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DC motor damping: A Strategy to increase Passive Stiffness of Haptic Devices

Physically dissipative damping can increase the range of passive stiffness that can be rendered by a haptic device. Unlike simulated damping it does not introduce noise into the haptic control system. The impedance based control system is the most common control system to be employed for haptic devices. However impedance based devices tend to become to non passive as the virtual wall stiffness increases. This is due to spatio-temporal quantization of the state variable of haptic control system which leads to an imbalance in the energy flow to and from the haptic device. The build of of excess energy in the haptic devices causes it to become non passive. If we can find a way to dissipate this energy in the haptic device, we can achieve a wider passive operating range for the haptic interaction.

Our method adds dissipative damping to the haptic control system. Damping is simulated in haptic control system by computing (within the host computer) the damping force. This damping is usually known as ‘computer damping’. While this method simulates the damping force, it does not contribute to the stability of the haptic rendering. An enhancement in the stability due to damping can only occur if the damping mechanism is capable of dissipating energy. Since simulated damping is not capable of this it does not contribute to the stability of the haptic rendering. In fact due to the noise content in the velocity estimation computed damping often contributes negatively to the stability of the haptic device.

DC motor damping or ‘electrical damping’ achieved by shunting the terminals or dissipating the back EMF of the motor, is a simple and efficient means to create physically dissipative damping. This is in addition to the mechanical damping due to friction and inertia of the moving parts which is present in the haptic device. In our experiments it was seen that this electrical damping contributed positively to increase the range of virtual wall stiffness that could be rendered at a particular sampling frequency.

We employ a configuration of the H-bridge which can cause this damping to impart stability to our haptic device. This results in an increase in passive wall stiffness of about 33.3% at a sampling rate of 100Hz and 16.6% at 1kHz over the performance of an un-damped DC motor. We have also attempted to implement the system on the hybrid haptic control system, it was seen that a perceivable change in the performance of this system was not observed by the use of DC motor damping.

Publications

  1. B.S.Manohar, H.Vasudevan, M.Manivannan, DC Motor Damping: Strategies to improve stiffness in virtual environments, Accepted in Eurohaptics 2008.


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Last Updated: May 26, 2008 4:45 PM Comments: Manivannan M