Fingerpad contact evolution under electrovibration

Abstract

Displaying tactile feedback through a touchscreen via electrovibration has many potential applications in mobile devices, consumer electronics, home appliances and automotive industry though our knowledge and understanding of the underlying contact mechanics are very limited. An experimental study was conducted to investigate the contact evolution between the human finger and a touch screen under electrovibration using a robotic set-up and an imaging system. The results show that the effect of electrovibration is only present during full slip but not before slip. Hence, the coefficient of friction increases under electrovibration as expected during full slip, but the apparent contact area is significantly smaller during full slip when compared to that of no electrovibration condition. It is suggested that the main cause of the increase in friction during full slip is due to an increase in the real contact area and the reduction in apparent area is due to stiffening of the finger skin in the tangential direction.

Keywords: contact mechanics; electroadhesion; electrovibration; haptic perception; tactile feedback.

Figure 1.

(a) The evolution of the stick ratio of one subject (S2) as a function of tangential force for a normal force of 2 N. The solid lines represent the average of five trials while the shaded areas around them represent the standard deviations. (b,c) Mean values (average of six subjects) of the initial apparent contact area and minimal tangential force to start a slip as a function of normal force. The error bars represent the standard errors of the means. (Online version in colour.)

Figure 2.

(a) Normalized apparent contact area of each subject as a function of finger displacement for a normal force of 2 N. Subjects are ordered according to their moisture level from S1 having the lowest level of moisture to S6 having the highest level of moisture. The thin lines are the individual trials while the thick lines represent the average of five trials. The vertical dashed lines mark the instant of full slip (SR = 0). (b) Mean values (average of six subjects) of the per cent reduction in the steady-state value of the apparent contact area (A_steady-state) with respect to the initial apparent contact area (A₀) for three different normal forces. The error bars represent the standard errors of the means. (c) Fingerprint images of subject 4 (S4). (Online version in colour.)

Figure 3.

(a) The coefficient of friction (μ) of each subject as a function of finger displacement for a normal force of 2 N (note that the subjects' finger was stationary and the touch screen was moved along a single direction, radial and from right to left). Subjects are ordered according to their moisture level from S1 having the lowest level of moisture to S6 having the highest level of moisture. The thin lines are the individual trials while the thick lines represent the average of five trials. (b) Mean values (average of six subjects) of the steady-state friction coefficient (μ_steady-state) for three different normal forces. (Online version in colour.)

Figure 4.

The tactile friction contrast (TFC) as a function of finger displacement of each subject having a different level of fingertip skin moisture for three different normal forces (a) 0.5 N, (b) 1 N, (c) 2 N. Subjects are ordered according to their moisture level from S1 having the lowest level of moisture to S6 having the highest level of moisture. (Online version in colour.)