On the Combination of PID control and Reinforcement Learning: A Case Study with Water Tank System

Reinforcement learning (RL) has attracted great interest from researchers in recent years. RL performs as human or better in many fields such as games and robot control. Although this technology is booming in computer science, it has not been practically applied in industrial process control. Up to now, proportional–integral–derivative (PID) control is still the most dominating and popular control method in industrial control. In this paper, we propose a combination of deep reinforcement learning (DRL) and PID control for better process control performance. The idea is generated by the following observations: for PID controller, its transient performance is not usually well enough to meet a strict requirement or in complex signal tracking tasks; For RL technology, a perfectly designed reward function is required for training. However, in practice, the reward function needs to be tested through trial and error, which will lead to a waste of computational power and time. By combining these two strategies, PID controller can help to improve the steady-state performance of RL control by its integral term, while the trained RL agent is able to improve the transient performance of PID controller. Several case studies with the water tank system are presented to demonstrate the effectiveness of the combined PID + RL control strategy.