JPS5856817B2 - Digital Netsden Ondo-K - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a digital thermocouple thermometer.

Thermocouples have been widely used as industrial temperature transducers.

This type of thermocouple requires cold junction compensation.

Conventionally, what has been considered is to use a potentiometer or electric bridge circuit between the thermocouple and the main instrument, as shown in Figure 3, and insert a temperature-sensitive element such as copper or platinum into the circuit. This compensated for the cold junction temperature.

This uses a cold junction compensating resistor with a large temperature coefficient in the circuit of a thermocouple thermometer to compensate for voltage changes due to temperature changes at the reference junction by adding them to the electromotive force of the thermocouple.

As described above, a thermocouple thermometer constructed in this manner has the disadvantage that a temperature error occurs because the voltage change due to temperature change of the compensation resistor is different from the electromotive force characteristic of the thermocouple.

However, in the present invention, a method is provided in which the temperature of the reference junction is measured with another thermometer and the temperature of the cold junction is corrected.

Now, if the measured thermoelectromotive force is E (t, to) and the reference junction temperature is 0°C, then the temperature of the temperature measuring junction is t.
Letting the thermoelectromotive forces at oC and tooC be Et and Eto, respectively, the temperature t'C of the temperature measuring junction is the temperature corresponding to the following equation Etc.

E t = E (t, to ) +E to ・−
..." (1) When calculating Eto in equation (1), first measure the temperature to of the reference junction using another thermometer, and then measure the temperature to according to the thermocouple characteristics to be used. Convert Eto and add E(t, to) g.

(Means the right-hand side of Equation 1) Therefore, even if the temperature of the reference junction changes over a wide range, Eto can be obtained in a form that is similar to the thermocouple characteristic, so the temperature error is much smaller than in the conventional compensation method.

Conventionally, it has been difficult to easily implement this method, but in the present invention, cold junction compensation and thermocouple nonlinear compensation are easily performed using one linearizer.

If the measured thermoelectromotive force E(ttto) is the above-mentioned (
1) It is necessary to measure and convert Eto using another thermometer based on the formula number.

The cold junction compensation of the present invention uses a resistance thermometer and places it as close to the reference junction of the thermocouple as possible to determine the temperature of the reference junction.

That is, in FIG. 1, the horizontal axis E shows the thermoelectromotive force, and the vertical axis T shows the temperature, and the relationship between the two is generally nonlinear.

Curve (2) is the case without cold junction compensation, and is a curve where the reference junction temperature is to and the temperature measuring junction is t°C.

On the other hand, curve ■ is the case when cold junction compensation is implemented;
This curve is obtained by translating the curve l along the coordinate axis E by Eto.

FIG. 2 is a block diagram showing one embodiment of the present invention.

C5 is a linearizer section whose output is gate G3
sent to.

Here, the term "linearizer" is a general name for a circuit having a function of performing digital σζ non-linear correction using polygonal line coefficient data written in a built-in ROM or the like using a method such as polygonal line approximation.

C4 is a counter section for the desired temperature T, C3 is a counter section for storing the reference junction temperature to, and C2 is a matching circuit for C3 and C4.

Dl is a register section that stores the converted value obtained by converting thermoelectromotive force, which is an analog quantity, into a digital quantity using a DVM (digital voltmeter), C2 is a monitor register section having the same configuration as Dl, and C1 is DB. It is a matching circuit for C2 and G1. G2 is clock gate, G3 is D
This is the gate for st DJ.

A detailed explanation of the present invention in the above configuration is as follows.

If you are trying to perform cold junction compensation first, set Dl in advance.

C2, C3, C4, etc. are all in a reset state (here, we will explain the 8 difficulties when first calculating the reference junction temperature.

The reference junction temperature to is obtained by converting the voltage Eto' measured using a resistance thermometer or the like into analog/digital conversion, and storing the resulting digital quantity Eto' in the following table.

When the gates G1 and G2 are opened together with the start pulse, the clock pulse is sent to the linearizer section D5c.

This is also sent to the counter section of D2.

Thereafter, when the contents of D2 match Dll, an output pulse is generated from the matching circuit C1, thereby inhibiting the gate of 01.

Meanwhile, a series of linearized output signals from C5 pass through gate G3 and are stored in C3.

As a result, the reference junction temperature to is stored in C3.

Next, a series of operations for performing cold junction compensation by translating the coordinate axis E in FIG. 1 by Eto will be described below.

First, the electromotive force E(t, to) from the thermocouple is A/D
After conversion, store C1 in advance.

Next, gate G1 is opened with a start pulse, and a series of output signals are generated from C5, linearized by the clock pulse.

This output signal is sent to C4c via gate G3.

Initially, D4 starts from zero, and the output of C4 becomes the output of C3 Eto? C4 is counted up in advance until these match.

When they match, a pulse is generated from the output of C2 and the gate G
Open 2.

What has been described above is equivalent to moving the origin in FIG. 1 by a point Al.

Then, while linearizing again, the counter of C4 is advanced by a series of output signals l from C5 until the output of C2 matches the output E(t, to)cc of Dl.

When the contents of Dl and C2 match, G1 is closed by the matching output from C1, and the calculation is completed.

As a result, in equation (1), E(t t t o )
This means that the temperature T for +EtoGC has been found.

Conventionally, in the reference junction compensation method formed by a bridge circuit including a temperature-sensitive element, which was considered indispensable in this type of thermocouple thermometer, when the reference junction temperature changes significantly, the relationship between the two is mutually CC% as described above. However, in the present invention, the electromotive force is digitally added to the reference junction temperature based on the given thermoelectromotive force characteristic. It has the feature of satisfying sufficient accuracy even if the reference junction temperature changes over a wide range.

In addition, the thermoelectromotive force characteristics of the measuring thermocouple and the characteristics of the resistance temperature detector (both are digitally linearized, so in principle, the temperature error is extremely small).

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram explaining the principle of the present invention, FIG. 2 is a block diagram of a digital thermocouple according to an embodiment of the present invention, and FIG. 3 is a wiring diagram showing a conventional example. Dl... Register section, C2... Monitor register section, C1, C2... Match circuit, C3, C
4...Counter part, C5...Linearizer part, G1. G2...Clock gate, G
3...Gate.

Claims (1)

[Claims] 1. A first counter that stores the reference junction temperature to, a second counter that stores the desired temperature T, a first matching circuit that matches the first and second counters, and a second counter that stores the thermoelectromotive force of the object to be measured. 1 register, a second register for monitoring having the same configuration as the first register, and the first register. Second
a digital linearizer which is driven by a clock pulse (1) and gives a non-linearly corrected output; the corrected output is counted by the second counter; means for inputting the clock pulse to the second register at the time when the coincidence of the contents of the counters is detected by the coincidence circuit; The second counter is provided with means for preventing the clock pulse from being applied to the second counter, and the reference junction temperature to is measured in accordance with the thermocouple characteristics, and the electromotive force corresponding to the temperature t, ol is digitally measured (electromotive force ( A digital thermocouple thermometer characterized by performing additional calculations and performing cold junction compensation.