How Capacitive Sensors work
How to use them effectively.
Compliments of Lion Precision. Copyright © 2007 Lion Precision. www.lionprecision.com
A Capacitive Measurement System
Capacitive sensor dimensional measurement requires three basic components:
a probe that uses changes in capacitance to sense changes in distance to the target,
driver electronics to convert these changes in capacitance into voltage changes,
a device to indicate and/or record the resulting voltage change.
Each of these components is a critical part in providing reliable, accurate measurements. The probe geometry, sensing area size, and mechanical construction affect range, accuracy, and stability. A probe requires a driver to provide the changing electric field that is used to sense the capacitance. The performance of the driver electronics is a primary factor in determining the resolution of the system; they must be carefully designed for a high-preformance applications. The voltage measuring device is the final link in the system. Oscilloscopes, voltmeters and data acquisition systems must be properly selected for the application.
What is Capacitance?
Capacitance describes how the space between two conductors affects an electric field between them. If two metal plates are placed with a gap between them and a voltage is applied to one of the plates, an electric field will exist between the plates. This electric field is the result of the difference between electric charges that are stored on the surfaces of the plates. Capacitance refers to the capacity of the two plates to hold this charge. A large capacitance has the capacity to hold more charge than a small capacitance. The amount of existing charge determines how much current must be used to change the voltage on the plate. Its like trying to change the water level by one inch in a barrel compared to a coffee cup. It takes a lot of water to move the level one inch in the barrel, but in a coffee cup it takes very little water. The difference is their capacity.
When using a capacitive sensor, the sensing surface of the probe is the electrified plate and what youre measuring (the target) is the other plate (well talk about measuring non-conductive targets later). The driver electronics continually change the voltage on the sensing surface. This is called the excitation voltage. The amount of current required to change the voltage is measured by the circuit and indicates the amount of capacitance between the probe and the target. Or, conversely, a fixed amount of current is pumped into and out of the probe and the resulting voltage change is measured.
How Capacitance Relates to Distance
The capacitance between two plates is determined by three things:
Size of the plates: capacitance increases as the plate size increases
Gap Size: capacitance decreases as the gap increases
Material between the plates (the dielectric):
Dielectric material will cause the capacitance to increase or decrease depending on the material
In ordinary capacitive sensing, the size of the sensor, the size of the target, and the dielectric material (air) remain constant. The only variable is the gap size. Based on this assumption, driver electronics assume that all changes in capacitance are a result of a change in gap size.
The electronics are calibrated to output specific voltage changes for corresponding changes in capacitance. These voltages are scaled to represent specific changes in gap size. The amount of voltage change for a given amount of gap change is called the sensitivity. A common sensitivity setting is 1.0V/100µm. That means that for every 100µm change in the gap, the output voltage changes exactly 1.0V. With this calibration, a +2V change in the output means that the target has moved 200µm closer to the probe.
Focusing the Electric Field
When a voltage is applied to a conductor, an electric field is emitted from every surface. For accurate gaging, the electric field from a capacitive sensor needs to be contained within the space between the probes sensing area and the target. If the electric field is allowed to spread to other items or other areas on the target, then a change in the position of the other item will be measured as a change in the position of the target. To prevent this from happening, a technique called guarding is used. To create a guarded probe, the back and sides of the sensing area are surrounded by another conductor that is kept at the same voltage as the sensing area itself. When the excitation voltage is applied to the sensing area, a separate circuit applies the exact same voltage to the guard. Because there is no difference in voltage between the sensing area and the guard, there is no electric field between them to cause current flow. Any conductors beside or behind the probe form an electric field with the guard instead of the sensing area. Only the unguarded front of the sensing area is allowed to form an electric field to the target.
Effects of Target Size
The target size is a primary consideration when selecting a probe for a specific application. When the sensors electric field is focused by guarding, it creates a field that is a projection of the sensor size and shape. The minimum target diameter for standard calibration is 30% of the diameter of the sensing area. The further the probe is from the target, the larger the minimum target size.
Range of Measurement
The range in which a capacitive sensor is useful is a function of the area of the sensing surface. The greater the area, the larger the range. The driver electronics are designed for a certain amount of capacitance at the sensor. Therefore, a smaller sensor must be considerably closer to the target to achieve the desired amount of capacitance. The electronics are adjustable during calibration, but there is a limit to the range of adjustment.
In general, the maximum gap at which a probe is useful is approximately 40% of the sensing surface diameter. Standard calibrations usually keep the gap considerably less than that.
Much more information available
Please view that complete tutorial at www.capacitive-sensing.com for detailed information on these important topics:
Multiple Channel Sensing
Effects of Target Material
Maximizing Accuracy: Target Size
Maximizing Accuracy: Target Shape
Maximizing Accuracy: Surface Finish
Maximizing Accuracy: Parallelism
Maximizing Accuracy: Environment
And find the precise definitions for these important terms:
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