Optimal Control of a Stepping Motor

Abstract

Over the past few years the stepping motor has been the subject of detailed study by manufacturers, users and research workers. This interest arises from the rapidly increasing demand for digital actuators, particularly in the machine tool and computer peripheral industries. The optimal control of a stepping motor is expected to play an important role in producing high performance positioning systems for these and other types of applications. Therefore the object of this thesis is to study some of the problems facing the systems engineer who has to design this type of system. After a general introductory chapter which reviews selected topics thought to be directly relevant to the production of an optimum system the work falls, broadly, into two parts. The first part (chapters 2-4) presents a method of modelling stepping motors in terms of electrical and magnetic equivalent circuits. ‘Exact’ and simplified models for a permanent magnet motor are presented, and simple linear end non-linear models are used to theoretically optimise the input variables. The second part (chapters 5-6) is devoted to the drive circuit and the control of stepping motors. A thyristor inverter circuit is presented, which could possibly be used to drive a power stepping motor, end which can be readily used for closed-loop control of the machine. Optical and electronic methods of phase angle control are discussed and a digital method of accurate phase angle control is developed. Finally, a computer controlled stepping motor drive system is described in which the computer is able to delegate various duties. Particularly the important function of switching in an optimal way to a simple smaller local controller.