Image Credits: Microsense, MTI Instruments, and RDP Electrosense

 

Capacitive linear position senors measure position, displacement, vibration, and run out (thickness), primarily in a noncontact manner. They have good accuracy and very high resolution, but may be sensitive to environmental factors.

 

Operation

Noncontact capacitive sensors measure changes in capacitance. Capacitance describes how two objects with a sapce between them respond to a voltage difference applied to them. This video illustrates:

 

Video Credit: Sensorlytics

Types

One type of capacitive sensor features conductive probe and target objects.

Image Credit: Machine Design.com

 

For nonconductive targets, a stationary target plate may be used. When the nonconductor is placed between the probe and the target, the thickness, density, and/or position of the material changes the dielectric constant of the interval space.

Image Credit: Lion Precision

 

In cases of nonconductive sensing without a reference, the sensing electric field wraps back to the probe, creating a "fringe field." When a nonconductive material nears the probe, its ectric changes the field.

Image Credit: Lion Precision

 

Specifications and Features

  • Measurement ranges vary (e.g. 0.004 to 0.35 inches).
  • Sensing frequency varies (e.g. 0.42 to 26 kHz).
  • Electrical output signals can be analog (current, voltage, or frequency), digital (serial, parallel, or other), an alarm, or a change in state of switches.
  • Packaging varies from none (a sensor element) to housed (transducer). Instruments or meters are self-contained and display output at or near the device. A gauge or indicator has an analog display and no electronic output.
  • Devices can tolerate maximum shocks, maximum vibrations, and certain temperature ranges (e.g. -100 to 1832°F).
  • Recorders and totalizerscan be added to collect and store data locally.

Applications

Capacitive sensors measure position (automation, semiconductor processing, disk drives), dynamic motion (tool spindles, disk drive spindles, high-speed drill spindles, ultrasonic welders), and thickness (silicon wafer, brake rotor, disk drive platter, label positioning). They also can perform assembly testing.

 

References

Capacitive Sensors-An Overview

Capacitive Sensor Operation and Optimization (How Capacitive Sensors Work and How to Use Them Effectively)

Capacitive Sensors

Finding the Right Sensor for Linear Displacement


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