Design and control of a haptic knob

Sensors and Actuators A 196 (2013) 78–85

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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

Design and control of a haptic knob Frédéric Giraud a,b,c,∗ , Michel Amberg a,b,c , Betty Lemaire-Semail a,b,c a

University Lille 1, Villeneuve d’Ascq F59000, France Laboratory of Electrical Engineering and Power Electronic, Villeneuve d’Ascq F59000, France c INRIA Lille Nord Europe, France b

a r t i c l e

i n f o

Article history: Received 18 September 2012 Received in revised form 11 March 2013 Accepted 13 March 2013 Available online 27 March 2013 Keywords: Piezoelectricity Force sensor Tactile plate Haptic Control

a b s t r a c t Introducing haptic into tactile input interfaces allow users to really feel their action on the device they control by this way. In this paper, we achieve tactile stimulation by using the squeeze film effect. It is applied on a haptic knob, as those which control an MP3 player for example. We present our design procedure of the active surface and of the position sensor based on force measurement which achieves a resolution of 3.6◦ . We also show that stimuli are damped by fingertip, and a specific control loop with a response time of 2.5 ms has been achieved to tackle this problem. Finally, a psychophysical experiment was conducted showing how the haptic feedback can increase user’s accuracy in a pointing task. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Our everyday life is rich of electronic devices which are controlled by the human finger. Touch interaction is now a standard user interface, and easily replaces a physical button or a knob. In many MP3 players for example, a knob is replaced by a wheel on which users move their thumb. This solution is easy to manufacture since it requires no moving part. However, users no longer receive any affordance cues, like the ‘click’ of a button for example, which reduces their performances [3]. To cope with this problem, the solution consisting in producing a haptic feedback proved to be useful in this context [4]. The haptic feedback can be achieved by several ways. Vibrotactile stimulation [6] can be produced by using rotating motors with an eccentric mass [7], or a linear actuator [10]. However, with these technics, the actuator takes time to start and stop, and it is difficult to calibrate the stimulation. Moreover, they produce a vibration of the whole object, so the hand holding the device perceives a stimulation as well as the touching finger. A piezoelectric bender attached to the case of the touched device is an other way to enhance interaction with vibrotactile stimulation [16,15]. Dynamic stimulation can be produced, but is still limited by the low resonating frequency of the actuator.

∗ Corresponding author at: University Lille 1, Villeneuve d’Ascq F59000, France. Tel.: +33 362531631. E-mail address: [email protected] (F. Giraud). 0924-4247/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2013.03.012

Friction reduction based tactile devices are a second way to achieve the tactile feedback. With this operating principle, we use an active surface in order to control the friction with user’s finger. By modulating it according to the fingertip’s displacement, it is possible to produce the illusion of touching various surfaces, like smooth or rough surfaces [18] or gratings [1]. In the paper, we used squeeze film air bearing to produce the friction reduction [17]. It consists in producing a high frequency vibration (above 25 kHz) at low vibration amplitude (∼1 ␮m), on a plate. The air trapped between the fingertip and the vibrating plate is subject to non-linear expansion and contraction. The squeezed film makes the friction decreasing as the vibration amplitude increases. In this paper, we present a haptic knob which is able to produce programmable haptic effects. There exist several designs in the literature of such device. For example, [12] uses a DC motor attached to a dial. In [5] a same principle is applied, but with a more sophisticated actuator. For both examples, users turn the dial and the response torque of the device can be controlled according to the knob’s position which is measured. These solutions require several moving parts, while the user’s hand is fixed on the knob. Our approach is different, and consists in designing a wheel on which user’s thumb slides. Friction reduction obtained from the squeeze film effect will produce a haptic feedback in order to reproduce the illusion of manipulating a rotating knob. The paper is organized as follows. In the following section, we present the design requirements. Then, the procedure to design and control such a knob is detailed, to finally present a user study in the last section.

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Fig. 3. Vibration modes of a ring.

We can see that all trajectories are inside a 40 mm circle. However, this concerns only one participant, and we also found that a 36 mm circle embraces the majority of the trajectories. Finally, taking into account the width of the contact area, a 38 mm diameter ring is found to be acceptable in our case. Fig. 1. CAD view of the tactile knob.

2.2. Vibration analysis 2. Design of the haptic knob The device has been designed in order to fit into user’s palm, and to be controlled by the thumb; a CAD view of the prototype is presented in Fig. 1. The haptic knob is made of a ring shape active area which vibrates in order to produce the squeeze film air bearing. It is built up with a copper ring attached by its center to the plastic case of the knob. The ring itself is not allowed to rotate. The size of the ring, the internal and outside radii, are determined to allow a free sliding motion of the thumb tip on its surface. When vibrating, the ring can produce friction reduction. A position sensor inside the knob can measure the angular position of the fingertip on the ring. This measurement can be used to modulate the vibration amplitude, and thus the tactile stimulus. Ring size of the haptic knob should fulfill two conditions. First, it should be large enough to allow a free movement of the thumb tip on the active area. Second, vibration of the ring should allow friction reduction by squeeze film air bearing. 2.1. Ring size In order to define the minimal size of the active area of the haptic knob, we asked 8 people to move their thumb around a 20 mm in diameter disk. Chalk on the finger was used to mark their trajectory. We then draw the medium line for each participant, as depicted in Fig. 2.

Fig. 2. Trajectories for each participants.

The method used to make the ring vibrate consists in using a flexural deformation of the substrate, as shown in [2]. However, vibration theory of circular plates shows that several bending modes can be produced: • radial vibration presents nodal line along radii of the ring as in Fig. 3a, • orthoradial vibration where nodal lines are concentric circles as in Fig. 3b, • a combination of both. In this paper, we used a radial vibration mode, because it produces good vibration amplitude at the periphery of the ring, where the thumb slides on the haptic ring. On the contrary, the orthoradial modes have a nodal circle close to thumb’s trajectory, and thus cannot produce squeeze film efficiently where it is needed. To produce the vibration, we used a piezoelectric hollow ring from Noliac. The material used is PCM41, because it has high piezoelectric constant: vibrations are obtained with less voltage, any other parameters kept constant [2]. Moreover, the thickness of the ring should be as small as possible in order to reduce the required amount of power needed for the vibration [9]. We used a 0.5 mm plate, since thinner plates would be too fragile. Simulations are then carried out. At this step, we focus our attention on the resonant frequency of each mode. We classify the results into the wanted radial modes, and the unwanted other modes. We then draw Fig. 4, on which quality of each mode (wanted or unwanted) is depicted according to their frequency. From the wanted modes, we choose those which are far enough from their unwanted neighbours. This is easily checked in Fig. 4, by choosing the largest centered single mode. Consequently, the vibration mode chosen for this application is 38.6 kHz.

Fig. 4. The frequency of the wanted and unwanted modes.

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Fig. 5. Centering the two rings during the manufacturing process.

2.3. Manufacturing When manufacturing the haptic knob, we must pay attention to center each part on the axis of the ring. Misalignment could result in problems to energize the wanted modes, or make unwanted modes closer to the selected one. For that purpose, we machined the ring into a copper-beryllium plate, and we added a centering hole of 3 mm. This hole is placed at the center of the plate, where no vibration occurs, and thus has no effect on the vibration. It will be useful later, when mounting the ring on the case. To center the piezo-ring on the copper disk, we manufactured a centering piece, also with a 3 mm hole at its center. This piece perfectly fits into the inner diameter of the piezo ring, and is thus used to center it on the mounting frame. Then, each part of the tactile ring is fixed and precisely positioned on one half of the mounting frame, as presented in Fig. 5. We then deposited a small amount of glue on the copper disk, removed the centering piece, and then put one half of the mounting frame onto the other. The two frames are perfectly aligned thanks to a small cylinder of 3 mm diameter placed through the mounting frame. By this way, we carried out a perfect centering of the piezoring on the copper disk. 2.4. Experimental evaluation Normally, the piezoelectric ring should have been divided into 14 positively and negatively poled region, one for each antinode of vibration, and we should apply a same voltage on them. However, the ring we were supplied is uniformly poled, we have then to separate the electrodes into two groups. For one group a voltage va = Vsin(ωt) is applied with ω = 2f, while a voltage vb = −Vsin(ωt) is applied on the other group. Fig. 6 presents the vibrating disk and its electrodes etched on the piezoelectric ring. Moreover, an electrode is used for feedback: the voltage measured is proportional to the vibration amplitude and serves for control. The output of this sensor is a voltage proportional to the deflection, and we measured a ratio of 34 V/␮m in our case. We then measure the impedance of the tactile disc on a large frequency span in order to identify the vibration mode. We identified our wanted mode at 39,920 Hz which is more than 1300 Hz above the calculated resonance frequency; this may be due to the precision of the thickness of the copper plate or the piezo ring. However, the measurement shows that our wanted mode is far enough of its unwanted neighbours, while Fig. 7, which presents a laser interferometer measurement of the vibration, shows that the vibration obtained is consistent with the results of the simulation.

Fig. 6. The piezoelectric ring fixed onto the copper disk, with the etched electrodes.

3. Design of the position sensor 3.1. Principle The position of the fingertip is used to modulate the vibration amplitude, and thus create the tactile pattern. Position should be detected with both accuracy and speed. In previous work [1], we found that it was possible to simulate fine gratings with spatial period as small as 2 mm with 25% of spatial resolution. Thus, the required resolution of the sensor can be estimated to be 0.5 mm. On the 36 mm mean circle of the touched area, this leads to a resolution of 1.6◦ . For this purpose, there exist several technological solutions, each have their own advantages and drawbacks: • Capacitive [11] sensors have good resolution but are not compatible with metal substrate, • Resistive sensors [14] would be easy to use in our application, but require a large pressure, • Optical solutions [13] have good resolution and time response, but are bulky. This is why, we achieve a position sensor from force measurement. The principle derived from [8], and we give in Fig. 8 a representation of the sensor and its mounting. The ring is fixed to the case of the knob through four rigid fixtures. We measure the force at each fixture thanks to four force sensors (FSS1500 from Honeywell), and we note Fi with i{1, 2, 3, 4} the output of each force sensor. When the finger moves on the ring, the equilibrium

Fig. 7. Experimental measurement of the disk’s vibration.

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Estimated Ref

1.2

m

F (N)

1 0.8 0.6 0.4 0.2 0

time (arbitrary unit) Fig. 10. Measured and reference force FN .

Fig. 8. Principle of the position sensor; 1 is the tactile ring, 2 are the rigid fixtures, F1,2,3,4 are the forces measured by the four sensors.

of the ring make Fi changing according to  – the position of the fingertip on the ring – and FN – the contact force of the fingertip. In Fig. 9, we show the haptic ring and the forces applied on it. In this figure, we represent the circle with radius R on which the user’s fingertip pushes the haptic ring, and the circle with radius Rs on which the four force sensors are located. The static equilibrium of the ring leads us to write that along axis z: F1 + F2 + F3 + F4 = FN

(1)

Moreover, the static equilibrium of the momentum of the force FN about axis x leads us to write: FN × Rcos() = Rs (F1 − F3 )

(2)

Finally, the equilibrium about axis y leads to: FN × Rsin() = Rs (F2 − F4 )

(3)

In Fig. 10, we compare the estimated value and the reference one of the force applied to the sensor. Both are consistent, and show that we can actually measure the force pressure with our sensor. We then compare ˜ to the actual position where the force is applied. Results are shown in Fig. 11, on which the actual position of the tip is compared to its estimation. Results are presented for several force pressure, from 0.2 N to 1.2 N. As it can be seen, there exists an error between the actual value ˜ This may be due to the nonlinearity of the  and the estimation . sensors, or different sensitivities. But from our experimental measurements, we have seen small differences between one sensor and another. It can also be due to a difference in the mechanical mounting. Parasitic forces applied on the pre-load mechanism may induce errors in measurements. However, the angular position of the fingertip is quite well estimated and we can calculate the standard deviation as follows:





 =

j − ˜ j

2 (5)

N

where i{1 . . . N} is the number of tries. With a force of 1 N, this gives   = 3.6◦ . Consequently, gratings not smaller than 4 mm should be simulated.

From Eqs. (2) and (3), we can now write the estimated position of ˜ from the force measured at each the fingertip on the ring, named , sensor:

4. Control

˜ = tan

A specific electronic board has been designed for controlling the haptic knob, and is described in Fig. 12, with an actual view

F − F  2 4 −1 F1 − F3

(4)

4.1. Description of electronic control circuit

3.2. Experimental runs

20

In order to measure the position sensor’s fidelity, we used a fine pin to apply a calibrated pressure FN at precisely known positions Mj (xj , yj ), j1 . . . N, with N the number of measurements. We then recorded the output of each sensor, and deduced the measured pressure force F˜Nj and the estimated contact positions M j . Actually, we calculate the average over 100 measurements for each position.

Y(mm)

10

0

−10

−20 −20

Fig. 9. Top view of the tactile ring and the forces applied on it.

−10

0 X(mm)

10

20

Fig. 11. Comparison between actual position (on the circle) and the estimated ones.

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W(µ m)

0,5

0,25

0 0

1

2

1

2

3

4

5

6

7

3

4

5

6

7

FN(N)

3 2 1 0 0

time (sec) Fig. 14. Force FN applied to the device and resulting vibration amplitude W as a function of time at V = 10 V and f = 39, 920 Hz.

the average of the measurements is calculated and sent to the PC. It also listens to the serial port, and waits for a command, i.e. the requested amplitude of vibration for example, or the working frequency. In fact, the eigenfrequency of the tactile ring may change due to different operating conditions, like temperature or aging of the bonding layer. 4.2. Control of the haptic knob

Fig. 12. Diagram of the electronic control circuit.

in Fig. 13. It is composed of an electronic DC/AC power converter which energizes the electrodes of the piezoelectric device with two 15 V/40 kHz voltages approximately. A DSP Piccolo F28027 is used to produce the pulse signals to the power converter thanks to its Pulse Width Modulation module. Moreover, an FTDI chip allows communication with a conventional PC through an emulated RS232 serial bus, via the USB port. Outputs of the force sensors are directly converted into a numeric signal by using the Analog to Digital module. We also convert the signal from the vibration sensor for control purpose as described in the next section. The piccolo achieves a Te = 5 ms loop. During Te , the 4 force signals and the feedback voltage are converted every Ts = 75 ␮s, and

The vibration amplitude W of the tactile ring is a function of the supply voltage amplitude V and frequency. Moreover, the fingertip has an influence on the vibration. In Fig. 14, we have measured the vibration amplitude as a function of FN at constant voltage V and frequency ω. Curves show that vibration can be reduced to 50% of its nominal value for FN > 1.5 N. Consequently, sensation can be dramatically changed from one user to the other, if they do not apply the same force on the device. This is why we achieved a control of W(t) in order to be robust against force variation. For that purpose, we first have to model vibration’s behavior. In this part, we will consider that the ring is excited at its eigenfrequency f0 , and we introduce ω0 = 2f0 . Frequency is constant, but the supplied voltage amplitude may be varying: hence, we note va (t) = V (t)sin(ω0 t − ϕ), where ϕ is the phase of v relatively to w. In the same way, the deflection of the ring is written as w(t) = W (t)sin(ω0 t) where W(t) is the vibration amplitude measured by the sensor: w(t) is chosen as a phase reference. Let introduce the complex notation: va = V ejω0 t with j2 = −1, and w = Wejω0 t ; we verify that w(t) = Im(w) while ϕ = arg(V ). In the vicinity of the eigenfrequency, the vibration amplitude follows a second order type behavior, and we can write: ¨ + ds w˙ + cw = N v mw

(6)

where m, c, ds and N are parameters depending on geometry, materials and vibration mode of the tactile ring. Moreover, by definition of the eigenfrequency we have: ω02 = c/m

(7)

We can now calculate time derivatives of w:

Fig. 13. The haptic knob and its control electronic circuits.

˙ )ejω0 t w˙ = (jω0 W + W

(8)

˙ − ω2 W )ejω0 t ¨ + 2jω0 W ¨ = (W w 0

(9)

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W(µm)

0.5

0.25

0 0

1

2

3

4

5

6

7

1

2

3

4

5

6

7

F (N) N

3 2 1 0 0

time (sec)

Fig. 18. Vibration amplitude W and force FN applied to the device as a function of time when the vibration is controlled.

Fig. 15. Step response of the vibration; comparison with simulation.

Fig. 16. Vibration amplitude control loop.

˙ , For sake of simplification, we assume that in our case ω0 W  W ˙ W ¨ . Eqs. (8) and (9) are then revised into (10) and (11): and ω0 W w˙  jω0 Wejω0 t

(10)

˙ − ω2 W )ejω0 t ¨  (2jω0 W w 0

(11)

Fig. 19. The haptic knob in use.

(12)

than expected; this may be due to saturation of the DC/AC converter, which is unable to produce the required amount of voltage in order to achieve a time response of 2.5 ms. However, we found that there is no overshoot nor static error, and this result is then acceptable in our case. We then applied normal pressure with a finger in order to test the robustness of the control against external force variation. As it can be seen in Fig. 18, the vibration amplitude is well controlled except when FN > 1.5 N: in this condition, the DC/AC controller cannot supply the required amount of voltage to compensate the damping of FN on the vibration. However, this level corresponds to a high pressure and is not a natural value for FN . Consequently, the control of the vibration amplitude independently of the normal force applied to the tactile ring is thus possible. Hence, we now have a knob the rendering of which can be programmed, which does not depend on user’s pressure force. It can be held into user’s palm, as depicted in Fig. 19. The next section deals with the programming of the haptic feedback in an example of a Human to Computer Interface.

By introducing (10), (11) and (7) into (6), we obtain Eq. (12): ˙ + ds ω0 W ) = NV j(2mω0 W

Consequently, at resonance, the transfer function between the voltage amplitude V and the vibration amplitude W is a first order type equation, and we write using the Laplace transform: G W (s) = 1 + s V (s)

(13)

with G = N/ds ω0 and  = 2m/ds . The parameters of Eq. (13) are deduced from the response of the vibration to a voltage step. In Fig. 15, we show the response when V is changed from 0 V to 22 V, and we compare the measured vibration deflection w(t) to the output of Eq. (13). From the measurements, we identified G = 0.0168 ␮m/V and  = 1.5 ms. We then programmed a simple numerical PI controller in order to achieve a control of the vibration. The control loop is described in Fig. 16; in our experimental study, we had a sampling period Ts = 300 ␮s, and we setup the controller in order to achieve a 2.5 ms time response with no overshoot. In Fig. 17 we present the experimental result of this control, when FN = 0. As it can be seen, the time response is a little longer w(t) and W(t) (µm)

5. Haptic rendering 0.5

0

−0.5

w(t) (measure) W (measure) W ref

0

2.5

5

time (msec) Fig. 17. Transitory operation of the vibration with the control loop.

It’s for purpose of illustration that the following test was conducted. It consists of a selection task, with or without haptic feedback. Without haptic feedback, user’s thumb slides on the wheel until a requested target is selected. In addition to this, and with haptic feedback, users feel the virtual sectors of the wheel as programmed on the interface. By this way, we are able to introduce more physicality in the touch interaction [13], and make users have an experience closer to their interaction with a real knob.

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Fig. 21. Pattern of friction reduction for TH, and example of variation in time of the friction.

Fig. 20. The list as displayed to the user, in the case of a trial where D = −3W.

5.1. Presentation of the experiments The experimental trials consist in selecting a target into a list of items, which were song titles in our example. To select an item, the user rolls the thumb on the knob to move back and forth the list under a selection pointer. At any time, the user can see 11 elements of the list, which contain 20 titles, the pointer being on the 6th element, and represented by a red rectangle, as depicted in Fig. 20. The target size is fixed, and was set as W = 1/16 of a complete rotation about the axis of the knob. This means that a complete turn of the thumb on the knob allows to select 16 different items in the list; Hence, each item is represented by a sector on the knob. For each trial, a new target is highlighted in the list with a red font, and we measure the selection time. Distance to the target is equal to { − 5W, − 4W, − 3W, − 2W, − 1W, 1W, 2W, 3W, 4W, 5W}, and is named D. A positive distance means an upward direction, a negative one means a downward direction. The distance to the target are presented randomly. During the test, two haptic rendering strategies were proposed. For the first condition NH, no haptic feedback is programmed on the device. For the condition TH, friction is decreased on each item of the list, with a pattern centred on each sector, with half a sector length. Fig. 21 provides an illustration of the haptic patterns. A total of 20 blocks (2 haptic strategies × 10 distances to the target) of 8 trials were completed by each participant; the first 2 trials were discarded for adaptation to the conditions, and participants had a pause of 8 seconds after 4 trials to avoid fatigue. The experience was proposed to four participants. For each trial and each participant, we measure the response time and also the number of overshoot. An overshoot occurs when the user go over the target. When interviewing participants after completion of the experiments, they all reported that the haptic knob was more comfortable to use, more precise, and they felt to be faster. Fig. 22, presenting the mean response time and number of overshoots for all the participants and as a function of the haptic strategy (NH or TH), depicts an other conclusion. In fact, we show that haptic feedback does not help to reduce the user’s response time. On the contrary, it can significantly increase it as found for subjects S1 and S2, who may have particularly focused their attention on the haptic feedback.

Fig. 22. Mean response time and number of overshoot during the exercise for all participants and for each haptic condition.

However, the number of overshoots are also significantly reduced for every subjects, showing how haptic feedback can really help to improve accuracy of this pointing task. These conclusions are different of participant’s experience, but are finally consistent with other studies within the field of haptic feedback [4,13]. This experiment shows an advantage of using the haptic knob instead of a classical wheel in this selection task. Other experiments could be conducted in order to confirm the conclusions, using more participants for example, and also by proposing other haptic patterns, like textures programmed in order to highlight each sector. 6. Conclusion We presented the design and the control of a haptic knob. It uses the squeeze film effect to produce friction reduction on a fixed wheel. Size selection and manufacturing process are presented step by step. It uses a position sensor, based on force measurement. The architecture allows a resolution of 3.6◦ as well as the measurement of the force exerted by the user. Moreover, since the user’s fingertip damps vibration, a control has been achieved in order to maintain the tactile feedback at any force. For that purpose, a modelling and an identification of the tactile device has been carried out. The device finally fits into user’s palm, and was used to select song titles more accurately in a psychophysical experiment.

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Participants reported that using the haptic knob was more comfortable, and measurements show that they more accurately selected targets with it. Despite the high voltage required by the piezoelectric ring, it has been possible to drive the haptic knob from the +5 V of a USB port, by using a boost-up converter with low required power (less that 0.25 W). These figures are promising if we look forward introducing such a technology into handheld devices like MP3 players for example. However, further work should be achieved in order to decrease the energy source’s voltage to comply with the level of common battery (from 1.5 V to 3.3 V typically), and to study the impact of the haptic on battery’s life. Acknowledgements This work has been carried out within the IRCICA Stimtac Project, and the INRIA Mint Project. The authors thank M. Messaoudi, A. Caroulle and J. Chlebicki for their work in the design of the haptic knob. References [1] M. Biet, G. Casiez, F. Giraud, B. Lemaire-Semail, Discrimination of virtual square gratings by dynamic touch on friction based tactile displays., in: Symposium on Haptic Interfaces for Virtual Environment and Teleoperator Systems, 2008. Haptics 2008, 2008 march, pp. 41–48. [2] M. Biet, F. Giraud, B. Lemaire-Semail, Implementation of tactile feedback by modifying the perceived friction, The European Physical Journal Applied Physics 43 (2008 May) 123–135. [3] S. Brewster, F. Chohan, L. Brown, Tactile feedback for mobile interactions, in: Proceedings of the SIGCHI Conference on Human Factors in Computing Systems. CHI ‘07., ACM, New York, NY, USA, 2007, pp. 159–162. [4] G. Casiez, N. Roussel, R. Vanbelleghem, F. Giraud, Surfpad: riding towards targets on a squeeze film effect, in: Proceedings of the 2011 Annual Conference on Human Factors in Computing Systems. CHI ‘11., ACM, New York, NY, USA, 2011, pp. 2491–2500. [5] D. Chapuis, X. Michel, R. Gassert, C.-M. Chew, E. Burdet, H. Bleuler, A haptic knob with a hybrid ultrasonic motor and powder clutch actuator., in: EuroHaptics Conference, 2007 and Symposium on Haptic Interfaces for Virtual Environment and Teleoperator Systems. World Haptics 2007. Second Joint, March, 2007, pp. 200–205. [6] J.V. Erp, Guidelines for the use of vibro-tactile displays in human computer interactions., in: EuroHaptics 2002, Edimburgh, 2002. [7] G. Ghiani, B. Leporini, F. Paternò, Vibrotactile feedback to aid blind users of mobile guides, Journal of Visual Languages and Computing 20 (5) (2009) 305–317. [8] F. Giraud, M. Amberg, B. Lemaire-Semail, G. casiez, Design of a transparent tactile stimulator., in: Haptics Symposium (HAPTICS), 2012 IEEE, 2012 march, pp. 485–489.

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Biographies Frédéric Giraud (BS’95 Paris-XI University, MS’97 Institut National Polytechnique de Toulouse) graduated from the Ecole Normale Supérieure de Cachan, France in 1996 in electrical engineering, and received his PhD from the University Lille1, in 2002. He is a member of the electrical engineering and power electronics laboratory of Lille where he works as an Associate Professeur. His research deals with the modelling and the control of piezo-electric actuators. Michel Amberg has been teaching electronics at the University of Lille, France. He graduated from Ecole Normale Supérieure de Cachan, France in 1981. He has tutored more than a hundred Bachelor Students during their projects in the field of telecommunications, computer science and electronics. He is now research engineer at IRCICA, and works on the electronic design of tactile devices. Betty Lemaire-Semail (PhD’90, University of Paris XI, Orsay). From 1990 to 1998, she was assistant professor in the Ecole Centrale of Lille and she is now professor at the University Lille1. She is a member of the electrical engineering and power electronics laboratory of Lille and head of the research axis on the control of electrical systems. She has studied electromagnetic motors and her main field of interest now deals with the modelling and control of piezoelectric actuators, for positioning and force feedback applications.