Use of a force plate versus armature accelerometer for measuring frequency response functions

Force measurement methods related to floor vibration modal testing are investigated. The basics of shaker-structure interaction and consequences of this interaction are presented. To illustrate the consequences of using the two force measurement methods, frequency response functions (FRF) are measured for four structures. The frequency content of the measured and computed force waveforms indicate large force notches which are not detected using the armature accelerometer method. Accelerance ratios for tests with mass ratios exceeding 1000 are found to be different by a maximum of 13.2% and an average of 2.9%. Accelerance FRFs are very different depending on the force measurement method due to the presence of large force drop-offs and large glitches in the force spectrum, which go undetected using the armature accelerometer method.