The electrical characteristics of shaker systems: Used for vibration testing of equipment

Abstract

Electro-dynamic shakers are generators of mechanical vibratory force used in testing the characteristics of structures or the endurance of components subjected to vibratory excitation. The design of adequate power supply and power distri bution systems for driving electro-dynamic shakers depends upon a knowledge of the impedance presented by the shaker armature. The design of the distribution system is complicated by the fact that the impedance varies widely, in both magnitude and phase, over the operating frequency range. In this type of shaker the vibratory force is derived from an alternating current passed through a cylindrical coil supported in a radial magnetic field. Figure 1 shows a cross sectional view of a typical shaker construction in which the radial flux is produced by means of a direct current coil exciting a pot-shaped field structure. The armature coil is supported in the annular field gap and constrained to move in an axial direction by means of a flexure system. The armature frame transmits the forces generated by the coil to a table or attachment for the mechanical system to be excited, Such a device converts an alternating current into an alternating force, and this conversion can be stated expli cit!*/ in terms of the shaV^r construction as explained in another paper**". The explicit nature of the conversion allows the shaker to be treated as an electro-mechanical transformer, and the effect of any mechanical system attached to the shaker on the armature impedance can be studied by reflecting the mobility"' of the mechanical system into the electrical system as a component of the armature impedance. Figure 2 shows diagramatically the electro-mechanical system^ and the resulting equivalent electrical system. Examination of this equivalent circuit makes it obviousxnax tr*Q impGciaSC^ ol tiie shaker armature xs depencienx on The characteristics of the mechanical system being driven. It is therefore impossible to evaluate the shaker impedance, for design purposes, without a careful consideration of the likely range of mechanical loadings to be encountered" This paper will attempt to show by example:A. The wide range of impedance match required of the power distribution system, and caused by changes in -o _