Tribometry

Surface haptic displays can modulate the friction of a touchscreen, via electrostatic attraction forces or ultrasonic vibration of the surface itself.

To aid the design of such displays, we must first understand the spatial resolution necessary to render distinguishable sensations on the fingertips. This requires careful characterization of finger skin deformation, when subjected to surfaces with different friction levels, as well as the transitions between them.

As a prerequisite, we often need to characterize the surfaces in the first place, to quantitatively assert the difference in friction levels. To do this, we developed a custom tribometer in our lab.

Two 6-axis force/torque sensors (ATI Nano43) are sandwiched between two custom-made aluminum plates. This assembly is mounted on top of a lateral stage (Thorlabs DDS300). Finally, the samples are mounted at the top, using a 3D printed casing. 

Friction levels can be determined by taking the ratio of measured lateral and normal forces, as the sample slides underneath the fingertip. Here, the test conditions are carefully matched to the experimental conditions.

Check out our paper on the effects of distributed friction during sliding touch, featuring friction characterization using the above tribometer. We found that the finger skin warped when friction was distributed across the sliding direction, but not when aligned with it.