I: Overview
Since the advent of analog load cells using adhesive resistance strain gauges in the early 1940s, after more than 60 years of improvements and developments, the structural design, manufacturing process, comprehensive performance indicators, stability and reliability have reached relatively high levels. High level, it has been widely used in various electronic weighing instruments and weighing measurement and control systems. With the advancement of science and technology and the improvement of the automation level of industrial processes, especially the development of digital technology and information technology, the application of digital technology and digital systems in weighing measurement and control systems has become more and more demanding. In order to achieve the digital and intelligent electronic weighing instrument, the digital weighing system is used to break through the limitations of the analog weighing system and other requirements. As we all know, digital weighing systems require weighing load cells and weighing instrument systems to output in digital form. The analog load cell currently in use, due to the resistance strain conversion principle, determines that no matter what kind of resistance strain gauge is used for manufacturing, it cannot produce an output signal with digital characteristics by itself, and the output analog signal is small, generally 20} 40mv; short transmission distance; poor anti-interference ability; complex weighing display control instruments; inconvenient installation and debugging and other congenital defects. It can’t adapt to the digital and intelligent requirements of electronic weighing instruments. Therefore, people pay more attention to the interface between analog weighing load cell and digital weighing system and digital intelligent weighing load cell. Some famous foreign weighing load cell manufacturers have carried out a lot of research work on this and achieved breakthrough technology. As a result, a variety of products with independent intellectual property rights have been developed.
As early as 1983, in order to meet the needs of industrial process automation, the American TOLEDO (now METTLER TOLEDO) company introduced the “digital” (DIGITOL) concept and gradually applied it to the weighing field. Committed to researching the use of microprocessor technology and digital compensation technology to combine it with the traditional strain-type load cell technology. After years of hard work, the rocker-type digital intelligent load cell has been developed. The US STS company also introduced an integrated digital intelligent weighing load cell at the 1988 National Weighing Apparatus Exhibition. Both are based on the basic principles of analog load cells, using modern electronic technology and computer software technology to develop new types of load cells. That is, more than 10 components such as amplification, filtering, A/D conversion, microprocessor chip and temperature sensitive components are added to the analog load cell. The use of scale conversion, digital filtering, digital zeroing, digital compensation and other technologies and Process manufacturing is completed. The manufacturing process, testing items, test methods, and supporting weighing instruments of digital intelligent load cells are quite different from those of analog load cells. Therefore, the digital intelligent weighing load cell is a system project jointly researched and developed with weighing instruments.
The digital intelligent load cell has a large output signal. The digital signal is a group of high and low level signals with a format and regular composition, generally ±SV; strong anti-interference ability such as radio frequency electromagnetic field radiation; long signal transmission distance, general It can be up to 150m, and it can exceed 600m with additional power supply; it is easy to install and debug; it is easy to realize intelligent control and other characteristics, which completely overcomes the shortcomings of analog weighing load cells. It is a digital electronic weighing instrument, automatic weighing measurement and control system, which requires automatic The first choice for calibrated weighing systems, various batching scales with complex structures, volume scales and super-large electronic weighing instruments.
From the perspective of the variety, structure and use of electronic weighing instruments, digital smart load cells cannot completely replace analog load cells. In a relatively long period of time in the future, analog load cells will still be the development of load cells. And the mainstream of the application. But we must pay attention to and study the digital intelligent weighing load cell and its application in the digital weighing system.
II: The difference between digital early and digital early weighing load cell
Although the measured weight parameters of the analog load cell are initially given in analog form by the sensitive element, they all need to be converted into analog voltage or analog current. A lot of experience has been accumulated in manufacturing process, circuit compensation and adjustment, signal conditioning, analog-to-digital conversion, etc., so it is widely used. However, its output signal is small, the transmission distance is short, the anti-interference ability is poor, the compensation items interact with each other, the circuit compensation and adjustment process is complicated, not only time-consuming and laborious, but also low compensation accuracy. The development of intelligence can only parasitize the digital conversion unit outside the strain bridge, changing the analog signal into a digital signal. Therefore, there are two digital conversion methods. One is to convert the output signal of the analog load cell into a digital signal output through a digital transmitter installed in it, which is usually called a digital load cell. which is:
Analog load cell + digital transmitter, digital load cell
The digital transmitter can be made very small, called a digital transmitter module, which is generally fixed in the junction box of the analog load cell, which is convenient for debugging and conducive to sealing. The mechanical and temperature performance indicators of digital load cells are based on the manufacturing process and circuit compensation and adjustment technology of analog load cells. The digital transmitter only digitizes the analog output signal and cannot improve the performance indicators. . On the contrary, if the quality of the digital transmitter is not good, some inherent performance will be lost. Therefore, manufacturers usually select those analog load cells with high circuit compensation accuracy and good comprehensive performance for digital processing.
The other is to completely break away from the manufacturing process of the analog load cell and the traditional circuit compensation and adjustment technology, and develop a new type of digital intelligent load cell. It departs from the manufacturing process of the analog load cell after the elastic element is patched, cured, post-cured, and the wiring group bridge. Through a digital circuit composed of amplification, filtering, A/D conversion, microprocessor chips, temperature sensitive components and other components, as well as software technologies such as scale conversion, digital filtering, digital zeroing, and digital compensation, the load cell The output digital signal becomes a group of formatted and regular high and low level signals, which can be transmitted over a long distance through an interface with high efficiency and good reliability. which is:
Elastic element bridge circuit + digital intelligent circuit + digital compensation technology and technology, digital intelligent weighing load cell
The analog-to-digital converter of a typical analog weighing load cell system is only 16-bit (bits), that is, there are 50,000 available counts, while the resolution of each digital intelligent weighing load cell in the digital weighing system is 20- bit, that is, there are 1,000,000 available counts. Therefore, a digital weighing system composed of 4 digital intelligent load cells can provide a resolution of 4 million usable counts. This high resolution is very important for digital weighing systems, especially for weighing systems with large dead loads and small live loads. This is difficult to achieve in a weighing system composed of traditional analog load cells.
Analog load cells are basically produced manually, and human factors have a greater impact on product quality. Zero temperature, sensitivity temperature, linearity, hysteresis, creep and other compensation methods and compensation processes are not perfect. The interaction between various compensations inevitably produces residual errors, which limits the further improvement of accuracy and stability. Digital intelligent weighing load cells are basically automated production, and human factors have little effect on product quality. Its digital zero adjustment, scale conversion, temperature compensation, linearity, hysteresis, creep compensation, etc., all collect, process and store various data through the internal microprocessor. Due to the use of “database” technology, the microprocessor can continuously identify and correct data, making it more intelligent and play a greater role. It can be seen that the digital processing circuit and software design are important links in the realization of digital compensation technology and craftsmanship.
III: Digital smart weighing load cell
There are two realization methods or two structural forms of digital intelligent load cell, namely integral type and separate type.
1. Integral digital intelligent weighing load cell
As mentioned earlier, digital processing circuits composed of amplification, filtering, A/D conversion, microprocessor chips and temperature sensitive components are installed inside the load cell. The software that has been stored in the microprocessor chip is used to implement various items. Digital compensation technology, comprehensive performance testing and verification, and finally using electron beam welding or laser welding for sealing. Because the digital conversion and compensation circuit and the load cell are integrated, it is called an integral digital intelligent load cell.
The manufacturing process of the digital smart load cell is completely different from the analog load cell. There are mainly two links. One is after the Wheatstone bridge circuit is composed of J-Wah components, through experiments, tests, and establishment of mathematical models of various compensation items required by digital compensation technology and process requirements, a formula that is convenient for procedural calculations is formed. , Easy to compile into compensation calculation software. Mainly mathematical models such as linear compensation, creep compensation, zero point and sensitivity temperature compensation. Because the digital compensation technology and process are based on reasonable physical and mathematical models, an accurate and reliable mathematical model is the prerequisite to ensure the accuracy of digital compensation. The second is to compile a simple, practical and effective compensation calculation software based on the mathematical model, and store it in the microprocessor chip for various error corrections and compensations. The software technology mainly includes: digital filtering technology. When the analog output signal of the load cell enters the microprocessor after A/D conversion, it is often mixed into the random noise interference such as sharp pulses, which must be weakened and filtered; Conversion technology: The analog signal of the load cell is converted by A/D to obtain a series of digital data, which must be converted into a number with steel to calculate and display; digital zero adjustment technology is implemented by various programs Calibration of deviations such as zero drift and gain drift; temperature compensation technology: establish a mathematical model (such as polynomial, etc.) expressing temperature, and use mathematical processing methods such as difference to effectively achieve temperature compensation; non-linear compensation technology based on measured Characteristic curve, carry out piecewise interpolation, as long as the number of interpolation points is reasonable and enough, a good line shape can be obtained.
The digital intelligent load cells developed by American TOLEDO, STS, CARDINAL and German HBM all have their own unique digital compensation technology and process. Eliminate electrical noise in digital output; digital circuit realizes automatic temperature compensation; solves anti-radio frequency interference (RF1) and electromagnetic interference (EMI); realizes adaptive, self-calibration, self-diagnosis functions and built-in overvoltage protection board to cope with electricity caused by lightning The technology and craftsmanship of transients are in a leading position. The characteristics of the integral digital intelligent load cell are:
(1) It is easy to generate, process and store digital signals, the output signal is large, and the output signal is a group of high and low level signals, generally ±SV;
(2) has a very high resolution, generally 20-bit, the available count is up to 1,000,000, and the accuracy is high, and the stability is good;
(3) High-level digital signal has strong ability to resist external radio frequency interference and electromagnetic interference;
(4) Strong signal output, long transmission distance, usually up to 150m, more than 600m after additional power supply;
(5) Software calculation method can be used to adjust the four-corner error, and use the number
word system realizes “self-calibration”, therefore, installation, debugging and verification are very convenient, which is only 1/4 of the traditional method;
(6) For large-range electronic weighing instruments and weighing devices that cannot be attached to the code, it can be calibrated without code. Using digital weighing system software, input the calibration parameters of each weighing load cell to ensure the weighing accuracy and working reliability of the electronic weighing instrument.
The problems of the integral digital smart weighing load cell are also more prominent, mainly:
(1) Due to the addition of A/D conversion pre-amplifier and post-stage microprocessor, more than 60 electronic components and 350 solder joints, such as temperature sensitive components, greatly exceed the internal only 11 electronic components and 30 solder joints. Point of analog load cell. The mean time between failures of the load cell is inversely proportional to the number of electronic components and solder joints, so the stability and reliability will inevitably decrease;
(2) The internal electronic components determine the small operating temperature range, generally -10400C. When working in an environment below 10°C, the cost of the low-temperature components that should be selected will increase several times or even 10 times;
(3) After the digital circuit is placed inside the load cell, in addition to the routine detection and calibration, the electrostatic discharge immunity, radio frequency electromagnetic field radiation immunity and electrical fast transient pulse group should be carried out according to the requirements of the weighing instrument. Immunity and other tests;
(4) The digital intelligent circuit is placed inside the load cell, which reduces the explosion-proof level. If the explosion-proof requirements are met, more expensive explosion-proof and explosion-proof measures must be adopted;
(5) The cost of manufacturing and maintenance is relatively high. Whether it is a failure of the resistance strain gauge bridge circuit or the digital intelligent circuit part, it is necessary to replace a new digital intelligent weighing load cell as a whole.
- Separate modular digital weighing load cellsystem
On the basis of analyzing the existing problems, application experience and market requirements of the integral digital intelligent load cell, the American STS company launched the separate 2200 series digital load cell module in 1992. It moves the A/D conversion digital processing circuit originally placed inside the load cell to an external junction box. The junction box with the A/D conversion module is called a digital junction box. After the ordinary analog load cell is connected to the digital junction box, its output will be transmitted to the supporting weighing display control instrument as a digital signal. Usually the analog load cell plus the digital junction box mode is called the modular digital load cell system. It not only retains the comprehensive performance indicators of the analog load cell, but also has all the characteristics and functions of the integral digital intelligent load cell. At the same time, the working environment of the A/D conversion circuit is greatly improved. The characteristics of the modular digital load cell system are:
(1) Under the condition that there is no change in the analog load cell, the analog signal output is changed to the digital signal output through the digital junction box (the correct use method of the weighing junction box), so that it has an integral digital intelligent weighing load cell The characteristics and purpose of education;
(2) Adopt modular digital weighing load cell system, which can realize the digitalization of electronic weighing instrument without changing the original electronic weighing instrument structure;
(3) The analog load cell can work normally under the ambient conditions of 4070℃, and the digital junction box can make the modular digital load cell system work in a wide temperature range;
(4) Compared with the integral type, the stability and reliability of the separate modular digital load cell system are greatly improved:
(5) The installation and wiring is very simple, there is only one cable from the digital junction box to the digital weighing instrument;
(6) It is easy to maintain and repair due to the connection of ordinary analog load cell, and the maintenance cost is also lower than the overall type.
Modular digital weighing load cell system of American STS Company, the main technical indicators are as follows:
(1) A/D channel: four independent 20-bit, cross-zero A/D conversion channels.
(2) Conversion rate: 100 samples/sec.
(3) internal resolution: 1048000
(4) Resolution of division number 200000
(5) Signal sensitivity: 0.1 }., v/division.
(6) Signal range: O.Smv/v to 6mv/v
(7) Excitation voltage: 10±O.SVDC
(8) Calibration: conventional/digital (no code removal required).
(9) Serial port 1: Bidirectional RS232 or 20MA current loop.
(10) Serial port 2: Bidirectional RS232 or RE485 or 20MA current loop.
(11) Digital port: 6 inputs, 7 outputs; TTL low voltage is valid.
IV: Application of integral and separate digital load cells
Integral digital intelligent weighing load cells are mainly used in digital weighing systems and digital electronic weighing instruments that realize industrial process automation. The most common applications are electronic hopper scales with more complex structures, electronic batching scales, electronic liquid tank scales, large-range electronic platform scales, and electronic steam scales.
Truck scales, dynamic rail scales and special electronic scales with large dead load and small live load. The former mainly values the digital weighing load cell system, which can use software to input the calibration parameters of the digital intelligent weighing load cell for non-decoding calibration, and the latter uses digital intelligent weighing to transmit
The high resolution of 1,000,000 usable counts provided by the load cell is something that an analog load cell simply cannot achieve.
The separate digital load cell has the characteristics of flexible configuration, wide application range, convenient installation and debugging, simple verification method, strong environmental adaptability, etc., and its market share is continuously increasing. The 2203, 2204 and 2224 digital weighing load cell modules in the 2200 series developed by the American STS company can form various digital weighing systems. The 2203, 2204 and 2224 digital load cell modules are only different in internal resolution and division resolution. The others are basically the same. They can be connected to 3mv/v analog load cells, up to 12. The analog output signal of each load cell is converted into a high-level digital signal through a module with a high-speed, high-resolution A/D converter and a microprocessor, and transmitted to the “digital process weighing control” via a serial communication interface. Through this controller, digital and analog data can be transmitted to the distributed control system, programmable controller and personal computer through the network, which constitutes a typical digital weighing system. It is not difficult to see from this that the outstanding feature of this solution is that it can change the analog weighing system to digital through the digital load cell module without making any changes to the original weighing system and without replacing the load cell. Weighing system. It can be said that this is a shortcut to digital transformation of the traditional weighing system, and it will definitely have a good development prospect.
