There are a considerable number of vibration testing apparatus that are well-known in the prior art. Such apparatus is used to mechanically shake an item for the purpose of diagnostically testing responses to certain driving forces. The item is physically attached to a moving portion of the apparatus and when the apparatus is activated, the item is subjected to a variety of test conditions. The moving portion of the vibration testing apparatus is typically driven by a force which may be continuous, cyclical or impulsed. One class of such apparatus employs the use of an electromagnetic field between field and armature windings. Various driving signals are impressed across the armature winding to control the movement of the armature. For convenience, the movable member of the vibration testing apparatus may be referred to from time to time hereinafter simply as the armature.
In many of the prior art vibration testing apparatus springs or static air pressure systems were relied upon to center the armature between its axial displacement limits. However, such systems are proving inadequate to meet the present demands for heavier pay loads, increased armature displacement, and unsymmetrical shock and random wave forms.