Introduction

We now derive state-variable models for several types of actuators that are used in automatic control systems to drive massive loads that must be precisely positioned. These models are used in subsequent chapters, where they appear as vital components in automatic systems.

One of the most commonly used actuators is the DC motor, an ideal subject for our study because of its immediate practical application in many engineering design problems. It is the simplest device that requires the simultaneous application of Newton's laws of mechanics, Kirchhoff's laws of electric circuits (subjects fundamental to Chapters 2 and 3), and the first principles of electromechanical energy conversion manifested in Faraday's law and Ampere's rule (which are reviewed in this chapter).

We then continue our study with the analysis of another commonly used system, the electromagnet driven by an electronic amplifier. This combination of two basically nonlinear devices is used to drive loads which require substantial forces but which are intended to travel only small distances from an equilibrium position. Because only limited displacements are required, our linear approximations to the nonlinear force–displacement–current relationships of the amplifier–magnet combination yield a useful linear state-variable model for this system.

The chapter concludes with the analysis of an electrohydraulic actuating system that combines the amplifier–magnet system with an hydraulic servovalve–ram system. This type of system is used in applications where high power (greater than several horsepower) is required in a small space and where the actuator is commanded by an electrical driving signal, in many cases delivered from a remote site.