Control Stabilization for Multiple Shaker Tests

Large test specimens often require multiple shaker excitation for realistic simulation of field vibration levels.At some frequencies, energy crossfeed between shakers causes control problems.Until recently no rnetltods existed for controlling multiple shaker tests at frequencies where the specimen .exhibited cross-coupling factors (CCF)* in excess of'about unity.In 1965 a method was described for controlling two shakers with CCFS1.(Reference 1)This paper describes a method for controlling any number of shakers with CCF>l.. I and it is desired to control this vibration input at each point of application (to conform with the expected final environment or to a standard test procedure), a major difficulty will usually be encountered.At one or more frequencies the" specimen will tend to vibrate in its natural mode shape in contradiction to the 1 i desired t e s t levels and phase.Trouble will occur whenever the coupled r t vibration from one driven point to another exceeds the level desired a t the , i second point, because "causett and "effect" are no longer separate and contained within each shaker-servo control loop.I * See definition of CCF, equation 2 on page 2 , I