JPH0548153A - Burnout detector circuit for thermocouple thermometer - Google Patents

Abstract

PURPOSE:To obtain a burnout detector circuit in which a measuring error does not occur by superposing a burnout detecting AC voltage on a thermal electromotive force of a thermocouple, and removing the AC voltage by a filter after amplifying it. CONSTITUTION:An AC voltage VAC from an AC signal source 6 is applied to a thermal electromotive force Vt of a thermocouple 1 to a composite voltage VX. After this is amplified by A times by an amplifier 2, an AC voltage AVAC is removed by a low pass filter 8 in a temperature measuring unit 7, a DC voltage ACt is extracted, applied to an A/D converter 3, and digitally converted. A measuring unit 4 calculates a measured temperature from digital data AVt, and displays it on a display unit 9. On the other hand, the voltage AVx of the amplifier 2 is also applied to a burnout signal generator 10, and converted to a DC voltage Va through a capacitor C and a rectifier 11. When a burnout occurs, the voltage Va becomes larger than a threshold voltage Vvef, a comparator 12 is turned ON, and a disconnection of the thermocouple 1 is informed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a burnout detection circuit for a thermocouple thermometer.

[0002]

2. Description of the Related Art A thermocouple element used as a temperature sensor in a thermocouple thermometer or the like may be deteriorated and disconnected, and this disconnection phenomenon is generally called burnout.
Therefore, in this type of device, a burnout detection circuit for monitoring whether or not the thermocouple element is disconnected is provided, and in the case of disconnection, the element is replaced.

FIG. 4 shows an example of a burnout detection circuit in a conventional thermocouple thermometer. For example, a current i is caused to flow from a positive DC voltage source + Vcc to a thermocouple 1 through a resistor R3, and the voltage thereof is applied. The drop Vi is the thermoelectromotive force V of the thermocouple 1.
It overlaps with t. If the combined voltage is Vx, then Vx = Vt + Vi (1). This voltage Vx is applied to the amplifier 2.

Now, assuming that the amplification degree of the amplifier 2 is A, the voltage of the equation (1) is multiplied by A in the amplifier 2 and the output voltage becomes A · Vx = A (Vt + Vi). This voltage is digitally converted by, for example, the A / D converter 3 and applied to the measuring unit 4. The measuring unit 4 calculates the value of the above formula (1) by making the data multiplied by A into 1 / A, for example, by calculation.

Assuming that the resistances of the two dissimilar metal wires constituting the thermocouple 1 and their lead wires are R1 and R2, the current i flowing from the voltage source + Vcc through the resistance R3 causes the thermocouple 1 to generate. Since the voltage drop Vi is Vi = i (R1 + R2) = {Vcc / (R1 + R2 + R3)} (R1 + R2), the combined voltage Vx is Vx = Vt + Vcc (R1 + R2) / (R1 + R2 + R3) (2). In this case, the value of the resistor R3 is usually set as R3 >> R1 + R2, so when the thermocouple 1 is operating normally, R1 in the second term denominator on the right side of the equation (2) is
Ignoring + R2, it can be written as Vx = Vt + Vcc (R1 + R2) / R3 (3).

When the thermocouple 1 is broken, the thermoelectromotive force Vt
Does not occur, Vt = 0. Therefore, from the formula (2), Vx = Vcc (R1 + R2) / (R1 + R2 + R3) Further, the sum R1 + R2 of the resistances of the elements can be regarded as infinity due to the disconnection, so that if R3 in the above formula is ignored, Vx = Vcc (4). Here, the output A · Vcc of the amplifier 2 is, for example, A
When the voltage is set to be equal to or higher than the full-scale input voltage of the / D converter 3, the same A / D converter is saturated and full-scale data (11 ... 1) is obtained from the measuring unit 4. If this data is displayed on a display unit (not shown), burnout of the thermocouple 1 can be detected.

FIG. 5 shows another example in which the differential amplifier 5 is used in place of the above-mentioned amplifier 2, but the operation is substantially the same as the example in FIG. 4, and therefore its explanation is omitted. ..

[0008]

According to the above-mentioned conventional burnout detection circuit, the disconnection of the thermocouple or the like can be surely detected. However, when the thermocouple is normal and the normal temperature measurement is performed, the temperature measurement value V is calculated as shown in equation (3).
The second term on the right side of x enters as an error component. Therefore, in general, a high resistance of several hundreds of KΩ or more is used for R3 to reduce the error, but this error is inevitably generated in the circuit configuration, and it is difficult to eliminate it. Further, if a high resistance is used, noise due to induction or the like tends to be picked up, which is not preferable.

The present invention has been made in consideration of the above circumstances, and an object thereof is to provide a burnout detection circuit configured so that a measurement error should not originally occur.

[0010]

Referring to FIG. 1, which illustrates an embodiment of the present invention, a thermocouple 1, an amplifier 2, an A /
The D converter 3, the measuring unit 4 and the like are configured in substantially the same manner as the above-mentioned conventional device, and are therefore provided with the same reference numerals. In this embodiment, the following means are provided to solve the above problems.

For example, an AC signal source 6 that superimposes an AC voltage Vac for burnout detection on the thermoelectromotive force Vt of the thermocouple 1 is provided.

For example, the temperature measuring unit 7 is provided after the amplifier 2.
And a burnout signal generator 10
The temperature measuring unit 7 includes a low-pass filter 8 that removes the AC voltage Vac and passes the thermoelectromotive force Vt.

The burnout signal generator 10 is
For example, when a capacitor C that blocks passage of the thermoelectromotive force Vt and passes the AC voltage Vac, a rectifying circuit 11 that converts the AC voltage Vac into a DC, and the rectified DC voltage exceeds a predetermined threshold level. Comparator 12 and reference voltage source 13 for issuing a signal notifying the disconnection of the thermocouple 1
It has and.

[0014]

When the AC voltage Vac for burnout detection is superimposed on the thermoelectromotive force Vt of the thermocouple 1 by the above means, the voltage applied to the amplifier 2 becomes the sum of the DC voltage Vt and the AC voltage Vac. Here, when the amplification degree of the amplifier 2 is A, a voltage obtained by multiplying the input voltage by A appears on the output side of the amplifier 2. In this case, the temperature measuring unit 7 is connected to the DC thermoelectromotive force A · Vt via the low-pass filter 7 by the above means.
Is taken in and the AC voltage that causes the measurement error is removed.

Further, the burnout signal generator 10 includes
By the above means, the AC voltage A
Vac is taken in. When the thermocouple 1 is operating normally, the taken-in AC voltage A · Vac has a relatively small value, so the rectified voltage also has a voltage corresponding to it. When the thermocouple 1 is broken, the AC voltage Vac becomes substantially equal to the signal source voltage V of the AC signal source 6, and the voltage taken in via the capacitor C becomes A · V. This voltage has a relatively large value, and the rectified voltage also has a magnitude corresponding thereto.

Therefore, the threshold voltage V of the reference voltage source 13
If ref is set to an appropriate value that is greater than the rectified voltage of the AC voltage A · Vac and smaller than the rectified voltage of the AC voltage A · V, the comparator 1 will operate when the thermocouple 1 is disconnected.
The output of No. 2 is turned ON, and the burnout can be notified by sending this ON output to the measuring unit 4.

[0017]

EXAMPLE In FIG. 1, the thermoelectromotive force of the thermocouple 1 is V
where t is the signal source voltage of the AC signal source 6 and VAC is the AC voltage applied from the signal source 6 to the thermocouple 1, a combined voltage Vx of the thermoelectromotive force Vt and the applied AC voltage Vac
Becomes Vx = Vt + Vac (5). In this case, since the magnitude of the applied AC voltage Vac is Vac = V (R1 + R2) / (R1 + R2 + R3), substituting this value into the equation (5) results in the combined voltage Vac.
x becomes Vx = Vt + V (R1 + R2) / (R1 + R2 + R3) (6).

Referring to FIG. 2 together, FIG. 2A shows the case where the thermocouple 1 is operating normally, and FIG. 2B shows the case where the thermocouple 1 is broken. Now, it is assumed that the thermocouple 1 is operating normally, and the voltage Vx of the above formula (5) is shown in (a) of FIG. This combined voltage Vx is amplified by A times, for example, by the amplifier 2 and becomes as shown in B of FIG. That is, AVx = AVt + AVac (7) In the temperature measuring unit 7, the low-pass filter 8 removes the AC voltage AVac from the amplified output AVx as described above, extracts the DC voltage AVt, and applies it to the A / D converter 3. The state is shown in C of FIG. The A / D converter 3 receives the voltage AVt applied from the low-pass filter 8.
Is digitally converted as shown in D of FIG. However, D is shown by converting the digital conversion data into an analog voltage value. The measuring unit 4 divides this digital data AVt by the amplification degree A of the amplifier 2 to obtain the thermoelectromotive force Vt, calculates the measured temperature corresponding to this voltage Vt, and displays it on the display unit 9.

Output voltage AVx of the amplifier 2 shown in equation (7)
Are also added to the burnout signal generator 10. In this case, the direct-current voltage AVt is blocked by the capacitor C, and the alternating-current voltage AVac passes through the same capacitor C so that the voltage of FIG.
It is added to the rectifier circuit 11 as shown in (A) -e. The rectifier circuit 11 rectifies this AC voltage AVac and converts it into a DC voltage Va representing an average value, for example. Figure 2 (A)
It is shown in F. Here, the threshold voltage V of the reference voltage source 13
When ref is set higher than the voltage Va as shown by the dotted line in the figure, the output of the comparator 12 is as shown in FIG.
It will be in the off state as shown in FIG.

Next, the case where the thermocouple 1 is broken will be described. In this case, since the thermoelectromotive force Vt is zero, Vx = Vac from the formula (5) and Vx = V (R1 + R2) / (R1 + R2 + R3) from the formula (6), but R1 + R2 → If it is regarded as ∞ and R3 is ignored, it becomes Vx〓V. Therefore, Vx = V = Vac is obtained. That is, the input voltage Vx of the amplifier 2 is the signal source 6
Voltage V. This state is shown in (a) of FIG.

The amplifier 2 multiplies this voltage Vac by A and sends it to the temperature measuring section 7 and the burnout signal generating section 10. The voltage multiplied by A is shown in B of FIG. 2 (B). Since the low-pass filter 8 of the temperature measuring unit 7 removes this amplified voltage AVac, its output is zero, and therefore the digital data of the A / D converter 3 is also zero. This state is shown in FIGS. 2B and 2C, respectively.

In the burnout signal generator 10,
The AC voltage AVac is applied to the rectifier circuit 11 through the capacitor C and converted into, for example, a DC voltage Va representing an average value. This state is shown in FIGS. In the figure, since the DC voltage Va is sufficiently higher than the threshold voltage Vref, the comparator 12 is turned on, and
As shown in (B) -G, an H-level voltage is generated on the output side. This H-level voltage is sent to the measuring unit 4 as a burnout signal indicating the disconnection of the thermocouple 1, and when the measuring unit 4 receives this signal, the thermocouple 1 is broken in the display (not shown) of the display unit 9, for example. It is supposed to be displayed. Alternatively, a burnout display lamp or the like may be turned on.

FIG. 3 shows another embodiment using the differential amplifier 5 in accordance with the conventional example of FIG. 5, but the operation of each part is the same as in the case of FIG. 1 described above. The description is omitted.

[0024]

As described above in detail, in the present invention, in order to detect the disconnection or the like of the thermocouple 1, for example, from the AC signal source 6 to the thermocouple 1, an AC voltage Va of a predetermined level is obtained.
One of the above DC voltage V from the combined voltage Vx of the DC thermoelectromotive force Vt of the thermocouple 1 and the added AC voltage Vac.
The temperature measuring unit 7 that separates and extracts t and measures the other alternating voltage Vac from the combined voltage Vx that is separated and extracted to monitor whether the level is equal to or lower than a predetermined threshold voltage. A burnout signal generating unit 10 is provided for outputting a warning to the temperature measuring unit 7 when the threshold voltage is exceeded.

Therefore, according to the present invention, the temperature measuring section 7 has a signal related only to the thermoelectromotive force Vt, and other signal components that cause an error do not enter, so that the temperature can be measured with high accuracy. Further, since the signal inside the burnout signal generator 10 is related only to the AC voltage Vac, when the AC voltage Vac increases due to the disconnection of the thermocouple 1 or the like, it is surely detected and the burnout signal is generated. Can be sent out.

[Brief description of drawings]

FIG. 1 is a block diagram showing an electrical configuration of a device example to which the present invention is applied.

FIG. 2 is a signal waveform diagram for explaining the operation of each unit in the above device example.

FIG. 3 is a block diagram showing an electrical configuration of another device example to which the present invention is applied.

FIG. 4 is a block diagram showing an electrical configuration of a conventional device.

FIG. 5 is a block diagram showing an electrical configuration in another example of the conventional device.

[Explanation of symbols]

 1 Thermocouple 3 A / D Converter 4 Measuring Section 6 AC Signal Source 7 Temperature Measuring Section 8 Low Pass Filter 9 Display Section 10 Burnout Signal Generation Section 11 Rectifier Circuit 12 Comparator 13 Reference Voltage Source C Capacitor Va Rectified DC Voltage Vac Partial AC Voltage Vref Threshold voltage Vt Thermoelectromotive force

Claims (1)

[Claims] 1. A thermocouple 1 is used as a temperature sensor, the temperature is measured based on digital conversion data of thermoelectromotive force Vt generated by the temperature sensor, and a predetermined voltage is applied to an output side terminal of the thermocouple 1 from a voltage source having a constant voltage. In the burnout detection circuit of the thermocouple thermometer, which detects a disconnection or the like of the thermocouple 1 by increasing the voltage between the terminals and displays the voltage, the burnout detection circuit of the thermocouple 1 detects the AC Divided voltage Vac
Is added, and the thermoelectric power Vt of the thermocouple 1 and the AC voltage Vac applied from the AC signal source 6 are combined and extracted, and the DC thermoelectromotive force Vt including the low-pass filter 8 is separated and extracted. A temperature measuring section 7 having an A / D converter 3 and a measuring section 4 for digitally converting and measuring the temperature, and a capacitor C for separating and extracting the AC voltage Vac from the combined voltage of the thermoelectromotive force Vt and the AC voltage Vac. A rectifying circuit 11 for rectifying the extracted AC voltage Vac to convert it into a DC voltage Va; and a predetermined threshold voltage Vref, and comparing the rectified DC voltage Va with the threshold voltage Vref. The burnout signal generator 10 including a comparator 12 that outputs an output to the measuring unit 4 when the DC voltage Va exceeds the threshold voltage Vref, and the measuring unit 4 includes a comparator. Burnout detection circuit of thermocouple, characterized in that a display unit 9 for displaying the disconnection of the thermocouple 1 by a command signal originating receiving an output from the 12.