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 weighing system. The traditional weighing system has used a high-precision multimeter to measure the output voltage of weighing sensor. The operation is complex and the error is large, so it is difficult to meet people's requirements.There are also many problems in the indirect measurement of volume and volume in many fields. For example:For ultra-thick and ultra-thick crude oil, because of its high viscosity, 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, so the volume method is adopted to measure heavy oil, the measurement time is long, the error is large, and the durability is bad.In this field, heavy oil production is measured by weighing method based on measurement principle.Therefore, it is urgent to develop a high precision and high stability weighing system.

1. Working principle of the weighing sensor

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 gage is pasted on the elastomer according to the strain effect, and the strain generated by the external force on the elastomer will be transmitted to the strain gage, so that the resistance value of the strain gage will change accordingly.Through the relationship between resistance and voltage, we can get the change of voltage quantity, from which we can deduce the mass of the substance to be measured.

2. Overall scheme design of the system

The weighing system adopts the electric resistance strain chip sensor measurement, direct current bridge output voltage signal after amplifying and filtering by the A/D conversion circuit into the single chip microcomputer system, the single-chip computer for data adopted multiple sampling and the extremum average filter processing, to avoid the instantaneous interference influence on sampling, in order to obtain more reliable data.The data processed by SCM is transmitted to PC (host computer) through RS232 serial interface, and the host computer uses VB software for further processing, so that the weighing system can 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 follow-up 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 the resistance, and the change of the resistance is difficult to be measured.The change in resistance can be converted into a voltage signal by choosing a DC differential bridge.However, the voltage signal output by the bridge is very weak and difficult to be measured by general testing instruments, so the precision instruments produced by Burr-Brown Company in the United States are amplified by the amplifier INA121, and the low-pass filter is used to eliminate the useless signal and retain the useful signal.

3.2 A/D conversion circuit

The 12-bit successive approximation ADC converter AD574 developed by AD Company of the United States is used in the conversion circuit. It has an internal integration of conversion clock, reference voltage source and three-state output latch.It is easy to use, can be directly with the computer interface, without external clock;The conversion time was 15-35 μs.The input analog voltage can be either unipolar or bipolar.0 ~ +10V or 0 ~ +20V for unipolar input, -5 ~ +5V or -10 ~ +10V for bi-polar input;The number of digits can be set to either 8 or 12 digits.The 12-bit data can be read once or read in two parts, so it is easy to connect to an 8-bit or 16-bit microcomputer.

3.3 Serial communication circuit

The digital signal which has been calculated and optimized needs to be sent to the upper computer for further processing and preservation. The chip MAX232 is used to realize the TTL←→RS232 bidirectional level conversion. In this way, the communication between AT89C51 microcontroller as the lower computer and PC as the upper computer can be realized.

4. Upper computer VB software design

The upper computer system uses VB language as the software development tool, and the serial communication control MSCOMM provided by VB can be connected with the lower computer conveniently through the serial port, so as to realize the sending and receiving of data.It is simple and efficient to realize the communication between devices, so that the programming efficiency is greatly improved.

5. Conclusion

The experiment and field application show that the weighing system can meet the requirements of high precision and high stability, so the feasibility and practicability of the system are further verified. In addition, in order to meet the requirements of high performance testing in a complex measuring environment, various considerations should be taken, such as:Add strain type bridge compensation circuit to solve the temperature change caused by the bridge equilibrium point shift, the sensitivity of the force sensor changes;The interface circuit should be as far as possible using electrical isolation technology;Strain gauge to ensure good adhesion process and so on.Determine the specific plan according to the specific measurement environment and accuracy requirements.

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