JPS59774B2 - temperature conversion circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a temperature conversion circuit that receives thermoelectromotive force from a thermocouple as input.

In the field of process measurement and control, thermoelectric temperature converters are used to measure temperature using a thermocouple as a detection end.

A conventional thermoelectric temperature converter has a configuration as shown in FIG.

That is, in the figure, the thermoelectromotive force Ei of the thermocouple 1 outputted via the compensation conductors L1 and L2 applied to the input terminals 2 and 3 is generated by three factors: cold junction temperature compensation, zero point transition, and zero point correction.
The combined resistances of the two actions R1, R2, R3, R4,
After being added to the output voltage Ep of the flip circuit A constituted by the variable resistor RVI, a unified desired electric signal is generated by the modulation type DC amplifier constituted by the preamplifier B and the feedback circuit C. is converted into and guided to output terminals 4 and 5. Here, the preamplifier B converts a DC low mV input signal into an AC signal via a low-pass filter circuit 6 and an orthogonal converter 7, then amplifies the AC signal with an AC amplifier circuit 8, converts it into an AC signal through an isolation transformer 9, and performs synchronous rectification. After being reconverted into a DC signal via the circuit 10, the DC amplification circuit 11 amplifies the DC signal to produce an output signal.

On the other hand, the feedback circuit C that receives the output signal passes through the linear rice circuit 12, converts it into an AC signal with the orthogonal converter 13, passes through the feedback transformer 14, converts it into a DC signal again with the rectifying and smoothing circuit 15, and then , are input to the voltage dividing circuit 16.

Then, the feedback signal from the voltage divider circuit 16 is sent to the preamplifier B.
The configuration is such that negative feedback is provided to the previous stage. The conventional thermoelectric temperature converter shown in FIG. 1 having such a structure has a drawback of having a complicated circuit structure.

In addition, the temperature-insensitive resistances R3 and R4 of the conventional thermoelectric temperature converter
are selected to have the same resistance value and a sufficiently large resistance value compared to resistors R1, R2 and variable resistor RVI, and are equivalently driven at a constant current. Resistance R2 depending on value and temperature measurement range
However, since the value of the variable resistor R1 varies widely and cannot be determined uniquely, it has the drawback that adjustment is extremely difficult.

In addition, the temperature-sensitive resistor R1 is made by winding a controlled copper wire around a bobbin, but it must be made to a specified resistance value while maintaining the ambient temperature at 0°C, which is extremely difficult and requires a long time. This method has the disadvantage of requiring time adjustment man-hours.

In addition, the parts-out operation when thermocouple 1 is disconnected is as follows.
The supply voltage E8 of the bridge circuit is divided by the resistors R2, R4 and the variable resistor RVl, and the changeover switch SWl
．． It is applied to the input end of the modulation type DC amplifier via the high resistance R5.

When the changeover switch SWl is on the "UP" side, the output signal swings out to the upper limit, and when it is on the "DOWN" side, it swings out to the lower limit.The operating time until the output signal swings out is the input gas span, or The input/output conversion gain C is determined by a time constant determined by the applied divided voltage, the value of the resistor R5, and the capacitance value (not shown) of the low-pass filter circuit 6.
On the other hand, since the resistance value of the divided voltage is selected as R4>R2, when the changeover switch SWl is set to "UP", the divided voltage almost becomes the value of the bridge circuit supply voltage E5 (several volts), but "D"
The divided voltage at the time of OWN is divided into small values ranging from several MV to several + m. Therefore, in the conventional thermoelectric temperature converter, from ``UP'' to DOWN (56D0WN''
In order to equalize the operating time when
cannot be switched.

Moreover, depending on the type of thermocouple and the measurement temperature range, R2, R
Since the values of Vl are different, the divided voltages are also different, and the value of the resistor R5 is not uniquely determined, which has the disadvantage that the resistance value must be selected each time. Furthermore, since the supply voltage E5 of the bridge circuit A is floating with respect to the reference potential of the AC amplifier circuit 8, it needs to be DC-insulated from the power supply for driving the AC amplifier.

This not only complicates the power supply circuit (not shown), but also generates stray voltage between the windings of the power transformer, which may adversely affect the output. In addition, in order to convert any measurement temperature range into a predetermined desired electrical signal, in addition to adjusting the zero point transition as described above, it is necessary to increase the input/output conversion gain of the modulating DC amplifier from several orders of magnitude to several thousand times. It must be possible to change the settings significantly.

This can be done by changing the amount of negative feedback by changing the voltage dividing ratio of the voltage dividing circuit 16 of the feedback circuit C. Note that the voltage dividing circuit 16 is also equipped with a span fine adjustment function (not shown). Furthermore, according to the circuit configuration of FIG. 1, the bridge circuit A
There is a relationship between the amount of zero point transition and the input/output conversion gain of the modulating DC amplifier. That is, the zero point adjustment and the span adjustment interfere with each other, and cannot be adjusted independently of each other, requiring repeated adjustment several times. Particularly, fine adjustment is required in the case of a signal including a bias component as the initial output signal, for example, 4 to 20 mA DC or 5 V DC. An object of the present invention is to provide a temperature conversion circuit with a simple circuit configuration and easy adjustment work.

The present invention is configured so that settings for input/output conversion gain, automatic cold junction temperature compensation, zero point transition, and parts-out operation can be changed independently, and each circuit has a common reference potential.
The purpose is to simplify the circuit configuration and simplify the adjustment work by allowing fine adjustment of the zero point and span to be made independently. Examples of the present invention will be described below.

FIG. 2 shows a temperature conversion circuit showing one embodiment of the present invention.

In the drawings, parts designated by the same reference numerals as those used in the conventional example shown in FIG. 1 are the same parts and have the same functions.

In the figure, A1 is a high-precision operational amplifier (for example, a highly stable 1C operational amplifier with an offset voltage of 0.5 microvolts/°C), and A2 is a general-purpose operational amplifier, both of which are stabilized positive and negative pairs. Constant voltage source (+) Vc・, (-)
It is driven by V. Q1 is a semiconductor temperature sensing element for sensing the ambient temperature (it is now available as an IC semiconductor temperature sensor at a low cost commercially).
A positive voltage source (+) V is applied to one terminal via the changeover switch SW3, and an output current of 1k with excellent linearity that changes in response to changes in ambient temperature is obtained from the other terminal. It is something. This current 1k and negative voltage source (-)V・
The constant current 1 s obtained from the resistor R7 and the changeover switch SW4 varies depending on the type of thermocouple connected between the input terminal 3 connected to the cold junction temperature side of the thermocouple 1 and the reference potential. The currents are configured to flow in opposite directions through reference resistors R6 having different values. (+)V・or (1)Ve
These temperature sensing elements Ql. A cold junction temperature automatic compensation circuit 100 is constituted by resistors R6 and R7. Then, the compensation conductor Ll applied to the input terminals 2 and 3,
The thermoelectromotive voltage E, of the thermocouple 1 is applied via L2 to the reference resistance R6.
After being subtracted from the voltage Ee generated across the resistor R8,
It is applied to the positive input terminal of operational amplifier A1 via an r-type CR low-pass filter circuit 101 consisting of R9 and capacitors Cl and C2. Furthermore, a resistor RIO for changing the input/output conversion gain setting is connected between the output terminal and the negative input terminal of the operational amplifier A1, and a resistor R1l is connected between the negative input terminal of the operational amplifier A1 and the reference potential. Variable resistor R for span adjustment
Feedback circuit 10 of operational amplifier A1 by series circuit of V2
2 are configured. Further, a positive voltage source (+) Vs is connected to the contact point "+B" of the changeover switch SW2, a negative voltage source (1) Ve is connected to the contact point "-B", and a reference potential is connected to the contact point "B.". Select one of them and set the resistor Rl
A zero point transition voltage Ea divided by a voltage dividing circuit 103 consisting of a resistor Rl3 and a resistor Rl3 is applied to the negative input terminal of the operational amplifier A1 via a resistor Rl4.

Here, the value of resistor Rl4 is equal to the value of resistor Rl3 and resistor R1.
It is selected to be sufficiently larger than the sum of l and variable resistor RV2. A bias circuit 104 for performing zero point transition is constituted by the changeover switch SW2 and the resistors Rl2Rl3, which use (+)Vc and (-)Ve as voltage sources.
Moreover, the output voltage ER of the operational amplifier A1 is the output voltage ER of the operational amplifier A2.
is applied to the positive input terminal of the positive and negative voltage source (+)Vc
, (-)Ve, the zero point correction voltage Ed obtained from the slider of the variable resistor RV3 of the zero point adjustment circuit 105 consisting of a series circuit of the resistor Rl5, the zero point adjustment variable resistor RV3, and the resistor Rl6 connected to both ends of is applied to the negative input terminal of operational amplifier A2 via resistor Rl7.

Further, a resistor Rl8 is connected between the negative input terminal and the output terminal of the operational amplifier A2, and a feedback circuit 106 of the operational amplifier A2 is constituted by the resistors Rl7 and Rl8. Here,
The value of the resistor Rl7 is selected to be sufficiently larger than the output resistance of the zero point adjustment circuit 105. In addition, the changeover switch SW
A positive voltage source (+) Vc is connected to the contact ``UP'' of I, and a negative voltage source (1) Ve is connected to the contact ``D0WN'', and either one is connected and a voltage dividing circuit is formed by resistor Rl9 and resistor R2O. The voltage Eb divided by is connected to the input terminal 2 to which the hot junction temperature side of the thermocouple 1 is connected via a high resistance R2l.

(+)V. A parts-out circuit 101 is constituted by a changeover switch SWI whose voltage source is (-)Ve and resistors Rl9 to R2l. Here, the value of the resistor 21 is selected to be sufficiently larger than the value of the resistor R2O, and also to prevent the current Eb/R2l from flowing to the thermocouple 1 side during steady state, and an error exceeding the allowable value due to the voltage drop will not occur. In addition, the value of the resistor R2l is selected to be sufficiently large. Since it is configured in this way, the part-out circuit 10 during normal operation.
T is equivalent to not connected, and operational amplifier A1
The output voltage ER of is expressed by the following equation.

Here, 0 is the position of the slider of the variable resistor RV2 (total resistance value RV2), which changes in the range of 020 to 1.

This allows fine adjustment of the span. Also,
Output voltage E. of operational amplifier A2. is expressed by the following equation.

Here, constant determination and adjustment of each function is performed in the following steps.

(a) Setting change of input/output conversion gain In equations (1) and (2), Es=0 (switch SW3
Both SW4 are set to ``0FF''), Ea20 (switch SW2゜゛B.''゜), input gas span voltage Eia, output gas span voltage E.

S, the input/output conversion gain G is given by equation (3). Here, [1+ RIO/( Rll+ O − R
V2): is the amplification degree of operational amplifier A1, (1+Rl8/R
l7) is the amplification degree of operational amplifier A2.

By appropriately distributing the amplification using two operational amplifiers in this way, sufficient feedback is provided even if the input/output conversion gain varies greatly depending on the input gas span, ensuring a loop gain that allows stable operation. can do. Note that in equation (3). In this case, the amplification degree of operational amplifier A2 is fixed to an optimum value.

Furthermore, the slider position of the variable resistor RV2 can be set to the center when 0=0.5. Then, the value of variable resistor RV2 is set to resistor R1l for fine span adjustment.
Since it is selected to be sufficiently smaller than the value of , it can be considered that ゜.

That is, equation (3) is given by equation (6).

Here, G=ED(s)/Ei(0, and the output gas span voltage ED(0) (for example, the desired output signal is DCI~5V
Since ED (024 V) is predetermined, the value of the resistor RIO can be calculated using equation (6) by giving the input gas span voltage El (s). Then, from the measurement temperature range, the input gas span voltage E
, (s), the value of the resistor RIO can be calculated in advance.

That is, the setting change of the input/output conversion gain is performed by the resistor RIO.

Note that errors caused by variations in circuit resistance can be corrected by performing span adjustment with the variable resistor RV2.
(b) Change of setting of zero point transition amount From equations (1) and (5), obtain the force equation.

The zero-point transition voltage Ea of the bias circuit that determines the amount of zero-point transition is expressed as Ee=O (switches SW3, S
Both W4 are 0FF), and when the input voltage corresponding to the lower limit of the measurement temperature range is E, (Mln), the output ER = 0
It is determined by

That is, in equation (8), the value of resistance Ra can be considered constant as shown in equation (5), the value of resistance RIO has already been selected in section (a), and the value of resistance Rl4 is set to a constant value. If it is fixed, the zero-point transition electric power E, can be calculated.

Once the value of Ea is determined, a bias circuit is used to determine the value, so if one of the resistors Rl2 and Rl3 is fixed to a constant value, the Ikekata resistance value can be calculated.

Note that in equation (8), (+)V. The selection of (-)Ve can be made by switch SW2, and which one is selected depends on the polarity of Ei (Rllin). For example, if the input signal range is 10-15mV (Ei(
Rrlin) = +10mV), switch SW2
The contact point should be on the 〃+B'' side.

In this way, the setting of the zero point transition amount can be changed. It should be noted that errors caused by assuming equation (5) and errors caused by variations in circuit resistance are both extremely small and can be corrected by the zero point adjustment circuit 105.

(c) If the cold junction temperature automatic compensation switch SW3 is set to 0N, the semiconductor temperature sensing element Q1
A voltage source (+) Vc is supplied to
Microamps/0k flows into reference resistor R6.

Here, 1k is obtained based on the absolute temperature of zero; for example, when the ambient temperature is 20'C, 1k = 293.2
It becomes microampere. In order to convert this to atmospheric temperature, switch SW4 is set to 0N" and a negative voltage source (-
)Ve and resistor R7, a constant current of 273.2 microamperes is obtained by Isu1'Ve/R721, and as shown by the arrow in the figure, it is applied to the reference resistor R6 in the opposite direction to Ik and subtracted. As a result, the voltage Ec generated across the resistor R6 becomes Ee2R6 (Ik-1s), which is converted into a value proportional to the atmospheric ambient temperature. Here, depending on the type of thermocouple 1, by selecting the value of resistor R6 so that a voltage equivalent to the thermoelectromotive force can be obtained from both ends of reference resistor R6, voltage Ee for cold junction temperature compensation can be adjusted. can get.

By adding this voltage with the input signal Ei, automatic cold junction temperature compensation is achieved. Here, it is important to note that the connection point of the compensation conductor LlL2, that is, the input terminals 2 and 3
If there is a temperature difference between the ambient temperature of the temperature sensing element Q1 and the ambient temperature of the temperature sensing element Q1, a cold junction compensation error occurs.

Therefore, care must be taken to prevent a thermal gradient from occurring between the two. Note that compensation errors caused by variations in the absolute value of the output current 1k of the temperature sensing element Q1 (equivalent to about 1 to 2'C) and variations in the constant current 1s are, for example, the temperature difference corresponding to the lower limit of the measurement temperature range. When the value obtained by subtracting the thermoelectromotive force value corresponding to the ambient temperature at that time from the electromotive force value is added as an input signal, the zero point adjustment circuit 105 corrects the output signal ED so that it becomes 0. be able to.

The zero point can be corrected by changing the zero point correction voltage Ed in equation (2). The value of Ed is determined by the zero point adjustment circuit 105.
This can be done by changing the variable resistor RV3. Finally, thermocouple 1 is disconnected, and input terminals 2 and 3
Let's discuss the case where the gap becomes open.

(d) Changing the operating time setting of the parts-out circuit 107 If the thermocouple 1 is disconnected, the divided voltage Eb of the parts-out circuit 107 is transmitted to the operational amplifier A1 via the high resistance R2l and the r-type CR low-pass filter circuit 101. is applied to the input terminal of

Therefore, the output signal E. quickly swings out of the defined standard signal range. Here, the operating time from the moment the thermocouple 1 is disconnected until the output swings is determined by the time constant determined by the value of the resistor R2l, the combined capacitance value of the capacitors Cl and C2 of the filter circuit 101, and the magnitude of the applied voltage Eb. It is determined by the input/output conversion gain, that is, the input gas span voltage. Therefore, in order to keep the operating time constant regardless of the measurement temperature range, if the time constant R2l.(C1+C2) is kept constant, the value of the voltage Eb can be changed depending on the input gas span voltage. The value of Eb is determined by the voltage dividing resistor of the part-out circuit and is given by the formula (CLO). In formula (10),
Since the value of the voltage Eb is determined if the input gas span voltage is given, by fixing one of the resistors R19 and R2O to a constant value, the resistance value of the other can be calculated.

In addition, in the AO) formula, the selection of (+)Vc and (1)Ve can be done with the switch SWI.If you set it to the "UP" side, the output will operate at the upper limit, and if you set it to the ``D0WN'' side, the output will change to the upper limit. It is possible to operate at the lower limit.

Furthermore, if the absolute values of (+)Vc and ()Ve are selected to be equal, the part-out operation direction can be freely selected and used using the switch SWI.

As described in sections (a) to (d) above, the settings of each function can be changed using four selected resistors: resistor R6, RIO, resistor Rl2 or Rl3, or resistor Rl9 or R2O. There are these resistors at the input gas span i.e.
Given the measurement temperature range, it can be calculated in advance.

Therefore, the adjustment only requires fine adjustment of the zero point and span using the variable resistors RV2 and RV3. Note that in this embodiment, the circuit configuration of the operational amplifier A2 is such that the output of the operational amplifier A1 is applied to the positive input terminal, and the output of the zero point adjustment circuit 105 is applied to the negative input terminal, but the present invention is not limited to this. In short, the output of the operational amplifier A1 is amplified to a desired value, and the output of the operational amplifier A1 and the zero point adjustment circuit 1 are
Any amplifier configuration that meets the two requirements of outputting the result of addition with the output of 05 can be applied.

In addition, the zero point adjustment circuit 105 also has positive and negative voltage sources (+) VO, (
-) o is connected to both ends, but it may be configured to use either one of the voltage sources. In addition, in this embodiment, each circuit is constructed using the stabilized voltage sources (+) o and (1) o, but the parts-out circuit 107
, the power supplies of the semiconductor temperature sensing element Q1 and the operational amplifier A2 do not need to be stabilized, and are stabilized voltage sources (+)E, (-)Ve.
Even if the rectified and smoothed output before stabilization is used as it is to obtain , the stability of this conversion circuit will not be affected in any way.

Furthermore, in this embodiment, a positive voltage (+) Ve is applied as the power source for the temperature sensing element Q1, and the polarity of the output current 1k is also in the direction of the arrow in the figure. Some operate on power.

In this case, the polarity of the output current 1k also flows in the opposite direction to the direction of the arrow shown in the figure, so the polarity of the thermocouple 1 is also opposite to that shown, and the hot junction temperature side is connected to the input terminal 3, and the cold junction temperature side is connected to the input terminal 2. Similarly, the same result can be obtained by connecting the positive and negative voltage sources (+) VO and (-) VO in opposite polarities. Furthermore, in the bias circuit 104 shown in this embodiment, the changeover switch SW
2, the positive and negative voltage sources (+) Ve and (1) Ve are selected, but both ends of the voltage divider circuit (series circuit) 103 of the resistor Rl2 and the resistor Rl3 are connected to both ends of the positive and negative voltage sources. You may also do so.

However, in this case, calculation of the circuit resistance becomes more troublesome than in this embodiment. Further, in the embodiment of FIG. 2, the operational amplifier A1
A series circuit consisting of a resistor R1l and a variable resistor R2 for span adjustment was connected between the negative phase input terminal of the R11 and the reference potential, but only the resistor R11 was connected between them, and one terminal of both ends of the variable resistor R2 is connected to the output terminal of operational amplifier A1, the other terminal is connected to a common potential via a resistor, and the terminal from the slider of variable resistor RV2 is connected to the positive phase input terminal of operational amplifier A2. , can be similarly applied by selecting the value of the resistor sufficiently larger than the resistance value of the variable resistor RV2.

In addition, in this embodiment, changeover switches were used as SW1 to SW4, but since these switches are rarely used, a land hole was provided on the printed board and a changeover circuit was constructed using zipper wiring. Good too.

Note that after completing the setting changes in items (a) and (5) above, the resistance R
If you follow the procedure of installing 7 and temperature sensing element Q1,
Switches SW3 and SW4 can also be omitted.
Therefore, according to this embodiment, the settings of the input/output conversion gain, zero point transition, automatic cold junction temperature compensation, and parts-out operation can be changed independently, making adjustment easy.

Further, according to this embodiment, fine adjustment of the zero point and span can be performed independently.

Further, according to this embodiment, the circuit configuration is simple because the reference potentials of the circuits constituting each function are all common, and the circuits operate using only a pair of positive and negative voltage sources.

Furthermore, according to this embodiment, even if the input/output conversion gain varies greatly depending on the input gas span, C can operate with sufficient stability by appropriately distributing the amplification using two operational amplifiers. Since the amplifier can be configured to obtain the desired result, special circuit design techniques are not required unlike the conventional example.

As described above, according to the present invention, the circuit configuration can be simplified and the adjustment work can be simplified.

The temperature conversion circuit of the present invention has a thermoelectric temperature conversion circuit by adding a standardized linear rice circuit and a human output insulation circuit (not shown) to the output side of the operational amplifier A2 shown in the embodiment of FIG. Applied as a converter.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a conventional thermoelectric temperature converter, and FIG. 2 is a circuit diagram of a temperature conversion circuit showing an embodiment of the present invention. 1...Thermocouple, 100...Cold junction temperature automatic compensation circuit, 101...π-type CR low-pass filter circuit, 102, 106...Feedback circuit , 103
...Voltage divider circuit, 104...Bias circuit, 105...Zero point adjustment circuit, 107...
・Parts out circuit.

Claims (1)

[Claims] 1 The ambient temperature is controlled by a pair of positive and negative constant voltage sources, an output current that changes in response to changes in the ambient temperature, and a constant current that flows in the opposite direction to the output current supplied from the constant voltage source. a cold junction temperature compensation circuit that obtains the thermoelectromotive force of the thermocouple compensated for, and an additional voltage of the voltage output from the cold junction temperature compensation circuit and the measured electromotive force corresponding to the temperature measured by the thermocouple. a low-pass filter circuit for passing a low-pass voltage, a bias circuit for outputting either a zero-point transition voltage obtained by dividing the positive and negative voltages of the constant voltage source and a reference voltage, and the low-pass filter circuit. A first circuit for amplifying the difference voltage between the output voltage from the bias circuit and the output voltage from the bias circuit.
an operational amplifier; a zero-point adjustment circuit that divides the output voltage from the constant voltage source into a predetermined voltage using a variable resistor and outputs the divided voltage; and a zero-point adjustment circuit that divides the output voltage from the first operational amplifier and the zero-point adjustment circuit. A second operational amplifier amplifies and outputs the difference voltage from the output voltage, and when the thermocouple does not output voltage due to disconnection etc., a voltage obtained by dividing the voltage of either of the pair of positive and negative constant voltage sources is also connected to the thermocouple. and a burnout circuit that outputs a voltage measured by the temperature converter to the low-pass filter circuit.