Wave Haptics: Building Stiff Controllers from the Natural Motor Dynamics

Haptics, like the fields of robotics and motion control, relies on high stiffness position control of electric motors. Traditionally DC motors are driven by current amplifiers and encoder-based position feedback creates virtual springs. Unfortunately, sensor quantization, discretization, and amplifier bandwidths impose performance limits, while the amplifiers work hard to cancel the motor’s electrical dynamics. We present an alternate approach noting the natural inductorresistor dynamics of the motor are indeed beneficial to the haptic task. We follow two main insights: First, the electrical inductance L can serve as a stiffness, providing a natural sensorless coupling between the virtual environment and the user. This physical effect is available at all frequencies and can be exploited in the bandwidth of human perception through an analog circuit that replaces the traditional current amplifier. Second, the analog circuit uses the electrical resistance R to create a natural wave transform. Controlling the system in wave variables creates robustness to servo delays and discretization. The resulting system requires only a simple voltage drive circuit. Built upon the motor’s physical behavior, it can outperform traditional approaches, by achieving higher virtual stiffness in the range of frequencies where human users are most sensitive. Encoder feedback and a slow digital control loop may be added to assure stable low frequency behavior and provide robustness to motor parametric uncertainty. In contrast to traditional designs, this auxiliary feedback path places reduced requirements on resolution and servo rates. A prototype 1-DOF system has been implemented and confirms the promise of this novel paradigm.