Capacitive sensor is of non-contact type sensor and is primarily used to measure the linear displacements from few millimeters to hundreds of millimeters.
It comprises of three plates, with the upper pair forming one capacitor and the lower pair another. The linear displacement might take in two forms:
a. one of the plates is moved by the displacement so that the plate separation changes
b. area of overlap changes due to the displacement.
Capacitive Element Sensor
Below Figure shows the schematic of three-plate capacitive element sensor and displacement measurement of a mechanical element connected to the plate 2.
The capacitance C of a parallel plate capacitor is given by,
C = εr εo A / d
where εr is the relative permittivity of the dielectric between the plates, εo permittivity of free space, A area of overlap between two plates and d the plate separation.
As the central plate moves near to top plate or bottom one due to the movement of the element/workpiece of which displacement is to be measured, separation in between the plate changes. This can be given as,
**C1 = (εr εo A) / (d + x) **
C2 = (εr εo A) / (d – x)
When C1 and C2 are connected to a Wheatsone’s bridge, then the resulting out-of-balance voltage would be in proportional to displacement x.
Capacitive elements can also be used as proximity sensor. The approach of the object towards the sensor plate is used for induction of change in plate separation. This changes the capacitance which is used to detect the object.
Applications of capacitive element sensors
• Feed hopper level monitoring
• Small vessel pump control
• Grease level monitoring
• Level control of liquids
• Metrology applications
o to measure shape errors in the part being produced
o to analyze and optimize the rotation of spindles in various machine tools such as surface grinders, lathes, milling machines, and air bearing spindles by measuring errors in the machine tools themselves
• Assembly line testing
o to test assembled parts for uniformity, thickness or other design features
o to detect the presence or absence of a certain component, such as glue etc.