JPH038481B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a Wheatstone bridge circuit (hereinafter simply referred to as a "bridge circuit") composed of strain gauges.
The present invention relates to a strain measuring device using the above-mentioned device, and more specifically to a strain measuring device capable of measuring the true amount of strain even when a bridge circuit is excited with an unstable bridge voltage.

In a bridge circuit that is originally composed of strain gauges, the unbalanced voltage of the bridge circuit output is determined by the gauge factor determined by the material of the strain gauge, the amount of strain proportional to the resistance change of the strain gauge, and the bridge circuit that excites the bridge circuit. It is expressed as the product of three voltage parameters.

FIG. 1 is a circuit diagram showing an example of the configuration of a bridge circuit for explaining the above relationship. In the same figure, G1 to G4 are strain gauges each having a resistance value of R, and 1 is the strain gauge G of these four sheets.
2 is a bridge power supply that excites the bridge circuit 1 with a bridge voltage E. Here, the input terminal c of the bridge circuit 1,
When the bridge voltage E is supplied from the bridge power source to d, the strain gauges G1, G3 and G2, G4 each have a resistance change R+ proportional to the amount of strain ε.
.DELTA.R and R-.DELTA.R occur, and as a result, an unbalanced voltage e expressed by the following equation is generated between the output terminals a and b of the bridge circuit 1.

e=Va−Vb=Ks・ε・E…(1) (Va: Voltage at terminal a, Vb: Voltage at terminal b, Ks: Constant determined by the material of the strain gauge and known constant over time and temperature. (E: bridge voltage, which is a parameter that can be selected depending on the circuit design of the strain measurement device) As is clear from equation (1) above, if the bridge voltage is constant under any conditions, By applying the stabilized bridge voltage E to the bridge circuit 1, it is possible to obtain an unbalanced voltage e that is proportional only to the amount of strain ε from (1) above, and by measuring the unbalanced voltage e, the amount of strain can be determined. ε can be found.

Although Fig. 1 shows a general bridge circuit consisting of four gauges, if the bridge circuit is configured with one or two gauges,
The value of the unbalanced voltage e shown in equation (1) is only 1/4 or 1/2 of the sensitivity, respectively, and is basically proportional to the product of the three types of parameters mentioned above, and is A bridge circuit may be constructed using a sheet gauge.

FIG. 2 is a block diagram showing the configuration of an example of a conventional strain measuring device. In this example, a stabilized bridge voltage E is applied inside the strain measuring device.
The bridge circuit 1 has a built-in bridge power supply 2 that outputs a voltage E, and is configured to supply the stabilized bridge voltage E to the input terminals c and d of the bridge circuit 1. 10
is a conventional strain measuring device, which is equipped with a bridge power supply 2, a voltmeter 3 for measuring the unbalanced voltage e of the bridge circuit 1, and a display 4 for displaying the unbalanced voltage e as a strain. . The bridge circuit 1 and the strain measuring device, which are provided at a point apart from the strain measuring device 10, are connected by four connecting cables Ca, Cb, Cc, and Cd each having a cable resistance r.

By the way, the conventional strain measuring device shown in FIG. 2 has the following problems.

(b) From the viewpoint of measurement accuracy, the bridge voltage E is not only stable against temperature changes and changes over time, but also the commonly used 60 to 1000 Ω.
The bridge power supply is required to have a current capacity that can be excited over the load range of the gauge resistance and to be sufficiently stable, but a bridge power supply that can satisfy these requirements is inevitably expensive.

(b) When the bridge circuit 1 and the strain measuring device 10 are installed far apart, the value of the cable resistance r cannot be ignored compared to the gauge resistance R, and even if a sufficiently stabilized bridge voltage E is supplied, The bridge voltage E' actually applied to the bridge circuit 1 decreases according to the relationship E'=E·R/(R+2r), and this decrease in bridge voltage causes a sensitivity reduction error. Measures to prevent this decrease in sensitivity include (i) a method of exciting the bridge voltage using a remote sensing method, (ii) a method of exciting the bridge voltage with a constant current, and (iii) a method of installing the bridge power supply near the bridge circuit side. , is taken.

However, in the above-mentioned remote sensing method (i), the number of cores in the input/output cable of the bridge circuit increases by two, and it is necessary to use a cable with a total of six cores. Further, in the constant current method (ii) above, the influence of cable resistance is eliminated, but the bridge voltage changes depending on the resistance value of the strain gauge, so the current value must be adjusted according to the gauge resistance value. Furthermore, if the bridge power supply 2 is installed near the bridge circuit 1 and directly connected to the input terminals c and d of the bridge circuit 1 as described in (iii) above, sensitivity errors due to the cable resistance r can be prevented. However, the temperature environment where the bridge circuit 1 is usually installed is harsher than the temperature environment where the strain measurement device is installed, and from the viewpoint of stability of the bridge voltage, the bridge power supply 2 is placed near the bridge circuit 1. It may not be possible to install it. As described above, problems arose regardless of which of the above methods was adopted.

The present invention has been developed in view of the above circumstances, and even if a bridge circuit is excited by an inexpensive bridge power supply having an unstable bridge voltage, it will not be affected by the instability of the bridge voltage and will be able to generate a true signal. It is an object of the present invention to provide a strain measuring device capable of measuring the amount of strain.

The present invention will be described in detail below with reference to the drawings.

FIG. 3 is a block diagram showing the configuration of one embodiment of the present invention. In the figure, parts given the same reference numerals as in FIG. 2 indicate the same or equivalent parts. In the figure, 2' is a bridge power supply installed near the bridge circuit 1, which in this embodiment is composed of dry batteries. The characteristics of a typical dry battery are such that its voltage decreases over time due to its discharging action, and it is also affected by temperature changes. Furthermore, the voltage value of a new dry cell battery itself varies considerably, and when used as a bridge power source for strain measurement, it can be regarded as an electrically unstable bridge voltage. Here, the bridge voltage is Ex
shall be. Reference numeral 10' denotes the main body of the strain measuring device (hereinafter referred to as the main body of the device), which is the essential part of the present invention, and inside thereof there are a voltmeter 3 and a voltmeter 3 for measuring the unbalanced voltage ex generated at the output terminals a and b of the bridge circuit 1. , a pair of buffer amplifiers 21 and 22 that are provided on the output side of the bridge circuit 1, that is, detect an unstable bridge voltage Ex from the output side of the bridge circuit 1, and a summing amplifier that adds the outputs of the buffer amplifiers 21 and 22. A bridge voltage detection circuit 20 consisting of a resistor R1, the output of the voltmeter 3 (that is, the unbalanced voltage value of the bridge circuit 1) Ex, the output of the bridge voltage detection circuit 20 (that is, the unstable bridge voltage value) Ex, and the predetermined Output of a reference voltage generator (not shown) that generates a reference voltage (i.e. reference voltage value)
Receives 3 inputs of Eo and outputs e′ = ex・Eo/Ex
A calculator 5 performs arithmetic processing to obtain e′ and performs automatic sensitivity correction processing, and an output e′ (=ex・
Eo/Ex) is compared with the output Ex of the bridge voltage detection circuit 20 to see if it is within the range of the upper limit threshold voltage VH and lower limit threshold voltage VL. When it is within the range, it is determined that the measured value is normal and outputs a set signal.
A comparator 6 determines that the measured value is abnormal and outputs a negative signal when it is higher than the upper limit threshold VH or lower than the lower limit threshold VL, and when the result of the judgment of this comparator 6, that is, a negative signal is output, It is composed of an alarm device 7 that emits an alarm sound (or alarm light). Note that the unstable bridge power supply 2' is separated from the device main body 10', and is connected to strain gauges G1 to G4.
between the output terminals a, b of the bridge circuit 1 composed of strain gauges G1 to G4 and the input terminals a', b' of the voltmeter 3 and the bridge voltage detection circuit 20. are connected by two common cables Ca' and Cb', and one cable Cd' as a common line is connected between the negative terminal d of the bridge circuit 1 and the ground point d' of the bridge voltage detection circuit 20. connected with.

Next, the operation of the embodiment configured as described above will be explained.

The unbalanced voltage ex when the bridge circuit 1 is excited by the bridge power supply 2 that outputs an unstabilized, so-called unstable excitation voltage (bridge voltage) Ex is calculated by the following equation (2) from equation (1). It is expressed as

ex=Va−Vb=Ks·ε·Ex (2) Next, the operation of the bridge voltage detection circuit 20 will be explained.

FIG. 4 shows an equivalent circuit diagram of the bridge circuit 1 shown in FIG. 1 when viewed from the terminals a and b. The voltages Va and Vb at terminals a and b are as follows:
It is expressed by equations (3) and (4).

Va=Ex/2+ex/2... (3) Vb=Ex/2-ex/2... (4) Note that the input terminals of the pair of buffer amplifiers 21 and 22 have large input impedance and small input bias current, so these input terminal to terminal
Impedance matching problems and strain measurement accuracy problems caused by connections to a' and b' can be ignored. Here, if the output voltages of the pair of buffer amplifiers 21 and 22 are E21 and E22, and the output voltage of the summing amplifier 23 is E23, then
E21 and E22 are expressed by the following equations (5) and (6) from equations (3) and (4).

E21=Va=Ex/2+ex/2 (5) E22=Vb=Ex/2-ex/2 (6) On the other hand, the output E23 of the summing amplifier 23 is expressed by the following equation (7).

E23=E21+E22…(7) E23 in equation (7) becomes the following equation (8) from equations (5) and (6).

E23=Va+Vb=(Ex/2+ex/2) +(Ex/2-ex/2)=Ex...(8) According to the above equation (8), the output of the bridge voltage detection circuit 20 is Ex, and the output of the bridge voltage detection circuit 1 is Ex. This shows that it is possible to extract the unstable bridge voltage Ex using a dry battery from the output side.

Next, the operation of the arithmetic unit 5 will be explained.

ex and Ex shown by equations (2) and (8) detected by the voltmeter 3 and the bridge voltage detection circuit 20,
When the reference voltage Eo is input from the reference voltage generation circuit to the input terminals X, Y, and Z of the arithmetic unit 5, the output terminal W of the arithmetic unit 5 produces an output as shown in the following equation (9). e′ is output.

e'=X.Z/Y=ex.Eo/Ex (9) By substituting equation (2) into equation (9) above, the following equation (10) is obtained.

e′x = (Ks・ε・Ex)・Eo/Ex =Ks・ε・Eo (10) Here, the reference voltage Eo is made equal to the stabilized bridge voltage E, that is, Eo=Eo
When set so that, equation (10) becomes the same as equation (1). This means that the output e' of the arithmetic unit 5 is not affected by the instability of the bridge voltage even if the bridge circuit 1 is excited with the unstable bridge voltage Ex. It is shown that the unbalanced voltage e is the same as when the circuit 1 is excited. The output e' of the arithmetic unit 5 is displayed as a strain (ε) on the display 4.

By the way, in actual use, the value of this unstable bridge voltage Ex cannot be selected infinitely, but is constrained to be within a permissible range of certain upper and lower thresholds. Therefore, in the second invention, the output Ex of the bridge voltage detection circuit 20 is inputted to the comparator 6 having an upper limit threshold (voltage) VH and a lower limit threshold (voltage) VL. The normality or abnormality of the measured value is determined based on this. That is, the comparator 6 outputs the output Ex of the bridge voltage detection circuit 20.
is compared with the preset upper limit threshold VH and lower limit threshold VL, and when the bridge voltage Ex is between the upper and lower thresholds, the measured value is determined to be normal and a positive signal is output. upper threshold
When it is higher than VH or lower than the lower limit threshold VL, it is determined that the measured value is abnormal and a negative signal is output. When the alarm 7 receives a negative signal from the comparator 6 indicating that the measured value is abnormal, the alarm 7
emits an alarm sound or alarm light, which allows the operator to recognize that the accuracy of the measured value at that time is not guaranteed.

Then this upper and lower threshold VH and
This section explains how to set up VL.

First, the upper limit threshold VH is determined by the lower one of VH limited by the conditions of the bridge circuit 1 and the strain measuring device 10'. The conditions of the bridge circuit 1 are limited by the heat capacity of the strain gauge and the specimen to which the strain gauge is bonded. The strain gauge itself has a certain resistance value, and when a bridge voltage is applied to the strain gauge, a current flows through the strain gauge. Then, the resistance value of the strain gauge changes due to self-heating of the strain gauge itself. As a result, the unbalanced voltage e changes and a measurement error occurs. Furthermore, applying an excessive voltage will eventually burn out the strain gauge. Therefore, the error due to self-heating of the strain gauge must be less than the bridge voltage, which can be ignored in terms of accuracy. Further, the conditions for the other strain measuring device 10' are limited by the maximum value of the measurement dynamic range of each circuit section inside the device. From equation (2), the strain sensitivity of the unbalanced voltage ex increases as the bridge voltage increases, so the unbalanced voltage at the maximum measured strain must be within the measurement dynamics range of the strain measuring device 10'. . The lower of the maximum bridge voltages limited by the above two factors is set as the upper limit threshold VH.

On the other hand, the lower limit threshold VL is limited by the relationship between the signal level of the unbalanced voltage of the bridge circuit 1 and the internal noise signal level of the distortion measuring device 10'. From equation (2), the distortion sensitivity of the unbalanced voltage becomes smaller as the bridge applied voltage is lower, and even if the amount of distortion is the same, the signal level becomes lower. On the other hand, since the main body 10' of the apparatus has its own internal noise level generated by the apparatus itself, if the signal level becomes lower than this noise level, distortion cannot be measured. The lower threshold VL is set (or selected) based on the factors mentioned above.
will be done.

It goes without saying that the present invention is not limited to the embodiments described above and shown in the drawings, and that various modifications can be made without departing from the spirit thereof.

For example, in the embodiment shown in the third figure, a dry cell battery is used as the bridge power source 2', but other unstable bridge power sources may be used.

Further, the internal circuit of the device main body 10' may be constructed from an analog circuit or a digital circuit, or may be constructed from a mixed analog and digital circuit.

As detailed above, the strain measuring device of the first invention provides the following advantages.

First, since the bridge voltage does not have to be stable against temperature changes, changes over time, and even load fluctuations, the circuit design of the bridge power supply becomes easy and can be manufactured at low cost.

In addition to the bridge power supply, the present invention uses a reference voltage generation circuit that generates a stable reference voltage, but this does not excite the Wheatstone bridge (in other words, the commonly used 60
(It does not excite in the load range of strain gauge resistance of ~1000Ω), so a small current is sufficient, and therefore it can be constructed inexpensively and compactly.

Furthermore, since this reference voltage generation circuit does not excite the bridge circuit, it is not necessary to place it near the bridge circuit, and therefore the temperature is more moderate and stable than the severe temperature environment in which the strain gauge is installed. Since it can be installed in a strain measuring device installed in the environment, an inexpensive voltage generating circuit can be used in this respect as well.

Second, even if the bridge circuit is excited with an unstable bridge voltage, the true amount of strain can be measured without being affected by the instability, so the usable temperature range of the bridge power supply is wider than that of conventional ones. result,
The bridge power supply can be installed near the bridge circuit. By installing the bridge power supply near the bridge circuit, sensitivity errors due to cable resistance between the bridge circuit and the main body of the device can be eliminated. Further, in the conventional method, a four-core cable had to be used, but with the present invention, signals can be sent and received using a three-core cable.

Further, according to the second invention (the invention set forth in claim 5), the detector determines whether the unstable bridge voltage detected by the bridge voltage detection circuit is within an appropriate level. Since the determination result is configured to be alarmed by an alarm, it is possible to prevent abnormal measurement values from being measured and to enable highly reliable measurements.

[Brief explanation of the drawing]

FIG. 1 is a circuit configuration diagram for explaining the principle of measuring unbalanced voltage in a bridge circuit, and FIG. 2 is a block diagram showing the configuration of an example of a conventional strain measurement device.
FIG. 3 is a block diagram showing the configuration of an embodiment of the present invention, and FIG. 4 is an equivalent circuit diagram of the bridge circuit shown in FIG. 1. 1... Wheatstone bridge circuit, 2, 2'
... Bridge power supply, 3 ... Voltmeter, 4 ... Display, 5 ... Arithmetic unit, 6 ... Comparator, 7 ... Alarm, 10, 10' ... Strain measuring device (device body), 20 ...Bridge voltage detection circuit, 21,2
2...Buffer amplifier, 23...Summing amplifier, G
1 to G4...Strain gauge, E...Stabilized bridge voltage, Ex...Unstable bridge voltage, r
...Cable resistance, R...Strain gauge resistance,
R1...Resistance, Eo...Reference voltage, VH, VL...
Upper and lower thresholds, Ca, Cb, Cc, Cd,
Ca′, Cb′, Cd′……cable.

Claims (1)

[Scope of Claims] 1. In a strain measurement device using a Wheatstone bridge circuit constituted by a strain gauge, a bridge power supply that outputs an unstable excitation voltage and supplies a bridge voltage to the input side of the Wheatstone bridge circuit; , a bridge voltage detection circuit provided on the output side of the Wheatstone bridge circuit and configured to detect the unstable bridge voltage, the circuit comprising a pair of buffer amplifiers and a summing amplifier that adds the outputs of the pair of buffer amplifiers to obtain a detection output; A voltmeter that measures the unbalanced voltage of the Wheatstone bridge circuit, a reference voltage generation circuit that generates a predetermined stabilized reference voltage, an output Eo of this reference voltage generation circuit, and an output Ex of the bridge voltage detection circuit. and the output e _x of the voltmeter as input, and based on these inputs, perform the calculation e′=e _x Eo/Ex, correct the distortion sensitivity error due to the unstable bridge voltage, and calculate the true strain 1. A strain measuring device characterized by comprising a computing unit that sends out an output e′ according to a quantity. 2. Separate only the unstable bridge power source from the device, install the separated bridge power source near the strain gauge, and connect it between the output end of the Wheatstone bridge circuit, the input end of the voltmeter, and the input end of the bridge voltage detection circuit. Connect with two common cables,
2. The strain measuring device according to claim 1, wherein one input end of said Wheatstone bridge circuit and a ground point of said bridge voltage detection circuit are connected by one cable. 3. A strain measurement device using a Wheatstone bridge circuit configured as a strain gauge, which includes: a bridge power supply that outputs an unstable excitation voltage and supplies a bridge voltage to the input side of the Wheatstone bridge circuit; a pair of buffer amplifiers; a bridge voltage detection circuit that is provided on the output side of the Wheatstone bridge circuit and detects the unstable bridge voltage; A voltmeter that measures the balanced voltage, a reference voltage generation circuit that generates a predetermined stabilized reference voltage, an output Eo of this reference voltage generation circuit, an output Ex of the bridge voltage detection circuit, and an output e of the voltmeter. _x as an input, and based on those inputs, perform the calculation _e ′=e The unstable voltage detected by the bridge voltage detection circuit is compared with a preset upper and lower threshold, and the bridge voltage is between the upper and lower thresholds. a comparator that determines that the measured value is normal and outputs a positive signal when it is higher than the upper threshold or lower than the lower threshold, and outputs a negative signal that determines that the measured value is abnormal when it is higher than the upper threshold or lower than the lower threshold; 1. A strain measuring device comprising: an alarm device that receives the output of the device and issues an alarm according to the determination result thereof.