JPH0217068B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention relates to a cold junction compensation circuit for a temperature measuring device using a thermocouple, that is, a cold junction compensation circuit that generates a signal corresponding to a thermoelectromotive force corresponding to the temperature of a cold junction of a thermocouple. be. The thermoelectromotive force of a thermocouple is generated by the temperature difference between the hot junction and the cold junction. That is, when the electromotive force of the thermocouple is converted into temperature, the temperature difference between the temperature measuring junction and the cold junction appears. However, when measuring temperature, it is usually not enough to detect only the temperature difference between the temperature-measuring junction and the cold junction; in most cases, it is necessary to detect the actual temperature of the temperature-measuring junction. Therefore, in order to measure the actual temperature of the temperature measuring junction in this way, a signal corresponding to the thermoelectromotive force corresponding to the temperature of the cold junction, that is, the ambient temperature, is obtained, and this signal and the signal corresponding to the above temperature difference are calculated. It is necessary to add it to the electromotive force and find the actual temperature of the temperature measuring contact, that is, the actual temperature of the temperature measured part, from this added signal amount. That is, in order to measure the actual temperature of the temperature-measured part, a so-called cold junction compensation circuit is required, which generates a signal corresponding to the thermoelectromotive force corresponding to the temperature of the cold junction as described above. The invention relates to such a cold junction compensation circuit. [Prior Art] A conventional cold junction compensation circuit of this type will be explained with reference to FIG. In FIG. 1, 1 and 2 are the first and second resistors whose resistance value is R ₁ , and 3 is the resistance _value where α is the temperature coefficient and t is the temperature.
t) is used to detect the temperature of a cold junction (not shown) of a thermocouple. 4 is a third resistor with a resistance value of R ₀ , 5 is an operational amplifier,
It has an inverting input terminal 5a and a non-inverting input terminal 5b. 6 is a reference voltage power supply with a voltage value of E _r , which is a DC power supply. 7 is an output terminal that outputs the DC output voltage _EOA , and 8 is the first to third resistor 1, 2, 4.
A bridge circuit composed of a temperature measuring resistor 3 and an operational amplifier 5 generates a reference voltage E _r from the reference voltage source 6.
is supplied. Reference numerals 8a and 8b are voltage comparison terminals of this bridge circuit 8. Next, the operation of the circuit shown in FIG. 1 will be explained. When the temperature of resistance thermometer 3 is 0℃, the resistance value is
Since _R0 , the bridge circuit 8 is in an equilibrium state. Therefore, the potential difference between the voltage comparison terminal portions 8a and 8b is 0, and the output voltage of the output terminal 7 is also 0. Next, when the temperature of the temperature measuring resistor 3 rises or falls from 0° C., the resistance value changes. That is, the resistance value of one branch of the bridge circuit 8 changes. Therefore, the bridge circuit 8 is in an unbalanced state, and a potential difference occurs between the voltage comparison terminals 8a and 8b in proportion to the change in the resistance value of the temperature measuring resistor 3, and a voltage signal is output from the operational amplifier 5 in accordance with this potential difference. E _OA is output.
This output voltage E _OA is expressed as follows. That is, E _OA =E _r・R ₀ /R ₁ +R ₀・α・t ……(1) =A・t (However, A=E _r・R ₀ /R ₁ +R ₀・α) ……(2 ), and as expressed by equation (2), it changes linearly with temperature. Next, the cold junction compensation error in the circuit shown in FIG. 1 will be discussed. The first one below is J (IC)
The standard thermoelectromotive force of thermocouples is shown by JISC1602,
Table 2 shows the voltage value corresponding to the standard thermoelectromotive force of the thermocouple in Table 1, which is output as A=0.0517 in the circuit of FIG. Here, in the case of the circuit shown in Figure 1, for example, -10° to
When calculating the compensation error of the cold junction over a wide temperature range of 100℃, the compensation error of the cold junction causes a maximum error of -0.098mV at 100℃, and when converted to temperature, this is an error of approximately 1.8℃. will occur. In other words, the circuit shown in Figure 1 has a small compensation error when performing cold junction compensation in a narrow temperature range such as the extremely common 23℃±20deg, but When using a thermocouple as a thermocouple, due to the wide range of vehicles used, the cold junction is affected by the ambient temperature over a wide temperature range, which increases compensation errors and makes it difficult to measure temperature with high precision. The drawback was that it was not possible.

【table】

[Summary of the invention]

This invention was made with the aim of eliminating the above-mentioned drawbacks of conventional circuits and suppressing detection errors caused by the influence of ambient temperature. By focusing on the change in temperature and providing a feedback circuit that feeds back the output of the bridge circuit to the bridge circuit via a compensation resistor, we can generate a signal that approximates the thermoelectromotive force corresponding to the temperature of the cold junction of the thermocouple. The present invention provides a cold junction compensation circuit for a thermocouple-based temperature measurement device that performs cold junction compensation. [Embodiment of the invention] Figure 2 is a connection diagram showing an embodiment of the invention.
In the figure, symbols 1, 2, 3, 4, 5, 5a,
5b, 6, 7, 8, 8a, and 8b are similar to the conventional circuit shown in Fig. 1, and 1 and 2 have resistance values.
The first and second resistance of R ₁ , 3 is the temperature coefficient α,
The resistance value when temperature is t is R ₀ (1+α・t)
This is a temperature measuring resistor expressed as , which detects the temperature of a cold junction part (not shown) of a thermocouple. 4 is a third resistor having a resistance value R ₀ , and 5 is an operational amplifier, which has an inverting input terminal 5 a and a non-inverting input terminal 5 b. 6 is a reference voltage power supply with a voltage value of E _r , which is a DC power supply. 7 is an output terminal for outputting the DC output voltage E _OB ; 8 is a bridge circuit composed of the first to third resistors 1, 2, 4, a temperature measuring resistor 3, and an operational amplifier 5; A reference voltage E _r is supplied from a source 6 . 8a and 8b are voltage comparison terminals of this bridge circuit 8, and 9 is a resistance value (n-1).
Compensation resistor with R ₂ , 10 is compensation resistor with resistance value R ₂ ,
Reference numeral 11 denotes a connection point between the compensation resistors 9 and 10, that is, a voltage dividing point. Reference numeral 12 denotes an adder circuit, which also has an inverting amplification function. 13 is a feedback circuit that feeds back the output _EOB of the operational amplifier 5 to the bridge circuit 8;
It consists of The output voltage E _OB of the bridge circuit 8 is divided by compensation resistors 9 and 10, and the output voltage E _OB of the bridge circuit 8 is further divided by the compensation resistors 9 and 10.
That is, the voltage at the voltage dividing point 11 and the voltage E _r of the reference voltage source 6 are added together by an adder circuit 12 and outputted with the polarity inverted, and the output of this adder circuit 12 is sent to the bridge circuit 8. The supply voltage is Next, the operation shown in FIG. 2 will be explained. In the circuit shown in FIG. 2, the resistance value of the temperature measuring resistor 3 in one branch of the bridge circuit 8 becomes R ₀ when its temperature is 0°C, the bridge circuit 8 is in an equilibrium state, and the voltage comparison terminals 8a, 8b The potential difference between them is 0, and therefore the output voltage at the output terminal 7 is also 0 [V].
As the temperature of the temperature measuring resistor 3 increases or decreases, the resistance value increases or decreases, and the bridge circuit 8 becomes unbalanced. Now, when the above-mentioned equilibrium state is reached, an output voltage E _OB is generated at the output terminal 7 according to the change in the resistance value of the temperature-detecting resistor 3, and a part of this output E _OB is connected to the bridge circuit 8. When added to the voltage E _r of the reference voltage source 6 by the addition circuit 12, the voltage of the reference voltage source 6 is equivalently increased in proportion to the temperature change, and the output E _OB of the bridge circuit 8 is: It changes in a hyperbolic curve with respect to temperature. Now, when we calculate the output E _OB in Figure 2, E _OB = E _r・R ₀ /R ₁ +R ₀・α・t/1−R ₀・α/n(R ₁
+R ₀ )・t……(3) =B・t/1−C・t……(4) (However, B=E _r・R ₀ /R ₁ +R ₀・α, C= R ₀・α/ n(R ₁ +R ₀ )). As shown in equation (4), the output E _OB of the bridge circuit 8 is expressed by a hyperbolic curve, and in equation (4),
By selecting appropriate values for B and C, it is possible to approximate the temperature characteristics of the thermoelectromotive force of a thermocouple (not shown). In the case of the above J (IC) thermocouple, for example, set B and C in the above equation (4) so that B = 5.0756 × 10 ^-2 and C = 3.6526 × 10 ^-4 , and as shown in Fig. 2 above. When the output E _OB in the case of the embodiment is determined, it is expressed as shown in Table 3 below. The maximum error in this case is -0.005 [mV] at -10°C, which is approximately 0.1°C when converted to temperature, which is approximately 1.8°C when compared with the maximum error of the conventional circuit shown in Figure 1. This is an 18 times improvement.

〔Effect of the invention〕

As described above, the present invention provides a cold junction compensation circuit for a thermocouple-based temperature measuring device, which has a bridge circuit having a temperature measuring resistor in one branch, and a reference voltage source that supplies a reference voltage to the bridge circuit. Since a feedback circuit is provided that feeds back the output of the bridge circuit to the bridge circuit through the compensation resistor, the compensation error can be sufficiently reduced compared to the conventional cold junction compensation circuit, and the cold junction compensation can be performed over a wide temperature range. It has the effect of

[Brief explanation of drawings]

FIG. 1 is a connection diagram showing a cold junction compensation circuit of a conventional temperature measurement device using a thermocouple, and FIG. 2 is a connection diagram showing an embodiment of the cold junction compensation circuit of a temperature measurement device using a thermocouple according to the present invention. FIG. 3 is a connection diagram showing another embodiment of the present invention. In the figure, 1 is the first resistor, 2 is the second resistor, 3 is the temperature measuring resistor, 4 is the third resistor, 5 is the operational amplifier, 5a is the inverting input terminal, 5b is the non-inverting input terminal, and 6 is the non-inverting input terminal. Reference voltage source, 7 is an output terminal, 8 is a bridge circuit, and 8a, 8b are voltage comparison terminals. Note that the same reference numerals in each figure indicate the same or corresponding parts.

Claims (1)

[Claims] 1. First and second resistors having the same resistance value and R ₁ ;
A third resistor with a resistance value R ₀ and a third resistor with a resistance value R ₀ (1+
αt) (α is the temperature coefficient, t is the temperature) A temperature measuring resistor for detecting the thermocouple cold junction temperature and an operational amplifier constitute a bridge circuit, and one end of the reference voltage source that provides a reference voltage to this bridge circuit The first resistor and the second resistor
A connection point between the first resistor and the temperature measuring resistor is connected to the inverting input terminal of the operational amplifier, and a connection point between the second resistor and the third resistor is connected to the non-inverting input terminal of the operational amplifier. a point, the other end of the second resistor is connected to the other end of the reference voltage source, and the other end of the third resistor is connected to the output terminal of the operational amplifier,
In the cold junction compensation circuit of the thermocouple-using temperature measuring device, which extracts a voltage signal corresponding to the cold junction temperature of the thermocouple from the output terminal of the operational amplifier, the output voltage of the operational amplifier is calculated by the above-mentioned calculation via the compensation resistor. A cold junction compensation circuit for a thermocouple-based temperature measuring device, characterized in that a feedback circuit for providing feedback is provided on the input side of an amplifier. 2. The feedback circuit has an addition circuit that adds the reference voltage and the divided output voltage of the operational amplifier, and the operational amplifier has a voltage corresponding to the product of the output of the addition circuit and the voltage proportional to the temperature of the cold junction of the thermocouple. 2. A cold junction compensation circuit for a thermocouple-based temperature measurement device according to claim 1, wherein the cold junction compensation circuit outputs a voltage that 3. The cold junction of the thermocouple temperature measuring device according to claim 1, wherein the feedback circuit has a compensation resistor between the output terminal of the operational amplifier and the non-inverting input terminal. Compensation circuit.