JPH02198327A - Input circuit of temperature measuring apparatus - Google Patents

Abstract

PURPOSE:To provide common use of an input circuit and to improve measuring accuracy by operating several units of switching means. CONSTITUTION:In the input circuit of this apparatus, two output lines of a thermocouple 2 which are selected with a first switch SW1 are commonly connected to two output lines among three output lines of a three-wire type temperature measuring resistor 3 which are selected with a second switch SW2. In this way, the analog temperature signals from the different kinds of temperature sensors, i.e., the thermocouple 2 and the resistor 3, can be processed in the single input circuit. Specified currents are made to flow to the resistor 3 in two directions. For this purpose, a third switch 3 which switches one output line of the commonly used output lines of the resistor 3 and one output line which is not commonly used is provided. A constant current source 6 which supplies the specified currents to the resistor 3 through a fourth switch 7 is provided at the front stage of the switch SW2. In this way, the measured temperature can be obtained without being affected by the wiring resistance of the resistor 3.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an input circuit for a temperature measuring device that inputs and processes analog temperature signals output from a thermocouple and a three-wire resistance temperature sensor.

[Conventional technology]

Thermocouples used for temperature measurement do not require any power source because they generate a thermoelectromotive force corresponding to the temperature, and can sense the temperature of the object being measured by measuring the thermoelectromotive force generated. It is configured as follows. For this reason, the measurement circuit is relatively simple to assemble, and is used to measure relatively high temperatures.

In addition, with a 3-wire RTD, a constant current is supplied from a constant current source to a reference resistor and a RTD, and the temperature of the object to be measured is determined by calculating the difference in the resistance values of both resistors. It is used for temperature measurements that require relatively low temperature measurement accuracy.

[Problem that the invention seeks to solve]

However, since the processing format of the detected analog temperature signal is completely different between a thermocouple and a 3-wire RTD, there is no way to use a thermocouple input circuit and a 3-wire RTD input circuit together. Because each device was configured separately, the configuration was complicated and resulted in restrictions on instrumentation.

[Purpose of the invention]

This invention was made to solve the above-mentioned problems, and uses a single circuit configuration to input and process analog temperature signals from thermocouples and analog temperature signals from 3-wire resistance thermometers. The purpose of this invention is to obtain an input circuit for a temperature measuring device that makes it possible to do this.

[Means for solving problems]

The input circuit of the temperature measuring device according to the present invention includes an output line of two thermocouples selected by the first switch means and a second output line of the thermocouple selected by the first switch means.
3 of the 3-wire resistance temperature detector selected by the switch means of
It is configured so that two of the output lines of the main body are connected in common, and the 3-wire resistance temperature detector is connected in common to allow a specified current to flow through the 3-wire resistance temperature detector from two directions. 31#A type selectively via the fourth switch means. It is equipped with a constant current source that supplies a specified current to the resistance temperature sensor.

[Effect]

The input circuit of the temperature measuring device according to the present invention includes two output lines of a thermocouple selected by a first switch means and three output lines of a three-wire resistance temperature detector selected by a second switch means. By configuring two of the output lines to be connected in common, analog temperature signals from different types of temperature sensors such as thermocouples and 3-wire resistance thermometers can be connected to a single input circuit. It is possible to process with

In addition, a specified current is passed through the 3-wire resistance thermometer from two directions (one of the common output wires of this 3-wire resistance thermometer
3Ivi! via a third switch that switches between the main output line and one output line that is not shared, and a fourth switch at the stage before the second switch. By providing a constant current source that supplies a specified current to the RTD, the above 3.
It makes it possible to determine the measured temperature without being affected by the wiring resistance of the wire type resistance thermometer.

〔Example〕

FIG. 1 is a block diagram showing the input circuit of a temperature measuring device according to an embodiment of the present invention. In FIG. The switch SW2 is composed of interlocking contacts 2a to 2C, and the switch SW3 is composed of interlocking contacts 3a to 3c.

A thermocouple 2 is connected to the contacts 1a and 1b, and the contacts 2a to 2
A 3@ type resistance temperature detector 3 is connected to c, and contacts 3a to 3C
A three-wire resistance temperature detector 4 (for cold junction temperature measurement when temperature is measured by a thermocouple) is connected to.

A plurality of contacts 72a, 72c, 7sa that interlock with each other
, 73C.

7aa, 74C, etc. 8 is an amplifier with a variable amplification factor; 9 is an A/D converter; 10 is a central processing unit (hereinafter referred to as CPU);
), a switch control signal C1 opens and closes the changeover switch 7, a multiplexer control signal C11 controls the opening and closing of the switches SWI to SW3, a switch control signal C3 controls the switching of the circuit changeover switch 5, and a gain control signal C2 of the amplifier 8. , outputs the A/D conversion control signal C1 of the A/D converter 9 at a predetermined timing. r, is the wiring resistance value of the 3-wire resistance temperature detector 3, r is the wiring resistance value of the 3-wire resistance temperature detector 4
is the wiring resistance value.

FIG. 2(a) shows the input circuit of FIG. 1, and the interlocking contacts 1a to 1c of the first switch SW1 as the first switch means.
is turned on, the circuit changeover switch 5 is turned to the terminal 5a side, and the switch 7 is turned off (these three switch states are held during the measurement period), and the analog temperature signal from the thermocouple 2 is measured. This is a circuit diagram in which only a portion is extracted, and FIG. 2 (bl is an equivalent circuit of the extracted circuit. In FIG.
On-resistance values of a and lb, rll& are on-resistance values when the circuit changeover switch 5 is turned on to the terminal 5a side, OP is an operational amplifier, rl and rf are gain determination resistance values, and this operational amplifier OP and gain determination. The amplifier 8 is constituted by the resistance value rm + rf.

Now, if the leakage current of the operational amplifier OP is i and the thermoelectromotive force of the thermocouple 2 is E, then this thermoelectromotive force E and the operational amplifier OP
The following relational expression holds true between the output voltage Vo and the output voltage Vo.

Here, rtc = rH+ r, b + rsa However, in general, the leakage current i is sufficiently small (and if the switch elements are selected so that the on-resistance value of all switches is sufficiently high, then the equation (1) above is The thermoelectromotive force E is E > rr
c・i, and the above equation (1) can be regarded as the following equation (2), and the thermoelectromotive force E is the output voltage Vo of the operational amplifier OP.
This can be determined by converting it with the /D converter 9.

When performing temperature detection measurement using the thermocouple 2, cold junction temperature measurement is required. Therefore, the first and second diagrams utilize the conventional configuration in which the L channel is assigned to the 3-wire RTD 4 for measurement, but for example,
A cold junction temperature measuring circuit with an independent amplifier and an A/D converter may be used. FIG. 3(a) shows the input circuit of FIG. 1 in which the second switch means is used in conjunction with the °C switch SW2. A circuit section that turns on contacts 2a to 2c and turns on switch 7 (the states of these two switches are maintained during the measurement period) to measure the analog temperature signal from the 3-wire resistance temperature detector 3. FIG. 3(C) is an equivalent circuit diagram of the extraction circuit when the circuit changeover switch 5 as the third switch means is turned on to the terminal 5a side. It is an equivalent circuit diagram of the said extraction circuit when the circuit changeover switch 5 is turned on to the terminal 5b side.

The resistance value measurement of the three-wire resistance temperature detector 3 is carried out in the following two steps. That is, measurement of the resistance value of the +113 i type resistance temperature detector 3 and wiring resistance value, and e) measurement of only the wiring resistance value.

In FIGS. 3(a) to 3(c), the same parts as in FIGS. 1, 2(a) and 2(b) are designated by the same reference numerals.

First, (1) the resistance value of the 3-wire resistance temperature detector and the wiring resistance value are measured. Close switch SW2 of multiplexer l
Close the circuit changeover switch 5 to the terminal 5a side. Further, the terminal 73a of the changeover switch 7 of the constant ti source 6 is closed, and the contact 73c is opened (the state of this switch is maintained during the period).

Now set the on-resistance values of contacts 2a and 2b of multiplexer l to rth m rtb1 and connect circuit changeover switch 5 to terminal 5a.
The on-resistance value when input to the side is rsa, and the operational amplifier O
The leakage current of P is 1. The constant current from the constant current source 6 is ic,
Gain determining resistance of operational amplifier OP rs rt s
When the output voltage of the operational amplifier OP is set as Vol, there is a relationship between the measurement part resistance value R1 of the 3-wire resistance temperature detector 3, the wiring resistance value r, and the output voltage Vo, as shown in FIG. 3(b). As can be seen, the following relational expression holds true.

(R+2r, ) (ic+i)+rnrol ・i
Here r RTDI: r2a + r tb +
However, in general, the leakage current i is sufficiently small compared to the current 1c from the constant current source 6 (, i < i c, (
It is considered that R+2r, +ic+i ))>rmtn, ·i Therefore, equation (3) is replaced as follows.

Here, the on-resistance of the changeover switch 70 can be ignored because the current from the constant current source 6 is flowing.

Next, (2) only the wiring resistance value is measured. That is, the third
Figure (a) [Keep the multiplexer switch SW2 closed and close the changeover switch 5 to the contact 5b side. Further, the contact 73a of the changeover switch 7 of the constant current source 6 is opened, and the contact 73C is closed. The states of these switches are maintained during the measurement period. Now, the on-resistance values of contacts 2b and 2c of multiplexer 1 are expressed as 'tb * rte s
The on-resistance value when the circuit changeover switch 5 is turned on to the terminal 5b side is rsb s The leakage current of the operational amplifier OP is i
, the constant current of the constant current source 6 is ic, the gain determining resistance of the operational amplifier OP is rsv rf, and the output voltage of the operational amplifier OP is O! Then, the wiring resistance value r of the 3-wire resistance temperature detector 3.

As can be seen from FIG. 3(C), the following relational expression holds between the output voltage Vol and the output voltage Vol.

2rl (ic+i)+r mto! @i=-V
owr@+rf (5) Here, r 1tTD1 = rtb+rte+ rab In general, the leakage current is sufficiently small, 1((ic.

2 rl (ic+i)> r RTD2 ・i. Therefore, equation (5) becomes as follows.

2r,・ic=-Vow...
(6) r brutality! Therefore, by subtracting each side of equations (4) and (6) and solving for R, we get the following.

Reic=−”−(Vol−Vow)r,
+H R= -7-X -1L-(Vo, -Vo, )
(7) tc rl+H Therefore, since IC@rl e rf is known, 1 Vol and VO2 are converted to CP via the A/D converter 9.
The difference between the output voltages, that is, Vol −
By determining VO2, it is possible to determine the measurement unit resistance value R without being influenced by the wiring resistance value r.

In addition, in the above embodiment, the terminal 5 of the circuit changeover switch 5
The amplification factor of the amplifier 8 when inputting to the a side and when inputting to the terminal 5b side is set to be the same as □, but it is not necessary to make them the same. Further, although the current ic from the constant current source is also set to the same value, it is not necessary to set it to the same value.

Figure 4 is a flowchart showing the operating procedure.
t) In 10, the manually specified input sensor is a thermocouple (T/
Step 4-1), YES (T/
In the case of C), the gain control signal C4 is output to the amplifier 8, and the operational amplifier O
A gain determining resistance value r@rl of P is determined (step 4-2).

Next, CPυ10 outputs the switch control signal C3 to
, set the changeover switch 7 to the OFF state in step 4-
3), Output the switch control signal C8 to connect the circuit changeover switch 5 to the terminal 5a side.Step 4-4), Output the multiplexer control signal C1, and connect the circuit changeover switch 5 to the terminal 5a side.
selectively turn on the interlocking contacts 13 to IC (step 4)
-5). As a result, an input circuit for the thermocouple 2 shown in FIG. 2(a) is constructed, and its equivalent circuit is as shown in FIG. 2(b).

Under the above conditions, after stabilizing the thermoelectromotive force E output from the thermocouple 2 according to the temperature, the output voltage Vo amplified by the amplifier 8 is A/D converted and measured by the A/D converter 9 ( Step 4-6).

Next, the CPυ10 uses the multiplexer control signal Ct to turn off the interlocking contacts 1a-1c of the switch SW1 (step 4-7), and converts the digital temperature data output from the A/D converter 9 based on the equation (2) above. Temperature calculation is performed (step 4-8) and control is ended. Note that in the above control, cold junction temperature compensation is omitted.

On the other hand, if the determination in step 4-1 is NO, that is, in the case of a 3i-type resistance temperature detector (RTD), the CPU 10 outputs the gain control signal C4 so as to satisfy the equation (4). Gain determining resistance value rs of operational amplifier OP *
Set rf (4-9).

Next, the CPυ10 outputs the switch control signal C8 to set only the terminal 7aa of the changeover switch 7 to the ON state (step 4-10), and outputs the switch control signal C8 to set the circuit changeover switch 5 to the terminal 5a side. When the connection is established (step 4-11), the multiplexer control signal C1 is output to selectively turn on the interlocking contacts 2a to 2C of the switch SW2 (step 4-12). As a result, the third
An input circuit for the three-wire resistance temperature detector 3 shown in FIG. 3(a) is constructed, and its equivalent circuit is as shown in FIG. 3(bl).

Under this condition, after stabilizing the input output from the 3-wire resistance thermometer 3 according to the temperature, the output voltage Vo amplified by the amplifier 8 is measured by A/D conversion by the A/D converter 9. (step 4-13), and the digital temperature data D1 from the A/D converter 9 is temporarily stored in the memory within the CPU10.

Next, only the terminal 73C of the changeover switch 7 is turned on (step 4-14). Next, the CPU 10 outputs the switch control signal C3 to switch the circuit changeover switch 5 to the terminal 5b.
(step 4-15).

As a result, the equivalent circuit shown in FIG. Reset the value rm+rf (step 4-16).

Under this condition, after the input output from the 3-wire RTD 3 stabilizes, the output voltage Vow amplified by the amplifier 8
A/D conversion measurement is performed using the A/D converter 9 (Step 4)
-17'), and then CPU10 sends the multiplexer control signal C! is output, and the interlocking contacts 2a~ of switch SW2
2c is turned off (step 4-18). Next, all the changeover switches 7 are turned off (step 4-19).

Next, digital temperature data D! is output from the A/D converter 9. and the temperature data D already stored in the memory, based on the above equation (7), ``C3-wire type resistance temperature detector 3
The resistance value R is calculated (Step 4-20), the desired temperature calculation is executed (Step 4-21), and the control is ended.

In the above embodiment, the case where the gain is set again in step 4-16 has been described, but the execution process may be omitted if necessary. In addition, each step 4-1~
Each order of 4-21 may be reversibly replaced.

Although the above embodiment describes the case where multi-channel input is processed, the process is not limited to multi-channel input, and as shown in FIG. The wire type resistance temperature detectors 3 may be connected interchangeably. In this case, the multiplexer l in the i@1 diagram is unnecessary.

〔Effect of the invention〕

As described above, according to the present invention, by operating several switch means, an analog temperature signal from a thermocouple and an analog temperature signal from a three-wire resistance thermometer are inputted using a single circuit configuration. Since the configuration is configured to process the data, input circuits can be shared, and the circuit configuration can be simplified. And 3
Since the measurement temperature is determined without being affected by the wiring resistance of the wire resistance temperature detector, measurement accuracy is improved. In addition, since the above-mentioned switch means are configured to be operated automatically by control signals from the central processing unit,
This has the effect of allowing temperature measurement input processing to be performed with simple operations.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the input circuit of a temperature measuring device according to an embodiment of the present invention, and FIG. 2(a) shows only the circuit portion of the input circuit shown in FIG. Figure 2(b) is an equivalent circuit diagram of the extracted circuit, and Figure 3(a) is the input circuit of Figure 1, in which the analog temperature signal from the 3-wire resistance thermometer is input. Figures 3(b) and 3(c) are equivalent circuit diagrams of the extraction circuit in Figure 3(a) that has been switched by turning on the circuit changeover switch. 70 is a chart showing the operating procedure, and FIG. 5 is a block diagram showing an input circuit of a temperature measuring device according to another embodiment of the present invention. 1...Multiplexer, 2...Thermocouple, 3...3
Wire type resistance thermometer, 5...Circuit selection switch, 6...
- Constant current source, 7... Changeover switch, 8... Amplifier, 9... A/D converter, 10... Central processing unit. 1. Indication of the case 1999 Patent application 2. Name of the invention Input circuit for temperature measuring device 3. Person making the amendment @ Relationship to case No. 18682 Patent applicant address Name (666) Yamatake Honeywell Co., Ltd. 5. In the specification subject to amendment, & Detailed explanation of the invention column 6, Contents of amendment 6.1 In the specification, Inl0 page 1, lines 16 to 1
7th line r(R+2 rl+ i c + i )>r
atD, m i J to r(R+2 rt) (i c
+i)>rmto, *i J. that's all

Claims (1)

[Claims] A thermocouple (2), a first switch means (SW1) for selecting the thermocouple, and a signal processing device (10) for determining the measured temperature from the thermoelectromotive force of the thermocouple selected by the first switch means. ), comprising a three-wire resistance temperature detector (3) and a second switch means (SW2) for selecting the three-wire resistance temperature detector, wherein the first switch means The two output lines of the thermocouple selected by the switch means and two of the three output lines of the three-wire resistance temperature detector selected by the second switch means are commonly connected. The wire is configured to be connected to one of the common output lines of the three-wire resistance temperature detector in order to cause a specified current to flow through the three-wire resistance temperature detector from two directions. A third switch means (SW3) for switching between one output line and the other
) and a constant current source (6) that supplies a specified current to the three-wire resistance temperature sensor, and between the constant current source and the thermocouple (2) and between the constant current source (6). ) and one of the common output wires of the three-wire resistance temperature detector (3).
A fourth switch means (7) is provided for selectively switching between the output line of the book and one output line that is not shared, and the signal processing device (10) is connected to the fourth switch means. and a temperature measurement device characterized in that the measured temperature of the three-wire resistance temperature detector (3) is also determined from two output values of the three-wire resistance temperature detector (3) determined by switching the third switch means. input circuit.