JPS6118687B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a thermocouple thermometer that converts measured temperature into a digital value and displays it.

Using a thermocouple whose output voltage changes depending on the temperature,
A digital thermocouple thermometer that digitally displays a measured temperature by converting its output voltage from analog to digital is well known. However, the output voltage characteristic of a thermocouple is non-linear with respect to temperature, so if the output voltage is directly converted to temperature, an error will occur in the measured value. As an example, chromel-aromel (C-
A) The error distribution for the temperature measured by the thermocouple is
As shown in the figure.

The C-A thermocouple has a voltage of 41 μV per 1°C near room temperature.
generates an electromotive force. However, when converting to temperature based on this 41 μV/℃, as shown in Figure 5, the measured temperature is only equal to the measured temperature at around 0℃, 100℃ and around 400℃, and at other temperatures. This results in a maximum error of 11.5℃. Therefore, in order to accurately display the measured temperature, it is necessary to linearize the output voltage of the thermocouple.

FIG. 1 is a diagram showing an example of an AD converter equipped with a digital linearizer used in a conventional thermocouple thermometer.

In Fig. 1, 101 is a thermocouple (not shown)
110 is a well-known integral type AD converter that converts the output of the buffer 101 from analog to digital, and 111 and 112 are decoders that display the output of the integral type AD converter. These are the driver and display section. The non-linearity of the thermocouple causes the time pulse width converted by the input signal-pulse width converter 102 of the integral type AD converter 110 to
It is corrected by modulating the reference pulse used to convert the pulse train. That is, integral type A
- Has a storage capacity according to the resolution of the D converter 110, and is addressed by the output of the counting section 103.
ROM104 (read only memory) and the above
It is equipped with a programmable frequency divider 106 controlled by the output of the ROM 104, and according to the contents of the counting section 103, the contents stored in the ROM 104 corresponding to the address are read out, and the contents stored in the ROM 104 are read out.
The programmable frequency divider 10 divides the output of
By changing the frequency division ratio of 6, linearization is performed. Therefore, it requires a ROM with a very large number of bits and a programmable frequency divider that requires a complex logic circuit, making it a very complicated and expensive device. However, in addition to the problem of integrating the ROM into a single chip, the number of elements becomes enormous, leading to increased costs.In particular, if the material of the thermocouple used changes, the distribution of errors with respect to temperature also changes. , the contents of the ROM must be changed accordingly, making the ROM and its peripheral circuitry extremely expensive.

In FIG. 1, 107 is a series of switches for switching the reference pulse, 109 is an inverter, and 10
A switch control circuit 8 controls the operation of the switch array 107 according to the operation sequence of the integral type AD converter 110.

Furthermore, the cold junction compensation circuit used in conventional thermocouple thermometers has a configuration as shown in FIG. 2, and requires a floating power supply 201 for the bridge, resulting in extremely large power consumption. In addition, since the adder circuit 202 is required, the circuit has disadvantages such as being complicated and expensive.

In FIG. 2, 203 to 205 are resistors, 206 is a thermistor, and 210 is a thermocouple, and the output of the adder circuit 202 is inputted from an output terminal 207 to an AD converter.

An object of the present invention is to solve the conventional drawbacks as described above and to provide a simple and inexpensive digital linearization method.

An object of the present invention is to simplify the power supply by common grounding the power supply for the cold junction compensation bridge with the normal power supply, and to provide a thermocouple thermometer that is inexpensive, has low noise, and has low power consumption. .

Still another object of the present invention is to provide an ultra-low power consumption thermocouple thermometer suitable for a hand-held type that can be used continuously for several hundred hours even with a small battery.

FIG. 3 shows an example of a thermocouple thermometer using the digital linearization method according to the present invention. In FIG. 3, 101 is a buffer consisting of an operational amplifier in front of a FET similar to the conventional one, 330 is an automatic zero correction circuit, 110 is an integral type AD converter using the well-known double integration method, and 111 is a decoder driver. , 11
2 is a display unit, 210 is a thermocouple, 321 is a low idling current reference voltage source, 322 is a cold junction compensation circuit according to the present invention, and 323 to 325 are integral type ADs.
The input resistance of the converter 110, 328 is an input resistance for inverse integration, and 327 is a digital linearization circuit according to the present invention.
3, 304 and an inverter 307, a switch drive circuit 305 that controls the operation of the switch circuit 331 according to the timing of AD conversion, a differentiator 306, a counter 308, and a logic circuit. section 309 and an oscillator 310
and a frequency dividing circuit section 332 consisting of timing counters 311, 312 and differentiators 313, 314,
A gate circuit section 333 consisting of AND circuits 315 and 316, an AND circuit 318, and an OR circuit 31
9 and an inverter 317.
It consists of 34.

When the above-mentioned C-A thermocouple is used as the thermocouple 210, the C-A thermocouple has a
Since an electromotive force of 41 μV is generated, this is multiplied by about 25 times by the buffer 101 to amplify it to a signal of 1 mV per 1° C., and is input to the integrating type AD converter 110 via the input resistor 323.

FIG. 4 shows a cold junction compensation circuit 322 according to the present invention.
401 is an input terminal 406
This is an inverter that inverts the output of the reference voltage source 321 input from the reference voltage source 321 with an amplification degree of 1. The output of this inverter 401 is a thermistor 402 and a resistor 403.
The voltage is divided by and output from the output terminal 407. At this time, a constant is determined so that the change is 1 mV per 1°C. Also, the output terminal 407 at 0°C
Constants are determined so that the output of the reference voltage source is divided by resistors 404 and 405 and output to output terminal 408, with the opposite sign and the same absolute value as the output of . The outputs of output terminals 407 and 408 are connected to input resistors 324 and 32 with the same resistance value as input resistor 323.
5, respectively, an integral type A-D converter 110
By inputting the built-in integrator as an addition/subtraction integrator, the integral type AD converter 110 performs AD conversion of the corrected input signal, and cold junction compensation can be realized.

Next, the linearization method according to the present invention will be explained.

Since the integral type A-D converter 110 is an A-D converter using the well-known double integration method, first, the corrected input signal inputted via the input resistors 323 to 325 is integrated for a predetermined time. After, input resistance 3
28, and count the time required for this inverse integration to obtain A.
-D conversion. When integrating the input signal, the switch 303 of the switch circuit section 331 is turned off and the switch 304 is turned on, and the output of the oscillator 310 is input to the integrating type AD converter 110 via the 1/2 frequency divider 302. It is used as a clock pulse to count the integration time of the input signal.

When the integration of the input signal is finished and the inverse integration begins,
Switch drive circuit 305 generates a signal to turn on switch 303 and turn off switch 304. At the same time, this signal via differentiator 306 resets 1/2 frequency divider 302, counter 308, and timing counters 311 and 312.
Timing counters 311 and 312 generate pulses at predetermined cycles via differentiators 314 and 315, respectively. In the case of this embodiment, the timing counter 311 outputs one pulse per 100 output pulses of the oscillator 310 at a rate of 1/100, and the timing counter 312 outputs one pulse per 32 outputs at a rate of 1/32. It is Zhou.

The logic circuit section 309 is configured so that the output of the counter 308 is 0.
The AND circuit 316 is on between 600 and 600.
-1600, a control signal is generated that turns on the AND circuit 315. The output of the AND circuit 315 is inverted by an inverter 317, and the output of the oscillator 310 is inverted by an inverter 317.
It is input to the AND circuit 318 together with the output of . Therefore, the output of the AND circuit 318 normally outputs the output pulse of the oscillator 310 as is, and turns off only when a pulse is output from the AND circuit 315, so the output of the AND circuit 318 is the output pulse from the oscillator 310 subtracted. Become.

Further, the output of the OR circuit 319 is the output of the AND circuit 3
18 and the pulse output when the AND circuit 316 is on, and this is sent as a clock pulse to the integrating type AD converter 110 via the 1/2 frequency divider 302 during inverse integration. is input.

In this way, by controlling the timing of adding the output pulses of the timing counters 311 and 312 for determining the interval of addition or subtraction by the logic circuit section 309, the number of clock pulses for counting the time of inverse integration is added or subtracted. ,
A linearized A-D conversion output is obtained.

According to the digital linearization circuit in this example, up to a measurement temperature of 300°C, the temperature is 0.5°C for every 50°C.
In addition, 0.5°C is subtracted for every 16°C between 300 and 800°C. FIG. 6 shows a comparison between the A-D conversion output linearized according to the present invention and the error distribution with respect to the measured temperature of the CA thermocouple. In the figure, 601 is the error distribution of the CA thermocouple,
602 is a linearized A-D conversion output.

As described above, according to the present invention, the general A
-By using a D converter (even one chip) and simply adding a small number of additional circuits, it is possible to provide a digital linearization type thermocouple thermometer that is stable and does not require adjustment. Even when thermocouples having output voltage characteristics are used, this can be easily accommodated by simply changing the configurations of the logic circuit section and the frequency dividing circuit section, and can be realized at a much lower cost than in the past.

In addition, since a floating power supply is not required, there is no need to use a DC-DC converter using a booster circuit, etc., and it is composed of only low power consumption elements, providing a handy type thermocouple thermometer with extremely low power consumption. I can do it.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of a conventional A-D converter equipped with a digital linearizer. FIG. 2 is a diagram showing a conventional cold junction compensation circuit. Third
The figure is a diagram showing an example of a thermocouple thermometer using the digital linearization method according to the present invention. Fourth
The figure is a diagram showing an example of a cold junction compensation circuit according to the present invention. FIG. 5 is a diagram showing the distribution of errors with respect to the measured temperature of the CA thermocouple. FIG. 6 is a diagram showing a comparison between the linearized AD conversion output according to the present invention and the error distribution of the CA thermocouple. 101...Batsuhua, 110...Integral type A-D
Converter, 111... Decoder driver, 112...
... Display section, 210 ... Thermocouple, 302 ... 1/2 frequency divider, 305 ... Switch drive circuit, 308 ...
Counter, 309...logic circuit section, 310...oscillator, 321...reference voltage source, 322...cold junction compensation circuit, 327...digital linearization circuit,
330... Automatic zero correction circuit, 331... Switch circuit section, 332... Frequency division circuit section, 333... Gate circuit section, 334... Control circuit section.

Claims (1)

[Claims] 1. A thermocouple, a buffer that amplifies the output signal of the thermocouple, an integral type AD converter that inputs the analog output of the buffer and converts it from AD to AD, and the A-D converter.
It comprises a reference voltage source that supplies a reference voltage for inverse integration to the converter, a cold junction compensation circuit that corrects the cold junction temperature of the thermocouple, and a display section that displays the output of the integral type AD converter, Further, an oscillator that generates a clock pulse, a counter that counts the output of the oscillator, and a logic circuit that determines the timing of adding and subtracting the correction pulse from the output of the oscillator based on the contents of the counter.
a gate circuit unit that selects which of the addition pulse train and the subtraction pulse train to pass through based on the output of the logic circuit unit and outputs either the addition or subtraction pulse train; and the addition and subtraction pulse trains. a frequency divider circuit that determines the interval of a pulse train, generates the pulse train at equal intervals, and supplies the pulse train to the gate circuit; and control that adds or removes pulses from the clock pulse output from the oscillator based on the output of the gate circuit. a circuit section, a switch circuit section for selecting which of the oscillator output and the control circuit output is to be supplied to the integral type A-D converter, and controlling the switch circuit section according to the timing of A-D conversion. and a digital linearization circuit that linearizes the output of the integral type AD converter by adding or subtracting at the timing determined by the logic circuit section. thermocouple thermometer.