JPH0656335B2 - Resistance temperature detector circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a resistance temperature detector circuit that handles a signal from a resistance temperature detector as a temperature sensor, and in particular, an improvement of a three-wire resistance temperature detector circuit. Regarding

[Prior Art] Conventionally, as a resistance temperature detector circuit of this type, for example, as shown in FIG. 2, of the three lead wires extending from both ends of the resistance temperature detector 1, the first lead wire 3 is used. Is connected to the plus (+) power source through the first constant current source 5 and the variable resistor VR,
The second lead wire 7 is grounded, and the third lead wire 9 is connected to a minus (−) power source via a second constant current source 11.

In this configuration, the constant current i1 from the first constant current source 5 is applied to the first lead wire 3, the resistance temperature detector 1 and the second lead wire 7.
A constant current i2 flows through the circuit ground through the second lead wire 7, the third lead wire 9 and the second constant current source 11
Flowing through.

Therefore, the resistance value of the resistance temperature detector 1 is Rx, and the wiring resistance values of the first to third lead wires 3, 7, and 9 are r1, r2, and r, respectively.
3, and these wiring resistance values r1, r2, r3
Are equal to each other (r = r1 = r2 = r3), the measured voltage Vx by the resistance temperature detector 1 is as follows.

Vx = r1 × i1 + Rx × i1 + r2 × (i1-i2) = Rx × i1 + r (2i1-i2) (1) Therefore, the variable resistance VR is operated so that the constant current becomes 2i1 = i2. If i2 is adjusted, the equation (1) becomes Vx = Rx × i1 (2), and the wiring resistance values r1 of the first to third lead wires 3 to 9 are obtained.
The temperature can be measured from the measured voltage Vx by canceling the influence of ~ r3.

Also, a configuration as shown in FIG. 3 is known.

That is, the resistance temperature detector 1 is connected to a positive power source via the first constant current source 13, and the resistance temperature detector 1 and the first lead wire 3 are connected.
The connection point with and is connected to the fixed contact of the switch SW, the second lead wire 7 is connected to the fixed contact of the switch SW ′, and the third
Is connected to the A / D conversion circuit 17 via the amplifier circuit 15, and the control circuit 19 for selectively switching and controlling the switches SW and SW 'is connected to the A circuit. It has a configuration in which the / D conversion circuit 17 is connected.

With this configuration, the measured voltages Vy and Vz by the resistance temperature detector 1 are measured.
If the wiring resistance values of the first to third lead wires 3 to 9 are equal to each other, the switch SW is ON (closed, the same applies hereinafter) and S
When W'is OFF (open, same hereafter) and switch SW
Is OFF and SW 'is ON, Vy = i1 * r1 + i1 * Rx + i1 * r3 = i1 * Rx + 2i1 * r (2) Vz = i1 * r3 = i1 * r ... .. (3) and becomes the following from the expressions (2) and (3).

i1 × Rx = Vy−2Vz (4) Therefore, it is not necessary to adjust the variable resistance VR as in the configuration of FIG. 2 and the wiring of the first to third lead wires 3 to 9 is performed. With the resistance canceled, the control circuit 19 can calculate the measured temperature.

[Problems to be Solved by the Invention] However, in the configuration shown in FIG.
The variable resistance VR must be strictly adjusted in the initial stage of the measurement operation in order to adjust the value of .DELTA.

Further, in the configuration of FIG. 3, the first to third lead wires 3 to
9 becomes longer and the wiring resistance values r1, r2, and r3 become larger than usual values, or a safety barrier is connected in the middle of the first to third lead wires 3 to 9 for intrinsic safety measures. Therefore, when the wiring resistance values r1, r2, and r3 are substantially increased, the second term “2i1 ×” in the above-described equation (2) is used.
r ”is a very large value.

On the other hand, the amplifier circuit 15 and the A / conversion circuit 17 are generally designed so that the measurement voltages Vy and Vz in the normal measurement range are input.

For example, when platinum (Pt100) is used as the resistance temperature detector 1, the resistance value Rx is 18.49Ω when the temperature around the resistance temperature detector 1 is −200 ° C., and 100.00Ω at 0 ° C.
At 660 ° C., it becomes 332.66Ω, and the amplifier circuit 15 and the A / D conversion circuit 17 are designed to be suitable for measurement in this range.

Therefore, the wiring resistance values r of the first to third lead wires 3 to 9 are
When 1, r2, and r3 have extremely large values, the input levels to the amplifier circuit 15 and the A / D converter circuit 17, especially the amplifier circuit 15 may overflow, which may make accurate measurement difficult. .

The present invention has been made to solve the above-mentioned conventional drawbacks. In addition to requiring no initial adjustment, the input level in the measurement circuit is unlikely to overflow, and an accurate and easy-to-handle resistance temperature detector circuit is provided. For the purpose of provision.

[Means for Solving the Problem] In order to solve such a problem, the present invention has a first lead wire at one end of a resistance temperature detector and a wiring resistance equal to that of the first lead wire at the other end. Second and third lead wires are provided, the second lead wire is grounded, the first lead wire is connected to the first constant current source, and the third lead wire is connected to the second constant current source. , A first switch is inserted between the first lead wire and the first constant current source, and a second switch is inserted between the third lead wire and the second constant current source, One end of the third switch is connected between the first lead wire and the first switch, and one end of the fourth switch is connected between the third lead wire and the second switch.

Furthermore, a third and second switch, a fourth and first switch, and a control circuit for simultaneously opening and closing them by a combination of the third, first and second switches;
And a calculation circuit that calculates the voltage across the resistance temperature detector based on the voltage value input from the other end of the fourth switch according to the opening / closing operation of the fourth switch. The current from the first constant current source and the current to the second constant current source are set in opposite directions.

The present invention can also be configured such that the second constant current source supplies a constant current that is approximately twice as much as the first constant current source.

[Operation] In the present invention provided with such means, when the control circuit controls the first and fourth switches to be ON and the second and third switches to be OFF, the first constant current becomes the first constant current. Of the constant current source, the first lead wire, the resistance temperature detector and the second lead wire, and the measured voltage is output to the arithmetic circuit via the fourth switch.

Further, when the control circuit controls the second and third switches to be turned on and the first and fourth switches to be turned off,
The second constant current flows through the second lead wire, the third lead wire, and the second constant current source, and the measured voltage is output to the arithmetic circuit via the third switch.

Further, when the control circuit controls the switching of the first to third switches to ON and the fourth switch to OFF, the above-mentioned first and second constant currents flow, but the first lead is connected to the second lead wire.
Current obtained by subtracting the second constant current from the constant current of
The measured voltage is output to the arithmetic circuit via the switch.

Therefore, the input to the arithmetic circuit is reduced by the current amount obtained by subtracting the second constant current from the first constant current.

Further, if the second constant current source has a relationship of supplying a constant current almost twice as much as that of the first constant current source, the decrease in the input to the arithmetic circuit becomes the largest.

Embodiments Embodiments of the present invention will be described below with reference to the drawings. In addition,
The same parts as those in the conventional example are designated by the same reference numerals.

FIG. 1 is a circuit diagram showing an embodiment of a resistance temperature detector circuit according to the present invention.

In the figure, the fixed contact of the first switch SW1 is connected from the positive power source through the first constant current source 5, and the movable contact of the first switch SW1 is measured via the first lead wire 3 for temperature measurement. It is connected to one end of the resistor 1.

The other end of the resistance temperature detector 1 is grounded via the second lead wire 7 and is also connected to the fixed contact of the second switch SW2 via the third lead wire 9. 1 and the first to third lead wires 3 to 9 form a three-wire resistance temperature detector.

The resistance value of the resistance temperature detector 1 is Rx, and the wiring resistance values of the first to third lead wires 3 to 9 are r1, r2, r3, and these wiring resistances r1, r2, r3 are equal to each other. It has a value r.

The movable contact of the second switch SW2 is the second constant current source 11
Connected to the negative power supply via.

The fixed contact of the third switch SW3 is connected to the connection point between the movable contact of the first switch SW1 and the first lead wire 3, the movable contact is connected to the amplifier circuit 15, and the third lead wire 9 The fixed contact of the fourth switch SW4 is connected to the connection point between the fixed contact of the second switch SW2 and the fixed contact of the second switch SW2, and the movable contact is also connected to the amplifier circuit 15.

The amplifier circuit 15 includes third and fourth switches SW3 and SW.
An analog circuit that amplifies four measured voltages, and an A / D conversion circuit 1 that converts the analog amplified signal into a digital signal
7, the A / D conversion circuit 17 is connected to the control circuit 19
It is connected to the.

The control circuit 19 includes first to fourth switches SW1 to SW4.
Functioning as an arithmetic circuit that measures and calculates the temperature of the resistance temperature detector 1 from the output signal of the A / D conversion circuit 17, and executes the switching control and the arithmetic function based on a predetermined program. CPU 19
a, RO storing the operation program of this CPU 19a
It is formed by a microcomputer including an M19b, a RAM 19c for temporarily storing data of control and calculation processes in the CPU 19a, and an I / O 19d which is an interface of input / output signals.

In particular, the control circuit 19 controls the switching of the first to fourth switches SW1 to SW4 by the following combinations, and performs the measurement calculation based on the output signal from the A / D conversion circuit 17 in each switching control state. have.

That is, the control circuit 19 turns on the first and fourth switches SW1 and SW4, and turns on the second and third switches SW1 and SW4.
2 and SW3 are turned off and controlled, the second and third switches SW2 and SW3 are turned on and the first and fourth switches SW1 and SW4 are turned off, and the first to third switches SW1 are controlled. .About.SW3 is turned on and the fourth switch SW4 is turned off to control.

Next, the operation of the resistance temperature detector circuit having such a configuration will be described.

The control circuit 19 includes the first and fourth switches SW1 and SW.
4 is turned on, and the second and third switches SW2, SW3
When is turned off, the constant current i1 flows from the first constant current source 5, the first lead wire 3, the resistance temperature detector 1 and the second lead wire 7 to the circuit ground, and the third lead wire 9 is connected. Does not flow the constant current i1, the measured voltage V to the amplifier circuit 15 is
x1 is Vx1 = i1 × r (5)

Next, turn on the second and third switches SW2 and SW3.
To turn off the first and fourth switches SW1 and SW4.
When switched to, the constant current i2 flows from the circuit ground through the second lead wire 7, the third lead wire 9 and the second constant current source 11 and the constant current i2 does not flow in the resistance temperature detector 1. The measured voltage Vx2 to the amplifier circuit 15 is Vx2 = -i2 * r (6).

Further, turn on the first to third switches SW1 to SW3.
Then, when the fourth switch SW4 is turned off, the constant current i1 flows through the first constant current source 5, the first lead wire 3 and the resistance temperature detector 1, and the second lead wire 7 receives i
Since a current of 1-i2 flows, the measured voltage Vx3 of the amplifier circuit 15 is Vx3 = i1r * i1Rx + (i1-i2) r = i1Rx + (2i1-i2) r ... (7) Formula (7) is i1Rx = Vx3- (2i1-i2) r = Vx3-2Vx1-Vx2 ... (8).

Then, the measured voltages Vx1 to Vx3 are taken into the control circuit 19 via the amplifier circuit 15 and the A / D conversion circuit 17 and calculated, and the temperature is measured.

As described above, in the resistance temperature detector circuit of the present invention, the constant currents i1 and i2 are selected to be 2i1 = i2 in advance, so that
~ Wiring resistance values r1, r2, r3 of the third lead wires 3 to 7
Even if (= r) becomes considerably large, the amplifier circuit 15 and A /
The input voltage to the D conversion circuit 17 does not become large, these overflows hardly occur, and accurate measurement becomes possible.

Moreover, unlike the configuration of FIG. 2 described above, initial adjustment is not required in advance, the handling is simplified, and it is necessary to change the amplifier circuit 15 and the A / D conversion circuit 17 in the configuration of FIG. Absent.

Note that the constant currents i1 and i2 do not have to be accurately set in the relationship of 2i1 = i2, and may be selected as 2i1≈i2.
By setting in this way, (2i1-i2) r≈
It becomes 0, Vx3≈i1 × Rx, and it is possible to almost certainly prevent the overflow.

However, even if the constant currents i1 and i2 are made different, overflow is less likely to occur in a considerable range compared to the conventional case.

By the way, in the above-described embodiment of the present invention, the amplifier circuit 1
3. Although the example in which the A / D conversion circuit 17 is formed separately from the control circuit (arithmetic circuit) 19 has been described, the arithmetic circuit may have any configuration in the present invention, for example, the amplifier circuit 13 and the A / D circuit.
It can be formed including the conversion circuit 17.

[Effects of the Invention] As described above, according to the present invention, initial adjustment is not required, and even if the wiring resistance value of the resistance temperature detector increases, the input level in the measurement circuit does not easily overflow, and the measurement becomes accurate. , It is easy to handle.

Further, if the constant current from the second constant current source is set to be approximately twice that of the first constant current source, the range where the overflow does not occur becomes maximum.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a resistance temperature detector circuit according to the present invention, and FIGS. 2 and 3 are circuit diagrams showing a conventional resistance temperature detector circuit. 1 ... Resistance temperature detector 3 ... First lead wire 5 ... First constant current source 7 ... Second lead wire 9 ... Third lead wire 11 ... Second constant current source 13 ...... Constant current source 15 ...... Amplifier circuit 17 ...... A / D conversion circuit 19 ...... Control circuit (arithmetic circuit) i1, i2 ...... First and second constant current (constant current) SW, SW '... Switches SW1 to SW4 ... First to fourth switches Vx, Vx1, Vx2, Vy, Vz ... Measured voltage

Claims (2)

[Claims] 1. A first lead wire is provided at one end, and second and third lead wires having a wiring resistance equal to that of the first lead wire are provided at the other end, and the second lead wire is grounded. A resistance temperature detector, a first constant current source connected to the first lead wire, a second constant current source connected to the third lead wire, the first lead wire and the first A first switch inserted between the first constant current source, a second switch inserted between the third lead wire and a second constant current source, the first lead wire and the first switch A third switch whose one end is connected between the first switch, a fourth switch whose one end is connected between the third lead wire and the second switch, and the third and second switches , The fourth and first switches, and the combination of the third, first and second switches to open them simultaneously. A control circuit for performing a closing operation; and an operation circuit for calculating the voltage across the resistance temperature detector based on the voltage values input from the other ends of the third and fourth switches according to the opening and closing operations thereof. A resistance temperature detector circuit, comprising: a current flowing from the first constant current source flowing through the second lead wire and a current flowing to the second constant current source in opposite directions. 2. The first and second constant current sources are the second
2. The resistance temperature detector circuit according to claim 1, wherein said constant current source has a relationship of supplying a constant current that is almost doubled.