An Analog Input Shaper for Stability Enhancement of Haptic Interfaces and Its Application to Energy-Bounding Algorithm

An analog input shaper is proposed for a haptic control system to improve stability when interacting with virtual environments. High frequency inputs to a haptic device, which usually occur in collision with virtual objects with high stiffness, can induce limit cycle oscillations and instabilities. In order to reduce these high frequency inputs to haptic devices, an analog input shaper is added to the control system. In order to minimize the delay, which is also one of major causes of instabilities, the analog input shaper is implemented with a high-pass filter instead of a low-pass filter. Since the input shaper has its own dynamics, when a haptic pointer leaves a virtual wall, a user may feel slow decrease of impedance. Moreover there may be negative impedance as if the wall is pulling. In order to solve these problems, we add linear half-wave rectifiers which allow fast decrease of impedance and no negative input to a haptic device. The rectifiers can also eliminate the undesirable energy. The input shaper reduces the total energy supplied to a haptic device by preventing high frequency inputs from flowing into a haptic device and by using rectifiers, therefore, it can be regarded as an artificial damping element. Energy-bounding algorithm, which is an energy-based method, can guarantee stable haptic interaction by limiting the input energy. So, it inevitably sacrifices transparency in haptic interaction. The analog input shaper is incorporated into energy-bounding algorithm to increase the impedance range that the algorithm can stably display. A digital filtering scheme, which uses the same structure as in an analog input shaper, is also tested. Through experiments, we show that the analog input shaper can enhance the stability and thus increase the impedance range which can be stably displayed by a haptic device.