Ripple compensation for torque sensors built into harmonic drives

Harmonic drives include a mechanically flexible component which transmits load torque, and therefore, a torque sensor can be built into harmonic drives by cementing strain gages on a flexible component of the gear. However, a periodic sensing error called ripple is generated by deformation of a flexible component during the gear operation. The ripple signal cannot be sufficiently compensated by pairs of strain gages that produce opposite phase signals, because the strain gages cannot be exactly positioned on the desired locations. In this paper, we present a method to compensate the ripple by a new approach of tuning the ripple amplitudes for separate strain gages. The ripple, caused by positioning errors of the strain gages, can be perfectly compensated, and, therefore, the requirement for accurate strain gage positioning is reduced. The method does not need any online calculations, and consequently, the torque signal is not delayed. The minimum number of strain gages needed to compensate a given number of frequency components of a ripple is derived. Some experimental results are shown.