  Touchscreens equipped with friction modulation can provide rich tactile
feedback to their users. To date, there are no standard metrics to properly
quantify the benefit brought by haptic feedback.The definition of such metrics
is not straightforward since friction modulation technologies can be achieved
by either ultrasonic waves or with electroadhesion. In addition, the output
depends strongly on the user, both because of the mechanical behavior of the
fingertip and personal tactile somatosensory capabilities. This paper proposes
a method to evaluate and compare the performance of haptic tablets on an
objective scale. The method first defines multiple metrics using physical
measurements of friction and latency. The comparison is completed with metrics
derived from information theory and based on pointing tasks performed by users.
We evaluated the comparison method with two haptic devices, one based on
ultrasonic friction modulation and the other based on electroadhesion. This
work paves the way toward the definitions of standard specifications for haptic
tablets, to establish benchmarks and guidelines for improving surface haptic
devices.
