Measuring with capacitance sensors: General Industry Coverage

Capacitance sensors are noncontact sensors used in machine tools to measure the distance between a noncontact probe and a target sensor. Because the probe does not physically touch the target, capacitance sensors are beneficial in instances where the target surface is moving, such as a rotating spindle, or delicate, such as a silicon wafer.

All images courtesy S. Smith

Figure 1. A flat-plate capacitor.

The operating principle is similar to that of a flat-plate capacitor. Figure 1 shows that when two conductive plates (blue) are connected to a voltage source (left) and separated by a nonconductive space (dielectric), positive charges collect on the top plate and negative charges on the bottom plate. A small current flows until the charge builds to its maximum value, and then no more current can flow. The amount of charge that builds up is related to the capacitance of the device, measured in farads.

If the voltage source is alternating rather than direct, the built-up charge reverses with the voltage, and some current is always flowing. With the plate area and the dielectric material fixed, a measurement of the current provides a measurement of the distance between the two plates. Larger plates that are closer together cause more current than smaller plates that are farther apart.

In machine tool sensor applications, one plate is the probe, and one plate is the target. The probe and the target are both connected to the voltage source and must be conductors.

Figure 2. A capacitance sensor with a guard circuit.