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Figure 9: MTII's Push/Pull Capacitance Sensor
Each
sensor is driven at the same voltage, however, there is a 180 degree phase shift between signais. This shift allows the current path to travel across the target surface rather than through the target to ground, eliminating any inaccuracies created by poorly grounded targets. Additionally, highly résistive targets can be measured with this technology allow-ing capacitance
sensors to be used on semi-insulating and semi-conducting targets.
Push Pull
_1_
_Target
4) Capacitive Sensor Advantages and Disadvantages i. Advantages
As with any sensing technology, capacitive systems have both advantages and disadvantages. Perhaps their greatest attribute is their ability to resolve measurements below one micro-inch (<25nm), at a fraction of the cost of other high performance technologies. Most capacitance
sensors are "passive" by design allowing them to be used in extreme environments while still maintaining stability. Sensors can be easily customized, allowing them to be adapted into a variety of applications or settings. They are immune to target composition and work equally well on all conductive targets, unlike eddy current probes. They are
immune to ultrasonic noises, lighting conditions, humidity and temperature for the most part.
ii. Disadvantages
Capacitance technology dictates that the probe be mounted close to the target. This increases the probability of crashing the
sensor or damaging the material being measured. MTII has provisions in the Accumeasure product line to extend the measure-ment range and standoff of the
sensor, however, this distance is rarely greater than 15mm.
Capacitance
sensors should also be kept clean. Dirt or other foreign debris can cause an offset in the measurement so frequent cleaning may be required depending on the application. These types of
sensors are typically not used in applications where
oil splatter or cutting fluids are present.
5) Applications
i. Position Sensing
General positioning is probably the most common application for capacitive
sensors. As mentioned about, the output of a capacitance
sensor is proportional to the distance to the target. If the probe remains stationary any capacitance change detected by the amplifier is directly related to the target position. Capacitive
sensors have highly linear response and low output phase shift making them ideally suited to be applied in both static and active feedback positioning applications.
MTII appnote: capacitancesensingtheory.pdf - Page 9 of 13
^^^^^^ ^^^^^^^^^J sales@mtiinstruments.com-www.mtiinstruments.com
