Development of a Force Feedback Controller with a Speed Feedforward Compensator for a Cable-driven Actuator

Cable-driven actuators (CDAs) are extensively used in the rehabilitation field because of advantages such as low moment of inertia, fast movement response, and intrinsic flexibility. However, velocity-induced disturbances pose challenges to accurate force control during dynamic movements. Several strategies for force control have been investigated in the literature, but repetitive time-consuming tests are often required. The aim of this study was to develop a force feedback controller and a speed feedforward compensator for a CDA utilising an experiment-based approach. The CDA comprised a motor, a cable drum, and a force sensor. The plant transfer function was estimated through an open-loop test. A PI force feedback controller was developed and evaluated in a static test. Subsequently, a dynamic test with a constant reference force was conducted, during which the cable was pulled to move at different speeds. The relationship between the motor speed and the cable force was determined, which facilitated further development of a speed feedforward compensator. Additionally, the system dynamics were simulated in MATLAB/Simulink. The static test showed that the PI force controller produced a mean force control error of 4.7 N, which was deemed very good force tracking accuracy. The model simulated the dynamic of CDA with the force output very similar to the experiment (RMSE error of 4.0 N). During the dynamic test, the PI force controller alone produced a force control error of 9.0 N. The additional speed feedforward compensator further reduced this error to 5.6 N. The combined force feedback controller and the speed feedforward compensator produced a satisfactory degree of accuracy during dynamic tests of the CDA at variable speeds. The experiment-based design of the force control strategy for the CDA shows potential to be a control approach for general CDAs, which establishes the foundation for precise movement control as required in cable-driven rehabilitation robotic systems. Future work will be integration of the speed compensator into better feedback algorithms for more accurate force control.