Capacitive Linear Displacement Sensors - An Overview
This single page will give you an overview of capacitive sensor operation, application, theory, and resources. Links to more detailed information are also provided.
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Capacitive sensors (capacitive linear displacement sensors) are noncontact devices capable of high-resolution position measurement and/or position change - displacement measurement - 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.
Below you will find:
Basic Theory, High-Performance Capacitive Sensors, Capacitive Sensor Advantages, Applications, and Products Overview.
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).
For a thorough explanation of how capacitive sensing works for displacement measurement as well as presence/absence and mass/density measurement, please visit the Capacitive Linear Displacement Sensor Theory page.
A high performance
capacitive sensor system
It is important to distinguish between "high-performance" capacitive linear displacement sensors and inexpensive capacitive 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 could be 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 displacement 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.
Capacitive Sensor Advantages
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 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-Current 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 can also be used with nonconductive targets in many applications.
Position Measurement/Sensing, Displacement Measurement
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
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 (15 kHz) to accommodate 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 is available 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 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
- Assembly testing
- Sensing glue or other applied material
Label sensor products and technical detail 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. Therefore, 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.
Capacitive Sensor Products Overview
High-performance 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."
TheCPA100 is a lower cost capacitive sensor. The output from this sensor is repeatable but not linear. The system is used in servo "zero-seeking" systems where the capacitive sensor outputs are used to drive a system to a known good condition rather than measuring the extent to which the system is out of tolerance. It is also used where a threshold voltage is established at which some conditional action is initiated.
For more detail on capacitive sensor products, visit the Capacitive Sensor Products page.
For more detail on the operation and theory of capacitive sensors, visit the Capacitive Sensor Theory page.