JPH0535978B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a temperature transducer using a thermocouple as a temperature sensing element.

[Conventional technology]

In a temperature transducer that uses a thermocouple as a temperature sensing element, the output voltage of the thermocouple does not have a linearly proportional relationship with the temperature, so error correction means is provided to correct this nonlinear error. is common. Conventional examples of this error correction means include one constructed of a microcomputer including an A/D converter and a ROM, and a polygonal approximation circuit constructed of resistors, capacitors, diodes, and the like. A conventional example of a thermometer using a thermocouple as a temperature sensing element is disclosed in Japanese Patent Application Laid-Open No. 129630/1983, and its configuration is shown in FIG. In the figure, 1 is a thermocouple, 2 is a reference junction compensation circuit,
3 is an error correction means, which includes a differential circuit 4 to which the output of the thermocouple 1 and the output of the reference junction compensation circuit 2 are connected;
It is composed of an A/D converter 5 and a microcomputer 6 having a ROM 6M. 7 is a decoder circuit to which the output of the error correction means 3 is connected, and 8 is a display circuit to which the output of the decoder circuit 7 is connected.

Next, the operation will be explained. During temperature measurement, the voltage of the thermoelectromotive force generated in the thermocouple 1 and the compensation voltage formed in the reference junction compensation circuit 2 are sent to the differential circuit 4 of the error correction means 3, and The difference between the two voltages mentioned above is obtained as the output. In thermometers using thermocouples,
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 determined by keeping the temperature of the reference junction at 0°C. There is a deviation from the value when The compensation voltage mentioned above is for canceling out this deviation. On the output side of the differential circuit 4, as described above, a voltage that is the difference between these compensation voltages and the voltage of the thermoelectromotive force appears. This difference voltage is applied from the differential circuit 4 to A/
Microcomputer 6 via D converter 5
The non-linear characteristics are converted into linear characteristics. Thereafter, it is applied via a decoder circuit 7 to a display circuit 8 having, for example, a liquid crystal element, so that the measured temperature is displayed. To convert this nonlinear characteristic into a linear characteristic, first, an analog temperature measurement signal inputted from the differential circuit 4 is converted into a digital signal by the A/D converter 5 and sent to the microcomputer 6. In this microcomputer 6, correction data for linearizing the thermoelectromotive force characteristics with respect to the temperature of the thermocouple 1 is stored in its ROM 6M, and this data is called up at a predetermined timing and sent from the A/D converter 5. The incoming temperature measurement signal is subjected to arithmetic processing for linearization, and then sent to the decoder circuit 7.

[Problem that the invention seeks to solve]

Thermometers that use a microcomputer, such as the conventional temperature transducer described above, can perform complex arithmetic processing on the temperature measurement signal, improving measurement accuracy, but they have many relatively expensive parts and the equipment is expensive. There was a problem that the amount was high. In addition, those using a line approximation circuit consisting of resistors, capacitors, diodes, etc. have low measurement accuracy due to drift due to temperature changes, and depending on the type of thermocouple, each constant that determines the line characteristic There were problems such as the need to experimentally determine the

This invention was made to solve the above-mentioned problems, and its purpose is to obtain an inexpensive and highly accurate temperature transducer.

[Means for solving problems]

The temperature transducer according to the present invention converts the voltage difference between the compensation voltage and the voltage of the temperature sensing element into a pulse width modulation signal, and calculates the average voltage E ₁ of the logical product of the reference DC voltage E _Z and the pulse width modulation signal. Made with the first circuit,
The average voltage E ₂ of the AND of the average voltage E ₁ and the pulse width modulation signal is created in the second circuit, and the average voltage up to the nth circuit... E _o is created up to the nth circuit, and the average voltage
It is configured to add and subtract E ₁ , E ₂ , . . . E _o at a predetermined ratio and correct the error to obtain a DC voltage proportional to temperature.

[Effect]

The temperature transducer in this invention has an average voltage E 1 of the logical product of a pulse width modulation signal and a reference DC voltage E _T corresponding to the difference between the compensation voltage and the voltage of the temperature sensing element, and an average voltage E ₁ of the AND of the pulse width modulation signal and the reference DC voltage E T . The average voltage E ₂ of the logical product of E ₁ and the average voltage up to nth below...
...E _o at a predetermined ratio, the error-corrected temperature is calculated.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. In FIG. 1, 1 is a thermocouple, 2 is a reference junction compensation circuit, 30 is an error compensation means, 4 is a differential circuit to which the output of thermocouple 1 and the output of reference junction compensation circuit 2 are connected, and 11 is a differential circuit to which the output of thermocouple 1 and the output of reference junction compensation circuit 2 are connected. A pulse width modulation circuit is connected to the output of the differential circuit 4, an AND circuit 13 is connected to the output of the pulse width modulation circuit 11, and an addition/subtraction circuit 14 is connected to each output of the AND circuit 13. Note that, as in FIG. 5, a decoder circuit and a display circuit for displaying the measured temperature are provided on the output side of the addition/subtraction circuit 14, but are not shown here. FIG. 2 is a diagram showing the AND circuit 13 in FIG. 1 in more detail. In the figure, E _Z is the reference DC voltage, S ₁ is the first switch to which the reference DC voltage E _Z is connected, and A ₁ is the first filter to which the output of the first switch S ₁ is connected, S ₂ is the second switch to which the output of the first filter A ₁ is connected, A ₂ is the output of the second switch S ₂ is connected to the second filter, and the switches and filters are connected in series, S _o is the nth switch to which the output of the n-1th filter (not shown) is connected,
A _o is the nth switch to which the output of the nth switch S _o is connected.
This is a filter. and pulse width modulation circuit 11
The outputs of the first to nth switches S ₁ , S ₂ , ...S _o
is connected to the control terminal of the

Next, the operation will be explained. In Figure 1,
Thermocouple 1, reference junction compensation circuit 2 and differential circuit 4
operates in the same way as the same circuit shown in FIG. , i.e. the voltage corresponding to the temperature to be measured
E _T is obtained. This voltage _ET is pulse width modulated by a pulse width modulation circuit 11 and converted into a pulse with a duty ratio proportional to the voltage _ET . This pulse satisfies the following equation (1), assuming that it has an on period T ₁ and an off period T ₂ .

E _T =T ₁ /T ₁ +T ₂ ...(1) This pulse is input to the next AND circuit 13. This AND circuit 13 is constructed as shown in FIG. 2, and its operation is as follows.
The reference DC voltage E _Z is at the first switch S ₁ ,
It is controlled on and off by the output pulse of the pulse width modulation circuit 11, and its output _l1 is converted into a pulse as shown in FIG. 3 <1>. The pulse output l ₁ is smoothed by the next first filter A ₁ and its average voltage E ₁ is expressed by the following equation (2).

E ₁ = T ₁ / T ₁ + T ₂・E _Z = E _T・E _Z ...(2) Next, the output voltage E ₁ of the first filter A 1 is applied to the pulse width modulation circuit in the second switch S _{2 .} 11 (for example, the output stage of the pulse width modulation circuit 11 is controlled on/off by the output pulse of the photocoupler, and its output _l2 is converted into a pulse as shown in Fig. 3 <2>.The pulse output _l2 is is smoothed by the second filter A ₂ , and its average voltage E ₂ is expressed by the following equation (3).

E ₂ = T ₁ / T ₁ + T ₂・E ₁ = E _T ²・E _Z ...(3) Similarly, switches and filters are connected in series in sequence, and the output l of the nth switch S _{o o} has the waveform shown in Figure 3 <4>, is smoothed by the nth filter A _o , and its average voltage E _o is as follows (4)
It is expressed by the formula.

E _o = T ₁ / T ₁ + T ₂・E _o-1 = E _T ⁿ・E _Z ...(4) However, in equation (4), E _o-1 is the (n-1)th filter (A _{o -1} ) indicates the output voltage. In this way, in the AND circuit 13, the _DC voltages E ₁ , E ₂ ,
...E _o is obtained. Each of the output voltages E ₁ , E ₂ , . . . E _o of the AND circuit 13 is added or subtracted at a predetermined ratio in the addition/subtraction circuit 14 to obtain the output voltage E ₀ expressed by the following equation (5).

E ₀ =K ₁ E ₁ +K ₂ E ₂ +...K _o E _o =E _Z {K ₁ K _T +K ₂ E _T ² +...+K _o E _T ⁿ } ...(5) Thermocouple temperature - When obtaining a DC voltage that is linearly proportional to temperature from the output voltage characteristics, each coefficient K ₁ ,
The magnitude and positive/negative sign of K ₂ , . . . K _o may be determined according to the curve of each thermocouple. Therefore,
In the circuits shown in FIGS. 1 and 3, the number of stages of switches and filters in the AND circuit 13 and the coefficients K ₁ , K ₂ , ...K _o of the addition/subtraction circuit 14 are determined according to the characteristic curve of the thermocouple. If selected, a DC voltage proportional to the input temperature can be obtained. The switch in Figure 2 is a C-MOS analog switch or
The FET and filter can be constructed with a simple circuit such as a C-R integration circuit or an integration circuit combining an operational amplifier, a capacitor, a resistor, etc.

Note that in the above embodiment, the reference voltage E _Z and the first switch S ₁ are provided independently; however,
As shown in Figure 4, this is a C-MOS inverter or NAND that operates using the reference voltage E _Z as a power source.
Similar effects can be achieved using gates and other logic.

〔Effect of the invention〕

As described above, according to the present invention, the error correction means pulse-width modulates the voltage from the thermocouple, and uses this pulse to drive an AND circuit consisting of a switch and a filter in series, and By obtaining the power of the voltage of Effects such as being able to calculate addition/subtraction coefficients can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a temperature transducer according to an embodiment of the present invention, FIG. 2 is a block diagram of the AND circuit in FIG. 1, and FIG. 3 is an explanation of the operation of the AND circuit in FIG. Figure 4 shows the first switch in Figure 2.
FIG. 5 is a block diagram showing another embodiment of _S1 , and FIG. 5 is a block diagram of a conventional temperature transducer. In the figure, 1 is a thermocouple, 2 is a reference junction correction circuit, 4 is a differential circuit, 11 is a pulse width modulation circuit, 1
3 is an AND circuit, 14 is an addition/subtraction circuit, 30 is an error correction means, S ₁ to S _o are switches, and A ₁ to A _o are filters. In each figure, the same reference numerals indicate the same or equivalent parts.

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 temperature transducer that outputs a DC voltage proportional to temperature by passing power through an error correction means, the error correction means outputs a compensation voltage from the reference junction compensation circuit and a voltage from the temperature sensing element. a differential circuit that outputs a voltage proportional to the difference _between
A reference power supply that outputs a voltage, a first switch connected to this reference power supply and controlled to open and close by the output pulse of the pulse width modulation circuit, a voltage with a peak value E _Z outputted from this first switch, which is smoothed and a first filter that outputs a DC voltage _E1 of 1; a second switch connected to the output side of the first filter and controlled to open and close by the output pulse of the pulse width modulation circuit;
A second filter smoothes the voltage with peak value E ₁ output from the second switch and outputs a second DC voltage E ₂ , which includes switches and filters that are sequentially arranged in series up to the nth filter. AND circuit,
The first, second, ... nth DC voltages E ₁ , E ₂ ,
...A temperature transducer comprising: an addition/subtraction circuit that receives E _o as an input and performs addition/subtraction at a predetermined ratio.