Optical rotating torque sensor with nano newton-meter resolution based on a hanging torsion wire

This article presents a novel torque measuring setup working in the nano newton-meter range. The central novelty is a long (1 meter range) thin (diameter of a few tens of micrometers) metallic wire used instead of the relatively stiff torsion zone of the shaft of classical rotating torque sensors. This thin wire is made straight by placing it vertically and suspending a mass at its lower extremity. The very low torsional stiffness of the wire is determined by measuring the frequency of the torsional pendulum constituted by the wire and its hanging mass. The twist-angle of the wire is measured optically. The setup can be used to measure torques in static and rotating modes. The performance of this setup has been demonstrated by measuring the output torque of a modified mechanical watch gear train constituted of two 10:1 speed multiplying stages in series, driven by a commercial torque meter. With outputs torques ranging from 40 to 160 n∙Nm, the measurements showed a linear input to output torque ratio of 108 for a total gear ratio of 100:1, corresponding to a load-dependent efficiency of 92.6%. The measured constant loss is 7.63 nN∙m.