JPS5832646B2 - pressure transmitter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a pressure transmitter using a pressure sensitive element, and more particularly to a pressure transmitter that always maintains constant linearity against changes in pressure and temperature.

FIG. 1 is a circuit diagram of a pressure transmitter using a pressure-sensitive element that has been commonly used in the past.

In the same figure, 1 and 2 are semiconductor pressure sensitive elements whose resistance values R1 and R2 change according to pressure, and this is a bridge circuit with reference resistors 3 and 4 which have a constant resistance value against changes in temperature and pressure. 5.

Reference numeral 6 denotes a constant voltage generating circuit that supplies a constant voltage to the power supply terminals a and b of the bridge circuit 5.

Reference numeral 7 denotes a differential amplifier which receives the voltage Vo output from the bridge circuit 5 as an input, and the voltage amplified here is converted into a current by a subsequent current conversion circuit 8 and amplified by a current amplification element 9.

A positive voltage is applied to the collector of the current amplifying element 9 from a power source 10 through the entire transmission line 11 .

Further, the positive polarity side voltage of the power supply 10 is made into a constant current by a constant current circuit 12, and is supplied to one feed terminal of the constant voltage generating circuit 6, the differential amplifier γ, and the voltage-current converting circuit 8. Similarly, the circuit 6. is connected via the constant voltage diode 13. T.

8 is supplied to the other power supply terminal.

The current signal amplified by the current amplification element 9 is transmitted to the transmission line 11 connected to the negative polarity side of the power supply 10 via a span adjustment resistor 14 that adjusts the span to an arbitrary span and an output confirmation resistor 15.

However, in the pressure transmitter with the circuit configuration as described above, the pressure sensing elements 1 and 2 themselves show pressure-resistance changes, and this is the case between the output terminals C2d of the bridge circuit 5 as shown in the solid line curve in FIG. As such, a non-linear voltage ■o appears with respect to changes in pressure P, and as a result, the receiving side ends up measuring pressure with an error.

Conventionally, as a means to eliminate this problem, it is possible to apply pressure to the pressure sensing elements 1 and 2 in advance and use them within a range with good linearity, or to transfer a part of the output signal of the differential amplifier 7 to the bridge circuit 5. Positive feedback is made to the power supply terminal a side of the power supply terminal A for correction.

However, with the former method, the range of good linearity is limited, resulting in a narrow operating pressure range, and with the latter method, where the pressure range is fixed, it is not possible to correct linearity. However, in the case where the pressure range moves due to the transition of the zero point, etc., there is a drawback that a span error occurs because the output voltage change range with respect to the pressure range fluctuates.

The present invention was made in order to eliminate the above-mentioned drawbacks, and the present invention has been made to obtain good linearity over a wide range of pressure changes, and to maintain constant linearity against ambient temperature changes. The present invention provides a pressure transmitter that measures changes with high precision.

Hereinafter, one embodiment of the present invention will be described with reference to FIG.

In the figure, 20 is a bridge circuit that converts changing pressure into an electrical signal and outputs it, and this has resistance values R1 and R2.
semiconductor pressure-sensitive elements 21 and 22 having the same characteristics, and reference resistors 23 and 24 that exhibit constant resistance values R8I and R82 with respect to temperature and pressure.
It is made up of.

The series resistance circuit of the resistors 23 and 24 also has a resistance value R.
83, R84f: The reference resistors 25 and 26 shown are connected in parallel.

27 is a resistance value R of the current made constant by the constant current circuit 28.
This is a constant voltage circuit that takes out a voltage in addition to a sensitivity compensation resistor 29 having C and applies it to the base of a voltage control transistor 32 via an operational amplifier 30 and a constant voltage diode 31, and takes out a constant voltage from the emitter side. The constant voltage taken out from this circuit 21 is supplied to the power supply terminals a, t) of the bridge circuit 20.

A differential amplifier 33 is connected to the output terminals c and d of the bridge circuit 20 and amplifies and outputs the current by converting the output voltage between the output terminals c and d.

34 inputs the difference voltage between the midpoint voltage of the pressure sensitive elements 21 and 22 of the bridge circuit 20 (voltage at the output terminal d) and the voltage at the midpoint terminal f of the reference resistor 25.26, and each resistance value R
This is an operational amplifier that generates an amplified output voltage at an output terminal g by an input resistor 35 and a feedback resistor 36 having i, , Rfk.

The reference voltage of this operational amplifier 34 is determined by resistors 37 and 38 indicating reference resistance values Ri' and Rf'.

39 is the output terminal g of the operational amplifier 34 and the bridge circuit 20
G-
This is a variable resistor for current control that has a resistance value RL that regulates the current that occurs when a potential difference occurs between h.

Further, this midpoint terminal is connected to the other input section of the operational amplifier 30.

Next, the operation of the pressure transmitter configured as above will be explained.

When pressure is applied to the pressure sensitive elements 21, 22, the same elements 21.2
The resistance values R1 and R2 of 2 change, and the voltage at the output terminal d also changes.

As a result, the voltage between terminals d and f changes, so the output voltage of the operational amplifier 34 also changes, which causes the variable resistor 3
The current flowing through 9 will change.

Since this current flows from the transistor 32 through the reference resistor 40, the voltage across the resistor 40 changes, and as a result,
The power supply voltage applied between the power supply terminals a and b of the bridge circuit 20 changes.

Therefore, the power supply voltage of the bridge circuit 20 changes in accordance with the change in pressure, but this can be done by changing the rate of change in the power supply voltage by adjusting the variable resistor 39.
1.22 It is possible to correct the nonlinear output of the Brifuji circuit 20 due to the nonlinearity of its own pressure-resistance change characteristics.

That is, the influence of the output voltage of the bridge circuit 20 due to the nonlinearity of the pressure sensitive element 2L22 itself can be removed by adjusting the variable resistor 39.

This point will be explained below using a mathematical formula.

That is, the resistance values R1 and R2 of the pressure sensitive elements 21 and 22 are Rl
-Ro-f(t) +lJ Roh(t)...
・・・・・・・・・(1) R2-Ro-f (t)-1R
It is expressed as oh(t) (2).

Here, f(t) and h(t) are each Ro. This is a function representing the temperature change of lRo.

In this case, if the bridge circuit 20 has a threshold voltage eVab between the power supply terminals a and b, a voltage between the terminals df and df, and a voltage between the terminals h and hb, the following equations are obtained.

By substituting equation (3) into equation (4) and transforming it, we get the following.

As a result, the nonlinearity of Vdf with respect to pressure is reduced by the variable resistance R
By setting L to an arbitrary value, f(t) can be approximately linearized.

In this case, a change in Vdf due to a temperature change can be corrected by providing a temperature gradient corresponding to vhb.

However, since the second term in the denominator similarly has a temperature gradient, even if the nonlinearity of Vdf with respect to pressure is compensated for by RL at a certain temperature, if the temperature changes, it is necessary to change the compensation value.

Therefore, by giving the following temperature characteristics 'kRi, Rf, the nonlinearity of Vdf with respect to pressure can be kept constant even if the temperature changes.

Generally h(t) h(t)−
becomes −〈1 as the temperature rises, so f(t)
f(t) f(t)>□&Cf, niwa (yoyoi.

. . 3. yo, operational amplifier h(t) It is only necessary to give the child gain G a positive temperature characteristic.

Therefore, by using an element with a positive temperature coefficient for the feedback resistor Rf, or by using an element with a negative temperature coefficient for the input resistor Ri, it is possible to obtain a pressure transmitter having a certain degree of linearity with respect to temperature changes. can.

As detailed above, according to the present invention, the difference between the midpoint terminal side output of the reference resistance circuit connected between the power supply terminals of the bridge circuit using the resistance of the pressure sensitive element and the output terminal side output of the bridge circuit. A correction resistor is inserted between the operational amplification element that inputs the input voltage and the common part of the voltage dividing resistor between the output terminals of the constant voltage circuit that supplies voltage to the bridge circuit. By adjusting this resistor, the ratio of the change in pressure applied to the bridge circuit with respect to the pressure change is changed, so even if the pressure-resistance change of the pressure-sensitive element is non-linear, the output voltage of the bridge circuit is linear. can be converted into

Furthermore, since a resistance element having a positive or negative resistance-temperature characteristic is provided as the input resistance or feedback resistance of the operational amplifier element provided on the input side of the correction resistance, the temperature dependence of the pressure-resistance change of the pressure-sensitive element can be reduced. It is possible to transmit an output voltage that can be compensated and has constant linearity with respect to temperature changes.

[Brief explanation of drawings]

Figure 1 is a circuit diagram of a conventional pressure transmitter, and Figure 2 is a circuit diagram of a conventional pressure transmitter.
The pressure-output voltage relationship characteristic diagram of the pressure transmitter shown in FIG. 3 is a circuit configuration diagram illustrating an embodiment of the pressure transmitter according to the present invention. 20... Bridge circuit, 21, 22...
・Pressure sensitive element, 23-26...Reference resistance, 27.
... Constant voltage circuit, 28 ... Constant current circuit,
29... Sensitivity compensation resistor, 30... Operational amplifier, 31... Constant voltage diode, 32...
... Voltage control transistor, 33 ... Differential amplifier, 34 ... Operational amplifier, 35 ...
...Input resistance, 36...Feedback resistance, 37,3B
...Reference resistance, 39...Variable resistance, 4
0,41...Reference resistance.

Claims (1)

[Claims] 1. In a pressure transmitter that configures a bridge circuit using the resistance of a pressure-sensitive element, converts the resistance change of the pressure-sensitive element corresponding to a pressure change into an electric signal, extracts it, amplifies it with an amplifier, and transmits it. A second reference resistance circuit is connected in parallel to the first reference resistance circuit constituting the bridge circuit.
an operational amplifier circuit that amplifies the difference voltage between the midpoint terminal voltage of the reference resistance circuit and the voltage obtained from the pressure-sensitive element side of the bridge circuit; a temperature compensation resistance circuit that compensates for the temperature dependence of the pressure sensitive element using a resistance element having resistance characteristics; and a voltage dividing reference connected between an output terminal of a constant voltage circuit that supplies a predetermined voltage to the bridge circuit. a resistor circuit; and a voltage correction resistor circuit connected between the output side of the operational amplifier circuit and a voltage dividing point of the voltage dividing reference resistor circuit, and adjusting the ratio of a change in power supply voltage to a change in pressure of the bridge circuit. A pressure transmitter comprising: