Capacitance type sensor works principle

Capacitance type sensor distributes type of only electric capacity and form of the capacitance that need branch 2 kinds. If pursue,1 is shown.

Pursue type of 1 only electric capacity and sensor of type of the capacitance that need branch

(A) is odd capacitance sensor

(B) differs minute of capacitance sensor

Graph 1(a) is the capacitor that two smooth andante comprise, the capacitance needing branch that graph 1(b) comprises to plate is inserted among two smooth andante is passed feeling. To graph 1(a) character, when the side effect of oversight capacitor:

The capacitance of capacitor is: 　

The A in type is the plate area of capacitor, d is the distance of plate, er, E0 is dielectric constant.

Capacitance sensor changes mediumly the C-d character of sensor of clearance style electric capacity is shown 2 times like the graph.

Pursue 2 change the C-d of sensor of clearance style electric capacity is characteristic graph

A plate of sensor of only electric capacity is fixed, call static plate, another plate and join of the object that be measured are use plate. Difference divides the fluctuation plate of capacitance sensor to all be secured, call static plate, intermediate plate is use plate. Move plate to follow when the object that be measured is mobile mobile, changed the capacitance C between plate, knowable C-d character is a curve:

When D0 reduces Dd, and Δ D<D0

(1)

By (1) type can get:

(2)

When Dd/d0<<1, get entering like linear relation;

The sensitivity of capacitance sensor:

(3)

If consider (the linear in 2) type and nonlinear:

;

Of capacitance sensor relatively nonlinear error:

(4)

From (3) type can see, want to heighten sensitivity, should reduce electric capacity initial clearance D0, but of D0 reduce get the limitation of capacitor puncture voltage, machine precision not only the demand is high, of capacitance sensor relatively nonlinear the error increases.

To raise the sensitivity K of sensor, raise precision, reduce nonlinear error & , capacitance sensor uses differential type structure mostly. In sensor of the capacitance that need branch, when the mobile space that uses plate is Dd, the clearance D1 of capacitance C1 turns into D0-Dd, the clearance D2 of capacitance C2 turns into D0 Dd.
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