Impact Hammer Test (TAP Testing)

Tap testing, also known as modal testing, is an experimental method that is used to excite the machine-tool system in order to extract its harmonic information such as natural frequencies, modal masses, modal damping ratios and mode shapes.  This is normally done in static conditions, using the impact hammer as the excitation mechanism, and an accelerometer as the sensor.  In theory, the tool tip should be given a perfect impulse which excites a range of frequencies with a constant amplitude in an infinitely short duration.  This allows us to obtain a clean frequency response function (FRF) over the full frequency range of interest.  In real life, however, this is not possible.  Sometimes, the FRF becomes noisy at higher frequencies, which is an indication that the hammer tip may be too soft, and the impulse is not strong enough to excite the higher frequency range.  If the FRF is noisy at lower frequencies it means the hammer tip is too hard and that all modes are excited far beyond the frequency range of interest.  Fortunately, we can alter the hammer mass and hammer stiffness by using different sized hammer and using different hammer tip stiffness respectively, to control contact time and impact force.  The combination is skillfully chosen to excite the frequency range of interest.