Effect of Waveform on Tactile Perception by Electrovibration Displayed on Touch Screens

Abstract

In this study, we investigated the effect of input voltage waveform on our haptic perception of electrovibration on touch screens. Through psychophysical experiments performed with eight subjects, we first measured the detection thresholds of electrovibration stimuli generated by sinusoidal and square voltages at various fundamental frequencies. We observed that the subjects were more sensitive to stimuli generated by square wave voltage than sinusoidal one for frequencies lower than 60 Hz. Using Matlab simulations, we showed that the sensation difference of waveforms in low fundamental frequencies occurred due to the frequency-dependent electrical properties of human skin and human tactile sensitivity. To validate our simulations, we conducted a second experiment with another group of eight subjects. We first actuated the touch screen at the threshold voltages estimated in the first experiment and then measured the contact force and acceleration acting on the index fingers of the subjects moving on the screen with a constant speed. We analyzed the collected data in the frequency domain using the human vibrotactile sensitivity curve. The results suggested that Pacinian channel was the primary psychophysical channel in the detection of the electrovibration stimuli caused by all the square-wave inputs tested in this study. We also observed that the measured force and acceleration data were affected by finger speed in a complex manner suggesting that it may also affect our haptic perception accordingly.

Development of a Fully Flexible Sheet-Type Tactile Display Based on Electrovibration Stimulus

“…However, the subjects were not able to perceive the sinusoidal wave at 5 Hz. Varder et al reported that the simulated input voltage applied to the skin decreased significantly under low frequency sinusoidal waveform conditions and that the resulting electrostatic force also decreased significantly [38]. It is believed that the decrease in electrostatic force under low frequency condition resulted in a low rate of perception and this rate also decreased with increase in the frequency.…”

“…Thus, the subjects answered with these words, which indicated a smooth surface. On the other hand, the response for the tactile sensation of pulse voltage and triangle waveform was either “bumpy” or “rough.” In the case of pulse voltage, the amplitude of the calculated electrostatic force was higher than that of the sinusoidal voltage and was edgy in nature [38]. This trend resulted in the above answers for the tactile sensation.…”

“…This attracts the finger toward the electrode and displays tactile sensation or surface textures to the users, as shown in Figure 2c. The electrostatic force can be expressed as follows [38]. $F=\frac{A\epsilon {\epsilon }_0}{2}{\left(\frac{V^{\prime }(t)}{d}\right)}^2$ where F is the electrostatic force that acts as the attractive force, ε 0 is the vacuum permeability, ε is the relative permeability of the stratum corneum, A is the contacting area of the fingerpad, V’ ( t ) is the applied voltage across the stratum corneum, and d is the thickness of the stratum corneum.…”

Electroadhesion for soft adhesive pads and robotics: theory and numerical results

“…The electrical attraction between a charged surface and a human finger was discovered by Johnsen and Rahbek. In 1953 Mallinckrodt et al [3] applied an alternating voltage to insulated metal electrodes and observed an alternating electrostatic force that periodically attract and release the finger from the surface; this is now denoted electrovibration [4][5][6][7], and forms the basis for electroadhesion based haptic devices such as touchscreens and tactile displays. For these applications, tactile sensations are produced by the application of a voltage to the conductive layer of an insulated haptic device such as a touchscreen, inducing electroadhesive forces between the device and the approached user finger.…”

“…For these applications, tactile sensations are produced by the application of a voltage to the conductive layer of an insulated haptic device such as a touchscreen, inducing electroadhesive forces between the device and the approached user finger. If the applied electric voltage is modulated in time the friction force acting on the finger will generate sensorial experiences [4][5][6][7].…”