JPH0447359B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an apparatus for measuring physical quantities such as strain and temperature of an object to be measured using a bridge circuit using a strain gauge or the like and a temperature-sensitive resistance element.

2. Description of the Related Art Conventionally, an apparatus shown in FIG. 1 is known as a device for measuring the temperature of an object to be measured and a certain physical quantity, such as strain, using a temperature-sensitive resistance element and a bridge circuit. This device is roughly divided into sensor section 1
0, an extension cord section 20, and a measuring device main body 30. The sensor unit 10 is a part that converts the amount of strain and temperature of the object to be measured into an amount of electricity, and includes a bridge circuit B consisting of four strain gauges SG ₁ , SG ₂ , SG ₃ , and SG ₄ , and a temperature sensing It has a resistance element R.
The extension cord section 20 has six core wires l ₁ , l ₂ , l ₃ , l ₄ ,
It is composed of l ₅ and l ₆ and is connected to the input terminals 1 and 2 of the bridge circuit B, the output terminals 3 and 4 of the bridge circuit B, and both terminals 5 and 6 of the temperature-sensitive resistance element R, respectively. Also, each core wire
l ₁ to _{l 6} have the same standard and the same length, and each has a resistance r. The measuring device main body 30 is a constant current source that supplies a constant current to the input side of the bridge circuit B.
_S1 , an amplifier _A1 that amplifies the voltage across the output side of the bridge circuit B, and a meter SM that is a voltmeter connected to the output terminal of the amplifier _A1 and that displays the strain of the object to be measured. . Also, the measuring device main body 30
is a constant current source that supplies a constant current to the temperature-sensitive resistance element R.
Combined resistance 2r of S ₂ , temperature-sensitive resistance element R, and core wires l ₅ and l ₆
an amplifier A ₂ that amplifies _the voltage drop caused by the
It has a meter TM for measuring the temperature of the object to be measured.

Next, the operation of the above conventional example will be explained. First, the strain gauges SG ₁ to SG ₄ are attached to the object to be measured by, for example, adhesive. For example, adjacent strain gauges of bridge circuit B are attached to the object to be measured in different expansion and contraction directions, and strain gauges on opposite sides of bridge circuit B are attached to each other in the same expansion and contraction direction of the object to be measured. In this case, if strain gauge SG ₁ is stretched at a certain point, strain gauge SG ₃ is also attached at a position where it will expand, and strain gauges SG ₂ and SG ₄ are attached at positions where it will be contracted. Then, when a current is supplied from the constant current source _S1 to the bridge circuit B, the voltage between terminals 3 and 4 changes depending on the strain of the object to be measured.
After this voltage is amplified by the amplifier _A1 , the meter SM is driven. Therefore, the amount of strain on the object to be measured is
Displayed on SM.

On the other hand, a temperature-sensitive resistance element R is also attached to the object to be measured, and the resistance value of the temperature-sensitive resistance element R changes greatly depending on the temperature of the object to be measured.
Therefore, as the temperature of the object to be measured changes, the input voltage of the amplifier A ₂ changes, and after amplifying this, the voltage value displayed on the meter TM determines the measurement error of the core wires l ₅ and l ₆ . By subtracting the voltage value corresponding to the voltage drop caused by the resistor 2r, the temperature of the object to be measured at that time can be determined.

However, in the above conventional example, when measuring only the strain of the object to be measured, four core wires are sufficient, but when measuring strain and temperature, two core wires are required.
There are more and more books. Of course, it is also possible to change the circuit configuration and share the core wires _l4 and _l5 , but even then, the number of core wires increases by one. Therefore, as the number of core wires in the extension cord 20 increases, the shape of the extension cord 20 becomes thicker and its weight increases. Furthermore, in the above conventional example, the temperature of the object to be measured is measured by subtracting the voltage drop caused by the current flowing through the resistance 2r of the core wires l ₅ and l ₆ , and the resistance 2r is the extension cord. It changes every time the length of 20 changes. Therefore, there is a problem in that if the length of the extension cord 20 changes, the correction value for temperature measurement must also be changed.

Therefore, the present invention reduces the number of core wires of an extension cord in a device that measures physical quantities such as strain and temperature of a measured object using a bridge circuit using a strain gauge or the like and a temperature-sensitive resistance element. In order to avoid being affected by the resistance of the extension cord when measuring temperature, a temperature-sensitive resistance element is installed in series on the output side of the bridge circuit, and two constant current sources that flow the same current are installed and switched. During temperature measurement, the input side of the bridge circuit is shorted, one of the constant current sources is connected between the input side and one end of the output side of the bridge circuit via a temperature-sensitive resistance element, and the The other end of the constant current source is connected between the other end of the output side.

Hereinafter, the present invention will be described in detail based on embodiments shown in the accompanying drawings. Note that the same members are given the same reference numerals. FIG. 2 is a circuit diagram showing one embodiment of the present invention.

The sensor section 11 is provided with a bridge circuit B including strain gauges SG ₁ to _{SG 4} and a temperature-sensitive resistance element R connected to one output terminal 4 of the bridge circuit B. Strain gauges SG ₁ to _{SG 4} are of the same standard. Note that 1 and 2 are input terminals of the bridge circuit B, and 3 and 4 are output terminals of the bridge circuit.

The extension cord section 21 is provided with four core wires _l1 to _l4 , all of which have core wire resistances r. Also, each core wire
l ₁ , l ₂ , l ₃ , and l ₄ are connected to terminals 1, 2, and 3 and temperature-sensitive resistance element R.

Bridge input side terminals 1', 2' and bridge output side terminals 3', 4' in the measuring device main body 31 are connected to core wires _l1 , _l2 , _l3 , _l4, respectively. Constant current source S ₁ ,
S ₂ are for passing the same current to each other, and switch SW ₁ connects terminal 2' to terminal 1' or constant current source S ₁
Switch SW ₂ is for connecting constant current source S ₁ to terminal 3' or terminal 2', and switch SW ₃ is for connecting constant current source S ₂ to terminal 4'. or no load (or connect to terminal 2'), and the switch
SW ₁ , SW ₂ , and SW ₃ are interlocked with each other.
In other words, switches SW ₁ , SW ₂ , and SW ₃ switch off input measurements 1' and 2' of bridge circuit B during temperature measurement, and switch between this input measurement 1' and one end 4' of the output side of bridge circuit B. A constant current source _S2 is connected to the input side 1' and the output side 3' via a temperature sensitive resistance element R, and a constant current source _S1 is connected between the input side 1' and the output side 3'. and switch
When measuring strain, SW ₁ , SW ₂ , and SW ₃ connect a constant current source S ₁ (and/or S ₂ ) to the input sides 1' and 2' of bridge circuit B, similar to conventional strain measurement circuits. , the constant current sources S ₁ and S ₂ are not connected to the output sides 3' and 4' of the bridge circuit B.

Amplifier _A3 amplifies the voltage between terminals 3' and 4', and has a sufficiently high input impedance. The meter M is for reading the amount of strain and temperature.

Next, the operation of the above embodiment will be explained. First, a case will be described in which the strain of the object to be measured is measured. This case is similar to strain measurement using the conventional constant current method. In other words, the switch
Switch all SW ₁ , SW ₂ , and SW ₃ to contact b side. An equivalent circuit in this case is shown in FIG. In the figure,
_{A constant current source S 1 ( and} /or
S ₂ ) is connected, so a constant current i flows through the resistor r, the bridge circuit B, and the resistor r.

According to the deformation of the object to be measured, the voltage between the output terminals 3 and 4 of the bridge circuit B changes. In addition, since the input impedance of amplifier A ₃ is sufficiently high, the combined resistance 2r of core wires l ₃ and l ₄ and the resistance of temperature-sensitive resistance element R
Despite the presence of Rx, terminals 3'-4'
The voltage between them is practically the same as that between terminals 3 and 4. Therefore, the needle of the meter M swings in accordance with the strain on the object to be measured, and the amount of strain can be visually confirmed from the indication of the needle.

Next, the case of measuring the temperature of the object to be measured will be explained. In this case, switches SW ₁ , SW ₂ , SW ₃
Switch all contacts to the a side. An equivalent circuit in this case is shown in FIG. In the figure, Z ₁ is the combined resistance between terminals 4 and 1', Z ₂ is the combined resistance between terminals 3 and 1', and E ₁
is the voltage between terminals 4' and 1', E ₂ is the voltage between terminals 3' and 1', and E ₀ is the voltage between terminals 4' and 3'. Therefore, E ₀ = E ₁ −E ₂ = (r・i+Rx・i+Z ₁・i) −(r・i+Z ₂・i) = Rx・i+Z ₁・i−Z ₂・i=Rx・i+(Z ₁ −
Z ₂ ) i ≒ Rx・i.

Here, (Z ₁ −Z ₂ ) is sufficiently negligible with respect to the amount of resistance change with respect to temperature of the temperature-sensitive resistance element R. For example, if four 350Ω strain gauges are used in bridge circuit B, and +1400×10 ^-6 and −1400×10 ^-6 strains are applied to the two opposing gauges, respectively, 100Ω and When the resistance temperature coefficient is 4000 ppm/°C and the core wire resistance r is 40Ω, (Z ₁ −Z ₂ ) is within the range of ±0.002Ω, and the temperature measurement error is ±0.005°C, which can be ignored.

Also, the voltage between terminals 4' and 3' at 0°C is E ₀
(0℃), and the same voltage at t℃ is E ₀ (t℃)
And the resistance value of the temperature sensitive resistance element R at 0℃ is
If R ₀ is the temperature coefficient of resistance of the temperature-sensitive resistance element R, and ΔE ₀ is the voltage change between terminals 4' and 3' when the measured object changes from 0°C to t°C, then ΔE ₀ =E ₀ (t°C)−E ₀ (0°C) R ₀ (1+αt)i−R ₀・i R ₀・α・i・t.

From this formula, the voltage change △E ₀ is the temperature t
There is a proportional relationship. By amplifying this voltage with amplifier _A3 and displaying it on meter M, the temperature of the object to be measured can be visually checked. In this case, measurement can be performed without being affected by the resistance r of the core wires l ₃ and l ₄ at all. Furthermore, although the strain and temperature of the object to be measured can be measured, since four core wires are sufficient, the cord has a thinner shape and is lighter in weight than conventional devices.

The embodiment described above is suitable for measuring the behavior of concrete, for example. That is, the stress in the concrete can be measured with a strain gauge, and the temperature at that time can be measured with a temperature-sensitive resistance element.

Note that the present invention is not limited to the above-mentioned embodiments, and various modifications can be made within the scope of the invention.

For example, in the above embodiment, four strain gauges SG ₁ to SG ₄ are used in the bridge circuit B, but in addition to this, the bridge circuit B may be used as only the strain gauge SG ₁ or only the strain gauges SG ₁ and SG ₂ . Fixed resistors may be used on the other sides.

Further, two temperature-sensitive resistance elements R may be provided in series with each of the terminals 4 and 3, or may be provided in series with only the terminal 3.

As this temperature-sensitive resistance element, for example, a thermistor, metal wire (platinum wire, nickel wire, indium wire, etc.) or temperature-measuring low antibody, which has been widely used in the past, can be used, but recently developed special It is desirable to use a temperature-sensitive resistor formed with a metal film and insulated with a special resin. In other words, thermistors have non-linear temperature characteristics, and their accuracy and reproducibility are not very high. In order to make the temperature characteristics linear, it is necessary to add a correction resistor or use a complicated correction circuit such as an analog calculation circuit. Become.

On the other hand, metal wire thermometers such as platinum wire and nickel wire have low resistance (for example, platinum thermometers are 50Ω and 100Ω as established in JISC1604).
Therefore, there are drawbacks such as the contact resistance and the wire resistance of the signal system circuit having a large influence, the possibility of being easily affected by external influences, the output signal being small, and the size being large, resulting in poor instrumentation.

In contrast to these, the above-mentioned insulation-coated type temperature-sensitive resistors are electrochemically stable at high temperatures, and are coated with a metal thin film (film material: The main components are nickel, iron, chlorine, etc. and several other metals are added as additives to obtain the desired resistance value and temperature coefficient of resistance) by vacuum evaporation, sputtering, etc. The top is coated with epoxy resin for protection, and the temperature/electrical resistance characteristics are linear, and resistances ranging from low resistance (several Ω) to high resistance (about 10KΩ) can be obtained, such as relatively high resistance. Contact resistance and line resistance of the signal circuit are not a problem when using the same, and the resistance change rate is relatively large against noise, which is advantageous and can also cope with minute temperature changes.

Furthermore, instead of strain gauges SG ₁ to _{SG 4} , other conversion elements such as differential transformers and differential inductances can be used to measure temperature as well as physical quantities other than temperature, such as load, force, pressure, acceleration, etc. It may be possible to do so. Incidentally, a differential transformer consists of a primary coil and a pair of secondary coils, and when the moving parts move depending on the physical quantity to be measured, the degree of magnetic coupling between the primary and the pair of secondary coils changes. ,
This detects the difference between voltages induced in the secondary coil in proportion to the amount of movement of the movable part. Furthermore, if a photoelectric conversion element such as a CDS (cadmium sulfide) element is used instead of the strain gauges _SG1 to _SG4 , the temperature as well as the amount of light can be measured.

Note that the contact b of the switch SW ₂ and the contact b of the switch SW ₃ may be connected in advance.

As described above, the present invention provides an apparatus for measuring the physical quantity and temperature of a measured object using a bridge circuit constituted by an element that converts a physical quantity other than temperature into an electrical quantity and a temperature-sensitive resistance element. A temperature-sensitive resistance element is connected in series to measure the output of the bridge circuit, and two constant current sources are provided that send the same current to each other. One of the constant current sources is connected between one end of the output measurement of the circuit via a temperature sensitive resistance element, and the other of the constant current sources is connected between the input measurement and the other end of the output measurement. As a result, the number of core wires of the extension cord can be reduced, and temperature measurement accuracy can be improved without being affected by core wire resistance.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing a conventional device, Fig. 2 is a circuit diagram showing an embodiment of the present invention, Fig. 3 is an equivalent circuit diagram of the above embodiment during strain measurement, and Fig. 4 is a circuit diagram of the above embodiment. It is an equivalent circuit diagram at the time of temperature measurement. 10, 11...Sensor section, 20,21...Extension cord section, 30,31...Measuring device main body, SG ₁ ~
SG ₄ ...Strain gauge, R...Temperature-sensitive resistance element, l ₁
~l ₆ ... core wire of extension cord, S ₁ , S ₂ ... constant current source, A ₁ ~ _{A 3} ... amplifier, M, SM, TM ... meter, B ... bridge circuit, SW ₁ , SW ₂ ,SW ₃ ...
Switch.

Claims (1)

[Claims] 1. A bridge circuit, an element that is provided on at least one side of the bridge circuit and converts a physical quantity into an electrical quantity, a temperature-sensitive resistance element that is provided in series on the output side of the bridge circuit, and two elements that mutually conduct the same current. one of the constant current sources, which shoots the input side of the bridge circuit during temperature measurement, and connects the input side and one end of the output side of the bridge circuit via the temperature-sensitive resistance element; and a switch for connecting the other constant current source between the input side and the other end of the output side.