An electromagnetic forcing device.

The paper describes the drawbacks of an electromagnetic forcing device, of the type commonly used to study forced vibration of structures, and presents details of a feedback control system designed to overcome them. The work described was initiated when attempting to generate sinusoidal forcing in a nonlinear beam-vibration study. Magnetic-material nonlinearities and spatial inhomogenieties in the magnetic field led to unwanted harmonics in the force the beam experienced, and feedback was used to reduce these effects. A brief description of the principles of feedback control is presented and the problems encountered in applying the concepts to the electromagnetic forcing device are discussed. Details of the system, its problems, operating characteristics and limitations are presented. [ABSTRACT FROM AUTHOR]