Ultra-Compact Joint Torque Sensor Units with Enhanced Resolution for Collaborative Robots

Measuring accurate force/torque is a key factor for cooperative robot to realize human-robot collaboration task. However, integrating torque sensors into robot joints are still a challenge due to limitations including size, cost, and mismatched communication protocol. Moreover, the crosstalk errors from undetermined external forces and the sinusoidal effect from the harmonic drive make it difficult to measure torque accurately. This paper proposed ultra-compact joint torque sensor units, addressing the contradiction of the cooperative robot to realize high resolution force sensing while ensuring proper stiffness. To achieve these goals, a novel spoked structure with double holes is proposed to increase strains and improve stress concentration. Meanwhile, an optimization model is formulated to enhance the resolution with desired stiffness. In addition, the torque ripple elimination method is introduced, which can properly handle the crosstalk suffer from the harmonic drive and thus accurately measure the interest torque. Finally, three sizes of joint torque sensor with different loads were implemented with the feature of modularization, light weight, low cost and easy to manufacture. Dynamic calibration was applied to a proposed joint torque sensor and the performance was also experimentally evaluated. Experimental results showed that the prototype can accurately measure the torque of interest in the range of 170Nm while eliminating crosstalk, and the torque resolution of the small, medium, and large joint torque sensor is 0.02Nm, 0.05Nm, and 0.1Nm respectively under the premise of ensuring the stiffness. These results and economical costs can promote the application of joint torque sensors in collaborative robots.