Wave haptics: Encoderless virtual stiffnesses

Abstract

Haptic rendering commonly implements virtual springs using DC motors with current amplifiers and encoder-based position feedback. In these schemes, quantization, discretization, and delays all impose performance limits. Meanwhile the amplifiers try to cancel the electrical motor dynamics, which are actually beneficial to the haptic display. We present an alternate approach that fully embraces and utilizes all electrical dynamics, following two insights: First, the electrical inductance L can serve as a stiffness, providing a natural sensor-less coupling between the virtual environment and the user. Second, the electrical resistance R can serve as part of a wave transformation. Implementing virtual objects in a wave domain provides complete robustness to servo delays or discretization. The resulting system requires only a simple voltage drive circuit. Built upon the physical behaviors, if can outperform traditional approaches achieving higher virtual stiffness. Encoder feedback is only required for absolute position information, with damping and velocity information inherently available from back-EMF effects. A prototype system has been implemented and confirms the promise of this novel paradigm.