LIMS Projects - Fingertip Haptics

Fingertip Haptics

The Relationship Between Tactile Sensations and Hand Motion

Fingertip haptics is a new branch of research in the field of haptics. Researchers at Northwestern University are studying tactile sensations at the fingertip and how they relate to larger hand and arm haptic sensations. We think that adding fingertip sensations to existing haptic feedback devices promises to increase the overall realism of haptic interfaces. Our main goal is to enable direct fingertip exploration of virtual objects - something that isn't currently possible with today's haptic feedback devices.

Exploration of an object using a probe or stylus takes advantage of distal attribution - the ability for humans to easily extend their perception to the end of an implement. This approach, however, diminishes or eliminates access to basic surface features such as temperature, contour, and even certain aspects of texture

Physical sensations we address in fingertip haptics include (but are not limited to) the relative slip between the finger and surface, the orientation of the surface, the temperature (and/or thermal conductivity), adhesive qualities, and vibration. These sensations share a common attribute: they all utilize a combination of all the afferent channels of the skin at the fingertip. We feel the most important thing to understand about the mechanics of the skin is that the four afferent channels act together in delivering sensations to the nervous system. The main sensation we concentrate on is the relative motion experienced by the fingertip in contact with the surface. This sensation requires some relationship between the tactile sensation and the finger movement.

Our initial work on fingertip haptics establishes the validity of pursuing relative motion at the fingertip, and lays the groundwork for subsequent tests. The preliminary tests reveal that our novel approach to rendering relative motion, rotating a drum beneath the finger, effectively recreates the sensation of moving one's finger along a surface. The work also reveals the importance of device dynamics and psychophysical influence on the perception. For more detail on the preliminary testing, click here.

This is a photo of a subject during preliminary testing. We are working towards defining the importance of relative motion between the finger and a surface.

Current testing makes use of a new apparatus. The new apparatus allows independent control of slip direction and speed, and at the same time allows control of large hand and arm movements. A servomotor controls the rotation of a plastic (Lexan) wheel with spherical curvature. Another servomotor actuates a cam assembly to raise and lower the plastic wheel which allows for pulling the surface away from the user between trials. The lift assembly and the plastic wheel rest on a turntable, also servomotor actuated. Finally, the entire apparatus rotates about the center of the table at an arbitrary radius.

The new apparatus enables independent control of slip direction and speed, and can raise and lower the "surface" to the user's fingertip. (The servomotor that actuates the turntable is not shown.)

The new apparatus enables independent control of slip direction and speed, and can raise and lower the "surface" to the user's fingertip.

A subject rests his or her hand on top of the apparatus with his or her finger beneath the finger guide. The resulting sensation is equivalent to a subject tracing a circle on a surface with his or her index finger. Both the tactile sensation of slip and the kinesthetic sensation of arm movement are apparent. Psychophysical experiments conducted during the Fall of 2002 and the Spring of 2003 establish the just noticeable difference for slip speed and direction, and qualify the relationship between the tactile sensation of slip and the kinesthetic proprioceptive sensation of arm movement. Over 60 subjects from the Northwestern Psychology 101 subject pool have participated to date.

As we suspected, the nature of the surface itself has a profound influence on the higher order cognitive perceptions such as surface velocity. Therefore, we designed the test apparatus to accommodate plastic wheels of differing surface characteristics. Our testing employs two fundamentally different surface types: one surface has a homogeneous texture, and the another has small features around the periphery. The two surfaces invoke different mechanoreceptors in the fingertip, and allow us to qualify which receptors are used during exploration.

Testing involves two different surface types: the first surface (on the left) has a sandblasted finish, and the other surface has a polished finish with small features, or "dots," around the middle.

Here are some preliminary results on slip direction discrimination. The plot on the left is for the homogeneous texture, while the plot on the right is for the "dotted" wheel. We performed tests on small angles about the two major axes of the finger, along (blue) and across (red), to identify a directional bias (if any) in perceiving surface velocity.