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Deformable surfaces
Previous research focused on modifying the geometry of a screen to create dierent shapes or even physical buttons [76],[33],[73]. The deformable surface is made of a exible material, such as rubber, which can be transparent. Dierent actuation methods have been experimented. In the case of Tactus [76], a deformable layer is superposed on a touchscreen, with small areas able to pop up to mimic buttons on the key locations, as shown in Figure 2.5. Fluid is pumped to in ate and de ate those areas on demand. By controlling the amount of uid pumped, the buttons stiness can be dynamically adjusted. Users can feel the buttons and press them.These interfaces are rather similar to pin arrays, as they are composed of a matrix of actuators. However, they can oer a continuous out-of-plane tactile information rather than the discrete tactile information provided by the pins. The integration in current touchscreen devices is potentially compatible on the long-term. However, this concept has some limitations. The buttons are not individually controlled. The tactile in atable layer is not recongurable due to the predened button pattern.
Other prototypes used pneumatic actuation, as in [33]. [73] added the ability of individually controlling the cells arranged in an hexagonal layout (2:54 cm distance between cells). The approach is more recongurable than the previous concept, at the expense of more actuation complexity.
Vibrotactile interfaces
As mentioned in subsection 2.2.1, Pacinian corpuscles’ sensitivity to vibrations lie between 50 and 500 Hz with peak sensitivity at around 250 Hz [75]. Most handheld devices use this frequency range to silently inform users of calls, to notify and enhance visual and audio feedback on tactile keyboards. The vibration is transmitted to the whole device, thus generating a \global feedback ». As opposed to this approach, the use of specic technologies such as time reversal allows to localise vibrations on a specic part of the interactive surface. This opened the possibility of multi-point haptic feedback and will be referred as \localised vibrations ». Global vibrations The common way to generate haptic feedback is relying on the use of cost-eective, compact rotating motors with an eccentric mass. It is an easy way to produce vibrations inside a device. Because of the direct link between the rotation frequency and the input voltage, the quality of the generated feedback by such motors is rather poor and limited. In addition, the reaction time is low (tens of milliseconds), which
generates a delay during the interaction.
Several technologies, such as electromagnetic actuators, voice-coils, and piezoelectric [67] actuators increase the controllability of the perceived vibrations. This greatly improves the key-click feedback, making it more realistic. Moreover, it is
also possible to simulate simple textures with those actuators.
The study of specic patterns in order to design and evaluate the most identi- able key-click signals for mobile devices is proposed in [11]. Dierent frequencies, amplitudes and repetition cycle are tested to identify the best haptically perceived signals.
Other experiments try to replicate force-displacement behaviour of real buttons with a haptic illusion based only on vibrations. A virtual button-type haptic feedback device built with a low-cost electromagnetic actuator and a pressure sensor is presented in [55], (see Figure 2.13a). The proposed method aims to provide haptic feedback, not only for key-clicks but also for the movement of the key before and after transition points in a force-displacement curve, with specic vibration patterns, as shown in Figure 2.13b. A user study concluded that six dierent virtual buttons were discriminated at a rate of 94.1%, and that the association of four virtual buttons with their physical counterparts was successful at a rate of 79.2%, showing that the resultant haptic feedback is realistic and distinctive.(a) Virtual button feedback device. (b) Force-displacement behaviour illusion.
By laterally vibrating the contact surface with the ngertip at a frequency within vibrotactile frequency range (50 500 Hz), it is possible to simulate a texture. The set-up in [83] is able to measure the force of interaction while a nger explores a texture and replay it (see Figure 2.14). The samples were placed on the tray A. The nger D position was measured by a linear variable dierential transformer sensor attached to the ngernail. The interaction forces induce exural deformations of the leaf springs H along x and in the piezoelectric bender B (500 Hz bandwidth). The position of the nger, the net force, and the tangential force are measured by the transducer (see Figure 2.14a), giving a texture spectrogram, as shown in Figure 2.15 (spectrogram of a triangular grating of a 1 mm spatial period). The transducer is guided by a linear bearing E which is located by an encoder F. The ngertip rests on the tray A and the transducer tracked the position of the nger D resting in a cradle G, thus relieving the ngertip from lateral loads. As the slider moves with the nger, the transducer stimulated the ngertip, as shown in Figure 2.14b. The device was used to record and reproduce 5 dierent texture samples. 3 out of 5 real samples were presented to users, who had to identify and match them either with real comparison textures or with simulated ones. The success rate for simulated textures was high (75 %) but not as high as for comparison to real textures (93 %).
Electrocutaneous stimulation
The previous devices focused on mechanical stimulations of the nger. Another alternative is to directly stimulate the nerve endings in the ngertip with an electric current. This process is called electrocutaneous stimulation. While [74] initially proposed an electrovibration device, the same electrode array was also tested without the insulating cover material. A current of a few mA is supplied to elicit a sensation on the nger. Compared to previous approaches, electrocutaneous stimulation has the advantage of being able to produce tactile feedback for both static and dynamic
ngers. Fabrication of electrode arrays with a transparent conductive material such as ITO allows to use this approach on touchscreens.On the other hand the diculties of building an ecient electrocutaneous display are pointed out in [50]: Abrupt motion of the nger on the electrode array can cause an electric shock sensation. Furthermore, sweat alters the contact conditions and therefore the sensation’s intensity (see Figure 2.33a). In addition, the two thresholds for tactile perception and pain are close to each other.Due to spatial variation of the perceived sensation, it is impossible to globally tune the intensity of the sensation without locally attaining the pain threshold (see Figure 2.33b). A real-time feedback of the contact impedance (1:45 s feedback loop is achieved) to be able to adjust the sensation at each electrode individually is proposed in [50].
Table of contents :
1 Introduction
1.1 Scope
1.2 Thesis overview
2 State of the art
2.1 Introduction
2.2 The sense of touch
2.2.1 Mechanoreceptors
2.2.2 Shape versus texture
2.3 Quasi-static interfaces
2.3.1 Pin-array interfaces
2.3.2 Deformable surfaces
2.4 Dynamic interfaces
2.4.1 Lateral motion
2.4.2 Vibrotactile interfaces
2.4.3 Variable Friction
2.4.4 Electrocutaneous stimulation
2.5 Hybrid interfaces
2.6 Discussion and motivation of this work
3 Time Reversal : theory and modelling
3.1 Introduction
3.2 Theory of Time Reversal
3.2.1 Principle
3.2.2 The fundamentals of time reversal
3.2.3 Signal processing
3.3 Engineering Trade-os
3.3.1 Contrast
3.3.2 Amplitude
3.3.3 Repetition
3.3.4 Energy Balance
3.3.5 Spatial Resolution
3.4 Spatial resolution model
3.4.1 Analytical model
3.4.2 Material properties estimation
3.4.3 Experimental validation
3.4.4 Discussion
3.5 Design guidelines
3.5.1 Plate’s material Y , ,
3.5.2 Plate’s area S and thickness
3.5.3 Bandwidth B
3.5.4 Characteristic time Tc
3.5.5 Attenuation constant
3.5.6 Reversal time T
3.5.7 Repetition time Tr
3.5.8 Time constant recommendation
3.5.9 Contrast C
3.5.10 Number of transducers Q, and material properties
3.6 Conclusion
4 Time Reversal Performance
4.1 Introduction
4.2 The tactile display prototype
4.2.1 The piezoelectric transducers
4.2.2 Driving electronics and amplication stage
4.2.3 Driving signals
4.3 Measurements : amplitude and spatial resolution
4.3.1 Attenuation constant and reversal time T
4.3.2 Displacement amplitude A
4.3.3 Spatial Resolution Rs
4.3.4 Discussion
4.4 Energy consumption
4.4.1 Experimental measurements
4.4.2 Analytical estimation
4.5 Noise emission
4.5.1 Noise emission in dBA
4.5.2 Wide bandwidth sound level measurements
4.6 Variability and integrability
4.6.1 Transducers’ location and generated amplitude
4.6.2 Focus point’s location and spatial resolution
4.7 Conclusion
5 Time Reversal and perception
5.1 Introduction
5.2 Time reversal and sensitivity to applied force
5.2.1 Experimental set-up
5.3 Detection threshold
5.3.1 Parameters
5.3.2 Experimental protocol
5.3.3 Participants
5.3.4 Results
5.4 Impact modulation and pattern perception
5.4.1 Impact amplitude modulation
5.4.2 Amplitude-modulated patterns
5.4.3 Experimental protocol and participants
5.4.4 Results
5.5 Summary of the results
5.5.1 Applied force eect on the displacement amplitude
5.5.2 Detection threshold
5.5.3 Perception
6 Electrovibration: theory and perception
6.1 Introduction
6.2 Fundamentals of electrovibration
6.2.1 Electrostatic force generation
6.2.2 Electrovibration model
6.2.3 Input signals
6.3 Electrovibration and perception
6.3.1 Experimental set-up
6.3.2 Exploration and force levels
6.3.3 Perception thresholds
6.4 Conclusion
7 General conclusion and outlook
7.1 Future work
A Pattern study detailed results
B.1 Experimental set-up
B.2 Result
