Sensor-less Back-driving Control of Direct-drivable RotLin Linear Series Elastic Actuator

In this paper, a novel direct-drivable RotLin (Rotary to Linear) linear serious elastic actuator is studied. The RotLin actuator converts rotary motion into linear motion through helical shaped permanent magnets on this rotor and translator. Because of removing mechanical transmissions of traditional electrical actuators, force density of the RotLin machine is high. In previous research, a load force sensor based back-driving system was implemented on the RotLin machine. In this research, a force-sensor-less back-drive control system is achieved. Firstly, magnetic force of the RotLin machine is analyzed based on linearized spring force model; then, mover side friction is measured and modeled; finally, a load force estimator is designed based on the spring force model and the friction model. Besides, the cogging torque is measured and compensated. In the end, a force-sensor-less force control system is designed based on rotor position control, and by setting the reference force to zero, a back-driving control was implemented on the prototype of the RotLin actuator. The experiment results verified accuracy of the designed load force estimator and back-drivability of the proposed system.