JPS6342330Y2 - - Google Patents

Description

[Detailed Description of the Invention] This invention relates to a capacitive electrical signal conversion device that converts the capacitance of a capacitor whose capacitance changes depending on a measured quantity such as displacement or pressure into a DC electrical signal.

Conventionally, in order to convert the capacitance of a capacitor whose capacitance changes depending on the quantity to be measured into an electrical signal, an AC power supply is used to obtain an AC current corresponding to the capacitance of the capacitor, and the AC current is passed through a rectifier circuit. Rectification is performed to obtain a DC signal corresponding to the above-mentioned capacitance. Such conventional capacitive electric signal converters require an AC power supply and have disadvantages such as poor stability due to the effects of linearity and temperature characteristics of the rectifier circuit.

In view of this, a capacitive electrical signal converter as shown in FIG. 1 has been proposed in the past. That is, the output of the operational amplifier 11 is switched to the switches 12 and 13.
This causes the capacitors 14 and 15 to be charged, and the switches 12 and 13 are turned off, and the charges in the capacitors 14 and 15 are supplied to the wave circuits 18 and 19 through the switches 16 and 17 for smoothing. The wave circuits 18 and 19 are each operational amplifier 2.
1, 22, smoothing capacitors 23, 24 and resistors 25, 2 connected to the feedback circuit of the amplifier.
6, and if necessary, nonlinear compensation capacitors 27 and 28 are connected to each input side of the wave circuits 18 and 19 between them and a common potential point.
Switches 12 and 13 are turned on at the same time, and switches 16 and 17 are turned on at the same time in opposite phases to these switches 12 and 13. The output of the wave circuit 19 is supplied to the inverting input side of the operational amplifier 11, and the reference voltage source 29 is supplied to the non-inverting input side of the operational amplifier 11.
A reference voltage is given by

At least one of the capacitors 14 and 15 14 changes depending on the measured quantity, and in this circuit, the output of the capacitor 15 is connected to the wave circuit 19.
The smoothed output is smoothed by the reference voltage source 29, and operates so that the smoothed output matches the reference voltage of the reference power supply 29. In other words, the output of the operational amplifier 11 operates as a reference voltage, and the capacitors 14 and 15 are periodically charged with that constant voltage, and the outputs of the capacitors 14 and 15 are smoothed by the wave circuits 18 and 19, respectively. circuit 1
8, an output proportional to the capacitance of the capacitor 14 can be obtained. The output of the wave circuit 18 is supplied to an output terminal 32 through an operational amplifier 31. Operational amplifier 3
1 may be, for example, voltage/current conversion. According to the converter shown in FIG. 1, neither an AC power source nor a rectifier circuit is required. Therefore, a product with relatively good stability can be obtained. However, in this device, for example, the wave circuit 1
When noise Δε is input between the output side of the wave circuit 8 and the input side of the operational amplifier 31, the output of the wave circuit 18 is
If E ₁ and the amplification degree of the operational amplifier 31 are A ₁ , then
The output of the operational amplifier 31 is (E ₁ +Δε)A ₁ .
In this way, the noise Δε is output in the form of being added to the signal E ₁ , and the error ratio ε ₀ with respect to the true signal is Δε/E ₁ .

The purpose of this invention is to provide a capacitive electrical signal converter that does not use an AC power supply or a rectifier circuit, and is less affected by noise.

According to this invention, an output is transmitted so that the output signal of the wave circuit to which discharge current of at least a capacitor whose capacitance changes is supplied to a pair of wave circuits becomes constant, and a DC power source is controlled by the output. In this way, the influence of noise is removed.

FIG. 2 shows a first embodiment of the capacitive electric signal converter according to this invention, and parts corresponding to those in FIG. 1 are designated by the same reference numerals. In this example, the output of the wave circuit 19 is fed to the inverting input of an operational amplifier 11, while the output of an operational amplifier 31 is fed to the non-inverting input of this operational amplifier 11. The output of the wave circuit 18 is supplied to the inverting input side of the operational amplifier 31, and the reference voltage source 29 is connected to the non-inverting input side of the operational amplifier 31. That is, in this example, an output is transmitted to the output terminal 32 so that the output E ₁ of the wave circuit 18 becomes equal to the reference voltage Es of the reference voltage source 29, and the operational amplifier 11, which is a DC power source, is controlled by the output. . Now, in this figure, the electrodes of the capacitor 14 change according to the measured quantity, the electrode spacing in the initial state when the measured quantity is zero is d ₀ , the amount of displacement is Δd, and the electrode spacing of the capacitor is d ₀ . The capacitance of the capacitor 14 at that time is C ₀ ,
Similarly, if the capacitance of the capacitor 15 is a fixed capacitance C ₀ , then each capacitance C ₁ and C ₂ of the capacitors 14 and 15 is
are respectively C ₁ =C ₀ d ₀ /d ₀ −Δd and C ₂ =C ₀ . Now, if the resistance values of the resistors 25 and 26 are R, then the output E ₁ of the wave circuit 18 and the wave circuit 19
The output E ₂ of can be expressed as follows.

E ₁ = Ec1/TC ₁ R, E ₂ = Ec1/TC ₂ R where T is the on/off cycle of the switches 12, 13, 16, and 17, and Ec is the output voltage of the operational amplifier 11. The amplification factor of the operational amplifier 31 is A ₁ ,
When the amplification factor of the operational amplifier 11 is _A2 , it can be expressed as (Es- _E1 )× _A1 = _E0 , ( _E0 - _E2 ) _A2 =Ec. E ₀ is the output voltage at terminal 32,
Since the amplification factors A ₁ and A ₂ are almost infinite, from the previous equation, Es−E ₁ = E ₀ /A ₁ ≒0, E ₀ −E ₂ = Ec / A ₂ ≒0, and from this, E ₀ = E ₂ /E ₁・Es is calculated. Therefore, _E0 = _C2 / _C1・Es= _d0 −Δd/ _d0Es . Therefore, the output voltage E ₀ of the terminal 32 is proportional to the displacement Δd of the measured quantity.

FIG. 4 shows a second embodiment of this invention. That is, in FIG. 2, the distance between the electrodes of the capacitor 14 changes depending on the amount to be measured, and the capacitor 1
5 is a fixed capacitor, but for example, as shown in FIG. 3, a movable electrode 36 is interposed between the fixed electrodes 34 and 35, and capacitors 14 and 15 are placed between this electrode 36 and the fixed electrodes 34 and 35, respectively. The movable electrode 36 is displaced according to the amount to be measured, and the capacitor 1
This idea can also be applied when the capacitances C ₁ and C ₂ of 4 and 15 change differentially. Now, when the amount of displacement is zero, these movable electrodes 36 and fixed electrodes 34, 3
Assuming that each interval between the wave circuits 18 and 19 is equal to d ₀ , in this case, the outputs E _{1 and 5} of the wave circuits 18 and 19 are
E ₂ is supplied to the inverting input side of the operational amplifier 31 through resistors 37 and 38, respectively, and the sum of these output voltages E ₁ and E ₂ is supplied to the operational amplifier 31, and the outputs E ₁ , _E2 is supplied to the inverting input side and the non-inverting input side of the operational amplifier 11 through resistors 39 and 41, respectively, and the difference between them is taken and the output of the operational amplifier 31 is
E ₀ is supplied to the non-inverting input of operational amplifier 11 through resistor 42 . The inverting input side of the operational amplifier 11 is connected through a resistor 43 to a common potential point. Now the resistance values of resistors 37 and 38 are R ₁
If the resistance values of the resistors 39, 41, 42, and 43 are R ₂ , then in the operational amplifier 31, {Es−(E ₁ +E ₂ )}
A ₂ =E ₀ , and in the operational amplifier 11, {1/2(E ₀ + E ₂ )−1/2E ₁ }A ₀ =Ec.

Since the amplification factors A ₁ and A ₂ are both equal to infinity, Es
= E ₁ + E ₂ , E ₀ = E ₁ − _{E 2} .

Further, the following relationship can be obtained from FIG. 3 regarding the capacitance of the capacitors 14 and 15, which differentially changes depending on the measured quantity. In other words, when the amount of displacement is zero, the movable electrode 36
and the distance between the fixed electrodes 34 and 35 is equal to d ₀ ,
Let the capacity at this time be C ₀ . When the movable electrode 36 is displaced by Δd, for example to the left, the capacitors 14 and 15
The capacitance of is C ₁ =C ₀ d ₀ /d ₀ −Δd, and C ₂ =C ₀ d ₀ /d ₀ +Δd.

Therefore, the output E ₀ can be determined in the same way as the relationship for FIG. 2 above. That is, from the relationship E ₀ = E ₁ − E ₂ / E ₁ + E ₂ · Es, E ₁ = R/TEcC ₁ , E ₂ = R/TEcC _2, E ₀ = Δd/d ₀ E
s, and the output is linearly proportional to Δd.

Circuit diagrams of the first and second embodiments (Fig. 2,
Figure 4) can be modeled and written as shown in Figures 5 and 6, respectively.

In the first embodiment, for example, if a noise of Δε enters between the wave circuit 18 and the operational amplifier 31, the noise-containing output output to the terminal 32 is called E ₀ ' as shown in the equivalent circuit of FIG. Then, operational amplifier 3
In the operational amplifier 11, {Es−(E ₁ +Δε}A ₂ =E ₀ ′) acts so that (E ₀ ′−E ₂ )A ₁ =Ec in the operational amplifier 11.The amplification factors A ₁ and A ₂ are Since both are almost infinite, the relationships Es=E ₁ +Δε and E ₀ ′=E ₂ hold true.Therefore, the output E ₀ ′ is E ₀ ′=Es/E ₁ +Δε·Es.

Therefore, the ratio of error (ε ₁ ) to the output E ₀ when noise Δε is not included is given by the formula ε ₁ = E ₀ −E ₀ '/E ₀ , E ₀ = E ₂ /E ₁・Es, E ₀ It is obtained by substituting ′=E ₂ /E ₁ +Δε·Es, respectively. That is, ε ₁ =Δε/E ₁ +Δε.

The error ratio ε ₁ =Δε/E ₁ +Δε in the case of the first embodiment is compared with the error ratio ε ₀ =Δε/E ₁ in the conventional example, and since ε ₀ >ε ₀ , the present invention It can be seen that the error ratio ε ₁ in the case of the first example is small.

In the second embodiment, for example, the wave circuit 18,1
When a noise of Δε enters between the synthesized output side of the amplifier 9 and the input side of the operational amplifier 31, the noise-containing output output to the terminal 32 is E ₀ ' as shown in the equivalent circuit of FIG. If so, the following relationship is obtained since the operation is exactly the same as in the case of the first embodiment.

In the operational amplifier 31, {Es−(E ₁ +E ₂ +Δε)}A ₂ =E ₀ ′ In the operational amplifier 11, {K ₂ (E ₀ ′+E ₂ )−K ₁ E ₁ }A ₁ =Ec However, K ₁ , K ₂ are proportional coefficients, and their values are all adjusted to 1/2.

Since both the amplification factors A ₁ and A ₂ are almost infinite,
The relationship Es=E ₁ +E ₂ +Δε, E ₀ ′=E ₁ −E ₂ holds true.

Therefore, E ₀ ′=E ₁ −E ₂ /E ₁ +E ₂ +Δε·Es.

Therefore, in the second embodiment, the ratio of error ε ₂ to the output E ₀ when noise Δε is not included is ε ₂ =
It is calculated as E ₀ −E ₀ ′/E ₀ .

That is, ε ₂ =1−E ₀ ′/E ₀ =Δε/E ₁ +E ₂ +Δε.

The error ratio ε ₂ of this example is compared with the error ratio ε ₁ of the first example, and ε ₁ −ε ₂ >0, so the error ratio ε ₂ of the second example is smaller. I understand that.

Therefore, the capacitive electrical signal converter according to this invention can reduce the influence of noise compared to the conventional device shown in Fig. 1, and moreover, it uses a pair of capacitors whose capacitance changes depending on the quantity to be measured. The second embodiment, which includes a pair of capacitors whose capacitance differentially changes depending on the measured quantity, can further reduce the influence of noise than the first embodiment. It is stable and highly accurate as it does not require a power supply or a rectifier circuit.

[Brief explanation of drawings]

Fig. 1 is a connection diagram showing a conventional capacitive electric signal converter, Fig. 2 is a connection diagram showing an example of a capacitive electric signal converter according to this invention, Fig. 3 is a diagram showing an example of the structure of a differential capacitor, Figure 4 is a connection diagram showing another example of the capacitive electric signal converter according to this invention, Figure 5 is an equivalent circuit diagram of the connection diagram of Figure 2, and Figure 6 is an equivalent circuit diagram of the connection diagram of Figure 4. be. 11: Operational amplifier, 12, 13, 16, 17:
Switch, 14, 15: Capacitor, 18, 1
9: wave circuit, 31: operational amplifier, 32: output terminal, 39: reference voltage source.

Claims (1)

[Scope of utility model registration request] A pair of capacitors whose capacitance at least one changes depending on the quantity to be measured, a DC power source for charging these capacitors, and a DC electrical signal that smoothes the discharge current of the pair of capacitors and corresponds to each capacitance. and a switch for alternately connecting the pair of capacitors to the DC power supply and the wave circuit, respectively, and periodically charging and discharging the capacitors. An operational amplifier is provided as
The output of the wave circuit that supplies the discharge current of the capacitor whose capacitance does not change is input to the inverting input side of the operational amplifier, and the output of the wave circuit that supplies the discharge current of the capacitor whose capacitance changes is connected to the reference voltage. The difference is input to the non-inverting input side of the operational amplifier, or the outputs of the pair of wave circuits are input to the inverting input side and the non-inverting input side of the operational amplifier, respectively, and the outputs of the wave circuits are input, A capacitive electrical signal conversion device in which the difference between the sum of the outputs of a pair of wave circuits and a reference voltage is input to the non-inverting input side of the operational amplifier.