Design And Development of High Precision Weighing System

With the continuous progress and development of computer and measurement and control technology, people have higher and higher requirements for symmetrical weight system. The traditional weighing system has used high-precision multimeter to measure the output voltage of weighing sensor. The operation is complex and the error is large, which is difficult to meet people's requirements, There are also many problems in some methods of indirectly measuring the volume to realize the side weight in many fields. For example, for ultra thick and extra thick crude oil, due to its high viscosity and poor fluidity, the gas in the oil is not easy to discharge, resulting in false volume, and the crude oil is easy to remain in the meter. Therefore, the volume method is used to measure heavy oil, which has long measurement time, large error and poor durability. In this field, weighing method is being used to measure heavy oil production from the metering principle. Therefore, it is urgent to develop a weighing system with high precision and high stability.
1 working principle of load cell
When the strain gauge is mechanically deformed under the action of external force, its resistance value will change. This phenomenon is called strain effect. The strain gauge is pasted on the elastomer according to the strain effect, and the strain generated by the elastomer under the action of external force will be transmitted to the strain gauge, so that the resistance value of the strain gauge changes accordingly. Through the relationship between resistance and voltage, the change of voltage can be obtained, from which the quality of the measured object can be deduced.
2 overall system scheme design
The weighing system uses a resistance strain gauge sensor for measurement. After amplification and filtering, the voltage signal output by the DC bridge enters the single chip microcomputer system from the A / D conversion circuit. The single chip microcomputer adopts the filtering processing of multiple sampling and de extremum and average value for the data, so as to avoid the influence of instantaneous interference on sampling, so as to obtain more reliable data. The single chip microcomputer transmits the processed data to the PC (upper computer) through RS232 serial interface. The upper computer uses VB software for further processing to make the weighing system meet the requirements of high precision.
3 hardware circuit design of lower computer
The hardware circuit of the lower computer is mainly composed of sensor subsequent signal conditioning circuit, a / D conversion circuit and serial communication circuit.
3.1 subsequent signal conditioning circuit of sensor
Because the resistance strain gauge produces the change of resistance, and the change of resistance is difficult to measure. The selection of DC differential bridge can convert the change of resistance into voltage signal. However, the voltage signal output through the bridge is very weak and difficult to be measured by general testing instruments. Therefore, the precision instrument amplifier ina121 produced by BURR-BROWN company of the United States is used for amplification, and the low-pass filter is used to eliminate the useless signals and retain the useful signals.
3.2a/d conversion circuit
The conversion circuit adopts 12 bit successive approximation ADC converter AD574 developed by American ad company. It integrates conversion clock, reference voltage source and three state output latch. It is easy to use and can directly interface with microcomputer without external clock; The conversion time is 15 ~ 35 μ s； The input analog voltage can be either unipolar or bipolar. 0 ~ + 10V or 0 ~ + 20V for unipolar input and - 5 ~ + 5V or - 10 ~ + 10V for bipolar input; The number of digits can be set to 8 or 12. 12 bit data can be read out once or twice, which is convenient for connection with 8-bit or 16 bit microcomputer.
3.3 serial communication circuit
After calculation and optimization, the digital signal needs to be sent to the upper computer for further processing and storage. The chip MAX232 is used to realize TTL ←→ RS232 bidirectional level conversion. In this way, the communication between AT89C51 single chip microcomputer as the lower computer and PC as the upper computer can be realized.
4 upper computer VB software design
The upper computer system selects VB language as the software development tool. The serial communication control MSComm provided by VB can be easily connected with the lower computer through the serial port, so as to realize the sending and receiving of data. It realizes the communication between devices simply and efficiently, which greatly improves the programming efficiency.
5 Conclusion
Experiments and field use show that the weighing system can meet the requirements of high precision and high stability. Therefore, the feasibility and practicability of the system are further verified. In addition, in order to meet the test requirements of high performance in complex measurement environment, we should also consider many aspects, such as adding strain bridge compensation circuit, Solve the drift of bridge balance point caused by temperature change and the change of sensitivity of force measuring sensor; Electrical isolation technology shall be adopted for interface circuit as far as possible; The strain gauge should ensure a better bonding process, etc. Determine the specific scheme according to the specific measurement environment and accuracy requirements.

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