JPH06105191B2 - Input circuit of temperature measuring device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an input circuit of a temperature measuring device that inputs and processes an analog temperature signal output from a thermocouple and a three-wire resistance temperature detector.

[Conventional technology]

The thermocouple used for temperature measurement does not require any power source because it generates a thermoelectromotive force corresponding to the temperature, and the temperature of the measured object can be sensed by measuring the thermoelectromotive force generated. Is configured. For this reason, the measuring circuit is also relatively simple to build and is used for measuring relatively high temperatures.

The 3-wire resistance temperature detector supplies a constant current from the constant current source to the reference resistor and the resistance temperature detector, calculates the difference in resistance between the two resistors, and obtains the temperature of the object to be measured. It is used for temperature measurement that requires relatively low temperature measurement accuracy.

[Problems to be solved by the invention]

However, since the processing form of the detected analog temperature signal is completely different between the thermocouple and the three-wire resistance temperature detector, there is one in which the input circuit of the thermocouple and the input circuit of the three-wire resistance temperature detector can be used together. However, since they were separately configured, there was a problem that the configuration was complicated and became a constraint on instrumentation.

The present invention has been made to solve the above-mentioned problems, and an analog temperature signal from a thermocouple and an analog temperature signal from a 3-wire resistance temperature detector are input and processed with a single circuit configuration. It is an object of the present invention to obtain an input circuit of a temperature measuring device that makes it possible.

[Means for solving problems]

The input circuit of the temperature measuring device according to the present invention comprises two output lines of a thermocouple selected by the first switch means and a second output line of the thermocouple.
3 of the 3-wire resistance temperature detector selected by the switch means of
The two output lines of the two output lines are commonly connected to each other, and the three-wire resistance temperature detector is commonly used so that a specified current is applied to the three-wire resistance temperature detector from two directions. The third switch means is provided for switching between one of the output lines set to the output line and one output line that is not commonly used, and the third switch means is used to selectively switch the output line by the third switch means. The wire type resistance temperature detector is equipped with a constant current source for supplying a specified current.

[Action]

The input circuit of the temperature measuring device according to the present invention comprises two output lines of the thermocouple selected by the first switch means and three outputs of the three-wire resistance temperature detector selected by the second switch means. Since the two output wires of the wires are connected in common, the analog temperature signals from the thermocouples and the different temperature measuring sensors of the three-wire resistance temperature detector are input into a single input circuit. It is possible to process with.

Also, one of the common output lines of the three-wire resistance temperature detector is not shared with one of the common output wires of the three-wire resistance temperature detector in order to apply a specified current to the three-wire resistance temperature detector from two directions. By providing a third switch for switching between the output line and a low current source for supplying a specified current to the 3-wire resistance temperature detector through the third switch, the 3-wire resistance temperature detector is provided. It is possible to obtain the measured temperature without being affected by the wiring resistance of.

〔Example〕

FIG. 1 is a block diagram showing an input circuit of a temperature measuring device according to an embodiment of the present invention. In FIG. 1, 1 is a multiplexer composed of switches SW1 to SW3,
The switch SW1 is composed of interlocking contacts 1a to 1c, the switch SW2 is composed of interlocking contacts 2a to 2c, and the switch SW3 is composed of interlocking contacts 3a to 3c.

The thermocouple 2 is connected to the contacts 1a and 1b, and the contacts 2a to 2c have 3
The wire-type resistance temperature detector 3 is connected, and the contacts 3a to 3c are connected to the 3-wire resistance temperature detector 4 (for cold junction temperature measurement at the time of temperature measurement by a thermocouple).

5 is a circuit changeover switch, 6 is a constant current source, 7 is a switch for opening and closing the output circuit of the constant current source 6, 8 is an amplifier with variable amplification factor, 9 is an A / D converter, 10 is a central processing unit (hereinafter, CPU), a switch control signal C ₁ for opening and closing the switch 7, a multiplexer control signal C ₂ for opening and closing the switches SW1 to SW3, a switch control signal C ₃ for switching and controlling the circuit changeover switch 5, and an amplifier 8 Gain control signal C ₄ , A /
The A / D conversion control signal C _{5 of the} D converter 9 is output at a predetermined timing. r ₁ is the wiring resistance value of the 3-wire resistance temperature detector 3, r ₂ is 3
It is a wiring resistance value of the linear resistance temperature detector 4.

FIG. 2 (a) shows the input circuit of FIG. 1 in which the interlocking contacts 1a to 1c of the switch SW1 serving as the first switch means are turned on, the circuit changeover switch 5 is turned on to the terminal 5a side, and the switch 7 is turned off (these). FIG. 2B is a circuit diagram in which only the circuit portion for measuring the analog temperature signal from the thermocouple 2 is extracted. FIG. 2B shows the extraction circuit. Is an equivalent circuit of. In FIG. 2 (b), r ₁ a and r ₁ b are the resistance values of the contacts 1a and 1b,
r ₅ a ON resistance value when the circuit changeover switch 5 is turned on to the terminal 5 a side, OP is an operational amplifier, rs, rf are gain determination resistance values, and this operational amplifier OP and gain determination resistance values rs, rf
And constitute the amplifier 8.

Assuming that the leak current of the operational amplifier OP is i and the thermoelectromotive force of the thermocouple 2 is E, the following relational expression holds between the thermoelectromotive force E and the output voltage V ₀ of the operational amplifier OP. .

Here, r _TC = r ₁ a + r ₁ b + r ₅ a However, the leak current i is generally sufficiently small, and
If the switch elements are selected so that the on-resistance values of all switches are sufficiently small, the thermoelectromotive force E in the above formula (1) becomes E >>
Since r _TC · i, the above equation (1) can be regarded as the following equation (2), and the thermoelectromotive force E can be known from the output voltage V ₀ of the operational amplifier OP.

When performing the temperature detection measurement by the thermocouple 2, it is necessary to measure the cold junction temperature. Therefore, in FIG. 1, a conventional configuration in which one channel is assigned to the 3-wire resistance temperature detector 4 for measurement is used.
A cold junction temperature measuring circuit using an A / D converter may be used.

FIG. 3 (a) shows that in the input circuit of FIG. 1, the interlocking contacts 2a to 2c of the switch SW2 serving as the second switch means are turned on and the switch 7 is turned on (the states of these two switches are maintained during the measurement period). FIG. 3B is a circuit diagram in which only a circuit portion for measuring an analog temperature signal from the 3-wire resistance temperature detector 3 is extracted, and FIG. 3B is a circuit as a third switch means. Equivalent circuit diagram of the above extraction circuit when the changeover switch 5 is turned on to the terminal 5a side, FIG. 3 (c)
FIG. 6 is an equivalent circuit diagram of the extraction circuit when the circuit changeover switch 5 is turned on to the terminal 5b side.

In FIGS. 3A to 3C, the same parts as those in FIGS. 1 and 2A and 2B are designated by the same reference numerals.
Now, the ON resistance value of the contacts 2a to 2c of the multiplexer 1 is set to r ₂
a, r ₂ b, r ₂ c, the on resistance value when the circuit changeover switch 5 is turned on to the terminal 5b side is r ₅ b, and the current from the constant current source 6 is i
c, the output voltage of the operational amplifier OP when the circuit changeover switch 5 is turned on to the terminal 5a side is V ₀₁ , and the output voltage of the operational amplifier OP when the circuit changeover switch 5 is turned on to the terminal 5b side is V ₀₂ At this time, as shown in FIG. 3 (b), the following relational expression holds between the resistance R of the measuring portion of the 3-wire resistance temperature detector 3, the wiring resistance r ₁ and the output voltage V ₀₁ . .

Here, r _RTD1 = r ₂ a + r ₂ b + r ₅ a However, in general, the leakage current i is the current from the constant current source 6.
It is sufficiently smaller than i _C (ic >> i).

Therefore, the above equation (3) is replaced by the following equation (4).

Next, when the circuit changeover switch 5 is turned on to the terminal 5b side, the circuit of FIG. 3 (a) becomes an equivalent circuit of FIG. 3 (c), the wiring resistance value r ₁ and the output voltage V _{02 of the} operational amplifier OP. The following relational expression holds between and.

Here, r _RTD2 = r ₂ b + r ₂ c + r ₅ b However, since (ic >> i) as described above, the above equation (5) is replaced by the following equation (6).

Therefore, by subtracting each of the above equations (5) and (6), the following equation (7) is obtained.

Here, if a switch element such that r ₂ a ≈r ₂ c, r ₅ a ≈r ₅ b is used, the above equation (7) is replaced as follows.

Therefore, from the equation (8) above, the output voltage V of the operational amplifier OP is
By measuring ₀₁ and V ₀₂ , it is possible to obtain the measurement portion resistance value R without being affected by the wiring resistance value r ₁ .

In the above embodiment, the terminal 5a of the circuit changeover switch 5 is
The amplification factor of the amplifier 8 when it is turned on and the terminal 5b is turned on. , But it does not have to be. Further, although the current ic from the constant current source has the same value, it does not have to be the same.

FIG. 4 is a flow chart showing the operation procedure.
The CPU 10 determines whether or not the input sensor designated manually is a thermocouple (T / C) (step 4-1). If YES (T / C), the gain control signal C ₄ is sent to the amplifier 8. Then, the gain determining resistance values rs and rf of the operational amplifier OP are determined so as to satisfy the expression (2) (step 4-2).

Next, the CPU 10 outputs the switch control signal C ₁ to set the input switch 7 to the off state (step 4-3), outputs the switch control signal C ₃ and connects the circuit changeover switch 5 to the terminal 5a side. (Step 4-4), the multiplexer control signal C ₂ is output, and the interlocking contacts 1a to 1c of the switch SW1 are output.
Are selectively turned on (step 4-5). This allows
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 the thermoelectromotive force E output from the thermocouple 2 according to the temperature is stabilized, the output voltage V ₀ amplified by the amplifier 8 is subjected to A / D conversion measurement by the A / D converter 9. (Step 4
-6).

The CPU 10 then switches to switch S with multiplexer control signal C _2.
Turn off the interlocking contacts 1a-1c of W1 (step 4-7),
The digital temperature data output from the A / D converter 9 is subjected to temperature calculation based on the equation (2) (step 4-
8) The control is ended. In the above control, the 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 3-wire resistance temperature detector (RTD), the CPU 10 outputs the gain control signal C ₄ to satisfy the equation (4). The gain determination resistance values rs and rf of the operational amplifier OP are set so that
-9).

Next, the CPU 10 outputs the switch control signal C ₁ to set the switch 7 to the ON state (step 4-10), outputs the switch control signal C ₃ and connects the circuit changeover switch 5 to the terminal.
It is connected to the 5a side (step 4-11), the multiplexer control signal C ₂ is output, and the interlocking contacts 2a to 2c of the switch SW2 are selectively turned on (step 4-12). As a result, an input circuit for the 3-wire resistance temperature detector 3 shown in FIG. 3 (a) is constructed, and its equivalent circuit is as shown in FIG. 3 (b).

Under this condition, the output voltage V ₀₁ amplified by the amplifier 8 is A / D converted and measured by the A / D converter 9 after the input output from the 3-wire resistance temperature detector 3 is stabilized according to the temperature. (Step 4-13), and the digital temperature data D ₁ from the A / D converter 9 is temporarily stored in the memory in the CPU 10.

Next, the CPU 10 outputs the switch control signal C ₃ to connect the circuit changeover switch 5 to the terminal 5b side (step 4-
14).

As a result, the equivalent circuit shown in FIG. 3C is constructed. Subsequently, the CPU 10 outputs the gain control signal C ₄ and resets the gain determining resistance values rs and rf of the operational amplifier OP so as to satisfy the expression (6) (step 4-15).

Under this condition, after the input output from the 3-wire resistance temperature detector 3 is stabilized, the output voltage V ₀₂ amplified by the amplifier 8 is set to A
A / D converter 9 performs A / D conversion measurement (step 4-16),
Then, CPU 10 may output a multiplexer control signal C _2, to turn off the interlocking contacts 2a~2c switch SW2 (step 4-17).

Next, from the digital temperature data D ₂ output from the A / D converter 9 and the temperature data D ₁ already stored in the memory,
The resistance value R of the three-wire resistance temperature detector 3 is calculated based on the equation (8) (step 4-18), the desired temperature is calculated (step 4-19), and the control ends. To do.

In the above embodiment, the case where the gain setting is performed again in step 4-15 has been described, but the execution process may be omitted if necessary. Also, the above steps 4-1 to 4
Each order of -19 may be reversibly interchanged.

In the above embodiment, the case of processing a multi-channel input has been described, but the present invention is not limited to the multi-channel input, and as shown in FIG.
The thermocouple 2 and the three-wire resistance temperature detector 3 may be exchangeably connected to 5a and 5b. In this case, the multiplexer 1 in FIG. 1 is unnecessary.

When performing temperature detection measurement by the thermocouple 2, cold junction temperature compensation is necessary. Therefore, conventionally, as shown in FIG. 1, a structure in which one channel is assigned to the 3-wire resistance temperature detector 4 is used for measurement, but an independent cold junction temperature circuit is used as shown in FIG. You may comprise.

In FIG. 5, reference numeral 11 is a cold junction temperature measuring resistor, one end of which is connected to the terminal 5c side of the circuit changeover switch 5 and the other end of which is connected to the ground. For cold junction temperature compensation,
When the CPU 10 outputs the switch control signal C ₃ , connects the circuit changeover switch 5 to the terminal 5c side, and outputs the switch control signal C ₁ to turn on the switch 7, the CPU 10 for cold junction temperature measurement shown in FIG. An equivalent circuit is constructed. In FIG. 6, R ₅ c is an ON resistance value when the circuit changeover switch 5 is turned on to the terminal 5c side.

When the resistance value R of the resistance temperature detector 11 is set sufficiently larger than the ON resistance value r ₅ c, that is, if R >> r ₅ c, the resistance value R
Considering that the following formula (9) holds, Becomes

Therefore, the CPU 10 outputs the switch control signal C ₃ , connects the circuit changeover switch 5 to the terminal 5c side, outputs the switch control signal C ₁ and turns on the switch 7, and the amplifier 8 is preset. The output voltage V ₀ amplified by the different amplification factor is output to the A / D converter 9 in the subsequent stage. This A
When the digital temperature data output from the / D converter 9 is calculated based on the equation (9), the desired cold junction temperature compensation can be performed.

Note that the ON resistance value R _5C is measured in advance (the resistance temperature detector 11 is short-circuited) and the value is stored in the memory in the CPU 10, so that the same is true even if the cold junction temperature compensation is performed by calculation processing. The result of is obtained.

In the above embodiment, when the output of the thermocouple 2 is measured, the switch 7 is turned off and the constant current ic is controlled so as not to flow, but the temperature of the thermocouple may be measured even if this switch 7 is excluded. Is possible.

However, except for the switch 7, since the constant current source 6 is always connected, the wiring resistance value and the interlocking contact 1a (1
If the resistance value of the sum of the on-resistance values of b, 1c) is r _X , an erroneous electromotive force V _R represented by the following equation (10) is generated.

V _R = r _X (i _C + i) (10) Therefore, as shown in FIG. 7, the resistance value r _X and the leakage current are set so that the above-mentioned false electromotive force V _R becomes a sufficiently small value. If i and the constant current i _C are determined, the switch 7 can be omitted and the circuit configuration can be simplified.

In addition, the resistance value r _X , constant current i _C , and leakage current i are sufficiently small, and the thermocouple part is short-circuited to measure the false electromotive force V _R, which is stored in the memory inside the CPU 10 and measured. It is also possible to read and correct it at this time.

[Effects of the Invention] As described above, according to the present invention, by operating a plurality of switch means, an analog temperature signal from a thermocouple and a 3-wire resistance temperature detector from a thermocouple can be obtained with a single circuit configuration. Since the input processing is performed on the analog temperature signal, the input circuit can be shared and the circuit configuration can be simplified. Moreover, since the measurement temperature is obtained without being affected by the wiring resistance of the 3-wire resistance temperature detector, the measurement accuracy is improved. Further, since the operation of each switch means is automatically performed by the control signal from the central processing unit, there is an effect that the temperature measurement input processing can be performed by a simple operation.

[Brief description of drawings]

FIG. 1 is a block diagram showing an input circuit of a temperature measuring device according to an embodiment of the present invention, and FIG. 2 (a) is only a circuit portion for inputting an analog temperature signal from a thermocouple in the input circuit of FIG. Fig. 2 (b) is an equivalent circuit diagram of the extraction circuit, Fig. 3 (a) is an analog temperature signal input from the 3-wire resistance temperature detector from the input circuit of Fig. 1. A circuit diagram in which only the circuit portion to be extracted is extracted, FIGS. 3 (b) and 3 (c) are equivalent circuit diagrams and FIG. 4 of the extraction circuit in FIG. 3 (a) switched by closing the circuit selector switch. Is a flow chart showing an operation procedure, FIG. 5 is a block diagram showing an input circuit of a temperature measuring apparatus according to another embodiment of the present invention, and FIG. 6 is a circuit in which a cold junction resistance temperature detector is connected from the circuit of FIG. An equivalent circuit diagram in which only the circuit part is extracted. It is a diagram. In the figure, 2 is a thermocouple, 3 and 4 are 3-wire resistance temperature detectors, 5 is a circuit changeover switch, 6 is a constant current source, 7 is a switch, 8 is an amplifier, 9 is an A / D converter, and 10 is Central processing unit (CPU).

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-59-212997 (JP, A) JP-A-61-223623 (JP, A) Practical application Sho-57-34098 (JP, U)

Claims (1)

[Claims] 1. A measurement temperature is obtained from a thermocouple (2), a first switch means (SW1) for selecting the thermocouple, and a thermoelectromotive force of the thermocouple selected by the first switch means. A temperature measuring device having a signal processing device (10) is provided with a three-wire resistance temperature detector (3) and a second switch means (SW2) for selecting the three-wire resistance temperature detector, Two of the three output lines of the thermocouple selected by the first switch means and the three output lines of the three-wire resistance temperature detector selected by the second switch means. Is connected in common, and one of the common output lines of the 3-wire resistance temperature detector is used so that a specified current is applied to the 3-wire resistance temperature detector from two directions. Third switch means (SW) for switching between the output line and one output line that is not shared
5) and a constant current source for selectively supplying a specified current to the 3-wire resistance temperature detector through the third switch means, and the signal processing device includes the third switch means. The temperature measuring device is characterized in that the measured temperature is also obtained from the two output values of the three-wire resistance temperature detector obtained by switching between the two, without being affected by the wiring resistance of the three-wire resistance temperature detector. Input circuit.