JPH0678951B2 - Automatic calibration device for temperature sensor - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic calibration device that automatically calibrates changes in temperature sensor characteristics, and particularly to a temperature in an electric furnace or the like used under severe temperature conditions. The present invention relates to a temperature sensor calibration device that improves the accuracy of the output performance of a temperature sensor and prolongs the life of the temperature sensor by periodically and automatically calibrating the deterioration of the sensor characteristics.

BACKGROUND ART Thermocouples, resistance temperature detectors, thermistors and radiation thermometers are known as examples of temperature sensors for electric furnaces and the like. Of these temperature sensors, most thermocouples are used in a severely high temperature state, so the thermocouple will be described as an example. When a thermocouple is exposed to a high temperature for a long period of time, the thermoelectromotive force of the thermocouple decreases with the passage of time, and its characteristics deteriorate.
The cause of deterioration is said to be evaporation and diffusion of a specific component at the temperature measuring junction, but the mechanism of deterioration has not been completely clarified. Therefore, although it is not possible to estimate the degree of deterioration over time, ISO type R (platinum-platinum rhodium 13
%) Thermocouple shows a general tendency of -20 ° C at 1400 ° C /
It is said that a decrease in electromotive force of -60 ° C / half year occurs at 1600 ° C for half a year.

This value is an average value and varies depending on each individual thermocouple, usage atmosphere, etc., so it is only a rough guide.

Errors in the furnace temperature control due to deterioration of the temperature sensor seriously affect the quality of the product.Therefore, the temperature sensor should be periodically removed and calibrated, or replaced with a new one at regular intervals, or the product yield should be fixed. It was exchanged every time it dropped to a level.

However, the removal calibration of the temperature sensor has a disadvantage that the operation of the electric furnace or the like needs to be stopped for removal or at least the temperature of the furnace suddenly changes.

The relatively frequent replacement of the temperature sensor not only requires the cost and labor of a new temperature sensor for replacement, but also causes at least a sudden change in the furnace temperature each time the replacement is performed, resulting in product quality deterioration at the beginning of replacement. There is a problem that the impact cannot be avoided.

It is also conceivable to use a temperature sensor, which is higher in class than the thermocouple, has high stability, and has a long life, instead of the thermocouple. For example, high quality “High hemperature optical fiber thermom
"eter" by RRDils in the Journal of Applied Physic
s, 54 (3), 3,1983, and "Temperature Measuring Device" is disclosed in JP-A-61-210922. However, this temperature sensor itself has a drawback that it is very expensive.

The present invention has been made in view of the above circumstances,
Therefore, it is an object of the present invention to provide a novel temperature sensor calibration apparatus that is not so expensive and that can eliminate the above-mentioned drawbacks inherent in the prior art.

Means for Solving the Problems In order to achieve the above object, the automatic calibration device for a temperature sensor according to the present invention inserts a protective tube accommodating the temperature sensor into the furnace, and the temperature inside the furnace is always kept by this temperature sensor. After measuring, the reference temperature sensor is inserted in the same protection tube according to a fixed schedule, and after the temperature of both sensors reaches equilibrium, measure the temperature with this reference temperature sensor.If there is a temperature difference, the temperature sensor output is used as the reference temperature sensor. The correction is performed so as to match the output, and when the correction work is completed, the reference temperature sensor is pulled out from the protective tube, the calibration work is completed, and the standby state is held until the next calibration command arrives.

The applicant has applied for a calibration method and apparatus having a similar idea to the present invention as Japanese Patent Application No. 1-131717. In the method described in this specification, the reference temperature sensor is moved in and out of the furnace at a position near the temperature sensor for calibration. For this reason, the distance between the temperature sensor and the reference temperature sensor tends to be large, and the temperature measuring contacts of both sensors do not always have the same temperature, which causes a calibration error. I was dissatisfied with the fact that it was structurally complicated.

The present invention aims to eliminate these remaining drawbacks by using the same protective tube.

Action It is assumed that the temperature inside the furnace is maintained at a constant value of t (° C). The output of the temperature sensor in this case is E ₁ (mV), which is t ₁ (° C) when converted to temperature. Generally, E ₁ and t ₁ are not in a direct proportional relationship, so the conversion from E ₁ to t ₁ is performed using an analog linearizer or a computer. If the temperature sensor is deteriorated, t ≠ t _s .

Next, a reference temperature sensor is inserted, and the output of the reference temperature sensor when the equilibrium is reached at the constant furnace temperature t is E _s (mV), and the temperature after conversion to temperature is t _s (° C). Assuming that the reference temperature sensor has no error, t ≠ t _s . The following proportional constant K is calculated.

K = t _s / t ₁ (1) In the case of a thermocouple, the thermoelectromotive force decreases due to deterioration, so there is a general relationship of t _s > t ₁ , so K is slightly larger than 1 Is the constant of proportionality. Using this K, t ₁ is multiplied by K in the computer and corrected to obtain the new temperature after correction.
Get t ₁ ′.

t ₁ ′ = t ₁ × K = t ₁ × t _s / t ₁ = t _s (2) Since t _s = t, t ₁ ′ = t and the corrected new temperature t ₁ ′
Becomes equal to the temperature t in the furnace, and the deterioration amount is corrected to show the correct temperature t.

Embodiment Next, the present invention will be specifically described with reference to the drawings with respect to a preferred embodiment thereof.

FIG. 1 is a sectional view showing an embodiment of an automatic calibration device to which the present invention is applied.

Referring to FIG. 1, reference numeral 1 is a furnace wall made of a heat insulating material, and 2 is a furnace wall 1.
The outer plates 3 for supporting the furnace are shown as flanges, and the furnace wall 1 is attached to the outer plate 2 by a method not shown. The base 11 of the sensor unit 10 is fixed to the flange 3 with screws 4. A protective tube 12 is fixed to the small diameter part of the base 11. A partition wall 13 is inserted inside the protective tube 12,
The inside of the protective tube 12 is divided into an upper sensor chamber 37 and a lower calibration chamber 34. Alumina-based ceramics are often used as the non-metallic high temperature protection tube. In this embodiment, the thermocouple 14 as a temperature sensor is housed in the sensor chamber 37. The wires of the thermocouple 14 are insulated from each other by an insulating tube 15. The thermocouple 14 forms a temperature measuring contact 16 at the right end and is connected to the terminal block 17 at the left end. From here, the wires of the thermocouple 14 are guided to an external measuring device through a conduit tube 18 by a compensating lead wire (not shown).

19 and 20 are spinning wheels, supported by shafts 21 and 22, and spinning wheels 19 and 20
A wire 23 is stretched around. The spinning wheel 20 and the gear 24 are integrated, and the rotation of the step motor 25 causes the pinion 26 to rotate.
The spinning wheel 20 rotates via. The wire 23 moves accordingly and the spinning wheel 19 also rotates.

The wire 23 has a screw 31 at the left end of the reference temperature sensor unit 30.
It is installed in. A reference temperature sensor (not shown) is housed in the reference sensor protection tube 32. The reference temperature sensor does not have to be a thermocouple, but may be a resistance temperature detector or the above-mentioned optical fiber type temperature sensor.

The left end of the reference temperature sensor unit 30 is the limit switch 33.
Then, the stepping motor 24 is stopped by the switch operation, and the calibration command standby state is set.

Reference numeral 27 is a printed wiring board (hereinafter abbreviated as “PCB”), and a circuit mainly controlling the operation of the stepping motor 25 is provided.

Next, to explain the operation of the present invention, the temperature in the furnace is t
It shall be held at a constant value of (° C). The output of the temperature sensor in this case is E ₁ (mV).
It becomes t ₁ (℃). Generally, E ₁ and t ₁ are not in a direct proportional relationship, so the conversion from E ₁ to t ₁ is performed using an analog linearizer or a computer. If the temperature sensor is deteriorated, t ≠ t ₁ .

When a calibration command from the outside is input through the conduit tube 18, the step motor 25 rotates according to the programmed speed curve, and the reference temperature sensor unit 30 is gradually inserted into the calibration chamber 34 of the protection tube 12. The left end of the reference temperature sensor unit 30 hits the limit switch 35, and the switch operation stops the insertion of the unit 30.

Next, the correction operation as described in the operation section is performed. That is, the reference temperature sensor output when the temperature equilibrium is reached at the constant furnace temperature t is E _s (mV), and the temperature after conversion to temperature is t _s
(° C). The reference temperature sensor Assuming no error, is t = t _s. The proportional constant K as described above is calculated.

K = t _s / t ₁ (1) In the case of a thermocouple, the decrease of thermoelectromotive force occurs due to deterioration, so there is generally a relationship of t _s > t ₁ , so K is slightly larger than 1 Is the constant of proportionality. Using this K, t ₁ is multiplied by K in the computer and corrected to obtain the new temperature after correction.
Get t ₁ ′.

t ₁ ′ = t ₁ × K = t ₁ × t _s / t ₁ = t _s (2) Since t _s = t, t ₁ ′ = t and the corrected new temperature t ₁ ′
Becomes equal to the temperature t in the furnace, and the deterioration amount is corrected to show the correct temperature t.

After completion of the correction, the reference temperature sensor unit 30 is pulled out to the position shown in FIG. 1 and waits for the next calibration command.

Reference numeral 36 is a curl wire for guiding the reference temperature sensor output to the terminal block 17. The protective case 28 is a protective case for protecting the sensor unit 10, and is detachably attached to the base 11.

In the calibration apparatus according to the present invention, the reference temperature sensor unit 30 is only exposed to a high temperature during calibration, and in most standby states, it is kept at a temperature near room temperature, and the progress of deterioration can be ignored.

The division of the formula (1), the multiplication of the formula (2), and the linearization operation described in the section of the operation can be calculated in an analog manner using an operational amplifier, but they can be calculated digitally using a computer. Is easier to process. The computer calibration command in this case is externally given via the conduit tube 18, but this is not an essential condition either. It is also easy to mount a microcomputer on the PCB 27 and process all operations inside the protective case 26.

As the deterioration progresses further, the proportional constant of equation (1) increases,
For example, when K = 1.10 is reached, a temperature sensor replacement alarm is issued. In this case, the reference temperature sensor unit 30 is manually inserted, and the output signal is used to temporarily control the furnace temperature. Meanwhile, replace the temperature sensor with a new one. Therefore, it is possible to avoid a sudden change in temperature when the temperature sensor is replaced.

If the proportional constant K at that time is recorded for each calibration operation, the state of deterioration can be understood at a glance.

The correction method is not limited to the methods of the formulas (1) and (2). Particularly in the case of constant temperature control, the temperature sensor output t ₁ may be left unchanged and the temperature set value may be changed instead. For example, if the calibration shows that the sensor output t ₁ is 5 ° C lower than the true furnace temperature t, the set temperature is also 5
If the temperature is lowered by 0 ° C, the furnace temperature is maintained at a constant value t. However,
Such an expedient is properly corrected near the temperature t when the furnace temperature is program-controlled, but is not correct at other temperatures.

Since the reference temperature sensor unit 30 is not necessarily deteriorated, the solution can be solved by a method such as performing verification at intervals of about once a year.

If the reference temperature sensor unit 30 is rapidly inserted at the time of calibration, the temperature of the temperature measuring contact 16 of the temperature sensor slightly drops. Since this becomes a kind of disturbance and disturbs the control system, the insertion speed must be controlled according to a fixed schedule. When inserting at a constant speed, it is necessary to make the insertion speed extremely slow.

When measuring at extremely high temperatures, the protective tubes tend to be separated from each other. Well-known technical means, such as spraying the alumina powder into the calibration chamber 34, or finely moving it back and forth to prevent separation even during calibration by inserting the reference temperature sensor unit 30, is effective.

FIG. 2A shows a cross section of the protective tube 12 of FIG. FIG. 2B shows a cross section of a protective tube according to another embodiment of the present invention.

In FIG. 2B, in another embodiment of the present invention, the sensor chamber 37 and the calibration chamber 34 are completely independent and mechanically and thermally integrated by ribs 13 'having a plate thickness t.

Although the influence when the reference temperature sensor is inserted is reduced, the temperature difference between the two sensors is inevitably large due to the distance, and there are advantages and disadvantages. Ribs 13 'with plate thickness t at the tip
However, the special protection tube tends to be expensive.

A stepping motor and a spinning wheel are used to insert the reference temperature sensor, but this is just an example. Of course, a servo motor, a linear motor, an air cylinder, etc. can also be used.

Needless to say, it is possible to perform interrupt calibration by a manual command at any time other than periodic calibration.

EFFECTS OF THE INVENTION The present invention is configured and operates as described above, and according to the present invention, the following various effects can be obtained.

(1). Since the deterioration of the temperature sensor over time is automatically corrected, the temperature control accuracy is good and the product yield is improved.

(2). Since the degree of deterioration is recorded every time automatic calibration is performed, the progress of deterioration can be grasped, and unnecessary and unnecessary temperature sensor replacement can be avoided.

(3). When a sudden deterioration occurs, it is detected early, so that a large amount of defective products can be avoided.

(4). There is no need for man-hours for manual calibration, and operation mistakes during calibration do not occur.

(5). Even when the temperature sensor is replaced, the reference temperature sensor acts on its behalf so that the temperature does not change suddenly.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of the present invention, and FIG.
FIG. 2B is a sectional view of the protective tube 12 in the figure, and FIG. 2B is a sectional view of the protective tube showing another embodiment of the present invention. 1 ... Furnace wall, 2 ... Exterior plate, 3 ... Flange, 10 ... Sensor unit, 11 ... Base, 12 ... Protective tube, 13 ... Partition wall, 13 '... Rib, 14 ... Thermocouple, 15 ... Insulation tube, 16 ... Measurement Hot junction, 17 ... Terminal block, 8 ... Conduit tube, 19, 20 ... Spinning wheel, 23 ... Wire, 24
… Gear, 25… Step motor, 26… Pinion, 27… Printed wiring board, 28… Protective case, 30… Standard temperature sensor unit, 32… Standard sensor protective tube, 33, 35… Limit switch, 34… Calibration room, 36 … Curl line, 37… Sensor room

Claims (3)

[Claims] 1. In a temperature measuring device in which a temperature sensor is housed in a protective tube and the temperature sensor is inserted into a furnace to measure the temperature in the furnace, the protective tube is divided into a sensor chamber and a calibration chamber, and a calibration command is issued. According to the automatic calibration of the temperature sensor, the reference temperature sensor is sequentially inserted into the calibration chamber, the temperature sensor is calibrated, the reference temperature sensor is pulled out from the calibration chamber to the standby position, and the next calibration command is awaited. apparatus. 2. The automatic calibration device for a temperature sensor according to claim 1, wherein the sensor chamber and the calibration chamber are independent protection tubes, and the sensor chamber and the calibration chamber are connected by ribs. 3. The automatic calibration device for a temperature sensor according to claim 2, further comprising cutting off a rib near the temperature measuring contact.