JP2008268234A - Burnout detection circuit - Google Patents

Abstract

An object of the present invention is to shorten the burnout detection time even when a temperature detection system is configured by connecting two or more burnout detection circuits to one thermocouple.
A temperature measurement is performed based on an electromotive force generated when a temperature of a thermocouple temperature sensor is detected, and a bias current capable of supply / non-supply control is supplied to the temperature sensor. In the burnout detection circuit that detects the disconnection of the temperature sensor when the voltage charged in the first capacitor 27 via the first resistor 26 during supply exceeds a predetermined threshold value, the bias current is detected in the burnout detection circuit. And a second capacitor 62 having a capacitance sufficiently smaller than that of the first capacitor 26, and the second capacitor 62 by the bias current. Comparing means 64 outputs a burnout detection signal when a voltage generated by charging the second capacitor 62 is applied to the voltage comparison terminal and the applied voltage exceeds the reference voltage.
[Selection] Figure 1

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 the controller is used as a part of a combustion plant, if a temperature lower than a normal value is output due to a sensor disconnection, fuel may be oversupplied to cause 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 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, and 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.

Such a sequence of a, b, c, and d is set as one A / D conversion cycle, and by continuing this cycle as 1, 2, 3,..., Temperature measurement by the thermocouple 11 can be continued. 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 description, the burnout detection circuit 10 that performs burnout detection and temperature measurement according to the ON / OFF operation timing of the semiconductor switch 13 has been described, but in addition to this, there is no semiconductor switch 13 illustrated in FIG. 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.

Further, 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 passed 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, even when one burnout detection circuit 10 is connected to one thermocouple 11 via connection terminals 11a and 11b, when the thermocouple 11 is disconnected, Since the burnout detection can be performed by causing the voltage by charging only the bias current to the capacitor 27 to exceed a predetermined threshold value, the capacitor 27 is charged with a voltage exceeding the threshold value. Cannot detect disconnection of the thermocouple 11, that is, burnout. In other words, there is a problem that the burnout detection time from the burnout state to the detection of the burnout is long.
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. An object of the present invention is to provide a burnout detection circuit that can perform temperature measurement and can shorten the burnout detection time.

  In order to achieve the above object, a burnout detection circuit according to claim 1 of the present invention performs temperature measurement based on an electromotive force generated at the time of temperature detection of a thermocouple temperature sensor and supplies the temperature sensor to the temperature sensor. A bias current that can be controlled without supply is supplied, and when the voltage charged in the first capacitor through the first resistor exceeds the predetermined threshold when the supply is performed, the temperature sensor In the burnout detection circuit for detecting disconnection, a second resistor having a capacitance sufficiently smaller than the first capacitor is connected between the bias current supply point and the ground via a second resistor. And a voltage obtained by charging the second capacitor with the bias current is applied to a voltage comparison terminal, and when this applied voltage exceeds a reference voltage, a burnout detection signal is output. It is characterized in that a means.

  According to this configuration, when the temperature sensor is disconnected at the time of supplying the bias current, the bias current is charged to both the first and second capacitors, but the capacitance of the second capacitor is greater than that of the first capacitor. Is sufficiently small, the voltage applied to the comparison means by charging the second capacitor exceeds the reference voltage in a very short time, and thereby a burnout detection signal is output. Thereby, disconnection of the temperature sensor is detected in a short time.

  The burnout detection circuit according to claim 2 of the present invention is the burnout detection circuit according to claim 1, wherein the first period for resetting the integration of the electromotive force of the temperature sensor and the second period for integrating the electromotive force are set. When a cycle having at least two periods continues, the bias current is supplied at the start of the second period, and after a time during which burnout can be detected by charging the second capacitor has elapsed, the bias It is characterized by stopping the supply of current.

  According to this configuration, if the temperature sensor is in a normal state at the time of stopping after the start of the bias current supply in the second period, the burnout detection circuit is switched to the original by the start of the first period of the next cycle. Therefore, it is possible to detect burnout while performing normal temperature measurement in one cycle. That is, temperature measurement and burnout detection can be performed efficiently.

  As described above, according to the present invention, when the temperature sensor is disconnected at the time of supplying the bias current, the capacitance of the second capacitor is increased when the bias current is charged to both the first and second capacitors. The voltage applied to the comparison means by charging the second capacitor exceeds the reference voltage in a very short time, and a burnout detection signal is output from the comparison means. Thereby, disconnection of the temperature sensor is detected in a short time.

Therefore, the burnout detection time can be shortened.
Further, when a cycle having at least two periods of a period for resetting the electromotive force integration of the temperature sensor and a period for performing the electromotive force integration continues, a bias current is supplied at the start of the electromotive force integration period. The supply of the bias current is stopped after a time during which burnout can be detected by charging the second capacitor has elapsed. As a result, if the temperature sensor is in a normal state at the stop after the bias current supply is started in the electromotive force integration period, the burnout detection circuit is switched to the original temperature by the start of the electromotive force integration reset period of the next cycle. Since it is possible to return to the measurement voltage, burnout detection can be performed while performing normal temperature measurement in one cycle. Therefore, temperature measurement and burnout detection can be performed efficiently.

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, portions corresponding to the respective portions of the conventional example shown in FIG.

  The burnout detection circuit 60 shown in FIG. 1 differs from the conventional burnout detection circuit 10 shown in FIG. 2 between the bias current supply line supplied from the bias circuit 16 to the thermocouple 11 and the ground. The capacitor 62 is connected via the resistor 61, and further, a voltage due to charge charging to the capacitor 62 due to the bias current when the thermocouple 11 is disconnected is applied to the voltage comparison terminal, and this applied voltage exceeds the reference voltage. In this case, a comparator 64 for outputting a burnout detection signal to the microprocessor 24 is provided.

However, the capacitance of the capacitor 62 is set sufficiently smaller than that of the capacitor 27. The capacitor 62 also serves to remove extraneous impulse noise.
In such a configuration, when the bias current is supplied to the thermocouple 11 by turning on the semiconductor switch 13, if the thermocouple 11 is disconnected, an operation is performed in which the bias current is charged to both the capacitors 27 and 62. .

However, since the capacitance of the capacitor 62 is sufficiently smaller than that of the other capacitor 27, the voltage applied to the comparator 64 due to charging of the capacitor 62 exceeds the reference voltage in a very short time, whereby the burnout detection signal is generated. It is output to the microprocessor 24. Thereby, the disconnection of the thermocouple 11 is detected in a short time by the microprocessor 24.
Specifically, for example, it is assumed that the resistor 26 and the capacitor 27 are set to values of 10 KΩ and 3.3 μF, respectively, in order to limit the commercial noise to 1/10. At this time, when the internal detection level is 100 mV and the bias current is 50 nA, the burnout detection time is 6.6 seconds. Further, at this time, if the capacitor 62 is set to a value of 3.3 nF, the burnout detection time is 1/1000, 6.6 milliseconds.

When the semiconductor switch 13 is turned from ON to OFF, the burnout detection circuit 60 can be returned to the original temperature measurement voltage in a very short time.
Here, a, b, c, and d representatively shown in the A / D conversion cycle 1 shown in FIG. 3 will be further described. A is an integration reset period, b is an input integration period, and c and d are reference voltages. Integration period. In each of these periods a to d, the semiconductor switch 13 is turned on at the same time as the period c is started to supply a bias current to the thermocouple 11, and after the time during which burnout can be detected by the capacitor 62 has elapsed, the semiconductor switch If 13 is OFF, burnout detection can be performed at the boundary between c and d.

Further, if the thermocouple 11 is in a normal state when the semiconductor switch 13 is turned off, the burnout detection circuit 60 is set to the original temperature measurement voltage by the start of the period a of the next A / D conversion cycle 2. It becomes possible to return.
That is, by such a sequence operation, burnout detection can be performed while performing normal temperature measurement in one A / D conversion cycle.

  As described above, according to the burnout detection circuit 60 of the first embodiment, when the thermocouple 11 is disconnected at the time of supplying the bias current, the bias current is charged to both the capacitors 27 and 62. Is sufficiently smaller than the capacitor 27, the voltage applied to the comparator 64 by charging the capacitor 62 exceeds the reference voltage in a very short time, whereby a burnout detection signal is output to the microprocessor 24. Is done. Therefore, since the disconnection of the thermocouple 11 is detected in a short time, the burnout detection time can be shortened.

  Further, if the thermocouple 11 is in a normal state at the stop after the bias current supply is started in the period c, the burnout detection circuit 60 is operated by the start of the period a at the beginning of the next A / D conversion cycle. Therefore, in one A / D conversion cycle, burnout detection can be performed while performing normal temperature measurement. That is, temperature measurement and burnout detection can be performed efficiently.

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, 61 Resistor 27, 62 Capacitor 28 LPF
29 Cold junction compensation input section 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 Amplified signal 37 Digital signal 64 Comparator

Claims (2)

The temperature measurement is performed based on the electromotive force generated when the temperature of the thermocouple temperature sensor is detected, and a bias current that can be controlled to be supplied / not supplied is supplied to the temperature sensor. In a burnout detection circuit that detects a disconnection of the temperature sensor when the voltage charged to the first capacitor via the detector exceeds a predetermined threshold value,
A second capacitor connected via a second resistor between the supply point of the bias current and the ground, and having a sufficiently smaller capacitance than the first capacitor;
Comparing means for outputting a burnout detection signal when a voltage generated by charging the second capacitor by the bias current is applied to a voltage comparison terminal and the applied voltage exceeds a reference voltage. A featured burnout detection circuit. The start of the second period when a cycle having at least two periods of a first period for resetting the integration of the electromotive force of the temperature sensor and a second period for integrating the electromotive force continues. 2. The burnout detection according to claim 1, wherein the supply of the bias current is stopped after a time in which the bias current is sometimes supplied and burnout detection by charging the second capacitor has elapsed. circuit.

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