CN103411525A - Cylindrical capacitance displacement sensor and conditioning circuit - Google Patents

Abstract

The invention provides a cylindrical capacitance displacement sensor. According to the cylindrical capacitance displacement sensor, a differential structure of three layers of cylinders is adopted, the measuring error because a distance d between an inner cylinder and an outer cylinder deviates, namely the center deviates by delta d when a movable cylinder is moved is eliminated, and the influence of dielectric constant of media is eliminated. Since the displacement is reflected by the difference value of upper and lower sensing capacitors, the sensitivity is improved.

Description

A kind of cylinder capacitance displacement sensor and modulate circuit

Technical field

The invention belongs to the displacement measuring technology field, more specifically say, relate to a kind of cylinder capacitance displacement sensor and modulate circuit.

Background technology

Capacitance displacement sensor is the change in displacement of non electrical quantity to be converted to a kind of sensor of electric capacitance change.Electric capacity is an electronic component that can charge and can discharge, and the basic structure of electric capacity is by two metal polar plates, and middle interval forms with insulator.Although capacitance displacement sensor difference in appearance is larger, organization plan is two classes substantially: parallel plate type and cylinder coaxial-type.

Fig. 1 is the structure principle chart of variable area cylinder displacement-capacitance sensor.

Variable area cylinder displacement-capacitance sensor is a kind of cylinder coaxial-type capacitance displacement sensor, in the situation that ignore edge effect, its capacity is:

$C_0=\frac{2\mathrm{\π\ϵ}}{\ln \frac{R}{r}}L---\left(1\right)$

In formula (1):

The length of L---out cylinder and inner cylinder cover part (m);

ε---the specific inductive capacity of medium (F/m) between capacitor plate;

R, r---outer utmost point cylinder inside radius and interior utmost point cylinder external radius (m).

Keeping between inner cylinder and out cylinder under the prerequisite constant apart from d, movable cylinder is inner cylinder translation x along its length, and capacity becomes:

$C=\frac{2\mathrm{\π\ϵ}}{\ln \frac{R}{r}}(L-x)=C_0(1-\frac{x}{L})=C_0-\mathrm{\ΔC}---\left(2\right)$

In formula (2), the variable quantity of capacity is:

$\mathrm{\ΔC}=\frac{x}{L}{C}_0---\left(3\right)$

Relatively being changed to of capacity:

$\frac{\mathrm{\ΔC}}{C_0}=\frac{x}{L}---\left(4\right)$

(it is linear that C～x) is the output characteristics of visible this sensor, and the range of measuring is not subjected to the restriction of the range of linearity, is suitable for measuring larger straight-line displacement.The sensitivity of this sensor is:

$k=\frac{\mathrm{\ΔC}}{x}=\frac{C_0}{L}=\frac{2\mathrm{\π\ϵ}}{\ln \frac{R}{r}}---\left(5\right)$

It is under the prerequisite constant apart from d, to draw keeping between inner cylinder and out cylinder that the output of existing variable area cylinder capacitance displacement sensor is linear conclusion.In the process that movable inner cylinder moves, if apart from d, can not accurately remain unchanged between inner cylinder and out cylinder, will cause measuring error.

Summary of the invention

The object of the invention is to overcome existing variable area cylinder capacitance displacement sensor and be subjected to the defect that between inner cylinder and out cylinder, change of distance affects, a kind of cylinder capacitance displacement sensor is provided, during with the elimination displacement, center of circle skew.

For realizing above purpose, cylinder capacitance displacement sensor of the present invention, is characterized in that, comprising:

The radius of horizontal alignment is R ₁, R ₃The inside and outside cylinder S of upper metal ₁₁, S ₁₂And the radius of horizontal alignment is R ₁, R ₃The inside and outside cylinder S of lower metal ₂₁, S ₂₂, the inside and outside cylinder S of upper metal ₁₁, S ₁₂And the inside and outside cylinder S of lower metal ₂₁, S ₂₂Height be L, and respectively vertical alignment be the center of circle point-blank;

Radius is R ₂Active cylinder S _m, its metal Partial Height is L+l, wherein, l is the distance between the inside and outside cylinder of upper and lower metal, active cylinder S _mMetal partly be placed between the inside and outside cylinder of metal, align and can move downward with the upper end of the inside and outside cylinder of upper metal in its upper end, until align with the lower end of cylinder inside and outside lower metal in lower end;

By the inside and outside cylinder S of upper metal ₁₁, S ₁₂On electric, be to link together as upper sensing capacitance C ₁An output terminal, active cylinder S _mAs upper sensing capacitance C ₁Another output terminal, by the inside and outside cylinder S of lower metal ₂₁, S ₂₂On electric, be to link together as lower sensing capacitance C ₂An output terminal, active cylinder S _mAs lower sensing capacitance C ₂Another output terminal;

Detect upper and lower sensing capacitance C ₁, C ₂Difference with and ratio k _pThat is:

$k_p=\frac{C_1-C_2}{C_1+C_2}---\left(6\right)$

According to active cylinder S _mDisplacement x and ratio k _pLinear relationship, displacement x is detected.

The object of the present invention is achieved like this:

Cylinder capacitance displacement sensor of the present invention, adopt the differential structure of three layers of cylinder, in the process that active cylinder is moved, and apart from the d skew, be that the measuring error that center of circle shifted by delta d brings is eliminated between inner cylinder and out cylinder, also eliminated the impact of the dielectric general knowledge of medium simultaneously.Due to the difference reflection displacement that is upper and lower sensing capacitance, sensitivity is improved.

The accompanying drawing explanation

Fig. 1 is the structure principle chart of the variable area cylinder displacement-capacitance sensor of prior art;

Fig. 2 is cylinder capacitance displacement sensor structure principle chart of the present invention;

Fig. 3 is that the capacitance of variable area differential generator shown in Figure 1 calculates schematic diagram;

Fig. 4 is the sectional view of cylinder capacitance displacement sensor shown in Figure 2 while being offset in the center of circle;

Fig. 5 is cylinder capacitance displacement sensor modulate circuit schematic diagram shown in Figure 2;

Fig. 6 is difference pulse width modulator electrical schematic diagram shown in Figure 5;

Fig. 7 is the working waveform figure of differential width modulation modulator shown in Figure 6.

Embodiment

Below in conjunction with accompanying drawing, the specific embodiment of the present invention is described, so that those skilled in the art understands the present invention better.Requiring particular attention is that, in the following description, when perhaps the detailed description of known function and design can desalinate main contents of the present invention, these were described in here and will be left in the basket.

Fig. 2 is cylinder capacitance displacement sensor structure principle chart of the present invention.

In the present embodiment, as shown in Fig. 2 (a) and (b), cylinder capacitance displacement sensor of the present invention is a kind of three cartridge type differential capacitor displacement transducers of changed area, comprising:

The radius of horizontal alignment is R ₁, R ₃The inside and outside cylinder S of upper metal ₁₁, S ₁₂And the radius of horizontal alignment is R ₁, R ₃The inside and outside cylinder S of lower metal ₂₁, S ₂₂, the inside and outside cylinder S of upper metal ₁₁, S ₁₂And the inside and outside cylinder S of lower metal ₂₁, S ₂₂Height be L, and respectively vertical alignment be the center of circle point-blank;

Radius is R ₂Active cylinder S _m, its metal Partial Height is L+l, wherein, l is the distance between the inside and outside cylinder of upper and lower metal, in the specific implementation, can be as much as possible little.Active cylinder S _mMetal partly be placed between the inside and outside cylinder of metal, aliging with the upper end of the inside and outside cylinder of upper metal in its upper end, as shown in Figure 2 (a) shows, and can move downward, as Fig. 2 (b), its displacement is x, until align with the lower end of cylinder inside and outside lower metal in lower end; Therefore, its range is L, i.e. the maximum displacement that can measure L length.And active cylinder S _mThe minimizing of cylinder partly equals active cylinder S inside and outside upper metal _mThe increase part of cylinder inside and outside lower metal, form one differential.

By the inside and outside cylinder S of upper metal ₁₁, S ₁₂On electric, be to link together as upper sensing capacitance C ₁An output terminal be A end, active cylinder S _mAs upper sensing capacitance C ₁Another output terminal C end, by the inside and outside cylinder S of lower metal ₂₁, S ₂₂On electric, be to link together as lower sensing capacitance C ₂An output terminal be B end, active cylinder S _mAs lower sensing capacitance C ₂Another output terminal be C end;

Detect upper and lower sensing capacitance C ₁, C ₂Difference with and ratio k _pNamely

$k_p=\frac{C_1-C_2}{C_1+C_2}---\left(6\right)$

According to active cylinder S _mDisplacement x and ratio k _pLinear relationship, just displacement x can be detected.

As shown in Fig. 2 (b), cylinder capacitance displacement sensor of the present invention consists of four electric capacity generally, and its equivalent electrical circuit is as shown in Fig. 2 (c).Its upper and lower sensing capacitance C ₁And C ₂(C wherein ₁=C ₁₁+ C ₁₂, C ₂=C ₂₁+ C ₂₂) formation differential structure, i.e. active cylinder S _mWhile pulling downwards, upper sensing capacitance C ₁Capacitance reduce, lower sensing capacitance C ₂Capacitance increase.

If active cylinder S _mThe pulled down distance is that displacement is x, can obtain according to formula (2):

$C_1=\frac{2\mathrm{\&pi;\&epsiv;}}{\mathrm{<figure-callout\; id="2R"\; label="ln\; R"\; filenames="HDA00003305870700011.png,HDA00003305870700031.png"\; state="\{\{state\}\}">ln</figure-callout>}\frac{R_{}}{}\frac{{}_2}{R_1}}(L-x)+\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{R_3}{R_2}}(L-x)---\left(7\right)$

$C_2=\frac{2\mathrm{\&pi;\&epsiv;}}{\mathrm{<figure-callout\; id="2R"\; label="ln\; R"\; filenames="HDA00003305870700011.png,HDA00003305870700031.png"\; state="\{\{state\}\}">ln</figure-callout>}\frac{R_{}}{}\frac{{}_2}{R_1}}x+\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{R_3}{R_2}}x---\left(8\right)$

In the present embodiment, displacement is with active cylinder S _mMetal part upper end and the upper end of the inside and outside cylinder of upper metal be aligned to starting point.Can certainly take lower end is starting point to it, and up motion, only need to again demarcate and get final product.

When in the scope of displacement x by 0 → L, changing, upper sensing capacitance C ₁Capacitance by maximal value

Fade to minimum value of zero, and lower sensing capacitance C ₂Capacitance by minimum value of zero, fade to maximal value $\frac{2\mathrm{\&pi;\&epsiv;}}{\mathrm{<figure-callout\; id="2R"\; label="ln\; R"\; filenames="HDA00003305870700011.png,HDA00003305870700031.png"\; state="\{\{state\}\}">ln</figure-callout>}\frac{R_{}}{}\frac{{}_2}{R_1}}L+\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{R_3}{R_2}}L.$

And have following linear relationship:

$\frac{C_1-C_2}{C_1+C_2}=(1-2\frac{x}{L}).$ Namely $x=\frac{1}{2}L-\frac{1}{2}\frac{C_1-C_2}{C_1+C_2}L---\left(9\right).$

It should be noted that, cylinder capacitance displacement sensor of the present invention is described and illustrates with vertical mode, but it also can place in other any modes, identical during its principle of work.

One, sensitivity and center of circle skew are analyzed

1, about the problem of sensitivity

The variable area cylinder displacement-capacitance sensor of prior art, as Fig. 3, when between inner cylinder and out cylinder apart from d under constant prerequisite, when inner cylinder was downward translation x along its length, capacitance was:

$C=\frac{2\mathrm{\&pi;\&epsiv;}}{n\frac{R_2}{R_1}}(L-x)=C_0(1-\frac{x}{L})=C_0-\mathrm{\&Delta;C}$

Its variable quantity is Δ C as can be seen here.

Cylinder capacitance displacement sensor of the present invention, as shown in Figure 2, when keeping active cylinder S _mIn mobile process, and between inner cylinder and out cylinder apart under the constant prerequisite of d, active cylinder S _mWhile being along its length downward translation x: upper and lower sensing capacitance C ₁, C ₂Capacitance be:

C ₁=C ₁₁+C ₁₂,C ₂=C ₂₁+C ₂₂

Therefore, $C_1=\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{{\mathrm{<figure-callout\; id="2R"\; label="R"\; filenames="HDA00003305870700011.png,HDA00003305870700031.png"\; state="\{\{state\}\}">R</figure-callout>}}_2}{R_1}}(L-x)+\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{R_3}{R_2}}(L-x)$

$C_2=\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{{\mathrm{<figure-callout\; id="2R"\; label="R"\; filenames="HDA00003305870700011.png,HDA00003305870700031.png"\; state="\{\{state\}\}">R</figure-callout>}}_2}{R_1}}x+\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{R_3}{R_2}}x$

By top as can be known, $\frac{C_1-C_2}{C_1+C_2}=\frac{\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{{\mathrm{<figure-callout\; id="2R"\; label="R"\; filenames="HDA00003305870700011.png,HDA00003305870700031.png"\; state="\{\{state\}\}">R</figure-callout>}}_2}{R_1}}(L-x)+\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{R_3}{R_2}}(L-x)-\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{{\mathrm{<figure-callout\; id="2R"\; label="R"\; filenames="HDA00003305870700011.png,HDA00003305870700031.png"\; state="\{\{state\}\}">R</figure-callout>}}_2}{R_1}}x-\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{R_3}{R_2}}x}{\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{{\mathrm{<figure-callout\; id="2R"\; label="R"\; filenames="HDA00003305870700011.png,HDA00003305870700031.png"\; state="\{\{state\}\}">R</figure-callout>}}_2}{R_1}}(L-x)+\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{R_3}{R_2}}(L-x)+\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{{\mathrm{<figure-callout\; id="2R"\; label="R"\; filenames="HDA00003305870700011.png,HDA00003305870700031.png"\; state="\{\{state\}\}">R</figure-callout>}}_2}{R_1}}x+\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{R_3}{R_2}}x}$

$=\frac{\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{{\mathrm{<figure-callout\; id="2R"\; label="R"\; filenames="HDA00003305870700011.png,HDA00003305870700031.png"\; state="\{\{state\}\}">R</figure-callout>}}_2}{R_1}}L+\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{R_3}{R_2}}L-2(\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{{\mathrm{<figure-callout\; id="2R"\; label="R"\; filenames="HDA00003305870700011.png,HDA00003305870700031.png"\; state="\{\{state\}\}">R</figure-callout>}}_2}{R_1}}x+\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{R_3}{R_2}}x)}{\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{{\mathrm{<figure-callout\; id="2R"\; label="R"\; filenames="HDA00003305870700011.png,HDA00003305870700031.png"\; state="\{\{state\}\}">R</figure-callout>}}_2}{R_1}}L+\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{R_3}{R_2}}L}$

$=\frac{C_0+C_0^{\&prime;}-2(\mathrm{\&Delta;C}+\mathrm{\&Delta;}{C}^{\&prime;})}{C_0+C_0^{\&prime;}}$

Its variable quantity is 2 (Δ C+ Δ C ') as can be seen here, and sensitivity of the present invention is improved.

2, about the problem of center of circle shifted by delta d

The variable area cylinder displacement-capacitance sensor of prior art, when if inner cylinder is downward translation x along its length, for sensor capacitance C is:

$C=\frac{2\mathrm{\&pi;\&epsiv;}(L-x)}{\ln [\frac{R_1^2+R_2^2-\mathrm{\&Delta;}{d}^2}{2R_1{R}_2}+\sqrt{{\left(\frac{R_1^2+R_2^2-\mathrm{\&Delta;}{d}^2}{2R_1{R}_2}\right)}^2-1}]}$

Now shifted by delta d in the center of circle is influential to two kinds of structure sensors.

The differential capacitance displacement transducer that cylinder capacitance displacement sensor of the present invention is the intermediate cylindrical changed area, when center of circle skew is Δ d, as Fig. 4, for this differential capacitance displacement transducer, as active cylinder S _mWhile being along its length downward translation x, upper and lower sensing capacitance C ₁, C ₂Capacitance be:

$C_1=\frac{2\mathrm{\&pi;\&epsiv;}(L-x)}{\ln [\frac{R_1^2+R_2^2-\mathrm{\&Delta;}{d}^2}{2R_1{R}_2}+\sqrt{{\left(\frac{R_1^2+R_2^2-\mathrm{\&Delta;}{d}^2}{2R_1{R}_2}\right)}^2-1}]}+\frac{2\mathrm{\&pi;\&epsiv;}(L-x)}{\ln [\frac{R_3^2+R_2^2-\mathrm{\&Delta;}{d}^2}{2R_3{R}_2}+\sqrt{{\left(\frac{R_3^2+R_2^2-\mathrm{\&Delta;}{d}^2}{2R_3{R}_2}\right)}^2-1}]}$

$C_2=\frac{2\mathrm{\&pi;\&epsiv;x}}{\ln [\frac{R_1^2+R_2^2-\mathrm{\&Delta;}{d}^2}{2R_1{R}_2}+\sqrt{{\left(\frac{R_1^2+R_2^2-\mathrm{\&Delta;}{d}^2}{2R_1{R}_2}\right)}^2-1}]}+\frac{2\mathrm{\&pi;\&epsiv;x}}{\ln [\frac{R_3^2+R_2^2-\mathrm{\&Delta;}{d}^2}{2R_3{R}_2}+\sqrt{{\left(\frac{R_3^2+R_2^2-\mathrm{\&Delta;}{d}^2}{2R_3{R}_2}\right)}^2-1}]}$

As can be known by above formula Therefore eliminate in such cases the impact that center of circle shifted by delta d brings fully, also eliminating the impact of the specific inductive capacity of medium simultaneously.

Two, cylinder capacitance displacement sensor modulate circuit of the present invention

1, circuit theory

As shown in Figure 5, it is comprised of difference pulse width modulator (PWM), low-pass filter, differential amplifier, AD converter, single-chip microcomputer cylinder capacitance displacement sensor modulate circuit schematic diagram of the present invention;

Fig. 6 is difference pulse width modulator electrical schematic diagram shown in Figure 5.

In the present embodiment, as shown in Figure 6, the difference pulse width modulator is by comparer A ₁, A _2,, RS trigger flip-flop and electric capacity charge and discharge control circuit and form;

U _RFor comparer A ₁, A ₂DC reference voltage, receive comparer A ₁, A ₂Anode, its value is greater than the diode forward forward voltage;

Electric capacity charges and discharge control circuit and comprises:

Resistance R ₁, R ₂, comparer A ₁, A ₂Negative terminal pass through respectively resistance R ₁, R ₂With power supply negative terminal, be connected;

Driving gate N ₁, N ₂, the Q end of RS trigger flip-flop, The end respectively with driving gate N ₁, N ₂Input end connect, driving gate N ₁, N ₂Output terminal is respectively by backward dioded D ₁, D ₂Be connected to comparer A ₁, A ₂Negative terminal; Driving gate N ₁, N ₂Input voltage is carried out exporting after negate;

The inside and outside cylinder S of the upper metal of cylinder capacitance displacement sensor ₁₁, S ₁₂Tie point meet driving gate N ₁Negative terminal, the inside and outside cylinder S of lower metal ₂₁, S ₁₂Tie point meet driving gate N ₂Negative terminal, active cylinder S _mGround connection;

Low-pass filter respectively to the Q of RS trigger flip-flop end,

After the PWM of end output carries out filtering, send into differential amplifier and amplify, then send into the AD converter conversion, finally conversion value sent into to single-chip microcomputer and process, calculate displacement x.

As shown in Figure 6, C ₁, C ₂For the sensing capacitance up and down of cylinder capacitance displacement sensor,, comparer A ₁, A ₂At its negative terminal input voltage, be greater than DC reference voltage U _RThe time, export respectively the R-S trigger flip-flop carried out to set and the S resetted, R pulse, two output terminal Q of rest-set flip-flop and

Be output as the pwm pulse ripple.

As shown in Figure 6,7, establish the t of power connection ₀Constantly, it is that the Q end is electronegative potential that the RS trigger flip-flop is in " 0 " state,

End is noble potential, therefore, and driving gate N ₁Output terminal G point be noble potential, reverse diode D ₁Cut-off, the 5V power supply starts to pass through resistance R ₁To upper sensing capacitance C ₁Charging; Driving gate N ₂Output terminal Point is electronegative potential, oppositely diode D ₂Conducting, lower sensing capacitance C ₂By reverse diode D ₂Electric discharge rapidly, the inside and outside cylinder S of lower metal ₂₁, S ₁₂Tie point B point be 0.7V, comparer A ₁, A ₂Output is high level, on the not impact of state of RS trigger flip-flop;

As upper sensing capacitance C ₁From the 0V charging, until the inside and outside cylinder S of upper metal ₁₁, S ₁₂Tie point A point potential rise to DC reference voltage U _RT ₁Constantly, comparer A ₁Output polarity changes (by high step-down), produces the S pulse of a negative polarity, make the upset of RS trigger flip-flop for one state (the Q end becomes noble potential,

End becomes electronegative potential).Driving gate N now ₁Output terminal G point become electronegative potential, make backward dioded D ₁Conducting, upper sensing capacitance C ₁Be discharged to rapidly 0.7V(backward dioded D ₁Forward voltage); Simultaneously,

The electronegative potential of end makes driving gate N ₂Output terminal

For noble potential, backward dioded D ₂Cut-off, so+the 5V power supply passes through resistance R ₂Downward sensing capacitance C ₂Charging, the inside and outside cylinder S of metal instantly ₂₁, S ₁₂Tie point B point voltage from+0.7V, charge to DC reference voltage U _RT ₂Constantly, comparer A ₂Produce the R pulse of a negative polarity, make rest-set flip-flop " 0 " state that overturns back again, (Q holds and becomes again electronegative potential,

End becomes again noble potential), repeat said process, so go round and begin again, make the RS trigger flip-flop two output terminal Q and

Produce separately a width and be subjected to sensing capacitance C ₁, lower sensing capacitance C ₂The pwm pulse ripple of modulation.Its course of work can be used table 1 explanation briefly.

Table 1

When upper and lower sensing capacitance, equate to be C ₁=C ₂The time, on circuit the each point voltage waveform as shown in Figure 7 (a), pulsewidth T in figure ₁=T ₂, Q,

2 are square wave, and they are after low pass electrofiltration ripple, and its average voltage is 1/2U _S, the DC voltage difference (E of point-to-point transmission ₁-E ₂) be zero.When upper and lower sensing capacitance is unequal, be C ₁≠ C ₂The time, if worked as the unequal C of upper and lower sensing capacitance ₁C ₂The time, C ₁, C ₂Discharge and recharge time constant and change, pulsewidth T ₁T ₂The each point voltage waveform as shown in Figure 7 (b) shows, Q,

Variation has occurred in the waveform width of 2, the average voltage E that Q is ordered after low-pass filtering ₁Be greater than 1/2U _S, The average voltage E of point ₂Be less than 1/2U _S, DC potential difference (E has appearred in point-to-point transmission ₁-E ₂) 0.Otherwise, if as the unequal C of upper and lower sensing capacitance ₁<C ₂The time, (E ₁-E ₂)<0.

2, performance evaluation

At t ₀-t ₁Interval, direct supply E(+5V) pass through resistance R ₁To upper sensing capacitance C ₁The voltage equation of charging is:

$u_{c1}=E(1-e^{-t/R_1{C}_1})---\left(10\right)$

At upper sensing capacitance C ₁Charging reaches DC reference voltage U _RThe time (t ₁Constantly), comparer A ₁Output S negative pulse, make RS trigger flip-flop state turnover (putting 1).At t ₁(t=T constantly ₁) have:

$u_{c1}=U_R=E(1-e^{-T_1/R_1{C}_1})---\left(11\right)$

Following formula solves and can obtain capacitor C ₁Duration of charging T ₁:

$T_1=R_1{C}_1\ln \frac{E}{E-U_R}---\left(12\right)$

In like manner can descend sensing capacitance C ₂Duration of charging T ₂:

$T_2=R_2{C}_2\ln \frac{E}{E-U_R}---\left(13\right)$

As seen from Figure 7, at Q and

The pwm pulse of end output, after the voltage stabilizing of stabilivolt amplitude limit, the formation amplitude is U _S, and width is respectively T ₁And T ₂Rectangular wave pulse (namely through upper and lower sensing capacitance C ₁And C ₂The PWM ripple of modulation), they are through low-pass filter, the cycle mean value U of output voltage ₀₁And U ₀₂Be respectively:

$U_{01}=\frac{T_1}{T_1+T_2}{U}_S,U_{02}=\frac{T_2}{T_1+T_2}{U}_S---\left(14\right)$

Their differential output voltage U ₀For:

$U_0=U_{01}-U_{02}=\frac{T_1-T_2}{T_1+T_2}{U}_S---\left(15\right)$

Work as resistance R ₁=R ₂The time, by formula (12) and (13) substitution formula (15) must this circuit output DC voltage U ₀:

$U_0=\frac{C_1-C_2}{C_1+C_2}{U}_S---\left(16\right)$

As can be known by following formula, the variation of differential capacitor makes the duration of charging difference, thus the square-wave pulse width difference that the trigger flip-flop output terminal is produced, and its output signal need only can obtain direct current output through low-pass filter.This metering circuit only needs a direct supply that voltage-regulation coefficient is higher, and this is than in other metering circuit, needing the AC power of the Frequency and Amplitude Stabilization of high stability easily to accomplish.

According to formula (7) and formula (8), can obtain:

$C_1-C_2=\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{{\mathrm{<figure-callout\; id="2R"\; label="R"\; filenames="HDA00003305870700011.png,HDA00003305870700031.png"\; state="\{\{state\}\}">R</figure-callout>}}_2}{R_1}}(L-x)+\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{R_3}{R_2}}(L-x)-\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{{\mathrm{<figure-callout\; id="2R"\; label="R"\; filenames="HDA00003305870700011.png,HDA00003305870700031.png"\; state="\{\{state\}\}">R</figure-callout>}}_2}{R_1}}x-\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{R_3}{R_2}}x---\left(17\right)$

$C_1+C_2=\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{{\mathrm{<figure-callout\; id="2R"\; label="R"\; filenames="HDA00003305870700011.png,HDA00003305870700031.png"\; state="\{\{state\}\}">R</figure-callout>}}_2}{R_1}}L+\frac{2\mathrm{\&pi;\&epsiv;}}{\ln \frac{R_3}{R_2}}L---\left(18\right)$

By formula (17) and (18) substitution formula (16), can obtain U ₀:

$U_0=\frac{C_1-C_2}{C_1+C_2}{U}_S=(1-2\frac{x}{L})U_s---\left(19\right)$

Following formula shows, when x=0, and U ₀=U _SWhen x=1/2L, U ₀=0; When x=L, U ₀=-U _S, adopt changed area differential capacitance sensor, can further offset the impact of Δ d output voltage U ₀X is linear with the input variable quantity.

Three, conclusion

(1), the linear measurement range of cylinder capacitance displacement sensor of the present invention is not limited in principle, applicable to measuring larger straight-line displacement;

(2), the structure of cylinder capacitance displacement sensor of the present invention can obtain higher sensitivity;

(3), intermediate plate parallel connection and the differential structure of cylinder capacitance displacement sensor of the present invention, have good counteracting characteristic, eliminate the impact that d changes;

(4), the modulate circuit parameter changes the compensation that can disappear mutually, eliminates the error that the many kinds of parameters variation causes, realizes that circuit is simple;

(5), cylinder capacitance displacement sensor of the present invention, convenience are independently debugged and calibrate.

Although the above is described the illustrative embodiment of the present invention; so that those skilled in the art understand the present invention; but should be clear; the invention is not restricted to the scope of embodiment; to those skilled in the art; as long as various variations appended claim limit and the spirit and scope of the present invention determined in, these variations are apparent, all utilize innovation and creation that the present invention conceives all at the row of protection.

Claims (2)

1. a cylinder capacitance displacement sensor, is characterized in that, comprising:

The radius of horizontal alignment is R ₁, R ₃The inside and outside cylinder S of upper metal ₁₁, S ₁₂And the radius of horizontal alignment is R ₁, R ₃The inside and outside cylinder S of lower metal ₂₁, S ₂₂, the inside and outside cylinder S of upper metal ₁₁, S ₁₂And the inside and outside cylinder S of lower metal ₂₁, S ₂₂Height be L, and respectively vertical alignment be the center of circle point-blank;

Radius is R ₂Active cylinder S _m, its metal Partial Height is L+l, wherein, l is the distance between the inside and outside cylinder of upper and lower metal, active cylinder S _mMetal partly be placed between the inside and outside cylinder of metal, align and can move downward with the upper end of the inside and outside cylinder of upper metal in its upper end, until align with the lower end of cylinder inside and outside lower metal in lower end;

By the inside and outside cylinder S of upper metal ₁₁, S ₁₂On electric, be to link together as upper sensing capacitance C ₁An output terminal, active cylinder S _mAs upper sensing capacitance C ₁Another output terminal, by the inside and outside cylinder S of lower metal ₂₁, S ₂₂On electric, be to be connected to as lower sensing capacitance C ₂An output terminal, active cylinder S _mAs lower sensing capacitance C ₂Another output terminal;

Detect upper and lower sensing capacitance C ₁, C ₂Difference with and ratio k _pThat is:

$k_p=\frac{C_1-C_2}{C_1+C_2}$

According to active cylinder S _mDisplacement x and ratio k _pLinear relationship, displacement x is detected.

2. the modulate circuit for the described cylinder capacitance displacement sensor of claim 1, is characterized in that being comprised of difference pulse width modulator (PWM), low-pass filter, differential amplifier, AD converter, single-chip microcomputer;

The difference pulse width modulator is by comparer A ₁, A ₂,, RS trigger flip-flop and electric capacity charge and discharge control circuit and form;

U _RFor comparer A ₁, A ₂DC reference voltage, receive comparer A ₁, A ₂Anode, its value is greater than the diode forward forward voltage;

Electric capacity charges and discharge control circuit and comprises:

Resistance R ₁, R ₂, comparer A ₁, A ₂Negative terminal pass through respectively resistance R ₁, R ₂With power supply negative terminal, be connected;

Driving gate N ₁, N ₂, the Q end of RS trigger flip-flop,

The end respectively with driving gate N ₁, N ₂Input end connect, driving gate N ₁, N ₂Output terminal is respectively by backward dioded D ₁, D ₂Be connected to comparer A ₁, A ₂Negative terminal; Driving gate N ₁, N ₂Input voltage is carried out exporting after negate;

The inside and outside cylinder S of the upper metal of cylinder capacitance displacement sensor ₁₁, S ₁₂Tie point meet driving gate N ₁Negative terminal, the inside and outside cylinder S of lower metal ₂₁, S ₁₂Tie point meet driving gate N ₂Negative terminal, active cylinder S _mGround connection; Low-pass filter respectively to the Q of RS trigger flip-flop end, After the PWM of end output carries out filtering, send into differential amplifier and amplify, then send into the AD converter conversion, finally conversion value sent into to single-chip microcomputer and process, according to active cylinder S _mDisplacement x and ratio k _pLinear relationship, displacement x is calculated.

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