One, foreword

 

The change that capacitance sensor is the blame n that will be measured changes a kind of sensor that changes for capacitance, it can measure the mechanical volume such as loading, displacement, oscillatory, angle, acceleration not only, still can measure content of fluid face, makings face, composition to wait for hot project parameter. This kind of sensor has tall impedance, small-power, dynamic range big, trends is answered faster, hardly 0 float, the structure is simple wait for an advantage by force with adaptability. Accordingly, capacitance sensor is detecting automatically very important place is had in the technology, receive wide application. But it also puts in a few problems in use process:

 

 

2, the problem of sensitivity

 

By a capacitor that two smooth andante comprise, be like oversight its edge effect, its capacitance can express with next type:

 

 

In type, s is plate mutual cover area, the unit is the distance between two smooth andante for M2;d, the dielectric constant of vacuum of;ε0 of opposite dielectric constant of the medium between the plate of dielectric constant;εr that the unit is the medium between plate for M;ε . From on type can see, when D the hour can make capacitance increases, and make sensitivity increases, but D Guo Xiaorong is easy cause capacitor breakdown, general we can be adopted mica is placed to improve between plate, right now capacitance C is two capacitance series connection, can write into:

 

 

In type, the;d0 of dielectric constant of airy of dielectric constant;ε2 of ε1 mica sheet is the distance between two plate for;d of air gap width. Micaceous dielectric constant is airy 7 times, micaceous puncture voltage not less than 103kv/mm, airy puncture voltage is 3kv/mm only. Ply is the mica sheet of 0.01mm only, its puncture voltage also is not less than 10kv/mm, because this had mica sheet, the distance between plate can be reduced greatly, the linear that still can make capacitance sensor outputs character gets ameliorative. Heighten sensitivity besides using the method that adds mica sheet, still can take the following step:

 

1. increases power source frequency.

 

2. reduces plate thickness to be able to weaken edge effect.

 

3, the span of a few quantities in capacitance sensor

 

Changing extremely in the capacitance sensor that span leaves, because reduce extremely span leaves can heighten sensitivity, multi-purpose the displacement that will measure micron level, the distance between general plate does not exceed 1mm, quantity of the biggest displacement should be restricted changing in the; inside the 1/10 limits of span in the sensor that plate working face accumulates, can measure the displacement of centimeter of class. In capacitance sensor, the size that chooses capacitance correctly is very important. Reasonable design can make sensor satisfies the requirement that measures range already, can heighten sensitivity again, reduce nonlinear error. General the change of its capacitance is inside limits of 103PF ～ 104PF, the change of relative value ΔC/C is inside 1 limits of 10-6 ～ . Impedance of capacitance component output is in commonly 108Ω ～ 106Ω , the frequency of alternating current source that this numerical value still uses with place is concerned. The influence to reduce insulation resistance and heighten sensitivity, power source frequency is used commonly in 50kHz above, but use high frequency power source to make signal magnifies, transmit when waiting for a problem to compare low frequency many complex.

 

4, the influence problem of distributinging capacitance

 

Capacitance sensor a very crucial question is distributinging capacitance capacitance and sensor capacitance are compared not only cannot oversight, and the effect is extremely serious still, it is to cause transmission loss of efficiency, sensitivity to drop if really ever since, measure an error to increase reach stability to become poor. In last few years, had new settlement to this problem way. Among them ” of law of “ whole shield is, it is with differential capacitance sensor exemple, show integral screen way, in next graphs, CX1, CX2 is differential capacitance, u is power source, a is amplifier, integral shield law is a whole bridge (include power source cable to wait) a screen rises, the crucial point of the design depends on receiving the reasonable setting of the place. Use a ground contact the dot is put between R1 of two balances resistor, R2 with integral screen in all, such, sensor is public the input impedance of the distributinging capacitance C1 between plate and screen and amplifier is shunt-wound, can input what C1 inspects amplifier thereby capacitance. Because the input impedance of amplifier has great value, the paralell connection of C1 also does not hope to exist, but it affects sensitivity only just, paralell connection of C3 of additionally two distributinging capacitance, C4 is on bridge arm R1, R2, can affect the initiative balance of bridge and integral sensitivity, do not affect the regular job of bridge. Accordingly, distributinging parameter is eliminated basically to the influence of sensor capacitance, integral shield law is the very good method that solves problem of capacitance sensor distributinging capacitance, defect is to make a structure complex.

 

Graph the 1 working circuit that eliminates distributinging electric capacity pursues

 

5, nonlinear problem

 

Change sensor of clearance style electric capacity is relative to output expression:

 

 

By on type is knowable change sensor of clearance style electric capacity is relative to output and input nonlinear relation. As a result of Δd/d<<1, the measure of below in order to two kinds of similar processing techniques often adopts on the project:

 

1. approximate linear handles: Take type right side namely the first approximate, have:

 

 

2, approximate nonlinear processing: Before taking what output expression relatively namely two approximate, have:

 

 

Of two kinds of afore-mentioned approximate generation relatively nonlinear the error is:

 

 

Above analyses a specification, relatively nonlinear error R0 and clearance D0 also concern into inverse proportion, the sensitivity that heightens sensor accordingly and reduce nonlinear the error is conflicting. In applying actually, contradict to solve this, use sensor of differential style electric capacity mostly.