Tactile Shape Displays for Small Scale Shape Feedback
Parris Wellman, Bill Peine, R. D. Howe
Support provided by the Whitaker Foundation
The tactile display in our prototype system consists of a line of 10
individually actuated pins that are raised against the fingerpad.
Shown below is a drawing of our design. A line configuration
was chosen since the palpation instrument will be scanned across the tissue
allowing motion to provide the other dimension. This also simplifies
the design. SMA wires are used to drive the pins. As electric
current heats the wire, it goes through a phase transformation and shortens,
thus pushing the pin up. With this design, each pin can move 3 mm
and produce over 1 N of force. A primary problem with SMA is the
slow response times. We overcame this by using water cooling and
position feedback for each pin from optical sensors. Figure
5 shows the response of the pin as a function of desired position frequency.
The output displacement drops by 30% (-3 db point) at 40 Hz. This
satisfies the design specification set by the finger speed experiments.
Shape Display Design. The upper figure shows the top view
of the display and a user’s finger. The lower figure shows the side
view of the display. Electrical current heats the SMA wires, which
undergo a phase transformation and shorten. This forces the pin up
against the fingerpad. The pin's position is measured optically.
Frequency Response of the Tactile Display. Maximum output
response of pin motion divided by desired maximum motion of the pin as
a function of temporal frequency.
Peine, W.J., Wellman, P.S., and Howe, R.D., Temporal
Bandwidth Requirements for Tactile Shape Displays, Proceedings of the IMECE Haptics Symposium, Dallas, TX, November 1997.
Wellman, P.S., Peine, W.J., and Howe, R.D., “Mechanical
Design and Control of a High-Bandwidth Shape Memory Alloy Tactile Display,”
proceedings of the International Symposium of Experimental Robotics,
Barcelona, Spain, June 1997.
Also see Simulating Soft Tissue with a Tactile Shape Display
Harvard BioRobotics Laboratory Home