C Édric

Di Loreto

Jean-R Émy Chardonnet
email: jean-remy.chardonnet@ensam.eu

Julien Ryard
email: julien.ryard@ensam.eu

Alain Rousseau
email: alain-r.rousseau@enedis.fr

WoaH: A Virtual Reality Work-at-Height Simulator

Keywords:

We present WoaH, a virtual reality work-at-height simulator aimed at (i) testing whether future workers are able to manage their stress when high up and thus easily detect susceptibility to vertigo, and (ii) training in a typical work-at-height engineering operation. The simulator is composed of a real ladder synchronized in position with a virtual one placed 11 meters above the ground in a virtual environment. Visualization is done through a head-mounted display (HMD). We conducted a first user study evaluating our simulator in terms of cybersickness, perceived realism and anxiety, through both subjective (questionnaires) and objective (electrodermal activity) measurements, and testing whether vibratory cues could enhance the level of anxiety felt. Results indicate that WoaH generates anxiety as expected and is perceived as realistic. Adding vibrations had significant impact on the perceived realism but not on the electro-dermal activity. These first results bring insights to future developments for a deployment in companies dealing with work at height.

INTRODUCTION

The use of virtual reality (VR), especially by companies, has become more and more common in the past few years, thanks to the availability of low cost devices, such as the Oculus Rift or the HTC Vive head-mounted displays (HMD). Virtual reality enables to immerse a user into a virtual environment and allows to evolve in a reproducible and safe way in situations that could be dangerous in reality. It can therefore be used as a tool to train, control, prevent or even treat disorders. For instance, on one hand, virtual reality exposure therapy was proposed a few decades ago to treat fear of heights. On the other hand, the entertainment field took the opportunity of low cost HMDs to propose experiencing fear of heights and vertigo in various situations such as walking on a board suspended high above the ground, or virtual rock climbing. From a more real applicative point of view, work-at-height is one of the dangerous situations encountered by workers that can be reproduced using virtual reality [START_REF] Hsiao | Human responses to augmented virtual scaffolding models[END_REF]. Companies can make use of such technology as a prevention tool to check the ability of their workers to work at height or to train them with full immersion in realistic conditions, without being exposed to possible fatal issues while reducing insurance costs and space needed compared to real work-at-height simulation mockups. However, still very few VR-based simulators are available for such use cases [START_REF] Hsiao | Human responses to augmented virtual scaffolding models[END_REF].

We present WoaH, a transportable virtual reality Work-at-Height simulator (see Fig. 1), aimed at testing whether users are able to manage their stress during the simulation of a typical work-at-height operation or not. Our motivation lies in investigating the parameters influencing the fear of heights in a virtual environment and allowing an accurate restitution of a real work-at-height situation. Therefore, in this study, we mainly focused on presence, more specifically the perceived realism, and anxiety, as they may represent strong indicators to judge the simulator's performance. For that purpose, multisensory cues were integrated. The final goal of WoaH is to be deployed in companies dealing with work at height, as an additional tool to hire workers, limitating risks and cost. Here, the use case of the WoaH simulator concerns the maintenance of high-voltage power lines.

Related Work 1.Vertigo and Psychology

Vertigo has been widely studied in the last decades. Two different kinds of vertigo can be distinguished [START_REF] Bles | The mechanism of physiological height vertigo. II. posturography[END_REF][START_REF] Brandt | The mechanism of physiological height vertigo. I. theoretical approach and psychophysics[END_REF]: the distance vertigo, seen as physiological, and acrophobia, seen as psychological and thus basically as a fear. Brandt et al. focus on the physiological height vertigo and show that the eye-object distance is the main parameter affecting vertigo. A conflict between visual and vestibular information was hypothesized to be responsible for vertigo. Humans continuously need visual cues to stabilize themselves. The parallax induced by natural displacements of the head helps stabilizing. When a subject is high up, the lack of close and stable visual references involves a parallax that is under the eye perception threshold. Therefore it imposes him to rely on his proprioceptive and vestibular information to stabilize himself, resulting in possible increased body sway. Coelho and Wallis further support this theory by asserting that a larger sway may generate a fear of falling and so the sensation of vertigo [START_REF] Coelho | Deconstructing acrophobia: Physiological and psychological precursors to developing a fear of heights[END_REF]. Acrophobic subjects may be persons with vestibular system deficiency or for whom visual information are too important. Therefore stabilization of their body becomes difficult and implies a larger body sway. Experimental studies revealed that vertigo may be a means for the body to indicate a situation inducing postural instability and so possibly leading to a fall. Moreover, Brandt et al. propose a trigonometrical model for distance vertigo [START_REF] Brandt | The mechanism of physiological height vertigo. I. theoretical approach and psychophysics[END_REF]. Based on this model, the distance of five meters is announced as a saturation distance for visual cues from which body sway does not increase anymore [START_REF] Coelho | Deconstructing acrophobia: Physiological and psychological precursors to developing a fear of heights[END_REF]. The effect of body postures has also been studied and results show that standing up in front of the void is the posture increasing the sensation of vertigo at most.

Coelho et al. highlighted the impact of another parameter on the variation of anxiety in height-involving situations, that is displacements [START_REF] Coelho | The role of self-motion in acrophobia treatment[END_REF]. Results show that especially lateral displacements induce more anxiety on acrophobic subjects. Adding lateral displacements in a virtual simulation may therefore increase anxiety on acrophobic subjects, but also the simulation susceptibility to trigger the sensation of vertigo, along with significant variations of physiological and behavioral reactions from subjects.

Cohen proposed a subjective method to evaluate the inclination to acrophobia, through a dedicated questionnaire [START_REF] Cohen | Comparison of self-report and overt-behavioral procedures for assessing acrophobia[END_REF]. Results show that a relationship does exist between the reported acrophobia score and the level of anxiety.

Acrophobia in Virtual Reality

Since virtual reality exists, several studies have been conducted on height perception in virtual environments and explored possibilities to use it to treat acrophobia.

Cleworth et al. conducted an experiment reproducing exactly the same anxiogenic situation in the real world and in virtual reality [START_REF] Cleworth | Influence of real and virtual heights on standing balance[END_REF]. Measuring electrodermal activity, anxiety, fear and movements of the center of pressure of participants, it appears that these measures significantly increase with height, but their variations are less important in a virtual exposure compared to an in vivo exposure. Using a CAVE system, Simeonov et al. showed that virtual reality appears as a good tool to reproduce the psychological and physiological effects of height-involving situations on subjects [START_REF] Simeonov | Height effects in real and virtual environments[END_REF]. Typical applications include prevention or control in situations where people work at height.

Other past work showed that HMDs can also reproduce the symptoms of a height exposure. Hüweler et al. submitted healthy and acrophobic subjects to a visual stimulus aiming at inducing a conflict between visual and somatosensory information [START_REF] Hüweler | The impact of visual flow stimulation on anxiety, dizziness, and body sway in individuals with and without fear of heights[END_REF]. Highly acrophobic subjects reported more anxiety and vertigo than healthy subjects, showing that receiving contradictory information is enough to induce anxiety on acrophobic subjects, thus supporting vertigo mechanisms theories previously exposed in [START_REF] Bles | The mechanism of physiological height vertigo. II. posturography[END_REF][START_REF] Brandt | The mechanism of physiological height vertigo. I. theoretical approach and psychophysics[END_REF].

As past work showed the efficiency of virtual reality in reproducing physiological symptoms of a real exposure to height, researchers explored possibilities to use virtual reality for the treatment of acrophobia. The first studies date back to 1995 and showed promising results [START_REF] Rothbaum | Effectiveness of computer-generated (virtual reality) graded exposure in the treatment of acrophobia[END_REF]. In this regard, virtual reality has shown to be at least as effective as an in vivo exposure for the treatment of acrophobia [START_REF] Coelho | The role of self-motion in acrophobia treatment[END_REF][START_REF] Emmelkamp | Virtual reality treatment in acrophobia: a comparison with exposure in vivo[END_REF][START_REF] Jang | The development of virtual reality therapy (VRT) system for the treatment of acrophobia and therapeutic case[END_REF]. Based on previous findings, Coelho et al. emphasize the role of lateral displacements during an acrophobia therapy, involving the role of parallax in the visuo-vestibular conflict which occurs in the distance vertigo [START_REF] Coelho | The use of virtual reality in acrophobia research and treatment[END_REF]. In this paper, we do not aim at providing a simulator for treating acrophobia, but just to use it as a detection tool of acrophobia and a training tool for real dangerous situations.

Presence and Anxiety

Presence is another parameter to take into account to ensure a simulator's fidelity. Slater and Wilbur define the presence as the sense of being here [START_REF] Slater | A Framework for Immersive Virtual Environments (FIVE): Speculations on the role of presence in virtual environments[END_REF]. Presence should make subjects act as if they were in a real environment. They should remember the experiment more as a place they visited than as pictures they saw. Slater further investigated this aspect and proposed two concepts contributing to realistic behaviors in immersive virtual environments: Place Illusion and Plausability Illusion [START_REF] Slater | Place illusion and plausibility can lead to realistic behaviour in immersive virtual environments[END_REF], further developed in [START_REF] Skarbez | A survey of presence and related concepts[END_REF].

Juan and Pérez compared the level of presence and anxiety in a height-involving situation with an HMD and a CAVE system [START_REF] Juan | Comparison of the levels of presence and anxiety in an acrophobic environment viewed via HMD or CAVE[END_REF]. Though with both systems a positive correlation between anxiety and presence was revealed, better results have been significantly obtained with the CAVE system. However, the HMD used in this work had an only 40 • field of view, which is much less than what current HMDs propose. In our case, we aim at providing a transportable simulator. Therefore, large-screen displays have not been considered for our simulator (compared to the one presented in [START_REF] Hsiao | Human responses to augmented virtual scaffolding models[END_REF]), as they are hardly transportable. Moreover, in work-at-height conditions, subjects may need to look up. When using a CAVE system or another large-screen display, it requires the display system to have at least a ceiling or high screens, which is not necessary the case of most existing systems. This issue does not arise when using an HMD.

Some studies found contradictory results on the relation between presence and anxiety [START_REF] Alsina-Jurnet | The role of presence in the level of anxiety experienced in clinical virtual environments[END_REF][START_REF] Bouchard | Anxiety increases the feeling of presence in virtual reality[END_REF][START_REF] Krijn | Treatment of acrophobia in virtual reality: The role of immersion and presence[END_REF][START_REF] Meehan | Physiological measures of presence in stressful virtual environments[END_REF][START_REF] Regenbrecht | Measuring the sense of presence and its relation to fear of heights in virtual environments[END_REF]. Early studies showed anxiety not to significantly correlate with presence [START_REF] Regenbrecht | Measuring the sense of presence and its relation to fear of heights in virtual environments[END_REF], however the hardware used consisted in a monoscopic color HMD which is not appropriate. Krijn et al. do not find a relation between anxiety and presence [START_REF] Krijn | Treatment of acrophobia in virtual reality: The role of immersion and presence[END_REF]. They rather suggest that a minimum of presence is required for subjects to develop anxiety but more presence does not necessarily generate more anxiety. However, Bouchard et al. found that anxiety increases the sense of presence in stressful environments [START_REF] Bouchard | Anxiety increases the feeling of presence in virtual reality[END_REF], though the phobia considered in their study was not related to a fear of heights. Alsina-Jurnet et al. suggest that presence and anxiety are not linked in non-stressful environments but this relation does exist in stressful environments, especially for acrophobic people [START_REF] Alsina-Jurnet | The role of presence in the level of anxiety experienced in clinical virtual environments[END_REF]. In stressful environments, the relationship between presence and anxiety would be, the more presence, the more anxiety, which was also found in previous studies (for example [START_REF] Meehan | Physiological measures of presence in stressful virtual environments[END_REF] where anxiety could be used as a proxy measure for presence). This study also reveals that the main parameter for anxiety to develop is the perceived realism of the virtual environment (which seems to be a factor corresponding more to Slater's Plausability Illusion). This factor has been well studied in past research related to social interactions (see for example [START_REF] Kwon | How level of realism influences anxiety in virtual reality environments for a job interview[END_REF]) showing that visual quality significantly influences anxiety.

Vertical Navigation and Impact of Multisensory Cues

In work-at-height situations, subjects may need to move up and down in the virtual environment, which has been hardly addressed in past work, as one of the issues is for instance to render physical efforts engendered by gravity while climbing. Slater et al. were one of the first to propose navigation methods based on body centered interaction to climb stairs and ladders, aiming at reducing the mismatch between visual and proprioceptive information [START_REF] Slater | Steps and ladders in virtual reality[END_REF]. Lai et al. used the march and reach technique to virtually climb a ladder [START_REF] Lai | March-and-reach: A realistic ladder climbing technique[END_REF]. For WoaH, we want users to feel real sensations of height and physical efforts. In the use case we address, only a few steps -two or three steps maximum -need to be climbed. Therefore, rather than tricking users with artificial travel techniques, we propose climbing a real ladder to strengthen immersion.

Multisensory (visual, haptic) cues may influence the simulator's performance on immersion and presence. In this regard, a huge piece of work in the literature has been provided for applications not implying vertical motion (see for example [START_REF] Dinh | Evaluating the importance of multi-sensory input on memory and the sense of presence in virtual environments[END_REF]). As we aim at providing a transportable simulator, light devices able to provide multisensory cues should be considered. For instance, Plouzeau et al. showed the efficiency of vibrations in classical navigation [START_REF] Plouzeau | Effect of footstep vibrations and proprioceptive vibrations used with an innovative navigation method[END_REF]. It also highlighted the contribution of vibrations on the sense of presence during navigation in virtual environments. Vasyleska and Kaufmann studied the effect of three different metaphors for vertical navigation [START_REF] Vasylevska | Influence of vertical navigation metaphors on presence[END_REF]. The best results in terms of presence were obtained by using a vibrant platform. They reproduced the atmosphere of an elevator which is used as a metaphor for subjects to navigate between floors.

Contributions

We present here first results of our work-at-height simulator. WoaH aims at being used by companies to detect acrophobia on potential future workers and to train them in real work-at-height operations. To the best of our knowledge, we are not aware of such VR-based simulators, apart from the one presented in [START_REF] Hsiao | Human responses to augmented virtual scaffolding models[END_REF] which has however different caracteristics and addresses a different work-at-height situation as we do. By providing the WoaH simulator, we try to fill this gap and match as much as possible real work-at-height situations. For that purpose, we investigate the parameters influencing the fear of heights in a virtual environment and allowing high fidelity in the simulation. We propose to use a real ladder allowing a perfect match in terms of body sensations and physical efforts between the real and the virtual environments, as past research showed an enhancement in the simulation when including real objects [START_REF] Hsiao | Human responses to augmented virtual scaffolding models[END_REF]. Compared to past work related to vertical navigation in virtual environments, we chose not to trick users as we believe that producing physical efforts will enhance the level of immersion, the perceived realism, and trigger anxiety much easier. We propose also to test multisensory modalities that can potentially increase the subjects' sense of presence and the likelihood of provoking anxiety among them. Here we integrated vibrations and the ability to see the hands and feet of the subjects' virtual avatar. The motivation behind studying the effect of vibrations are the past results showing the increase of the sense of presence. Moreover, it is expected that, as vertigo is closely linked to postural stability, such a modality could also induce an increase in the anxiety felt by the subjects. Visual feedback was added as a visual guide to facilitate the tasks to achieve (especially climbing on a ladder), as subjects are totally immersed in the virtual environment leading them to use proprioception.

THE WOAH SIMULATOR

Hardware

We aim at providing a transportable simulator. The reasons for such feature lie in a request from a company dealing with the maintenance of electrical power lines we worked with, to be easily installed and uninstalled in various locations of the company throughout the country, enabling efficient on-site training. Pros of a transportable simulator compared to a stationary one include its light weight yet still enabling full immersion, cost effectiveness, ease of use and easy duplicability. Cons are the need to calibrate the simulator each time it is installed, and less visual quality in terms of resolution than with large systems with high resolution capabilities.

To be transportable, we opted for an HTC Vive HMD as a visualization device, coupled with a VR-Ready Windows 10 PC. Though HMDs shield users from reality, it allows to be totally immersed in realistic situations, which would not have been possible with a real ladder simulator or using see-through displays as users would have been influenced by the surrounding environment. An audio headset supplements the system for phonic immersion. Four Vive trackers are placed on the tibia and the hands of the user with a simple home-made support, since they represent a very good compromise between cost, robustness and low latency (10 ms), avoiding its effect on presence, especially in stressful environments [START_REF] Meehan | Effect of latency on presence in stressful virtual environments[END_REF]. The trackers are mainly used to provide visual feedback of the hands and feet positions to users.

A home-made 3-degree-of-freedom dynamic platform [START_REF] Plouzeau | Dynamic Platform for Virtual Reality Applications[END_REF] was integrated to enhance the performance of the simulator in terms of immersion, by reproducing the vibrations of a virtual aerial device during its motion. This dynamic platform is composed of four electric actuators (8 cm of maximal displacement) and each of them can be controlled independently from the others. The platform is able to perform motion along the vertical axis and around the pitch and roll axes. The actuators can also reproduce vibrations at frequencies up to 3 kHz.

A real ladder is attached to a pole so that users can climb it for real during the operation. The real ladder is synchronized in position with its virtual representation in the virtual environment during the simulator's installation. Users are equipped with a harness for their safety that allows them to evolve hands-free on the ladder. We chose to put a real ladder to get a perfect matching between the real and the virtual environments in terms of body sensations (e.g., sensation of gravity) and physical efforts. This also explains the reason to choose an HMD as a visualization device rather than a CAVE system for instance, as the total setup takes some space in terms of height.

The whole hardware setup is depicted in Fig. 2. As mentioned, WoaH has been designed to be transportable. Therefore, the ladder can be easily fixed on and removed from the pole while the pole is part of a lightweight structure that is easily mountable and demountable. The whole simulator takes about 45 minutes to be set up and calibrated.

The Virtual Environment Design

The WoaH simulator uses a specific scene developed under Unity (see Fig. 3). In this scene, subjects are placed on the platform of a virtual aerial device. The dynamic platform and the platform of the aerial device match in position, therefore subjects can feel for real the void with their feet on the edge. During the rise of the aerial device, vibrations can be set on the dynamic platform. Here, they have been set to an amplitude of 1 cm and at a frequency of 1200 Hz. These parameters were chosen empirically from several trials with a real aerial device. The motion of the aerial device's platform integrates also a lateral component, which is prone to anxiety according to past work [START_REF] Coelho | The role of self-motion in acrophobia treatment[END_REF] and may affect the perception of height [START_REF] Coelho | The use of virtual reality in acrophobia research and treatment[END_REF]. As soon as the aerial device's platform is in its upper position, the dynamic platform jerks once in a short time to reproduce the stop of the aerial device, and then starts swaying in every direction (amplitude: 2 cm, frequency: 0.3 Hz). This sway has been implemented to reproduce the effect of the wind on the aerial device, generating more stress to subjects as they may feel less stable. The aerial device's motion is set so that once elevated, subjects can step physically onto a virtual ladder placed on an electric pole. As mentioned earlier, the virtual ladder is synchronized in position with the real ladder so that subjects can climb the real ladder even though they do not see it because of the HMD. The virtual aerial device rises to an altitude of 11.6 meters. The surrounding environment is above 10 meters in distance in order not to give any static spatial references that could be used to stabilize [START_REF] Bles | The mechanism of physiological height vertigo. II. posturography[END_REF][START_REF] Brandt | The mechanism of physiological height vertigo. I. theoretical approach and psychophysics[END_REF].

Moving clouds have been added to the sky to strengthen the effect of loss of static spatial references. The surrounding environment is composed of buildings, rocks, hills and trees that are far enough from the subjects, again not to provide any static spatial references. Dynamism is included in the scene thanks to moving cars and 3D sounds emitted by cars, birds and the aerial device.

Finally, because subjects are cut from reality due to the use of an HMD, we included the possibility for subjects to see a virtual representation of their hands and feet as a visual guide. We chose as a first implementation to represent only hands and feet to simplify motion tracking computation. These parts were just tracked in position and not kinematically animated.

USER STUDY

A within-subject study was designed to assess the performance of the simulator. The tasks to achieve have been planned in cooperation with the company we worked with and were based on typical operations performed by their employees. We hypothesized the following:

H1 Multisensory cues increase the level of immersion and the sense of presence H2 Multisensory cues increase the level of anxiety felt

Participants

14 participants (13 males and 1 female, ages: 21-27 (mean: 23.4)) took part to the study . Only one of them had never experienced a VR system before the experiment. They were recruited within the university members and through word of mouth. No compensation was given at the end of the experiment. A briefing was provided individually to give enough information about the test procedure and possible risks before each experiment. Note that in this study, we voluntarily chose to recruit participants who were not familiar with work-at-height situations to test our simulator's performance in terms of immersion and anxiety. All participants were asked to fill a demographic form with personal information and questions on particular health issues. None of them reported having particular health issues and were in their usual state of fitness.

They were asked to answer the Acrophobia Questionnaire from Cohen [START_REF] Cohen | Comparison of self-report and overt-behavioral procedures for assessing acrophobia[END_REF] to detect participants with possible acrophobia. From this questionnaire, 11 participants got an acrophobia score under 15, which is low considering that the maximum score for this questionnaire is 120.

Experimental Protocol

Two modalities were tested: without any vibration from the dynamic platform (M1) and with vibrations (M2). Participants did both sessions, one per day with a different modality each time to prevent cumulative aspects. The modalities were distributed among the participants to avoid any bias of learning effects on the results. Each session lasted around 15 minutes on average. The protocol was as follows:

1. participants are helped to put on all the equipment (harness, trackers, head-mounted display, audio headset); 2. they are then placed on the dynamic platform and a one-minute familiarization session begins to get used to the trackers and virtual reality. After familiarization ends, a fade-in/fade-out effect allows a smooth transition to the main scene;

3. participants are placed on the platform of the virtual aerial device. They are able to watch the environment of the scene for 20 seconds before the rise of the aerial device starts. The rise of the aerial device is triggered by the experimenter through a user interface displayed on a separate screen;

4. once the aerial device's platform reaches its final position, participants are asked to step physically onto the ladder and climb one step;

5. the experimenter comes to harness the participants to the ladder and the pole. Though it may not look realistic, in real training, trainees may be assisted by an experienced person. Once harnessed, participants are able to move hands-free safely;

6. the aerial device moves off the ladder so that participants cannot step again onto the platform of the aerial device;

7. participants are asked to try to catch a kite placed behind them using both hands (see Fig. 4). Though typical operations on electrical poles are not to catch kites, this task has been included to check whether participants are confident in the harness, which is a requisite for workers to work at height. Indeed, to catch the kite, participants need to turn around, completely removing both hands off the ladder (see Fig. 1). Also, we voluntarily chose a simple task, following the company's recommendation as by experience, strongly acrophobic people would stop the session even before performing this task;

8. the experiment ends as soon as the kite is caught.

At anytime, participants were able to stop the experiment if necessary (e.g., sickness felt, vertigo).

Measurements

Throughout the experiment, participants' electrodermal activity (EDA) was measured using an E4 Empatica wristband that can be remotely controlled by Wifi, with a sampling rate of 4Hz and a precision about 0.01µS [START_REF] Mccarthy | Validation of the empatica E4 wristband[END_REF]. Other physiological measurements, such as blood pressure, heart rate, or behavioral measurements, such as postural stability, could have been considered. However, these require more intrusive devices than EDA wristbands, though in a fear of heights scenario, heart rate has shown to perform better than skin conductance [START_REF] Meehan | Physiological measures of presence in stressful virtual environments[END_REF]. Moreover, for example, the analysis of postural sway may be difficult to perform as: (i) this measure can be an indicator of combined factors such as cybersickness [START_REF] Chardonnet | Features of the postural sway signal as indicators to estimate and predict visually induced motion sickness in virtual reality[END_REF], anxiety [START_REF] Redfern | Visually induced postural sway in anxiety disorders[END_REF],

(ii) we created a situation where steady visual references may be partially lost, thus implying potential destabilization of participants' body [START_REF] Brandt | The mechanism of physiological height vertigo. I. theoretical approach and psychophysics[END_REF], and so potentially biasing the measures. The EDA wristband records the participants' sweating, which gives an objective indication of the anxiety felt by the participants [START_REF] Birket-Smith | Electrodermal activity in anxiety disorders[END_REF]. The wristband records data throughout the experiment.

At the end of the experiment, participants were asked to fill out two questionnaires: Kennedy et al.'s Simulator Sickness Questionnaire (SSQ) [START_REF] Kennedy | Simulator Sickness Questionnaire: An enhanced method for quantifying simulator sickness[END_REF] to measure potential cybersickness induced by vertical navigation, and a perceived realism questionnaire derived from Witmer-Singer's presence questionnaire [START_REF] Witmer | Measuring presence in virtual environments: A presence questionnaire[END_REF] to measure the level of the perceived realism of the scenario (we used only the questions related to the realism factor from the presence questionnaire). Oral comments were also recorded to get participants' feelings regarding anxiety.

At the end of the last session, participants were asked to rank the modalities according to their preference in terms of realism, level of immersion, and to freely comment their ranking.

RESULTS

Cybersickness

From the SSQ questionnaires, sickness scores were calculated using the method provided by Kennedy et al. [START_REF] Kennedy | Simulator Sickness Questionnaire: An enhanced method for quantifying simulator sickness[END_REF]. A Mann-Whitney test was conducted to compare the effect of vibratory cues. No significant effect was found on cybersickness (p = .359) between the modality with vibrations (M = 1.57, SD = 1.45) and the one without any vibrations (M = 1.21, SD = 1.89) (see Fig. 5 top).

SSQ scores are very low. In fact, many participants reported on the SSQs no sickness at all after the sessions. Moreover, observations made during the sessions showed that none of the participants complained about sickness or looked sick or confused after the experiment. This shows our simulator does not generate any strong sickness, which is an important feature for the simulator to be deployed.

Perceived Realism

We used only the realism factor questions from the presence questionnaire, as from the literature, the main parameter for anxiety to develop is the perceived realism of the virtual environment [START_REF] Alsina-Jurnet | The role of presence in the level of anxiety experienced in clinical virtual environments[END_REF], and the perceived realism was found to have a significant impact on presence [START_REF] Bouchard | Perceived realism has a significant impact on the feeling of presence[END_REF]. A Mann-Whitney test was conducted to compare the effect of vibrations on the perceived realism. There was a significant effect of vibrations on the perceived realism (p = .023). The perceived realism was significantly higher with vibrations (M = 41.1, SD = 5.6) than without any vibrations (M = 35.1, SD = 7.7) (see Fig. 5

bottom).

According to the rankings that were asked to the participants at the end of the sessions, most of the participants preferred the modality including vibrations in terms of immersion (13 participants out of 14). The presence of a visual feedback on the hands and feet was appreciated by the participants as they could rely not only on their proprioception to move in the virtual environment, thus facilitating them in achieving the required tasks.

Electrodermal Activity

Recall that we measured the electrodermal activity to evaluate the level of anxiety. As the data did not follow a normal distribution, they were transformed using a Box-Cox transformation. A Student's t-test was conducted to compare the effect of vibrations on the EDA. No significant effect was found on the EDA (p = .302) between the modality with vibrations (M = 7.7µS, SD = 11.7) and the one without any vibrations (M = 2.6µS, SD = 2.7) (see Fig. 6A).

Five participants orally indicated that the vertigo they felt during the sessions was higher when with vibrations. During the experiment, tremors have been observed on three participants while on the ladder, indicating not only a fear of height but also strong immersion in the virtual environment. For these participants, high EDA variations along with a high increase in the EDA have been observed, showing that our simulator indeed induced anxiety. Figures 6B andC show the evolution of the EDA for one non-acrophobic participant and for the four participants who reported being acrophobic from the Acrophobia Questionnaire. We clearly see that for the non-acrophobic participant, the EDA remains low with very low variations throughout the experiment. Whereas, for the acrophobic participants, the EDA increases much more with high variations, especially when climbing to the ladder, turning around and removing both hands off the ladder to catch the kite.

As past literature showed that the EDA could be an indicator of cybersickness [START_REF] Kim | Characteristic changes in the physiological components of cybersickness[END_REF], we checked whether here the level of the EDA could be correlated to the SSQ scores. No correlation was found between the increase in the EDA and the SSQ scores (r = -.03), showing that the increase in the EDA is rather due to anxiety and not to cybersickness.

DISCUSSION

Regarding cybersickness, low scores have been observed. Therefore our simulator can be used without cybersickness interfering with the user experience. In the way the WoaH simulator has been designed, cybersickness may occur only during the motion of the virtual aerial device to the ladder, as subjects do not move physically during this time. The motion speed of the aerial device to get to the ladder is rather low both in the vertical and horizontal directions (v vert = 0.2m.s -1 , v lat = 0.217m.s -1 , it takes 52 seconds to go from the ground to the ladder). The motion speed is clearly under the thresholds proposed in the literature for the occurence of cybersickness though most of past work focused on horizontal motion in virtual reality (see for example [START_REF] Mirzaei | Influence of interaction techniques on VIMS in virtual environments: estimation and prediction[END_REF][START_REF] So | Effects of navigation speed on motion sickness caused by an immersive virtual environment[END_REF]). Also, the motion of the aerial device lasts less than one minute (the time subjects are steady on the platform and feel vibrations), which is too short to have an effect on cybersickness. When the aerial device reaches its upper position (near the ladder), the dynamic platform oscillates at a frequency of 0.3 Hz. According to the literature, for vertical motion, oscillations that may highly provoke sickness are at frequencies about 0.2 Hz [2]. However, as subjects are asked to step onto the ladder as soon as the aerial device stops, there is not enough time to provoke cybersickness. Finally, when stepping onto the virtual ladder after the rise, subjects have to step physically onto a real ladder and climb it. Therefore, after stepping onto the ladder, the occurence of cybersickness due a visuo-vestibular and proprioceptive conflict that is usually observed when navigating in virtual reality should be almost non-existent.

Regarding the perceived realism, results reveal a significant effect of vibrations on the perceived realism. These results are confirmed by the feedback from the participants, who all preferred the modality including vibrations over the one without. These results are even more significant because vibrations are not set for the whole time of the experiment. Also, according to the literature, the mean score for the realism subpart of the presence questionnaire is 29.45 [START_REF] Robillard | Validation canadienne-franc ¸aise de deux mesures importantes en réalité virtuelle : l'Immersive Tendencies Questionnaire et le Presence Questionnaire[END_REF]. Here on average the scores were higher, independently from the modalities, showing that our simulator copes with the initial requirement of being realistic.

As for electrodermal activity, an increase in the EDA for the modality including vibrations was expected but no significant differences have been observed between each modality. Though on average, the EDA increased when adding vibrations, the deviation is too large to draw a strong conclusion regarding the impact of such sensory cues on anxiety. Several reasons can explain these results:

• there were only 14 participants in this first experiment, which is not enough to get a significant evolution. Experiments with many more participants should be planned;

• from Cohen's acrophobia questionnaire [START_REF] Cohen | Comparison of self-report and overt-behavioral procedures for assessing acrophobia[END_REF], 11 participants out of 14 got an acrophobia score under 15, which is low considering that the maximum score for this questionnaire is 120. According to the literature, no EDA increase should therefore be found for people who do not suffer from acrophobia [START_REF] Alsina-Jurnet | The role of presence in the level of anxiety experienced in clinical virtual environments[END_REF][START_REF] Krijn | Treatment of acrophobia in virtual reality: The role of immersion and presence[END_REF]. On the other hand, four participants got higher scores with the acrophobia questionnaire and for them, higher EDA variations along with tremors have been observed during the experiment;

• participants were already experienced in VR systems except one, therefore results on anxiety may have been biased. Future experiments should include future employees of the company we worked with who are expected to be totally inexperienced in VR.

Regarding the link between presence and anxiety, past research found a correlation only for acrophobic users [START_REF] Alsina-Jurnet | The role of presence in the level of anxiety experienced in clinical virtual environments[END_REF]. Here, considering only the acrophobic participants, between both modalities, the variation of the perceived realism score seems to be positively correlated to the variation of the EDA (r = .31). But again, the number of participants was too low to be significant. Nonetheless, past literature suggested that the main parameter for anxiety to develop is the perceived realism of the virtual environment [START_REF] Alsina-Jurnet | The role of presence in the level of anxiety experienced in clinical virtual environments[END_REF], the perceived realism has a significant impact on presence [START_REF] Bouchard | Perceived realism has a significant impact on the feeling of presence[END_REF], and a minimum of presence is required to develop anxiety [START_REF] Krijn | Treatment of acrophobia in virtual reality: The role of immersion and presence[END_REF], which seems to be confirmed with our simulator. We can therefore imagine that the more presence (or the more perceived realism) the better to ensure the simulator can trigger anxiety. Consequently, adding sensory cues such as vibrations is essential in a work-at-height simulator. Future studies should investigate this correlation in more detail.

LIMITATIONS

Though our simulator meets the first industrial requirements (realistic work-at-height situation, induced cybersickness, anxiety felt by acrophobic participants), it still has some limitations, which are planned to be addressed in future work.

First, our simulator is aimed at being used indoor. Though we already provided sensory cues through the dynamic platform (motion of the aerial device and sway due to the wind), it still lacks other sensory cues that are typical from outdoor environments, that could enhance the level of immersion and anxiety. For instance, air motion could be added to simulate gusts of wind while at height. Also, the virtual scene lacks elements such as a virtual representation of the harness or of the subject's full body. These have not been added in this work for complexity reasons but they will have to be considered to enhance the level of realism. Last, most of the operations on power lines are performed with live lines. Therefore, WoaH could integrate a haptic feedback when users get close to the lines to increase stress, combined with a fear of heights.

Second, though our simulator has been designed with expert workers, in this work, we considered only non-expert subjects as test subjects. However, expert workers were asked to evaluate our simulator and provide orally feedback in terms of realism. The experts reported finding WoaH enough realistic, though they are looking forward to seeing further functionalities included as described above. Also, comparisons with reality have not been performed for safety reasons at that moment, however these are planned to make clear the effect of virtual reality, which is also a request from the expert workers. Especially, as WoaH is aimed at being used with potential future workers, experiments should be performed with applicants to work-at-height-related jobs.

CONCLUSION

In this paper, WoaH, a transportable virtual reality work-at-height simulator was proposed, aimed at reproducing real situations with high fidelity. An optional dynamic platform that simulates the vibrations of an aerial device's platform and its sway was included. A first user study was conducted to test whether the functionalities of our simulator met the requirements of a company dealing with work at height in terms of presence and anxiety, as WoaH aims at being deployed in such companies. Results revealed no significant relation between the modalities and cybersickness as well as the EDA, but showed a significant effect of vibrations on presence.

From this study, several guidelines can be derived for both the design and the evaluation of simulators such as the one we developed:

• according to the literature, maximizing the sense of presence among participants is essential, as a certain level of presence is a sine qua non condition for the anxiety to develop [START_REF] Krijn | Treatment of acrophobia in virtual reality: The role of immersion and presence[END_REF]. Thus, sensory modalities that provide a higher sense of presence have to be considered for such simulators; for instance, vibratory cues as presented here fit the design of a simulator involving vertical navigation, as reported also in [START_REF] Vasylevska | Influence of vertical navigation metaphors on presence[END_REF].

• the evaluation of a work-at-height simulator in terms of anxiety should be performed with both acrophobic and non-acrophobic subjects. These subjects should also be inexperienced in VR.

As mentioned above, several limitations still exist on WoaH. These are planned to be addressed in future work, especially the comparison with real training without using virtual reality, mixing both experienced and inexperienced workers. Finally, future investigation should be carried out to clearly set the link between presence and anxiety in height-involving situations and clearly define which parameters encounter for both presence and anxiety in such environments.

Figure 1 :

 1 Figure 1: The WoaH simulator in operation.

Figure 2 :

 2 Figure2: The WoaH simulator setup. In the foreground, the HTC Vive with the trackers to be attached to the user; in the background, the dynamic platform with the harness to be attached to the user and the real ladder fixed on a lightweight structure.

Figure 3 :

 3 Figure 3: Global view of the scene.

Figure 4 :

 4 Figure 4: Position of the kite to be caught.

Figure 5 :

 5 Figure 5: For each modality (M1: without any vibrations; M2: with vibrations), Top: SSQ scores; Bottom: Realism scores.

Figure 6 :

 6 Figure 6: A: EDA values (µS) for each modality (M1: without any vibrations; M2: with vibrations); B: EDA evolution (µS) for one non-acrophobic participant; C: EDA evolution (µS) for the four participants who reported being acrophobic. In B and C, there was no vibration; the different phases are a: rise of the aerial device, b: ladder climbing, c: catch-a-kite task, d: end of the experiment.

ACKNOWLEDGMENTS

The authors wish to thank Enedis Bourgogne for their financial support and their help in designing the simulator.


J Ér

Émy Plouzeau
email: jeremy.plouzeau@ensam.eu

Jean-R Émy Chardonnet
email: jean-remy.chardonnet@ensam.eu

Member, IEEE Fr éd Éric Merienne
email: frederic.merienne@ensam.eu

Using Cybersickness Indicators to Adapt Navigation in Virtual Reality: A Pre-study

Keywords:

We propose an innovative method to navigate in a virtual environment by adapting the acceleration parameters to users in real time, in order to reduce cybersickness. Indeed, navigation parameters for most navigation interfaces are still determined by rate-control devices. Inappropriate parameter settings may lead to strong sickness, making the application unusable. Past research found that especially accelerations should not be set too high. Here, we define the accelerations as a function of a cybersickness indicator: the Electro-Dermal Activity (EDA). A pre-study was conducted to test the effectiveness of our approach and showed promising results where cybersickness tends to decrease with our adaptive navigation method.

INTRODUCTION

Navigation interfaces and techniques are required to explore virtual environments that can be infinite [START_REF] Plouzeau | Effect of proprioceptive vibrations on simulator sickness during navigation task in virtual environment[END_REF]. However, undoubtedly, navigation in virtual reality differs from that in the real world. For instance, navigation tasks are the source of a well-known virtual reality issue: cybersickness. Past studies showed that some navigation methods were more efficient in terms of navigation performance while they should use some specific acceleration parameters to reduce cybersickness [START_REF] Cirio | Walking in a Cube: Novel Metaphors for Safely Navigating Large Virtual Environments in Restricted Real Workspaces[END_REF]. However, with specific parameters, these navigation methods loose their effectiveness for navigation.

We aim at reducing cybersickness while still allowing efficient navigation in virtual reality. We propose to adapt in real time navigation parameters from online cybersickness indicator's measures.

Related Work

Cybersickness

Many studies attempt to understand cybersickness while others seek to decrease its effect. Cybersickness occurs during or following virtual immersion and is similar to the discomfort caused by motion sickness. It can be evaluated through both subjective and objective ways. Subjective measurements include the well-known Simulator Sickness Questionnaire (SSQ) [START_REF] Kennedy | Simulator Sickness Questionnaire: An Enhanced Method for Quantifying Simulator Sickness[END_REF]. Postural stability allows to estimate and predict cybersickness [START_REF] Chardonnet | Features of the Postural Sway Signal as Indicators to Estimate and Predict Visually Induced Motion Sickness in Virtual Reality[END_REF]. Objective evaluation include measurements of the Electro-Dermal Activity (EDA) [START_REF] Kim | Characteristic changes in the physiological components of cybersickness[END_REF] and the calculation of the Motion-Sickness Dose Value (MSDV) [START_REF] So | The Search For A Cybersickness Dose Value[END_REF].

Navigation Methods

Many works explore how to tune navigation parameters such as the acceleration and the maximum speed in order to reduce cybersick-ness. In any case, smaller linear and rotational accelerations are recommended [START_REF] Mirzaei | Influence of interaction techniques on VIMS in virtual environments: estimation and prediction[END_REF]. Sargunman et al. [START_REF] Sargunam | Guided head rotation and amplified head rotation: Evaluating semi-natural travel and viewing techniques in virtual reality[END_REF] evaluated a semi-natural navigation method with amplified rotation factors and found that their method works well when users are seated. Basting et al. [START_REF] Basting | The effectiveness of changing the field of view in a HMD on the perceived self-motion[END_REF] exposed that the field of view (FOV) changes the perceived vection. From this statement, Fernandes and Feiner [START_REF] Fernandes | Combating VR sickness through subtle dynamic field-of-view modification[END_REF] explored an innovative solution where the field of view is dynamically modified according to the displacement in the virtual environment. Argelaguet Sanz [START_REF] Sanz | Adaptive Navigation for Virtual Environments[END_REF] proposed and evaluated a speed adaptation algorithm that automatically adjusts the navigation speed from the spatial relationship between the user, the environment and the optical flow.

Contributions

Cybersickness effects are user-dependent but from the literature, virtual reality applications use navigation methods with static parameters, i.e., that do not depend on the user. Here, we propose to explore the possibility to adapt in real time the parameters depending on the user's actions and physiological state. The objective is to match the best navigation parameters for each user in every situation in order to decrease cybersickness while still allowing efficient navigation. Compared to past work [START_REF] Sanz | Adaptive Navigation for Virtual Environments[END_REF], we contribute in adapting the navigation method to users and not to the virtual environment, allowing us to propose a truly personalized navigation method.

ADAPTIVE NAVIGATION 2.1 Navigation Method

We use a Razer Hydra device, a joystick-based device, coupled with an Oculus Rift CV1 head-mounted display. We allocated the right joystick of the Razer Hydra to move forward/backward (z axis) and strafe left and right (x axis), while the left joystick allows rotation around the vertical axis (y axis).

We implemented an acceleration-based control, as it is the most natural way to move in a virtual environment. With this control, a position of a joystick corresponds to an acceleration, linear or rotational, implying to reach a speed limit depending on the properties and the topology of the virtual environment.

Cybersickness and Parameters Adaptation

To objectively evaluate cybersickness in real time, we chose the EDA as it is a simple value to measure and monitor in real time, and the EDA was shown to increase in the presence of cybersickness [START_REF] Kim | Characteristic changes in the physiological components of cybersickness[END_REF]. To measure the EDA, we use an E4 Empatica wristband [START_REF] Mccarthy | Validation of the Empatica E4 wristband[END_REF].

We then need to adjust the navigation accelerations according to the change in the EDA. Adaptation is performed as follows: when the EDA increases, the acceleration factors decrease proportionally to the EDA's increase speed; and when the EDA decreases, the acceleration factors increase, which is described as follows:

A(t) = A (t -1 ) -0.5 × dEDA(t) dt (1)
where A (t -1 ) and A(t) are the accelerations, linear or rotational, at two successive frames, and the 0.5 factor has been determined empirically following simulations with different frequencies and amplitudes of the EDA variation in order to be adapted to the physiological reaction time of the EDA [START_REF] Nishisato | Reaction time as a function of arousal and anxiety[END_REF].

PRE-STUDY

Hypotheses

The purpose is to explore whether using adaptive navigation can have a effect on cybersickness, thus our hypotheses are: (H1) Adapting acceleration parameters from the EDA evolution limits EDA variations, and (H2) The adaptive control decreases cybersickness.

Virtual Environment

We designed a virtual environment so that cybersickness can be easily evaluated and induced by navigation tasks and not by any other factors. The virtual environment consists in a virtual forest with a gravel path. The path is 1.180 km long, so that the experiment can last enough time regarding cybersickness, and 5 m wide. The test application runs at 90 frames per second.

Procedure and Measurements

Seventeen participants (ages: 19-36) took part to the pre-study.

They were asked to navigate in the virtual environment through two separate sessions in random order corresponding to two modalities, the normal modality (N) (without adaptation) as a reference and our adaptive modality (A). For each modality, participants were immersed the same amount of time, around 8 minutes.

During the simulation, the head movements, the virtual positions and orientations, and the time were logged. We computed the Motion-Sickness Dose Value (MSDV) [START_REF] So | The Search For A Cybersickness Dose Value[END_REF], an EDA score computed over time considering the EDA variations between two frames, and the evolution of the Postural Stability (PS), defined as the ratio between the mean surfaces described by the projection of the center of gravity after and before exposures. Last, we asked participants to fill an SSQ before and after exposures.

Results

One participant could not finish the experiment and had to stop. Therefore, the results are expressed with 16 participants.

The mean values of the MSDV decreased from modality N to modality A for each motion axis: MSDV x = 37.7 ± 7.6 ⇒ 33.3 ± 7.6, MSDV y = 36.0 ± 7.1 ⇒ 32.1 ± 7.4 and MSDV z = 42.7 ± 8.2 ⇒ 38.6 ± 7.4. Therefore, the manipulation of navigation parameters tends to affect the MSDV.

We observed a decrease in the mean values of the EDA from 2.19 ± 2.24µS.s with modality N to 1.09 ± 2.18µS.s with our method. Figure 1 shows an example for one participant of the EDA evolution for both modalities. It clearly shows that the EDA level is better controlled with adaptive navigation. It also shows that the manipulation of navigation parameters driven by the EDA level could have an effect on the EDA itself, which validates our proposal.

The PS did not present any evolution between both modalities, suggesting that adaptive navigation has no effect on postural stability.

Finally, the SSQ scores show an overall decrease of around 71% from modality N to our proposed modality.

DISCUSSION

This pre-study provides promising results, as these seem to validate our hypotheses. If we observe the evolution of the acceleration parameters over time, some of the participants finished the adaptive navigation session with higher accelerations than they began, which proves that the adaptive modality adjusted navigation to the user and not to the environment, without changing the user experience.

CONCLUSION AND FUTURE WORK

We showed that adapting navigation as a function of users' EDA could allow to decrease cybersickness without changing users' virtual reality experience.

Future studies include considering more participants in order to perform in-depth statistical analysis as well as investigating other cybersickness indicators to adapt navigation parameters.

Figure 1 :

 1 Figure 1: EDA evolution for one participant between both modalities

ACKNOWLEDGMENTS

This work was supported in part by a grant from the Bourgogne Franche-Comté council.


Abdul Salam

Al Al Atem

Victor Bratus
email: v_bratus@isp.kiev.ua

Bruno Canut
email: bruno.canut@insa-lyon.fr

Jérémie Lefèvre
email: jeremie.lefevre@polytechnique.edu

G Guillot
email: gerard.guillot@insa-lyon.fr

Jean-Marie Bluet
email: jean-marie.bluet@insa-lyon.fr

) B Canut

J Lefevre

J-M Bluet

Combined EPR and photoluminescence study of electron and proton irradiated 3C-SiC

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

In past few years, point defects in silicon carbide (SiC) have been identified as promising for applications in quantum technologies [1]. A variety of point defects in hexagonal SiC [2], including VSi and VSiVC have been optically isolated and used as single defect-based spin qubits with long coherence time [3][4][5]. All of this, proves that these point defects allow the SiC to be a very favorable candidate for quantum applications especially, solid state quantum bits (Qubits) and single photon source (SPS). Most of these studies were carried out on the hexagonal polytypes 4H-SiC and 6H-SiC, although the 3C-SiC polytype presents the unique advantage of integration possibility on standard Si wafer. This is due to the amount of defects (dislocation mainly) in the 3C-SiC heteroepitaxy on Si which are detrimental for long coherence time considering Qubit application.

Consequently, the goal of the present study is the investigation of point defects formation after implantation by proton H + (300 keV) and irradiation by electron e-(0.8 and 2 MeV) in 3C-SiC (respectively 3C-SiC<p> and 3C-SiC<e>) for SPS application purpose. Toward this end, we have combined two characterization techniques, the photoluminescence (PL) and the electron paramagnetic resonance (EPR). PL (12K) and EPR (70-300K) measurements will be presented in order to analyze precisely the signatures of point defects generated after these two types of irradiations. The effects of the thermal annealing (500 -1000°C) were also investigated.

PL spectra both for 3C-SiC<p> and 3C-SiC<e> are presented in figure 1 for the annealing temperatures giving the highest PL signal (1000°C for 3C-SiC<p> and 750°C for 3C-SiC<e>). We notice first that the whole PL signal is higher for 3C-SiC<p>. In this case the spectrum is dominated by the DI defect line (possibly related to antisite pair [6]) and the E line (attributed to Si vacancy [7,8]) while in electron irradiated sample the  line (attributed to CSiVC in a 3C-SiC nanocrystal [8]) dominates as previously reported [9]. A strong zero phonon line at 1.6 eV also appears for 3C-SiC<p> with its phonon replica. This line was previously observed also for neutron and proton irradiated 3C-SiC and at present is of unknown origin [10,11]. In the infrared range, the proton implantation is also more efficient to produce the VcVsi PL line with an optimum luminescence for 750°C annealing. All together, these results show that, even if the energy transferred to the host atoms during electron irradiation is quite above the displacement thresholds for both C and Si, defects involving Si vacancy are more pronounced in 3C-SiC<p> samples. EPR spectra for 3C-SiC<p> for isochronal annealing (30 min.) at different temperatures are presented in figure 2. The spin-three-half negatively-charged Si vacancy (the T1 center) is a dominant defect in 3C-SiC<p> epitaxial layers corresponding to the E line in the PL spectra. Similar to neutron irradiated 3C-SiC crystals the as-implanted not annealed 3C-SiC<p> samples demonstrate an isotropic spectrum with the g-value of 2.0029 and a superhyperfine doublet with a splitting typical to the T1 center superimposed on a broad more intense structureless background (BG) line. An appearance of the intense BG line in the 3C-SiC<p> samples can be related with the defect regions formed as a result of cascade displacements. The total concentration of paramagnetic defects determined by EPR is equal to 3.1x10 19 cm -3 . Such high defect concentration can reduce spin-lattice relaxation time and be a factor of EPR line broadening. It is likely that for this reason we didn't find EPR spectra of other defects. Considerable decrease in intensity of the BG spectrum after annealing at 500°С turns the T1 spectrum to the symmetric shape and correlates with a distinct gain in the PL intensity. The total concentration of paramagnetic defects becomes four times lower. The EPR spectrum of spin-one neutral divacancy, the Ky5 center [12], has been also detected both at room temperature and 77K (this correlates with the observation of the 1.12 eV line in PL). In addition, an axial spin S=1/2 defect labeled Ky6 and tentatively assigned to the negatively charged carbon vacancy-carbon antisite pair [13] has been observed within the temperature range 77K-200K. The number of paramagnetic centers in the range of g-value g~2.0 decreases by more than one order of magnitude after annealing at 750°C as compared with samples annealed at 500°C. The T1 defects are quenched, the Ky6 centers are the dominant defects at low temperature measurement up to 200K. The Ky5 centers are still detected, although their intensities are about 2 times smaller than ones after anneals at 500°C. Simultaneously with annealing of paramagnetic defects a succeeding growth of PL intensity has been found. This is significant of the domination of non radiative process for the PL signal: for high temperature annealing the non radiative rate decreases due to crystallinity recovery. This induces PL signal enhancement even if radiative defect density vanishes as observed in EPR.

Figure 1 :

 1 Figure 1 : Low temperature (12 K) photoluminescence spectra of proton implanted and electron irradiated 3C-SiC/Si respectively after 1000°C and 750 °C annealing.

Figure 2

 2 Figure 2 Room temperature EPR spectra of proton implanted 3C-SiC/Si after different annealing steps.


J Alejandro Betancur
email: j-betancur@javeriana.edu.co

Jérémy Plouzeau
email: jeremy.plouzeau@ensam.eu

Frederic Merienne
email: frederic.merienne@ensam.eu

Daniel Suárez
email: d-suarez@javeriana.edu.co

Human behaviour analysis when using Head-Up Display systems in autonomous driving situations

Keywords: Head-Up Display, Autonomous Driving, Human Machine Interaction, User Experience

This paper covers an autonomous driving situation, in which a fully automated vehicle was proposed for driving along a specific route, while some safety-related information was projected in a Head-Up Display (HUD) system. This situation was tested by 10 subjects in a driving simulator, in order to estimate how the mental discomfort in autonomous driving situations could be influenced when using or not the proposed HUD system. The above, was performed firstly by measuring and analysing for all test subjects their pupil size, blink rate, heart rate, temperature and electro-dermal activity as discomfort estimators; and secondly, by developing a categorical questionnaire for identifying anxiety and stress. According to the results, there was not enough evidence for confirming a significant reduction on the measured discomfort variables (p-value<0.05) while using the proposed HUD visual interfaces; these results were also supported by all categorical questions developed after the proposed experimentation. In conclusion, under the experimental considerations here proposed the HUD systems do not influence the mental discomfort in autonomous vehicles.

Introduction

The development and evolution of the Advanced Driver-Assistance Systems (ADAS) define the capabilities of the autonomous driving vehicles [Alt17], therefore the user understanding of an autonomous driving situation depends, in turn, on how the users understand the ADAS communication interfaces; in this sense, some studies expose the implementation of Head-Up Display (HUD) systems as necessary for visually understanding the ADAS systems influences [Wie17], specifically those systems related to hazard pedestrians maneuvers [Göt15]. According to the above, this research is focused on comparing the driver behavior when using or not different visual inputs in a specific HUD system, during an autonomous hazard pedestrian situation. The above is especially remarkable for autonomous driving, due to there is always a user discomfort for do not controlling the cars' maneuvers [Noy18]. Recalling the above, there is an academic and also an industrial discussion about how a HUD interface could help the driver to always feel in control and keep calm when autonomous driving is active.

Therefore, this research work explores the main human factors involved during autonomous driving, measuring these human factors when different HUD visual interfaces are applied to a specific hazard situation. These interfaces are mainly related to driving route indications or driving alerts, as shown respectively in Figure 1. Furthermore, even if there is an always bigger quantity of HUD systems focused on improving driving experiences [Bet16], a research question arises about whether or not there is any impact on the driving behaviour when using autonomous automobiles equipped with HUD systems, specifically during hazard situations related to pedestrians; on the other hand, regarding that the visual configuration of the information projected in a HUD could influence the driving behaviour [Kno15, Alv13], some specific driving route indications and driving alerts were proposed, in order to enhance the probabilities for finding significant differences on the driving behavior. Finally, according to the autonomous driving levels defined by the Society of Automotive Engineers (SAEsee at the International Standard J3016 for motor vehicles), this paper is focused on the fourth level (high automation); thereby, the results here obtained are replicable just for that level of autonomous driving.

Related works

Currently, simulations of autonomous and semiautonomous vehicles includes a limited quantity of driving scenarios [Sad15,Kim18], this because every one of these simulations involve their own evaluation objectives, and methodologically is more comprehensible to board just one driving situation at time, instead of boarding all of these at the same time. Then, the development of these driving scenarios is proposed according to accidental data provided by traffic police reports and assurance organizations.

For instance, Sportillo et al. [Spo17] describe semiautonomous situations such as line-keeping and static and dynamic obstacle avoidance, providing real-time collection of relevant user and vehicle data, looking for warning the driver throughout a HUD system when it was necessary to take-over the driving control of the car. This paper describes a straight-line driving situation, in which the driver activates the autonomous driving (80 km/h as maximum speed), looking for developing infotainment tasks; and then, re-taking the car control when an obstacle is detected by the autonomous driving (130 km/h as maximum speed), avoiding a possible crash by using the lane-change maneuver.

Similarly, Charissis and Papanastasiou [Cha08] proposed a sudden brake scenario in which drivers travelled a distance of 2km in a motorway, and then the lead vehicles brake abruptly, causing the following vehicles to decelerate rapidly. Also, another scenario was composed based on the previous one, but in this case the driver follows a leading vehicle's group for 5 km, and then a 120 degrees curve takes place, finishing in an underneath bridge with traffic congestions; additionally, heavy fog and weather conditions become worse the driver´s visibility. Therefore, this study determined the driver's performance with and without the assistance of a proposed HUD interface. In this way, Sportillo et al.,Charissis and Papanastasiou [Spo17,Cha08] describe driving scenarios in which throughout a HUD system the user is forced to braking or maneuvering around a possible hazard situation.

In general, some basic driving interfaces have been broadly explored in autonomous driving situations. For instance, Sadigh et al. [Sad15] suggest a method for exposing driving information according driver data feedback; these data were related to driver gaze tracking and steering wheel touch sensors, looking for estimating the driver behavior by using kmeans clustering algorithms.

Methodology

Problem understanding

When an autonomous automobile is involved in a hazard situation, the occupants could experiment a high mental discomfort because of not having the driving control of the automobile [Duf15]. In this case study, the HUD systems represent an alternative for reducing this mental discomfort by showing critical information about the faced hazard situations; therefore, these situations are related to the main car-to-pedestrians statistic, and the mental discomfort suffered by the test subjects is estimated by measuring the following variables: In this way, the following research questions are addressed by this paper:


 (1) During autonomous driving: can the HUD system affect the discomfort estimators by exposing information about the upcoming pedestrian hazard situation?  (2) If the above-mentioned question is affirmative: among driving route indications and driving alerts which one produce a lower discomfort in autonomous driving?

The methodological approach for solving these research questions is presented in this paper as follows:



Description of the proposed experiment

Proposed hazard driving situations

Regarding previous researches about implementation trends in automotive HUD systems "(…) from the driver interaction point of view the most included information in HUD systems is related to traffic sign recognition systems, collision warning systems and pedestrian detection systems (…)" [Bet2016]; from which, the last two could involve autonomous maneuvers. However, the collision warning systems are too wide and extremely random to be analyzed by using just one single driving situation, therefore the pedestrian detection systems allows to develop a more specific analysis. Consequently, for the pedestrian detection systems, there are maneuvers that could prevent or avoid carto-pedestrian collisions, some of the most known are: steering pad (from ISO 7401), Sine with Dwell (from FMVSS n. 126 and reg.UNECE13-H), and plenty more; but, regarding the most common car-topedestrian accident scenario 1 proposed by APROSYS [Lan11], under an imminent collision, the Sine with Dwell maneuver is the most suggested one for being applied, as presented in Figure 2.

1 According to Lange [Lan11], the proposed car-topedestrian accident represents the most common one (59%). According to Lange [Lan11], for the scenario depicted in Figure 2, the characteristics indicated in Table 1 must be applied.

Environment

Good visibility conditions without traffic, and dry surfaces. Some pedestrians were randomly included on the track, looking for simulating the Sine with Dwell maneuver described in Figure 2.

Experimental approaches for the hazard driving situation

As described in section "Problem understanding", during the driving scenario here proposed a driving indication or driving alert was projected throughout a HUD system, as indicated in Figure 3. Then, the instant for projecting the information in the HUD system is related to the Perception Reaction Time (PRT), which in this case describes the time needed to identify a potential risky situation (0.8 seconds) [Ber17]. Therefore, according to Table 1 the HUD information must be displayed at least at 17.76 meters before the car-to-pedestrian collision. Also, the red color was implemented for the information projected in the HUD system; the above is related to Alves et al. [alv13], which propose that "the severity of the danger is shown based on colour changes from green (safety) to red (danger)".

Technical testing bench specification

The proposed testing bench involve 2 main components, a TV display and a HUD combiner. The position in which must be placed, and its constructive parameters are indicated in Figure 4 and Figure 5. Where: M: middle point of the TV display, which coincides with the drivers' sight line. E: virtual image position of the proposed HUD system. At this point the combiner is also placed. θ: maximum eccentricity viewing angle that subtend all the HUD virtual image=10°. θc; combiner inclination angle=45°. TV display coordinates (x1,y1) = (6.0 m, 0.0 m). HUD virtual image coordinates (x2,-y2): these distances are based on the driver´s Depth Of Field (DOF) 2 = (2.3 m, -0.4 m).

Test subject selection

For this experiment 10 test subjects (8 males and 2 females) were proposed, aged between 22 and 29 years old (mean=23.6 years, std=2.67 years), all of them have a valid driving license at least during 3 years (mean=4.7 years, std=2.83 years). Then, every test subject was evaluated in terms of Visual Acuity (VA), looking for determining ophthalmological illnesses; according to the results, all test subjects had normal or corrected-to-normal adult human looking at the optical infinity, his depth of focus is 0.43D (2.3m) [Cam57]. sight, i.e. no significant differences were identified among the VA values for both eyes (Right eye: mean= 0.589, std=0.118; Left eye: mean= 0.576, std=0.137; Kruskal-Wallis test: Chi-square=0.12, df=1, p-value=0.914). Also, every test subject was provided with a written consent to participate in these experiments, declaring s/he has not history of psychiatric. Finally, during the proposed experiments all test subjects were naïve about the scope of the obtained results.

Data acquisition procedure

During the autonomous driving context every test subject faced one time the proposed hazard pedestrian situation, using or not the HUD system. Then, every time the data3 from the BR, PZ, HR, T, EDA and the MW questionnaire was obtained. The test subjects drove for 3 minutes every one of the proposed Scenario (S) (S1: scenario without HUD, S2: scenario using HUD driving route indications, and S3: scenario using HUD driving alerts), and for all these the proposed hazard pedestrian situation was included at least one time, lasting every time at least 1 second. Therefore, all test subjects were prompted for driving during 9 minutes. Also, during the proposed experimentation every test subject was prompted to be focused just on the driving route, looking for avoiding any secondary task. In this sense, for solving the first research question indicated in Section 2.1, it is necessary to compare the PZ, BR, HR, T, EDA, and the MW for all test subjects when using an autonomous vehicle equipped or not with a HUD system. Then, for solving the second research question it is necessary to compare the PZ4 , BR5 , HR6 , T7 , EDA8 , and the MW 9 for all test subjects when using driving route indications or driving alerts projected by the HUD system while a hazard pedestrian situation take place.

Data analysis & discussion

Regarding the obtained data, from the statistical point of view the Analysis Of Variances test (ANOVA) was implemented; therefore, for every single Scenario (S) a set of data were defined. The statistical description of the obtained results are indicated in Table 2. In the same way, from a graphical point of view the information reported in Table 2 is finally, all variables were taken randomly, in order to do not affect the independence assumption.

Consequently, according to the ANOVA test, for the PZ, BR, HR, T, EDA and MW data there were not significant differences among the proposed S (pvalue PZ, BR, HR, T, EDA, MW= 0.915, 0.947, 0.886, 0.969, 0.885, 0.450>0.05); from which it is possible to state that there is not enough evidence to confirm that there are significant differences in the drivers PZ, BR, HR, T, EDA and MW, when facing a hazard pedestrian situation in an autonomous vehicle equipped with HUD system.

In this sense, even if [START_REF] Kim | Driver behaviour and performance with augmented reality pedestrian collision warning: An outdoor user study[END_REF] exposed significant advantages for pedestrians' collisions warnings applied to automotive HUD systems, the driving context was mainly focused on nonautonomous driving, conformal graphics, and the Limit Braking manoeuvre instead of the Sine with Dwell manoeuvre. Consequently, the conclusions stated by this study are not comparable to the results here obtained. Then, the implementation of the HUD systems in autonomous driving as a discomfort reducer during hazard situations is not recommended by this study.

Conclusions & future steps

According to the indicated hazard situation, for autonomous driving the proposed HUD system does not influences significantly the PZ, BR, T, HR, EDA and MW variables, neither for the whole driving test (1 minute) nor for the specific hazard situation (1 second); the above is consistent for the driving route indication and the driving alert here described. Therefore, regarding the same colour (red), size (>2cm) and exposition time (1 second), the shape of the hazard information projected in the HUD system does not produce any significant effect on the abovementioned variables, which are considered in this study as mental discomfort estimators. Additionally, by using the ANOVA analysis, comparing the MW variable among S1, S2 and S3 not as an average but independently as the MD, TD and F indicators, there were not found significant differences for any of these indicators in any S (pvalue MD = 0.147, p-value TD = 0.817, p-value F= 0.624).

In this sense, an analysis not about hazard situations, but about infotainment interfaces could represent a more valuable point of discussion when considering autonomous driving; consequently, the above is proposed as a future research work. Finally, it could be also interesting to implement the methodological approach here indicated to other autonomous driving situations, including physical feedbacks as reactions forces and vibration.

Financial disclaimer: this study has been financially supported by the Pontificia Universidad Javeriana (Bogota, Colombia). Project "Análisis del comportamiento del conductor cuando se utilizan sistemas automotrices Head-Up Display", ID 07228.

Fig 1 .

 1 Fig 1. Driving route indications and driving alerts in HUDs.

  Pupil size (PZ): there is a trend between the increasing of the visual effort by discomfort and the reduction of the pupil size [Atc12].  Blink rate (BR): usually while test subjects become more concentrated, there is a blinking frequency reduction [Bor14].  Heart rate (HR): under different driving situations the heart rate measures are largely unaffected [Sha18].  Temperature (T): this type of information is usually adopted as a basic control measurement [Bor14, Hea05].  Electro-dermal activity (EDA): according to Healey and Picard [Hea05], specific changes in the electro-dermal activity are correlated with different driver stress levels.  NASA-TLX Questionnaire as mental workload indicator (MW): this type of information is usually adopted for quantifying the driver experience [Har88, Kre77].

  Description of the proposed experiment o Proposed hazard driving situations o Experimental approaches for the hazard driving situation o Technical testing bench specification o Test subject selection o Data acquisition procedure  Data analysis & discussion.  Conclusions & future steps.

Fig 2 .

 2 Fig 2. Proposed hazard driving situation

Fig 3 .

 3 Fig 3. Proposed HUD information.

Fig 4 .

 4 Fig 4. Proposed testing bench.

Fig 5 .

 5 Fig 5. Schematic view of the proposed testing bench.

Figure 6

 6 Figure 6 represents the common view of proposed driving scenario indicated in Table 1, which was developed throughout Unity-3D ®, version 2018.1.

Fig 6 .

 6 Fig 6. Proposed driving environment.

9

  Fig 7. a) Distribution of the PZ data obtained during the proposed driving scenarios. b) Distribution of the BR data obtained during the proposed driving scenarios. c) Distribution of the HR data obtained during the proposed driving scenarios. d) Distribution of the T data obtained during the proposed driving scenarios. e) Distribution of the EDA data obtained during the proposed driving scenarios. f) Distribution of the MW data obtained during the proposed driving scenarios.

Table 1 . Development of the most common car-to- pedestrian accident scenario.

 1

	Actor		Description
		A	constant	pedestrian
		speed about 5.3 km/h is
	Pedestrian	assumed, considered	which under	is low
		attentional	awareness
		[Mus14].
		A compact car, drove at an
		average speed of 80 km/h,
	Vehicle	following the procedures stated by the from FMVSS
		n. 126 and reg.UNECE13-
		H.
		A specific driving route
		previously selected from
		point A to point B, with good
		visibility conditions, 2 km of
	Track	path distance interrupted by
		5	traffic	lights	and
		acceptable constrains for
		driving at 80 km/h (as
		maximum speed).

Table 2 . Data set collected from the proposed driving situation.

 2

	S	Value	PZ	BR	HR	T	EDA	MW
		Mean	4.07	18.8	75.9 32,8	0.21	7.23
		Median	4.13	16.3	72.9 32,9	0.19	6.33
	S1	Std.	0.92	10.0	12.8	2,4	0.14	3.43
		Min. val.	2.35	7.9	52.8 29,7	0.01	3.0
		Max. val.	5.19	32.6	94.7 36,0	0.50	12.33
		Mean	4.14	18.6	76.2 33,1	0.22	9.0
		Median	4.17	19.6	74.4 32,6	0.19	9.33
	S2	Std.	0.81	10.2	14.2	2,5	0.17	3.14
		Min. val.	2.63	4.1	51.4 30,3	0.01	3.66
		Max. val	5.45	34.0	101	36,4	0.53	12.66
		Mean	4.24	16.1	73.7 33,0	0.25	7.7
		Median	4.29	13.0	73.1 32,4	0.22	8.5
	S3	Std.	1.01	8.4	10.2	2.2	0.17	2.97
		Min. val.	2.95	4.2	56.2 29.8	0.06	2.66
		Max. val.	6.20	31.6	91.6 36.2	0.57	11.33

The BR and the PZ variables were measured by using the SMI Eye tracking glasses; then, the EDA was measured by using the E4 Empatica wristband.

Measured in millimetres.

Rated in minutes.

Measured in Beats Per Minute (BPM).

Measured in Degree Centigrade (°C).

EDA is also noun as the skin conductance, measured in MicroSiemens (µS).