JPH03156981A - Input processor for thermocouple - Google Patents

Abstract

PURPOSE:To quickly test thermistors by checking disconnections in the thermocouples based on the output from a CPU that calculates the difference in output between the low-pass filter selected previously and the low-pass filter selected at present. CONSTITUTION:Low-pass filters 3-1 to 3-n include capacitors C1 to Cn, which are corresponding to thermocouples 1-1 to 1-n. A multiplexer 6 selects outputs from the low-pass filters sequentially. Selected outputs from the multiplexer are converted to digital values by an A/D converter 9. A charger 8 sequentially charges the capacitors C1 to Cn corresponding to the filters, the outputs from which have been converted. A CPU 5 calculates the difference in output between the filter selected previously and the filter selected currently. The thermocouples 1-1 to 1-n are checked for disconnections based on the difference determined by the CPU 5.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a thermocouple input processing device for processing thermocouple input.

[Conventional technology]

Conventionally, when processing a thermocouple input, a circuit configuration as shown in FIG. 7 has been used to enable detection of disconnection of the thermocouple. In the same figure, ■ is a thermocouple, 2 is a measuring device, and 2 is a thermocouple.
-1 is a battery, 22 is a low-pass filter, R, is M
A high resistance on the order of Ω, r is a resistance component of the thermocouple 1, and the low-pass filter 2-2 is composed of a capacitor C and two resistors R.

In such a circuit configuration, a minute current is caused to flow from the battery 21 to the thermocouple 1 via the high resistance R. R,
If is large enough, the minute current becomes a thermoelectromotive force■. The impact on

When the thermocouple 1 is disconnected in such a state, the minute current starts charging the capacitor C of the low-pass filter 2-2. Therefore, ■ as a thermoelectromotive force. The value of V increases. It is detected that the thermocouple 1 has exceeded a predetermined value, and the thermocouple 1 is deemed to be disconnected.

[Problem to be solved by the invention]

However, according to such a circuit configuration, the thermocouple 1
It takes a long time of several tens of seconds to several minutes before a disconnection is detected. Also, V gradually increases until it is considered as a disconnection. is treated as a normal thermoelectromotive force.

Furthermore, if the measuring instrument 2 is of the input point type, a battery 2-1 is required for each thermocouple input.

[Means to solve the problem]

The present invention has been made to solve such problems, and includes a low-pass filter that includes a capacitor provided corresponding to each thermocouple input, and a system that sequentially selects the passing outputs of these low-pass filters. a selection means for converting the selected passing output of the selection means into a digital value;
A D conversion means, a charging means that sequentially charges the capacitors of the low-pass filter whose passed output was converted into a digital value by the A/D converting means, and a passed output of the low-pass filter which was selected last time and was converted into a digital value. and a calculation means for calculating the difference between the output of the low-pass filter that was selected this time and converted into a digital value, and a thermocouple disconnection is determined based on the output difference calculated by this calculation means. .

[Effect]

Therefore, according to the present invention, after the passed output of the low-pass filter is converted into a digital value, the capacitor of the low-pass filter is charged.

Then, if the thermocouple is not disconnected, until the passing output of that low-pass filter is selected in the next cycle,
Assuming that the charges stored in the capacitor of the low-pass filter are discharged,
The difference between the passed output of the low-pass filter selected last time and converted to a digital value and the passed output of the low-pass filter selected this time and converted to a digital value is usually obtained as a small value.

On the other hand, if the thermal lightning pair is disconnected, the charges stored in the capacitor of the low-pass filter will be discharged until the passing output of the low-pass filter is selected in the =4th cycle. If this is not performed, the difference between the passed output of the low-pass filter selected last time and converted into a digital value and the passed output of the low-pass filter selected this time and converted into a digital value will be obtained as a large difference.

〔Example〕

Hereinafter, the thermocouple input processing device according to the present invention will be explained in detail.

FIG. 1 is a circuit diagram showing an embodiment of this thermal lightning pair input processing device. In the same figure, 1-1 to 1-n are thermocouples, H/W is a measuring device, and 3-1 to 3-n are thermocouples 1-1 to l.
4-1 to 4-n are low-pass filters 3-1 to 3;
-n passing output (analog value) is sent to the A/D converter 9, 5 is a processor (CPU) as a control unit using a microprocessor, 6 is a switch 4-1 under the control of this CPU 5 ~4-n is a multiplexer that sequentially selects and turns on, 7 is a constant current circuit that flows a constant current i, 8 is an analog switch, 10 is a bath resistor,
11 is a RAM, 12 is a ROM, and 13 is an interface (1/F). CPU5. Multiplexer 6, A
/D converter9. Bumper register 10. RAMI 1.
ROM12. The I/Fs 13 are interconnected via a bus line BUS, and the CPU 5 performs operations as described below in accordance with programs stored in the ROM 12 or RAMII.

That is, as explained with reference to the flowchart shown in FIG. 2, the CPU 5 controls the operation of the multiplexer 6, and turns on the switch 4I, for example. As a result, the manual input from the thermocouple 1-1, that is, the output passed through the low-pass filter 3-1 is selected (step 201), and A
The signal is applied to the /D converter 9 and converted into a digital value V011 (step 202). And this A/D conversion value■
. , is the current value, and in the same way as this 6 A/D conversion value obtained last time (previous value) Difference from VoIo Δ
(step 203), and this difference Δ is a predetermined setting value Δ. If it is below, the current A/D conversion value ■. 11
is set as a database (step 204). Thereafter, the analog switch 8 is turned on for a certain period of time, and the current i is supplied to the capacitor C1 of the low-pass filter 31 via the switch 4-1, which is still on, to charge the capacitor C1 (
Step 205). Then, the operation of the multiplexer 6 is controlled, the switch 4-2 is turned on instead of the switch 4-1, and the next thermocouple input, that is, the output passed through the low-pass filter 3-2 is selected (step 206).
By repeating the above-described operations, a database is obtained for all thermocouple inputs.

After obtaining the database for the input from the thermocouples 1-n, the process returns to step 201 to obtain the database for the two-cycle period for the thermocouples 1-1 to 1-n. Here, when obtaining the database of two-period mouths,
Assuming that the thermocouple 1-1 is disconnected, the electric charge charged in the capacitor C8 in the low-pass filter 3-1 will be discharged if the thermocouple 1-1 is not disconnected; If the pair 1-1 is disconnected, it will not be discharged. Therefore, step 20
The A/D conversion value V of the second period obtained in step 2. , 2 is the previous A/D conversion value V. It becomes larger than ++. Then, in step 203, the current A/D conversion value ■. 1□ and the previous A/D conversion value ■. , 1 is the set value Δ. If it is determined that Δ has exceeded Δ, the process proceeds to step 207.

Check whether it has exceeded 2 cycles in a row. in this case,
The difference Δ between the current value and the previous value is the set value Δ. Since it has not exceeded V for two consecutive cycles, the current A/D conversion value V is determined at step 208. 1□ is temporarily stored in the buffer register 10, and the previous A/D conversion value ■. Let ++ be the database (
Step 209). After this, the analog switch 8 is turned on for a certain period of time, and the switch 4, which is still on, is turned on for a certain period of time.
-1, a current i is supplied to the capacitor C8 to charge it (step 205). The charge charged in this capacitor CI is not discharged in the next cycle, and the charge is further increased, so the A/D conversion value v at the beginning of the third cycle is
. The difference Δ between +3 and the previous A/D conversion value VOI□ temporarily stored in the buffer register 10 is set as the set value Δ in step 203. , and Δ becomes Δ. Based on the judgment result that 2 cycles have been exceeded (Step 7, Step 20)
7), thermocouple 1-1 is considered to be disconnected for the first time here (
Step 210).

Note that in the above, Δ is Δ. Although it is assumed that a disconnection is determined by detecting that the line exceeds 2 consecutive cycles, it is also possible to determine a disconnection after only one cycle, although the accuracy of the determination will be lower. As in this embodiment, Δ is Δ for two consecutive periods. If the wire breakage is determined by detecting that the temperature exceeds the set value Δ, it is possible to prevent false detection of a wire breakage due to a sudden rise in the measured temperature. It becomes possible to reduce the value of . That is, if the measured temperature is assumed to rise rapidly during the above-mentioned one scan period (for example, 2 seconds),
Δ is Δ. It is possible to exceed the However, in the next cycle, the amount of increase is Δ. If you consider that it does not exceed
Δ is Δ for two consecutive periods. This prevents a sudden rise in the measured temperature from being mistakenly determined to be a disconnection.

As described above, according to the thermocouple input processing device according to the present embodiment, the time from the time of wire breakage until the wire breakage is detected is two to three times the scan period, compared to the conventional wire breakage detection method. Therefore, the disconnection detection time can be shortened. Also,
Until the disconnection is detected, the database A/
The D conversion value does not deviate significantly. Furthermore, it is only necessary to provide one constant current circuit 7 and one analog switch 8 for a plurality of thermocouple inputs, and there is no need to provide a battery for each thermocouple input.

Figure 3 shows the change characteristics of the A/D conversion value when the thermocouple is normal (when there is no disconnection).If the measured temperature does not change to 0, the electric charge stored in the capacitor of the low-pass filter will be lost by the next cycle. Since it is discharged, the A/D variation obtained in each cycle is #! The value remains constant.

Figure 4 shows the change characteristics of the A/D conversion value when the thermocouple is abnormal (broken).
The difference Δ7 between the current value and the previous value at the beginning of the n period is Δゎ〈Δ. ,
Difference Δ between the current value and the previous value in the n+1th cycle. . 1 is Δ1
゜1≧Δ. , the difference Δ between the current value and the previous value in the n+2th cycle
□. 2 is Δ. If 2≧Δ0, then n4-2
At the beginning of the cycle, Δ becomes Δ for two consecutive cycles. This is the first time that the line is considered to be broken.

Figure 5 shows that the temperature measured by the thermocouple is one scan period (for example,
This shows the change characteristics of the A/D conversion value when it is assumed that the value rises rapidly during a period of DeA
/D conversion value rises rapidly, and the difference between the current value and the previous value is Δ. ,l
is □. , ≧Δ. So, Δ7. Δ. . 2kuΔ. There are times when this happens. In this case, according to the device of this embodiment, n+
In the second period, Δ,,□≧Δ. Therefore, it is assumed that there is a possibility that the thermocouple is disconnected, and the A/D conversion value at that time is temporarily stored. Then, the previous value (A of the n,100th cycle
/D conversion value) is bolded to represent the data. Next, in the n1003rd cycle, Δ, l+2<Δ. Therefore, Δ becomes Δ for two consecutive periods. Since it does not exceed , it is assumed that there is no disconnection, and the A/D converted value of the n + 3 period is considered valid and used as the data base. The example in Fig. 5 corresponds to, for example, when a thermocouple is placed in a high temperature state from room temperature. (after), the increase is Δ. The basic rule is that it will never exceed. Therefore, 80 should be set as a value that satisfies this condition, that is, a value that is slightly larger than this, assuming the rise width in the next cycle when the measured temperature suddenly rises. Accordingly, the set value Δ. It is possible to make the value as small as possible, which is 1 2 . Note that Δ0 can be approximately calculated from the type of thermocouple, the time constant of the low-pass filter, the heat capacity of the thermocouple itself, the maximum rate of temperature change in the measurement object (prediction is also possible), the A/D conversion period, etc.

FIG. 6 is a diagram illustrating a case where disconnection is erroneously detected even when using the device of this embodiment. L
JIn+1 and the temperature rise as shown by the dotted line in the figure between period fi+l and period n+2, that is, A/D
Assuming that a change in the converted value actually occurs (Δ., 80, 1 ≧Δ.), Δ becomes Δ for two consecutive periods. , it is considered a disconnection. However, temperature changes in the temperature measurement target in process control etc. are usually measured by A/
This is sufficiently slow compared to the D conversion period, and the possibility of false detection using the mechanism of this embodiment is extremely low. Furthermore, if a function is provided to continue checking for a disconnection even after a disconnection is detected, even if the above-mentioned erroneous detection occurs,
It is possible to escape from this false detection state.

In addition, in this embodiment, it is assumed that a disconnection is determined by detecting that Δ exceeds Δ0 for two consecutive periods.
The number of cycles may be increased to 3 cycles, 4 cycles, and so on, and these parameters can be arbitrarily selected in order to distinguish possible fluctuations in the detected process quantity from disconnections.

〔Effect of the invention〕

As explained above, according to the thermocouple input processing device according to the present invention, low-pass filters each including a capacitor provided corresponding to each thermocouple input, and outputs passed through these low-pass filters are sequentially selected. A selection means, an A/D conversion means for converting the passed output selected by the selection means into a digital value, and a capacitor of a low-pass filter whose passed output is converted into a digital value by the A/D conversion means are sequentially charged. and a calculation means for calculating the difference between the passed output of the low-pass filter selected last time and converted to a digital value, and the passed output of the low-pass filter selected this time and converted to a digital value, Since a thermocouple disconnection is determined based on the output difference calculated by the calculation means, the disconnection detection time can be shortened, and a large deviation in the A/D conversion value can be avoided until the disconnection is detected. Furthermore, excellent effects such as no need to provide a battery for each thermocouple input can be achieved.

[Brief explanation of the drawing]

Fig. 1 is a circuit configuration diagram showing an embodiment of the thermocouple input processing device according to the present invention, Fig. 2 is a flowchart for explaining the operation of this thermocouple input processing device, and Fig. 3 is a normal thermocouple input processing device. Figure 4 shows the change characteristics of the A/D conversion value when the thermocouple is abnormal (when the wire is not disconnected).
Figure 5 is a diagram showing the change characteristics of the A/D conversion value when the temperature measured by the thermocouple rises rapidly during one scan period, and Figure 6 is a diagram showing the change characteristics of the A/D conversion value. FIG. 7 is a diagram illustrating a case in which disconnection is erroneously detected even when the device of this embodiment is used, and FIG. 7 is a circuit configuration diagram showing a conventional thermocouple input processing device. 1-1 to 1-n...Thermocouple, 3-1 to 3-n...Low pass filter, 01 to C7... Capacitor, 4-1
~4-n...Switch, 5...CP [J, 6...
Multiplexer, 7... Constant current circuit, 8... Analog switch, 9... A/Dim device, 10... Buffer register, 11/W... Measuring device.

Claims (1)

[Claims] A low-pass filter including a capacitor provided corresponding to each thermocouple input, a selection means for sequentially selecting passing outputs of these low-pass filters, and converting the selected passing output of the selection means into a digital value. A/D conversion means for converting, charging means for sequentially charging the capacitors of the low-pass filter whose passed output was converted into a digital value by the A/D conversion means, and a charging means for sequentially charging the capacitor of the low-pass filter whose passed output was converted into a digital value by the A/D conversion means; Calculating means for calculating the difference between the passing output of the low-pass filter and the passing output of the low-pass filter selected this time and converted into a digital value, and determining whether the thermocouple is disconnected based on the output difference calculated by the calculating means. A thermocouple input processing device characterized by: