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Capacitive Sensor Theory: How They Work, How to Use Them Effectively on 2008-04-19
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Area and Dielectric are held constant for ordinary capacitive sensing so only the Gap can change the capacitance.
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Focusing the Electric Field
Probes use a guard to focus 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 probe’s 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
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The sensor’s electric field covers an area about 30% larger than the sensing area of the probe.
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In general, the maximum gap at which a probe is useful is approximately 40% of the sensor diameter. Standard calibrations usually keep the gap considerably less than that.
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The greater the area, the larger the range.
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Using multiple probes on the same target requires that the excitation voltages be synchronized. This is accomplished by configuring one driver as a master and others as slaves.
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Capacitive sensors measure all conductors: brass, steel, aluminum, or even salt-water, with equal accuracy.
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target thickness does not affect the measurement.
Surface finish can affect the measurement. Capacitive sensors will measure the average position of the target surface within the spot size of the sensor.
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Nonconductors can be measured by passing the electric field through them to a stationary conductive target behind.
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Fringing can be used to measure nonconductive targets without a conductive background target.
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Accuracy requires that the measurements be made under the same conditions in which the system was calibrated.
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flat target more than 30% larger than the sensing area
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the probe must be closer to the target for the same zero point.
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An additional problem of an undersized target is that the system becomes sensitive to X and Y location of the probe relative to the target.
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Alternatively, when flat calibrations are used with curved surfaces, multipliers can be provided to correct the measurement value.
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the capacitive sensor will average over the area covered by the spot size of the sensor.
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Parallelism must be considered when designing a fixture for the measurement.
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More temperature related errors are due to expansion and contraction of the measurement fixture than probe or electronics drift.
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The dielectric constant of air is affected by humidity. As humidity increases the dielectric constant increases. Humidity can also interact with probe construction materials. Experimental data shows that changes from 50%RH to 80%RH can cause errors up to 0.5% of full scale.
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Capacitive Sensors - An Overview with Resources on 2008-04-19
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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.
- Dirty or wet environment (eddy-current sensors are ideal)
- Large gap between sensor and target is required (optical and laser are better)
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- Automation requiring precise location
- Semiconductor processing
- Final assembly of precision equipment such as disk drives
- Precision stage positioning
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- Precision machine tool spindles
- Disk drive spindles
- High-speed drill spindles
- Ultrasonic welders
- Vibration measurements
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two-channel differential system
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- Silicon wafer thickness
- Brake rotor thickness
- Disk drive platter thickness
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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.
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- Label positioning during application
- Label counting
- Glue detection
- Glue thickness
- Assembly testing
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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.
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