JP4321167B2 - Burnout detection circuit - Google Patents

Description

The present invention relates to a burnout detection circuit used for a thermocouple temperature measurement circuit used in industrial process measurement equipment such as a temperature converter, a recorder, and a temperature controller.

The burnout detection circuit is a sensor disconnection detection circuit of a thermocouple that is a temperature sensor. If disconnection (burnout) occurs in the thermocouple temperature sensor, the instruction and recording of the receiver may become indefinite values. For example, when a controller is used as part of a combustion plant, if a temperature lower than a normal value is output due to a sensor disconnection, fuel may be oversupplied, causing abnormal combustion, resulting in a serious accident. In addition, even when used for experiments, the data is incorrect. In order to prevent this, it is used to distinguish between data measured when the sensor is in a normal state and data measured when the sensor is disconnected. Hereinafter, a thermocouple will be described as a representative temperature sensor.

FIG. 2 shows a circuit configuration of a conventional burnout detection circuit, which will be described.
The burnout detection circuit 10 shown in FIG. 2 is connected to the thermocouple 11 via connection terminals 11a and 11b, and detects disconnection of the thermocouple 11. The semiconductor switch 13, the high impedance resistor 14 and the internal voltage A bias circuit 16 having a source 15, a semiconductor multiplexer 18, an amplifier 20, an A / D (Analog / Digital) converter 22, and a microprocessor 24 are provided, and an end opposite to the ground end of the thermocouple 11 is a resistor. It is connected to the input side of the semiconductor multiplexer 18 through an LPF (Low Pass Filter) 28 composed of a capacitor 26 and a capacitor 27. A cold junction compensation input unit 29 is further connected to the input side of the semiconductor multiplexer 18.

The bias circuit 16 is a circuit used to detect that the thermocouple 11 is disconnected by supplying a bias current to the thermocouple 11. The supply of the bias current is controlled by the ON / OFF operation of the semiconductor switch 13. The ON / OFF control of the semiconductor switch 13 is controlled by the ON / OFF control signal 30 output from the microprocessor 24. Controlled by splitting.

The thermocouple electromotive force signal 31 output from the thermocouple 11 is input to the semiconductor multiplexer 18 after low frequency noise such as commercial noise is removed by the LPF 28.
A thermocouple temperature compensation signal 32 for performing temperature compensation of the thermocouple 11 is input from the cold junction compensation input unit 29 to the semiconductor multiplexer 18. Further, the semiconductor multiplexer 18 is supplied with reference signals 33 and 34 that are used to softly correct the thermocouple electromotive force signal 31 and the thermocouple temperature compensation signal 32 to appropriate signals.

The semiconductor multiplexer 18 selects the thermocouple electromotive force signal 31, the thermocouple temperature compensation signal 32, and the reference signals 33 and 34 by time division according to the time division control signal 35 output from the microprocessor 24.
The selected signal is amplified by the amplifier 20, and the analog amplified signal 36 is converted into a digital signal 37 by the A / D converter 22 and output to the microprocessor 24. The microprocessor 24 obtains the temperature physical quantity from the digital signal 37 by calculation.

At this time, if the integration method is adopted for the A / D converter 22, the digital signal 37 can be obtained as a pulse width signal. Therefore, the microprocessor 24 measures the time of the pulse width of the pulse width signal, thereby measuring the thermocouple. 11 can measure the temperature electromotive force generated at both ends.
FIG. 3 is an operation sequence diagram of the burnout detection circuit 10.
3, the horizontal axis indicates time t, and 1, 2, 3,... Indicate the A / D conversion cycle of the A / D converter 22. In each A / D conversion cycle 1, 2, 3,..., The thermocouple electromotive force signal 31, the thermocouple temperature compensation signal 32, and the reference signals 33 and 34 are processed by the microprocessor 24 to obtain a temperature physical quantity. It has been.

Further, in each A / D conversion cycle 1, 2, 3,..., A, b, c, d shown as representative of the A / D conversion cycle 1 correspond to the ON / OFF control signal 30 from the microprocessor 24. The thermocouple electromotive force signal 31, the thermocouple temperature compensation signal 32, and the reference signals 33 and 34 are controlled by the semiconductor switch 13 that is ON / OFF controlled and the semiconductor multiplexer 18 that performs time division control according to the time division control signal 35. The timing at which any signal is output to the amplifier 20 is shown.

It is possible to continue temperature measurement by the thermocouple 11 by making such a sequence of a, b, c, d one A / D conversion cycle and continuing this cycle as 1, 2, 3,... Become.
For example, in the A / D conversion cycle 1, the semiconductor switch 13 of the bias circuit 16 is turned on to perform burnout detection. In the A / D conversion cycles 2 and 3, the semiconductor switch 13 is turned off and is generated at both ends of the thermocouple 11. It is designed as a sequence that measures the temperature electromotive force.

By such ON / OFF operation, burnout detection and temperature measurement can be performed accurately. In other words, the burnout detection circuit 10 also has a function of a temperature measurement circuit, in other words, it can be said to be a temperature measurement circuit having a burnout detection function.
In the above, according to the timing of the ON / OFF operation of the semiconductor switch 13 has been described burnout detecting circuit 10 to perform burn-out detection and temperature measurement, In addition, there is no semiconductor switch 13 shown in FIG. 2, There is another burnout detection circuit configured to always supply a bias current to the thermocouple 11. In this circuit, when the thermocouple 11 is disconnected, the burnout detection can be performed by causing the voltage resulting from the charging of only the bias current to the capacitor 27 shown in FIG. 2 to exceed a predetermined threshold. Has been.

Furthermore, in terms of temperature detection system, the thermocouple electromotive force signal 31 is used for industrial temperature control or the like in process measurement. Therefore, when the thermocouple 11 is abnormal, the system must be controlled in a safe direction (fail-safe). Don't be. However, when a minute bias current is allowed to flow through the thermocouple 11 as described above, an error signal corresponding to a voltage drop obtained by multiplying the bias current by the resistance value of the thermocouple 11 itself is generated in the thermocouple electromotive force signal 31. Since the values are added and measured by the microprocessor 24, an error has occurred in the temperature measurement result.

In the burnout detection circuit 10 shown in FIG. 2 , such a malfunction is detected when the semiconductor switch 13 for controlling the supply of the bias current is ON, and when the semiconductor switch 13 is OFF, the temperature is measured. Is solved by.
An example of this type of conventional burnout detection circuit is disclosed in Patent Document 1.
Japanese Patent No. 2567878

By the way, in the conventional burnout detection circuit, there is a case where two or more burnout detection circuits 10 are connected to one thermocouple 11 via connection terminals 11a and 11b to constitute a temperature detection system. In this case, a bias current for performing burnout detection is supplied from each burnout detection circuit 10 to one thermocouple 11. However, since the amount and direction of each bias current are different, the bias current is The influence of the voltage drop multiplied by the resistance value of the thermocouple 11 itself causes an error in temperature measurement between the burnout detection circuits 10, and accurate temperature measurement cannot be performed in each burnout detection circuit. is there.

The present invention has been made in view of such problems. Even when two or more burnout detection circuits are connected to one thermocouple to constitute a temperature detection system, each burnout detection circuit is accurate. and its object is to provide a burnout detecting circuit can be determined promptly by using an temperature measurement.

In order to achieve the above object, a burnout detection circuit according to claim 1 of the present invention detects disconnection of a temperature sensor of a thermocouple and measures temperature based on an electromotive force generated when the temperature of the temperature sensor is detected. In the burnout detection circuit, the bias means for supplying at least two bias currents having different current values to the temperature sensor, and the at least two bias currents from the bias means to the temperature sensor. Bias control means for performing either one of the first control to be individually supplied and the second control to not supply all the bias currents, and whether or not another burnout detection circuit is connected to the temperature sensor A connection presence / absence detection means for detecting whether or not the connection of another burnout detection circuit is detected by the connection presence / absence detection means. The difference between a plurality of temperature measurement values each measured at the time of individual supply of the at least two or more bias currents and a difference between the at least two or more bias currents is divided to provide the temperature sensor. A resistance value is obtained, an error corresponding to a voltage drop obtained by multiplying the resistance value by the bias current of the other burnout detection circuit is stored as a correction value, and the third temperature measured during the second control is measured. And a correction calculation means for performing correction by subtracting the stored correction value from the measured value.

According to this configuration, when one or more other burnout detection circuits other than the self burnout detection circuit are connected to the temperature sensor, the resistance value of the temperature sensor is multiplied by the bias current of the other burnout detection circuit. An error corresponding to the voltage drop is obtained and stored as a correction value. Then, when the temperature is measured without supplying at least two or more bias currents of the self burnout detection circuit, if the previously stored correction value is subtracted from this temperature measurement value, it is included in the temperature measurement value. Since the above-described error of the other burnout detection circuit is eliminated, an appropriate temperature measurement value can be obtained.

  As described above, according to the present invention, when one or more other burnout detection circuits are connected to the temperature sensor in addition to the self burnout detection circuit, the bias current of the other burnout detection circuit is added to the resistance value of the temperature sensor. An error corresponding to the voltage drop multiplied by is obtained and stored as a correction value. Then, when the temperature is measured without supplying the first and second bias currents of the self burnout detection circuit, if the previously stored correction value is subtracted from this temperature measurement value, it is included in the temperature measurement value. Since the above-described error of the other burnout detection circuit is eliminated, an appropriate temperature measurement value can be obtained.

Therefore, even when two or more burnout detection circuits are connected to one thermocouple to constitute a temperature detection system, there is an effect that accurate temperature measurement can be performed in each burnout detection circuit.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(First embodiment)
FIG. 1 is a circuit configuration diagram of a burnout detection circuit according to the first embodiment of the present invention. However, in the present embodiment shown in FIG. 1, the same reference numerals are given to portions corresponding to those of the conventional example shown in FIG. 2, the description thereof is omitted.
The burnout detection circuit 40 shown in FIG. 1 is different from the conventional burnout detection circuit 10 shown in FIG. 2 in the bias circuit 41 and the microprocessor 42.

The bias circuit 41 connects the first and second semiconductor switches 13a and 13b in parallel with the internal voltage source 15 via the first and second high impedance resistors 14a and 14b having different resistance values. Thus, the first and second bias currents having different current values are supplied to the thermocouple 11.
The microprocessor 42 is configured by connecting a bias current switching control unit 46, an external circuit connection presence / absence detection unit 47, a correction calculation unit 48, and a correction value storage unit 49 to a main control unit 44 that performs overall control. Has been.

The bias current switching control unit 46 performs control to turn on / off the semiconductor switches 13a and 13b individually in a time-sharing manner by the bias current switching control signal 51. In normal temperature measurement, each bias current is not supplied in the same manner as described with reference to FIG. 3 in the conventional example, so both semiconductor switches 13a and 13b are turned off. Further, at the time of detecting burnout, one of the semiconductor switches 13a and 13b is turned on.

The external circuit connection presence / absence detection unit 47 is a burnout detection circuit (not shown) other than the own burnout detection circuit 40 to be connected to the connection terminals 11a and 11b of the thermocouple 11 (hereinafter referred to as other burnout detection circuit). Is to detect whether or not is connected.
The correction value storage unit 49 stores a correction value obtained by the correction calculation unit 48 as described later.

When the external circuit connection presence / absence detection unit 47 detects that the other burnout detection circuit is connected, the correction calculation unit 48 multiplies the bias current in the other burnout detection circuit by the resistance value of the thermocouple 11 itself. An error corresponding to the voltage drop is calculated and calculated as described later. The obtained error is stored in the correction value storage unit 49 as a correction value, and when the temperature is measured by the thermocouple electromotive force signal 31 in the own burnout detection circuit 40, the temperature measurement value is used by using the stored correction value. Correction is performed.

Next, various calculations performed by the correction calculation unit 48 until the correction is performed will be described.
First, as shown in FIG. 1, it is assumed that only the self burnout detection circuit 40 is connected to the thermocouple 11. In this case, when two types of bias currents are supplied at different timings, a voltage drop obtained by multiplying each of the first bias current iB1 and the second bias current iB2 by the resistance value r (tc) of the thermocouple 11 itself. Each error r (tc) × iB1 and r (tc) × iB2 corresponding to the minute is added to the electromotive force mV (tc) by the thermocouple electromotive force signal 31, and these results are measured by the microprocessor 42. . The temperature measurement value in this case is obtained as in the following formulas (1) and (2).

First temperature measurement value = mV (tc) + r (tc) × iB1 (1)
Second temperature measurement value = mV (tc) + r (tc) × iB2 (2)
Further, in the normal temperature measurement, each bias current is not supplied in the same manner as described with reference to FIG. 3 in the conventional example. Therefore, the temperature measurement value measured by the microprocessor 42 is expressed by the following equation (3). Desired.
Third temperature measurement value = mV (tc) (3)

Since each bias current iB1, iB2 is a value determined in advance at the design stage, the resistance value r (tc) of the thermocouple 11 itself as shown in the following equation (4) under the condition that the environmental temperature at the time of temperature measurement can be regarded as constant. Is required.
r (tc) = (first temperature measurement value−second temperature measurement value) ÷ (iB1-iB2) (4)
Next, it is assumed that another burnout detection circuit is connected to the thermocouple 11 in addition to the self burnout detection circuit 40. In this case, an error r (tc) × iB corresponding to a voltage drop obtained by multiplying the bias current iB of the other burnout detection circuit by the resistance value r (tc) of the thermocouple 11 itself is represented by the above equations (1) and ( 2), and the result is measured by the microprocessor 42. The temperature measurement value in this case is obtained as in the following formulas (5) and (6).

Fourth measured temperature value = mV (tc) + r (tc) × iB1 + r (tc) × iB (5)
Fifth temperature measurement value = mV (tc) + r (tc) × iB2 + r (tc) × iB (6)
Here, in order to simplify the calculation, if the bias current of the other burnout detection circuit is constant, each bias current in the self burnout detection circuit 40 is not supplied in normal temperature measurement. The measured temperature value is obtained as in the following formula (7).
Sixth temperature measurement value = mV (tc) + r (tc) × iB (7)

Even when other burnout detection circuits are connected, the bias currents iB1 and iB2 are values determined in advance at the design stage. Therefore, under the condition that the environmental temperature at the time of temperature measurement can be regarded as constant, Thus, the resistance value r (tc) of the thermocouple 11 itself is obtained.
r (tc) = (fourth temperature measurement value−fifth temperature measurement value) ÷ (iB1-iB2) (8)

Therefore, an error r (tc) × iB corresponding to a voltage drop obtained by multiplying the bias current iB of the other burnout detection circuit by the resistance value r (tc) of the thermocouple 11 itself is stored as a correction value. If stored in the unit 49, an appropriate temperature measurement value can be obtained by subtracting the correction value from the result of the above equation (7) in the temperature measurement when the other burnout detection circuit is connected. Become.
However, in the above description, it is assumed that there is one other burnout detection circuit. However, even when there are two or more burnout detection circuits, an appropriate temperature measurement value can be obtained by performing the same correction.
Thus, according to the burnout detection circuit 40 of the first embodiment, the external circuit connection presence / absence detection unit 47 includes one or more other burnout detection circuits in addition to the self burnout detection circuit 40 in the thermocouple 11. Is connected, the correction calculation unit 48 obtains an error corresponding to a voltage drop obtained by multiplying the resistance value of the thermocouple 11 by the bias current of the other burnout detection circuit, and stores the correction value as a correction value. Stored in the unit 49.

When the bias current switching control unit 46 performs temperature measurement without supplying the first and second bias currents of the self burnout detection circuit 40, the correction calculation unit 48 first determines the temperature measurement value. The stored correction value is subtracted. As a result, the above-described error of the other burnout detection circuit included in the temperature measurement value is eliminated, so that an appropriate temperature measurement value can be obtained.
Therefore, even when two or more burnout detection circuits are connected to one thermocouple 11 to constitute a temperature detection system, accurate temperature measurement can be performed in each burnout detection circuit.

1 is a circuit configuration diagram of a burnout detection circuit according to a first embodiment of the present invention. It is a circuit block diagram of the conventional burnout detection circuit. It is an operation | movement sequence diagram of a burnout detection circuit.

Explanation of symbols

10 burnout detection circuit 11 thermocouple 11a, 11b connection terminal 13 semiconductor switch 13a first semiconductor switch 13b second semiconductor switch 14 high impedance resistor 14a first high impedance resistor 14b second high impedance resistor 15 Internal voltage source 16, 41 Bias circuit 18 Semiconductor multiplexer 20 Amplifier 22 A / D converter 24, 42 Microprocessor 26 Resistor 27 Capacitor 28 LPF
29 Cold junction compensation input unit 30 ON / OFF control signal 31 Thermocouple electromotive force signal 32 Thermocouple temperature compensation signal 33, 34 Reference signal 35 Time division control signal 36 Amplification signal 37 Digital signal 44 Main control unit 46 Bias current switching control unit 48 Correction calculation unit 49 Correction value storage unit 51 Bias current switching control signal

Claims (1)

In the burnout detection circuit that detects the disconnection of the temperature sensor of the thermocouple and measures the temperature based on the electromotive force generated when the temperature of the temperature sensor is detected,
Bias means for supplying the temperature sensor with at least two bias currents each having a different current value;
Bias control means for performing either one of the first control for individually supplying the at least two or more bias currents from the bias means to the temperature sensor and the second control for not supplying all the bias currents. When,
Connection presence / absence detection means for detecting whether or not another burnout detection circuit is connected to the temperature sensor;
A plurality of temperature measurement values each measured at the time of individual supply of the at least two or more bias currents by the first control when connection of another burnout detection circuit is detected by the connection presence / absence detection means A voltage drop obtained by dividing a difference between the two and the difference between the at least two bias currents to obtain a resistance value of the temperature sensor and multiplying the resistance value by a bias current of the other burnout detection circuit Correction calculation means for storing an error corresponding to the minute as a correction value, and subtracting the stored correction value from the third temperature measurement value measured during the second control. A burnout detection circuit characterized by the above.

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