JPS5847002B2 - Digital strain measurement device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in digital strain measuring devices.

FIG. 1 is a block diagram showing the configuration of a conventional device, and FIG. 2 is an explanatory diagram of the operation of the conventional device shown in FIG.

In FIG. 1, a Wheatstone bridge composed of strain gauges R1-R4 is supplied with a bridge voltage E□ via a bridge power switch S1.

When the strain gauge receives mechanical strain, the Wheatstone bridge output proportional to the mechanical strain is R8, and the extension cable L0 is connected to the Wheatstone bridge circuit.
The unnecessary thermoelectromotive force present at the connection end of ~L4 and the input circuit of the amplifier 2 is collectively expressed as ET.

For convenience of explanation, amplifier 2 has an amplification degree of 1 (actually 100
~500), assuming that there is no polarity reversal of input and output,
An unnecessary voltage ED is generated at the output terminal of the amplifier 2 due to the zero point variation of the amplifier 2, and an unnecessary voltage ED is generated at the output of the code converter 1 due to the zero point variation of the code converter 1 for reversing the polarity of the output of the amplifier 2. Let the unnecessary voltage be EO (
These E T yED and Ec are shown with polarity added temporarily).

Switch S2 is the output of the code converter 1, and the output of the amplifier.
, the output of the reference power source 3 in this order, and selects the input of the integrator 4 in a time-division manner.

The comparator, gate 5, clock oscillator 6, counter 7, etc. arranged on the output side of the integrator 4 together with the integrator 4 and the basic power source 3 constitute an integral type A/D converter 8 shown by a dashed line. There is.

The output of the naori counter 7 is sent to a display and printing device 9.

The operation with the switch S1 selected as 0FF1 and S2 selected as ■ is as shown in the section T1 in FIG.
The voltage ET-ED+EC, which is the sum of -ET-ED and the zero-point fluctuation EC of the code converter, which has the opposite polarity, is integrated.

Subsequently, the switch S1 is turned on and the switch S2 is switched to the state (■), resulting in the operation in the second homogeneous division section indicated by T2 in Fig. 2. At this time, the strain gauge is detected by the output of the Wheatstone bridge. The added value of the voltage E8 proportional to the mechanical strain generated, the equivalent thermoelectromotive force ET, and the zero-point variation ED of the amplifier is integrated, and when the second homogeneous component is completed, the integrator output E2-1 becomes the second homogeneous component. Minute result (E
8+ET+ED) and the result of the first similar type (-ET-BD+
It is the sum of E and o).

In other words, by carrying out the third homogeneous test indicated by T3 in Fig. 2, we obtain R2
After replacing the voltage -1 with the time T3, R2-1 is converted into a digital quantity using a conventionally known integral type A/D conversion method, and then displayed and printed as a mechanical strain quantity.

FIG. 3 is an explanatory diagram showing the principle of a conventional device different from that in FIG. 1, in which the polarity changeover switch S3. Other than S4, the same symbols as in FIG. 1 are used.

In this case, the Wheatstone bridge output E when the bridge power supply is set to positive polarity.

1 and Wheatstone bridge output E when the bridge power supply is set to negative polarity.

2 to obtain Es as shown in equation (2), and then performs division to obtain Es (divided by 2).

The conventional device described above has the following drawbacks.

In the device shown in Figure 1, (1) Zero point fluctuations of the code converter are not removed.

(2) When measuring in a place where the Wheatstone bridge and its extension cables L1 to L4 are subject to heavy commercial frequency common mode noise or normal mode noise, the degree of influence of these external noises can be adjusted by turning switch S1 ON or OFF'. The integration time T1. Even if T2 is selected to be an integer multiple of the commercial power supply period, it is difficult to completely eliminate its influence, which induces measurement errors.

(3) The power supply voltage is not applied to the Wheatstone bridge during the first homogeneous minute, and the bridge power voltage is applied only during the second homogeneous minute, so the effect of self-heating of the strain gauge is only affected by the Wheatstone bridge during the second homogeneous minute. Since this occurs as a zero point fluctuation of the bridge, if this component is α, equation (1) is transformed into equation (3), which further increases the measurement error.

E knee 1 = E8 + E,.

+α ・・・・・・・・・・・・ (3) Furthermore, in the device based on FIG. 3, (1) an arithmetic circuit for performing division is essential and becomes expensive.

(2) The bridge power supply is normally 5 to 12V at DCZV.
However, for this reason, the influence of self-heating of the strain gauge cannot be sufficiently avoided, and zero point fluctuations occur when measuring strain on materials such as plastics with poor thermal conductivity.

The present invention eliminates these drawbacks of conventional devices and enables accurate strain measurement without being affected by unnecessary thermoelectromotive force, zero point fluctuation of the amplifier, self-heating of the strain gauge, etc. The purpose of this invention is to provide a strained Oki-dōdō device that is both possible and inexpensive.

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

FIG. 4 is a block diagram showing one embodiment of the present invention.

In Fig. 4, a pair of input terminals of the Wheatstone bridge composed of four strain gauges R1-R4 are connected to a polarity changeover switch S from a variable voltage bridge power supply.
3. Through S4, the Wheatstone bridge power supply voltage E
I is applied, while an output voltage E8 exactly proportional to the mechanical strain ε detected by the strain gauge appears at the pair of output terminals, but at the same time, the electrical connection point in the Wheatstone bridge to which the strain gauge is connected, The connection points between the extension cables L2 and L'4 and the amplifier input terminals and the connection points of the wiring in the amplifier input circuit form a type of thermocouple, and each generates an unnecessary equivalent thermoelectromotive force determined by the ambient temperature during use. ET is constantly occurring.

Also, the zero point of an amplifier fluctuates over time,
Similar to the thermoelectromotive force described above, harmful voltage ED components are generated, and these unnecessary and harmful voltage components exist mixedly with R8, which is proportional to mechanical strain, and cannot be illustrated separately. Because it is difficult, how many songs have Es, ET, ED etc.
, the polarity is temporarily added.

For convenience of explanation, the amplifier has an amplification degree of 1 (actually 100 to 5
00) and there is no polarity reversal of input and output.

Now, polarity changeover switch S3. Considering the case where S4 is on the positive polarity side as shown in FIG. 4, the digit #iM indicated by the A/D converter 10 connected to the amplifier output is:
As shown in FIG. 5a,

On the other hand, if the mechanical strain applied to the strain gauge remains constant and the polarity selector switch of the bridge power supply is switched to the negative polarity side and measurements are performed in the same manner as above, the digital quantity M2 indicated by the A/D converter is As shown in Figure 5,

By switching the polarity of the Wheatstone bridge's power supply, only the component E8 proportional to mechanical strain can have its polarity reversed while its absolute value remains unchanged. However, this is unnecessary due to the equivalent thermoelectromotive force ET at each connection point and the zero point fluctuation of the amplifier. , it is shown that the harmful voltage ED does not change in absolute value or polarity.

Here, Ml of equation (4) obtained in the first measurement is stored in advance in the register 11 in FIG. When the difference between the two is calculated digitally using the calculation circuit 12 (hereinafter referred to as "later measurement"), the calculation circuit 12
The output is as follows, which means that twice R8, which is proportional to the required mechanical strain, is obtained.

Therefore, this value 2E8 is divided by 2 by the function of the arithmetic circuit,
By obtaining R8 and displaying and printing it using the display and printing device 9, unnecessary and harmful ET and ED can be removed and mechanical strain can be determined.

Up to this point, it is the same as the conventional device shown in FIG. 3, and therefore zero point fluctuations due to self-heating of the strain gauge remain.

However, focusing on the fact that 2E8 shown in equation (6) is twice the amount of mechanical strain required, the number of "previous measurements" and "later measurements" can be reduced while lowering the bridge voltage. The control circuit 13 and arithmetic circuit 12 can be configured so that even simple addition and subtraction can immediately indicate R8 itself. The gist of the present invention is that it is possible to easily and reliably measure strain while simultaneously eliminating zero point drift due to self-heating of the strain gauge itself by applying it to the control circuit 13.

In other words, as is well known, the output BS of a Wheatstone bridge composed of strain gauges is calculated by setting the bridge voltage EI to 2■ and the gauge factor of the strain gauge to 8 to 2.00. Therefore, for convenience in circuit configuration and use, the bridge voltage E is usually adjusted to 2, and the 2 in equation (6) is
E8 is also based on this kind of thinking.

Therefore, suppose the voltage of the variable voltage bridge power supply is E, =
If it is set to IV, the arithmetic circuit output shown by equation (6) can be obtained directly as R8, which is convenient because no division is necessary, and at the same time, the power supply voltage applied to the strain gauge is As shown in Fig. 6, the amount of heat generated by the strain gauge itself is P 2 = E, /
R = 4/R, and in b, P, =E□'/R = 1/R, the latter being K in the former, and the zero point fluctuation due to self-heating of the strain gauge is reduced accordingly.

In addition, in FIG. 6, R0 to R4 in FIG. 4 are replaced by R1-R.
2=R3=R4=R, and Pl and P2 are the power consumption (heat amount) of the entire Wheatstone bridge when the bridge voltage is IV and 2V, respectively.

If this idea is further developed, the bridge voltage can be further reduced, and at the same time, the number of "previous measurements" and "later m1ffl" can be increased accordingly, for example, the bridge voltage E
If □ is 0.5V, perform the process twice ("previous measurement" and "later measurement") and simply add the results, and as a result, Es will be Moreover, as shown in Table 1, the result has the same meaning as the average of many measured values, so the 1 variation in measured values is reduced and the measurement accuracy is statistically improved. It becomes possible to do so.

As explained above, the digital strain measuring device of the present invention has the following advantages over conventional devices.

(1) By setting the bridge power supply voltage to DCI V and making two measurements each by switching the polarity, you can eliminate unnecessary thermoelectromotive force existing in each part and the zero point of the amplifier without performing division. Variations can be reliably removed and strain can be directly read.

(2) Since the arithmetic circuit does not require division, it can be constructed from a simple counter and can be made inexpensive.

(3) The zero point of the amplifier need only remain unchanged for a very short time, and accurate strain measurement can be performed with inexpensive equipment even under harsh environmental conditions.

(4) After setting the voltage of the bridge power supply, only the polarity is switched and the two measurements before and after are performed as one set, so the measurement conditions of the Wheatstone bridge circuit are always unified, and the common mode and normal mode noise caused by the commercial power supply is Even if there is an effect, it is easy to eliminate.

(5) By reducing the bridge voltage to 1■ or less and increasing the number of measurements, not only can zero point fluctuations due to self-heating of the strain gauge be minimized, but also one measurement value can be obtained from multiple measurements. Therefore, the measurement values can be statistically highly accurate.

Moreover, many modifications can be made to the present invention.
For example, in the text explanation, the difference between the measured value M1 when the bridge power supply is set to positive polarity and the measured value M2 when set to negative polarity is calculated digitally by the functions of registers and arithmetic circuits, but this Ml - M2 It is not necessarily necessary to calculate the calculation digitally, but it is also possible to calculate the calculation analogously using the characteristics of the integrator of a conventionally known integral type A/D converter.

That is, in FIG. 7, at the first homogeneous division stage indicated by tl, when the bridge power supply has positive polarity, the formula (4) M any E8
-E 'r-ED is integrated, and then in the second homogeneous division stage indicated by t2, M2-- BS of equation (5) when the bridge power supply has negative polarity, -E 'i'-B, is polarized. In the third homogeneous division stage shown at t3, E8 is immediately digitized (distortion amount Furthermore, it is easily possible to use a constant current power supply instead of a constant voltage power supply for the bridge power supply, or to prepare two positive and negative power supplies and switch between them. be.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing the configuration of a conventional device, Fig. 2 is an explanatory diagram of the operation of the conventional device, Fig. 3 is an explanatory diagram of a conventional device different from Fig. 1, and Fig. 4 is an embodiment of the present invention. FIG. 5 is an explanatory diagram showing the principle of the present invention. FIG. 6 is an explanatory diagram of power consumption (heat generation) of the Wheatstone bridge.
The figure is an explanatory diagram of an example of an operation of an arithmetic circuit according to the present invention. 2...Amplifier, 9...Display and printing device, 10...A/D converter, 11...
・Register, 12... Arithmetic circuit, 13...
...Control circuit, R0-R4...Strain gauge,
SL, S2...Polarity changeover switch, E□...
...Wheatstone bridge power supply voltage (voltage variable)
, E8... Output voltage of the Wheatstone bridge proportional to mechanical strain, ET... Unnecessary equivalent thermoelectromotive force present in the Wheatstone bridge, at the connection point of the extension cable and in the amplifier input circuit, ED'...
...Unnecessary voltage that appears at the amplifier output due to the zero point fluctuation of the amplifier, L1 to L4...Extension cable.

Claims (1)

[Claims] 1. When a bridge voltage E0 is applied to the input terminal and mechanical strain ε is applied, the resistance values vary from R to R+ and J, respectively.
It changes to R, and is composed of a strain gauge with gauge factor constraint = 2, and from the output end E, , JR-view, R8, ε-shadow,
A Wheatstone bridge, a variable voltage bridge power supply that supplies a predetermined bridge voltage E to the input terminal of the Wheatstone bridge, and an output E that receives the output E of this bridge power supply. a polarity changeover switch that switches the polarity of □ between positive and negative and applies it to the input terminal of the Wheatstone bridge; an amplifier that amplifies the output voltage Es that appears at the output terminal of the Wheatstone bridge; and an amplifier that receives the output of this amplifier and converts the output to A. /D converter and this A/D converter
A register temporarily stores a digital amount proportional to the mechanical strain ε obtained by the /D converter, and the output of this register and the output of the above-mentioned converter are subjected to a predetermined calculation to calculate the mechanical strain ε. an arithmetic circuit that obtains the mechanical strain ε; a display and printing device that receives the output of the arithmetic circuit and displays and prints the mechanical strain ε; The output of the D converter is input to the register, and in the subsequent "later measurement", the polarity selector switch is switched to the other polarity, the output of the A/D converter is input to the arithmetic circuit, and the output of the A/D converter is input to the arithmetic circuit. In the calculation circuit (previous measured value - subsequent measured value) /
(bridge voltage), or when the bridge voltage E0 of the bridge power supply is set to be smaller than 1■, the "previous measurement" and the "after measurement" are determined according to the degree of voltage drop. Increase the number of times inversely proportionally, (
The A/D converter, the register,
The display and printing device and the arithmetic circuit are each controlled according to a predetermined procedure. A digital strain measuring device characterized in that it is configured to measure true mechanical strain by removing measurement errors due to zero point fluctuations.