Instrumented Impact Hammer Calibration

Using Newton's Second Law

Instrumented impact hammers are a useful and common tool in the structural dynamics and vibration field trouble shooting fields of technology. The hammers, available in sizes from as small as a 0.10 oz (2.9 gm) pencil up to 12 lb (5.44 kg) sledges, are typically configured for a given test by adding an impact tip of specific hardness (soft to nylon to steel) and occasionally a mass extender to add additional inertia for broadening the impact energy pulse in low frequency situations. Due to the unique nature of impact measurements, it is advised by manufacturers that users should calibrate the hammers in the tip/extender configuration that they will be used. For this reason, it is common to calibrate impact hammers a number of times for different tip/extender combinations.

Fortunately, there is a simple calibration technique for instrumented impact hammers that relies on Newton’s second law, F=Ma, and requires very little extra equipment or fixturing.  The basis for the calibration is a constitutive method which simply relies on the straight line rigid body behavior of a mass.  This means that since a rigid mass has no resonant frequencies (below some very high frequencies), it has a straight line frequency response function comparing a measurement of acceleration over force.  By adding a calibrated accelerometer to the test mass and suspending it pendulously, a simple calibration rig can be constructed.  When a user strikes the mass with an impact hammer and measures the unscaled force and resultant scaled motion, the F=Ma equation can be simplified to the single unknown, the sensitivity of the force sensor on the impact hammer. Click here for an explanation of the simple math.  Users can use their digital signal analyzer (DSA) or data acquisition card in capture mode to measure and calculate the frequency response of impacting the instrumented pendulous mass.  To calculate the scaling for the hammer, simply multiply by the mass (in appropriate units) by the FRF at the desired reference frequency and divide this result into the sensitivity of the accelerometer.  This is the same method that is used at the manufacturer and is available as an option on some automated accelerometer calibration workstations.

We hope this short description hits the point and expands your calibration knowledge.