is located in the north-central part of Namibia and as early as the late 19th century it received much attention on account of its base metal reserves which include copper, lead, zinc and vanadium, [START_REF] Cairncross | The Otavi Mountain Land Cu-Pb-Zn-V Deposits[END_REF][START_REF] Gebhard | Tsumeb II[END_REF][START_REF] Schwellnus | Vanadium deposits in the Otavi Mountains, South-West Africa[END_REF]. The OML is composed predominantly of Neoproterozoic carbonate rocks that have been modified by extensive chemical weathering processes that resulted in a rough karstic terrain endowed with many caves and fissures in which sediments and breccias accumulated. The spelean sediments of the OML have yielded abundant fossils of Middle Miocene to Recent age [START_REF] Pickford | Karst geology and palaeobiology of northern Namibia[END_REF].

At diverse sites in the OML, [START_REF] Pickford | The Fossil Record of Namibia[END_REF][START_REF] Pickford | Karst geology and palaeobiology of northern Namibia[END_REF] identified fossil remains among which micromammals predominated. The following groups were listed : Myriapoda (centipedes), Gastropoda (terrestrial snails -Xerocerastus), Amphibia, Squamata (lizards and snakes), Aves, Carnivora, Primates (Galagidae (bush babies), Hominoidea, Cercopithecidae), Tenrecidae, Macroscelididae (elephant shrews), Hyracoidea, Proboscidea, Chiroptera, Chrysochloridae (golden moles), Soricidae (shrews), Rodentia (Sciuridae, Gerbillinae, Gliridae, Muridae, Bathyergidae, Thryonomyidae, Pedetidae), Leporidae (hares), and Ruminantia (Giraffidae, Bovidae).

The mammalian fauna from the Prospekteerkop breccias, the existence of which was first mentioned by [START_REF] Schwellnus | Vanadium deposits in the Otavi Mountains, South-West Africa[END_REF], comprises two species of soricids (crocidurines), a macroscelidid (elephant shrew), three species of bat (two rhinolophoids and one vespertilionid), a hare (Pronolagus humpatensis), a high diversity of rodents (Graphiurus, Mystromys, Mastomys, Petromus, Steatomys, Malacothrix, Petromyscus, Tatera, Gerbillurus, Otomys, Rhabdomys, Zelotomys, Mus, Praomys), a leopard (Panthera pardus), a dassie (Procavia capensis), two bovids (Oryx, Antidorcas) and a baboon (Papio) [START_REF] Pickford | Karst geology and palaeobiology of northern Namibia[END_REF][START_REF] Mein | African Micromammal Palaeontology Archives of Pierre Mein, Unpublished compilation by M. Pickford. PDF copies in author's personal library[END_REF]. This fauna indicates correlation to the Plio-Pleistocene.

In 2022 and 2023 members of the Namibia Palaeontology Expedition discovered small ovoid fossils amongst the microfauna in the breccia occurrence at Prospekteerkop on Farm Rietfontein. Preliminary research indicates that 67 they represent cocoons of an invertebrate, most probably of a species of coleopteran. [START_REF] Stehr | Cocoon[END_REF] described a cocoon as a protective covering made of silk within which caterpillars of most moths, a few butterfly species, and several other insect orders such as Siphonaptera (fleas), Hymenoptera (ants, bees and wasps), Neuroptera (lacewings and antlions), and Trichoptera (caddis flies) pupate. [START_REF] Lea | Notes on some calcareous insect puparia[END_REF] and [START_REF] Veatch | Ancient weevil pupal cases: Trace fossils from Australia's Pleistocene[END_REF] reported on fossilised cocoons of the Australian weevil Leptopius duponti (Coleoptera) from Pleistocene localities in Australia.

According to [START_REF] Brasier | Earliest Cretaceous cocoons or plant seed structures from the Wealden Group, Hastings, UK[END_REF] endogenous materials associated with pupation, such as cocoons, are not common in the fossil record. Therefore, these fossilised cocoons from Rietfontein are important and they may yield information about the environment of the area during the Plio-Pleistocene and the animals that lived here.

Moths, together with butterflies, comprise part of the order Lepidoptera. Today there are 481 genera and 890 species of Lepidoptera in Namibia [START_REF] Kopij | Invertebrate Fauna of Namibia, Biodiversity and Bibliography[END_REF]. However, according to the Namibia Biodiversity Database Web Site [START_REF] Irish | Namibia Biodiversity Database Web Site. Page: Order Lepidoptera in Namibia. Interpretive collation based on the combined published sources for all included taxa, as listed[END_REF] 1,789 lepidopteran species currently live in Namibia and there are currently 4,058 species of Coleoptera in the country.

Out of the 4,593 lepidopteran fossil specimens described worldwide, 92.8% are body fossils and 7.2% are trace fossils. According to [START_REF] Sohn | The fossil record and taphonomy of butterflies and moths (Insecta, Lepidoptera): implications for evolutionary diversity and divergence-time estimates[END_REF] only about 7% of these fossils have been formally named, and of the 145 lepidopteran fossil localities listed, 31 are Miocene, 23 are Pleistocene and Holocene, 22 are middle and late Eocene and 15 are early Oligocene. Geological setting The Prospekteerkop fossil site is located in the Otavi Mountainland on Farm Rietfontein (19°41'36"S 17°52'38"E) (Fig. 1,2). It is close to the foot of a kopje which lies approximately 10 km north-north-east of the farmhouse. The fossiliferous breccia blocks lie next to and inside an old prospecting trench that is ca 2 metres wide, approximately 8 m long and 1.5 m deep (Fig. 3,4). The trench exposes flowstone remnants (speleothems) in its southern end and the host rock surrounding the breccias is dolomite.

MATERIALS AND METHODS

In 2022 and 2023, palaeontological field surveys were carried out on Farm Rietfontein in the Otjozondjupa region, Grootfontein District.

Blocks of breccia containing fossilised micro-and macrofauna were collected for scientific analysis at the National Earth Science Museum of the Geological Survey of Namibia. Among the material collected were several insect cocoons.

DESCRIPTION OF THE FOSSIL COCOONS

The most complete fossil cocoon described herein is whitish, oval in shape and is preserved in a reddish-brown breccia. Its length is approximately 2.5 cm and its width is approximately 1.3 cm. It has a somewhat rough surface texture with several small cavities visible, which may be due to weathering, or possibly damage to the cocoon when it was still fresh. Small patc hes of sediment are stuck to the cocoon (Fig. 5).

A sectioned cocoon in breccia from Prospekteerkop is of interest on account of the fact that it is surrounded by a layer of greyish sediment ca 7 mm thick, that differs in colour from the surrounding redder sandy breccia (Fig. 6). This coating is itself covered by an irregular and incomplete layer of calcite crystals and the inner wall of the cocoon is lined with a similar layer of small crystals of calcite. The halo of sediment contains small angular fragments of petrified vegetation (Fig. 6). The length of the cocoon, as preserved, is 20.8 mm and the diameter in the middle is 12.5 mm. The surrounding halo of greyish sediment is 36.5 mm long x 22.8 mm in diameter, making for a coating that is ca 7 mm thick.

The way that the specimen is preserved suggests that it had a coating of organic matter, only parts of which were fossilised, but which persisted long enough for calcite to precipitate over parts of the surface. Most of the organic matter then rotted away slowly, leaving space for finegrained sand to infill the void, much of the iron in the sand being reduced as it was replacing the organic matter. If this scenario is correct, then it suggests that the cocoon belonged to a beetle such as the garden fruit chafer (Pachnoda) that coats its cocoons in a relatively thick layer of organic debris (Fig. 12). These beetles typically construct their cocoons in compostlike debris in the soil profile.

INTERPRETATION OF THE COCOONS

The entomological collections of the National Museum of Namibia were studied to determine whether any cocoons have been accessioned into the collections. The cocoons of the moths Gonometa postica and Schausinna regia were photographed and compared to the Rietfontein fossil cocoons. However, these were larger, more elongated and more irregularly shaped than the fossil ones.

Two entomologists who are experts on Namibian insects, Dr John Irish and Dr Rolf Oberprieler were consulted for advice on the identification of the fossil cocoons.

Dr Irish postulated that we may well be dealing with a cocoon, possibly of a Saturniidae moth, such as Gonimbrasia (Mopane moth), because its size and shape correspond well with the fossils from Rietfontein. However, he questioned what he saw as "a possible longitudinal seam, especially visible along the left and bottom sides of the object". We interpret this as an artefact of weathering.

Dr Oberprieler observed that the find is interesting, but that it may be difficult to propose a definite identification of the cocoon or cocoonmaker. He emphasised that there are many moth caterpillars that produce cocoons, but most of these cocoons look quite different from the Prospekteerkop cocoons. They tend to be more elongate, larger and less symmetrical. The smaller ones are soft, and as moth cocoons are made of silken fibre, they might not fossilise at all. According to the shape of the Rietfontein cocoons and the fact that they needed to be durable in order to be fossilised, he postulated that the most likely candidate, if indeed we are dealing with a moth cocoon, would be the slug moth, Coenobasis amoena of the family Limacodidae, which has also been recorded in Namibia. Its cocoon however is smaller, about 10 mm in length and it is attached to a tree branch. However, no fossil twigs have been observed at Prospekteerkop. Dr Oberprieler is currently studying weevils in Australia and alerted us that there are beetles whose larvae produce cocoons underground. Their cocoons become hard and can easily fossilise if the ground chemistry is suitable, especially if calcium is present. Articles about fossilised beetle cocoons, in particular weevils, have been published about Australian specimens, but so far little has been reported from Africa. In contrast to moth cocoons, those of coleopterans, such as the garden fruit chafer, Pachnoda sinuata, have larger, rounder and more symmetrical cocoons that the beetle constructs in compost. The cocoon is approximately 2.5-3 cm in length and 2.5 cm in diameter. Plant matter is attached to the surface of the cocoon, as can be seen in Fig. 12.

DISCUSSION

The literature on the fossil record of terrestrial arthropods from Africa is not extensive, but what has been described comprises body fossils [START_REF] Leakey | Lower Miocene invertebrates from Kenya[END_REF][START_REF] Paulian | Three fossil dung beetles (Coleoptera, Scarabeidae) from the Kenya Miocene[END_REF][START_REF] Morris | A new fossil terrestrial isopod with implications for the East African Miocene landform[END_REF]McKay & Rayner, 1986;Rayner & McKay, 1986;Rayner & Waters 1991;Rayner et al. 1991;[START_REF] Pickford | The Fossil Record of Namibia[END_REF][START_REF] Pickford | Karst geology and palaeobiology of northern Namibia[END_REF] as well as bioconstructions made by insects (termite hives, termite mounds, ant nests, diverse cocoons) and spiders (spider webs) and other traces (foraging tunnels) [START_REF] Coaton | Fossilised nests of Hodotermitidae (Isoptera) from the Clanwilliam District, Cape Province[END_REF][START_REF] Coaton | National survey of the Isoptera of southern Africa. 3. The genus Psammotermes Desneux[END_REF][START_REF] Coaton | National survey of the Isoptera of southern Africa. 10. The genus Hodotermes Hagan (Hodotermitidae)[END_REF][START_REF] Corbett | The Sedimentology of Diamondiferous Deflation Deposits within the Sperrgebiet, Namibia[END_REF][START_REF] Crossley | Sedimentation by termites in the Malawi Rift Valley[END_REF][START_REF] Duringer | First discovery of fossil dung beetle brood balls and nests in the Chadian Pliocene Australopithecine levels[END_REF][START_REF] Harrison | Paleontology and biochronology of fossil localities in the Manonga Valley, Tanzania[END_REF][START_REF] Moore | Heuweltjies (earth mounds) in the Clanwilliam District, Cape Province, South Africa: 4,000 year old termite nests[END_REF][START_REF] Pickford | Fossil spider's webs from the Namib Desert and the antiquity of Seothyra (Araneae, Eresidae)[END_REF][START_REF] Pickford | The Namib's amazing fossil spider webs[END_REF][START_REF] Pickford | Arthropod bioconstructions from the Miocene of Namibia and their palaeoclimatic implications[END_REF][START_REF] Ritchie | Trace fossils of burrowing Hymenoptera from Laetoli[END_REF][START_REF] Sands | Ichnocoenoses of probable termite origin from Laetoli[END_REF][START_REF] Seely | Termite casts in Tsondab Sandstone?[END_REF]Tessier, 1959aTessier, , 1959b;;[START_REF] Thackeray | Fossil nests of sweat bees (Halictinae) from a Miocene palaeosol, Rusinga Island, western Kenya[END_REF][START_REF] Ward | Eolian, fluvial and pan (playa) facies of the Tertiary Tsondab Sandstone Formation in the Central Namib Desert, Namibia[END_REF][START_REF] Wilson | A fossil ant colony: new evidence of social antiquity[END_REF]. Among the body fossils described in the literature, beetles predominate [START_REF] Paulian | Three fossil dung beetles (Coleoptera, Scarabeidae) from the Kenya Miocene[END_REF] whereas among the trace fossils, those attributed to termites are the most commonly described [START_REF] Seely | Termite casts in Tsondab Sandstone?[END_REF][START_REF] Pickford | Arthropod bioconstructions from the Miocene of Namibia and their palaeoclimatic implications[END_REF]. Soft tissue fossils are exceptionally rare in Africa but are known from Cretaceous crater infillings at Orapa, Botswana (Insects : Rayner & McKay, 1986) and from aeolianites of the Namib Desert (Spider webs : [START_REF] Pickford | Fossil spider's webs from the Namib Desert and the antiquity of Seothyra (Araneae, Eresidae)[END_REF][START_REF] Pickford | The Namib's amazing fossil spider webs[END_REF] Kitching (1959) described a fossilised puparium from Plio-Pleistocene cave breccias at Makapansgat, South Africa, and the presence of insect cocoons has been listed at various Early Miocene localities in Kenya (Rusinga, Mfwanganu, Koru, Songhor, Kirimun : [START_REF] Pickford | Report on invertebrates from Kirimun[END_REF]Pickford, , 1986aPickford, , 1986b) ) and Uganda (Napak : 74 [START_REF] Musalizi | Geological and palaeontological archives relating to Early Miocene localities of Uganda, 1957-1969[END_REF][START_REF] Pickford | Nouvelles découvertes dans le Miocène inférieur de Napak, Ouganda Oriental[END_REF]) but none of the material has been described in detail, although measurements of the Kirimun specimens were published [START_REF] Pickford | Report on invertebrates from Kirimun[END_REF]. For many years, East African fossil cocoons were misidentified as snake eggs [START_REF] Musalizi | Geological and palaeontological archives relating to Early Miocene localities of Uganda, 1957-1969[END_REF].

In most of the publications dealing with fossil arthropods and their traces, there were discussions about their palaeoenvironmental significance. For this reason, it is important to put on record the discovery of fossilised cocoons at Prospekteerkop, Namibia.

CONCLUSION

The fossil cocoons from Prospekteerkop, Rietfontein, in northern Namibia, are not attributed to a specific invertebrate maker, although the available evidence suggests that two of them may have been made by a beetle such as the fruit chafer. Nevertheless, the specimens do indicate that an organism was living in northern Namibia during the Plio-Pleistocene that needed a cocoon to protect the pupa against adverse environmental conditions (e.g. aridity, heat), parasites or predators.

Taking into account the size, shape and symmetry of the cocoons, the most plausible conclusion is that they may have been constructed by a beetle which deposited its resistant casing, coated in vegetable matter, in the soil so that the pupa could pupate safely.

To date, there is no evidence that the cocoons were attached longitudinally to twigs, which, had this been the case, would have indicated that the cocoons might belong to a species of moth that attached its cocoons to the branches or twigs of trees well above ground level. Therefore, on the basis of the available information, especially the symmetry and dimensions of the specimens and the halo of greyish sediment containing angular fragments of petrified plant debris, and the calcite crystals that surrounds one of them, we conclude that the most likely maker of the Prospekteerkop fossil cocoons was a species of coleopteran that constructed its cocoon in compost in the soil profile, and that attached organic debris to the surface of the cocoon, as is done by fruit chafers. Until the maker of the cocoons is securely identified it will be difficult to base reconstructions of the palaeoenvironment upon them.

Other research methods such as X-ray Tomography may shed light on the possible contents and immediate surroundings of the cocoons. If so this could narrow down the identification of their maker.

Figures 7 -

 7 Figures 7-11 show various extant moth species from Namibia and their cocoons.

Figure 1 .

 1 Figure 1. Prospekteerkop fossil locality on Farm Rietfontein in the Otavi Mountainland (map produced by Freddy Muyamba).

Figure 2 .

 2 Figure 2. Position of the Prospekteerkop breccia dumps on Farm Rietfontein, Otavi Mountainland, Namibia. Map modified from Google Earth.

Figure 3 .

 3 Figure 3. Prospekteerkop fossil site layout (figure prepared by Anna Upindi).

Figure 4 .

 4 Figure 4. View of the old prospecting trench at the foot of Prospekteerkop (in the background). Grey blocks are dolomite, red ones are cave breccia.

Figure 5 .

 5 Figure 5. Fossilised cocoon from Prospekteerkop, Rietfontein.

Figure 6 .

 6 Figure 6. Stereo images of a sectioned cocoon in cave breccia from Prospekteerkop, Otavi Mountains, Namibia. Note the greyish sediment 'halo' (in which there are fragments of petrified vegetation : yellow arrows) enclosing the cocoon for a thickness of ca 7 mm and which is partly coated by calcite crystals (blue arrows) (scale in cm).

Figure 7 .

 7 Figure 7. Argema mimosa moth and cocoon (images from Oberprieler, 1995b).

Figure 8 .

 8 Figure 8. Epiphora bauhiniae moth and cocoon (images from Oberprieler, 1995a).

Figure 9 .

 9 Figure 9. Gonometa postica moth and cocoon.

Figure 10 .

 10 Figure 10. Schausinna regia moth and cocoons.

Figure 11 .

 11 Figure 11. Coenobasis amoena moth, caterpillar and cocoon on a twig (images from Staude et al. 2023).

Figure 12 .

 12 Figure 12. Pachnoda sinuata A) adult beetle, B) cocoons coated in vegetable debris and C) larva inside broken cocoon (photo credits : Alexander Mocke).

ACKNOWLEDGEMENTS

The authors would like to thank Mr and Mrs Cronjé, the owners of Rietfontein for access to their property, Mr Francois Becker, Chief Curator of Natural Science at the National Museum of Namibia, for access to the entomology collections, and Drs Irish and Oberprieler for their insights. The fieldwork and study would not have been possible without the issuance of a research permit by the National Commission on Research, Science and Technology and the National Heritage Council of Namibia with the support of the Geological Survey of Namibia.

We thank the Scientific Society of Namibia for information and references, and the National Museum of Namibia for providing access to their collections. Thanks to Freddy Muyamba and Anna Upindi for drafting figures 1 and 3 respectively. Field survey was partly financed by the project ATM 2022 PalPrim Namibie (OE-7207) of the Muséum National d'Histoire Naturelle, Paris, with support from the Geological Survey of Namibia.


Gabriela Rus

Nadim Al Hajjar

Paul Tucan

Andra Ciocan

Calin Vaida

Corina Radu

Damien Chablat

Doina Pisla

Mixed-Reality-Guided Teleoperation of a Collaborative Robot for Surgical Procedures

Keywords: Collaborative robot, Teleoperation, Laparoscopic Holder, Mixed reality

The development of advanced surgical systems embedding the Master-Slave control strategy introduced the possibility of remote interaction between the surgeon and the patient, also known as teleoperation. The present paper aims to integrate innovative technologies into the teleoperation process to enhance workflow during surgeries. The proposed system incorporates a collaborative robot, Kuka IIWA LBR, and Hololens 2 (an augmented reality device), allowing the user to control the robot in an expansive environment that integrates actual (real data) with additional digital information imported via Hololens 2. Experimental data demonstrate the user's ability to control the Kuka IIWA using various gestures to position it with respect to real or digital objects. Thus, this system offers a novel solution to manipulate robots used in surgeries in a more intuitive manner, contributing to the reduction of the learning curve for surgeons. Calibration and testing in multiple scenarios demonstrate the efficiency of the system in providing seamless movements.

Introduction

The concept of teleoperation in surgery marks a significant advancement in medical technology, allowing surgeons to manipulate instruments from a remote location with precision. This innovative approach enables the execution of intricate surgical procedures through robotic systems [START_REF] Pisla | Kinematical analysis and design of a new surgical parallel robot[END_REF], enhancing the capabilities of medical professionals, especially for master-slave systems [2][3].

According to this, it is important to underline how critical are the aspects related to precision and guidance. Surgeons need high precision to achieve goals while guidance offers real-time information for informed decisions. As medical professionals continually seek innovative solutions to improve patient outcomes, the integration of cutting-edge technologies becomes primary [START_REF] Kazanzides | Teleoperation and Visualization Interfaces for Remote Intervention in Space[END_REF] [START_REF] Gerup | Augmented reality and mixed reality for healthcare education beyond surgery: an integrative review[END_REF] [6] [START_REF] Vaida | Preliminary Assessment of Artificial Intelligence Agents for a SILS Robotic System[END_REF]. A significant component in this effort is the existent collaborative robotics systems.

The advantages of using collaborative robots in surgical procedure include (1) safety futures-force and torque sensors, vision systems, and software algorithms that allow them to detect the presence of humans and respond by slowing down or stopping; [START_REF] Luo | Combined perception, control, and learning for teleoperation: key technologies, applications, and challenges[END_REF] adaptability and flexibilitydue to their configuration can be easily reconfigured for various surgical procedures; (3) human-robot collaboration -these system being designed to interact with humans. Considering the presented aspects these systems showed their potential to be used in various complex surgery procedures such as Esophagectomy (esophageal resection), Pancreatectomy (pancreatic resection) and Hepatectomy (liver resection), which share a common issue, referring to the location of the tumors in the proximity of critical internal structures imposing highly accurate resection techniques.

The use of collaborative robots in surgery was discussed in many articles [START_REF] Tucan | Design and Experimental Setup of a Robotic Medical Instrument for Brachytherapy in Non-Resectable Liver Tumors[END_REF][START_REF] Man | Telemedicine, telementoring, and telesurgery for surgical practices[END_REF][START_REF] Schwaner | MOPS: A Modular and Open Platform for Surgical Robotics Research[END_REF][START_REF] Su | Safety-enhanced Collaborative Framework for Teleoperated Minimally Invasive Surgery Using a 7-DoF Torque-controlled Robot[END_REF]. For example, the authors from [START_REF] Schwaner | MOPS: A Modular and Open Platform for Surgical Robotics Research[END_REF] used the UR5 robot to hold and move the surgical instruments and the camera during surgeries. The authors of [START_REF] Su | Safety-enhanced Collaborative Framework for Teleoperated Minimally Invasive Surgery Using a 7-DoF Torque-controlled Robot[END_REF] used KUKA LWR 4+ robot to develop a collaborative control framework for teleoperations in Minimally Invasive Surgery (MIS).

Another innovative technology increasingly used in recent years, demonstrating real advantages in surgery is related to the augmented reality (AR) and virtual reality (VR) devices. These devices have the potential to improve surgeries in many aspects such as: surgical planning and visualization, training and simulation, enhanced navigation, remote assistance, reduced radiation exposure and postoperative monitoring and rehabilitation [START_REF] Abdullah | Evaluation of marker tracking using mono and stereo vision in Microsoft HoloLens for surgical navigation[END_REF] [START_REF] Birlo | Utility of optical see-through head mounted displays in augmented reality-assisted surgery: A systematic review[END_REF].

The paper aims to analyze the potential of utilizing collaborative robots and an AR device (Hololens 2) to enhance teleoperation. The proposed system is designed to enable the manipulation of surgical robots without the need for additional joysticks such as 3D space-mouses or haptic devices, offering an intuitive interface. Simultaneously, it provides the capability for multi-user operation of the robot, allowing multiple users with HoloLens devices to use the system. Considering these aspects, the system can be used in intraoperative phase, having the potential to reduce learning curves and enhance the overall surgical process for surgeons.

Succeeding the introduction section, the paper is structured as follows: Section II describes the methodology for development of the proposed system including calibration and the protocol for testing. Section III describes the results, and Section IV presents the discussions and the conclusions of the study.

Methodology

The teleoperation setup proposed for this study is composed of a collaborative robot, a laparoscopic camera attached to the robot through a customized flange, and the augmented reality device, HoloLens 2, as can be seen in Fig. 1. The transfer of data between systems is based on TCP/IP (Transmission Control Protocol/Internet Protocol) communication.

Fig. 1 Schematic representation of the proposed system

Materials

The chosen collaborative robot was KUKA IIWA (Intelligent Industrial Work Assistant) [14]. The configuration of the system (seven joints) includes integrated force torque sensors for safe and efficient operation, which enables suitable control of movement. Its collaborative features not only ensure safety but also make it easy for tasks which demand precise manipulation. A laparoscopic camera is connected to a specially tailored flange, this assembly being fixed to the end effector of the robot.

HoloLens 2, a Microsoft augmented reality device, introduces a hands-free advantage to the described setup. The HoloLens 2 hands free control makes it easier to perform precise movements based on hand movement, as part of teleoperation set-up.

Communication

As can be seen in Fig. 2 the communication is based on TCP/IP connection, assuring real-time transfer of data.

In the described teleoperation framework, the HoloLens device hosts a standalone application that provides several functions including a client socket. This application is designed to transmit real-time wrist coordinates when a certain gesture (the pinching) is recognized, these coordinates being then directed to a server socket, created in Python. Thus, upon initiating the application, it verifies the connection with the Python server, and the application proceeds only when the connection between the client and server is established. The same prerequisite applies to the connection between Python and the robot. If the connection is successfully established among all participants, the pinch gesture from HoloLens is verified, and the data transfer is initiated upon the recognition of the gesture. The coordinates are further processed and sent to the end effector of the KUKA robot after being received by the Python server, enabling realtime control and manipulation. After the pinch gesture is finished, HoloLens does not transmit any more coordinates until the gesture is performed again, which stops unintentional robot movements. In order to assure the transfer of all of required data for a movement, a validation of the movement is implemented in the user interface, thus the user will validate if the movement was executed by the robot or not.

Calibration

Hololens has been configured to consider the KUKA coordinate system throughout the calibration process, ensuring seamless integration and accurate interpretation of coordinates between the augmented reality device and the collaborative robot.

To address the unit disparity between the KUKA system, which operates in millimeters, and the Hololens, which uses meters, a normalization step was implemented.

Additionally, as coordinates are transmitted to the end effector of the KUKA collaborative robot, the calibration process incorporates the utilization of inverse kinematics.

For this procedure, the coordinates from the End-Effector are required. The origin of the coordinate system is positioned in the characteristic point of the End-Effector, and the axes aligned with the robot native ones enabling a fast computation with respect to the base of the robot. The integration of robot singularities and trajectory planning has been realized using the Kuka Sunrise Toolbox.

As part of the calibration process, the robot was moved to a known point in space. The coordinates of the tip of the laparoscopic camera were then registered for comparison using specific markers with locations defined accurately with respect to the fixed coordinates system of the robot. A similar process was carried out for the cursor from HoloLens.

Ultimately, the robot is positioned in a specific configuration, serving as the starting point for task execution. This strategic stance facilitates the seamless initiation of various tasks.

Protocol for testing

The protocol for testing and ensuring the functionality of the collaborative robot, integrated with the Hololens, involves several key steps presented in Table 1. In the initial setup, the correct configuration and connection of the robot, as well as verifying the proper functioning of the Hololens is performed. Basic movement tests are then conducted in an open space to guarantee accurate motion. Calibration between the robot and Hololens is tested, and the recognition of pinch gestures with different hand orientations, is verified. Subsequently, specific movement tests are executed, with emphasis on the insertion of the laparoscopic camera through the trocar. The laparoscopic camera has 3 degrees of freedom (DOF) and can perform 2 rotational movements around the X-axis, and respectively around the Y-axis, as well as translation along the Z-axis (Fig 3 .a).

Table 1.Testing protocol.

Steps of protocol

Task 2

Move laparoscopic camera on 3D space. (The movement is performed on the 3D space). Task description: In proximity of the robot are placed both physical objects (a phantom torso) and holograms (projected from Hololens) The robot should be able to gentle touch these holograms, respectively to introduce the laparoscopic camera in torso.

Task 3

Following a trajectory. Task description: The robot should be able to move based on certain trajectory drawn by the hand in order to enhance maneuverability and accuracy.

Results

Considering that the main purpose of this paper is to test the feasibility of a system composed from an AR device and a collaborative robot, in teleoperation surgery, aspects such as accuracy, delays and time to complete a task are critical metrics. The accuracy was calculated taking into account the coordinates of the cursor from Hololens and the coordinate of the tip of laparoscopic camera (coordinates of the End-Effector + the offset caused by the length of the camera -30 cm). The average positional error was measured at 0.7 mm during the execution of movement 1, presented in protocol.

The positional errors registered for the next step in protocol (movement 2), were 0.8 mm when the user had to touch the holograms with the laparoscopic camera and 1.1 mm when the task was the introduction of laparoscopic camera in torso ( The average time to complete the tasks was 1 minute and 45 seconds for the first one, 2 minutes and 15 seconds for touching all holograms, 1 minute and 52 seconds for introducing the camera in the torso, and 40 seconds for task 3. These tasks were completed by three users with different levels of experience with the Hololens device.

Discussion and conclusion

Based on the tests performed with the proposed system, Kuka collaborative robot demonstrated the capability to effectuate accurate movements based on real time commands from Hololens, for all proposed tasks. However, tasks of varying complexity showed proportional errors, as can be seen between Task 1 and Task 2, reflecting the demand for increased dexterity. Data transmission was executed with a small delay, which indicates the potential of this system to be used in real-time applications such as surgery.

The proposed system is feasible for teleoperation, bringing in addition the advantage of allowing multi-user manipulation if there are more Hololens devices, facilitating collaborative decision-making and enhancing overall surgical efficiency. This real-time, intuitive control system has the potential to optimize surgical procedures, reduce learning curves, and improve outcomes for the surgical team.

Another advantage is given by its collaborative design, thanks to which potential collisions do not lead to damage. Additionally, the robot provides the option to restrict each joint within a virtual plane, ensuring that the robot stays within predefined spatial boundaries.

To respond to the specific task of holder for laparoscopic camera, there are other aspects that should be also taken into consideration. First of all, in order to avoid unwanted behavior, the robot executes a linear displacement to the insertion point. Before the insertion, the camera is aligned with the insertion point. Following this alignment, the insertion is carried out incrementally, with minimal increment and minimal velocity (both the velocity and the increment are selected by the user from the interface). Another important factor is the scaling between the movement of the human operator and the robot. This component is also implemented in the user interface and allows the user to scale the movement, in accordance with his needs.

In conclusion, the teleoperation setup combining Kuka IIWA and HoloLens 2 can bring a significant advancement in surgical technology. The collaborative system, together with HoloLens device, used in intraoperative phase holds promise for improving surgical precision, safety, and overall outcomes. This system, built around the collaboratives robot and the augmented reality device HoloLens 2, can be easily integrated into diverse collaborative systems.

Fig. 2 .

 2 Fig. 2. Schematic representation of the data transfer between Hololens and Kuka

1 Move

 1 Description Initial setup • Ensure proper installation and connection of the collaborative robot • Ensure Hololens functionality Basic Movements Tests • Perform basic movement tests in open space to ensure the robotic moves smoothly and accurately. • Test the calibration between the robot and Hololens • Test the recognition of the gesturewith pinch gesture, without pinch gesture, with different orientation of the hand Task laparoscopic camera on plane (The movements are performed on a 2D plane). Task description: A paper where diverse geometrical figures are drawn, is placed in front of Kuka. The robot, based on the hand's movement, should be able to move to a certain point of a figure or to execute movements throughout the figures from the paper.

  Fig 3.b).

Fig 3 .

 3 Fig 3. a) Motions of camera Fig 3. b) Execution of task 2. Inserting the laparoscopic camera in the phantom torso (image from Hololens)For the final test the positions of the end effector and the positions of the hand have been registered to analyze the trajectories. As can be seen in Fig.4, the collaborative robot has been able to follow closely the trajectory of the hand.

Fig 4 .

 4 Fig 4. Graphical representation of the trajectory registered from the wrist of the hand (red line) and End-Effector trajectory (blue line) Considering the protocol for data transmission, the average delay registered between the hand and the robot was almost 0.1 milliseconds. It has been observed that sudden movements tend to create an additional delay caused by the large amount of data that should be processed.The average time to complete the tasks was 1 minute and 45 seconds for the first one, 2 minutes and 15 seconds for touching all holograms, 1 minute and 52 seconds for introducing the camera in the torso, and 40 seconds for task 3. These tasks were completed by three users with different levels of experience with the Hololens device.

Acknowledgment

This work was supported by the project New smart and adaptive robotics solutions for personalized minimally invasive surgery in cancer treatment -ATHENA, funded by European Union -NextGenerationEU and Romanian Government, under National Recovery and Resilience Plan for Romania, contract no. 760072/23.05.2023, code CF 116/15.11.2022, through the Romanian Ministry of Research, Innovation and Digitalization, within Component 9, investment I8 and by a grant of the Ministry of Research, Innovation and Digitization, CNCS/CCCDI-UEFISCDI, project number PN-III-P2-2.1-PED-2021-2790 694PED-Enhance, within PNCDI III.


Fabien Malbet

Romain Petrov

Gerd Weigelt

Olivier Chesneau

Armando Domiciano

De Souza

Anthony Meilland

Florentin Millour

Eric Tatulli

Armando Domiciano De Souza

First AMBER/VLTI Science

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The AMBER instrument installed at the Very Large Telescope (VLT) combines the beams from three telescopes to produce spectrally dispersed interference fringes with milli-arcsecond angular scales in the near infrared. Three years after installation, first scientific observations have been carried out mostly during the Science Demonstration Time and the Guaranteed Time. The first science has mainly focused on the environment of various types of stars. Because AMBER has dramatically increased the number of measures per baseline, this instrument brings strong constraints on morphology and models.

AMBER is one of the two science instruments of the Very Large Telescope Interferometer (VLTI) described in Petrov et al. (2007). AMBER is an interferometric beam combiner for the VLTI working in the near-infrared J-, H-, and K-bands and able to simultaneously mix three beams coming from three identical telescopes. AMBER interferograms are spectrally dispersed with a resolution of about 35, 1500, or 12 000. Therefore the instrument can measure visibilities and a closure phase in a few hundred different spectral channels. The spectral coverage, the spectral resolution, and the better sensitivity compared to small-aperture interferometers give access to many new astrophysical fields that we describe in this paper.

Discs and winds in young stars

The young stellar object MWC 297 is an embedded Herbig Be star exhibiting strong hydrogen emission lines and a strong near-infrared continuum excess. MWC 297 was observed with AMBER during its first commissioning run (Malbet et al. 2007). MWC 297 has been spatially resolved in the continuum as well as in the Brg emission line where the visibility decreases to a lower value (see Figure 1). The interpretation of this result is that the gas emitting the Brg emission line is located in a region larger than the disc from which the dust continuum emission arises. A picture emerges in which MWC 297 is surrounded by an equatorial, optically thick disc, that is possibly still accreting, and by an outflowing wind located just above it. AMBER's unique capability to measure spectral visibilities allowed Malbet et al. (2007), for the first time, to compare the apparent geometry of a wind with the disc structure in a young stellar system.

A lower-mass, less active system, the Herbig Ae system HD104237, was also observed with AMBER (Tatulli et al. 2007). The central emission line star is surrounded by a circumstellar disc that causes the infrared excess emission and that drives a jet. The visibility of this object measured by AMBER does not vary between the continuum and the Brg line region, even though the line is strongly detected in the spectrum. This result demonstrates that the line and continuum emission have similar size scales. Assuming that the K-band continuum excess originates in a puffed-up inner rim of the circumstellar disc, Tatulli et al. (2007) conclude that this emission most likely arises from a compact disc wind very close to the inner rim location.

2.15

2.16 2.17 and an outflowing wind (in blue). The wind geometry has been computed to both fit the visibility drop in the Brg line and reproduce the object spectrum. The apparent size of the wind is larger than the apparent size of the disc. From Malbet et al. (2007).

These two results show that AMBER on the VLTI is going to be a major tool for understanding the very close environment of young stars and will disentangle the regions of emission of dust and gas, especially those coming from the disc and the wind.

Rotating gas envelopes around hot active stars

Several emission-line stars have been scrutinised by AMBER: two Be stars, a Arae, one of the closest Be stars (Meilland et al. 2007b) and k Canis Majoris, one of the brightest ones (Meilland et al. 2007a), and one B[e] supergiant star, CPD-57˚2874, which is one of the rare hot stars showing forbidden lines and IR emission from dust (Domiciano de Souza et al. 2007).

The AMBER instrument, when operating in the K-band, provides a gain in spatial resolution of a factor of five compared to previous VLTI/MIDI observations of a Arae. Moreover, high angular resolution is combined with medium spectral resolution which allows the kinematics of the gas envelope inner part to be studied and its rotation law to be estimated (see Figure 2). Meilland et al. (2007b) obtained, for the first time, direct evidence that the gas envelope is in Keplerian rotation, answering a question that has existed since the discovery of the first Be star, g Cassiopae, by Father Secchi in 1866. The envelope around a Arae is compatible with dense equatorial matter confined in the central region, whereas a polar wind is outflowing along the rotational axis of the central star. Between these two regions the density must be low enough to reproduce the large visibility amplitudes obtained for two of the four VLTI baselines.

Using differential visibility amplitudes and phases across the Brg line, Meilland et al. (2007a) detected an asymmetry in the circumstellar structure around k Canis Majoris. However, this star is difficult to fit within the classical scenario for Be stars, i.e., fast rotating B star close to its breakup velocity surrounded by a Keplerian circumstellar gas envelope with a strong polar wind. We found that k CMa does not seem to be a critical rotator, the rotation law within the envelope is not Keplerian, and the detected asymmetry seems to be hardly explained within the one-armed viscous disc framework.

The first high spatial and medium spectral observations of the circumstellar envelope of a B[e] supergiant, CPD-57˚2874, were performed with the VLTI using both the AMBER and MIDI instruments. Thanks to these observations Domiciano de Souza et al. (2007)

Mass loss from massive stars

One of the most luminous, most massive, and unstable Luminous Blue Variable stars, h Carinae, is suffering from an extremely high mass-loss rate. A variety of observations suggest that the central source of this object is a binary, even if it is still a matter of debate. h Car was observed with AMBER (Weigelt et al. 2007) at two different epochs using three Unit Telescopes and both medium and high spectral resolutions in the spectral regions around the He I and Brg emission lines.

The visibility measurements revealed and resolved the h Car's optically thick wind region (see left part of Figure 3). Comparing the AMBER continuum visibilities with recent NLTE radiative transfer models, a very good agreement is found. In both the Brg and the He I emission lines, nonzero differential phases and non-zero closure phases were measured, indicating a complex and asymmetric object structure. Weigelt et al. (2007) developed a model which shows that the asymmetries measured within the wings of the Brg line with differential and closure phases are consistent with the geometry expected for an aspherical, latitudedependent stellar wind (see right part of Figure 3).

Colliding wind binary in late stellar evolution

The Wolf-Rayet (WR) and O star binary system g 2 Velorum was observed using AMBER in medium-resolution mode (Millour et al. 2007). Signals strongly varying through the broad Wolf-Rayet (WR) emission lines were observed and interpreted in the framework of a simple model that consists of two unresolved sources, whose flux ratio (close to one) is strongly wavelength dependent due to the strong emission of WR lines. Millour et al. (2007) demonstrated that the combination of differential visibility, differential phase and closure phase, as a function of the wavelength, allows both the angular separation of the binary components to be retrieved and their respective spectra to be extracted, leading to a direct measure of the distance of the system. It is found to be significantly larger than the Hipparcosdetermined distance, placing the target in the Vela OB association. One of the byproducts is a direct and model-independent measurement of the spectrum of the WR component, improving the modelling of this star. Furthermore, the signature of the circumstellar material is revealed in tens of spectral channels by a 5 to 10 s residual between the AMBER measurements and the binary model. These significant residuals allow speculations on the nature of the corresponding emission, probably associated with the wind-wind collision zone that contributes both to the emission lines and to the free-free continuum.

The outburst of the recurrent nova RS Oph

The famous recurrent nova RS Ophiuci exploded on 12 February 2006, an event expected since the previous outburst that occurred only 21 years ago. The extension of the expanding milliarcsecondscale emission was measured by AMBER only five days after the discovery using three telescopes and the medium spectral resolution in the K-band continuum, the Brg 2.17 µm line and the He I 2.06 µm line (Chesneau et al. 2007). Unfortunately, the 200 km s -1 spectral resolution was unsufficient to get a deep insight into the kinematics of the outflowing material ejected at high velocities. The low visibilities in the lines, compared to their values in the nearby continuum, are consistent with extended line-forming regions, the He I emission being formed in the fastest ejecta, close to the shock front. Both the continuum and the line emissions are highly flattened, sharing apparently the same global geometry, at different scales (see Figure 4). In addition, two radial velocity fields were detected in the Brg line: a slow (~ 1800 km s -1 ) expanding ringlike structure and a fast (~ 3 000 km s -1 ) structure extended in the East-West direction, a direction that coincides with the jet-like structure seen in the radio domain. These results demonstrate the capabilities of the VLTI to study the geometry and the kinematics of the earliest stages of nearby, i.e. few kiloparsec distant, recurrent or classical nova explosions.

mas

Figure 1 :

 1 Figure 1: Left: Spectral dependence of the visibility as measured with AMBER for MWC297 around the Brg line. Right: Edge-on view of the model including an equatorial optically thick disc (in red/yellow) and an outflowing wind (in blue). The wind geometry has been computed to both fit the visibility drop in the Brg line and reproduce the object spectrum. The apparent size of the wind is larger than the apparent size of the disc. From Malbet et al. (2007).

Figure 2 :

 2 Figure 2: Relative visibility (left plot) and differential phases (right plot) of a Arae across the Brg line profile for several VLTI baselines. The upper left subpanel is the Brg line profile. The plain lines are the fits we ob-

Figure 3 :

 3 Figure 3: Left panel: hCar's AMBER visibilities (red lines) compared to model predictions (green lines) in the spectral regions of the Brg and He I emission lines (from left to right: Brg, high spectral resolution 12 000; Brg, medium spectral resolution 1500; and He I , medium spectral resolution 1500). The figure shows the spectra (upper row) and the wavelength dependence of the visibilities (lower three rows; three different projected baseline lengths). Upper right panel: Illustration of the components of the geometric model for an optically thick, latitude-dependent wind (for the weak aspherical wind component, we draw the lines of latitudes to illustrate the 3Dorientation of the ellipsoid). Bottom right panels: for two representative wavelengths, the total brightness distribution of the model including the aspherical wind component and the contributions from the two spherical constituents. From Weigelt et al. (2007).

Figure 4 :

 4 Figure 4: Sketch of the fitted elliptical extension in the near-IR for RS Oph nova at t = 5.5 d compared with the radio structure observed at t = 13.8 d (thick extended ring). The continuum ellipse is delimited by the solid line, the ellipse that corresponds to the core of Brg by the dotted line and the one corresponding to the core of He I by the dashed line. The outer small dotted line delimits the Brg ellipse scaled at t = 13.8 d. North is up, East left. From Chesneau et al. (2007).

  estimated the size and geometry of the circumstellar regions re-

	Phase (degree)			B0
		2.15	2.16	2.17	2.18
		20
	Phase (degree)	-10 10 0		B1
		2.15 -20	2.16	2.17	2.18
		20
	Phase (degree)	-10 10 0		B2
		2.15 -20	2.16	2.17	2.18
		20
	Phase (degree)	-10 0 10		B3
		2.15 -20	2.16	2.17	2.18
			Wavelength (micron)

Acknowledgements

We are grateful to the AMBER consortium and to the ESO staff for their help in making such observations possible.


Romain G Petrov
email: petrov@unice.fr

Amber Consortium

INTRODUCING THE NEAR INFRARED VLTI INSTRUMENT AMBER TO ITS USERS

Keywords: astronomy, optical interferometry, instrumentation, high angular resolution, very large telescope, differential interferometry

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Introduction

AMBER is a near infrared, three beam, dispersed fringes, single mode, focal instrument for the interferometric mode of the Very Large Telescope (VLTI). The VLTI [START_REF] Glindemann | Very large telescope interferometer: a status report[END_REF] feeds AMBER with two or three beams produced by any of the telescopes present on the Paranal Mountain top. Each beam has been partially corrected for atmospheric turbulence and is stabilized in direction, position and optical path. Besides a very wide scientific program (Malbet et al., 2002a), the near infrared offers the possi bility to use nearly diffraction limited images. The three beams yield closure phases, which have interesting calibration properties and offer the possibility to reconstruct images. Still, AMBER will mainly be an efficient 'model fitting' instrument thanks to its large spectral coverage, from 1.1 to 2.4 µm and spectral resolutions (35, 1000 and 10000). The dispersed fringes increase: the instantaneous frequency plane coverage; the instrument accuracy when a (set of) spectral channel(s) can be used as a reference. They allow measures on unresolved and/or very high dynamics systems. The spatial filtering with single mode fibers is intended to achieve the highest possible accuracy on absolute as well as on differential visibility measurements.

Figure 1 summarizes the key elements of the AMBER concept. AMBER has a multi axial beam combiner (1) which produces fringes in the Airy pattern common to several non-redundantly spaced parallel beams. A cylindrical optics system (2) feeds this Airy pattern into the entrance slit (3) of a spectrograph (4), which disperses the fringes on an array detector (5). The medium (1000) and highest (10000) spectral resolutions imply to cool the spectrograph and to equip it with cold slits and pupil stops. Each incoming beam goes through a single mode optical fiber (6) which reduces all atmospheric and optic aberrations into an unknown optical path difference (OPD), which can be frozen, and an unknown coupling factor into the fibers. The coupling factors are monitored in real time by photometric beams ( 7) which analyze at each wavelength the transmitted flux. Since a single mode fiber is efficient only over a limited wavelength range, before feeding the beams in the fibers we split the total spectral range with dichroics and we have different spatial filters for the J, H and K bands. In addition to these basic features AM-BER is equipped with many beam cleaning tools (polarization filters, refraction and dispersion correctors...) and calibration devices (artificial coherent sources, calibrated phase delays, beam commuters...). More details about the instrument can be found in (Petrov et al., 2002a;Malbet et al., 2002b;[START_REF] Chelli | AMBER data processing[END_REF].

Amber Measurements and Science

By reconfiguring the VLTI ATs over the available Paranal summit positions, good use of super synthesis and of the increase in u-v sampling due to our large wavelength coverage permits reconstructing an image at the diffraction limit of the interferometer. However, such a process will be quite slow and future AMBER users should mainly think about it as an instrument designed to constrain models of astrophysical sources thanks to a limited number of angular measurements with a rich wavelength information. The following paragraphs explain what are the key AMBER measurements, together with some examples of the kind of information they can provide and with their main calibration requirements and methods.

AMBER MEASUREMENTS

For each elementary frame and in each AMBER spectral channel, we have an interferometric and three photometric signals processed by the same optics and the same dispersive elements. After the correction of detector cosmetics and background subtraction, a generalization of the ABCD algorithm is used [START_REF] Chelli | AMBER data processing[END_REF] to establish a linear relation between the values measured in each pixel and the complex coherence for each baseline. The matrix is calibrated using artificial sources and records of various interferograms affected by different known phase delays. It combines the effects of many parameters such as the detector gain table and bad pixels, the exact shape of the beams after the fiber output and the pupil stop in the spectrograph. For each baseline l-m, we measure the visibility V lm (λ) and the phase lm (λ).

Calibrated visibility as a function of wavelength: V lm (λ)

After time averaging and various bias corrections and calibration steps [START_REF] Chelli | AMBER data processing[END_REF], V lm (λ) yields the calibrated visibility in each spectral channel. For a simple object, V lm (λ) will give constraints on its angular size in the direction of the baseline B lm . If a certain range of spatial frequencies B lm /λ is explored, thanks to baseline changes or to the spectral coverage, visibility measurements yield the different scales of the structures present in the source. Several levels of attenuation at different spatial frequencies can indicate that structures of various sizes are present. For example the super giant star Betelgeuse shows at first very fast visibility attenuation with frequency, which is the signature of an extended dust envelope. At higher frequencies, a limb darkened disk model can fit a fraction of the visibility curve. Finally, at the highest frequencies, where the disk contribution is expected to be very low, the visibility remains significantly higher than zero, which can be interpreted as the signature of a structure smaller than the stellar disk such as a stellar spot.

The use of measurements at different wavelengths increases the frequency coverage and can constrain the interpretation. For example, in a circumstellar envelope or an AGN BLR, one can compare the visibility in two narrow spectral channels in a spectral line. Each spectral channel corresponds to an area of given radial velocity. The comparison between the measures constrains the relative sizes of the equal velocity areas and the velocity field [START_REF] Marconi | Extragalactic astronomy with the VLTI: a new window on the universe[END_REF].

The accuracy of the visibility measurements is critical. A few % accuracy yields gross size information (which in a limited number of cases can be very valuable). But, for example, limb darkening or different radial distribution of material in an envelope or in a dust torus will produce visibility variations of the order of 1% or less. Detecting features which are faint or small with regard to a main source (binary stars with a strong magnitude difference and ultimately extra solar planets, star spots, structures in envelopes or disks...) is particularly demanding. Even the hottest giant extra solar planets such as 51 Peg, have visibility signatures smaller than 10 -4 . The same will apply for 'planetary gaps' in accretion disks or for stellar structures on otherwise quite unresolved stars. In spite of all precautions: single mode spatial filtering, frozen piston, short exposures and dispersion correction, the visibility will still be sensitive to instrumental and atmospheric effects. The main tool to calibrate it is to observe as often as possible a reference star with unit or known visibility. The fundamental limit comes from the speed with which is it possible to switch between science and reference source, and to lock AOs and fringe tracker on both sources. Our best expectation for the period of such a calibration cycle is 10 minutes (closer to 30 minutes in the first months or years of VLTI operation).

Differential visibility and phase

One or several spectral channels can be defined as being a reference channel (when several wavelengths are used, the information is averaged over λ). Then the ratio between the visibility in each spectral channel and the visibility in the reference channel yields the differential visibility V lm (λ)/V lm (λ 0 ). The interesting point is that V lm (λ) andV lm (λ 0 ) are measured simultaneously which increases the accuracy of the calibration since many effects will affect all the channels in the same way (or in ways related by known equations). The absolute phase (λ) has no meaning unless an absolute metrology [START_REF] Glindemann | Very large telescope interferometer: a status report[END_REF] is available. However, it is immediately possible to measure the absolute phase differences between any spectral channel and the reference channel: lm (λ)lm (λ 0 ). The Differential Phase can bring specific astrophysical information. For example, if one observes in different channels of a line as described above, the Differential Phase will constrain the angular distance between the two different velocity areas. For sources with relatively simple or partially known velocity fields combining the variation of the visibility modulus and phase with wavelength is particularly interesting. The modulus variation will constrain the relative sizes and intensities of the equal velocity zones while the phase will constrain their relative position. Combining these values with some knowledge of the velocity field can allow to almost reconstruct images. A good example is likely to be circumstellar envelopes or AGN observed in emission lines [START_REF] Marconi | Extragalactic astronomy with the VLTI: a new window on the universe[END_REF]. Among many other applications we have multiple system with small number of stars, envelopes combining a global rotation and a global expansion, knots in a jet with a similar global direction but different velocities...Both Differential measurements are particularly interesting when the object is known to be unresolved in some spectral channels and resolved in others. Then, if the channel containing an unresolved source is used for reference, the differential complex visibility in the other channels is the exact object visibility and the image reconstruction process can be as efficient as if we had a 'phase referencing system'. A specific interest of the differential phase is that, unlike the differential modulus, it can be calibrated using an internal modulation instead of an external reference star. The measured phase difference lm (λ) will be the sum of:

lm (λ) = lm, * (λ) + lm,a (λ) + lm,I (λ) (1)
where lm, * (λ) is due to the source, lm,a (λ) is the contribution of the atmosphere, which is very strongly dominated by the differential dispersion in the tunnels and lm,I (λ) is the instrumental chromatic phase difference between the beams l and m. The dominant and most difficult to eliminate term is the instrumental one lm,I (λ). AMBER plans to solve this problem by inverting the beams as close as possible to the entrance of the instrument, before any chromatic optics in AMBER (dichroics, fibers, beam splitters, cryostat windows, spectrograph chamber, detector...).Then weobtain anew phase difference ml (λ):

ml (λ) =- lm, * (λ) - lm,a (λ) + lm,I (λ) (2)
The difference between equations (1) and (2) eliminates the instrumental term lm,I (λ). The atmospheric term lm,a (λ) results from the differences in average temperature, pressure and water vapor be tween the two beams. The first two terms have a globally multiplicative effect, except for saturated water vapor lines, and can be fitted in the data itself particularly if we use a large spectral coverage.

It has been shown [START_REF] Vannier | Optimizing a Differential Interferometric Instrument[END_REF]) that if we have differential phase measurements with the UTs, at low spectral resolution over the full J,H,K spectral coverage, limited only by photon, background and detector noise, the variation of phase with wavelength produced by the presence of a Jupiter mass hot extra solar planet around a solar type star at 10 pc can be detected. The SNR will be higher than 20 for 51 Peg (separation = 0.05 AU) and the SNR will be around two for a separation of about 0.2 AU. Such an observation will yield: the orbital parameters and thus the mass of the planet; the spectrum and thus the temperature and composition of the planet atmosphere. These measurements are one of the most ambitious AMBER goals, since the corresponding phase variation ranges between a few 10 -5 and a few 10 -4 radians. The conditions to achieve such an extreme accuracy are that the instrumental and atmospheric variation must be smaller than the photon noise over the calibration period. After a systematic study of all factors affecting this measurement [START_REF] Vannier | Optimizing a Differential Interferometric Instrument[END_REF] it appears that the result is achievable with a 30 s calibration cycle based on an internal beam commutation and out of reach if one has to use the 300 s cycle using a reference star. A specific attention has to be dedicated to the atmosphere term, which is not corrected by the beam commutation. As explained above, with two telescopes we expect to compute this term from the data itself. If this fails, with three telescopes this dispersion term can be eliminated using the closure phase relation [START_REF] Segransan | How AMBER will contribute to the search of brown dwarfs and extrasolar planets[END_REF].

Closure phase 123 (λ)

In the closure phase relation:

123 (λ) = 12 (λ) + 23 (λ) + 31 (λ)
all atmospheric and most of instrumental terms are cancelled. The exceptions result from the fringe detection process (error in the 'pixel to visibility' matrix (PTVM) due to a change in the detector gain table, in the beams shape or overlap after the fibers, in the spectrograph geometry...). Fortunately, we recently discovered (Petrov et al., 2002b) that commuting two of the three beams calibrates the closure phase relation. Let's assume that we measure the phases for the three baselines B12, B23 and B31. Each beam i is affected by an optical path difference (OPD) which translates in a phase i . The spatial frequency u lm = B lm /λ yields the sum of source phase φ * (u lm ) with a detection phase error φ d (u lm ), as illustrated in Figure 2.

Since φ * (u ij ) =-φ * (u ji ) and φ d (u ij ) =-φ d (u ji ), the difference between the closure phase 123 (λ) without and where φ * ,123 (λ) is an astronomical closure phase measurement free from any atmospheric or instrumental effect, if the beam commutation has been made fast enough for the instrumental terms to remain unchanged. This way to obtain a perfectly calibrated closure phase is unique to the VLTI because of its three large telescopes and to AMBER because of its beam commuting device. In addition to this very interesting calibration 'perfection', the closure phase is the key for image reconstruction. Moreover, even a limited number of closure phases can strongly constrain a model. For example a zero closure phase at medium frequencies and a non zero one at higher frequencies may indicate (and allow to locate) a small non symmetric feature on an otherwise globally symmetric object.

Differential photocenter

When the source is unresolved (i.e. smaller than B/λ) the phase tends to be proportional to the photocenter of the object at that given wavelength: lm (λ) = 2π( B lm /λ). ǫ(λ),w h e r e B lm . ǫ(λ) is the scalar product of the two vectors. The photocenter is defined as the barycenter of the object brightness distribution o(r, l) at the wavelength λ:

ǫ(λ) = ro(r,l)dr o(r, l) dr (4)
Since phase differences can be measured even when they are extremely small compared to 2π , this allows to extract, from differential phases, the variation of objects photocenter as a function of wavelength; even when the source is much smaller than the resolution limit. For example, for a magnitude 5 source observed with a resolution 1000 during 5 hours with an UT, one can measure photocenter displacement of about 0.1 µas. For a magnitude 10 star with UTs or a magnitude 7 star with ATs the accuracy on the photocenter displacement remains 1 µas for the same resolution 1000. This allows obtaining information on sources or structures much smaller than the resolution limit, such as stellar spots [START_REF] Petrov | Differential interferometry[END_REF]. One of the most interesting applications is likely to be the study of the Broad Line Regions (BLR) of Active Galactic Nuclei [START_REF] Marconi | Extragalactic astronomy with the VLTI: a new window on the universe[END_REF]. The study of the photocenter has numerous other applications, including stellar rotation, differential rotation, position of stellar rotation axes in multiple systems, stellar diameters of otherwise unresolved objects, asteroseismology, cepheids, structure and kinematics of unresolved disks and envelopes...The key fact to remember is that for objects with spatially structured spectral features, the limiting resolution of the AMBER/VLTI can be much smaller than the classical B/λ (1 or 2 mas) limit. Photocenter measurements of unresolved objects are linked to differential phase measurements because it is easy to show that when a source of angular size a is unresolved, the photocenter and the differential phase decrease proportionally to a, while the visibility modulus approaches 1 like a 2 and the closure phase decreases like a 3 .

OBSERVING AND CALIBRATION CYCLES OF AMBER

The observing modes of AMBER are set by: the number of telescopes (2 or 3); the spectrograph set up (resolutions ≈ 35; ≈ 1000 ; ≈ 10000 ; spectral coverage; central wavelength); the availability of a fringe tracker. It seems that, from the very beginning, AMBER observations will benefit from an on axis fringe tracker (FINITO [START_REF] Gai | FINITO: A Fringe Sensor for the VLTI in Two and Three Beam Configuration, ESO symposia Scientific drivers for ESO future VLT/VLTI instrumentation[END_REF]).

In all cases the observing and data processing sequences remain the same and are defined by the following parameters.

-The required accuracy: for very precise measurements of the visibility modulus, one should use very short frame times, even with a fringe tracker. The minimum frame time is of the order of 10 ms for 40 spectral channels (J,H and K band in the low resolution mode). For higher sensitivity (and lower accuracy except in the differential modes) one could use longer frame times (50 to 100 ms) even with poor fringe tracking. Finally for high spectral resolution, correct fringe tracking must support frame times up to a few tens of seconds. -The kind of calibration selected: visibility measurements will give priority to frequent calibration using one or more reference stars. Differential measurement will use relatively fast internal beam commutation and use external reference stars only from time to time to check the instrument. In all these very similar modes, we set up the instrument and perform the corresponding PTVM calibration. Then we read frames, with frame times selected as explained above, and we combine them in exposures (successive frames recorded on the same source with the same instrument set up). Each exposure yields measurements of the complex coherence but a complete calibration needs a combination of exposures of different sources (science target, reference star, sky background, calibration lamps...) or with different beam commutations. All sequences of operations needed to perform a given measurement, i.e. a given exposure cycle, are automatically executed by the instrument. The sequences are described in standard templates, with parameters set by the User as a function of its scientific objectives. For each given exposure cycle there is a standard way to reduce the data for calibrated measurements, which will be delivered to the standard User. However, all data will be recorded to allow more sophisticated post processing.

SUMMARY OF AMBER TOP LEVEL SPECIFICATIONS AND EXPECTED

PERFORMANCES

Table I summarizes the main specifications of AMBER after the various tradeoffs made during the design phases.

The fringe tracker sets the limiting magnitude of AMBER. The current conservative estimate for λ/20 fringe tracking is H = 13. Table II gives the SNR which can be expected in each spectral channel on a source with visibility close to 1. We assumed 4 hours observations divided in 50 to 100 s frames for a star of magnitude H=12 with the UTs or H=9 with the ATs, for various spectral resolutions. It is important to remember that: the error in the differential phase can be estimated to be σ φ = 1/( √ 2SNR) radians; the error in the photocenter displacement is given by σ ǫ = σ φ (λ/B)/2π ; for lower visibility sources, the SNR decreases with the visibility modulus.

For example, for a magnitude H=12, at resolution 1000 we will get σ φ = 3 × 10 -3 radians and σ ǫ = 2 µarcsec (K band, 100 m baseline), which is suf-

V = 0.01 σ V = 10 -4 Differential visibility accuracy 3σ V/V 0 = 10 -3 3σ V/V 0 = 10 -4 Differential phase accuracy 3σ = 10 -3 rad σ = 10 -5 rad
Instrument contrast stability 10 -2 over 5 minutes 10 -3 over 5 minutes Differential phase stability 10 -3 rad over 1 minute 10 -4 rad over 1 minute ficient to resolve all quasars and Seyfert 1 BLRs with these kind of magnitudes. Another example, is that for a magnitude 7 star, we should have at resolution 35, a differential phase error σ φ = 6 × 10 -5 radians, sufficient to resolve many 51 Peg like planets, if of course we manage to be limited only by these fundamental noises.

Eventually, a field separator will allow tracking of fringes and measuring wavefronts on stars up to 1 minute away from the science source. When such a reference source exists, the limiting magnitude of AMBER can approach 20 in a few hours. However, with the current performance of fringe trackers, the corresponding sky coverage will be very poor. To have good sky coverage, we need fringe sensors with detectors limited by photon noise in the near infrared. This might be achievable in a few years.

Conclusion

All subsystems of AMBER have been delivered and are now in the final global integration and test phase. The first fringes in the K band with all instrument components have been obtained in January 2003. We expect to ship the instrument to Paranal during the summer 2003, after a full laboratory commissioning of the hardware and the software. The commissioning with two large telescopes (UTs with Adaptive Optics) should take place before the end of the year and a partial access to the instrument offered to the Consortium and to the Community starting in April 2004. The full operation with three telescopes will probably be open in October 2004. The summary of the AMBER consortium Guaranteed Time Program will be released in April 2003. The initial pressure factor was of about two on the UTs and three on the ATs and we had to make a severe selection. We now have two proposals about the distance scale in the Universe, 15 extra galactic programs, mainly about Quasars and other AGNs, 22 programs about Star formation covering many topics such as parameters of PMS stars, disks, outflows, jets, young binaries and young stellar clusters. Ten programs are about low-mass companions, with a very developed program on Pegasides planets. More than 22 programs tackle various aspects of the late stages of stellar evolutions and about 15 programs are dedicated on fundamental properties of stars (age, diameters, rotation, activity, seismology, magnetism) and of stellar envelopes (structure and kinematics). The UTs are mainly asked for extra galactic sources and extra solar planets. This leaves a lot of objects and room for additional programs and ideas and we are quite confident that AMBER will permit a large harvest of new results.

Figure 1 .

 1 Figure 1. Basic concept of AMBER. 1: multi axial beam combiner; 2: cylindrical optics; 3: anamorphotic image in spectrograph slit; 4: spectrograph; 5: dispersed fringes on 2D Hawaii detector; 6: spatial filtering of incoming beams with single mode fibers; 7: channels for photometric monitoring.

Figure 2 .

 2 Figure 2. Closure phase before and after beam commutation.

† The AMBER Consortium has been established by the five Institutes in the affiliation list and is supported by more than 50 engineers and researchers. More information can be found on the web site: www.obs-nice.fr/amber


Abderrahman Benchekroun
email: abderrahman.benchekroun@yncrea.fr

Arnaud Davigny

Vincent Courtecuisse

Léo Coutard

Kahina Hassam-Ouari

Benoit Robyns

Demand-Side Management Strategy for Electric Vehicles and Electric Water Heaters Connected to Distribution Grids

Keywords: Demand-side management, Electric vehicles, Electric water heaters, Renewable energy, Distribution grids. Nomenclature

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Due to recent energy transition policies trying to limit the alarming effects of global warming, the use of renewable energy instead of fossil fuels is very encouraged alongside other green technologies such as electric vehicles.

According to the French General Commissariat for Sustainable Development, renewable energy constitutes 16.3% of the gross final energy consumption in France in 2017 [1]. These numbers have been growing steadily in the past ten years, and significant growth of 62% in primary renewable energy production has been recorded between 2005 and 2017. This growth is mainly related to the growth of biofuels, heat pumps and the wind energy sector.

On the other hand, the global stock of electric cars surpassed the 3 million mark in 2017 [START_REF]Towards cross-modal electrification[END_REF] according to the International Energy Agency, with a similar growth of the charging infrastructure worldwide. This massive EV integration aggravates the electricity peak loads noticed during the day, leaving night hours with relatively low electricity demand. This significant increase in renewable energy production on one side, and the growing integration of EVs on the other side can cause many problems on the distribution network mainly because of their stochastic nature. The electrical grid will have a higher risk of having overloading problems for example, which may require expensive reinforcements for equipment such as transformers and cables. The impact of EV integration has been broadly studied in the literature, showing that problems like congestions and voltage problems may appear [START_REF] Anastasiadisa | Effects of Increased Electric Vehicles into a Distribution Network, Technologies and Materials for Renewable Energy[END_REF] if there is no proper charging strategy for EVs and that the reliability of distribution systems may become compromised [START_REF] Reddy | Impact of plug-in electric vehicles and distributed generation on reliability of distribution systems[END_REF] [START_REF] Min | Study on Influence of Large-scale Electric Vehicle Charging and Discharging Load on Distribution System[END_REF]. However, the author of [START_REF] Nour | Mitigation of Electric Vehicles Charging Impacts on Distribution Network with Photovoltaic Generation[END_REF] showed that the use of decentralized photovoltaic generation in distribution networks could alleviate the impacts of uncontrolled EV charging on transformer and cable loading, the voltage profile and daily energy losses.

Another way to remedy previous problems is to use demand-side management strategies. A review of the DSM framework and methods is presented in [START_REF] Meyabadi | A review of demand-side management: Reconsidering theoretical framework[END_REF]. Among previous studies involving EV load control, [START_REF] Xu | Multi-energy Coordination and Schedule Considering large-scale electric vehicles penetration[END_REF] used a multi-energy scheduling strategy to minimize the grid power fluctuation, [START_REF] Milano | Optimal Load Management With Inclusion of Electric Vehicles and Distributed Energy Resources[END_REF] used an optimal load management strategy based on quadratic programming to schedule EV charging, and [START_REF] Sarabi | Supervision of Plug-in Electric Vehicles Connected to the Electric Distribution Grids[END_REF] used dynamic programming to reduce power losses and improve the voltage profile within the grid.

DSM strategies for home applications also control electric water heaters, heating, air conditioning, and other home appliances generally using dynamic pricing strategies [START_REF] Hu | Hardware Design of Smart Home Energy Management System With Dynamic Price Response[END_REF]. These methods try to encourage users to avoid using all their appliances in peak times and shift demand towards less busy hours or periods when renewable energy production is abundant. The acceptance and involvement of consumers and energy producers to participate in these DSM methods were also studied recently in [START_REF] Durillon | Integration of Consumers' Sensitivities and Preferences in Demand Side Management[END_REF].

In this paper, an energy management strategy is proposed to monitor EVs and EWHs within the distribution network, with a primary goal of minimizing the energy transmission cost for the distribution system operator. A fuzzy logic supervision system is used to generate reference power signals for EV charging and EWH activation. Fuzzy logic was particularly chosen for its effectiveness in managing complex systems such as electrical distributions grids without the necessity to model all their components. The proposed supervisor's parameters are determined first empirically and then optimized using a genetic algorithm for better performance. The paper is structured as follows: First, the adopted EV and EWH load models are presented. Then, the proposed energy management strategy is explained, starting with ETC calculation since it's the objective to minimize, and going through the different components of the supervision system that are presented alongside the operating rules. The system's performance is finally evaluated through simulation results, and some perspectives are given in the last section.

Load Modeling

This section presents the proposed load models for EVs and EWHs that are used in the distribution grid's energy management system.

Electric Vehicles. The load profile and energy requirements of EVs are determined using deterministic and probabilistic parameters that define their charging process: -Battery specifications: especially the battery capacity (kWh) and the standard consumption (kWh/km) of the EV. In this study, all EVs are considered to have the same characteristics as the Renault ZOE [START_REF] Renault | [END_REF]. -Charging mode: since the EVs can charge at home and at the workplace, the normal charging mode (16A/230V) is considered for its versatility. Faster charging modes could be considered in places where the necessary infrastructure (charging stations) is available. -Daily travel distance: the average travel distance recorded in France in 2017 is around 35km per day [START_REF]Parcours moyens annuels des voitures particulières en France de[END_REF]. -Arrival/departure times: based on the traffic habits of French department Deux-Sèvres [START_REF] Sarabi | Contribution of Vehicle-to-Grid (V2G) to the energy management of Plug-in Electric Vehicles' fleet on the distribution network[END_REF] in which this study is conducted, the home to office journey takes place around 08:00 and the return journey is around 17:00.

The heterogeneity between EV users is represented by a normal distribution, which characterizes a random variable x by its average value µ and its standard deviation σ, as expressed in (1). The chosen values in this study are given in Table 1.

𝑓(𝑥) = 1 𝜎√2 • 𝜋 𝑒 ( )
.

(1) These arrival/departure times vary on weekends since people have different dynamics. Traffic-based modeling of working days and weekends was proposed in [START_REF] Sarabi | Traffic-based Modeling of Electric Vehicle Charging Load and its Impact on the Distribution Network and Railway Station Parking Lots[END_REF], where probability distribution functions were fitted to real traffic data through a linear optimization problem. In this study, simpler assumptions are used: the office charging is not considered on weekends, and the home arrival and departure times are shifted two hours forward since people do not wake up early for work and return home later in the evening.

Electric Water Heaters. The load profile and energy requirements of EWHs are determined using a single element thermal model inspired by [START_REF] Shaad | Parameter identification of thermal models for domestic electric water heaters in a direct load control program[END_REF] and depend mainly on the users' hot water demand which is determined statistically.

The incoming cold water enters the EWH at the temperature Tin = 15°C, is heated through an electric resistance, and the outgoing hot water is delivered at the temperature Tout = 60°C as explained in Fig. 1. The water temperature is considered uniform in the entire EWH.

𝑃 (𝑡) = 𝜌𝑐 𝑉 𝑑𝑇(𝑡) 𝑑𝑡 . (2)
𝑃 (𝑡) = 𝜌𝑐 𝑊 (𝑡)(𝑇(𝑡) -𝑇 ) . (3)
𝑃 (𝑡) = 𝑆 𝑅 (𝑇(𝑡) -𝑇 ) . (4)
where:

-ρ and cp are respectively the mass density (kg/m 3 ) and the specific heat (J/(kg.°C)) of water. -V, S, and R are respectively the volume (m 3 ), the surface (m 2 ) and the thermal resistance (m 2 .°C/W) of the tank. -Wflow is the outgoing water flow (m 3 /s).

The EWH is switched on or off by a thermostat, which means Pelectric is either null or equal to the nominal power Pnom of the EWH. The only remaining parameter that varies through time is the hot water demand (Wflow).

The hot water usage in France has been determined with a one-hour step by a study [START_REF]Les besoins d'eau chaude sanitaire en habitat individuel et collectif -Guide technique[END_REF] conducted by the French environmental and energy efficiency agency (ADEME). An average household of three people consumes around 150±50 L of water at 40°C daily, which corresponds to 83±28 L at 60°C. The hot water consumption profile is then obtained by modulating this average daily consumption (Wcons day average) with hourly (αhour), daily (αday) and monthly (αmonth) coefficients, as expressed in [START_REF] Nour | Mitigation of Electric Vehicles Charging Impacts on Distribution Network with Photovoltaic Generation[END_REF]. The water flow is considered constant within every hour as it is suggested in Fig. 2.

𝑊 = 𝑊 * 𝛼 * 𝛼 * 𝛼 . ( 6
) Finally, taking into account all the elements of (2) and knowing that Pelectric and Wflow are considered constant during every hour, the temperature evolution is calculated in [START_REF] Meyabadi | A review of demand-side management: Reconsidering theoretical framework[END_REF]. , where: ( 7)

𝜏 = 𝜌𝑐 𝑉𝑅 𝑆 + 𝜌𝑐 𝑊 𝑅 . ( 8
)
𝐾 = 𝑃𝑅 + 𝑆𝑇 + 𝜌𝑐 𝑊 𝑅𝑇 𝑆 + 𝜌𝑐 𝑊 𝑅 . (9)
To obtain the temperature evolution on a long period, t0 is reset every hour, T(t0) gets the final value of the previous hour, and the temperature evolution within the hour is determined using [START_REF] Meyabadi | A review of demand-side management: Reconsidering theoretical framework[END_REF].

Energy management strategy

This section starts by presenting the computation of the energy transmission cost aimed to be minimized in this study, then describes the specifications of the proposed energy management system, its structure, and implementation.

Energy transmission cost computation. As users of the electricity transport network operated by RTE, the French Transport System Operator, Distribution System Operators like GEREDIS pay an annual Energy Transmission Cost that represents an important financial expense. The computation of this cost is set by the public electricity network user tariff (TURPE 5) [START_REF] Rte | TURPE 5 -Tarification des réseaux -Distributeurs[END_REF], which is regulated by CRE, the French energy regulatory commission, to guarantee economic and secure access to electricity within the network. The considered components of the energy transmission cost are mainly related to the energy extracted by the DSO, and the periods where the subscribed power with the TSO is exceeded. The cost computation [START_REF] Sarabi | Supervision of Plug-in Electric Vehicles Connected to the Electric Distribution Grids[END_REF] is done for each High Voltage / Medium Voltage (HV/MV) substation.

𝐸𝑇𝐶 = 𝑏 𝑃 + 𝑏 (𝑃 -𝑃 ) + 𝑐 𝐸

+ 𝛼 𝑏 𝑃 -𝑃 .

(

) 10
where:

-Psc i is the subscribed power (in kW) at the i th time frame (each time frame corresponds to peak or off-peak hours during different seasons, as defined in [START_REF] Rte | TURPE 5 -Tarification des réseaux -Distributeurs[END_REF]). -Ei is the extracted energy (in kWh) during the i th time frame.

-Pj is the measured power (in kW) exceeding the subscribed power. -bi and ci are weighting coefficients for power and energy respectively. They depend on the voltage range on the substation and tariff version considered. -α is a weighting coefficient for the exceeding power that also depends on the voltage range on the substation.

System specifications. The supervision system aims to control EV charging and EWH activation within a distribution network. Its objectives, constraints, means of actions and performance indicators are presented in Table 2. Supervisor structure. The energy management strategy aims to supervise EV charging and EWH activation in a real distribution network operated by GEREDIS. Real power flow data is collected every 10 minutes from HV/MV substations through telemetry devices.

As shown in Fig. 3, the supervision system takes two main inputs. First, there is the measured power at the substation (Psub) expressed in [START_REF] Hu | Hardware Design of Smart Home Energy Management System With Dynamic Price Response[END_REF] that includes PV and wind energy producers (PPV, Pwind), EVs and EWHs, and other consumers (Pcons) connected to the HV/MV substation.

𝑃 = (𝑃 + 𝑃 + 𝑃 ) -(𝑃 + 𝑃

) .

(11) Fig. 3. Structure of the supervision system

The measured power can be positive in case the consumption within the substation is greater than the local production and can be negative in the opposite case.

The second input is the State of Charge (SoC) of EVs and EWHs, which is estimated at each time step within the system. The system output is the global power reference for charging EVs and EWHs, which is finally distributed into individual references for EVs and EWHs. It should be noted that in the entire supervision strategy, EWHs are treated the same way as EVs by reasoning on their daily energetic needs instead of thermodynamic parameters such as temperature.

Supervision strategy. The supervision system includes two stages, as illustrated in the functional graph in Fig. 4 [20]: -A "Fuzzy Mode" that aims to maximize EV and EWH charging during the periods of excess PV/Wind production, and limit it in the opposite case. -A "Boolean Mode" that guarantees the fulfillment of EVs and EWHs daily energy requirements by forcing the charging power reference if necessary.

Fig. 4. Functional graph of the supervision system

The supervision system generates the charging power references for EVs at home/office and EWHs depending on Psub, which can be Negative (N) or Positive (P), and the SoC of EVs and EWHs, which can be Small (S), Mean (M) or Big (B). The proposed operating rules are illustrated in Fig. 5.

Fig. 5. Operational graph of the supervision system [START_REF] Courtecuisse | A methodology to design a fuzzy logic based supervision of Hybrid Renewable Energy Systems[END_REF] The Fuzzy Mode's inputs and outputs are described using membership functions (Fig. 6) that are determined empirically at first. On the other hand, the Boolean Mode duration was also fixed empirically at 3 hours for EVs and EWHs. Optimization of supervisor parameters. The supervision system's performance depends directly on the parameters of the Fuzzy Mode and the Boolean Mode. In this study, these parameters are optimized using a Genetic Algorithm (GA) as presented in [START_REF] Bouallaga | Optimization of fuzzy supervisor for electric vehicle load in distribution grid[END_REF]

Results and Discussion

The energy management strategy is tested using data from an HV/MV substation operated by GEREDIS that contains 2 90/20 kV transformers of 36 MVA and a 225/20 kV transformer of 40 MVA. Around 5000 customers and six energy producers (PV and wind) with a total power of 83 MW are connected to this substation. The controllable loads considered in this study are 2700 EVs (according to the 2030 scenario in the department of Deux-Sèvres [START_REF] Bouallaga | Gestion énergétique d'une infrastructure de charge intelligente de véhicules électriques dans un réseau de distribution intégrant des énergies renouvelables[END_REF]), and approximately 3400 EWHs that are identified from the global load profile in the substation. The total energy flow is recorded every 10min using telemetry devices and communicated to the supervision system.

The load profile of EVs and EWHs, as well as the resulting power profile at the HV/MV substation, are represented in Fig. 8 and 9 respectively for a day where renewable energy is available and another one when it is not.

Fig. 8. Simulation results on a day with available renewable energy

In the first case illustrated in Fig. 8, the supervisor shifts most of the EV and EWH load to the time when an excess of renewable energy is detected in the substation (Psub < 0). In the second case illustrated in Fig. 9, there is no excess production that appears throughout the day, therefore EVs are activated only by the Boolean Mode in the last hours before their departure, and the same thing happens for EWHs at the end of the day.

Fig. 9. Simulation results on a day with no available renewable energy

The annual results of the simulation are used to calculate the optimal subscribed power with the TSO and the resulting annual energy transmission cost shown in Table 3. The introduction of EVs and EWHs into the grid without any supervision increases the ETC by approximately 13.1%. The energy management system reduces this percentage to only 8.6% of the original cost.

Conclusion and Perspectives

This paper proposed a supervision strategy to manage EV charging and EWH activation in a distribution grid, with a particular goal of minimizing the energy transmission cost. The performance of the system was confirmed through simulation by testing it on real power flow data from an HV/MV substation and showed that proper supervision of EVs and EWHs leads to an important reduction of the energy transmission cost. However, some additional improvements are expected to be introduced in future work. The integration of production and consumption forecasting into the supervision strategy can greatly improve the system since it adds the possibility to use optimization methods to calculate the best load profile for EVs and EWHs one day ahead. Another phenomenon to be considered is the impact of these strategies on electrical grid parameters like voltage and loading. Therefore, another important perspective of this work is to integrate the electrical constraints in the supervision strategy and modify the outputs accordingly. Finally, these strategies will be implemented in a real supervision system to test their performance and evaluate their acceptability by users that are participating in a demonstrator project in the Nouvelle-Aquitaine region in France.

Fig. 1 .

 1 Fig. 1. Operating diagram of an EWH The energy exchanges in the EWH are expressed in the equations (2-5). The energy given by the heating element (Pelectric) is used to raise the temperature T of the water (Pheat), a part is lost due to the consumption of hot water (Pwater flow), and another one is lost as heat exchanges with the outside environment through the tank (Ploss). 𝑃 (𝑡) = 𝑃 (𝑡) + 𝑃 (𝑡) + 𝑃 (𝑡) .(2)

Fig. 2 .

 2 Fig. 2. Hourly hot water demand of 100 EWHs

Fig. 6 .

 6 Fig. 6. Membership functions of the empirical fuzzy supervisor

  . The objective function to minimize is the annual energy transmission cost and the parameters are the membership functions (MF) limits and the duration of the Boolean Mode for EVs and EWHs. The optimized MFs are represented in Fig. 7, and the optimized Boolean Mode durations for EVs at home/office and EWHs are ∆tBM EV office = ∆tBM EV home = 3h10 and ∆tBM EWH = 4h30.

Fig. 7 .

 7 Fig. 7. Membership functions of the optimized fuzzy supervisor

Table 1 .

 1 Arrival/departure times to home/office.

		Daily travel distance, km	Office arrival, hh:mm	Office departure, hh:mm	Home arrival, hh:mm	Home departure, hh:mm
	µ	35	08:15	16:45	17:15	07:45
	σ	5	00:30	00:30	00:30	00:30

Table 2 .

 2 Supervision system specifications.

		Reduce the DSO's energy transmission bill
	Objectives	by promoting local consumption of renewable energy through EV charging and
		EWH activation.
		-Renewable power intermittency.
		-Availability of EVs at home/office.
	Constraints	-Full charging of EVs before departure.
		-Complete heating of EWHs before the
		end of the day.
	Means of action	Load shifting.
	Performance indicators	Energy transmission cost reduction.

Table 3 .

 3 Performance indicators.

		Subscribed Power, kW	Energy Transmission Cost, k€	Energy Transmission Cost, pu
	No EVs/EWHs	21738	786	1.000
	Unsupervised EVs/EWHs	24351	889	1.131
	Supervised EVs/EWHs	22438	853	1.086

Acknowledgment

This work has been supported by SEOLIS and GEREDIS, the energy provider and distribution system operator of the department of Deux-Sèvres respectively.