Fuzzy self-tuning PI control of pH in fermentation

Abstract

Controlling the pH value of small-scale laboratory fermentation processes with a conventional PI controller is difficult. This is due to the highly non-linear response of the pH to the addition of acid or base, the time-varying behaviour of the microorganism, various reactor volumes, and the buffer capacity of the system. It is also very difficult for a biochemist without a control engineering background to adjust the PI parameters of the controller when a fermentation process is changed. This article describes the development and implementation of a fuzzy self-tuning PI controller for a low-cost laboratory pH control system with digital peristaltic pumps. The controller was designed to have a wide range of applications in batch, fed-batch and continuous processes in a laboratory reactor of a working volume ranging from 2 to 15 l. The essential idea is to tune the controller gains on-line by means of a parameter that results from a fuzzy inference mechanism. The tuning of the controller gains is based on the response of the pH system. The controller was first tested in simulations and then experimentally on 2- and 7-l reactors. This experimental validation confirmed that the pH controller is capable of controlling the pH value of a fermentation process within the required range of ±0.05 pH units. The self-tuning capability makes the controller robust with respect to the varying buffer capacity and the working volume of the fermenter.