A capacitive sensor is a proximity sensor that detects nearby objects by their effect on the electrical field created by the sensor. Capacitive proximity sensors use an alternating voltage which causes the charges to continually reverse their positions. The moving of the charges creates an alternating electric current which is detected by the sensor. The amount of current flow is determined by the capacitance, and the capacitance is determined by the area and proximity of the conductive objects. Larger and closer objects cause greater current than smaller and more distant objects. The capacitance is also affected by the type of nonconductive material in the gap between the objects.
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Capacitive Proximity Sensor Design QualitiesThe design of a simple capacitance meter is often based on a relaxation oscillator. The capacitance to be sensed forms a portion of the oscillator's RC circuit or LC circuit. A capacitive cable sensor functions by charging the unknown capacitance with a known current. The capacitance can be calculated by measuring the charging time required to reach the threshold voltage (of the relaxation oscillator), or equivalently, by measuring the oscillator's frequency.
Capacitor Sensor ApplicationsIn typical capacitive sensing applications, the probe or sensor is one of the conductive objects; the target object is the other. The sizes of the sensor and the target are assumed to be constant as is the material between them. Therefore, any change in capacitance is a result of a change in the distance between the probe and the target. Capacitive sensors have some similarities to radar in their ability to detect conductive materials, while seeing through insulating materials such as wood or plastic. Capacitive sensors are also often used to detect fill levels of liquids, pellets, and powders through containers. Some other related capacitive sensor applications, include:
- Proximity sensing for industrial machine shut-off
- Touch sensing / button replacements (isometric buttons on microwave ovens, stovetops etc., rotary input like on the iPod)
- Material properties - most famous is probably the Stud Sensor patented in the seventies, that allows you to sense studs, water and mains lines in walls.
- Vehicle/Personnel detection
- Capacitor/Condenser microphones
- Liquid level sensing
- Tilt/Rotation sensing
- X-Y Input (Tablets/Laptop computer touch pads, Smart Phones
Capacitive Proximity Sensor BenefitsCompared to other noncontact sensing technologies such as optical, laser, eddy-current, and inductive, high-performance capacitive sensors provide a wide range of benefits, some of which include:
- Cost efficient
- Minimal maintenance
- Contains no moving parts
- Good range of measurement
- Simple to use
- Easy to clean
- Perfect for high temperature and high pressure applications