Design of Strain Gauge DC Bridge in Wireless Data Acquisition System

This paper analyzes the problems existing in the resistance strain gauge DC bridge measurement circuit in the complex system wireless data acquisition circuit in detail. According to the sensitivity and power requirements of the actual system, the gain resistor and filter capacitor are reasonably selected, and an effective design scheme of the resistance strain gauge DC bridge wireless data acquisition measurement circuit is proposed. The test proves that the scheme is stable and reliable in the data acquisition system.

0 Introduction

In complex mechanical systems, people have paid attention to studying power consumption and performance, designing their structures, studying the lubrication status between modules, measuring friction between components and other important parameters for many years. Since the internal movement of the mechanical system is complex, the environment is harsh, and the friction is relatively small, it brings great difficulties to the measurement. How to accurately measure these data is particularly important.

By adopting wireless transceiver mode and using sensor signals to transmit signals through wireless transceiver circuits, data can be stored first and then read into the computer for analysis, providing a new method for data acquisition in complex systems with precise data requirements. In addition, when the acquisition frequency is high, the amount of data is relatively large, which places high demands on the processor processing speed, RF wireless transmission speed, interface transmission speed, A/D conversion speed and power consumption in the acquisition system. In addition, the internal size of the mechanical system is limited, which makes it more difficult. In this way, the design of the data acquisition circuit board becomes the key to the data acquisition system. We need to design a special data acquisition and wireless transceiver.

1 Measurement system principle

The system consists of sensors, power supplies , signal conditioning circuits, signal processing circuits and PCs. During actual measurements, the sensors are installed on moving parts. Since the use of lead devices to transmit signals will limit the movement of mechanical parts, wireless transceiver circuits can be used to transmit data. Data can also be collected in a storage manner, that is, data is first saved to a memory card, and after data collection is completed, the memory card is taken out and read into the computer. The principle of the measurement system is shown in Figure 1.

The pressure sensor and strain gauge output a voltage of 0 to 2.5V through the signal conditioning circuit. The signal processing circuit can convert the analog signal into a digital signal and then store it in the memory card. The thermocouple outputs a 12-bit SPI digital signal through the signal conditioning circuit, and the signal can be directly stored in the memory card by the single-chip microcomputer . The capacity of the memory card should be able to ensure the time requirement for signal acquisition (when the acquisition frequency is 3000Hz, a memory card of 512M or more can ensure that the acquisition time is not less than 25 minutes). The design of the DC bridge measurement circuit of the resistance strain gauge in this measurement system is a key. We will analyze and design this part in detail below.

2 Design of DC bridge measurement circuit for resistance strain gauge

2.1 Strain gauge conditioning circuit

The strain gauge conditioning circuit consists of a boost chip (providing operating voltage for the chip), a voltage reference (voltage regulator), a bridge, filtering, and amplification, as shown in Figure 2.

The strain gauge model selected for the measurement circuit is 350-2AA, with a strain sensitivity coefficient K=2.18 (dimensionless positive number), a nominal resistance of 350Ω, and a wire grid size of 2×2mm. Considering the small space on the connecting rod body, as few sensors as possible are installed, so a DC single-arm bridge is used to realize the conversion from resistance to voltage.

2.2 Sensitivity and power of DC bridge

From formula (2-6), it can be seen that the larger the power supply voltage U of the bridge is, the higher the sensitivity of the bridge is. U depends on the maximum current allowed to pass through the resistance elements that make up the bridge. However, from formula (2-8), it can be seen that when the power supply voltage U of the bridge is larger, the power consumption of the bridge is greater, so a suitable power supply voltage should be selected.

After the power supply voltage of the bridge is determined, the coefficients before equations (2-7) and (2-8) can be used to draw the curves shown in Figures 3 (a) and 3 (b), which respectively reflect the changes in bridge sensitivity and power consumption with n values. It can be seen that when n=1.5, the power consumption of the bridge is reduced by 20%, while the sensitivity of the bridge is only reduced by 4%. Therefore, when designing the bridge, n=1.5 is selected, which not only reduces the power consumption but also ensures the sensitivity of the bridge. At this time, 4 R = 525Ω can be obtained, and 1 R = 2KΩ and 2 R = 3KΩ can be selected at the same time.

Since the environment where the circuit board is located has great acceleration and vibration, no variable resistor is used in the whole circuit. 4 R can be realized by connecting fixed resistors of different resistance values ​​in series and parallel.

2.3 Calculation of amplification factor and selection of gain resistor

After voltage reference stabilization, the bridge supply voltage U=2.500V. In order to retain a certain margin, the output voltage of the circuit is set at [0.1V, 2.4V].

After determining the value of R7, the value of R8 can be calculated according to formula (2-10), and then the resistor closest to this resistance value can be selected from the nominal series. Here, R = 5.1KΩ 7 is selected. According to the strain on the connecting rod, R = 360KΩ 8 can be selected.

2.4 Selection of filter capacitor

The interference signal of the data measurement circuit comes from electromagnetic interference and the influence of the measurement system itself, so the circuit needs to adopt filtering measures to improve the signal-to-noise ratio of the system. Considering the small size of the circuit board, simple resistance-capacitance filtering is mainly used here.

2.5 Temperature Error and Its Compensation

The ambient temperature of the strain gauge is changing, so the strain gauge must be temperature compensated. Due to the small space between the sensor and the circuit board, in order to minimize the number of sensors, the differential method is not used for temperature compensation. However, during calibration, the stress-strain and temperature-strain relationships at the same point are obtained, so that the error caused by temperature can be compensated during calculation.

3 Conclusion

So far, the design of the resistance strain gauge DC bridge measurement circuit in the wireless transceiver data acquisition system has been completed. Of course, when we design the data acquisition system, according to the actual measurement needs, such as the environment in which the system operates is affected by temperature, frequency and some other environments, the different measurement parameters will lead to the data acquisition system being not exactly the same. The designer can modify it as needed.

Innovation of this paper: This paper focuses on the design of strain gauge DC bridge in wireless data acquisition system, and proposes a feasible strain gauge DC bridge design scheme for complex mechanical control systems when performing difficult data acquisition.