A Dual-Loop Dual-Frequency Torque Control Method for Flexible Robotic Joint

The impedance control of most flexible joints generally has a cascaded structure with both inner torque feedback and outer position feedback loops. The torque control accuracy and bandwidth have great effects on their impedance control performance. But in practical applications, the improvement of the torque control performance are limited by many factors, such as the high noise level of torque sensor and the model uncertainty and nonlinearity. This paper proposes a dual-loop torque control structure with a dual-frequency control in the inner loop and a disturbance observer in the outer loop. The dual-frequency control uses a frequency-separated controller design method that employs two controllers in low and high frequency, respectively. The high frequency controller with relatively conservative gains is designed to ensure the system stability. Thus, the low frequency controller can focus on improving the torque control accuracy, without experiencing the limitations of system stability and torque sensor noise. Moreover, the disturbance observer is introduced to compensate the model uncertainty and nonlinearity. Simulations are conducted to verify the effectiveness of the proposed dual-loop dual-frequency torque controller.