“According to the current application of Electronic scales, this paper analyzes the use of single-chip microcomputer and AD574 to design a simple electronic scale device. On the basis of analyzing the measurement principle, a relatively simple 51 series single-chip microcomputer is selected as the main control system, and the sensor circuit is designed according to the measurement requirements. , AD conversion circuit, over-range alarm circuit, Display circuit, button circuit, etc. According to the hardware circuit, the corresponding software design has been completed.
According to the current application of electronic scales, this paper analyzes the use of single-chip microcomputer and AD574 to design a simple electronic scale device. On the basis of analyzing the measurement principle, a relatively simple 51 series single-chip microcomputer is selected as the main control system, and the sensor circuit is designed according to the measurement requirements. , AD conversion circuit, over-range alarm circuit, Display circuit, button circuit, etc. According to the hardware circuit, the corresponding software design has been completed.
1. Measurement principle
An electronic scale is generally composed of three parts, a load cell, a load-bearing system, and a force-transmission reset system. On this basis, it is divided into other hardware circuit sub-units, such as single-chip minimum system circuit, sensor circuit, AD conversion circuit, over-range alarm circuit, display circuit, key circuit, storage circuit, etc. The measurement principle is: when the object is placed on the weighing platform of the load-bearing system, its weight parameter will pass through the sensor, produce a pressure-electric effect, convert it into an electrical signal corresponding to its weight, and then amplify the electrical signal through the amplifying circuit and pass it through. AD processing, and finally the signal is input to the single-chip microcomputer for processing, and the input information is displayed on the LCD after being processed by the single-chip microcomputer. The accuracy of the measurement is generally determined by the load cell.
2. Hardware circuit design
2.1. Overall design block diagram
According to its measurement principle, the overall hardware block diagram of the design is shown in Figure 1. It mainly includes pressure sensor circuit module, amplifying circuit module, AD conversion module, LCD display module, threshold alarm module, and single-chip control system module.
2.2, sensor circuit design
The design adopts SP20C-G501 resistance strain sensor. The load cell is composed of a combined S-beam structure and a metal foil strain gauge, and has an overload protection device. In the process design, the Wheatstone bridge is used for voltage acquisition and conversion, which can suppress the influence of temperature changes, has a strong ability to suppress interference, and is convenient and simple to compensate. Therefore, the selected sensor has high precision, small zero drift, and stable operation. The schematic diagram of the sensor is shown in Figure 2:
How it works: When measuring with a strain gauge, stick it on the elastomer. When the elastic body is deformed by force, the sensitive grid of the strain gauge is also deformed, and its resistance value changes accordingly, which is converted into a voltage or current change through the conversion circuit. Since the internal circuit adopts Huygens bridge, when the elastic body is deformed under load, the output signal voltage can be given by the following formula (1):
2.3, AD conversion circuit design
AD574 is a 12-bit monolithic A/D converter produced by American AnalogDevice Company. It adopts successive approximation type A/D converter, the maximum conversion time is 25us, and the conversion accuracy is 0.05%, so it is suitable for high-precision fast conversion sampling system. The chip contains microprocessor interface logic (with three-state output buffer), so it can be directly connected with various types of 8-bit or 16-bit microprocessors without additional logic interface circuits, and is compatible with CMOS and TTL circuits . AD574 adopts 28-pin dual in-line standard package.