JPH039229A - Burn-out circuit for thermocouple - Google Patents

Abstract

PURPOSE:To take industrial measurements effectively by performing deflection to the upper limit of a normal input voltage within a constant time when the thermocouple is disconnected with an input span voltage and suppressing an error due to the applied voltage below a constant permissible value when the thermocouple is connected normally and operates through a high resistance. CONSTITUTION:An input device 1 inputs the switching state of the amplification factor of a high-input-impedance amplifier 2 to a microprocessing unit 5 and the digital signal from and A/D converter 3 which varies with the amplification factor of the amplifier 2 from the unit according to the switching state is inputted to the microprocessing unit MPU 5 and processed as specified to output a signal corresponding to the signal from the thermocouple 1 by a unit 7. In this case, if the thermocouple is disconnected, the deflection to the upper limit of the normal input voltage is performed within the constant time and when the thermocouple is connected normally and operates through the high resistance, the error due to the applied voltage is suppressed below the constant permissible value.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a thermocouple burnout circuit, and in particular, when the thermocouple is disconnected, the output of a temperature/electrical signal converter using the thermocouple is burnt out. The present invention relates to a burnout circuit suitable for achieving out-of-amplification scale.

[Conventional technology]

In a typical automatic process control system, if a thermocouple becomes disconnected due to fatigue or damage, it must be detected and some processing must be done to ensure that the control system operates safely. The temperature/electrical signal converter is preferably equipped with a circuit that operates to swing the output signal outside the standard signal range at either the upper or lower limit in order to detect a disconnection of the thermocouple. This is generally called a burnout circuit.

On the other hand, as a burnout circuit configuration, a voltage is applied to the thermocouple through a high resistance so that it is within the error range with respect to the input span voltage in normal operation, and when the thermocouple is disconnected, the input section of the amplifier Generally, the applied voltage is applied to the capacitor of the provided filter circuit through a high resistance, and the potential of the capacitor rises, causing the capacitor to operate outside the normal operation input range. .

When the high resistance is R and the input span voltage of the filter circuit capacitor C1 is V, the input span voltage will be outside the normal operation range after the wire is disconnected when the input span voltage changes by more than V. The current flowing to capacitor C via i
Then, from equations (1) and (2), we obtain, and from equation (3), ■ 1-eRC! is obtained, and the applied voltage E that reaches the input span voltage V after the time is determined.

Next, during normal operation, the error caused by applying a voltage through the high resistance R is as follows: If R is the line resistance of the compensation conductor connecting the thermocouple to the voltage signal converter, the applied voltage E of the high resistance R1 is From the input span voltage V, ■ becomes an error, and the line resistance R in the − shell is less than several hundred Ω, and if the high resistance R is several tens of MΩ, when ■ is several mV,
If E is set to several hundred mV, the error becomes a negligible value. Therefore, each constant and applied voltage of the burnout circuit is determined by the input span voltage.

In recent years, with the remarkable development of microcomputers, there has been a demand for converters that use microprocessors and can freely select the input span voltage. When applied, the smaller the input span voltage is, the larger the error with respect to the electromotive force of the thermocouple during normal operation becomes, according to equation (6), and cannot be ignored. Furthermore, as the input span voltage increases, if the thermocouple is disconnected, it will take more time to shake it out of the normal operating range according to equation (3), and some kind of countermeasure is required.

[Problem to be solved by the invention]

In the above conventional technology, the constants and applied voltage of the burnout circuit are determined by the input span voltage, so it is necessary to change the constants and applied voltage every time the input span voltage changes. No consideration was given to making changes.

It is an object of the present invention to maintain a normal applied voltage for any input span voltage by applying a voltage corresponding to the switching state of the amplification factor, so that the error is within a certain tolerance value, and furthermore, when a thermocouple is disconnected, it can be applied for a certain period of time. An object of the present invention is to provide a burnout circuit for a temperature/electrical signal converter having a thermocouple input, which causes the temperature/electrical signal converter to operate outside the normal operation input range within a short period of time.

[Means to solve the problem]

The above object is to provide an input means for inputting a switching state of the amplification factor of an amplifier selected corresponding to an input span voltage into a microprocessing unit (hereinafter referred to as MPU), A digital/analog converter is provided to convert a digital output signal that changes in accordance with the amplification factor into an analog signal, and the output from this digital/analog converter that changes in accordance with the amplification factor of the amplifier is passed through a high resistance. is applied to one end of the thermocouple to ensure that the output of the amplifier swings out to the upper limit within a certain period of time when the thermocouple is disconnected, and even when the thermocouple is normally connected. This is achieved by operating the analog output error from the amplifier due to the applied voltage from the high resistance within a certain allowable value regardless of the setting state of the amplification factor.

[Effect]

When an abnormal condition occurs due to a thermocouple disconnection, the applied voltage applied through the high resistance causes current to flow through the capacitor of the filter circuit installed in the front stage of the amplifier, and the input span voltage <applied voltage> As a result, the potential across the capacitor, that is, the input voltage of the amplifier, rises and becomes higher than the input span voltage, and eventually the input voltage of the amplifier reaches the applied voltage, which reduces the input range during normal operation. You can change the input to the outside.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to FIGS. 1 to 4.

FIG. 1 is a block diagram showing an embodiment of a temperature/electrical signal converter equipped with a thermocouple burnout circuit according to the present invention, and FIG.
The figure is the equivalent circuit diagram of Figure 1 in normal operation, and Figure 3 is the equivalent circuit diagram of Figure 1.
In FIG. 1, the voltage signal due to the electromotive force of the thermocouple 1 is smoothed by a filter circuit 8 composed of a resistor RL + a capacitor Cs.
The output signal from the filter circuit 8 is input to the high input impedance amplifier 2 whose amplification factor is selectively switched in advance by the amplification factor switching circuit 4 so that analog/digital conversion can be performed under optimal conditions depending on the input span voltage.

The output of the amplifier 2 is input to an analog/digital converter 3, which converts the analog signal into a digital signal, and is converted into a digital value.

The signal converted into a digital value is taken in by the MPU 5, performs predetermined calculations, processing, storage, etc., and outputs an output signal corresponding to the signal from the thermocouple 1 from the output circuit 7.

By periodically repeating this operation, the temperature/electrical signal conversion operation is performed. During this operation, the amplification factor switching circuit 4
Then, the selected state of the amplification factor of the amplifier 2 is imported into the MPU 5, the amplification factor is calculated, the on time and off time of one point of digital output are calculated so that the voltage to be applied via the high resistance R2 is obtained, and the MPU 5 The pulse @ voltage output, which outputs a pulse with a wider pulse width, is smoothed by a filter circuit 9 consisting of a resistor R3 and a capacitor C2, voltage amplified by an amplifier 6, and applied to one end of the thermocouple 1 via a high resistor R2. .

Next, the output operation of the applied voltage for burnout operation corresponding to the amplification factor will be explained using FIGS. 1 and 4. FIG. 4 is a processing operation flow diagram of the MPU which outputs a variable voltage applied for burnout operation in accordance with the amplification factor of the amplifier shown in FIG. In Figure 4, step 1
The selection state of the amplification factor of the amplification factor switching circuit 4 is read at o. Next, in step 11, the amplification factor G is calculated from the selected state.

Next, in step 12, pulse output on time n/G=
Calculate X. Here, n is a value obtained by dividing the pulse width voltage output period time by the minimum pulse width time, that is, the number of divisions of the pulse width. When the minimum value of the amplification factor G is 1, n n -=-=n 1 (7), the pulse output is always on, the applied voltage is maximum, and the maximum value of the amplification factor G is n. In this case, n -=-= 1 n...(8), the pulse output is on for 1/n, and (n
-1)/n, the pulse output is off and the applied voltage is at its minimum. In other words, the applied voltage is variably output in inverse proportion to the amplification factor G. Next, in step 13, the pulse output period n and pulse output on@X are set, and the pulse output operation begins.

First, in step 14, the D/A output, which is a pulse output, is turned on, and then in step 15, the counter value is changed from (X-1) to X, (n-
1) → Decrement each n by operation 0 Next, in step 17, determine whether X=O. If X≠0, return to step 15 again and repeat until x=0, and if Then, in step 18, the D/A output is turned off.

Next, in step 19, it is determined whether n=o, and if n≠O, return to step 15 and repeat until n=0.
When n=o, the process returns to step 13, repeats the series of operations, and outputs a pulse width voltage corresponding to the amplification factor G. The output voltage e of the filter circuit 9 when the voltage when the pulse is on at this time is V and the voltage when the pulse is off is 0.
o is, and in this case, the circuit time constants of the resistor R3 and capacitor C2 of the filter circuit 9 are sufficiently large compared to the product of n and T.

Next, if the amplification factor of the amplifier 6 is A, the applied voltage E is So here it is.

Since X=-...(11), substituting equation (11) into equation (10), we get
Therefore, the smaller the amplification factor G, the larger E becomes.

This means that the amplification factor of the amplifier 2 is small and the input span voltage is large, so that the burnout operation applied voltage corresponding to the input casp voltage can be automatically varied. In other words, the applied voltage is determined and applied at a constant ratio to the input span voltage, so that when thermocouple 1 is operating normally, it is within a certain error range for any input span voltage, and even when thermocouple 1 is disconnected. It is possible to change the input by more than the input span voltage within a certain period of time and operate outside the normal operating input range.

Next, the operation of the thermocouple 1 in the embodiment shown in FIG. 1 when it is normally connected will be explained using FIG. 2. The thermocouple shown in FIG.

The electromotive force is shown as V', and the line resistance of the compensating lead wire is shown as r, and the other symbols are the same as in FIG. Due to the analog voltage output of the amplifier 6, the applied voltage E is applied to one end of the thermocouple via the high resistance R2, and the current j1 is caused by the high resistance Rz, the line resistance r, the thermocouple, #! flows through the road resistance r. The conditions for the current 11 to flow at this time are E > v...(
13), and the current 11 is as follows. Furthermore, the voltage error e' input to the amplifier 2 due to the applied voltage E is as follows, and as mentioned above, the line resistance 2r of the compensation conductor is several hundreds of Ω.
If R2 is set to several tens of MΩ, v'3>e
' and can be completely ignored.

Next, the operation when the thermocouple 1 is disconnected in the embodiment shown in FIG. 1 will be explained using FIG. 3. If a break occurs at one or more points of the thermocouple including the compensation conductor, (1
4) Formula 1s=o, and a current iz flows through the high resistance RZ, the resistance R1 of the filter circuit 8, and the capacitor C1.

The current iz is the same as equation (4), i−+iz, R→R
The only difference is 2+R1. Therefore, E(1-8-door can τ・)>v'-ax-(16
), that is, after t time, the input span voltage becomes higher than or equal to the input span voltage, and the input can be reliably operated outside the normal input range.As a result, if the thermocouple is disconnected during temperature/electrical signal conversion operation, the output can be caused to swing out of the normal operating range.

Furthermore, in this embodiment, the MPU 5 acquires the switching state of the amplification factor of the amplifier 2 to learn the amplification factor, and changes the applied voltage to the corresponding thermocouple 1. However, the method is not limited to this. Instead, the circuit configuration is such that the amplification factor of the amplifier 6 can be switched by the output from the MPU 5, and the amplification factor is calculated so that analog/digital conversion can be performed under optimal conditions using the input zero and span voltage specified for the MPU 5. A method may also be adopted in which the MPU 5 performs predetermined processing such as controlling output, and determines the voltage to be applied for the burnout operation based on the obtained amplification factor.

In addition, in this embodiment, the MPU 5 performs a pulse width control output that can be realized at low cost as a means to variably output the applied voltage applied for burnout operation, and the output is smoothed by the filter circuit 9 and then output by the amplifier 6. Although the method is amplification, the method is not limited to this, and the method is to generate a reference voltage, divide the output with a resistor ladder, switch it with a switch, and take out the voltage divided by the resistor ladder.
A method may also be adopted in which the applied voltage is varied by performing the switching operation of the switch from the MPU 5.

Furthermore, in this embodiment, when the thermocouple 1 is disconnected, the input voltage of the amplifier 2 is increased from the input span voltage during normal operation, so-called burnout upscaling, but this is not limited to this. For example, if the voltage applied for burnout operation is a negative voltage,
In the figure, if the amplifier 6 is configured as an inverting amplifier,
During normal operation, there will be a negative error in the electromotive force from thermocouple 1 through high resistance Rz, and if thermocouple 1 is disconnected, if the input zero voltage is greater than the applied voltage, the input voltage will be lower than the input zero voltage. Therefore, the input voltage can be lowered below the normal operating range.

Furthermore, the applied voltage for burnout operation is a positive voltage.

Both negative voltages are output, and either one of the outputs can be applied to one end of the thermocouple 1 via the high resistance R2, and the switching is done between positive voltage and negative voltage using the output control signal from the MPU 5. As a circuit configuration for switching and applying, M
It is also possible to automatically switch the polarity of the applied voltage by specifying the burnout operation direction to the PU 5.

〔Effect of the invention〕

According to the present invention described above, the MPU automatically calculates the applied voltage to be applied through the high resistance for burnout operation in accordance with the amplification factor of the amplifier that amplifies the electromotive force signal from the thermocouple. Since variable output is possible, if the thermocouple is disconnected at any input span voltage, it can be made to swing out to the upper limit outside the normal operating input voltage range within a certain period of time, and even if the thermocouple is connected normally. Even when the MPU is operating, the error due to the applied voltage applied through a high resistance can be kept below a certain tolerance value at any input span voltage. The pulse width variable voltage output from a single on/off output is smoothed by a filter circuit consisting of one resistor and one capacitor, amplified by an amplifier, and the output voltage is applied to one end of the thermocouple via a high resistance. It can be realized with a simple circuit configuration and is useful for industrial measurement.

[Brief Description of the Drawings] Fig. 1 is a block diagram showing an embodiment of a temperature/electrical signal converter equipped with a thermocouple burnout circuit according to the present invention;
The figure is the equivalent circuit diagram of Figure 1 in normal operation, and Figure 3 is the equivalent circuit diagram of Figure 1.
FIG. 4 is an equivalent circuit diagram at the time of an abnormality as shown in the figure, and FIG. 4 is a processing operation flow diagram of an MPU that outputs a variable voltage applied for burnout operation in accordance with the amplification factor of the amplifier shown in FIG. 1...Thermocouple, 2.6...Amplifier, 3...Analog/digital converter, 4...Amplification factor switching circuit, 5...
・・8q Fui 1 and nu day τ

Claims (1)

[Claims] 1. A thermocouple that generates an electromotive force corresponding to temperature, and a high input impedance amplifier that amplifies the electromotive force of the thermocouple;
The thermoelectric converter consists of an analog/digital converter that converts the output signal of the amplifier into analog/digital, and a microprocessing unit that performs predetermined processing such as calculation, storage, and output based on the output data of the analog/digital converter. In the temperature/electrical signal converter capable of automatically switching the amplification factor of the amplifier according to the signal level of a pair of electromotive forces, an input means for inputting a switching state of the amplification factor of the amplifier to the microprocessing unit; , a digital/analog converter is provided for converting a digital signal from the analog/digital converter into an analog signal, which changes in accordance with the amplification factor of the amplifier from the microprocessing unit based on the switching state; Applying an output from an analog converter corresponding to the amplification factor of the amplifier to one end of the thermocouple through a high resistance, and ensuring that the output of the amplifier swings to the upper limit within a predetermined time when the thermocouple is disconnected. Make it work like this,
Moreover, even when the thermocouple is connected normally, the analog output error from the amplifier due to the applied voltage from the high resistance is configured to operate within a certain tolerance regardless of the setting state of the amplification factor. Features a thermocouple burnout circuit. 2. As a conversion means for converting the digital signal into an analog signal corresponding to the digital signal, the microprocessing unit converts the voltage difference between the voltage when the output is on and the voltage when the output is off by one point of on/off output, and every predetermined period. A digital/analog converter consists of a digitally variable output using so-called pulse width control that varies the output on time and output off time, a filter circuit that smoothes the digitally variable output, and an amplifier that amplifies the output signal of the filter circuit. and applying a voltage corresponding to the amplification factor of the amplifier that amplifies the electromotive force of the thermocouple by applying the output of the digital/analog converter to one end of the thermocouple via the high resistance. A burnout circuit for a thermocouple according to item 1.