This application note from MTI Instruments compares two types of non-contact sensors: capacitive and Eddy current. It describes their basic operating principles, includes a quick comparison table, and offers recommendations for use.

Capacitance Sensors: Basic Operating Principles

Capacitive probes use electric fields to measure the distance from the sensor face to the target surface. A guard ring around the capacitive sensor in the probe steers the direction of sensing. This steering is practically straight through to the target and ignores nearby influences of adjacent surfaces that might disrupt the reading.

Typically, the targets are conductive and grounded; however, capacitive push-pull probes can work with ungrounded targets as well. The electric field senses only the surface layer of the conductive target and is not affected by the type of metal nor by its magnetic properties. However, the electric field is affected by anything besides air in the gap. Moisture, oil, heavy dust, or grease can cause an error reading. Measurement is not adversely affected by temperature, vacuum, air pressure, or magnetic fields.

Figure 1: Typical capacitive probe with stainless steel body

Figure 2: Only the target area under the field is measured

Figure 3: Step height of a thin conductive film on a roller

Figure 3 depicts a typical application for a capacitive probe. The user wanted to measure the thickness of a thin conductive film for electric vehicle (EV) battery production. The step height, the difference between probe and roller and probe film height, was used to determine the film thickness.

Unlike Eddy current probes, capacitive probes only sense conductive surfaces. They do not sense subsurface variations such as those found in rollers because of magnetic effects, and in the film itself because of its thinness.

In this example, multiple capacitive probes were used to compensate for roller runout and roller tilt. MTI Instruments provided software called Basic Measurement that allowed the user to setup a step height measurement by selecting the measurement application from a menu of standard choices.

Eddy Current Probes: Basic Operating Principles

Eddy current probes use an alternating AC current to induce a magnetic field in a conductive target. In turn, the induced magnetic field produces a back magnetic field that is proportional to the gap between the probe face and the target.

Figure 4: Typical Eddy current probe with plastic face and threaded body

Figure 5: Typical magnetic fields around an Eddy current probe

Eddy current probes have several disadvantages. The magnetic fields can sense nearby objects such a screw heads, ridges, and mounting influences (if sunk in a flush-mounting hole). Compared to a capacitive probe, the sensing footprint of an Eddy current probe is also much larger. However, an Eddy current sensor is immune to non-conductive contamination in the gap from oil, water, dust, etc.

With an Eddy current probe, the calibration (sensing slope factor) changes as the target material changes. Therefore, an aluminum target will have a different calibration factor than a steel one. Eddy current probes do not work on laminated surfaces such as transformer cores and are affected by rotating targets where the magnetic field changes, such as with large rotating turbine shafts. Thin targets also present a problem since the magnetic field penetrates the surface. Therefore, the use of opposing probes to measure the thickness of a thin metallic sheet probably wouldn’t work well.

All of these conditions must be taken into consideration when deciding whether to use a capacitive sensor or an Eddy current sensor. The table in the next section provide comparisons.

Quick Comparison Table

Conclusion

In general, a capacitance probe is a better choice than an Eddy current probe. However, if the sensing environment is dirty, contaminated, or wet, an Eddy current probe may be required. Before making a final decision then, consider all of the factors in your application.

Learn more MTI’s non-contact hybrid probe that combines a¬†capacitance probe and an Eddy current probe to measure the thickness of a non-conductive material on top of a grounded metal substrate.

Are you looking for process control metrology products?  MTI’s systems can measure thickness, total thickness variation, bow and warp as part of in-process monitoring or as a quality station in production.