A wearable haptic ring for the control of extra robotic fingers

Irfan Hussain, Giovanni Spagnoletti, Claudio Pacchierotti, and Domenico Prattichizzo

Dept. of Information Engineering and Mathematics, University of Siena, Siena, Italy. Dept. of Advanced Robotics, Istituto Italiano di Tecnologia, Genova, Italy.

Abstract. This paper present a wearable haptic ring called "hRing" to control the soft extra robotic finger and to render haptic informations to provide cutaneous stimuli. The "hRing" is a novel wearable cutaneous device for the proximal finger phalanx. The soft extra robotic is finger designed to be used by chronic stroke patients to compensate for the missing hand function of their paretic limb.

The hRing consists of two servo motors, a vibro motor, two pairs of push button and a belt. The servo motors move the belt placed in contact with the user's finger skin. When the motors spin in opposite directions, the belt presses into the user's finger, while when the motors spin in the same direction, the belt applies a shear force to the skin. The soft finger is an underactuated modular robotic finger worn on the paretic forearm by means of an elastic band. The device and the paretic hand/arm act like the two parts of a gripper working together to hold an object. It has been designed to be wearable, Two chronic stroke patients took part to our experimental evaluation on how the proposed integrated robotic system can be used for hand grasping compensation.

The hRing enabled the patients to easily control the motion of the robotic finger while being provided them with haptic feedback about the status of the grasping action. The patients found the system useful for ADL tasks, the hRing easy to use, and the haptic feedback very informative.

Overview of the system

We propose a novel wearable cutaneous device for the proximal finger phalanx, called "hRing". It consists of two servo motors, a vibrotactile motor, two pairs of push buttons, and a belt. The servo motors move the belt in contact with the user's finger skin. When the motors spin in opposite directions, the belt presses into the user's finger, while when the motors spin in the same direction, the belt applies a shear force to the skin. The device structure is symmetrical and each side features two push buttons, enabling the hRing to be used both on the left and right hand. A preliminary version of this wearable device has been presented in [START_REF] Pacchierotti | The hRing: a wearable haptic device to avoid occlusions in hand tracking[END_REF]. With respect to the version proposed in [START_REF] Pacchierotti | The hRing: a wearable haptic device to avoid occlusions in hand tracking[END_REF], the new hRing presented in this work offers enhanced mobility of the finger phalanx, ambidexterity, a vibrotactile motor, and a lighter body structure.

In this work, we use the hRing as an interface to interact with a novel underactuated soft robotic finger (SRF), presented in [START_REF] Hussan | The soft-sixthfinger: a wearable emg controlled robotic extra-finger for grasp compensation in chronic stroke patients[END_REF] (see Fig. 1). The proposed SRF is an extra robotic finger to be worn on the forearm by means of an elastic band. The robotic device has been devised for compensating hand function in chronic stroke patients to re-gain the grasping capabilities of their paretic hand. Preliminary results with chronic stroke patients has been proposed in [START_REF] Hussain | Using the robotic sixth finger and vibrotactile feedback for grasp compensation in chronic stroke patients[END_REF]. The robotic finger and the paretic hand act like the two parts of a gripper, working together to hold an object. The proposed finger device consists of two main parts: a flexible finger and a support base. The flexible finger has a modular structure. The SRF has a single actuator that can flex/extend the whole finger through a tendon. It houses the actuator, and an elastic band allows the user to secure the device at the forearm. Similarly to the hRing, the structure of the base is symmetrical, enabling the robotic finger to be worn both on the left and right arm. A FSR force sensor is placed at the tip of the robotic finger.

Methods and Experiments

Two chronic stroke patients took part to our experimental evaluation on how the proposed integrated robotic system can be used for hand grasping compensation. The subjects were required to wear the robotic finger on their paretic hand and the hRing on the healthy hand index finger (see Fig. 1). The proposed compensatory tool can be used by subjects showing a residual mobility of the arm. Each patient uses the SRF for bimanual tasks typical in Activities of Daily Living (ADL), such as unscrewing a cap of tomato jar, opening a popcorn bag, or opening a can of beans. The hRing interface is used to both control the flexion/extension motion of the robotic finger and to provide the patient with haptic feedback about the forces exerted by the robotic finger on the environment.

The patient can press the blue button on the external side of the hRing with the thumb of the same hand to initiates the flexing procedure of the robotic finger.

The finger will close until a contact with an object through the FSR mounted on its fingertip is detected. As soon as the contact happens, the robotic finger stops its flexion and the hRing generates a short vibration burst to notify the patient. If the patient presses the blue button again, the finger increases the grasping force on the object. During this process, the hRing belt squeezes the patient's finger proportionally to the grasp force applied by the robotic finger on the environment. When the patient is satisfied with the grasping configuration, he or she can proceed with the task. Finally, pressing the yellow button on the hRing will initiate the opening procedure of the robotic finger. The hRing releases its belt and provides a vibration burst when the opening procedure is completed.

Fig. 1 .

 1 Fig. 1. The integrated system used by a patient in Activities of Daily Living (ADL).


Francesco Chinello

Claudio Pacchierotti

Monica Malvezzi

Domenico Prattichizzo

A novel

  L'archive ouverte pluridisciplinaire

Introduction

Wearable haptic devices are recently gaining great popularity among researchers. Such interfaces will indeed enable natural forms of communication between the wearer and the environment. Humans will be free to move and interact with the surrounding world while being provided with rich and unobtrusive haptic information.

In this respect, fingertip cutaneous technologies have shown promising results. Prattichizzo et al. [START_REF] Prattichizzo | Towards wearability in fingertip haptics: a 3-dof wearable device for cutaneous force feedback[END_REF] presented a wearable cable-driven 3-degrees-of-freedom (3-DoF) cutaneous device for the fingertip. It consists of two parts: one is located on the back of the fingertip, supporting three DC motors, and the other one is placed in contact with the volar surface of the fingertip. The two parts are connected by three cables, actuated by the motors. The mobile platform can press into the user's fingertip and re-angle to simulate contacts with slanted surfaces. Three force sensors placed on the platform measure the contact force at the fingertip. The maximum force applicable is 1.5 N and the device weights as little as 30 g.

More recently, Tsetserukou et al. [START_REF] Tsetserukou | Linktouch: A wearable haptic device with five-bar linkage mechanism for presentation of two-dof force feedback at the fingerpad[END_REF] presented a 2-DoF wearable fingertip display composed of a five-bar linkage system. Two DC motors, placed on the back of the finger, actuate the linkage mechanism. When the two motors spin in the same direction, the linkage system applies shear stimuli at the finger pulp; when the two motors spin in different directions, the linkage system applies normal stimuli at the finger pulp.

3RRS wearable device

This paper presents a novel 3RRS wearable fingertip device. It is composed of a static upper body and a mobile end-effector. The upper body (F in Fig. 1) is located on the nail side of the finger, supporting three small servo motors (C), and the mobile end-effector (E) is in contact with the finger pulp. The two parts are connected by three articulated legs (A), actuated by the motors. The endeffector can move toward the user's fingertip and rotate it to simulate contacts with arbitrarily-oriented surfaces. A piezoresistive sensor (D) can also measure the force applied by the mobile platform to the fingertip. Finally, a clamp (B) enables the user to easily fasten the device on the finger. Each leg, connecting the endeffector to the upper body, is composed of two rigid links connected to each other, the body, and the end-effector, according to a RRS (Revolute-Revolute-Spherical) kinematic chain. Specifically, three spherical (S) joints connect the distal links of the legs to the end-effector, one revolute (R) joint connects the distal and proximal links of each leg, and another revolute (R) joint connects the proximal link of each leg to the body. The three revolute joints between the proximal links and the body are actuated by the servo motors. In each leg, the axes of the two revolute joints are parallel, so that it constitutes a 2-DoF planar articulated mechanism, constraining the motion of the center of each spherical joint on a plane fixed w.r.t. the body. The mobile end-effector has therefore 3-DoF w.r.t. the body. The proposed device weights 25 g for 35×50×48 mm dimensions. A preliminary version was presented in [START_REF] Chinello | Design and development of a 3rrs wearable fingertip cutaneous device[END_REF].

With respect to the cable-driven fingertip devices presented in [START_REF] Meli | Wearable haptics and hand tracking via an rgb-d camera for immersive tactile experiences[END_REF][START_REF] Pacchierotti | Cutaneous haptic feedback to ensure the stability of robotic teleoperation systems[END_REF][START_REF] Pacchierotti | Cutaneous feedback of fingertip deformation and vibration for palpation in robotic surgery[END_REF][START_REF] Prattichizzo | Towards wearability in fingertip haptics: a 3-dof wearable device for cutaneous force feedback[END_REF][START_REF] Prattichizzo | Cutaneous force feedback as a sensory subtraction technique in haptics[END_REF], this device solves the indeterminacy due to the underactuation of the platform. This mechanical structure decouples position and force control problems, simplifying the control structure with respect to the solution proposed in [START_REF] Prattichizzo | Towards wearability in fingertip haptics: a 3-dof wearable device for cutaneous force feedback[END_REF].

2 FFig. 1 :

 21 Fig. 1: The 3RRS thimble.

Table 1 :

 1 3RRS thimble features.

Acknowledgment: The research has received founding from European Union's Horizon 2020 Research and Innovation Programme, Grant Agreement No. 688857 (SoftPro) and from the European Union Seventh Framework Programme FP7/2007-2013, Grant Agreement No. 601165 (WEARHAP).


Vincent Luboz

Christelle Mathieu Bailet

Boichon Grivot

Michel Rochette

Bruno Diot

Marek Bucki

Yohan Payan

Subject-specific modeling for real-time pressure ulcer prevention in sitting posture

Introduction: Ischial pressure ulcer is an important risk for every paraplegic person and a major public health issue. Pressure ulcers appear following excessive compression of buttock's soft tissues by bony structures, and particularly in ischial and sacral bones. Current prevention techniques are mainly based on daily skin inspection to spot red patches or injuries. Nevertheless, most pressure ulcers occur internally and are difficult to detect early.

Methods: Estimating internal strains within soft tissues could help to evaluate the risk of pressure ulcer. A subject-specific biomechanical model could be used to assess internal strains from measured skin surface pressures. However, a realistic 3D non-linear Finite Element buttock model, with different layers of tissue materials for skin, fat and muscles, requires somewhere between minutes and hours to compute, therefore forbidding its use in a real-time daily prevention context. In this work, we propose to optimize these computations by using a reduced order modeling technique (ROM) based on proper orthogonal decompositions of the pressure and strain fields coupled with a machine learning method.

Results: ROM allows strains to be evaluated inside the model interactively (i.e. in less than a second) for any pressure field measured below the buttocks. In our case, with only 19 modes of variation of pressure patterns, an error divergence of one percent is observed compared to the full scale simulation for evaluating the strain field.

TexiSense, Montceau-les-Mines, France, {vincent.luboz, marek.bucki}@texisense.com;

Univ. Grenoble Alpes, CNRS, Grenoble INP, TIMC-IMAG, F38000 Grenoble France, {Mathieu.Bailet, Yohan.Payan}@univ-grenoble-alpes.fr;

ANSYS, Lyon, France, {michel.rochette, christelle.boichon}@ansys.com;

IDS, Montceau-les-Mines, France, b.diot@ids-assistance.com;

Discussions: This reduced model could be the first step towards interactive pressure ulcer prevention. The all on-line computations could be ported to a micro-controller embedded within a pressure mat placed on a wheelchair, thus providing a daily pressure ulcer prevention set-up.

Clinical relevance: Coupled with the TexiMat textile pressure sensor (www.texisense.com) that continuously measures pressure frames under the sitting subject, the real-time ROM strain estimation inside the gluteal soft tissues should help to provide an interactive pressure ulcer risk assessment.


Manish Chauhan
email: manish.chauhan@iit.it

Nikhil Deshpande

Giacinto Barresi

Claudio Pacchierotti

Domenico Prattichizzo

Darwin G Caldwell

Leonardo S Mattos

Design and control of a novel robotic microsurgical forceps for Transoral Laser Microsurgery

This paper presents a master-slave configuration robotic microsurgical forceps, which is capable of performing micro tissue manipulation. The master, i.e., 7 degree of freedom (DOF) device (Sigma.7), tele-operates the slave device which is a combination of a 6-DOF serial robotic arm and a 1-DOF (open/close) microsurgical forceps device. The serial robotic arm is used for positioning and orienting the slave device, which is integrated with a force/torque sensor for tissue gripforce measurement. This integrated system is analyzed for its (i) functional, (ii) usability, and (iii) haptic performance through user trials. The proposed system offers improved tool placement, enhanced tissue perception, safety, and accuracy with respect to the state of the art. This study the feasibility of replacing traditional manual forceps with easy-to-use and ergonomic robot-assisted devices.

I. INTRODUCTION

Transoral Laser Microsurgery (TLM) deals with treatment of malignancies in the laryngeal region such as cysts, polyps, nodules, carcinoma by accessing the vocal cords (surgical site) through the larynx, which has the shape of an irregular closed cylinder. This shape offers a wide range of challenges to the surgeons which are broadly due to three factors, (i) Surgical site: difficulty in accessing its areas due to its limited size ranging between 17-21mm for males and 11-15mm for females [START_REF] Hirano | Morphological Structure of the Vocal Cord as a Vibrator and its Variations[END_REF], (ii) Surgical tools: one DOF long (length 200 -240mm exclusive of tool handle, diameter 2-2.5mm ) rigid structure tools, and (iii) Surgeon interface: manual handling makes it unwieldy for surgeons to perform intraoperative task ergonomically.

Figure 1 shows the exposure method of the surgical site using a laryngoscope (approximate length of 180mm and cross-section 16 × 23mm 2 ) which is inserted orally for obtaining direct line-of-sight through the surgical microscope. A CO 2 laser beam is aimed from a distance of 400mm with the help of a mechanical micro-manipulator joystick. The combination of the laryngoscope and micro-manipulator consumes a depth of 300 -340mm. This leaves the surgeons with a narrow range of 50 -60mm range for microsurgical tool movement and tissue manipulation. Additionally, manual handling of these tools induces hand tremors which makes it cumbersome and non-ergonomic for accessing and resecting the vocal folds [START_REF] Liverneaux | Telemicrosurgery: robot-assisted microsurgery[END_REF].

With a focus on improving the surgeon-machine interface, the research in this paper extends the benefits of robot-Fig. 1: Traditional constraints in TLM operating room. assisted technologies to the critical aspect of tissue manipulation in TLM and presents the design and development of a novel, robotic microsurgical forceps with the surgeon using an ergonomic interface with haptic feedback for improved surgical perception and task outcome. Earlier research in this context resulted in the design of a robot-assisted forceps device presented in [START_REF] Deshpande | Robot-assisted microsurgical forceps with haptic feedback for transoral laser microsurgery[END_REF]. However, in using the device in a realistic surgical scenario, key issues were identified related to the size of the device, which caused occlusion of the surgical site. The device presented in this paper focuses on resolving these issues and meeting the dimensional and operational constraints of TLM through the design of a next generation robotic forceps for enhancing the performance, accuracy & safety of the surgery.

II. RELATED WORK

Various robot-assisted surgical tools have been developed by different groups. Snake-like manipulators with high tip dexterity for tissue manipulation and suturing [START_REF] Simaan | A Dexterous System for Laryngeal Surgery[END_REF] have been explored by Simaan et al. A cooperatively controlled bimanual teleoperation robot having 3-DOF wrists with surgical tools attached was developed by He et al. [START_REF] He | A New ENT Microsurgery Robot: Error Analysis and Implementation[END_REF]. Wang et al. [START_REF] Wang | Kinematic Design for Robotassisted Laryngeal Surgery Systems[END_REF] presented a new robot-assisted master-slave laryngeal surgery system consisting of two symmetrical 9-DOF manipulators, with quick-change interface for surgical tools. Rivera-Serrano et al. [START_REF] Rivera-Serrano | A Transoral highly flexible robot: Novel technology and application[END_REF] presented a highly articulated robot in a follow-the-leader mechanism using a master controller. [START_REF] Solares | Transoral Robot-assisted CO 2 Laser Supraglottic Laryngectomy: Experimental and Clinical Data[END_REF] and Desai et al. [START_REF] Desai | Transoral Robotic Surgery using a Carbon-dioxide Flexible Laser for Tumors of the Upper Aerodigestive Tract[END_REF] have explored the utility of the da Vinci Surgical System [START_REF]da Vinci Surgical System[END_REF] for TLM but found size of the da Vinci tool shafts as a major limitation along with constant change of attendant during surgery. An effective solution was presented by Maier et al. [START_REF] Maier | A New Micromanipulator System for Middle-Ear Surgery[END_REF] where standard surgical tools could be attached to a lightweight manipulator without any modification. Despite extensive research efforts, these instruments are not particularly focused towards TLM application. Also, surgeon performance can be enhanced with haptic feedback for enhanced intra-operative perception [START_REF] Meli | Experimental evaluation of magnified haptic feedback for robot-assisted needle insertion and palpation[END_REF], [START_REF] Meli | Sensory subtraction in robot-assisted surgery: fingertip skin deformation feedback to ensure safety and improve transparency in bimanual haptic interaction[END_REF]. These benefits lend themselves readily towards facilitating and improving the complex suite of otolaryngological techniques of tool control involved in TLM. Having understood these needs, it is important to create a device with such capability.

Solares and Strome

III. ROBOT-ASSISTED MICROSURGICAL FORCEPS DESIGN

With reference to above discussion, a standalone device for improved motion accuracy, reduced operator dependence, and simpler tissue manipulation with surgical exposure was created while complying with design constraints as noted in Fig. 1. The key design specifications of these components are based on the constraints offered by TLM which are listed in Table I. The novel design consists of three modules: (i) the tool shaft; (ii) the tool actuation mechanism; and (iii) the tool-shaft holder. For avoiding interference between tool and other surgical setup.

Tool footprint under microscope of 5mm.

To maintain minimum vision occlusion through the microscope.

A. The tool shaft

This component is a modified version of the traditional microsurgical tools, made up of an outer shaft (φ 2mm) which holds an inner translating wire (itw,φ 1mm). The translation of this wire (≈ 3mm) provides the open-close DOF of the tool jaw. The tool shafts are of two types: (i) where pushing action of the itw closes the tool jaws, and (ii) where pulling action of the itw closes the jaws. One adaptation, namely docking interface (DI) is introduced at the proximal end of the tool shaft which is a short hollow tube with M3 external threading. (Refer Fig. 2(a)).

B. Tool actuation mechanism

The tool actuation mechanism consists of a set of five linkages ("L1","L2", "L3", "L4", "L5"), which are designed to provide linear translation of the itw through kinematic inversion. Here, the link "L1" is treated as ground, i.e., the hinge link. Link "L2" forms the input link (i.e., slider) along the "actuator axis" for transfering motion to link "L3" which in-turn transfers motion to link "L4" through the ground/hinge link "L1". Links "L3" and "L4" have an inverse relationship and Link "L4" is directly coupled with link "L5". Link "L5" (i.e., wire pusher) is connected to and actuates the itw of the tool shaft along the "tool axis" (Refer Fig. 2(b)). A force sensor (ATI Nano17) is located with its measurement axis coincident with the "actuator axis" of link "L2" optimally for allowing the sensing of the tissue gripping force. The closing of the gripper jaws (along "tool axis") on tissue transmits a reaction force on to the surface of the sensor through the link "L2" (along "actuator axis"). The force sensor, attached to link "L2", outputs a signal in direct proportion to the tissue gripping force.

C. Tool-shaft holder

Figure 2(c) shows the design of the tool-shaft holder which connects the tool shaft with tool actuation mechanism. It's frame comprises of three sub-frames "F1", "F2", and "F3". The tool shaft itself is attached rigidly to sub-frame "F1" through DI at point "P1" such that the "tool axis" is at an offset of 200mm from the "actuator axis". The crosssectional thickness of "F1" is designed to be 5mm. Both these aspects are maintained to comply with the design constraints in Table I. Further, the sub-frames "F1" and "F2" are rigidly connected at "P2" (i.e. "support axis"). The sub frame "F3" is designed as an attachment bracket for robot end effector upon which a linear actuator (Nanotec L2018 series) is placed to drive the open/close DOF.

IV. THE NOVEL STRAIGHT-LINE MECHANISM AND ITS

VALIDATION WITH ADAMS SIMULATION The tool open/close DOF is actuated by a mechanism which is designed as a graphical synthesis method in twostages. This five-link mechanism is synthesized as a Function Generator [START_REF] Kinzel | Function generation with finitely separated precision points using geometric constraint programming[END_REF] problem (where output motion is linearly related to input actuation) to provide straight line motion of the itw.

1) In the first stage of synthesis, a suitable three link mechanism is formed which can allow linear displacement of the slider (i.e., link "L2": F-E) within limits x i , x f along the "actuator axis". Here x i and x f are the initial and final positions of slider motion, i.e., 0 and 3mm respectively. The kinematic synthesis begins with an arbitrary choice of hinge point A at an offset of 50mm from the "actuator axis" in Ydirection. (Refer Fig. 3). On the "support axis" another point C is chosen at a distance of 10mm from point A in X-direction. The end point E of link "L2" is chosen on the "actuator axis" such that its position lies on the vertical line passing through point A. Joining the three points A -C -E, a triangular shaped link is achieved, named as "L3". The linear displacement of link "L2" causes link "L3" to translate in angular motion θ i , θ f . For ensuring straight-line trajectory of link "L2" between x i and x f , three additional intermediate Chebyshev's precision points [START_REF] Meli | Experimental evaluation of magnified haptic feedback for robot-assisted needle insertion and palpation[END_REF] are chosen using the equation 1.

x j = a -hcos[(2 j -1) • π/2n] j = 1, 2, 3 (1)
The variables a and h are defined as x i + x f /2 and x fx i /2 respectively. The choice of three Chebyshev's precision points (n) is sufficient for allowing the slider to follow an exact straight line path. The three precision points are x 1 = 0.2mm, x 2 = 1.5mm and x 3 = 2.799mm. The link "L2" starts from x i passes through the three precision point x 1 , x 2 , x 3 & finishes its stroke at its final position x f . Since link "L2" Fig. 3: Kinematic Synthesis of Mechanism for surgical tool actuation.

is rigid, points F and E have identical movement such that displacement of link "L2" causes angular motion of link "L3" through corresponding positions θ i ,θ 1 , θ 2 , θ 3 , θ f . A closer observation of link "L3" (A -C -E) shows that a displacement of 1mm of end point E in X direction (Fig. 4(a)) causes its simultaneous displacement of 0.0130mm in Y direction (Refer Fig. 4(b)). This negligible Y displacement is because of angular motion of link "L3" with reference to the hinge point A which is compensated with a small cavity in link "L3" at end point E.

1) In the second stage of synthesis, two links are fabricated such that the motion from "actuator axis" can be transferred to "tool axis". First, a hinge point D is chosen along "support axis" at a distance of 10mm from point C in X-direction. Another point B is assumed on the "tool axis" which is at a distance of 150mm from "support axis" in Y-direction. The triangular link B -C -D so created is termed as link "L4". Point C serves as the common engagement point for links "L3" and "L4" such that the angular motion of link "L3" through θ i ,θ 1 , θ 2 , θ 3 , θ f is mirrored by link "L4" . The fifth link is synthesized to be connected at the end point B, link B -G or "L5". The angular motion of link "L4" is transferred to "L5" such that it produces the corresponding straight line translation of the itw through the precision x 1 , x 2 , x 3 . The ratio of the link lengths "L4" and "L3" results in a link-ratio On performing the mobility analysis using the Grbler's criterion [START_REF] Gogu | Chebychev grubler kutzbachs criterion for mobility calculation of multi-loop mechanisms revisited via theory of linear transformations[END_REF], it was found that the mechanism's DOFs were two. Here, the first DOF is the linear translation of link "L5" (B -G) and the second DOF can be explained as the negligible motion in the Y direction for the end points E and B. Finally, the angular movements of links "L3" and "L4" are possible in both clockwise and anticlockwise direction which makes it adaptable for both the types of microsurgical forceps as discussed in sec. III-A.

V. INTEGRATION AND CONTROL OF ROBOT ASSISTED DEVICE WITH SERIAL MANIPULATOR

In continuation with the discussion in sec. III-C, the above designed device was integrated with a 6 DOF Univeral Robot serial manipulator (UR5 [START_REF]Universal Robot 5. Universal Robots[END_REF]) in order to position the tool exactly at the surgical site. Figure 5(a) and (b) shows the integration of this device with control through 7 DOF teleoperation master device (Force Dimension Sigma.7 [START_REF]Force Dimension: Sigma 7, Product Overview[END_REF]).

A. Control and haptic feedback Design

Linux-based software was written using the Robot Operating System platform for controlling the 7-DOF manipulator. The velocities of the robotic manipulator joints q ∈ R 6 can be expressed with unilateral velocity based control as:

qr = J -1 qh ζ (2)
where J -1 is the Moore-Penrose pseudo-inverse of the manipulator Jacobian matrix J ∈ R 7x6 and qh ∈ R 7 are the velocities of the Sigma.7's end-effector. The gesture scaling factor ζ is tunable to allow coarse and fine gestures in different stages of operation. The qh velocities of the master end-effector are scaled through a low-pass filter in equation ( 3), with a tunable factor β to control the level of highfrequency tremor suppression.

qk h = (1 -β ) • qk-1 h + β • qencoder h (3)
Sigma.7's gripper DOF was mapped directly to the open/close DOF of the microsurgical forceps such that while a tissue is gripped with the forceps, the sensed value from the ATI sensor is filtered using a low-pass filter and then scaled for rendering to the 7 th (gripper) DOF of the Sigma.7. The following equations are used.

f k g = (1 -β ) • f k-1 g + β • f sensor g f sigma g = κ • f k g (4)
The values used for the constants are emperically obtained as β = 0.001 and κ = 4. The haptic rendering loop is run at 500 Hz.

VI. EVALUATION & USER TRIALS

User trials were conducted for establishing (i) the performance; (ii) the usability; and (iii) the gripping force feedback of the integrated device. 12 different subjects (divided in two sets A and B of 6 each) operated the master device for performing an experiment with ring-to-peg transfer (Refer inset, Fig. 6). This task was chosen due to its similarity in nature to the general surgical tasks like gripping, pulling and orienting a tissue out of the larynx. These trials were conducted under two conditions: (i) C1: with haptic feedback; and (ii) C2: without haptic feedback for obtaining a comparative analysis between the two conditions. "Set A" subjects performed trials in the sequence C2-before-C1, while "Set B" performed C1before-C2 for avoiding subjective bias in the results coming from the order of presentation of the haptic condition. Each subject in both the set's performed 6 trials for accounting the learning aspects.

For uniform results, the UR5 robot was programmed to start from the home position in each trial where the following measurements were made, (i) Time required to lift the Oring from a peg T li f t , (ii) Total time for peg to peg tansfer (T total ), (iii) Number of attempts for lifting the ring, (iv) Number of ring drops during transfer, (v) the Trajectory executed for the task, and (vi) Tissue gripping force on the O-ring surface for C1 and C2 conditions (only for Set B). S4. The control/use of the device was easy. S5. I found the device was easy to learn, so I could start using it quickly. S6. I was stressed, irritated, and annoyed using this device during the task. S7. I would like to use this device again for this kind of task.

S8. My performance in this task with this device was satisfying S9. It was easy to make errors with this device.

CP. Preference of condition 1 or condition 2 (Age of subjects. Mean=27.75 years with SD=2.95 years. Gender. 9 Male, 3 Female)

A subject-wise experience evaluation was collected through a questionnaire of 9 statements (Refer Table II)

where their degree of agreement with each statement was collected along a 7-point Likert-type scale [START_REF]Universal Robot 5. Universal Robots[END_REF] (the score "1" means "I strongly disagree", while "7" implies "I strongly agree"). At the end, the subjects compared their overall experience by expressing a degree of condition preference (CP) for C1 or C2. Based on all the above measurements and subjective evaluations, the following section explains the results of the experiment.

A. Device Performance Analysis 1) Trajectory Analysis : Change in trajectory lengths (improvement or deterioration) over the trials performed by the subjects was chosen as a metric for making a comparison among subjects in order to establish the device performance. Since each subject performed 6 trials, the length ratio of the subsequent 5 trials was calculated against the first trial. It was found that the ratio of the last 6 th trial was 0.8273, while the average ratio over the 5 trials was 0.9218. Both values being less than 1 demonstrate quick learnability and skill acquisition of subjects over a small set of trials.

2) Execution Time Analysis: Analysis of task execution time showed that T li f t reduced from 72.17 seconds to 42.25 seconds over the 6 trials with an improvement of 41%. The same trend for T total was obtained where it went from 94.4 seconds to 56.8 seconds, giving an improvement of almost 40%. This demonstrated that the device allowed quick learning and skill acquisition for subjects.

B. System Usability Analysis 1) User Accuracy evaluation:

The subjective evaluation scores in Table III (S1, S7, S8, and S9) are observed to be greater than 4 on 7-point scale, indicating a positive evaluation of the device accuracy. For S1, the subjects evaluated the device at (5.455 / 7), giving high marks for its precision in the tasks. This aspect is also confirmed by S8 where the subjects are comfortable and satisfied with their performance using the device (5.33 / 7). 2) Usability Analysis: The overall scores for statements S2, S3, S4, S5, and S6 pertain to the overall usability, ergonomics, and usage fatigue. Low values on S2 and S6 indicate the reduced mental effort required by subjects, while the high scores on S4 and S5 indicate device easy to use and learnability.

C. Haptic Performance Analysis 1) Comparison of robot trajectory under C1 and C2: The comparative analysis show that the overall trajectory ratio in C1 was much lower than that for C2 condition. Also, irrespective of the order of presentation of the device, the average ratio of robot trajectory was smaller in C1 (Set A (C1:0.77, C2:0.9654) and Set B (C1: 0.8926, C2: 1.0592)). Though this value is not significantly smaller, it indicates a trend where haptic feedback prompts better performance with the robot.

2) Time comparison for task under C1 and C2: 1) T li f t : There was no significant difference between time response for each subject to lift a ring from a peg in both conditions with mean time of 55.7s for C1 and 57.8s for C2. A comparison of the number of attempts done to lift the ring (errors) show that the condition C1 is favorable (with 6 occurrences) with respect to C2 (with 9 occurrences). 2) T total : No significant difference was seen in comparison of time response for pick-n-place of ring for both conditions with mean time of 71.8s for C1 and 73.5s for C2. Also, insignificant difference in number of ring drops (errors) after lifting were observed with 2 occurrences in C1 and 1 occurrence in C2. In order to get a better perception while gripping with forceps, a comparison of average gripping force applied by subjects showed that the mean force applied on the ring was less in C1 (p = 7.4189e-04) according to Student's t-test with respect to C2 (2.158N vs 5.606N). This result indicates that gripping force feedback can allow enhanced perception as well as safety, limiting the force applied for tissue gripping. This result agrees with the conclusion in [START_REF]Force Dimension: Sigma 7, Product Overview[END_REF]where the mean applied force was less under haptic feedback condition.

4) Subjective evaluation with questionnaire: The questionnaire scores in Table III were evaluated through Friedman test [START_REF] Dawes | Do data characteristics change according to the number of scale points used an experiment using 5 point, 7 point and 10 point scales[END_REF] to understand the effect of gripping force feedback on task completion time. It can be seen from statements S1, S2, S4, S5, S6 and S7 that the subjects were unable to judge the performance of device with respect to the two conditions. The performance parameters of the device i.e. its learning and induction of fatigue got higher scores in condition C1. But this score had no significant difference with respect to C2. Also, a comparison of scores for statement S3, S8 and S9 shows that device performance suffered marginally in context of errors occurring in condition C1.

VII. CONCLUSION AND FUTURE WORK

This paper presents a novel design of an integrated robotassisted microsurgical forceps tool for intraoperative use in TLM. The new design complies with the TLM constraints and replaces the traditional manually operated tools with a teleoperation system consisting of an integrated: (i) 7 DOF robotic microsurgical forceps; (ii) a 7-DOF teleoperation master device; and (iii) a force/torque sensor for tissue gripping feedback. A comparative performance analysis of the device was done through user trials under C1 and C2 conditions which resulted in better performance with force feedback. The system provides: (i) improved controllability, safety; (ii) reduced task completion time; (iii) enhanced surgical site perception; and (iv) intuitive and ergonomic use with a common surgeon interface providing gesture scaling, reduced hand tremors and wrist excursions. In future, an attempt to increase the tool workspace shall be investigated by addition of rotational DOF to tool shaft which shall be tested in collaboration with expert surgeons through ex-vivo and cadaver trials.

Fig. 2 :

 2 Fig. 2: Robot assisted Microsurgical forceps design.

of 3 .Fig. 4 :

 34 Fig. 4: Curvi-linear motion of Link 3 and 4.

Figure 4 (

 4 d) shows this negligible displacement of 0.0075mm in Y direction which is compensated with a cavity in link "L4" at point B (Refer Fig. 2(b)).

  (a) Closeup view under surgical microscope. (b) Haptic feedback integration.

Fig. 5 :

 5 Fig. 5: Microsurgical forceps integration control by master slave configuration.

Fig. 6 :

 6 Fig. 6: Ring-in-peg experiment.

  S3. I had to work hard to accomplish the task with this device.

3 )

 3 Comparison of Gripping force in C1 and C2: Figure 7 show a subjective comparison of O-ring gripping force under conditions C1 and C2 for subjects in Set B. It can seen that the gripping force applied under condition C1 is significantly less in comparison to C2 indicating an awareness of the force required to grasp the ring in condition C1.

Fig. 7 :

 7 Fig. 7: Subject-wise comparison of gripping force.

TABLE I :

 I Design specifications of the microsurgical forceps

	Design Limits	Remarks
	Tool maneuverability	To be operable under the surgical
	range of 70-80mm.	microscope.
	Displacement from
	microscope line-of-sight
	of 200mm.

TABLE III :

 III Scores for Questionnaire

		Subject Trials
		C1		C2		Overall Scores
		m	sd	m	sd	m
	S1	5.58	0.64	5.33	1.59	5.455
	S2	3.00	1.73	2.41	0.86	2.705
	S3	3.91	1.49	3.16	1.46	3.535
	S4	4.91	1.11	4.58	1.65	4.745
	S5	5.75	1.16	5.5	1.44	5.625
	S6	2.25	1.23	2.66	1.02	2.451
	S7	5.25	1.42	5.16	1.28	5.205
	S8	4.91	1.55	5.75	1.16	5.33
	S9	4.08	1.38	3.91	1.60	3.995
	CP	m=4.08; sd = 2.09


Irfan Hussain

Leonardo Meli

Claudio Pacchierotti

Domenico Prattichizzo

A soft robotic supernumerary finger and a wearable cutaneous finger interface to compensate the missing grasping capabilities in chronic stroke patients

Stroke survivors who experience severe hemiparesis often cannot completely recover the use of their hand and arm. Many of the rehabilitation devices currently available are designed to increase the functional recovery right after the stroke when, in some cases, biological restoring and plastic reorganization of the central nervous system can take place. However, this is not always the case. Even after extensive therapeutic interventions, the probability of regaining functional use of the impaired hand is low. In this respect, we present a novel robotic system composed of a supernumerary robotic finger and a wearable cutaneous finger interface. The supernumerary finger is used to help grasping objects while the wearable interface provides information about the forces exerted by the robotic finger on the object being held. We carried out two experiments, enrolling 16 healthy subjects and 2 chronic stroke patients. Results showed that using the supernumerary finger greatly improved the grasping capabilities of the subjects. Moreover, providing cutaneous feedback significantly improved the performance of the considered task and was preferred by all subjects.

I. INTRODUCTION

Long-term disabilities of the upper limb affects millions of stroke survivors [START_REF] Go | Heart disease and stroke statistics-2014 update: a report from the american heart association[END_REF]. More than 80% of individuals who experience severe hemiparesis after stroke cannot completely recover hand and arm use [START_REF] Nakayama | Compensation in recovery of upper extremity function after stroke: the copenhagen stroke study[END_REF]. The improvement of the paretic hand functionality plays a key role in the functional recovery of stroke patients with a paretic upper limb [START_REF] Faria-Fortini | Upper extremity function in stroke subjects: relationships between the intl. classification of functioning, disability, and health domains[END_REF]. Different motor impairments can affect the hand both at motor execution and motor planning/learning level, including weakness of wrist/finger extensors, increased wrist/finger flexors tone and spasticity, co-contraction, impaired finger independence, poor coordination between grip and load forces, inefficient scaling of grip force and peak aperture, and delayed preparation, initiation, and termination of object grip [START_REF] Balasubramanian | Robot-assisted rehabilitation of hand function[END_REF]. In the last two decades, several rehabilitation teams have started integrating robotic-aided therapies in their rehabilitation projects. Such treatments represent a novel and promising approach in rehabilitation of the post-stroke paretic upper limb. The use of robotic devices in rehabilitation can provide high-intensity, repetitive, task-specific and interactive treatment of the impaired upper limb, and can serve as an objective and reliable means of monitoring patient This research has received funding from the European Union Seventh Framework Programme FP7/2007-2013 under grant agreement n • 601165 of the project "WEARHAP" and from the European Union's Horizon 2020 Research and Innovation Programme under Grant Agreement n • 688857 of the project "SoftPro". 1 Dept. Information Eng. and Mathematics, Univ. Siena, Siena, Italy. 2 CNRS at Irisa and Inria Rennes Bretagne Atlantique, Rennes, France. 3 Dept. Advanced Robotics, Istituto Italiano di Tecnologia, Genoa, Italy.

(a) Opening a jar of coffee (left) and tomato (right).

Fig. 1. The system is composed of a supernumerary robotic finger and a wearable cutaneous finger interface, called hRing. The picture shows the integrated system used by a patient in Activities of Daily Living (ADL). The hRing is used to control the opening/closing motion of the robotic finger and to provide the wearer with information about the forces exerted by the robotic finger.

progress [START_REF] Masiero | Robotic-assisted rehabilitation of the upper limb after acute stroke[END_REF], [START_REF] Chiri | Mechatronic design and characterization of the index finger module of a hand exoskeleton for post-stroke rehabilitation[END_REF]. Most of the proposed devices for hand and arm rehabilitation are designed to increase functional recovery in the first period after the stroke when, in some cases, biological restoring and plastic reorganization of the central nervous system may take place [START_REF] Lum | Robotic approaches for rehabilitation of hand function after stroke[END_REF]. However, even after extensive therapeutic interventions in acute rehabilitation, the probability of regaining functional use of the impaired hand is low [START_REF] Kwakkel | Probability of regaining dexterity in the flaccid upper limb impact of severity of paresis and time since onset in acute stroke[END_REF].

For this reason, our group has recently started to study innovative robotic devices for the compensation of hand functions in chronic stroke patients [START_REF] Prattichizzo | The sixthfinger: a modular extra-finger to enhance human hand capabilities[END_REF], [START_REF] Hussain | Using the robotic sixth finger and vibrotactile feedback for grasp compensation in chronic stroke patients[END_REF], [START_REF] Hussain | An emg interface for the control of motion and compliance of a supernumerary robotic finger[END_REF], [START_REF] Hussain | A soft robotic extra-finger and arm support to recover grasp capabilities in chronic stroke patients[END_REF]. We devised wearable robotic supernumerary fingers that can be used as an active compensatory tool for grasping objects. These devices can be easily worn on the paretic forearm by means of an elastic band, so that the robotic finger and the paretic hand can act like the two parts of a gripper working together to hold an object. To further improve their wearability, these fingers can be also shaped into a bracelet when not used. The user can then control the flexion/extension of the robotic finger through different means, such as an EMG interface placed on the patient forehead [START_REF] Salvietti | Compensating hand function in chronic stroke patients through the robotic sixth finger[END_REF] or a hardware switch [START_REF] Hussain | Using the robotic sixth finger and vibrotactile feedback for grasp compensation in chronic stroke patients[END_REF], [START_REF] Hussain | Vibrotactile haptic fedback for intuitive control of robotic extra fingers[END_REF], [START_REF] Hussain | A wearable haptic ring for the control of extra robotic fingers[END_REF].

These above mentioned solutions have shown promising results, re-enabling impaired users to grasp objects they would have not been able to grasp otherwise. Despite this, they do not provide any information about the forces exerted by the supernumerary limb on the environment, which is known to be useful [START_REF] Meli | Sensory subtraction in robot-assisted surgery: fingertip skin deformation feedback to ensure safety and improve transparency in bimanual haptic interaction[END_REF], [START_REF] Pacchierotti | Cutaneous feedback of fingertip deformation and vibration for palpation in robotic surgery[END_REF], [START_REF] Pacchierotti | Cutaneous haptic feedback in robotic teleoperation[END_REF]. Haptic feedback is particularly important for post-stroke patients suffering from tactile anesthesia, or anaphia [START_REF] Son | Deficits of movement accuracy and proprioceptive sense in the ipsi-lesional upper limb of patients with hemiparetic stroke[END_REF], in the hand contralateral to the stroke, which is the one that should provide force information about the supernumerary limb. For this reason, there have been attempts to restore the sense of touch in patients affected by this deficit. A very first result has been presented in [START_REF] Hussain | Using the robotic sixth finger and vibrotactile feedback for grasp compensation in chronic stroke patients[END_REF], [START_REF] Hussain | Vibrotactile haptic fedback for intuitive control of robotic extra fingers[END_REF], where the authors developed a vibrotactile ring to provide information about the status of a robotic supernumerary finger.

Although promising, the design of the supernumerary fingers presented in [START_REF] Salvietti | Compensating hand function in chronic stroke patients through the robotic sixth finger[END_REF], [START_REF] Hussain | An emg interface for the control of motion and compliance of a supernumerary robotic finger[END_REF], [START_REF] Prattichizzo | The sixthfinger: a modular extra-finger to enhance human hand capabilities[END_REF], [START_REF] Hussain | Design guidelines for a wearable robotic extra-finger[END_REF] only considered fully-actuated supernumerary devices, which used as many motors as joints, resulting in quite complex and cumbersome systems. Moreover, the haptic interfaces for supernumerary limbs presented in [START_REF] Hussain | Using the robotic sixth finger and vibrotactile feedback for grasp compensation in chronic stroke patients[END_REF], [START_REF] Hussain | Vibrotactile haptic fedback for intuitive control of robotic extra fingers[END_REF] considered only very simple vibrotactile interfaces, which prevented them from conveying any rich force information.

For this reason, in this paper, we present an innovative soft supernumerary underactuated robotic finger coupled with a wearable skin stretch cutaneous device, shown in Figs. 1 and2. The supernumerary finger is controlled by only one motor, and its soft structure enables it to adapt to the object being grasped, resulting in a gentle but stable grasp. The haptic interface is able to provide normal, skin stretch, and vibrotactile stimuli at the wearer's finger, relying rich information about the forces exerted by the supernumerary finger on the environment. A live demonstration of this system during the 2016 IEEE Haptics Symposium in Philadelphia, PA, USA, received the "Best Demonstration Award".

II. THE SUPERNUMERARY ROBOTIC FINGER (SRF)

The supernumerary robotic finger is a wearable robotic extra finger, that can be used by chronic stroke patients to compensate for the missing functions of their paretic hand [START_REF] Hussain | The soft-sixthfinger: a wearable emg controlled robotic extra-finger for grasp compensation in chronic stroke patients[END_REF]. It is composed of an underactuated modular structure to be worn on the paretic forearm by means of an elastic band.

The final prototype used in this work is the result of a long interaction between our group of engineers, the clinicians of the Azienda Ospedaliera Universitaria Senese, and a small group of keen chronic stroke patients.

1) Description: The device consists of two main parts: a modular flexible finger and a support base, as shown in Fig 2b . The support base is realized in ABS (Acrylonitrile butadiene styrene) using rapid prototyping techniques. It houses one servo motor and an elastic band used to wear the device on the forearm. Rubber spacers are used to increase the comfort and grip of the device on the forearm. The flexible finger is composed of seven identical modules. Each module consists of an ABS part that acts as a rigid link and a 3D printed TPU (thermoplastic polyurethane) part that acts as a flexible joint. Finally, a soft rubber layer increases the friction at the contact area. The modules are connected to each other by sliding the thermoplastic polyurethane inside the ABS, making the assembling process very easy. 2 mm holes in the rigid ABS links allows the passage of a tendon that connects all the modules to the servo motor fixed to the support base. The servomotor, a Dynamixel servo MX-28T, drives the flexion/extension of the finger by pulling/releasing the tendon.

The robotic finger has been also designed thinking about its wearability. In fact, it can be easily worn by means of an elastic band, as shown in Fig. 1. It can be also wrapped up on the arm as a bracelet when not used. The switching between working and resting positions is achieved through a passive rotatable locking mechanism.

2) Compliance: Due to the flexible modules, the robotic finger is compliant, making it capable to easily adapt to different shapes during the grasp [START_REF] Dollar | Joint coupling design of underactuated hands for unstructured environments[END_REF].

During grasping, the mechanical compliance of the soft modules leads to an adaptation of the finger's configuration to the object's shape. Shape adaptation is known to increase grasp performance by compensating for the uncertainties in sensing and actuation, helping to achieve a stable grasp [START_REF] Eppner | Grasping unknown objects by exploiting shape adaptability and environmental constraints[END_REF].

In our previous works, we considered a fully-actuated fully-sensorized robotic device with rigid links [START_REF] Salvietti | Compensating hand function in chronic stroke patients through the robotic sixth finger[END_REF], [START_REF] Hussain | An emg interface for the control of motion and compliance of a supernumerary robotic finger[END_REF]. The closing trajectory of the finger was controlled by the motion of each independent actuator. Advanced control techniques, based on the finger sensory information, were needed to achieve a suitable grasping trajectory for shape adaptation during grasping [START_REF] Hussain | Vibrotactile haptic fedback for intuitive control of robotic extra fingers[END_REF]. With the proposed soft finger, thanks to its flexible design, it is easier to adapt to the grasped object, ensuring a more stable grasp. Moreover, thanks to its underactuated structure, we were able to achieve a more compact and robust design, as well as a simpler control action. Table I shows the technical details of the proposed supernumerary robotic finger.

III. THE HRING WEARABLE INTERFACE

To provide the patient with information about the forces exerted by the supernumerary robotic finger to the environment, we used a cutaneous skin stretch device for the proximal finger phalanx. Wearability, comfort, ease of use, and effectiveness were the main requirements for the design of this haptic device [START_REF] Pacchierotti | Wearable haptic systems for the fingertip and the hand: taxonomy, review, and perspectives[END_REF]. 1) Description: The hRing is shown in Fig. 2a. It is composed of a static part, that houses two servo motors and two pulleys, and a fabric belt, that applies the requested stimuli to the finger. A strap band is used to secure the device on the finger proximal phalanx. We used two PWMcontrolled HS-40 servomotors (HiTech, Republic of Korea). The device weights 38 g for 30×43×25 mm dimensions.

2) Haptic feedback: The working principle of the device is depicted in Fig. 3. Similarly to the principle proposed by Minamizawa et al. [START_REF] Minamizawa | Gravity grabber: wearable haptic display to present virtual mass sensation[END_REF], when the two motors rotate in opposite directions, the belt is pulled up, providing a force normal to the finger (left side of Fig. 3). On the other hand, when motors spin in the same direction, the belt applies a shear force to the finger (right side of Fig. 3). Of course, these two movements can be combined together to provide at the same time shear and normal stimuli.

However, although the hRing can apply both normal and shear forces, in this paper, we used it to only apply forces normal to the finger skin, similarly to [START_REF] Prattichizzo | Cutaneous force feedback as a sensory subtraction technique in haptics[END_REF]. This is mainly due to the fact that we have not added any multi-DoF force sensor on the robotic finger, with the objective of improving its wearability and portability. As detailed below, we estimate the contact force solely from the load of the finger's motor. Nonetheless, as also mentioned in Sec. VI, in the future, we plan to add an ATI 6-DoF force/torque sensor on the distal module of the supernumerary finger, to be able to accurately measure the forces exerted by the finger on the environment.

The relationship between the belt displacement and the force applied to the human finger proximal phalanx, f h , is

∆d = f h K , (1)
where ∆d ∈ R is the displacement of the belt toward the skin since it first made contact with the fingertip, and K ∈ R is the finger phalanx stiffness value [START_REF] Pacchierotti | Cutaneous haptic feedback in robotic teleoperation[END_REF], [START_REF] Park | Development of a softfingertip and its modeling based on force distribution[END_REF]. The force exerted by the supernumerary finger on the environment is evaluated by considering the load of the motor τ m ,

f h = f h,M τ m,M τ m , (2)
where τ m ∈ [0, τ m,M ] Nm, τ m,M = 2.5 Nm is the maximum torque the motor can provide (see Tab. I), and f h,M = 9.4 N is the maximum force the hRing can apply on the finger skin. f h,M was measured positioning an ATI Nano force/torque sensor between the belt and the rigid structure of the hring and applying the maximum opposite rotation of both servo motors, i.e., the maximum squeezing force.

IV. CONTROL

Pressing the blue (proximal) button on the external side of the hRing (see Figs. 1 and2) initiates the flexing procedure of the robotic finger. The finger will then close until a contact with an object is detected. As soon as the contact happens, the robotic finger stops its flexion and the hRing generates a short vibration burst to notify the subject. If the subject presses the blue button again, the finger increases the grasping force on the object. During this process, the hRing belt squeezes the user's finger proportionally to the grasp force applied by the robotic finger on the environment (see Sec. III). If the grasping force exerted by the robotic finger reaches the maximum force applicable by the motor, a double vibration burst alerts the user. When the patient is satisfied with the grasping configuration, he or she can proceed with the task. Finally, pressing the yellow (distal) button on the hRing will initiate the opening procedure of the robotic finger. A video of the integrated system is available as supplemental material.

V. EXPERIMENTS

In order to demonstrate the effectiveness of our system, we carried out two experiments, enrolling sixteen healthy subjects and two post-stroke patients.

A. Experiment #1: pick and place with healthy subjects 1) Subjects: Sixteen healthy participants (12 males, 4 females, age range 21 -32) took part to the experiment, all of whom were right-handed. Nine of them had previous experience with haptic interfaces. 2) Setup: The experimental setup is shown in Fig. 4. It is composed of our supernumerary finger, worn on the right forearm, and the hRing cutaneous interface, worn on the proximal phalanx of the left index finger. As for this experiment we enrolled only healthy subjects, we asked them to mimic the post-stroke hemiparesis of their right hand by keeping the hand closed in a fist. Subjects were then asked to stay in front of a table where 5 different objects were placed. Details on the objects are reported in Table II. The target locations for the objects were marked on the table. Subjects had to pick, lift, and place all the objects being as fast as possible and using only their right (impaired) arm. Subjects could the objects in any order they prefer. To prevent subjects from dragging the objects, we placed an obstacle between the starting and target objects' locations. A video of the experiment is available as supplemental material.

3) Methods: We carried out the pick and place task in three different experimental conditions: no grasping compensation (N), grasping compensation using the supernumerary finger (S), grasping compensation using the supernumerary finger and the hRing haptic interface (SH). Subjects performed the experiment two times per condition, ending up with 6 randomized repetitions of task per subjects.

When no grasping compensation (N) was considered, subjects did not wear the supernumerary finger and the hRing interface. When grasping compensation using the supernumerary finger (S) was considered, subjects were asked to wear the supernumerary finger on their right forearm and the hRing on the proximal phalanx of their left index fingers. The hRing was only used to control the opening/closing motion of the supernumerary finger, as described in Sec. IV, and provided no haptic feedback.

When grasping compensation using the supernumerary finger and the hRing haptic interface (SH) was considered, subjects were asked again to wear the supernumerary finger on their right forearm and the hRing on the proximal phalanx of their left index fingers. This time the hRing was used to both control the opening/closing motion of the supernumerary finger, as described in Sec. IV, and to provide haptic feedback about the forces exerted by the supernumerary finger to the environment and about the status of the grasping procedure, as described in Secs. III and IV.

In all these conditions, subjects were asked to keep their right hand closed in a fist to mimic the hemiparesis of the hand.

4) Results: As a measure of performance, we evaluated (1) the success rate of the pick and place task, (2) the completion time, (3) the force applied by the supernumerary finger to the object being grasped, and (4) the perceived effectiveness of the three conditions.

Table III shows the success rate for the considered task. The task was considered successful if all the objects were correctly placed within a time limit of 3 minutes. No one was able to pick up the two small cubes without the supernumerary finger (condition N). Due to this result, for the next two metrics, we only considered conditions using the supernumerary finger (S and SH).

Fig. 5a shows the completion time. The collected data passed the Shapiro-Wilk normality test. We ran a pairedsamples t-test and the condition S elicited a statistically significant increase in completion time compared to condition SH, t(15) = 2.153, p = 0.048.

Fig. 5b shows the mean load exerted by the supernumerary finger on the environment, registered by the servo motors, considering only instants when the load was more than zero. Fig. 5c shows the same mean load split object by object (objects are listed in Tab. II). We ran a two-way repeatedmeasures ANOVA. Feedback conditions (S vs. SH) and objects were treated as within-subject factors. The data passed the Shapiro-Wilk normality test. Mauchly's Test of sphericity indicated that the assumption of sphericity had not been violated for the objects factor (χ 2 (9) = 10.066, p = 0.349). Sphericity was assumed for the other factor, as it has only two levels. The two-way repeated-measure ANOVA did not reveal a statistically significant two-way interaction between feedback conditions and objects (F 4,60 = 1.914, p = 0.113). The main effect showed a statistically significant in motor load between conditions (F 1,15 = 25.326, p < 0.001) and objects (F 4,60 = 93.587, p < 0.001). According to the pairwise comparison, all objects lead to significantly different loads except dice vs. glass. Moreover, we ran a Pearson product-moment correlation test to study the relationship between the motor load exerted to lift each object and its weight. Regardless of the feedback condition considered, we found a strong positive correlation between these two variables: r(3) = 0.969, p = 0.007 and r(3) = 0.964, p = 0.008 for the conditions S and SH, respectively.

In addition to the quantitative evaluation reported above, we also measured users' experience. At the end of the experiment, subjects were asked to rate, on a slider going from 0 to 10, the effectiveness of each feedback condition in completing the given task. Figure 5d shows the perceived effectiveness for the three feedback conditions. A Friedman test showed a statistically significant difference between the means of the three feedback conditions (χ 2 (2) = 27.763, p < 0.001). Post hoc analysis with Bonferroni adjustments revealed a statistically significant difference between N vs. S (p = 0.008) and N vs. SH (p < 0.001).

B. Experiment #2: ADL tasks with post-stroke patients

Two chronic stroke patients (both males, age 35 and 56) took part to our experimental evaluation. The two subjects were required to wear the robotic finger on their paretic hand and the hRing on the healthy hand index finger (see Fig. 6).

The proposed compensatory tool can be used by patients showing a residual mobility of the arm. For being included in this experimental evaluation, patients had to score ≤ 2 when their motor functions were tested with the National Institute of Health Stroke Scale (NIHSS), item 5, "paretic arm". Moreover, the patients had to show the following characteristics: normal consciousness (NIHSS, item 1a, 1b, 1c = 0), absence of conjugate eyes deviation (NIHSS, item 2 = 0), absence of complete hemianopia (NIHSS, item 3 ≤ 1), absence of ataxia (NIHSS, item 7 = 0), absence of completely sensory loss (NIHSS, item 8 ≤ 1), absence of aphasia (NIHSS, item 9 = 0), absence of profound extinction and inattention (NIHSS, item 11 ≤ 1).

The patients used the soft finger for bimanual tasks typical of Activities of Daily Living (ADL), such as opening a jar of jam, breaking a piece of bread, and opening a can of beans. In all the tasks, the patients used the robotic finger and the paretic limb to hold the object, while the healthy hand was used to perform the manipulation and to interact with the hRing. For example, the patient can hold the bread with the help of supernumerary robotic finger and non-functional arm, while using the healthy hand to break it (see Fig. 6b).

As described in Sec. IV, the hRing interface was used to both control the flexion/extension motion of the robotic finger and to provide the patient with haptic feedback about the forces exerted by the robotic finger on the environment and the status of the grasping procedure.

Both patients found the soft robotic finger useful in the considered ADL tasks. Moreover, they found the hRing intuitive to use and unobtrusive. Finally, they reported haptic feedback to be a valuable information to estimate the quality of the grasp. Both patients would like to be able to use it at home.

VI. DISCUSSION, CONCLUSION, AND FUTURE WORK

This paper introduced the design of a novel soft supernumerary robotic finger integrated with a wearable skin stretch cutaneous interface, called hRing. The supernumerary finger aims at restoring the grasping capabilities of post-stroke patients with a paretic hand. The hRing enables patients to easily control the motion of the robotic finger while providing them with haptic feedback about the status of the grasping action.

To demonstrate the effectiveness of our system, we carried out two experiments, enrolling sixteen healthy subjects and two post-stroke patients in pick-and-place and ADL tasks, respectively. Healthy subjects were asked to mimic a paresis on their right hand by closing it in a fist. Results with the healthy subjects showed that the supernumerary finger can significantly help to improve the grasping capabilities of paretic hands. Indeed, no subject was able to complete the pick-and-place task without using the supernumerary finger. Moreover, results also show that providing cutaneous feedback through the hRing significantly improve the performance of the considered pick-and-place task in terms of force applied on the environment and perceived effectiveness. It is worth to point out that the heavier the object, the larger the improvement of performance when using the haptic feedback. Finally, the two chronic stroke patients found the very useful for ADL tasks, the hRing easy to use, and the haptic feedback very informative. However, it is also important to notice that the benefits of using the proposed system are not always so evident. In fact, in our task, the only objects that all users were not able to grasp were the small cubes.

In the future, we are planning to add an ATI 6-DoF force/torque sensor on the supernumerary finger, in order to be able to accurately measure the forces and torques exerted by the finger on the environment. Moreover, using a 6-DoF sensor will enable us to fully exploit the capabilities of the hRing of providing both normal and skin stretch stimuli at the fingertip. In fact, in this work, since we only had the torque information coming from the load of the servomotor, we provided solely stimuli normal to the finger pad. We are also going to investigate how providing vibrotactile stimuli to other parts of the body (e.g., the wrist, the forearm) affects the performance of the considered tasks. Finally, we will extensively test our device in more challenging real-life scenarios, such as the Frenchay Arm Test.

  (a) hRing haptic interface. (b) Supernumerary robotic finger.

Fig. 2 .

 2 Fig. 2. The hRing and the supernumerary robotic finger.

Fig. 3 .

 3 Fig.3. Device actuation principle. When the motors spin in opposite directions, the belt presses into the user's finger (left), while when the motors spin in the same direction, the belt applies a shear force to the skin (right).

Fig. 4 .

 4 Fig. 4. Experiment #1: pick and place. Subjects were asked to pick, lift, and place all the objects correctly, being as fast as possible and using only their right (impaired) arm. The hRing was used to open/close the supernumerary finger and to receive haptic feedback.

  Completion time (s).

  Motor load per object (Nm).

Fig. 5 .

 5 Fig.5. Experiment #1: pick and place. Overall mean and 95% confidence interval of (a) completion time and (b) motor load; (c) means of motor load split by object; (d) perceived effectiveness of the feedback conditions (the rating given by the subject could be in the range 1 -10, the higher values, the better performance).

  (a) Opening a jar of jam. (b) Breaking a piece of bread.

Fig. 6 .

 6 Fig. 6. One chronic stroke patient performing ADL bi-manual tasks.

TABLE I TECHNICAL

 I DETAILS OF THE SUPERNUMERARY ROBOTIC FINGER

	Dimensions
	Module	20 × 31 × 12 mm 3
	Total length of finger (on arm)	180 mm
	Diameter smallest graspable obj.	14 mm
	Weights
	Total (finger + support base)	180 g
	Actuator
	Max. torque	2.5 Nm @ 12 V
	Pulley radius	8 mm
	Max. current	1.4 A @ 12 V
	Max. operating angles	300 deg, endless turn
	Max. non-loaded velocity	684 deg/sec
	Max. Force at fingertip	40 N
	Max. payload	2.4 kg
	Max. horizontal resistive force	13 N @ dia=65 mm