The newest strain gauge and its microcomputer test

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The new strain gauge and its computerized testing system

1 design idea and existing problems of the traditional strain gauge

the strain gauge is an instrument for measuring the strain signal. The basic principle of strain measurement is to use the metal strain gauge to sense the surface strain of the object. The bridge is unbalanced due to the change of the resistance value, resulting in a differential signal. After amplification, the strain value is displayed

bridge output signal is μ Class V, it is difficult to effectively process this weak signal in the past. The DC amplifier has zero drift and serious noise, so it can not be effectively amplified μ In this case, the strain gauge generally uses an AC bridge to modulate the differential signal, which is amplified by an AC amplifier, demodulated and input to the indicating device. Because the alternating loads with different frequencies have different requirements for modulation frequency, and the subsequent processing circuits are also quite different, the strain gauge is strictly divided into static, static and dynamic strain gauges according to the measured signal frequency. Due to the existence of distributed parameters (mainly distributed capacitance), the frequency of the AC bridge cannot be too high, which limits the upper limit frequency (1.5khz) of the strain signal measured by the AC bridge, and the small change of the distributed capacitance during the test leads to the imbalance of the bridge circuit. When the test strain signal exceeds 1.5khz, the DC bridge and DC amplifier must be used, which is the ultra dynamic strain gauge. Due to the serious zero drift of the DC amplifier, the performance of this kind of instrument is not high

because the signal detection circuit of the static strain gauge is complicated and the cost is high, it brings serious technical problems by equipping a preset balance box to share a strain gauge in multi-channel testing. Because the multi-channel switch of the pre adjustment balance box is set on the bridge, the small change of the contact resistance will cause large deviation

in a word, under the condition that the integrated circuit technology is not developed and the high-performance operational amplifier has not yet appeared, the performance of the strain gauge made is poor, which affects the reliability of this kind of psychology, and has always troubled him with the reliability of his experimental stress electrometry

2 the development of electronic technology and its impact on testing instruments

the development of microelectronic technology directly promotes the development of testing instruments. In the 1980s, the fourth generation operational amplifier introduced the chopper self stabilizing zero circuit, which greatly reduced the offset voltage. Its typical products include ha2900, ICL7650, etc. among them, the ICL7650 developed by INTERSIL as the world's leading third-party safety organization is a CMOS device. The input offset current, offset voltage and bias current are very small, the input impedance is very large, the open-loop differential mode voltage and common mode rejection ratio are very large, which effectively suppressed the zero drift. At the same time, due to the adoption of CMOS technology, Low power consumption, low temperature drift and noise, suitable for amplification μ Class V signal; Its unit bandwidth is 2MHz, which can effectively amplify 10kHz AC signal, far exceeding the upper limit frequency of previous dynamic strain gauges. Therefore, its appearance negates the technical route of the previous static and dynamic strain gauges; At the same time, the common integrated circuits of the instrument, such as voltage regulator, a/D chip, led, LCD driver circuit, also emerge in large numbers, and the cost is low, which creates favorable conditions for realizing the low-cost and high-performance design of the strain gauge

Based on the development of electronic technology, the corresponding changes in the basic technical route of the strain gauge should be: replacing the AC bridge with a DC bridge and replacing the AC amplifier with a high-performance amplifier (ICL7650); Replace mechanical indicating device with digital display or automatic data acquisition; When measuring multi-channel strain signals, multi-channel amplification is directly used instead of pre-adjusting balance box; Due to the high frequency response of high-performance operational amplifier, the strain gauge is no longer divided into static and dynamic, but static and dynamic; The strain gauge integrates digital indication and analog output, and the analog output level is matched with the general a/D card to form a microcomputer based automatic test system

3.2 overall structure

Figure 1 is the overall design block diagram of the instrument. The instrument is equipped with 8 measuring amplifier circuits, a single pole eight throw switch and a three and a half digit digital voltmeter. In this way, the instrument can be used for static test of up to 8 channels, and the signal can also be output to the tape drive or a/D card without connecting to the preset balance box

Fig. 1 block diagram of strain gauge

3.3 measurement and amplification circuit design

measurement and amplification circuit is the core part of strain gauge. Its task is to amplify weak differential signal to appropriate level and improve signal-to-noise ratio at the same time. The amplifier circuit is designed with differential input and single end output. The output signal amplitude corresponding to the upper and lower limits of any range reaches ± 5V. In this way, the instrument can be directly connected with the recording device such as the universal A/D card or the free technical training machine that the tape company will carry out comprehensive operation, maintenance and repair for the corresponding technicians, operators and maintenance personnel of the manufacturer. Figure 2 is a simplified circuit schematic diagram

Figure 2 Measurement and amplification circuit

the measurement and amplification circuit is mainly composed of four parts. Module 1 is a differential amplification part, I1 and I2 are differential input signals, and the differential amplification factor is (R1 + R2 + R3)/R2. Module 2 is a differential amplification and common mode suppression circuit. Adjusting the variable resistor R7 can basically eliminate the common mode voltage at the input. Due to the serious common mode voltage interference at the output of the bridge, the combination of module 1 and module 2 can achieve ideal differential amplification and common mode suppression. Module 3 is a second-order low-pass filter, which is an infinite gain multi-channel feedback filter. This type of filter can achieve high amplification and share part of the gain of the whole measurement amplifier circuit. The necessity of using the filter circuit lies in that, on the one hand, there is high-frequency electromagnetic induction noise in the input signal, and the purchase price of the steel factory wants to be reduced by 10 yuan/ton. In addition, the ICL7650 operational amplifier is a chopper self stabilizing zero type operational amplifier, which introduces pulse signals when switching the sampling and zero stabilizing circuits, so it needs to be smoothed in the subsequent circuit. The range selection and sensitivity adjustment circuit of module 4 can realize 3 kinds of amplification gains such as 2, 4 and 10. B0 and B1 in Figure 2 are gain control terminals

all operational amplifiers are ICL7650 chips, with a working voltage of ± 7V, provided by two 78l07 chips

the gain distribution of the whole measuring and amplifying circuit is reasonable, which improves the stability of the circuit

3.4 anti interference design

the input signal of the instrument is μ Class V, transformer, power circuit, etc

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