Cascade Control System Design and Auto Tuning Using Relay Feedback Technique

Abstract

One of the basic functions of instrumentation engineers is tuning of controllers. In the industries there are no of process loops (SISO and MIMO) necessitate tuning. The auto tuned proportional –integral –derivative controllers are designed for application where load change are expected and need control action fast and accurately. Cascade controllers are designed for fast and accurate response. Cascade system/controller provides good control over single loop controller system. Relay feedback technique is used for tuning of PID controller and this method can be extended to auto tune of cascade controller system. The conventional on –off relay oscillation for a single loop feedback controller is extended to the relay tuning of cascade controllers. In this paper a well established cascade PID controller design using arm controller and relay feedback techniques is presented for cascade controller tuning. In past year the primary and secondary loop both are tuned manually. First secondary tuned and then primary. There are many methods for tune cascade controller like fuzzy logic based self tuning method , and Bi and CAI et al method for inner loop. By using relay feedback method the tuning of the secondary loop to be done without necessarily placing the primary controller in manual mode. The method presented in paper is used for the tuning of cascade controller to obtain its parameter with a minimum computing complexity. Controller parameters KP, KI and KD are calculated using classical Ziegler-Nichols tuning rules or redefined tuning rules. The measurement of relative speed of system is done using limit cycle oscillation.