Quasi-Direct Drive Actuators are built by combining such high torque density Permanent magnet synchronous motor with a low gear ratio planetary reducer. It has high open loop force controller bandwidth, high force transparency, highly backdrivability, and is widely used in robots. The output torque of Quasi-Direct Drive Actuators is generally estimated by motor’s stator current, but there are errors in the estimated actuator output torque due to the friction and inertia of the planetary gears. To solve this problem, an optimization method of torque estimation using stator current was proposed in this paper. By establishing a mathematical model of planetary reducer in the actuator, the reflection inertia of planetary reducer was calculated quantitatively, and the torque constant and offset of actuator were obtained through the repeated experiments. The relationship between the stator current and the output torque can be obtained by compensating the offset and inertia value to the model. The experimental results indicate that the error between the optimized output torque and the actual torque measured by the sensor is small, and the estimation accuracy is high.

Quasi-Direct Drive Actuators; Torque estimation; Planetary reducer model; Reflected inertia