Composite LADRC and deadbeat predictive current control for a limited-angle torque motor position servo system

Abstract

Limited-angle torque motors (LATMs) feature a compact mechanical configuration, rapid dynamic behaviour, and direct torque delivery, making them well-suited for precision servo valves, optical scanning systems and fuel-control systems. In practical LATM position servo systems, however, cogging torque, load disturbance, friction torque and parameter uncertainty can degrade tracking performance. Conventional cascaded position proportional-integral (PI) and current-PI control is therefore often unable to provide both fast dynamic response and strong disturbance rejection. This paper proposes a composite control strategy that combines a linear active disturbance rejection controller (LADRC) in the outer position loop with a deadbeat predictive current controller in the inner current loop. The LADRC treats internal unmodelled dynamics and external disturbances as a total disturbance and estimates it online using a linear extended state observer, while the predictive current controller computes the control voltage from the discrete electrical model of the motor. Simulation results show that the proposed method shortens the step rise time by 48.9%, reduces the 1 Hz sinusoidal tracking root mean square error (RMSE) by 54.2%, and decreases the maximum position drop under a sudden load disturbance from 7.16 degrees (deg) to 1.28 deg compared with conventional PI+PI control. Additional four-controller ablation simulations further indicate that the high-dynamic LADRC outer loop benefits from a sufficiently fast current loop; replacing the deadbeat current loop with a well-tuned PI current loop gives a close but slightly weaker response, especially under sudden load disturbance. These results show the potential of the proposed composite strategy for high-precision LATM position-servo applications requiring both rapid response and disturbance immunity.