When the weighing object is placed on the weighing pan, the resistance strain gauge is deformed by the influence of gravity, and the resistance value changes. The bridge circuit built by the strain gauge will output a weak voltage signal. The weak voltage signal is amplified by the HX711 internal integrated amplifier, sampled and converted by the internal ADC, and sent to the MCU. The received data information is linearly fitted and sent to the LCD. LCD. The system can complete the basic functions such as weighing, setting the unit price calculation amount and amount accumulation, peeling and peeling range limitation, and also realize the extended functions such as time, temperature, alarm and QR code payment. The overall block diagram of the system is shown in Figure 1.
Figure 1: Overall block diagram of the system
2, theoretical analysis and calculation 2.1, bridge drive circuit analysis
Figure 2: Differential full bridge measurement circuit
As shown in Fig. 2, when the measured load acts on the elastic arm beam to make it elastically deformed; the resistive strain piece attached to the pressure sensing device is deformed by the viscous substance, thereby transforming into the strain piece, and the resistance value changes; The dynamic circuit converts the resistance change of the strain gauge into an electrical signal output proportional to the load. The output voltage is:
It can be known from the formula 1 that the measurement circuit is theoretically measured by differential full bridge, and the output voltage signal is linearly related to the magnitude of the resistance deformation resistance. The national standard strain gauge resistance is 60, 120, 200, 350, 500, 1000 Ω. In view of the large resistance and small current, long running period and high signal-to-noise ratio, the system uses a resistance strain gauge with a resistance of 1KΩ.
2.2, analog to digital (A / D) converter analysisThe HX711 provides an excitation voltage for the load cell. When a heavy object is loaded onto the load cell, it outputs a relatively small voltage signal. After receiving the voltage signal, it amplifies the signal and then performs A/D conversion. It is directly converted into a digital signal and output to an external device.
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