JPS6362689B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention detects a change in capacitance of a variable capacitor constructed between a moving electrode such as a diaphragm that is displaced by the pressure of a fluid and a fixed electrode facing one surface of the moving electrode. to measure the pressure.
This paper relates to an improvement of a so-called single-capacity pressure transmitter.

In such a pressure transmitter, in addition to the variable capacitance capacitor, a fixed capacitance capacitor whose capacitance does not change even if the moving electrode is displaced is provided adjacent to the variable capacitor.
Devices with improved detection characteristics have been proposed in the past.

FIG. 1 is a circuit diagram of a detection section of such a conventional pressure transmitter. An alternating current is supplied from an oscillator 3 to the variable capacitor 1 and the fixed capacitor 2 in parallel. The current flowing through the variable capacitor 1 is rectified by the diode D ₁ and flows to the resistor 4, where a DC voltage e ₁ is generated. On the other hand, the current flowing through fixed capacitor 2 is diode D ₂
It is rectified and flows to the resistor 5, where a DC voltage e ₂ is generated. These voltages e ₁ and e ₂ are input to a ratio calculation circuit 6, and a division calculation is performed in this circuit.

Now, if the capacitance of variable capacitor 1 is Cx, the capacitance of fixed capacitor 2 is Cs, the output amplitude of oscillator 3 is E, the oscillation angular frequency is ω, and the resistance values of resistors 4 and 5 are R ₁ and R ₂ , respectively. , the following formula holds.

Here, assuming that the circuit conditions 1/ωCx≫R ₁ and 1/ωCs≫R ₂ hold true, equations (1) and (2) become as follows.

e ₁ =E/π・ωCxR ₁ ...(3) e ₂ =E/π・ωCsR ₂ ...(4) Then, the output of the ratio calculation circuit 6 is as follows.

e ₂ /e ₁ = CsR ₂ /CxR ₁ ... (5) Here, if Cx = Co (1/1 - kP) and Cs = Co, the equation (5) becomes as follows.

e ₂ /e ₁ = R ₂ /R ₁・(1-kP) ...(6) where k is a constant determined by the structure of variable capacitor 1, and P is the pressure applied to the moving electrode of variable capacitor 1. .

Therefore, the output signal of the ratio calculation circuit 6 becomes an electrical signal proportional to the pressure P to be measured.

However, when trying to obtain a sufficient voltage output from resistors 4 and 5 to ensure accuracy,
Setting the circuit conditions such as 1/ωCx≫R ₁ and 1/ωCs≫R ₂ is difficult from a practical standpoint. If such circuit conditions cannot be set, e ₂ /e ₁ can be calculated from equations (1) and (2) as follows.

Here, in equation (7), Cx=Co(1/1-kP), Cs=
Substituting Co, we get the following:

Thus, according to the conventional pressure transmitter, e ₂ /
The output signal of the ratio calculation circuit 6 that calculated e ₁ is the pressure P
It becomes out of proportion to.

This invention was made to solve these conventional problems, and its purpose is to provide pressure transmission that can accurately obtain an electrical signal output proportional to pressure with a simple configuration. It is about providing the equipment.

In order to achieve such an object, the present invention configures a variable capacitance section by adding an additional fixed capacitance capacitor whose capacitance does not change even with displacement of the pressure receiving element in parallel to the variable capacitor. The capacitance detection circuit outputs a voltage signal corresponding to the capacitance of the variable capacitance section, and the ratio calculation circuit calculates the ratio between the voltage signal output from the fixed capacitance detection circuit corresponding to the capacitance of the fixed capacitance section. It is something.

Hereinafter, this invention will be explained in detail based on examples.

FIG. 2 is a sectional view of a detection section of an embodiment of the pressure transmitter according to the present invention. In the figure, 10a,
10b is a main body, 11a and 11b are electrode supports made of insulating material provided in the main body 10a, 12 is a metal ring held between the electrode supports 11a and 11b, and 13 is an outer periphery fixed to the metal ring 12. 14 is a supported leaf spring; 14 is a movable electrode fixed concentrically to the leaf spring 13; 15 is a connecting shaft whose one end is attached to the center of the leaf spring 13 and the movable electrode 14 via an insulating ring 16; A bellows serves as a pressure receiving element for supporting the other end of the connecting shaft 15 on the main body 10b, 18 is a fixed electrode formed in a ring shape on the electrode support 11a, and 19 is also formed in a ring shape on the outside of the fixed electrode 18. Fixed electrode 18 inside
20 is a fixed electrode that is also formed in a ring shape on the outside of the fixed electrode 19, 21a is a metal ring 12, and a plate spring 13.
terminal, 2 connected to the moving electrode 14 via
1b, 21c are fixed electrodes 18 and 19, fixed electrode 2
0 and 22 are diaphragms. An incompressible liquid such as silicone oil is sealed in a sealed chamber within the main body 10a.

Here, a variable capacitor is formed by the moving electrode 14 and the fixed electrode 18 facing it, and an additional fixed capacitance capacitor is formed by the metal ring 12 and the fixed electrode 19 facing it, and these are connected in parallel. Both capacitors constitute a variable capacitance section. Further, a comparison fixed capacitor is formed by the metal ring 12 and the fixed electrode 20 facing thereto, and this capacitor constitutes a fixed capacitor section.

In such a configuration, when pressure is applied to the bellows 17, the leaf spring 13 and the movable electrode 14 are displaced via the connecting shaft 15, and the movable electrode 14 is thereby displaced.
The opposing gap between the fixed electrodes 18 changes, and the capacitance of the variable capacitor changes. At this time, since the metal ring 12 is not displaced, the capacitances of the additional fixed capacitance capacitor and the comparison fixed capacitance capacitor do not change. Note that due to the displacement of the leaf spring 13, the main body 10a
Although the volume of the sealed chamber inside the diaphragm 2 increases or decreases,
2 is displaced to absorb this increase and decrease, allowing the bellows 17 to expand and contract without difficulty.

Next, the operation of the electric circuit connected to such a detection section will be explained.

FIG. 3 is a circuit diagram of this electric circuit. In the figure, 23 is the movable electrode 14 and the fixed electrode 1 in FIG.
8 is a variable capacitance capacitor, 24 is an additional fixed capacitor consisting of a metal ring 12 and a fixed electrode 19, and 25 is a metal ring 12 and a fixed electrode 2.
It is a fixed capacitance capacitor for comparison consisting of 0,
The common terminal 21a of each capacitor is connected to one end of the oscillation circuit 27, the connection terminal 21b of the capacitors 23 and 24 is connected to the diode 29 and the resistor 34, and the terminal 21c of the capacitor 25 is connected to the diode 28.
and the other end of the oscillation circuit 27 via the resistor 31,
That is, they are connected to a common potential point. Therefore, alternating current is supplied from the oscillation circuit 27 to each capacitor, the current flowing through the parallel circuit of capacitors 23 and 24 flows to the resistor 34, and the current flowing to the capacitor 25 flows to the resistor 31. There is. Note that the diodes 35 and 36 and the resistor 37
constitutes a circuit that allows an alternating current to flow in the opposite direction. 32, 33, and 38 are smoothing capacitors.

Here, from the oscillation circuit 27, the capacitors 23 and 2
The alternating current supplied to the variable capacitance section consisting of four parallel circuits is rectified by the diode 29, and a DC voltage e ₁ corresponding to the capacitance of the variable capacitance section smoothed by the capacitor 32 is generated across the resistor 34. This resistance 3
4 and the capacitor 32 constitute a variable capacitance detection circuit. The alternating current supplied to the fixed capacitance section, which also consists of the capacitor 25, is rectified by the diode 28, and a DC voltage e ₂ corresponding to the capacitance of the fixed capacitance section smoothed by the capacitor 33 is generated across the resistor 31. Then, the DC voltages e ₁ and e ₂ are respectively input to the ratio calculation circuit. That is, the voltage e ₁ is inputted to the voltage duty cycle conversion circuit 42 after passing through the buffer circuit 44 , and is then input to the voltage duty cycle conversion circuit 42 , where it is connected to a resistor 45 and a switch 46 .
The signal is supplied to the inverting input terminal of a comparator 43 having hysteresis. A capacitor 47 is connected in parallel to the series circuit of a resistor 45 and a switch 46, and the inverting input terminal is connected to a common potential point via a switch 48 operating in the opposite direction to the switch 46 and a resistor 49. On the other hand, voltage e ₂ is supplied to the non-inverting input terminal of comparator 43.

Switches 46 and 48 are controlled by the inverting operation of the output of comparator 43. When switch 48 is on, voltage _e1 is charged to capacitor 47 through resistor 49, and when switch 48 is off and switch 46 is on, voltage e1 is charged to capacitor 47. The charged electric charge is transferred to the switch 46,
It is discharged through the resistor 45. If the duty cycle of the output pulse of the comparator 43 is D, the average value of the voltage charged in the capacitor 47 is De ₁ , and since both inputs of the comparator 43 operate to match, De ₁ = e ₂ Then, the relationship D=e ₂ /e ₁ is obtained.

The output pulse train of the voltage duty cycle conversion circuit 42 is converted into a voltage by a duty cycle voltage conversion circuit 51, further converted into a current by a voltage/current conversion circuit 52, and transmitted to a load. In the duty cycle voltage conversion circuit 51, the voltage of a constant voltage source 53 is sampled by a switch 54 controlled by the output pulse of a comparator 43, and is supplied to a wave reducer of a resistor 55 and a capacitor 56. Ru. Then, a voltage proportional to the duty cycle D is obtained in this wave reducing device, and this voltage is generated by the resistor 57.
to the non-inverting input terminal of the operational amplifier 58 of the voltage-current converter 52. The inverting input terminal of the operational amplifier 58 is connected to a common potential point via a resistor 59, and the output terminal is connected to the base of a transistor 61. The collector of the transistor 61 is connected to a DC power supply 64 via a terminal 62 and a transmission line 63, and the emitter is connected to a resistor 65, a current detection resistor 66,
It is connected to the other end of the DC power supply 64 via a terminal 67, a transmission line 68, and a load 69. Resistor 66 and terminal 6
The connection point 7 is connected to the non-inverting input terminal of the operational amplifier 58 via a resistor 71.

The voltage drop across the load 69 due to the current flowing therein matches the output voltage of the duty cycle voltage conversion circuit 51 and has a value proportional to e ₂ /e ₁ .

Note that the voltage at the terminal 62 is made into a constant voltage by a constant current device 72 and a Zener diode, and is supplied to each circuit.

In this way, this circuit calculates the ratio between voltages e ₁ and e ₂ without being affected by level fluctuations of the oscillation circuit 27.

Here, the capacitance of the variable capacitance section consisting of capacitors 23 and 24 is Cx = Co (1/1-kP) + βCo, and the capacitance of the fixed capacitance section consisting of capacitor 25 is Cs = Co.
The output amplitude of the oscillation circuit 27 is E, the oscillation angular frequency is ω, and the resistance values of the resistors 34 and 31 are R ₁ and R ₂ respectively.
Then, the voltages e ₁ and e ₂ are as follows.

Therefore, the ratio of e ₁ and e ₂ is as follows.

Now, as an example, if f=ω/2π=50KHz and _R1 = _R2 =20KΩ, then Cx=100(1/1−kP+β)pF Cs=100pF.

Here, in the range where kP = 0 to 1/3, that is, the movable electrode is displaced by 1/3 with respect to the initial gap with the fixed electrode due to span pressure, the relationship between β and the nonlinearity of e ₂ /e ₁ Calculating the relationship is as follows.

When β = 0 (conventional example): 0.64% When β = 0.1: -0.09% When β = 0.2: -0.71% In this way, add the additional fixed capacitance capacitor 24 of an appropriate value in parallel to form the variable capacitance section. With this configuration, even if the resistance values of the resistors 31 and 34 are made sufficiently large, the nonlinearity of e ₂ /e ₁ can be largely compensated for.

As described above, according to the pressure transmitter according to the present invention, it is not necessary to set the circuit condition of 1/ωC≫R, and a sufficiently large detection voltage can be obtained with the capacitance detection circuit, and the straight line of e ₂ /e ₁ can be obtained. This has the effect of outputting an electrical signal proportional to pressure with high accuracy with a simple circuit configuration.

[Brief explanation of drawings]

Figure 1 is a circuit diagram of the detection section of a conventional pressure transmitter.
FIG. 2 is a sectional view of a detection section of an embodiment of a pressure transmitter according to the present invention, and FIG. 3 is a circuit diagram of an electric circuit. 10a, 10b...Main body, 11a, 11b...
Electrode support, 12... Metal ring, 13... Leaf spring, 14... Moving electrode, 15... Connection shaft, 17...
...Bellows, 18, 19, 20...Fixed electrode, 2
3... Variable capacitance capacitor, 24... Additional fixed capacitance capacitor, 25... Fixed capacitance capacitor for comparison, 27... Oscillation circuit, 42... Voltage duty cycle conversion circuit, 44... Buffer circuit, 51
...Duty cycle voltage conversion circuit, 52...
Voltage current conversion circuit.

Claims (1)

[Claims] 1. A variable capacitance section consisting of a variable capacitor whose capacitance changes according to the displacement of the pressure receiving element, and an additional fixed capacitor that is configured in parallel with the variable capacitor and whose capacitance does not change even with the displacement; A fixed capacitance section is provided adjacent to the variable capacitance section and includes a comparison fixed capacitance capacitor whose capacitance does not change even with the displacement, and AC signals are supplied to each of the variable capacitance section and the fixed capacitance section. an oscillation circuit, a variable capacitance detection circuit that outputs a voltage signal corresponding to the capacitance of the variable capacitance section, a fixed capacitance detection circuit that outputs a voltage signal corresponding to the capacitance of the fixed capacitance section, and the variable capacitance detection circuit. A pressure transmitter comprising a ratio calculation circuit that calculates a ratio of voltage signals output from a fixed capacitance detection circuit.