JPS6352685B2 - - 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.

For such pressure transmitters, in addition to the variable capacitance capacitor, a fixed capacitor configured by providing another fixed electrode that does not displace due to pressure on the outer periphery of the variable capacitor has been proposed, which improves the detection characteristics. has been done.

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. The current flowing through the fixed capacitance capacitor 2 is rectified by the diode D ₂ and flows to the resistor 5, where a DC voltage e ₂ is generated. These voltages e ₁ and e ₂ are input to the ratio calculation circuit 6, where a division calculation is performed.

Now, the capacitance of variable capacitor 1 is C _x , the capacitance of fixed capacitor 2 is C _s , the output amplitude of oscillator 3 is E, the oscillation angular frequency is ω, the value of resistor 4 is R ₁ ,
If the value of resistor 5 is _R2 , then the following formula holds.

Here, assuming that the circuit conditions 1/jωC _x ≫R ₁ and 1/jωC _s ≫R ₂ are satisfied, (1) and (2) become as follows.

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

e ₂ / e ₁ = C _s R ₂ / C _x R ₁ ...(5) Here, if we set C _x = C _p (1/1-kP) and C _s = C _p , equation (5) becomes It will look like this:

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 the resistor to ensure accuracy, 1/jωC _x
Setting the circuit conditions such as ≫R ₁ , 1/jωC _s ≫R ₂ is difficult from a practical standpoint.

Therefore, unless the circuit conditions are set such that 1/jωC _x ≫R ₁ , 1/jωC _s ≫R ₂ , e ₂ /e ₁ obtained from equations (1) and (2) will be as follows. .

Here, in equation (7), C _x = C _p (1/1-kP), C _s = C _p
Substituting , 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
will no longer be proportional to.

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

In order to achieve such an object, the present invention configures a variable capacitor by adding a fixed capacitor whose capacitance does not change according to the displacement of the pressure receiving element in parallel to the variable capacitor to form a variable capacitor. The detection circuit outputs an electrical signal corresponding to the capacitance of the variable capacitance section, and the output of the oscillator is controlled so that this electrical signal is held at a constant value.

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

FIG. 2 is a cross-sectional structural diagram of a detection section of an embodiment of the pressure transmitter according to the present invention. In the figure, 10
a and 10b are the main body, 11a and 11b are the main body 10a
a fixed electrode support made of an insulating material provided therein;
12 is a metal ring held between the fixed electrode supports 11a and 11b; 13 is a leaf spring whose periphery is supported by the metal ring 12; 14 is a movable electrode attached to the leaf spring 13; and 15 is the movable electrode 14 as a main body. 10
16 is a fixed electrode formed in a ring shape on the fixed electrode support 11a; 17 is also formed in a ring shape on the outer periphery of the fixed electrode 16; The fixed electrode 18 is further connected to the fixed electrode 17
A fixed electrode also formed in a ring shape on the outer periphery of the
19 is a terminal drawn out from the moving electrode 14, 20 is a terminal drawn out from the fixed electrode 18, and 21 is a terminal drawn out from the fixed electrode 1.
Terminals pulled out from 6 and 17, 22 is the main body 10
This is a diaphragm provided on the outer surface of a.

Here, a variable capacitor is formed by the fixed electrode 16 and the moving electrode 14, an additional capacitor is formed by the fixed electrode 17 and the metal ring 12, and a variable capacitor is formed by these two capacitors connected in parallel. be done. In addition, the fixed electrode 18
A fixed capacitance capacitor for comparison is formed by the metal ring 12 and the capacitor, and this capacitor constitutes a fixed capacitance section.

In such a configuration, when pressure P is applied to the diaphragm 22, this pressure is applied to the leaf spring 13 via the internally sealed fluid and displaces it. As a result, 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.

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, 30 is the detection section explained in FIG. 2, 31 is a variable capacitance section consisting of a variable capacitance capacitor and an additional fixed capacitance capacitor connected in parallel to this, and 32 is a fixed capacitance section consisting of a fixed capacitance capacitor for comparison. be. 40 is an oscillator that supplies an alternating current signal to the detection unit 30, 41 is a control circuit consisting of an operational amplifier 42 and a reference voltage source 43, 45 is an operational amplifier, 46 is a transistor, and 47 is a receiver of a two-wire transmitter. . Also, R ₁ , R ₂ , R ₃ , R ₄ , R ₅ , R ₆ , R ₇ , R ₀
is a resistor, VR is a potentiometer, C ₁ , C ₂ , C ₃ are capacitors, and D ₁ , D ₂ , D ₃ , D ₄ are diodes.

In such a circuit configuration, an AC signal supplied from the oscillator 40 to the variable capacitance section 31 is rectified by the diode _D1 , and a capacitor is connected at both ends of the resistor _R1 .
A DC voltage e ₁ corresponding to the capacitance change of the variable capacitance section 31 smoothed by C ₁ is generated. Here, the resistor _R1 constitutes a variable capacitance detection circuit. Similarly, fixed capacity section 3
The AC signal supplied to the circuit 2 is rectified by the diode _D2 , and a DC voltage _e2 is generated across the resistor _R2 , which is smoothed by the capacitor _C2 . And the control circuit 41
In the operational amplifier 42, the voltage e ₁ is input to its inverting input terminal, and the reference voltage E _p of the reference voltage source 43 is input to its non-inverting input terminal, and both inputs are compared and _calculated . As it increases, its output becomes oscillator 4
Since it acts to reduce the amplitude of 0, the control circuit 41 controls the output of the oscillator 40 so that e ₁ =E _p at all times. In this state, voltage e ₂ , resistance R ₄ ,
A voltage-to-current conversion circuit composed of R ₅ , R ₆ , R ₇ , R ₀ , potentiometer VR, operational amplifier 45, and transistor 46 converts into current I _p flowing through the two-wire transmission line. By doing this, this output current I _p is changed to the differential pressure P applied to the moving electrode of the detection unit 30.
can be changed in proportion to That is,
If the common connection point of resistors R ₇ , R ₀ and potentiometer VR is taken as the reference potential of the voltage-current conversion circuit, and R ₄ = R ₅ = R ₆ = R ₇ , the following equation holds true.

V−e ₂ = −αE _f …(9) E _f =−R _p I _p +E ₁ …(10) However, V is the power supply voltage and E _f is the potentiometer.
The voltage applied across VR, α, is the voltage division ratio of voltage E _f . Note that the potentiometer VR has a function for setting the span of the voltage-current conversion circuit.

On the other hand, the capacitance of the variable capacitor 31 is calculated as follows: C _x = C _p (1/
1−kP)+βC _p , the capacitance of the fixed capacitance section 32 is C _s =C _p ,
When the output amplitude of the oscillator 40 is E and the oscillation angular frequency is ω, the voltages e ₁ and e ₂ are as follows.

Therefore, Now, as an example, f=ω/2π=50KHz, R ₁
= R ₂ = 20KΩ, and further set C _x = 100 [1/(1-kP) + β] pF C _s = 100pF, and the moving electrode is initially set at kP = 0 to 1/3, that is, span pressure. With the gap displaced by 1/3,
As a result of examining the relationship between the correction coefficient β and the nonlinearity NL of e ₂ /e ₁ , the following data were obtained.

When β = 0 (conventional example), NL = 0.64% When β = 0.1 (present invention), NL = -0.09% When β = 0.2 (present invention), NL = -0.71% In this way, appropriate values can be added. By adding fixed capacitance capacitors in parallel to form a variable capacitance section, the nonlinearity of e ₂ /e ₁ can be largely compensated for even if the resistors R ₁ and R ₂ are made large.

Furthermore, if we set e ₂ /e ₁ = X in equation (13), then e ₂ = e ₁
= E _p X, and the following relationship is obtained from equations (9) and (10).

V ₋ E _p X=αR _p I _p −αE ₁ ...(14) I _p =V− _{αE 1} −E _p , a voltage signal proportional to the receiving resistance R _L can be obtained.

As described above, according to the pressure transmitter according to the present invention, there is no need to establish the circuit condition of 1/jωC≫R, and a sufficient detection voltage can be obtained by the capacitance detection circuit, so that pressure can be detected accurately with a simple circuit configuration. It has the excellent effect of outputting a proportional electrical signal.

[Brief explanation of drawings]

Figure 1 is a circuit diagram of the detection section of a conventional pressure transmitter.
FIG. 2 is a cross-sectional structural diagram 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...
Fixed electrode support, 12... Metal ring, 13...
Leaf spring, 14... Moving electrode, 16, 17, 18...
... fixed electrode, 22 ... diaphragm, 30 ... detection section, 31 ... variable capacitance section, 32 ... fixed capacitance section, 40 ... oscillator, 41 ... control circuit, 42 ...
...Operation amplifier, 43...Reference voltage source.

Claims (1)

[Claims] 1. A variable capacitor whose capacitance changes according to the displacement of the pressure receiving element, and a fixed capacitor whose capacitance does not respond to the displacement, which is connected in parallel to the variable capacitor and whose capacitance value is selected so that the displacement signal with respect to the displacement has linearity. A variable capacitance section consisting of a capacitance capacitor, a fixed capacitance section consisting of a fixed capacitance capacitor that is provided adjacent to the variable capacitance section and does not respond to the displacement, and alternating current signals are supplied to each of the variable capacitance section and the fixed capacitance section. a variable capacitance detection circuit that outputs a variable capacitance signal corresponding to the capacitance of the variable capacitance section; a control circuit that controls the output of the oscillator to maintain the variable capacitance signal at a constant value; A pressure transmitter comprising a displacement detection circuit that detects the displacement signal from a current flowing through the capacitor.