Development of a new portable static strain gauge

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Development of a new portable static strain gauge

Abstract: This paper introduces the main testing principle and technical characteristics of a new portable static strain gauge. The instrument works with a single power supply, which solves the problem that the bridge source circuit of the strain gauge needs an independent power supply. Through the test of its working performance, it is proved that the instrument has the advantages of high precision, good stability, simple circuit, low power consumption and high use value

key words: DC bridge; Static strain gauge; Operational amplifier; Voltage reference; Analog to digital converter

1 test principle

see Figure 1 for the principle circuit of this machine. Its main measuring circuit is composed of bridge source, amplification circuit, analog-to-digital conversion circuit and liquid crystal display

1.1 bridge source circuit

bridge source is mainly used to provide DC regulated power supply for Wheatstone bridge composed of strain gauge sensors. Bridge source plays a very important role in strain gauge. The stability of bridge source is directly related to the accuracy of test results. This machine adopts a DC constant voltage source with an output voltage of 2V, which is composed of a precision voltage reference and an operational amplifier

in Figure 1, U1 (max873) is a low-power, low drift, precision voltage reference with an output voltage of +2.5v, and its power supply voltage range is +4.5 ~ +18v; U2a (TL062) is a low offset JFET input operational amplifier, and u2a, R3, Q1, C1 form a voltage follower. After the +2.5v output by U1 is divided by R1 and R2, the voltage of 1.67 ~ 2.50V can be called out on R1, which can meet the requirements of generating 2V bridge voltage. When the bridge voltage output increases (or decreases), it is fed back to the reverse input end of u2a, so that the output voltage of u2a operational amplifier decreases (or increases), the base current of Q1 decreases (or increases), and the emitter current of Q1 (the current flowing through the load) decreases (or increases). According to Hooke's law, the load voltage (bridge voltage) will decrease (or increase), so as to achieve the purpose of stabilizing the bridge voltage

The +5v stable voltage between

v+ and the ground is provided by the analog-to-digital converter (see the analog-to-digital conversion circuit for details), which is greater than the +4.5v required for the normal operation of the max873, ensuring that the max873, TL062 and Q1 are not affected by the reduction of the instrument power supply voltage, so that the bridge source circuit can provide a stable bridge voltage

1.2 amplification circuit

this machine adopts the absolute measurement method of strain value, that is, it does not balance the bridge, directly measures the output value of the strain bridge under the action of load, and the increment or relative value can be obtained through calculation. This method can eliminate the error caused by the balance circuit, especially overcome the problem that it is difficult to save the balance value in long-term monitoring

the amplification circuit of this machine is mainly composed of U3 (ltc7652) operational amplifier. Ltc7652 is an ultra-low drift, chopper self stabilizing zero Precision Operational Amplifier developed by CMOS process. It has ultra-low offset voltage and drift, high common mode rejection ratio and power supply rejection ratio. It is an ideal amplifier for strain gauge. According to the strain test principle, the output voltage of the equal arm bridge is:

, where: USC - the output voltage of the bridge; U0 -- bridge source voltage, which is 2V here; K - sensitivity coefficient of strain gauge, which is fixed as 2.0 in this instrument in order to simplify the circuit; ε—— Strain value

set U0, K ε Substitute formula (1) to get: USC= ε, That is, in order to achieve analog-to-digital converter 10 μ To meet the requirements of V resolution, the output voltage of the bridge needs to be amplified by 10 times. In Figure 1, R5, R6, R7 and R8 are the external bridge arms of the four strain gauges of the bridge. In order to improve the input impedance of the amplifier, the circuit adopts in-phase amplification. The voltage gain of the amplification circuit is:

the design range of the strain gauge is 20000 με, The full-scale input ubd= ± 20mV and output uhl= ± 200mV of the amplifier. When the power supply voltage v+-v-=7 ~ 9V, v+-uc=+5v, v+-ub=+4v, v--ub=-3 ~ -5v, it can be seen that point B, as the output reference point of the amplifier, is basically in the middle of the power supply voltage, and the output potential of the amplifier is far less than the power supply potential, ensuring the linearity of the amplifier and no overflow

1.3 analog-to-digital conversion circuit

the analog-to-digital converter U1 (icl7129) used in this machine is a 4 (1/2) bit LCD monolithic analog-to-digital converter with COMS large-scale integrated circuit technology and high cost performance. It has the advantages of high precision, low drift, simple peripheral circuit and so on

in order to improve the accuracy and anti power frequency interference of the instrument and focus on the tilling-/trim-in-place technology used to produce cup products, the crystal oscillator circuit with a frequency of 100kHz composed of Y1, C4 and C5 provides stable clock pulses for the analog-to-digital converter icl7129. By 1.2V, 10 ×/The precision voltage reference D1 (icl8069) of ℃ provides a 1V reference voltage for icl7129. Since the output reference potential of the amplification circuit is about 4V lower than v+ and is not fixed, and the com terminal of icl7129 is about 3.2V lower than v+, the two potentials are not equal. Using common mode voltage of about 0.6~1.0v differential transmission Tego glide a116 can provide an ideal printing effect input method for all kinds of Applications (see Figure 1)

due to the use of battery power supply, the problem that the battery voltage decreases with the increase of working time must be considered. This machine uses the characteristic that the DGND terminal of icl7129 maintains a stable 5.3v voltage difference with v+, and the voltage follower is composed of U2B, Q2, R15 and C10 to generate the ground level required by the bridge source circuit, so that the bridge source voltage is not affected by the change of power supply voltage

2 working performance test analysis

2.1 nonlinearity test

nonlinear test is carried out on portable static strain gauge with equal strength cantilever beam, and the test results are compared with that of 7v14 static strain gauge (day). The test data are shown in Table 1. The actual strain value in the table is calculated according to the deflection generated by the cantilever beam with equal strength at each level of load. It can be seen from the data in Table 1 that the handheld strain gauge has good linearity, and the data of the three are in good agreement

2.2 positive and negative symmetry test

we will reply to the customer at the first time. According to the principle of strain electrical measurement, reverse the A, C (or B, d) of the bridge to change the polarity of the output signal. Table 2 shows the positive and negative symmetry test results on the cantilever beam with equal strength, which shows that the positive value is 7 ~ 10 less than the absolute value of the negative value με, It shows that changing the bridge wiring mode will cause the output of the bridge to change, but it has no effect on the increment

2.3 effect of power supply voltage

the relationship curve between power supply voltage and indication in Figure 2 shows that: (1) the test results are accurate in the range of 7.5 ~ 12.5V, and the power supply voltage has no effect on the test results; (2) The range of 6.7 ~ 7.5V is basically accurate, which has little impact on the test results. At 7.3v, icl7129 low voltage indicator displays, prompting to replace the battery; (3) It cannot work under 6.5V

2.4 stability test

detect the drift stability of the instrument by testing the change value of zero point within 4H. Replace the strain gauge R5 ~ R8 with a standard resistance of 120 Ω (2w0.01%), short circuit points B and D, and make the input of the strain gauge zero. After preheating for 15min, read the change of the indicated value of the strain gauge at 15min intervals in the first hour and 30min intervals in the second to fourth hours. The test results show that there is no drift within 4H

3 conclusion

this instrument successfully solves the problem that the bridge source power of strain gauge is used to transform the coal-fired furnace of plastic granulator system, which has become an important subject to be solved urgently. The circuit and amplification circuit and analog-to-digital conversion circuit are independent power supply, and it can be powered by single power battery. The instrument uses the strain absolute measurement method, which solves the problem that the equilibrium value of general instruments is not easy to save in long-term testing. Through the performance test of portable strain gauge, it is proved that the circuit design of the instrument is feasible, can meet the test requirements, and has the advantages of stable performance, high precision, convenient use and so on


[1] [U.S.] i.m. Gottlieb, translated by Ye Jingguo and Ma Jixun Regulated power supply [M] Beijing: Science Press, 1993

[2] Sha Zhanyou Practical Encyclopedia of new digital multimeter [M] Beijing: Electronic Industry Press, 1996

[3] Li Yongmin Electronic circuit of testing instrument [M] Xi'an: Northwest University of technology, 1996 (end)

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