Capacitive sensor:
A transducer that uses variation in capacitance is known as capacitance sensors. The elastic deflection of a membrane due to applied force is detected by a change in capacitance. Highly sensitive displacement and proximity transducers can be built because the capacitive transducer detects very small deflections accurately. Capacitive sensors can directly detect a variety of things such as motion, displacement, chemical composition, electric field and indirectly detect many other variables that can be converted into motion or dielectric constant such as pressure, acceleration, fluid level and fluid composition .
A capacitance sensor consists of two metal plates separated by an air gap. The capacitance C between the terminals is given by the expression
capacitive sensor equationCapacitive proximity sensor:
One form of capacitive proximity sensor is shown in Figure 1.31, where one plate of a capacitor is connected to the center conductor of a coaxial cable while the other plate is formed by a target object. The principle of operation is based on geometry (i.e. distance, d) or capacitance variations in the presence of conductive or dielectric materials.
Capacitive proximity sensor diagramForms
This type of sensor can be used to measure position, displacement, gauging or any other similar parameter in a machine tool.
Advantages of capacitive sensor:
(i) Excellent linearity over the entire dynamic range when the area is changed (since stray electric fields are small)
(ii) It has high sensitivity
(iii) Capacitive displacement detectors can detect displacements of 10-14 m with good stability, high speed and wide extreme environmental conditions,
(v) It has freedom of materials and electrode (plate) geometry for demanding environments and applications.
(vi) The fractional change in capacitance can be large.
(vii) Capacitive sensors can be made to respond to displacement in only one direction.
(viii) The forces exerted by the measuring device are electrostatic and generally small enough that they can be disregarded
(ix) Capacitors are silent
(x) High accuracy and resolution are possible. A resolution of 2.5 x 1O-3 can be achieved.
Disadvantages of capacitive proximity sensor
(i) The performance of these sensors is likely to be affected due to environmental conditions such as dust, humidity, vibration, etc.
(ii) The metallic parts of the capacitor must be isolated from each other.
