JPS6245485B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a displacement transducer having a variable impedance element whose impedance changes depending on the amount of displacement and a fixed impedance element that does not respond to displacement.

In the field of industrial measurement, pressure, temperature, and flow rate are the main measurement elements, and there are also devices such as differential pressure flowmeters that detect flow rate using pressure differences, and as a whole, differential pressure detectors was used very often. This differential pressure detector is an open-loop type electronic differential pressure detector that does not apply feedback to the input displacement, unlike the conventional force balance type.
Because the structure is relatively simple and high accuracy can be obtained,
It's becoming mainstream. This electronic differential pressure detector includes a differential capacitance type and a differential strain gauge type. As mentioned earlier, pressure is often measured, and so-called gauge pressure is pressure based on atmospheric pressure, and the measurement involves detecting the differential pressure between atmospheric pressure and the measured pressure using a differential pressure detector. It is done by

In a conventional capacitive differential pressure detector, a diaphragm, for example, is provided as a pressure receiving element at both ends of a cylindrical container, the pressure receiving diaphragms are connected by a connecting shaft, a moving electrode is attached to the center of the connecting shaft, and the moving electrode is sandwiched between the two pressure receiving diaphragms. Fixed electrodes are held on both sides insulated from the container, and electrical signals corresponding to the difference in capacitance between the moving electrode and one fixed electrode and between the moving electrode and the other fixed electrode are extracted. The pressure difference between one pressure receiving diaphragm and the other pressure receiving diaphragm is measured.

In this way, the conventional capacitive differential pressure detector has a symmetrical structure with respect to the central moving electrode. Adjustment between the moving electrode and one fixed electrode changes the distance between the moving electrode and the other fixed electrode, making it difficult to adjust. Creating such a symmetrical structure with high dimensional accuracy required advanced machining techniques.

From this point of view, a displacement detector has been proposed that has a variable capacitance section whose capacitance changes according to displacement and a fixed capacitance section that does not respond to displacement (this is called a single capacitance type detector). . This single capacitive detector does not require a symmetrical structure with respect to the moving electrode and is relatively simple to manufacture. However, this single capacitance type detector has the disadvantage that the output with respect to displacement is non-linear, and the output is relatively small.

Therefore, the purpose of this invention is not a symmetrical structure, but
It is an object of the present invention to provide a displacement transducer that is relatively easy to manufacture, has a linear output with respect to displacement, and can obtain a relatively large output.

According to this invention, in a displacement transducer having a variable impedance element that changes depending on the amount of displacement and a fixed impedance element that does not respond to displacement,
The output of the power source is supplied to the variable impedance element and the fixed impedance element, and the difference between the electric signals obtained by the variable impedance element and the fixed impedance element is detected by a difference detection circuit, and the power source is controlled by the detected output, and the power source is controlled by the detected output. The difference in electrical signals is maintained at a constant value.
The sum of the electrical signals obtained from each of the variable impedance elements is extracted by a sum detection circuit and is used as the output of the converter. As will be understood from a detailed explanation later, the above control provides an output that is linearly proportional to the displacement, and by using the above sum as the output, a relatively large output can be obtained.

Next, an embodiment of the displacement transducer according to the present invention will be described with reference to the drawings. First, an example of a single capacitance type detector used in this displacement transducer will be explained with reference to FIG.

This displacement detector has a hole 2 formed at one end of a container 1, and a pressure-receiving membrane 3 is placed in the hole 2. A movable electrode 5 is attached to the pressure-receiving membrane 3 via a connecting shaft 4 . The periphery of the moving electrode 5 is connected to a metal ring 7 via a spring material 6.
Supported by The metal ring 7 is held insulated within the container 1 by insulating materials 8 and 9. Fixed electrodes 10 and 11 are formed on the surface of the insulating material 9 facing the movable electrode 5 and the metal ring 7. The fixed electrode 10 has a disk shape, and its peripheral edge extends to a position facing the metal ring 7. Fixed electrode 1
1 is formed into a ring shape and is formed concentrically on the outside of the disk-shaped fixed electrode 10. This ring-shaped fixed electrode 11 faces only the metal ring 7. Metal ring 7, ring-shaped fixed electrode 11, and disk-shaped fixed electrode 1
A lead wire is connected to 0, and this lead wire is led out of the container 1 and connected to terminals 12, 13, and 14. A through hole 16 is formed in the insulating material 9 and the closed surface 15 on the other end side of the container 1, and a diaphragm 17 is stretched on the outside of the closed surface 15. That is, an incompressible liquid such as silicone oil is filled in a space surrounded by the surface on which the moving electrode 5, the metal ring 7, and the fixed electrodes 10 and 11 are formed. This sealing liquid is communicated through the through hole 16 with the empty space surrounded by the diaphragm 17 and the closed surface 15 of the container 1, and when the movable electrode 5 moves, the volume change of the space inside the container 1 is controlled by the expansion and contraction of the diaphragm 17. Absorb. It is also configured to respond to expansion and contraction of the sealing liquid due to temperature changes.

By configuring the displacement detector in this manner, the initial gap between the movable electrode and the fixed electrode can be easily adjusted by simply adjusting the gap between the movable electrode 5 and the fixed electrode 10. In other words, in the conventional differential capacitance type displacement detector, fixed electrodes are arranged on both sides of a movable electrode, so that when one gap is adjusted, the other gap also changes, making adjustment cumbersome. For this reason, each component of a differential capacitance type displacement detector must be manufactured with high precision so that the movable electrode can be mounted at the center between the fixed electrodes.
This increases manufacturing costs, making the device expensive as a whole.

However, the displacement detector described in FIG. 1 has a simple structure and is easy to manufacture and adjust, so it can be manufactured at low cost.

By the way, the capacitive element formed between the moving electrode 5 and the metal ring 7 and the fixed electrodes 10 and 11 of the displacement detector explained in FIG. 1 can be represented as shown in FIG. ₁ indicates a fixed capacitance element formed between the metal ring 7 and the fixed electrode 11, and C ₂ indicates the movable electrode 5 and the metal ring 7.
A variable capacitance element formed between the fixed electrode 10 and the fixed electrode 10 is shown. The variable capacitance element C ₂ is formed by a parallel circuit with a variable capacitance C _x formed between the moving electrode 5 and the fixed electrode 10 and a fixed capacitance C _s2 formed between the metal ring 7 and the fixed electrode 10. configured. Here, assuming that the opposing areas between the electrodes are equal to each other S ₀ , the capacitance of the fixed capacitance element C ₁ is C 1 and the capacitance of the variable capacitance element C ₁ is S 0 .
The capacitance of C ₂ is C ₂ , the displacement of the moving electrode 5 is zero, C ₁ = C _x C _s2 = C ₀ , the initial gap between the moving electrode 5 and the fixed electrode 10 is d ₀ , and the pressure-receiving membrane 3 is
If the moving electrode 5 is displaced by Δd when pressure P is applied to , C ₁ =C ₀ (1) C ₂ =C ₀ +C ₀ d ₀ /d ₀ −Δd (2). Therefore, if the difference between the capacitance value C ₀ of the fixed capacitance element C ₁ and the capacitance C ₀ +C ₀ d ₀ /d ₀ -△d of the variable capacitance element C ₂ is extracted as an electrical signal, the electrical signal is C ₀ d ₀ /d ₀ −△
d. In this way, since the displacement Δd due to the pressure P is in the denominator of the above equation representing the output electrical signal, the relationship between the electrical output and the pressure P becomes non-linear, resulting in a disadvantage that the display becomes difficult to read.

The present invention uses a circuit configuration as shown in FIG. 3 to obtain an electrical output that varies linearly with the measured pressure P and therefore with the displacement. In FIG. 3, reference numeral 18 indicates the displacement detector described in FIG.
Between terminals 12-13 and 12-1 of displacement detector 18
A fixed capacitance element C ₁ and a variable capacitance element C ₂ explained in FIG. 2 are connected between the capacitors 4 and 4, respectively. A series circuit of a diode _D1 and a resistor _R1 is connected between the terminal 13 of the displacement detector 18 and the common potential point 19, and a diode is connected between the terminal 14 and the common potential point 19.
Connect the series circuit of D ₂ and resistor R ₂ . The output terminal of an oscillator 20 serving as a power source is connected between the terminal 12 of the displacement detector 18 and the common potential point 19, and a series circuit consisting of a fixed capacitance element C ₁ -diode D ₁ -resistor R ₁ and a variable capacitance are connected. An alternating current signal is applied across each of the series circuit consisting of element C ₂ - diode D ₂ - resistor R ₂ . Therefore, fixed capacitance element
The alternating current signals flowing through C ₁ and variable capacitance element C ₂ are half-wave rectified by diodes D ₁ and D ₂ , respectively, and the impedances of fixed capacitance element C ₁ and variable capacitance element C ₂ are changed by resistors R ₁ and R, respectively. ₂ , DC voltages proportional to the respective capacitance values of the fixed capacitance element C ₁ and the variable capacitance element C ₂ are generated in the resistors R ₁ and R ₂ . Furthermore, displacement detector 18
One ends of diodes D ₃ and D 4, which are in opposite directions to the diodes D 1 and D ₂ , are connected to terminals 13 and ₁₄ , and the other ends of these diodes D _{3 and} D ₄ are commonly connected and connected through a resistor R ₃ . It is connected to the common potential point 19. A circuit consisting of these diodes D ₃ and D ₄ and a resistor R ₃ constitutes a circuit that discharges the electric charge charged in the fixed capacitance element C ₁ and the variable capacitance element C ₂ during the positive half cycle, and the resistor For example, a half-wave rectified current of a positive half cycle is made to flow through _R1 and _R2 . In addition, capacitors C ₃ , C ₄ , and C ₅ are connected in parallel to resistors R ₁ , R ₂ , and R _{3 ,} respectively, and resistors R ₁
The half-wave rectified voltage generated at ~ _R3 is smoothed.

In the embodiment of this invention, the fixed capacitance element C ₁
A control means 21 is provided to detect an electrical signal corresponding to the difference in capacitance between the variable capacitance element C2 and the variable capacitance element _C2 , and to control the amplitude of the output of the oscillator 20 so that the detected signal always has a constant value. In this example, the control means 21 includes an operational amplifier 22 that calculates the difference between the DC voltages generated across the resistors _R1 and _R2 , and an amplifier 23 that compares the output of the operational amplifier 22 with a reference voltage _E0 . Indicate the case. That is, the voltage generated across the resistor R ₁ is supplied to, for example, the inverting input terminal of the operational amplifier 22 through the resistor R ₄ . Further, the voltage generated across the resistor R ₂ is supplied to the non-inverting input terminal of the operational amplifier 22 through the resistor R ₆ . This non-inverting input terminal is connected to the common potential point 19 through a resistor _R7 ,
The output of operational amplifier 22 is negatively fed back to the inverting input terminal via resistor _R5 . Here, resistor R ₄ ,
If the resistance values of R ₅ , R ₆ , and R ₇ are chosen equally, this operational amplifier 22 operates as a subtraction circuit with a gain of 1, and the output side of the amplifier 22 has resistors R ₁ and R _{2 .}
The difference voltage between the DC voltages generated respectively can be obtained. This differential voltage is supplied to, for example, an inverting input terminal of the amplifier 23, compared with a reference voltage E ₀ of a reference voltage source 24, and the comparison output is supplied to the amplitude control terminal 20a of the oscillator 20.

Further, in this embodiment, the sum of each electric signal obtained according to the fixed capacitance element C ₁ and the variable capacitance element C ₂ is extracted. That is, the DC voltages e ₁ and e ₂ obtained across the resistors R ₁ and R ₂ are supplied to the inverting input terminal of the operational amplifier 25 through the resistors R ₈ and R ₈ ', respectively.
A sum voltage proportional to the sum of the DC voltages e ₁ and e ₂ is available at the output of the operational amplifier 25, which controls the transistor 26. The collector emitter of transistor 26 is connected through a two-wire transmission line 27-27 to a receiver 28 located at a remote location. A constant bias voltage is applied to the non-inverting input terminal side of the operational amplifier 25 by resistors _R10 and _R11 , and the opposite side of the resistor 11 from the resistor 10 is connected to the mover of the sliding resistor _VR1 . Ru. A sliding resistor VR ₁ is connected in parallel with a current detection resistor R ₁₃ inserted in series between the common potential point 19 and the transmission line 27 .

In this configuration, the initial state, that is, pressure P = 0
When _{, the fixed} capacitance element _{C 1 and the variable capacitance} element _C Select the capacitance value of ₂ or the resistance value of resistors R ₁ and R ₂ . In this initial state, when pressure P is applied to the pressure receiving membrane 3, the variable capacitance element
The capacitance value of the variable capacitor C _x of C ₂ becomes large. Therefore, the value of the current flowing through the resistor R ₂ increases, and the voltage e ₂ generated across the resistor R ₂ increases. As a result, operational amplifier 2
The output potential of No. 2 shifts in the positive polarity direction in this example. In other words, the output voltage e ₃ of the operational amplifier 22 is e ₃ +△
It rises by e in the positive direction. Therefore, the output of the amplifier 23 becomes -A△e (A is the gain of the amplifier 23) and is biased in the negative direction, and the output amplitude of the oscillator 20 is controlled to be small. The amplitude of this alternating current signal decreases. As a result, the voltages e ₁ and e ₂ generated across the resistors R ₁ and R ₂ both decrease, and the difference between them, e ₂ −e ₁ =e ₃ , becomes stable as e ₃ =E ₀ .
This operation will be explained in more detail using FIG.

A straight line A shown in FIG. 4 shows the change characteristic of the voltage e ₁ generated in the resistor R ₁ with respect to the output voltage E of the oscillator 20. In other words, the voltage e ₁ generated across the resistor R ₁ maintains a constant slope and changes linearly with respect to changes in the output voltage E of the oscillator 20. On the other hand, the straight line B ₁ is the change characteristic of the voltage e ₂ generated across the resistor R ₂ when the pressure P is zero, with respect to the output voltage E of the oscillator 20, and the straight line B 2 is the change characteristic of the voltage e 2 generated across the resistor R 2 when the pressure P is zero, and the straight line B ₂ is the change characteristic of the voltage e 2 generated across the resistor R 2 when the pressure P is zero. ₂ shows the characteristics of the voltage e ₂ generated at oscillator 20 with respect to the output voltage E of the oscillator 20. That is, when pressure is applied to the displacement detector 18, the variable capacitance C _x capacitance of the variable capacitance element C ₂ increases, so the voltage e ₂ generated in the resistor R ₂ is equal to the pressure even if the output voltage E of the oscillator 20 is the same. is higher than when it is zero, and the slope becomes steeper. Therefore, if the output voltage of the oscillator 20 that yields e ₂ −e ₁ = e ₃ = E ₀ is E ₁ when the pressure is zero, then when pressure is applied, the output voltage E ₂ is lower than E _1. e ₂ −e ₁
= e ₃ = E ₀ is obtained. In this way, fixed capacitance element
By controlling the output voltage of the oscillator 20 so that the voltage e ₂ −e ₁ corresponding to the difference between C ₁ and the variable capacitance element C ₂ always matches the reference voltage E ₀ , a voltage is generated across the resistors R ₁ and R ₂ . The DC voltages e ₁ and e ₂ change in proportion to the pressure applied to the displacement detector 18. The reason for this will be explained below. Here, to simplify the explanation, the resistance values of resistors R ₁ and R ₂ are assumed to be equal, R ₀ . resistor R ₁
Let the voltages generated at and R ₂ be e ₁ and e ₂ respectively. If the capacitive reactance of fixed capacitance element C ₁ and variable capacitance element C ₂ is large relative to the resistance value R ₀ of resistors R ₁ and R ₂ , respectively, then e ₁ and e ₂ are the capacitance values of C ₁ and C ₂ , the output voltage E of the oscillator 20, and the angular frequency ω, respectively.
Since it is proportional to the product of , the following equation (3) holds true.

e ₁ = ωEC ₁ R ₀ , e ₂ = ωEC ₂ R ₀ (3) Now calculate and see the following equation (4).

e ₁ +e ₂ /e ₂ −e ₁ =ωER ₀ (C ₁ +C ₂ )/
ER ₀ (C ₁ −C ₂ )=C ₁ +C ₂ /C ₂ −C ₁ (4) Substituting C ₂ =C ₀ +C _x and the relationship of equations (1) and (2) into equation (4), It will look like this:

Since e ₂ −e ₁ is controlled to a constant value E ₀ , equation (5) becomes as follows.

e ₁ +e ₂ =E ₀ (3-2Δd/d ₀ ) (6) The output shown in equation (6) is obtained as the output of the operational amplifier 25. This output is proportional to 2Δd, that is, it is linearly proportional to twice Δd. This output is converted into a current by the feedback action of the transistor 26 and the current detection resistor R ₁₃ on the operational amplifier 25, that is, the current I ₀ flowing through the two-wire transmission line 27 is controlled by e ₁ + e ₂ .

In FIG. 3, the difference between e ₁ and e ₂ is detected by the operational amplifier 22, and the difference between the difference output and the reference voltage E ₀ is detected by the operational amplifier 23, but as shown in FIG. It is also possible to connect the non-inverting input terminal of the operational amplifier 22 to the reference voltage source 24 through the resistor R ₁₂ so that the output of the difference between e ₂ −e ₁ and E ₀ is obtained at the output side of the operational amplifier 22 . In Fig. 5, the common potential point sides of resistors R ₁ and R ₂ are connected to common potential point 19 through a common resistor R ₁₄ , and the voltage corresponding to e ₁ + e ₂ obtained at resistor R ₁₄ is This is the case when the voltage is supplied to the operational amplifier 25 through the resistor _R8 . Power supply 3 is connected to the inverting input terminal of operational amplifier 25 through resistor _R15 .
1 is connected to add a constant voltage.

Obtaining e ₁ +e ₂ with a common resistor R ₁₄ in this way may be done in the example shown in FIG.

In the above description, the present invention was applied to a capacitive type detector, but it can also be applied to a storage gauge type detector. An example is shown in FIG. That is, one end of a strain gauge _R16 whose resistance value changes with displacement is connected to the common potential point 19 through a resistor _R2 , and one end of a resistor _R17 whose resistance value is fixed and does not respond to displacement is connected through a resistor _R1 . Common potential point 19
connected to. Strain gauge R ₁₆ and resistor
The other end of _R17 is connected to the output side of the variable constant voltage source 32 to drive the strain gauge _R16 and resistor _R17 . A resistor R ₁₆ is connected in parallel with the strain gauge R ₁₆ . Resistor at the connection point of resistors R ₁ and R ₁₇
A voltage e ₁ corresponding to the resistance value of R ₁₇ is obtained, and the resistor
A voltage _{e 2} corresponding to the resistance value R _s of the strain gauge R ₁₆ is obtained at the connection point between R 2 and the strain gauge R ₁₆ .

The difference between these voltages e ₂ −e ₁ and the reference voltage E ₀ is obtained by the operational amplifier 22, and its output controls the variable constant voltage source 32 so that e ₂ −e ₁ is maintained at a constant value. be made into Further, the voltages e _{1 and} e ₂ obtained across the resistors R 1 and R ₂ are supplied to the operational amplifier 25 through the resistors R ₈ and R ₈ ', and an output proportional to the sum e ₁ +e ₂ is obtained. In this case as well, an output linearly proportional to the displacement can be obtained.

Let me explain this point a little. For ease of explanation, the resistance values of resistors R ₁ , R ₂ , R ₁₇ , and R ₁₈ are expressed as R ₀
and the resistance value R _s of the strain gauge R ₁₆ is R ₀ (1
−kΔd). k is a constant and Δd is a displacement. Let E be the voltage of the variable constant voltage source 32. The parallel resistor R _p of resistor 16 and R ₁₈ can be expressed by the following equation.

R _p = (1-1/2-k△d) R ₀ The voltages e ₁ and e ₂ are as shown in the following equation.

e ₁ = 1/2E e ₂ = 2-k△d/3-2k△dE When the following equation is calculated from these, it becomes e ₂ +e ₁ /e ₂ -e ₁ = 7-4k△d. The voltage source 32 is set so that e ₂ −e ₁ is a constant value.
is controlled, so the output e ₁ + e ₂ is proportional to the displacement △d.
is obtained.

This invention similarly provides an inductance element whose inductance value changes according to the amount of displacement and an inductance element whose inductance value does not respond to the amount of displacement and has a fixed inductance value, obtains an electric signal according to the inductance value of these elements, and obtains the electric signal. It is also possible to control the voltage applied to the inductance element so that the difference between them is constant, and to extract the sum of electrical signals according to each inductance value as a conversion output.
In the capacitive converter shown in Figure 1, the variable capacitance
The fixed capacitor C _s2 provided in parallel with C ₂ may not be provided. In other words, the present invention can be applied even when the fixed electrode 10 is provided so as not to face the metal ring 6.

As described above, according to the present invention, the detector is not a differential type but one in which the impedance of one impedance element changes according to displacement, and therefore it is relatively simple without having a symmetrical structure. It can be configured as follows, and an output proportional to the displacement can be obtained. In addition, in FIG. 3, the operational amplifier 25
Even if only e ₁ or e ₂ is supplied to , an output linearly proportional to the displacement can be obtained, as can be easily understood by considering equations (4) and (5). In this case, the output is, for example, e ₁ = E ₀ (1-△d/d ₀ ), but since e ₁ + e ₂ is obtained in this invention, the output is E ₀ (3 -2Δd/d ₀ ), the term Δd is doubled, and a correspondingly larger output can be obtained. Furthermore, in the example shown in Fig. 6, if the voltage e ₁ is output, 3
-2k△d, and when outputting e ₁ + e ₂ , it becomes 7-4k△d as shown earlier, and the term △d is 2
It will be doubled. Note that when using a capacitive detector, in the above description, for convenience of explanation, 1/jωC ₁ and 1/jωC ₂
We used the condition that R is sufficiently larger than R ₀ . However, in actual circuits, it may be difficult to satisfy this condition. Therefore, e ₁ + e ₂ is actually as follows,
Displacement Δd is not completely linearly proportional.

Now, ω/2π = 50KHz, R ₀ = 40KΩ, C ₁ = 100pF, C ₂ = 100 (1 + d ₀ /d ₀ −△d)pF, d ₀ =0.3mm, △d
= 0 to 0.1 mm, e ₁ +e ₂ exhibits 0.85% nonlinearity with respect to Δd. This nonlinearity can be corrected by adding a fixed capacitance C _s2 to the variable capacitance C _x as shown in FIGS. 1 to 3. That is, C _s2 =(α+
1) When C ₀ , the above equations (1) and (2) become as follows.

C ₁ =C ₀ C ₂ =(α+1)C ₀ +C ₀ d ₀ /d ₀ −Δd By appropriately determining the value of α, the value of nonlinearity changes. For example, in the above numerical example, the relationship between α and nonlinearity is as follows.

Nonlinearity α=0 0.85% α=0.1 0.10% α=0.2 −0.55% Therefore, by selecting this α, that is, the fixed capacitance C _s2 , the nonlinearity can be significantly reduced.

In addition to achieving linearity in this way,
You can do it like this: That is, now the resistors R ₁ , R ₂
For each resistance value R ₁ and R ₂ , set the condition of R ₁ = R ₂ = R ₀ .
If R ₁ = (1-α) R ₀ and R ₂ = R ₀ , then becomes. By appropriately determining the value of α,
The value of nonlinearity can be changed.

ω/2π=50KHz R ₀ =40KΩ C ₁ =100pF C ₂ =100(1+d ₀ /d ₀ −△d)pF d ₀ =0.3mm△d
= 0 to 0.1 mm, the relationship between α and nonlinearity is as follows.

Nonlinearity α ₀ =0 0.85% α=0.1 0.17% α=0.2 −0.40% From this, it can be seen that the nonlinearity can be corrected by selecting α.

In addition to the method described above, non-linear correction can also be easily performed by calculating after dividing e ₁ =βe ₁ (0<β<1).

[Brief explanation of the drawing]

Fig. 1 is a sectional view showing an example of a displacement transducer used in the present invention, Fig. 2 is a sectional view illustrating its electrical configuration, and Fig. 3 is an embodiment of the electric circuit portion of the invention. 4 is a graph for explaining its operation, FIG. 5 is a connection diagram showing another embodiment of the electric circuit portion, and FIG. 6 is an example in which the present invention is applied to a strain gauge type detector. FIG. C ₁ : Fixed capacitance element, C ₂ : Variable capacitance element, 1
6: Displacement detector, 20: Oscillator, 21: A circuit that detects an electrical signal corresponding to the difference in capacitance value and controls the output amplitude of the oscillator so that the difference signal always has a constant value, 22: Difference detection calculation Amplifier 25: Operational amplifier for sum detection.

Claims (1)

[Claims] 1. In a displacement converter having a variable impedance element that changes according to the amount of displacement and a fixed impedance element that does not respond to the displacement, a power supply that supplies output to the variable impedance element and the fixed impedance element, and a power supply that supplies the output to the variable impedance element and the fixed impedance element, and a difference detection circuit that detects a difference between electrical signals according to each impedance of the impedance element; a control means that controls the power source so that the difference signal is held constant according to the detected difference signal; A displacement converter comprising an element and a sum detection circuit that detects a sum signal of electric signals according to each impedance of the fixed impedance element and outputs the signal as a converter output.