JPS625292B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in temperature measurement circuits using resistance temperature detectors, and in particular to improvements in measurement errors caused by fluctuations in measurement current caused by changes in the resistance value of resistance temperature detectors, and by currents flowing into differential amplifiers. It is an object of the present invention to provide a temperature measurement circuit using a resistance temperature sensor, which can reduce errors and the like that occur due to the temperature measurement to a sufficiently small value.

FIG. 1 shows a conventional temperature measuring circuit using a resistance temperature detector. This temperature measuring circuit connects a resistance bridge composed of resistors 1, 2, 4 and a resistance temperature detector 3 between a constant voltage source V _E and a reference potential point D, This circuit amplifies and extracts the differential voltage generated between the voltages using the differential amplifier 5. This temperature measurement circuit is widely used in general, and its characteristic feature is that the resistance temperature detector 3 is of a three-wire type, and current is passed in the same direction through the lead wires 6 and 7, and the sum of the currents is passed through the lead wires 6 and 7. In this way, the voltages E ₁ and E ₂ generated in the line resistance r of the lead wires 6 and 7 are canceled out between the terminals 9 and 10.

In general, resistance temperature detector 3 is connected by resistors 1 and 2.
The current flowing through the resistor 4 is set, and the zero point of the output voltage outputted by the resistor 4 to the output terminal 12 of the differential amplifier is set. Therefore resistors 1 and 2
is called a measurement current setting resistor, and resistor 4 is called a zero suppression resistor. Resistor 1, 2,
4 are R ₁ , R ₂ , R ₄ , and the resistance value at 0% of the measurement range of the resistance thermometer 3 is R ₃ . Generally, R ₁ = R ₂ , R ₃ = R ₄ It is set as follows. Also, the operational resistors 13, 14 and 15 of the differential amplifier 5,
When the resistance values of 16 are R _a , R _b , R _a ′, and R _b ′, R _a =R _a ′, and R _b =R _b ′ are selected.

However, the circuit shown in FIG. 1 has the following drawbacks.

(1) As the measured temperature changes, the resistance temperature detector 3
However, due to _the change in R ₃ , the measurement current flowing through the resistance temperature sensor changes, causing an error. That is, when the resistance value R _{3 of the resistance temperature detector 3} changes in accordance with the measured temperature, the value of the current flowing through the resistor 1 and the resistance temperature detector 3 changes. Due to this current fluctuation, the voltage drop E ₁ in the lead wire 6 changes, and the amount of the change becomes an error. Furthermore, the relationship between the resistance value R ₃ of the resistance temperature detector 3 and the voltage drop across the resistance temperature detector 3 becomes nonlinear. This measurement error can be reduced to some extent by selecting the relationship R ₁ >>R ₃ , but there is a limit to this.

(2) The measurement current flowing through the bridge is shunted to the operational resistors 13 and 15 of the differential amplifier 5, which causes a change in the measurement current, and this change in measurement current causes an error.

In other words, the current flowing through the operational resistors 13 and 15 of the differential amplifier is the resistance value R of the operational resistors 13 and 14.
The larger _a and R _a ' become, the smaller the error becomes. However, the resistance values of the operational resistors 13 and 15 are limited by the bias current value required by the amplifier, and cannot be increased indefinitely at present. Therefore, as long as a differential amplifier is used, there is a drawback that the error caused by the current shunted to the differential amplifier cannot be sufficiently reduced.

(3) If the lengths of the lead wires 6 and 7 of the resistance temperature detector 3 change and their resistance r changes, there is a drawback that a span error occurs.

That is, if the lengths of the leads 6 and 7 change, the measured current flowing through the bridge will change. As a result, even if the resistance temperature detector brings about a predetermined change in resistance value within a predetermined temperature range, the change in resistance value makes it impossible to obtain a predetermined voltage change, resulting in a span error.

This span error can be reduced to a certain extent by selecting the relationships R ₁ ≫ R ₃ and R ₂ ≫ R ₄ as in item (1), but lead wires 6 and 7 If the length of the span changes significantly, a corresponding span error will occur and cannot be ignored. In other words, there is usually a large difference in length between the lead wires 6 and 7 connected for adjustment during manufacturing and the lengths of the lead wires 6 and 7 when actually installed in the field. Therefore, it has the disadvantage that the span must be adjusted again while it is installed in the field.

(4) Operational resistors 13 to 1 forming the differential amplifier 5
Due to temperature fluctuations in the mutual ratio of 6, output terminal 1
2, a zero point variation of approximately R _a +R _b /R _a times proportional to the voltage drop across the zero suppression resistor R ₄ occurs.

An object of the present invention is to provide a temperature measuring circuit using a temperature measuring resistor that can eliminate the various errors and zero point fluctuations described in sections (1) to (4).

An embodiment of the present invention will be described in detail below with reference to FIG.

In this invention, a current setting resistor 2 and a zero suppression resistor are connected to a resistance bridge constituted by measuring current setting resistors 1 and 2, a temperature sensing resistor 3, and a zero suppression resistor 4. A positive feedback resistor is provided to control the potential at the connection point A between the measurement current setting resistor 1 and the temperature sensing resistor 3 so that the potential at the connection point A with the measuring current setting resistor 1 always becomes the potential at the reference potential point D. The signal is amplified by an amplifier 19 having 18.

The means 17 can be constituted by an operational amplifier. For example, an inverting input terminal of the operational amplifier is connected to a point A of the bridge, and a non-inverting input terminal is connected to a reference potential point D. Furthermore, the output of the operational amplifier is connected to terminal 1.
1 and configured to draw the current flowing through the bridge by an operational amplifier constituting means 7.

Further, the amplifier 19 applies negative feedback from the voltage dividing point of resistors 20 and 21 connected between the output terminal and the reference potential point D, and connects the positive feedback resistor 18 between the output and the non-inverting input terminal. Let the positive feedback point be point B of the bridge. Therefore, the amplifier 19 operates as an unbalanced input type amplifier with sufficiently high input impedance.

With this configuration, if the gain of the means 17 is sufficiently large, the potential at the point A of the bridge is equal to the reference potential point D.
1 is controlled. Therefore, _the current flowing through the measurement current setting resistor 2 and the zero suppression resistor 4 is V/R 2, where the resistance value of the resistor 2 is R ₂ and the voltage of the constant voltage source V _E is V. The current flowing through the resistor 2 is maintained at a constant value without being affected by the line resistance r of the lead wire 7 of the temperature resistor 3. Therefore, if the potential at point C at the lower end of the bridge is V _C , then V _C =-V/R ₂ ×(R ₄ +r) ……(1).

When the potential at point B is V _B , the output voltage V _O of the output terminal 12 is V _O =R _a +R _b /R _a ×V _B (2).

However, R _a is the resistance value of the resistor 20 and R _b is the resistance value of the resistor 21 .

The potential V _B at point B is a series circuit of the current setting resistor 1, the resistance temperature detector 3, and the line resistance r of the lead wire 6,
Since it is the potential at the connection point of the Y-shaped circuit constituted by the positive feedback resistor 18, _VB can be rewritten as follows.

Here, R _C is the resistance value of the positive feedback resistor 18.

Solving this for V _B , we get Here, if R ₁ = R ₂ = R, becomes.

In the fifth formula, (R ₃ +r) {1/R _C +1/R ₁ −
If the term 1/R _C ·(R _a +R _b )/R _a } can be made zero, there will be a proportional relationship between V _B and R ₃ .

That is, 1/R _C -1/R _C・(R _a +R _b )/R _a +1/
R ₁ =0 Therefore, R _C =R ₁・R _b /R _a =R ₁ (k-1) However, k is k=R _a +R _b /R _a , which indicates the gain of amplifier 19. .

The resistance value R _C of the positive feedback resistor 18 is R _C = R ₁ (k
-1), V _B = V/R (R ₃ - R ₄ ) Therefore, V _O = R _a + R _b /R _a・V _B = R _a +R _b /R _a・V/ R・(R ₃ − _{R 4} )…………(6
), and the output terminal 12 has the resistance value of the resistance temperature detector 3.
An output voltage V _O proportional to R ₃ can be obtained.

Therefore, according to the present invention, the point A of the bridge is held at the potential of the reference potential point D by the control means 17,
Furthermore, the resistance value R _C of the positive feedback resistor 18 of the amplifier 19
By selecting R _C =R ₁ (k-1), the positive feedback resistor 18 can compensate for changes in the measurement current due to changes in the resistance value of the temperature-measuring resistor.

In other words, even if the current flowing through the resistance temperature detector 3 decreases when the resistance value R3 of the resistance temperature detector ₃ increases, the decrease in current is compensated for by the current flowing into point B through the positive feedback resistor 18. Ru. Therefore, the current flowing through the temperature measuring resistor 3 always operates at a constant value. Therefore, as explained in section (1) of the conventional example, the voltage drop E ₁ in the lead wire 6 based on the change in the measured current
Errors resulting from changes in can be eliminated.

Furthermore, since the connection point A between the current setting resistor 2 and the zero suppression resistor 4 is held at the reference potential, an operational amplifier with an unbalanced input can be used and a differential amplifier need not be used. The input bias current of the unbalanced input type operational amplifier can be made sufficiently small compared to the measurement current flowing through the bridge. Therefore, unlike in the case of a differential amplifier, unnecessary current is not shunted from the bridge, so the disadvantages described in item (2) of the conventional example can also be eliminated.

Furthermore, since the output voltage V _O does not include the line resistance r of the lead wires 6 and 7 at all, it is not affected by large changes in the line resistance r. Therefore, even if the line resistance r changes, a span error does not occur, and the conventional drawback of item (3) can be overcome.

Further, when the resistance value R ₃ of the temperature-measuring resistor 3 is at 0% of the measurement range, the resistance value R 3 of the temperature-measuring resistor ₃ is not involved in the calculation of the gain of the amplifier 19 at all. Therefore, no zero point shift occurs due to temperature fluctuations in the operational resistors 18, 20, 21 of the amplifier 19. Therefore, the drawbacks of the conventional item (4) can also be eliminated.

As explained above, according to the present invention, the resistance change of the temperature measuring resistor 3 can be accurately converted into a voltage signal. Therefore, temperature can be measured with high accuracy. Moreover, since the circuit configuration does not have a large increase in circuit elements compared to the configuration explained in FIG. 1, it can be manufactured at low cost. Therefore, an inexpensive and highly accurate temperature measuring device can be provided.

[Brief explanation of the drawing]

FIG. 1 is a connection diagram for explaining a conventional temperature measuring device using a resistance temperature sensor, and FIG. 2 is a connection diagram showing an embodiment of the present invention. 1, 2: Measurement current setting resistor, 3: Three-wire resistance temperature detector, 4: Zero suppression resistor, V _E :
constant voltage source, 17: means for controlling the potential at point A to be the reference potential, 18: positive feedback resistor, 19:
amplifier.

Claims (1)

[Claims] 1. Two measurement current setting resistors with the same resistance value and whose one ends are commonly connected to each other, a three-wire resistance temperature detector whose one end is connected to one end of the measurement current setting resistor, and the above measurement current. One end is connected to the other end of the setting resistor, the other end is connected to the other end of the three-wire resistance temperature detector, and the value is selected as appropriate depending on the measurement temperature range. a resistance bridge composed of a
A constant voltage source that provides a constant voltage between the connection point of the two measurement current setting resistors and the reference potential point, and a constant voltage source that provides a constant voltage between the connection point of the two measurement current setting resistors and the reference potential point, and the connection point of the zero suppression resistor and measurement current setting resistor so that the reference potential is always maintained. means for controlling the potential at the connection point between the above-mentioned resistance temperature detector and the zero suppression resistor, and the potential at the connection point between the above-mentioned resistance temperature detector and the measurement current setting resistor are input, and the input point is used as a positive feedback. A temperature measuring resistor comprising an amplifier with an amplification factor of K times the connecting end of the resistor, and a resistance value of the positive feedback resistor selected to be (K-1) times the resistance value of the measuring current setting resistor. Body temperature measurement circuit.