This paper uses four foil strain gauges to form a Wheatstone bridge. The analog signal of the integrated chip HX711 symmetric weight sensor is used for A/D conversion to complete the measurement data acquisition, and then the high-precision measurement is performed by the single-chip computer for data processing, and then various algorithms are added to complete the calculation of various functions; With the LCD display, touch input, voice broadcast, etc. to complete friendly human-computer interaction.
2, system compositionAccording to the design requirements, the hardware needs to have load cells, AD conversion circuit, MCU minimum system, key input, display output, voice broadcast, and power module, the system block diagram of which is shown in Figure 1.
Figure 1 Electronic scale system block diagram
3. Design of load cell 3.1. Design of cantilever beamMade of high-quality aluminum alloy (length, width, height: 190mm, 20mm, 3mm), aluminum alloy has strong molding, moderate hardness, good elasticity, etc., often used as a cantilever beam of resistance strain gauge load cell. In this design, the cantilever beam is treated as follows to make the mechanical deformation more reasonable.
(1) A small groove of about 1.5 mm deep is opened on the lower surface of the cantilever beam near the fulcrum (about l/3). Concentrate the deformations produced on one line for better linear strain.
(2) At the other end of the fulcrum, a 5 cm hole is fixed in the middle and a screw with a through hole at the lower end. The advantage is that the center of gravity of the measuring object can be concentrated at one point, which is convenient for improving the accuracy.
3.2, the design of the measurement circuitThe design uses the four-arm measurement wiring method in the Wheatstone bridge. This wiring method can compensate the system for external disturbance factors including temperature and humidity. Make the system less susceptible to interference from external factors and improve measurement accuracy.
The R1 and R4 strain gauges are attached to the upper surface of the groove, and the R2 and R3 strain gauges are attached to the lower surface, thus completing the design of the load cell.
4, AD converter designThe HX711 is a 24-bit A/D converter chip designed for high-precision load cells. Compared with other chips of the same type, the chip integrates peripheral circuits required by other types of chips including a regulated power supply and an on-chip clock oscillator, and has the advantages of high integration, fast response, and strong anti-interference.
The HX711 module is connected to the Wheatstone bridge and the single chip microcomputer; through the HX711, the weakly deformed signal measured by the Wheatstone circuit can be converted into a digital signal and transmitted to the STM32 single chip for data processing.
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