Design of high-precision electronic balance based on MSP430F4250

Introduction

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

The development of electronic balances in China, following closely behind the international developed countries, began in the early 1980s. Now it has formed large-scale production and is widely used in various fields in China, and is also partially exported to many countries in the world.

Electronic balances have the characteristics of digital display, direct reading, fast weighing, light weight, easy operation, strong anti-interference ability, etc., and use microcomputer technology to make it intelligent and multifunctional. It can be connected with printers and computers for online measurement, data statistical analysis, etc., which makes electronic balances have advantages that mechanical balances cannot match, so the scope of application is becoming more and more extensive. The

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

Hardware Design

This system consists of LCD display, keyboard input, preamplifier, MCU, alarm output, ISP download, RS-232 interface and power supply, as shown in Figure 1.

Figure 1 Electronic balance block diagram

MCU

This system uses the MSP430F4250, a new MSP430 series microcontroller released by TI. The biggest difference between this microcontroller and previous models is that it has an internal integrated 16-bit SD analog-to-digital conversion module (Figure 2).

Figure 2 Structure of the SD analog-to-digital conversion module inside the MSP430F4250

The SD analog-to-digital conversion module is different from the traditional analog-to-digital converter. It uses oversampling technology to sample the signal and then filters it through a digital filter to obtain the final conversion result. Therefore, the oversampling rate must be set before starting the analog-to-digital converter. The oversampling rate of the analog-to-digital conversion module inside this chip can be selected from 32 to 1024. The larger the oversampling rate, the better the filtering effect, but the slower the response speed. The electronic balance designed in this paper requires accurate and stable measurement results, and does not require high system response speed, so it is set to a maximum oversampling rate of 1024.

The sensitivity of the system is different when the internal PGA (programmable amplifier) ​​amplification factor is set differently, but the larger the amplification factor, the smaller the signal-to-noise ratio. In this design, the amplification factor is set to 1 to achieve the highest conversion accuracy.

In order to further improve the stability of the conversion and reduce the conversion error caused by temperature changes. In practical applications, we compensate for temperature drift by detecting the internal temperature sensor and achieved good results.

LCD

This design uses a dedicated electronic balance LCD display module, which displays rich and targeted information compared to the general LCD display module. In addition, it has the advantages of simple functions, large production volume, and low price.

The dedicated LCD display module adopts a one-to-one display mode of storage unit and 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. The interface between the LCD module and the single-chip microcomputer is serial, consisting of a data line and a read-write signal line. Since the MSP430F4250 does not have a read-write signal line, it is also very convenient to use the I/O port to control the read-write timing. Preamplification The

preamplification

circuit is the key part to ensure the measurement accuracy. In this design, the basic circuit of the instrument amplifier is used, which is composed of two OP07s, as shown in Figure 3.

Figure 3 Preamplifier circuit

OP07 is a low zero drift operational amplifier with low price. The key to this circuit is that the two resistors R13 and R14 need to be accurately matched. In the assembly process, R13 and R14 use precision resistors with an accuracy of 0.1%, and with manual accurate matching, the drift and common mode rejection ratio of this circuit can reach a very high standard. At the same time, the output signal is filtered by the RC network and transmitted to the input end of the A/D converter, which can ensure the stability of the measurement signal. This is also confirmed in actual use.

Software Design

This design uses C language programming, and the compilation and debugging tool uses IAR's EW430 3.30a, which efficiently completes the program design of this electronic balance.

Conclusion

The electronic balance with the MSP430F4250 microcontroller with an internal 16-bit SD analog-to-digital converter as the core eliminates the need for an independent SD analog-to-digital converter, reduces the cost of the product, and can achieve sufficient accuracy. The electronic balance using this design has been put into formal production, and has achieved good economic benefits in the current fiercely competitive market with its good performance-price ratio.

References:

1. Shen Jianhua, Yang Yanqin, Zhai Xiaoshu, “MSP430 Series 16-bit Ultra-low Power MCU Principles and Applications”, Tsinghua University Press, 2004.11.
2. Hu Dake, “MSP430 Series MCU C Language Programming and Development”, 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