JPS6333631A - Temperature transducer - Google Patents

Abstract

PURPOSE:To correct non-linear errors at a high accuracy, by correcting the non-linear errors of a thermocouple with a circuit in which a plurality of switches and filters are connected in concatenation. CONSTITUTION:An output voltage of a thermocouple 1 subjected to a temperature compensation with a differential circuit 3 for a reference contact temperature through a reference contact correction circuit 2 is converted into a modulation signal with a pulse width modulation circuit 11 to be inputted into an AND circuit 13. The circuit 13 produces a mean voltage E1 in the ANDing of a fixed DC voltage and filter A1 and a switch S1 and a mean voltage E2 in the ANDing of the mean voltage E1 and the pulse width modulation signals with a circuit comprising a filter A2 and a switch S2. The same operation is performed to its (n)th to produce a similar mean voltage En sequentially with a circuit comprising a filter An and a switch Sn. The mean voltages E1, E2... and En thus obtained are added or subtracted at such a specified ratio as to be made proportional to temperature to obtain a DC voltage.

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.

[Prior Art] 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 to the temperature. It is common to provide a correction circuit. Conventional examples of this error correction circuit include one constructed of a microcomputer including an A/D converter and a ROM, and a broken line approximation circuit constructed of resistors, capacitors, diodes, and the like. An example of the former is JP-A-60-129.
There is a circuit shown in Japanese Patent No. 630, and the block diagram showing the configuration of the circuit is shown in Figure 5.
1) and the reference junction compensation circuit (2) are connected to the input of the differential circuit (3). The output of the differential circuit (3) is input to an error correction circuit (4) composed of an A/D converter (7) and a microcomputer (8).

The output of the error correction circuit (4) is displayed on the display section (6) after passing through the decoder n (5).

Next, 173 works will be explained. The voltage of the thermoelectromotive force generated in the thermocouple (1) and the compensation voltage created by the reference junction compensation circuit (2) are applied to the differential circuit (3), and the difference between the above two voltages is obtained as the output. 0, i & quasi contact for example at 0°C
When using a thermometer that uses a thermocouple set to measure the temperature of an object when its reference junction is at a temperature other than 0°C, for example at the same temperature as room temperature, the measured value will be the temperature of 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 room temperature. The above compensation voltage is for canceling this deviation.
A voltage corresponding to room temperature is applied to one input terminal of the differential circuit (3). A voltage that is the difference between the compensation voltage and the voltage of the thermoelectromotive force appears at the output of the differential circuit (3). This difference voltage is inputted from the differential circuit n (3) to the error correction circuit (4), where the non-linear characteristics of the thermoelectromotive force are converted into linear characteristics, and then passed through the decoder circuit (5), for example. It is added to a display section (6) having a liquid crystal display element etc. for display. The error correction circuit (4) is composed of an A/D converter (7) and a microcomputer (8) including a ROM, and the analog temperature measurement signal input from the differential circuit (3) is It is converted into a digital value by a D converter (7) and input to a microcomputer (8). The ROM of the microcombi (8) stores correction data for linearizing the thermoelectromotive force characteristics with respect to the temperature of the thermocouple (1), and this data is called at a predetermined timing and the A/ The temperature measurement signal sent from the D converter (7) is subjected to iIf line calculation processing and is input to the decoder circuit (5).The decoder circuit (5) converts the input digital signal into an analog value. It is designed to be converted and displayed.

[Problems to be solved by the invention] In the error correction circuit using the microcomputer described above, high measurement accuracy can be obtained by performing advanced and complicated arithmetic processing, but this requires expensive components. There was a problem that the price of the device was high. In addition, although the explanation is omitted, the circuit using the well-known broken line approximation circuit has low measurement accuracy due to unavoidable circuit drift due to temperature changes, and the values of resistors, capacitors, etc. that determine the broken line characteristics depending on the type of thermocouple. There was a problem in that it had to be determined experimentally, which required a great deal of time and effort.

The object of this invention is to obtain an inexpensive and highly accurate thermocouple temperature transducer.

[Flat membrane for solving problems] The temperature transducer of the present invention converts the output voltage of a thermocouple, which is temperature-compensated with respect to a reference junction, into a pulse width modulation signal, and generates a constant DC voltage and pulse width. The first circuit generates an average voltage E1 of the logical product of the modulated signals, the second circuit generates the average voltage E2 of the logical product of the average voltage E1 and the pulse width modulated signal, and then the average voltage E2 of the logical product of the average voltage E2 and the pulse width modulated signal. Conjunction is made in the third circuit and similar average voltage E is obtained up to the nth circuit.
n sequentially by the n-th circuit, and each average voltage E+
, E2. . . , the DC voltage is obtained by adding and subtracting Eo at a predetermined ratio proportional to temperature.

[Function] The average voltage El of the AND of the pulse width modulation signal corresponding to the output voltage of the thermocouple and the reference DC voltage El, the average voltage E2 of the AND of the pulse width modulation signal and the average voltage E1, and the average voltage E2 of the AND of the pulse width modulation signal and the average voltage E1. Average voltage E8. By adding and subtracting .

[Example] Correction of non-linearity of the temperature-output voltage characteristic of a thermocouple is shown in JIS C1602-1981 as an interpolation formula for a reference thermoelectromotive force. This interpolation formula is determined for each fixed temperature range and is expressed by a power series as shown in formula (1).

Here, E is the output voltage, t is the temperature, and each value is determined for al.

In the present invention, in order to obtain the DC voltage EO proportional to the temperature t, the JIS interpolation formula described above is transformed into the form t=ΣkiE, which is a formula for calculating temperature, and the DC voltage EO is expressed by a power series. The equation (2) was calculated.

EO position 0.07 + 182.836E-56,948
E"+ 24.345E3- 5.741E'+
0.541E5・ ・ ・ ・ ・ (2) ! @Equation (2) represents an approximate value in the temperature range of 0 to 400''C for an R-type thermocouple according to JIS.

Voltage E of each term expressed by the sum and difference of the above power series
, E, . . . , E is shown in the block diagram of FIG. 1.

In the figure, the outputs of the thermocouple (1) and the reference junction correction circuit (2) are input to two input terminals of a differential circuit (3).

The output of the differential circuit 11 (3) is input to the pulse width modulation circuit (11). The output of the pulse width modulation circuit (11) is input to an AND circuit (13) via a photocoupler (12).

The output of the AND circuit (13) is applied to the addition/subtraction circuit.

FIG. 2 shows the detailed configuration of the AND circuit (13).

A reference DC voltage (Ez) having a constant voltage is applied to the first filter (A+) via the switch (Sl) @1. The output of the first filter (AI) is configured to be applied to the second filter (A2) via the second switch (S2).

Thereafter, filters and switches are arranged alternately in the same way, and the output end (on) of the nth switch (S,) is the nth
The filter (A,) of FIG.

The output of the photocoupler (12) is connected to the first to nth switches (S
I), (S2), ・+, (S,) control terminal (k+
), (k2), ..., (k・,).

Next, the operation of this embodiment will be explained. The voltage of the generated thermoelectromotive force and the compensation voltage created by the reference junction compensation circuit (2) are applied to the differential circuit (3), and the difference between the two voltages is obtained as its output. For a temperature needle using a thermocouple, if the reference is not 0°C, for example, when measuring the temperature of the object being measured at the same temperature as room temperature, the measured value will be 0°C above the reference junction temperature. There is a deviation from the value obtained when the temperature is maintained at room temperature by an amount corresponding to the above-mentioned room temperature. The above-mentioned compensation voltage is for canceling out this shift and is applied to one input terminal of the differential circuit (3). The differential circuit (3) outputs a voltage (ET) that is the difference between the compensation voltage and the voltage of the thermoelectromotive force.

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) of 1 cfi. The on period of this pulse is TI
, the average value of the voltage (ET) is (3
) is expressed by the formula.

ET=T+/(T 10 T2) ・ ・
・ ・ (3) Next, this pulse is transmitted through the photocoupler (12) and input to the next AND circuit (13). As shown in FIG. A switch (SI) that obtains the logical product of the output pulse of the pulse width modulation circuit and a voltage of a predetermined constant value, and a filter circuit that is connected afterwards to smooth the output voltage of the switch (!9+) are connected by MI. The operation is as follows: The reference DC voltage (Ez) is turned on and off by the first switch (SI) controlled by the output pulse of the photocoupler (12). The output e! is converted into a pulse shown in the timing chart of Fig. 3 (1).The pulse output e!
), and the average voltage El is expressed by the following equation (4).

E+=[T+/ (TI+T2)]-EZ=ET-Ez
・ ・ ・ (4) Next, the output voltage (E+) of the first filter (A+) is
The switch (S2) is controlled on and off by the output pulse of the photocoupler (12), and its output (θ2) is
It is converted into a pulse as shown in FIG. 3(2). The pulse output (e2) is smoothed by the next second filter (A2). The average voltage (Ez>) is expressed by the following equation (5).

E2=CT I/ (T ++ T2)'3・E1=
E T2・E z...(5) If the above processing is repeated in the same manner, the output (e n) of the nth switch (S,) will have the waveform shown in Figure 3 (4). The average voltage (E7) after being smoothed by the filter (A11) is expressed by the following equation (6).

E, = [T+/ ('I”++T2)l・En-1=
ET" -Ez...(6) (Ere-1 indicates the output voltage of the (n-1)th filter A n -+) In this way, the AND circuit (13)
Then, the DC voltage (E+), (E2), which is proportional to the power of the voltage (ET) corresponding to the temperature to be measured, is:
.

(En) is obtained. Each output voltage (E+), (E2),
..., (Ell) is added or subtracted at a predetermined ratio in the addition/subtraction circuit (14), and an output voltage (Ea) shown by the following equation (5) is obtained.

Eg= to +E++ to 2E2+・・・+knE, 1=
Ez(k+E'I'+ to 2ET +...+ to E'
r″)...(5) When obtaining a DC voltage that is linearly proportional to temperature from the temperature-output voltage characteristics of the thermocouple, each coefficient (K+) in equation (5),
(K23, . . . +, the magnitude and positive/negative sign of (Kn) can be determined according to the interpolation formula for each thermocouple shown in JIS. Therefore, in the circuit shown in Fig. 1,
The number of stages of switches and filters of AND circuit II (13) and each coefficient (k+) of addition/subtraction circuit (14).

If (k2), . . . , (kn) are selected according to the characteristic curve of the thermocouple, a DC voltage proportional to the input temperature can be obtained. In addition, the switch in the gN3 diagram is C-MO
The filter can be constructed by a simple circuit such as a C-R integrating circuit, an integrating circuit combining an operational amplifier, a capacitor, a resistor, etc. In addition, in the above embodiment, the reference voltage (Ez
) and a switch (Sl) are provided independently, but as shown in Figure 4, this is because a C-MOS inverter or NA
A similar effect can be obtained by using a logic circuit (2θ) consisting of an ND gate or the like.

In addition, a photocoupler (12
), which can transmit pulsed voltage,
For example, a transformer or the like can produce a similar effect.

〔Effect of the invention〕

According to this invention, non-linear errors of thermocouples are corrected by a circuit in which a plurality of switches and filters are connected in cascade, the switches are made of semiconductor elements, and the filters are made of a C-R integration circuit or an operational amplifier. Since the temperature transducer can be constructed from a relatively simple and inexpensive circuit such as an integrating circuit combined with a capacitor resistor, it is possible not only to obtain an inexpensive temperature transducer but also to correct non-linear errors with high accuracy.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a circuit according to an embodiment of the present invention, and FIG. 4 is a detailed diagram of the AND circuit (13) in FIG.
Fig. 3 is a timing chart showing the operation of the embodiment of this invention, Fig. TS4 is a circuit diagram showing another embodiment of the reference voltage and switch portion of this invention, and Fig. 5 is error/difference correction of the conventional example. FIG. 2 is a block diagram showing a circuit. l: Thermocouple Sl: Switch A1: Filter 11: Pulse width modulation circuit 13: AND circuit 14: Addition/subtraction circuit

Claims (1)

[Claims] (1) A thermocouple with temperature compensation for the reference junction, a pulse width modulation circuit that outputs a pulse with a duty ratio proportional to the output voltage of the thermocouple, and logic between the output pulse of the pulse width modulation circuit and a voltage of a predetermined value. An AND circuit formed by cascading a predetermined number of sets of a switch means for obtaining a product and a filter circuit connected after the switch means for smoothing the output voltage of the switch means, and a smoothed output of at least one filter circuit. A temperature transducer equipped with an arithmetic circuit that adds and subtracts at a predetermined ratio.