Measuring machine vibration

Measurements of machine tool vibration can help control unbalance, detect chatter, detect tool breakage and predict bearing failures, among other uses. Vibration measurements are often made using piezoelectric accelerometers. As vibration transducers, they are inexpensive and easy to use. They can be mounted temporarily or permanently, come in a variety of sizes and sensitivities and have low power requirements. Piezoelectric accelerometers measure motion against the inertial reference frame, so they do not need a nonmoving reference structure like displacement probes do.

All images courtesy S. Smith

Figure 1. Schematic of a piezoelectric accelerometer.

These accelerometers use materials, such as quartz crystal, that exhibit the piezoelectric effect. That is, they produce an electric charge in response to mechanical deformation. The idealized structure of a piezoelectric accelerometer is shown in Figure 1. Inside a metal housing, a small mass is mounted atop a piezoelectric crystal, and electrical leads are attached. The mass has inertia and resists acceleration.

When the housing experiences acceleration, the crystal is squeezed between the mass and the housing with a force that is proportional to the acceleration. The crystal produces an electric charge, which is also proportional to the acceleration. An electrical circuit called a “charge amplifier” converts the charge into an easily measurable and transmittable voltage. Sometimes, a miniature charge amplifier is contained inside the accelerometer in a configuration called “ICP” (integrated circuit piezoelectric), and the equipment that records the data also powers the circuit. Because the charge dissipates quickly, piezoelectric accelerometers do not measure low-frequency vibrations well and do not measure static (DC) accelerations at all.

Figure 2. Placement of the accelerometer at the position indicated by the red line measures spindle bending vibration but not tool bending vibration.

Care must be taken when selecting, mounting and placing piezoelectric accelerometers.