Ripple compensation of harmonic drive built-in torque sensing

Built-in torque sensing in Harmonic Drives enables torque sensing without assembling additional torque sensors into mechanisms where Harmonic Drives are already present. The sensing principle has been known for about ten years, but it is not widely utilized yet, mainly because of a relatively high ripple signal in the sensing output, generated by the gear operation. Ripple is difficult to be compensated due to inaccuracies of the strain gages positioning and in geometrical properties of the gear and its assembly. Increased number of applied strain gages reduces the ripple, but does not eliminate it. In this paper, the authors present a new method to effectively compensate the ripple, it is based on a periodic characteristic of the ripple signal itself and proposes use of separate amplifiers for each of the signals from the strain gages. Gains of the amplifiers are tuned so that the ripple signal is compensated. A mathematical model of the ripple signal, and a method to calculate the tuned gains is studied. Minimum number of strain gages needed to compensate the ripple signal is derived. The method is successfully confirmed by experiments.