Application of Fuzzy Control Technique to Thyristor DC Drive Krzysztof Zawirski Institute of Industrial Electrical Engineering Poznañ University of Technology Poznañ, Poland Konrad Urbanski Institute of Industrial Electrical Engineering Poznan University of Technology Poznan, Poland Abstract. In the paper a study of fuzzy logic controller (FLC) application for solution of some control problems of thyristor DC drive is presented. The control problems, which are taken into account during FLC synthesis, consist in variable structure of the current control plant and variation of moment of inertia in the speed control loop. Comparison between fuzzy control system and traditional digital cascade control, carried out by simulation method, proved that FLC as a robust control gives better performance in the range where non-linearity and parameter variation is observed. I. INTRODUCTION Fuzzy logic controllers (FLC) based on fuzzy logic principle and expert knowledge, converted into automatic control systems, still increase their industrial application. In recent years this new technique has been successfully implemented in control system of power electronic converters and converter-fed drives [1,2,3]. Most of these systems are strongly non-linear with variable parameters and structure, what makes them a difficult plant for traditional control technique. Application of fuzzy logic controllers in these conditions provides an efficient tool to design robust control systems [4,5]. In the paper a study of FLC application for solution of some control problems of thyristor DC drive is presented. Improvement of DC drives performance is still an important task because of their application in many industrial branches. Most of the control problems of these drives were solved on the base of traditional control technique. Interesting research tasks is to investigate how the new fuzzy technique can deal with some difficult control problems of DC drives, consisting in non-linear characteristics and parameter variation. Implementation of FLC system to a solution of problems solved yet in traditional way, should provide to better control performance or similar performance but achieved on the more simple way of realisation [4,5,6]. In the presented work a comparison of two control systems for DC drive was provided. One investigated system is a digital control system with connected in cascade controllers of current and speed. Second one is a FLC system designed with similar general structure, decomposed into speed and current controllers connected in cascade, but realised in fuzzy technique. This decomposition gives a chance to apply in synthesis of FLC some expert knowledge, collected during analysis and synthesis of traditional control system. General structure of this FLC system is presented in fig.1. The control problems, which should be taken into account during FLC synthesis, consist in variable structure of the plant in current control loop, being a result of continuous and discontinuous converter current, and variation of moment of inertia in speed control loop. Fig.1 Structure of FLC system for DC drive II. CURRENT CONTROL LOOP A. Control problems in current loop It is well known that as a result of two possible mode of converter operation, with continuous and discontinuous current, the current control plant has variable structure. This is a reason that in many application adaptive current controller - with variable structure PI/I is applied [7]. Proposed in some papers more sophisticated control algorithms require time consuming complex calculations and are more sensitive on error of system parameters identification [7,8]. In the paper such a structure of current FLC was assumed, which can be comparative with traditional adaptive PI/I current controller. Among many known solutions of digital current controller that one was chosen, which concept bases on calculation of current average value only once per converter period [6,7]. This simple solution has structural limit of its dynamic behaviour but it is not important for comparative character of provided analysis. Each structure of adaptive current controller is optimised individually at its range of operation: PI in the range of continuous current and I in the range of discontinuous current mode [7]. Exemplary simulation results for investigated controllers during separate operation of each structure (PI/I) are shown in fig. 2. Investigations were provided with exemplary system consisting of DC motor 15kW/220V/8A/1450rpm supplied from proper 3-phase thyristor rectifier. The problem of adaptive control consists in proper changing over PI and I structures during the changes of converter mode. The simple changing over, at the point of boundary between both ranges of converter operation, provides sometimes to