JPH0151133B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a digital thermometer using a thermocouple as a temperature sensing element.

When heat is applied to the temperature-measuring junction side of a thermocouple, a thermoelectromotive force is generated on the reference junction side according to the temperature difference between the temperature-measuring junction side and the temperature-measuring junction side. It is known that there is no linear proportional relationship with respect to the temperature difference between the contacts, and that it exhibits so-called non-linear characteristics. Some conventional digital thermometers that use thermocouples are equipped with an error correction circuit called a linearizer to cancel out errors caused by this nonlinear characteristic. An example of this is shown in Figure 1 of the attached drawings. There is.

To give an overview of such a conventional digital thermometer with reference to the figure, what is the voltage of the thermoelectromotive force generated in the thermocouple 1 and the compensation voltage formed by the reference junction compensation circuit 2 during temperature measurement? The voltage is sent to the differential circuit 3, and the difference between the two voltages is obtained as its output. In a thermometer that uses a thermocouple, if the reference junction is at a temperature other than 0°C, for example the same temperature as room temperature, and the temperature of the object to be measured is measured in that state, the measured value will be at the reference junction. There is a deviation from the value obtained when the temperature is maintained at 0° C. by an amount corresponding to the above-mentioned room temperature. The compensation voltage mentioned above is for canceling this deviation.
As described above, a voltage that is the difference between these compensation voltages and the voltage of the thermoelectromotive force appears on the output side of the differential circuit 3. This difference voltage is sent from the differential circuit 3 to the error correction circuit 4 as a temperature measurement signal, and after its non-linear characteristics are converted into linear characteristics, it is sent via a decoder circuit 5 to a display having, for example, a liquid crystal element. It is added to the circuit 6 to display the measured temperature.

In this conventional example, the error correction circuit 4 is composed of, for example, an A/D converter 7 and a microcomputer 8 including a ROM. The analog temperature measurement signal input from the differential circuit 3 is
The signal is converted into digital data by the A/D converter 7 and sent to the microcomputer 8. In this microcomputer 8, correction data for linearizing the thermoelectromotive force characteristics with respect to the temperature of the thermocouple 1 is stored in its ROM, and this data is called at a predetermined timing and sent from the A/D converter 7. The incoming temperature measurement signal is subjected to linearization processing and sent to the decoder circuit 5.

Conventional digital thermometers like this, which use a microcomputer, can perform complex arithmetic processing on the temperature measurement signal, which helps improve measurement accuracy, but the equipment is expensive because it has many relatively expensive parts. Furthermore, there is a drawback that the price cannot be easily reduced even in cases where high precision is not required in practice.

In the case of a digital thermometer that does not use a microcomputer, the absolute level of the temperature measurement signal sent from the differential circuit 3 is generally extracted at an appropriate timing by a sampling circuit (not shown), etc.
This extraction level is linearized via, for example, a function generating circuit (not shown), and then A/D conversion is performed. In this case, as the temperature measurement range widens, the temperature measurement signal also covers a wide range from low level to high level, and it has been difficult to avoid complication of the sampling circuit and function generation circuit.

This invention was made in view of the above points, and its purpose is to provide a digital thermometer that has sufficient accuracy for practical use without using a microcomputer, is simple in construction, and is inexpensive. .

Hereinafter, the present invention will be explained in detail with reference to an embodiment shown in FIG. 2 of the accompanying drawings. In this figure, the same reference numerals are given to the same structural units as those of the conventional example shown in FIG. 1 above.

Comparing the embodiment of the present invention shown in FIG. 2 with the conventional example described above, the configuration of the error correction circuit is greatly different. That is, this error correction circuit 10
For example, the buffer circuit 12 and the broken line approximation circuit 13
, A/D converter 14, and other units are easily available, and do not include a relatively expensive microcomputer or the like. Next, step by step, the voltage of the thermoelectromotive force generated in the thermocouple 1 by temperature measurement is sent to a differential circuit 3 using, for example, an operational amplifier. On the other hand, the compensation voltage formed by the reference junction compensation circuit 2 is
The voltage is sent to the differential circuit 3 via the differential circuit 3, and a voltage corresponding to the difference between the voltage of the thermoelectromotive force and the compensation voltage appears on the output side, as in the case of the conventional example. This voltage difference is sent to the next stage A/D converter 14 as a temperature measurement signal.

In this embodiment, the reference junction compensation circuit 2
For example, a resistor and a diode for detecting the reference junction temperature of thermocouple 1 are connected in a bridge configuration, and when the reference junction temperature fluctuates above or below a preset room temperature value, the fluctuation The polarity of the above-mentioned compensation voltage is reversed depending on the polarity of the compensation voltage. In addition, the buffer circuit 12
is composed of a so-called voltage follower type circuit in which, for example, the output side of an operational amplifier is connected to its (-) input side, and when circuits are connected to the input side and output side of this buffer circuit 12, respectively, The impedances of both circuits are prevented from influencing each other.

The voltage of the thermoelectromotive force and the compensation voltage sent out from the buffer circuit 12 are respectively
3 is also input. This polygonal line approximation circuit 13 is composed of, for example, an operational amplifier, a resistor, a capacitor, a diode, etc., and calculates a composite voltage of a thermoelectromotive force voltage and a compensation voltage whose voltage level changes in a curved manner with respect to temperature changes. It is designed to be replaced with a voltage having a polygonal characteristic along the above-mentioned curve and output. This substituted output voltage is sent to, for example, an integral type A/D converter 14, and is used as a reference voltage for converting the temperature measurement signal inputted from the differential circuit 3 into a digital value. .

To explain the functions of the broken line approximation circuit 13 and the A/D converter 14 with reference to FIG. 3, as shown in FIG. In response to this, a thermoelectromotive force as shown by the solid line is generated. In this case, the thermoelectromotive force may be regarded as a temperature measurement signal after temperature compensation of the reference junction, and a temperature difference of 0 to t3° C. may be considered as the temperature measurement range. The virtual line shown in the figure represents a case of ideal characteristics in which the magnitude of thermoelectromotive force changes linearly with respect to temperature difference.

In the above-mentioned broken line approximation circuit 13, this temperature measurement range is, for example, 0 to t1℃, t1 to t2℃, t2
When divided into three regions from t3°C to t3°C, a reference voltage for each region is formed. In other words, when converting a thermoelectromotive force with an ideal linear characteristic as shown by the imaginary line in A/D, the reference voltage may be a constant level voltage as shown in the imaginary line in B of the figure. Regarding the thermoelectromotive force having the non-linear characteristic shown by the solid line in (b), it is desirable to use a voltage that changes in a curved manner as shown by the solid line (b) in response to deviation from the ideal linear characteristic as a reference.
However, in this case, it is necessary to subdivide the temperature measurement range and form a reference voltage for each region, making the circuit complicated. In this embodiment, the desired curved reference voltage is replaced by approximating the curved line in three regions as shown by the dotted lines in (b). If this is used as the reference voltage of the A/D converter 14, when a thermoelectromotive force with nonlinear characteristics is directly input as a converted signal to the A/D converter 14, the nonlinear part will be canceled by the reference voltage. The converted digital value has a substantially linear characteristic with respect to the measured temperature. An example of the error after the above conversion is shown by the dotted line in Figure C, but it is natural that if the measured temperature range is divided into more regions and a broken line approximation is performed, this error will become smaller accordingly. .

The digital output corrected to the above linear characteristic is
For example, a decoder circuit 1 composed of logic elements, etc.
1 to a display circuit 6, and the measured temperature is displayed as in the conventional example.

As described above in detail, the digital thermometer according to the present invention includes an error correction circuit 10 consisting of a buffer circuit 12, a broken line approximation circuit 13, and an A/D converter 14, and when measuring temperature, Temperature compensation voltage is buffer circuit 12
is added to the thermoelectromotive force via the voltage, and a temperature measurement signal is formed by combining both voltages.
On the other hand, in the polygonal line approximation circuit 13, a reference voltage for A/D conversion is formed from the temperature measurement signal. This reference voltage and the temperature measurement signal are each sent to an A/D converter 14, and the temperature measurement signal is digitally converted. This error correction circuit 10 has a simple configuration using a known circuit and does not include a microcomputer, which can greatly contribute to reducing the cost of the apparatus. Furthermore, compared to the case where the temperature measurement signal to be A/D converted is linearized in advance at its absolute voltage level, the configuration of the broken line approximation circuit 13 in this error correction circuit 10 is also extremely simple.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a conventional digital thermometer, FIG. 2 is a block diagram of a digital thermometer according to the present invention, and FIG. 3 is an explanatory diagram of thermoelectromotive force, reference voltage, error, etc. In the figure, 1 is a thermocouple, 2 is a reference junction compensation circuit, and 3 is a thermocouple.
is a differential circuit, 6 is a display circuit, 10 is an error correction circuit, 12 is a buffer circuit, 13 is a broken line approximation circuit,
14 is an A/D converter.

Claims (1)

[Claims] 1. A thermocouple whose reference junction is temperature-compensated by a compensation voltage from a reference junction compensation circuit is used as a temperature-sensing element, and the thermocouple generated by heat applied to the temperature-sensing element is In a digital thermometer that converts electric power into a digital signal via an error correction means and displays the temperature thereof, the error correction means includes a buffer circuit that receives a compensation voltage from the reference junction compensation circuit and outputs it. , a differential circuit that forms a temperature measurement signal from two voltages, the compensation voltage output from the buffer circuit and the voltage from the temperature sensing element; and a nonlinear characteristic of the temperature measurement signal that receives the two signals. a polygonal line approximation circuit that replaces and outputs a voltage with a polygonal characteristic along the non-linear characteristic; and a polygonal line approximation circuit that uses the output from the polygonal linear approximation circuit as a reference voltage, and converts and inputs the temperature measurement signal from the differential circuit. A digital thermometer characterized by comprising an A/D converter for generating a signal.