Nearly all vibration tests cover a frequency range where mechanical resonances occur in a system consisting of payload, fixture and armature. In this context, the test is controlled by acceleration, based on the following basic equation at constant mass:
force = mass × acceleration (f = ma)
However, under resonance conditions, the effective mass does not remain constant. Therefore, poor vibration control can lead to underloading or overloading of the payload and damage due to overdriving of the armature. Choosing where to place the control accelerometers is one of the most critical parts of any vibration test.
There are no universally suitable active vibration control positions. Nevertheless, the wrong positions can damage the vibration equipment or affect the accelerations applied to the payload. The following principles should therefore be observed:
- All mechanical structures have resonances
- The larger a structure, the lower the resonant frequency
- For increased mass without increased stiffness, the resonant frequency will reduce
- For increased stiffness without increased mass, the resonant frequency will increase
- In a free system, when a purely axial resonance occurs, the liveliest points will always be the ends