Capacitive proximity sensor | Diagram , Advantages , Disadvantages

Capacitive Sensor : 

A transducer that uses capacitance variation is known as capacitance sensors. The elastic deflection of a membrane due to the applied force is detected by a capacitance variation. A highly sensitive displacement and proximity transducers can be constructed because the capacitive transducer senses very small deflections accurately. Capacitive sensors can directly sense a variety of things such as motion, displacement, chemical composition, electric field and indirectly sense many other variables which 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 terminals is given by the expression

capacitive sensor equation
capacitive sensor equation

Capacitive Proximity Sensor : 

One form of capacitive proximity sensor is shown in Figure 1.31 where one plate of a capacitor is connected to the central conductor of a coaxial cable while the other plate is formed by a target object. The operating principle is based on either the geometry (i.e. the distance, d) or capacitance variations in the presence of conductive or dielectric materials.

capacitive proximity Sensor Diagram
capacitive proximity Sensor Diagram


This type of sensor can be employed for measuring position, displacement, gauging or any other similar parameter in a machine tool.

Advantages capacitive Sensor : 

(i) Excellent linearity over entire dynamic range when area is changed (since stray electric fields are small)
(ii) It has high sensitivity
(iii) Capacitive displacement detectors can detect 10-14 m displacement with good stability, high speed and wide extremes of environment ,
(v) It has freedom of electrode (plate) materials and geometry for demanding environments and applications.
(vi) Fractional change in capacitance can be made large.
(vii) Capacitive sensors can be made to respond to displacement in one direction only.
(viii) The forces exerted by the measuring apparatus are electrostatic and usually for small enough so that they can be disregarded
(ix) Capacitors are noiseless
(x) High accuracy and resolution are possible. A resolution of 2.5 x 1O-3 can be obtained.

Disadvantages of Capacitive proximity Sensor 

(i) The performance of these sensors is likely affected due to the environmental conditions such as dust, moisture, vibration etc.
(ii) The metallic parts of the capacitor must be insulated from each other.

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