Capacitive sensors are noncontact devices capable of high-resolution measurement of the position and/or change of position of any conductive target. The nanometer resolution of high-performance capacitive sensors makes them indispensable in today’s nanotechnology world. They can also be used to measure the position or other properties of nonconductive targets. Capacitive sensors use the electrical property of “capacitance” to make measurements. Capacitance is a property that exists between any two conductive surfaces within some reasonable proximity. Changes in the distance between the surfaces changes the capacitance. It is this change of capacitance that capacitive sensors use to indicate changes in position of a target. High-performance displacement sensors use small sensing surfaces and as result are positioned close to the targets (0.25-2 mm).

Capacitive Sensor Advantages and Limitations

  • Compared to other noncontact sensing technologies such as optical, laser, eddy-current, and inductive, high-performance capacitive sensors have some distinct advantages.
    • Higher resolutions including subnanometer resolutions
    • Not sensitive to material changes: Capacitive sensors respond equally to all conductors
    • Less expensive and much smaller than laser interferometers.
  • Capacitive sensors are not good choice in these conditions:
    • Dirty or wet environment (eddy-current sensors are ideal)
    • Large gap between sensor and target is required (optical and laser are better)

Applications of Capacitive Sensors

It is important to distinguish between “high-performance” capacitive sensors and inexpensive capacitive sensors. Simple capacitive sensors, such as those used in inexpensive proximity switches or elevator touch switches, are simple devices. Proximity type sensors are tremendously useful in automation applications, but they are not suited to precision metrology applications. In contrast, capacitive sensors for use in precision displacement measurement and metrology applications use complex electronic designs to execute complex mathematical algorithms. Unlike inexpensive sensors, these high-performance sensors have outputs which are very linear, stable with temperature, and able to resolve incredibly small changes in capacitance resulting in high resolution measurements of less than one nanometer.

  • Position Measurement/Sensing – Capacitive sensors are basically position measuring devices. Their outputs always indicate the size of the gap between the sensor’s sensing surface and the target. When the probe is stationary, any changes in the output are directly interpreted as changes in position of the target. This is useful in:
    • Automation requiring precise location
    • Semiconductor processing
    • Final assembly of precision equipment such as disk drives
    • Precision stage positioning
  • Dynamic Motion Measurement – Measuring the dynamics of a continuously moving target, such as a rotating spindle or vibrating element, requires some form of noncontact measurement.
    • Precision machine tool spindles
    • Disk drive spindles
    • High-speed drill spindles
    • Ultrasonic welders
    • Vibration measurements
  • Thickness Measurement – Measuring material thickness in a noncontact fashion is a common application for capacitive sensors. Capacitive sensor technology is used for thickness measurement in these applications:
    • Silicon wafer thickness
    • Brake rotor thickness
    • Disk drive platter thickness
  • Nonconductive Thickness Measurement – Capacitive sensors are sensitive to nonconductive materials which are placed between the probe’s sensing area and a grounded back target. If the gap between the sensor and the back target is stable, changes in the sensor output are indicative of changes in thickness, density, or composition of the material in the gap. This is used for measurements in these applications:
    • Label positioning during application
    • Label counting
    • Glue detection
    • Glue thickness
    • Assembly testing
  • Assembly testing – Capacitive sensors have a much higher sensitivity to conductors than to nonconductors. Therefore, they can be used to detect the presence/absence of metallic subassemblies in completed assemblies.