High Performance Haptic Displays and Increasing Z-width

Students:
David Weir

Haptic Display Professors:
Michael Peshkin
Ed Colgate
Kevin Lynch

The goal of this project is to investigate the use of analog electrical damping to increase the Z-width of a haptic display.

The dynamic range of impedances that can be stably rendered by a haptic display is termed its Z-width and can be considered a measure of quality. Colgate and Brown (1994) showed that the maximum attainable Z-width for a given sampling rate depends on the physical damping present in the mechanism: greater physical damping allows a greater Z-width. Simulated damping (i.e. virtual damping added by a digital controller) cannot substitute for physical damping in the mechanism.

Physical damping can be added through a variety of means. Physical damping may be provided by a mechanical damper on a motor, with typical practical problems such as fluid leakage and Coulomb friction in seals. In this project, we explore the alternative of providing physical damping on the electrical side of the motor.

We designed a linear current amplifier with added analog circuitry to add damping. By adding damping to the motor, we can provide actual physical damping that can be used to expand the Z-width of the haptic display. Prior knowledge of the parameters and dynamics of the motor allow us to design this analog circuitry in the current amplifier to estimate the back EMF of the motor. The back EMF generated by the motor windings is proportional to velocity. This velocity signal is fed back to add damping to the system.

The level of analog damping can be dynamically modulated by the digital controller. Therefore more damping can be added in situations when instability is likely and less damping added during stable interactions with the virtual environment.

To test the electrical damping, we designed and built a high fidelity, low mass, linear haptic display to render small 1-DOF motions, shown in the picture above. It has a peak force output of 8.5 Newtons, continuous force output of 1.3 Newtons, range of motion of approximately 15 millimeters, maximum linear sensing resolution of 0.5 microns, and a -3dB bandwidth of approximately 200 Hz. By having low apparent mass of approximately 5 grams, we can realistically render linear switches, which themselves have a moving mass of only a few grams. This device utilizes a low inertia rotary motor, but over only a limited range of motion, allowing it to be driven without commutation.



Selected Publications:

"Measuring and Increasing Z-Width with Active Electrical Damping", Weir, D.W.; Colgate, J.E.; Peshkin, M.A.; IEEE Proc. Symposium on Haptic Interfaces for Virtual Environments and Teleoperator Systems 2008. 13-14 March 2008. pp169-175. (PDF, Abstract)
Finalist (1 of 2) for Best Paper: Haptic Technology -- IEEE Haptics Symposium 2008.

"Chapter 7: Stability of Haptic Displays" in Haptic Rendering: Foundations, Algorithms and Applications Edited by Ming Lin and Miguel Otaduy, A.K. Peters, May 2008, pp. 123-156, ISBN: 978-1568813325

"Design and Performance of a High Fidelity, Low Mass, Linear Haptic Display", Weir, D.W.; Peshkin, M.A.; Colgate, J.E.; Buttolo, P.; IEEE Proc. First Joint Eurohaptics Conference and Symposium on Haptic Interfaces for Virtual Environment and Teleoperator Systems, 2005 (WHC 2005). 18-20 March 2005. pp177-182. (PDF, Abstract)

David Weir's Masters Research Page...



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Last updated: DW 03/17/2008