Fuzzy control is a practical alternative for a variety of challenging control applications since it provides a convenient method for constructing non-linear controllers via the use of heuristic information. Since heuristic information may come from an operator who has acted as "a human in the loop" controller for a process. In the fuzzy control design methodology, a set of rules on how to control the process is written down and then it is incorporated into a fuzzy controller that emulates the decision making process of the human.
In other cases, the heuristic information may come from a control engineer who has performed extensive mathematical modelling, analysis and development of control algorithms for a particular process. The rest of the process is the same as the earlier case. The ultimate objective of using fuzzy control is to provide a user-friendly formalism for representing and implementing the ideas we have about how to achieve high performance control. Apart from being a heavily used technology these days, fuzzy logic control is simple, effective and efficient. In this paper, the structure, working and design of a fuzzy controller is discussed in detail through an in-depth analysis of the development and functioning of a fuzzy logic pH controller.
To illustrate the application of fuzzy logic, the remaining section of the paper is directed towards the design and working of a pH control system using fuzzy logic.
PH is an important variable in the field of production especially in chemical plants, sugar industries, etc. PH of a solution is defined as the negative of the logarithm of the hydrogen ion concentration, to the base 10. I.e., PH= -log 10 [H+]
Let us consider the stages of operation of a sugar industry, where PH control is required. The main area of concern is the clarification of raw juice of sugarcane. The raw juice will be having a PH of 5.1 to 5.5. The clarified juice should ideally be neutral. I.e., the set point should be a PH of 7. The process involves addition of lime and SO2 gas for clarifying the raw juice. The addition of these two are called liming and sulphitation respectively. Since the process involves continuous addition of lime and SO2 ; lime has a property of increasing the PH of the clarified juice. This is the principle used for PH control in sugar industries. The PH of the raw juice is measured and this value is compared to the set point and this is further used for changing the diameter of the lime flow pipe as per the requirement.
The whole process can be summarised as follows. The PH sensor measures the PH. This reading is amplified and recorded. The output of the amplifier is also fed to the PH indicator and interface. The output of this block is fed to the fuzzy controller. The output of fuzzy controller is given to the stepper motor drive. This inturn adjusts the diameter of lime flow pipe as per the requirement. Thus, the input to the fuzzy controller is the PH reading of the raw juice.
The output of the fuzzy controller is the diameter of the lime flow pipe valve or a quantity that controls the diameter of the lime flow pipe valve like a DC current, voltage, etc. The output obtained from the fuzzy controller is used to drive a stepper motor which inturn controls the diameter of the value opening of the lime flow pipe. This output tends to maintain the pH value of sugar juice to a target value. A detailed description of the design and functioning of the fuzzy controller is given in the following section.