Unwarping Encoder Ripple in Low Cost Haptic Interfaces

This paper describes a technique for unwarping encoder ripple in a low cost haptic interface prototype. We previously demonstrated how using a low-resolution optical encoder to electrically commutate a stepper motor can produce high torque levels with low ripple. The optical encoder scheme produces a pair of quadrature voltages that are approximately sinusoidal functions of rotor position. However, because of variations in the optical return from the low-precision encoder disk, quadrature phasing and signal amplitude vary over a mechanical turn. In this paper, we discuss an effective self-calibration technique for unwarping the encoder ripple. The technique is used in a one degree-of-freedom haptic interface prototype, and experiment results quantifying the improved performance due to encoder unwarping are presented.