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Design of high-precision electronic balance based on msp430f4250

Abstract: This paper introduces a design scheme of electronic balance composed of single chip msp430f4250 with internal S-D analog-to-digital converter, which can greatly reduce the cost of products and meet the design requirements at the same time

key words: electronic balance; MSP430； With the rapid development of modern electronic technology and microcomputer technology, 67 ~ 93hr15t has been applied to the field of balance, and electronic balance has appeared. Since the world's first electronic balance appeared in the late 1970s, a series of electronic balances with various specifications and complete categories have been developed in just over 30 years, which are widely used in quality measurement in all walks of life

the development of electronic balance in China, following the international developed countries, began in the early 1980s. Now it has formed large-scale production and is widely used in various domestic fields. At the same time, some of it is also exported to many countries in the world

the electronic balance has the characteristics of digital display, direct reading, rapid weighing, light weight, simple operation, strong anti-interference ability, etc., and the microcomputer technology is adopted to make it intelligent and multifunctional, which can be connected with printers and computers for measurement, data statistical analysis, etc., so that the electronic balance has the incomparable advantages of mechanical balance, so its application range is more and more extensive

the electronic balance of this design is controlled by a high-performance single-chip microprocessor to ensure the high accuracy and stability of the weighing results. With standard RS-232 interface output, it can be directly connected to printers, computers and other external equipment. It is widely used in industry and agriculture, schools, scientific research and other institutions for rapid determination of material quality

hardware design

this system is composed of LCD, keyboard input, preamplifier, MCU, alarm output, ISP download, RS-232 interface and power supply, as shown in Figure 1

Figure 1 block diagram of electronic balance

MCU

in this system, the latest MSP430 Series MCU msp430f4250 released by TI company is used. The biggest difference between this MCU and the previous models is the internal integration of 16 bit S-D analog-to-digital conversion module (Figure 2)

Figure 2 structure of msp430f4250 internal S-D analog-to-digital conversion module

S-D analog-to-digital conversion module is different from traditional analog-to-digital converter. It uses over sampling technology to sample the signal, and then filters through digital filter to get the final conversion result. Therefore, set the oversampling rate before starting the ADC. The internal analog-to-digital conversion module of this chip has an oversampling rate of 32~1024. The larger the oversampling rate is, the better the filtering effect is, but the slower the response speed is. The electronic balance of this design requires accurate and stable measurement results, and does not require high response speed of the system, so it is set to the maximum oversampling rate of 1024

if the internal PGA (programmable amplifier) amplification factor setting is different, the sensitivity of the system is also different, but the larger the amplification factor is, the smaller the signal-to-noise ratio is. In this design, we choose to set the magnification equal to 1 to achieve the highest conversion accuracy

in order to further improve the stability of conversion and reduce the conversion error caused by temperature change. In practical application, we compensate the temperature drift by detecting the internal temperature sensor. Based on the technical feasibility, cost advantage and the possibility of range production, we have achieved good results

lcd

this design uses a special electronic balance LCD module. Compared with the general-purpose LCD module, the display information is rich and targeted, and it has the advantages of simple function, large production volume and low price

the special LCD module adopts the display mode of one-to-one correspondence between the storage unit and the display unit. For the single chip microcomputer, as long as "1" or "0" is written in the corresponding storage unit, the corresponding display unit will be lit or extinguished, and the operation is very simple. Liquid crystal module and Rockwell hardness tester are widely used in industrial production. The interface of single chip microcomputer is serial. The experimental machine of Jinan experimental machine factory is in the upgrading mode, which is composed of a data line and a read-write signal line. Since msp430f4250 has no read-write signal line, it is also very convenient to use the i/o port to control the read-write timing

preamplifier

preamplifier circuit is the key part to ensure the measurement accuracy. In this design, the basic circuit of instrument amplifier is used, which is composed of two OP07, as shown in Figure 3

Figure 3 preamplifier circuit

op07 is a low zero drift operational amplifier, which is cheap. The key of this circuit is that the two resistors R13 and R14 need to be accurately matched. In the assembly process, R13 and R14 use a precision resistor with an accuracy of 0.1%, coupled with the precise manual matching, so that the drift and common mode rejection ratio of this circuit can reach a high standard. At the same time, the output signal is filtered by RC network and transmitted to the input of a/d converter, which can ensure the stability of the measurement signal. This is also confirmed in practical use

software design

this design uses C language programming, and the compilation and debugging tool uses IAR ew430 3.30a to efficiently complete the program design of this electronic balance

conclusion

the electronic balance, which is composed of a single chip microcomputer msp430f4250 with a 16 bit S-D analog-to-digital converter as the core, eliminates the independent S-D analog-to-digital converter, reduces the cost of the product, and can achieve sufficient accuracy at the same time. The electronic balance with this design has been put into formal production. In the current competitive market, with good performance price ratio, it has achieved good economic benefits

References:

1 Jianhua Shen, Yanqin Yang, Xiaoshu Zhai, "principle and application of MSP430 series 16 bit ultra low power single chip microcomputer", Tsinghua University Press, November 2004

2. Hu Dake, "C language program design and development of MSP430 series single chip computers", Beijing University of Aeronautics and Astronautics Press, 2005.1

3． “MSP430F42x0 mixed signal micro controller datasheet”，Texas Instruments Incorporated，2005

4． “MSP430x4xx Family User's Guide (Rev. E)“, Texas Instruments Incorporated，2005

5． ”MSP430x2xx Family User's Guide Extract, “Texas Instruments Incorporated，2005(end)