Saturation Compensation for Nonlinear Unstable Plants Based on Coprime Factorization

Plants often have constraints on actuator inputs due to physical limitations and protection mechanisms. Input saturation can lead to plant instability and increase the risk of accidents. Particularly in unstable plants, when the input saturates, the output can exceed the controllable range, potentially making it impossible to recover control performance. This presents a significant issue. In this paper, we propose a saturation compensator for nonlinear and unstable plants that compensates before the output exceeds the controllable range. This compensator extends a disturbance observer, functioning as the disturbance observer under normal conditions, and as the saturation compensator when the output approaches uncontrollable regions. Additionally, the compensator is designed based on coprime factorization, ensuring stability. This approach allows for improved control performance while maintaining system stability. The effectiveness of the proposed compensator was verified through simulations of a simple pendulum. The simulation results confirm that the compensator functions as the disturbance observer under normal conditions and compensates for input saturation when the output approaches uncontrollable regions. The effectiveness of the compensator was also confirmed by comparing cases with and without the compensator. Future research will focus on implementing the compensator for experiments with actual equipment.