Capacitive displacement sensors

Capacitive displacement sensors are non-contact devices that are used for measurements at a high-resolution. It can also be used to check the position of any conductive object as well as measure the width or density of non-conductive materials. Capacitive displacement sensors have a large variety of uses including precision thickness measurements, assembly line testing, assembly metrology, and semiconductor processing. Capacitive displacement sensors can be found in manufacturing facilities all around the globe.

There’s more to selecting the proper type of displacement sensor and measurement system than meets the eye. To select the appropriate type of device for your specific needs you need to know the application, the environment, and accuracy. There are a number of factors when it comes to determining which device would best suit your purposes.

  • Number of devices to use
  • Installation costs
  • Target shape
  • Target Application
  • Target Material
  • Target characteristics
  • Desired measurement accuracy
  • Usage environment

Basic capacitive theory

Knowing how the machinery in your shop works is only one part of the equation. It’s also important to understand the theory behind it. Capacitance is a property of an electric field that’s created when an electrical charge is applied to two objects that are conductive in nature. An excellent example would be two conductive plates of the same type that is charged with an electrical charge.

Capacitance sensors are ideal for measuring small precise distances over ranges from 25 µm (.001”) up to 12.5mm (0.5 inch) and custom sensors that can measure over an inch are available. Resolution in the sub-nanometer range is standard, with range being an inverse factor.

Capacitance sensors have a probe and amplifier. The probe is usually a small passive stainless steel cylinder. Round probes are easier to mount (and build) than square ones and have become the industry standard. However, both types are available upon request.

Since it is passive, (contains no electronics) it is highly stable, and probes can be built that operate in vacuum, high heat, or cryogenic temperatures down to 4° Kelvin (Liquid Helium). They can also be built of non-magnetic materials so they can operate in high magnetic fields (> than 2 Tesla) such as found in generator rotor to stator gaps. Capacitance probes are so stable that they rival laser interferometers resolution, while not prone to fringe jumping.

Capacitance works on the two plate parallel capacitance theory.

distance-between-plates

Figure 3

Where the distance between plates d= ε*A/C

ε= dielectric constant of air, A = area of the plates, C = the capacitance (charge) between the plates.

the-amplifier-injects-a-very-small-current-through-the-probe

Figure 4

The amplifier injects a very small current through the probe; the voltage produced across the capacitance of the gap is proportional to the distance of the gap. In this case the target is grounded; MTI Instruments also makes probe and amplifiers (Push-Pull) that don’t need to be grounded. The amplifier has a frequency response from 0-5kHz.

Limitation of capacitance sensors :

Capacitance sensors cannot be used in environments that have oil or water droplets and other types of contamination such as brake dust or airborne machined debris.

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The selection process

When calculating your measurement accuracy you have to keep in mind that the results can vary based on the measurement system that you’re using. Ideally, the measurement will vary based on the range of measurements sought. Furthermore, understanding the capabilities of the measurement system that you’re using is vital. Can you view the measurement with the naked eye or do you need to use software to view the measurement? These are the types of questions you must answer before making your selection.