Capacitive Displacement Sensors - An Overview of Selection and Usage
- Capactive Products
- Capacitive Sensor Applications and TechNotes
- Detailed Capacitive Sensor Theory Whitepaper
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 sensors makes them indispensible in today's nanotechnology world. Capacitive sensing can also be used to measure the position or other properties of nonconductive targets.
Basic Capacitive Sensor Theory
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 (0.25-2mm).
For a thorough explanation of capacitive sensor theory, please visit the Capacitive Sensor Theory page. Or you can visit our Capacitive Sensors Product page for detailed information on existing capacitve sensing solutions.
High-Performance Capacitive Sensor Systems
It is important to distinguish between "high-performance" sensors and inexpensive sensors. Simple capacitive sensors, such as those used in inexpensive proximity switches or elevator touch switches, are simple devices and in their most basic form are often designed in a high school electronics class. Proximity type sensors are tremendously useful in automation applications and many commercially available models are well made, 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 based on 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.
A high performance capacitive sensor system
Advantages of Capacitive Displacement Sensors
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
- Inexpensive compared to laser interferometers.
Capacitve sensors are not a 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)
Visit the Eddy-Curent vs. Capacitive Sensors page for a more detailed comparison between the two technologies.
Capacitive sensors are useful in any application requiring the measurement or monitoring of the position of a conductive target. They are also useful in many applications with nonconductive targets.
Position Measurement and Sensing
Capacitive sensors are basically position measuring devices. The 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 interpretted 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
Measuring the dynamics of a continuously moving target, such as a rotating spindle or vibrating element, requires some form of noncontact measurement. Capacitive sensors are ideal when the environment is clean and the motions are small, requiring high-resolution measurements. Lion Precision capacitive sensors also have high frequency response (15kHz) to accomodate high-speed motion.
- Precision machine tool spindles
- Disk drive spindles
- High-speed drill spindles
- Ultrasonic welders
- Vibration measurements
Measuring material thickness in a noncontact fashion is a common application for capacitive sensors. The most useful application is a two-channel differential system in which a separate sensor is used for each side of the piece being measured. Details on thickness measurements with capacitive sensors are availabe in the Conductive Material Thickness Measurement with Capacitive Sensors Application Note. Capacitive sensor technology is used for thickness measurement in these applications:
- Silicon wafer thickness
- Brake rotor thickness
- Disk drive platter thickness
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 indicitive 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
Label sensor products and technical details are available at www.labelsensors.com.
For details on the measurement of nonconductive targets, please see the more detailed Capacitive Sensor Theory page.
Capacitive sensors have a much higher sensitivity to conductors than to nonconductors. For this reason, they can be used to detect the presence/absence of metallic subassemblies in completed assemblies. An example is a connector assembly requiring an internal metallic snap ring which is not visible in the final assembly. Online capacitive sensing can detect the defective part and signal the system to remove it from the line.
High-perfromance capacitive sensors generally consist of an electronics module and probe which is connected to the module by a cable. These systems are available in different configurations and price points.
The Elite Series capacitive sensors feature a modular design in which single or multiple channels of electronics modules are enclosed in a modular rack. All of the channels are synchronized and a meter/display can be added to the system. Each capacitive sensor module has a "calibrated range" display and coarse and fine zero adjustments for adjusting the DC level of the sensor output. The Elite Series are the highest performing capacitive sensors.
The Compact Driver contains from one to six capacitive sensor channels in a small box with no user controls. Separate connectors are provided for power in and sensor channel outputs. Connectors are also provided for the probes. This system is ideal for OEM applications in which the capacitive sensor system is embedded within a larger system. The lack of user adjustments prevents inaccurate readings resulting from "operator error."