JPH0719808A - Apparent distortion compensation circuit for high temperature distortion gauge - Google Patents

Abstract

PURPOSE:To reduce the amount of generated apparent distortion to such level as no actual problem occurs even if temperature distribution condition of a lead wire is subjected to high temperature changes. CONSTITUTION:A series circuit consisting of an active gauge 2 and a dummy gauge 3, another series circuit consisting of two fixed resistors 9 and 10, and, the first and second lead wires 4a and 4b which connect bath ends of series circuits constitute a bridge circuit. The output of the bridge circuit is derived from both an output terminal 16 on the other side of the third lead wire 4c and an output terminal 17 connected to the connection points of the fixed resistors 9 and 10. Between the first lead wire 4a or the second lead wire 4b and bridge power supply input terminals 18 or 19, apparent distortion compensation resistors 12 or 13, which is caused by distortion gauge's temperature, is connected. Between the output terminal 16 and the power supply input terminals 18 or 19, resistors 14 or 15 that compensates apparent distortion that follows resistance changes caused by temperature distribution changes of lead wires 4a-4c is connected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparent strain compensation circuit for a high temperature strain gauge, and more specifically, a resistance value of a lead wire changes depending on a change in temperature distribution, and an apparent strain amount generated by the change in the resistance value. The present invention relates to an apparent strain compensation circuit for suppressing high temperature strain gauges.

[0002]

2 and 3 are a longitudinal sectional view and a plan view showing the construction of a welding type high temperature strain gauge which is a kind of high temperature strain gauge. The high temperature strain gauge is composed of a tube 1 made of stainless steel or the like, an active gauge 2 and a dummy gauge 3 arranged in the center thereof.

That is, the active gauge 2 is formed by bending a wire material such as a nickel-chromium wire which changes its resistance value in response to strain into a substantially U-shape, and the same around the active gauge 2. Dummy gauge 3 made of material
Is wound so that it is insensitive to strain. Tube 1 with active gauge 2 and dummy gauge 3
Magnesium oxide MgO powder is tightly enclosed therein as an insulator. The tube 1 is sealed at its tip, and a lead wire 4 which is an input / output wire connected to the active gauge 2 and the dummy gauge 3 is led out from the base end, and its lead-out port is a ceramic high temperature resistant adhesive 5 It is sealed by. The sensitive portion 1a of the tube 1 is fixed to the base 6 having the flange portions 6a and 6b, and the sensitive portion 1a is attached to the object 7 to be measured by spot welding via the flange portions 6a and 6b of the base 6. To be

FIG. 3 shows a state in which a high temperature strain gauge is fixed on an object to be measured 7, and the object to be measured is located at 8 positions on the flanges 6a and 6b protruding in both directions of the tube 1. It is fixed to the object 7 by spot welding.

The output from the high temperature strain gauge is the MI cable (Mineral Insulated Metal Sheathed C
can be led to the measuring instrument via the lead wire 4.

The strain generated in the object 7 to be measured is detected as a resistance change of the active gauge 2 which is subjected to a compression or tension action through the flange portions 6a and 6b, the sensing portion 1a of the tube 1 and magnesium oxide MgO. . The change in the resistance value due to the temperature of the active gauge 2 itself is electrically offset and compensated by the change in the resistance value sensitive to only the temperature of the dummy gauge 3 inserted on one side of the Wheatstone bridge circuit.

However, in the general method of using the high temperature strain gauge, since the object 7 to be measured and the active gauge 2 have different linear expansion coefficients, even if no stress is applied to the object 7 to be measured, the strain gauge is affected by temperature. Apparent distortion occurs. In order to compensate for the apparent strain due to this temperature, conventionally, a compensation resistor (hereinafter referred to as "gauge compensation resistor" and also referred to as "RTC" in the mathematical expression) is inserted in one piece of the Wheatstone bridge circuit.

Therefore, an example of a conventional circuit for compensating the apparent distortion due to temperature is shown in FIG. The Wheatstone bridge circuit in the same figure has an active gauge 2, a dummy gauge 3,
It is composed of fixed resistors 9 and 10 and gauge compensation resistors 12 and 13. In FIG. 6, 11, 11, 11
Is the lead wire resistance (internal resistance) of the lead wires 4, 4, 4.

Further, a terminal 16 connected to the other end of the lead wire 4 extended from the connection point of the active gauge 2 and the dummy gauge 3 and a terminal 17 connected to the connection point of the fixed resistors 9 and 10 are output. Configure terminals. The other ends of the fixed resistors 9 and 10 having one ends connected to each other are connected to the input terminals 18 and 19 of the bridge power supply.

If the apparent strain due to the temperature of the strain gauge is positive among the gauge compensation resistors RTC12, 13, the resistor 12 is inserted on the positive voltage side of the bridge power source Ein and the resistor 13 on the negative voltage side is short-circuited. If the apparent strain due to temperature is negative, reverse this. Normally, the gauge compensation resistors 12 and 13 are provided at two temperatures, for example, room temperature and 100
The apparent strain amount at 0 ° C. selectively determines the optimum value that can cancel them.

[0011]

The temperature characteristic of the apparent strain as described above will be verified based on the equivalent circuit of FIG. For example, set one gauge compensation resistor to 1
20Ω (ohm) and the resistance value RTC of the other gauge compensation resistor 13 is set to 4.582Ω. The output voltage Eout0 at room temperature is the bridge power supply voltage Ein, the resistance value of the active gauge 2 is RA, and the resistance value of the dummy gauge 3 is R.
When the internal resistance value of D and the lead wire 11 is RL, it is expressed by the following equation (1).

Eout0 = {(RD + RL + RTC) / (RA + RD + 2RL + RTC)} Ein (1) Further, the output voltage EoutT at high temperature is the resistance value of the active gauge 2 at high temperature, RAT, and the dummy gauge 3 Is the resistance value of RDT, the internal resistance value of the lead wire 11 is RDT
When it is LT, it is expressed by the following equation (2).

EoutT = {(RDT + RLT + RTC) / (RAT + RDT + 2RLT + RTC)} Ein (2) In the equation (1), the strain is 0 (με = 1 × 10).
^-6 strain. Hereinafter, the output voltage Eout (0) in the case of “με”) and the output voltage Eout (0.1) in the case of 1,000 [με], that is, 0.1% strain are obtained. Prior to obtaining this numerical value, the conditions are set as follows. The gauge factor is 2, the bridge power supply voltage to the Wheatstone bridge circuit is Ein, and the initial resistance value RA0 of the active gauge and the resistance value RD of the dummy gauge are equal (RA0 = RD).
The following expression (3) is established between the initial resistance value RA0 of the active gauge and the resistance value RA0.1 at 0.1% strain.

RA0.1 = RA0 (1 + 0.1 × 0.01 × 2) = 1.002RA0 (3) Under the above conditions, the output voltages Eout (0) and Eout (0.
1) is obtained by the following equations (4) and (5).

Eout (0) = {RD / (RA + RD)} × Ein = 0.5Ein (4) Eout (0.1) = {RD / (1.002RA + RD)} × Ein = 0.4995004Ein (5) 0 The output voltage eout (0.1) when 1% strain is generated is the value obtained by subtracting the output voltage when strain is applied from the output voltage when the amount of strain is 0, and is obtained by the following equation (6). .

Eout (0.1) = Eout (0) −Eout (0.1) = 0.5Ein−0.49950Ein = 0.005Ein (6) Output voltage 0.0005 [V] is a strain amount of 1,000 [με] Therefore, the magnitude of the apparent strain MTC [με] is expressed by the following equation (7).

MTC = {Eout (0) -Eout (T)} × 1,000 / 0.0005 (7) A concrete example in which the parameters of the above equation (7) are digitized is shown in FIG.
Is a graph of using a copper wire as a lead wire type and using a lead wire 4
FIG. 6 is a characteristic diagram showing the relationship between the length (m) of the heated portion of and the apparent strain MTC. The numerical value of each parameter is the same as the numerical value described in the section of the embodiment of the present invention described later. However, the gauge compensation resistance value (RTC of the conventional example = 4.
582Ω) is different from that of the embodiment in that the lead wire resistance change compensation resistance is added in the embodiment. Further, FIG. 8 shows a case where a nickel wire is used for the lead wire, and although the description of the parameter numerical value is omitted, the influence due to the change in the length of the heated portion of the lead wire is largely shown.

As can be seen from these figures, the conventional compensating method can be said to be conditioned on that the resistance value of the lead wire is constant. However, when measuring strain in a high temperature environment, the temperature distribution of the lead wire changes depending on the environment in which it is used, so there is a major problem that the above-mentioned apparent strain compensation circuit due to temperature cannot completely compensate. There is.

That is, in strain measurement under an environment of generally 300 ° C. or higher, a so-called MI cable, which holds magnesium oxide MgO in a metal tube as described above, is used as a lead wire derived from a high temperature strain gauge. used. The resistance value of this MI cable changes about 4 to 6 times when the environmental temperature reaches about 1,000 ° C., and in particular, the portion of the entire length of the MI cable exposed to high temperature depends on the measurement environment (condition). If it changes, the resistance of the lead wire also changes greatly, so that the conventional compensation circuit described above cannot compensate.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an apparent strain compensation circuit for a high temperature strain gauge in which the apparent strain amount is small even if the temperature distribution condition of the lead wire is changed. To provide.

[0021]

In order to achieve the above object, the present invention provides an electric signal according to the strain when it is attached to an object to be measured under high temperature and is strained from the object to be measured. In the apparent strain compensation circuit for outputting high temperature strain gauge, an active gauge that changes its resistance value when it receives strain from the object to be measured, and an active gauge that is insensitive to the strain and is placed under substantially the same environment as the active gauge. A dummy gauge having one end connected in series with one end of the active gauge, one end connected in series and connected to one output terminal, and the other end connected to one and the other bridge power input terminal Two fixed resistors and one end connected to the other end of the active gauge and the other end to the one bridge power input terminal directly or apparent strain due to temperature And a first lead wire connected through a compensation resistor, one end of which is connected to the other end of the dummy gauge and the other end of which is connected to the other bridge power supply input terminal directly or through an apparent strain compensation resistor due to temperature. A second lead wire, a third lead wire having one end connected to a connection point between the active gauge and the dummy gauge and the other end connected to the other output terminal, and the one bridge power supply input terminal and A lead wire resistance change compensating resistor that is connected between one or both of the other bridge power supply input terminal and the other output terminal, and that compensates an apparent distortion due to a resistance change of the lead wire; It is also characterized in that the generation of the apparent strain due to the difference in the length of the portion exposed to the high temperature is compensated by the lead wire resistance change compensation resistance. It is.

[0022]

[Operation] The apparent strain compensation circuit of the high temperature strain gauge configured as described above calculates the apparent strain due to the temperature of the strain gauge when the resistance of the lead wire is constant,
Compensation is performed by inserting an apparent strain compensation resistance circuit in the extension end side of one or both of the two lead wires. On the other hand, the lead wire resistance changes significantly depending on the length of the lead wire exposed to high temperature relative to the total length of the lead wire, and the apparent strain due to temperature also changes significantly. Between the other end side of the lead wire of No. 3 and the other end side of one or both of the first and second lead wires whose one ends are connected to the other ends of the active gauge and the dummy gauge, respectively. Compensation is achieved by inserting a lead wire resistance change compensating resistor that suppresses the apparent strain associated with the resistance change caused by the change in temperature distribution.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The structure of the present invention will be described in detail below with reference to the embodiments shown in the drawings. FIG. 1 is a circuit diagram showing an embodiment of the apparent strain compensation circuit of the high temperature strain gauge of the present invention. The circuit of the present embodiment is configured by adding a lead wire resistance change compensating resistor for compensating the apparent strain accompanying the resistance change due to the change in temperature distribution of the lead wire to the Wheatstone bridge circuit.

On the high temperature environment side I, the active gauge 2 in the tube 1 of the high temperature strain gauge, the dummy gauge 3 and the lead wires 4a, 4b, 4c consisting of the MI cable, and on the normal temperature environment side II, the strain Apparent strain compensation resistor 12, which compensates for the apparent strain due to gauge temperature,
A Wheatstone bridge circuit is configured by including 13 and fixed resistors 9 and 10.

As described above, the active gauge 2 and the dummy gauge 3 are placed under substantially the same environment, and their one ends are connected to each other to form a series circuit. Also, 2
The fixed resistors 9 and 10 also have one ends connected to each other to form a series circuit.

The other end of the active gauge 2 and the other end of the one fixed resistor 9 are connected to each other via a first lead wire 4a and an apparent strain compensation resistor 12 due to temperature. However, if the apparent strain due to temperature is positive, it is omitted or both ends are short-circuited.

Fixed resistor 1 on the other end of dummy gauge 3 and the other
The other end of 0 is connected to the second lead wire 4b through an apparent strain compensation resistor 13 due to temperature. However, the apparent strain compensation resistance 13 due to temperature is omitted if the apparent strain due to the strain gauge is positive,
Both ends are short-circuited.

The other ends of the two fixed resistors 9 and 10 are
The bridge power source input terminals 18 and 19 which are supplied with the bridge power source voltage Ein are respectively connected. One output terminal 17 of the bridge circuit has two fixed resistors 9 and 10.
It is connected to the connection point with.

The other output terminal 16 of the bridge circuit
Is connected to the extension end of the third lead wire 4c, one end of which is connected to the connection point between the active gauge 2 and the dummy gauge 3.

Between the other output terminal 16 and one of the bridge power source input terminals 18 and between the one of the bridge power source input terminals 19, changes in the temperature distribution of the lead wires (lead wires 4a, 4b, 4c). Lead wire resistance change compensating resistors 14 and 15 are inserted in the circuit for compensating for the apparent distortion caused by the change of the internal resistance 11 (resistance value: RL) caused by the change in length exposed to high temperature. However, only one of the lead wire resistance change compensation resistors 14 and 15 is usually used, but both may be used.

Next, the effect of the resistance value RLC of the lead wire resistance change compensation resistance will be described in a concrete example. Now, in the circuit of FIG. 1, only one of the apparent strain compensation resistors 12 and 13 depending on the temperature of the gauge is used and the other 12
Is not used because it is short-circuited as shown by the broken line.
Further, it is assumed that only one of the lead wire resistance change compensation resistors 14 and 15 is used and the other 15 is opened and not used.

Under the above conditions, the active gauge 2 (gauge resistance value RA
) And the dummy gauge 3 (gauge resistance value RD) at the connection point voltage E1 is obtained by the equation (8).

E1 = {R2 / (R1 + R2)} * Ein = [(RD + RL + RTC) / {(RA + RL) (RL + RLC) / (RA + 2RL + RLC) + (RD + RL + RLC)}] * Ein (8) However, the above (8) R1 and R2 in the formula are as follows.

R1 = (RA + RL) (RL + RLC) /
(RA + 2RL + RLC) R2 = RD + RL + RTC In the circuit of the embodiment of FIG. 1, the output voltage Eout (0 ') when the active gauge 2 is not distorted is obtained by the bridge power supply voltage Ein and the equation (7). Potential difference (Ein-E1) from the applied voltage E1 and lead wire resistance change compensation resistance R
It is obtained by substituting LC and the resistance RL of the internal resistance 11 of the lead wire into the following relational expression.

Eout (0 ′) = {RL / (RL + RLC)} × (Ein−E1) + E1 (9) Output voltage Eout (T ′ when the high temperature environment region I in FIG. 1 is exposed to high temperature ) Is obtained by the equations (10) and (11) when the internal resistance value of the lead wire at high temperature is RLT based on the equation (9).

Eout (T ′) = {RLT / (RLT + RLC)} × (Ein−E1T) + E1T (10) E1T = {R2T / (R1T + R2T)} × Ein (11) R1T in the equations (10) and (11) And R2T are the resistance values RA and RL of the equation (9) at room temperature to the resistance values RA and RL at room temperature.
It is obtained by the following relational expression based on T and RLT.

R1T = (RAT + RLT) (RLT + RLC) / (RAT + 2RLT + RLC) (12) R2T = RDT + RLT + RTC (13) Further, the apparent strain MTC is obtained by the equation (14) by the same procedure as the equations (5) and (6). To be

MTC = {Eout (0 ′) − Eout (T ′)} × 1,000 / 0.0005 (14) When the parameter of the equation (14) is set to the following numerical values, A specific example showing the change in the magnitude of the apparent strain with respect to the change in the length is the graph of FIG. 4, and is a characteristic diagram when a copper wire is used as the lead wire type. Further, FIG. 5 is also a characteristic diagram in the case of a nickel wire, and the description of parameter numerical values in this case is omitted.

(1). Parameters at normal temperature (room temperature) RA = 120Ω, RD = 120Ω, RL = 2Ω, RTC = 8.75Ω, RLC = 6,190Ω (2). Parameters at high temperature (for example, 1,000 ° C.) RAT = 129.5532Ω, RDT = 130Ω, temperature coefficient of resistance of lead wire = 0.8 / 1,000 ° C. (3). The relationship between the lead wire length, the lead wire resistance value and the apparent strain output value in the high temperature heating section is as shown in the table below.

[0040]

[Table 1] 4 and 5 are characteristic diagrams to be compared with the characteristic diagrams of the conventional example of FIGS. 7 and 8 described above, respectively. Comparing these opposing figures, it can be seen that in the example according to the present invention, the influence of the lead wire length of the high temperature heating section is greatly alleviated. In addition, as shown in FIG. 5, the characteristic curve when a nickel wire is used is inferior in compensation effect to a copper wire, but it may be used in advantages such as heat resistance and mechanical strength.

The present invention should not be limited to the above embodiments, but various modifications can be made without departing from the gist of the present invention. For example,
Two lead wire resistance change compensation resistors 14 and 15 may be used, or one or both of the apparent strains 12 and 13 due to the temperature of the gauge may be omitted.

[0042]

As is apparent from the above description, by inserting a lead wire resistance change compensation resistor having a predetermined resistance value between the bridge power source input terminal and the strain gauge side output terminal of the Wheatstone bridge circuit, The change in the apparent strain caused by the change in the resistance value of the lead wire due to the change in the temperature distribution state of the wire is suppressed and compensated, and by this compensation effect, the change in the temperature distribution condition of the lead wire,
Apparent strain of high temperature strain gauge that can measure strain of the measured object at high temperature at desired measurement temperature with high accuracy without the trouble of compensating the apparent strain each time the lead wire length is changed. A compensation circuit can be provided.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a circuit configuration of an embodiment of the present invention.

FIG. 2 is a vertical sectional view showing the structure of a high temperature strain gauge.

FIG. 3 is a plan view of the high temperature strain gauge shown in FIG.

FIG. 4 is a graph showing a change characteristic of a length of a heating portion of a lead wire and an apparent strain when a copper wire is used as the lead wire of the circuit of the embodiment shown in FIG.

5 is a graph showing the change characteristics of the length and the apparent strain of the heated portion of the lead wire when a nickel wire is used as the lead wire of the circuit of the embodiment shown in FIG.

FIG. 6 is a circuit diagram showing an example of a conventional apparent distortion compensation circuit considering a linear expansion coefficient.

FIG. 7 is a graph showing the change characteristics of the length of the heated portion of the lead wire and the apparent strain when a copper wire is used as the lead wire of the apparent strain compensation circuit considering the conventional linear expansion coefficient shown in FIG. is there.

FIG. 8 is a graph showing the change characteristics of the length of the heated portion of the lead wire and the apparent strain when a nickel wire is used as the lead wire of the apparent strain compensation circuit considering the conventional linear expansion coefficient shown in FIG. is there.

[Explanation of symbols]

1 Tube 1a Sensing part 2 Active gauge 3 Dummy gauge 4, 4a, 4b, 4c Lead wire 5 High temperature resistant adhesive 6 Base 6a, 6b Flange part 7 Object to be measured 8 Spot welding point 9, 10 Fixing resistance 11 Lead wire Resistance 12,13 Apparent strain compensation resistance by strain gauge temperature 14,15 Lead wire resistance change compensation resistance 16,17 Output terminal 18,19 Bridge power supply input terminal

Claims (1)

[Claims] 1. An apparent strain compensating circuit for a high temperature strain gauge, which is attached to an object to be measured under high temperature and outputs an electric signal according to the strain when the object to be measured is strained, An active gauge that changes its resistance when subjected to strain, and a dummy gauge in which one end of the active gauge is connected in series with one end of the active gauge in a state insensitive to the strain and is disposed under substantially the same environment as the active gauge. And two fixed resistors, one end of which is connected in series and which is connected to one output terminal and the other end of which is connected to one and the other bridge power supply input terminal, and one end of which is connected to the other end of the active gauge And the other end is connected to the one bridge power supply input terminal directly or via an apparent strain compensation resistor due to temperature, and one end is connected to the front. A second lead wire connected to the other end of the dummy gauge and the other end of which is connected to the other bridge power supply input terminal directly or via an apparent strain compensation resistor due to temperature; and one end of the active gauge and the dummy gauge. A third lead wire connected to the connection point of and the other end connected to the other output terminal, and one or both of the one bridge power input terminal and the other bridge power input terminal and the other output terminal A lead wire resistance change compensating resistor that is connected between the lead wire and compensates for the apparent strain due to the resistance change of the lead wire, and the apparent strain due to the difference in the length of the portion where the lead wire is exposed to high temperature. An apparent strain compensating circuit for high temperature strain gauge, characterized in that the generation is compensated by the lead wire resistance change compensating resistor.