Design Of Fuzzy Logic, Sliding Mode, Dynamic Sliding Mode And PID Controllers For pH Control In A Continuous Stirred Tank ReactorABSTRACT

In recent years the industrial application of advanced control techniques for the process industries has become more demanding, mainly due to the increasing complexity of the processes themselves as well as to enhanced requirements in terms of product quality and environmental factors. Therefore the process industries require more reliable, accurate, robust, efficient and flexible control systems for the operation of process plant. This project presents PID, Fuzzy logic, sliding mode and dynamic sliding mode control for pH in a stirred tank reactor. Process modeling approach adopted in this paper is based on the Physico-chemical principles and fundamental laws. A conventional mathematical modeling process is incorporated.

This paper intends to control the pH of a by adding acid and/or base (alkali) into a process tank in a closed tank reactor to maintain the pH to a desired value. PID, Fuzzy logic, Sliding mode and Dynamic Sliding mode controllers are applied to the process and a comparison as to the best possible controller is arrived at. This paper uses LabVIEW for simulation of the system. LabVIEW provides a graphical presentation that is easily understood and hence required inferences can be obtained effortlessly. The interfacing is performed using National Instruments Compaq Data Acquisition System (NI-CDAQ).

PID controller provides control output by calculating the error between the actual value and the desired value. The error is integrated, differentiated with respect to time and multiplied with a constant gain to obtain the required controller output value. The output is then used to regulate the control valve position. Fuzzy logic controller is based on a mathematical system that analyzes analog input values in terms of logical variables. Fuzzy logic has the advantage that the solution to the problem is cast in terms that human operators can understand. Sliding mode controller is a nonlinear control method that alters the dynamics of a system by application of a discontinuous control signal that forces the system to slide along a cross-section of the systems normal behavior. Dynamic sliding mode controller provides control action by differentiating the output provided by sliding mode controller with respect to time. The intelligence of the fuzzy logic controller increased the decision making efficiency of the split range plant. The sliding mode controller increased the robustness of the system under study and the changes in real time were incorporated satisfactorily. The dynamic sliding mode controller reduced the chattering effects of the sliding mode control, thereby increasing system stability.

The dynamic sliding mode controller was observed to provide the best performance as it integrated stability and robustness. The set point was reached at in the fastest time and the oscillations were found to be reduced.