1. Development and technological innovation of strain-type load cell
In 1938, Professor E. Simmons (Simmons) of the California Institute of Technology and Professor A. Ruge (MIT) of the Massachusetts Institute of Technology simultaneously developed paper-based wire-wound resistance strain gauges. The assistant is named SR-4 and is produced by the patent of American BLH company. It has laid the theoretical and material foundation for the development of strain load sensors.
In 1940, A. Thurston, the chief engineer of American BLH and Revere, developed a strain-type load cell of cylindrical structure using SR-4 type resistance strain gauge, which was used for engineering force measurement and weighing measurement, and became a strain type load. The founder of the sensor. Strain-type load cells have been mass-produced in the United States in 1942 and have a history of more than 60 years.
For the first 30 years or so, it was the domination of column, tube, ring, and beam structure load sensors using normal stress (tensile, compression, and bending stress). During this period, the British scholar Jackson developed a metal foil resistance strain gauge, which provides an ideal conversion element for the load sensor, and created a new process of using thermosetting glue to paste the resistance strain gauge. After years of practice, American BLH Company and Revere Company have created load sensor circuit compensation and adjustment technology, which has improved the accuracy and stability of the load sensor, and the accuracy has been improved from a few percent in the 1940s to 0.05 in the early 1970s. Magnitude. But the problems in the application process are also very prominent, mainly: the change of the force point will cause a relatively large sensitivity change; the sensitivity deviation when loading at the same time in the tensile and compression cycles is large; the anti-eccentricity and lateral load capacity are poor; Load measurement. The above shortcomings severely restrict the development of load sensors.
After more than 30 years, it has experienced two major technological breakthroughs of the shear stress load sensor and the aluminum alloy small-range load sensor in the 1970s; in the 1980s, the load cell was completely separated from the load cell, R60 international recommendations were formulated and a digital intelligent load cell was developed Two major changes; in the 1990s, high-tech was continuously incorporated into structural design and manufacturing processes to meet new challenges, accelerating the development of load cell technology.
In 1973, in order to overcome the inherent shortcomings of the normal stress load sensor, the American scholar Hogstrom proposed the theory of designing the load sensor without using normal stress and using shear stress independent of bending moment, and designed a circular truncated I-shaped cantilever shear. Beam type load cell. It breaks the dominance of normal stress load sensors and forms a new development trend. This is a major breakthrough in the structural design of load sensors.
Around 1974, the American scholar Stein and the German scholar Edom proposed the establishment of a more complex mechanical model of the elastic body. The finite element calculation method was used to analyze the strength, stiffness, stress field and displacement field of the elastic body to obtain the optimal design. . It opens up a new way to design and calculate load sensors using modern analysis methods and calculation methods.
In the early and mid-1970s, weighing instrument manufacturing companies in the United States, Japan, and other countries began to develop commercial electronic pricing scales, and urgently needed small-scale load sensors. Neither the conventional normal stress nor the newly developed shear stress load sensor can achieve the measurement in the range of several kilograms to tens of kilograms. American scholar Chatters proposed to use aluminum alloy with low elastic modulus as the elastomer, and adopt the multi-beam structure to solve the contradiction between sensitivity and rigidity. Designed a small-range aluminum alloy parallel beam load sensor, and pointed out that the parallel beam load sensor is based on the principle of constant bending moment, so that the normal stress structure using the bending stress on the surface of the parallel beam has the characteristics of the shear stress load sensor. The design and calculation of load sensors laid a theoretical foundation and formed another trend of development.
Creep is a key problem that resistance strain gauges and aluminum alloy load sensors often encounter and must solve. Before 1978, Soviet scholar Kolokova proposed to control the ratio of the width of the grid head of the sensitive grid of the resistance strain gauge to the width of the grid wire through the analysis of the one-dimensional mechanical model and the strain transfer coefficient. Theory, and successfully developed a series of creep compensation resistance strain gauges. It plays a vital role in reducing creep error and improving accuracy of low-capacity aluminum alloy load sensors, making it possible to produce a variety of aluminum alloy load sensors for electronic price scales in large quantities.
Due to the rapid development of electronic weighing technology, the evaluation method of load sensor performance has been unable to meet the needs of using a stepped tolerance band to evaluate the accuracy of electronic scales. There is an urgent need for measurement procedures that are compatible with the accuracy evaluation methods of electronic scales. In the early 1980s, the International Organization for Legal Metrology (OIML) Quality Measurement Guidance Secretariat decided to completely separate the sensors used for electronic weighing from the sensors used for force measurement. The 8th report secretariat drafted by the U.S. procedure". After a written vote by OIML member countries, it was formally approved at the 7th Congress of Legal Metrology in October 1984 and promulgated in 1985 with OIML, R60 international recommendations and distributed to member countries. The 2000 version of R60 is currently being implemented in various countries. It can be said that the R60 "Measurement Regulations for Load Cells" is a "pass" for the load cells of various countries to enter the international market.
With the development of digital technology and information technology, there is an increasing demand for digital electronic weighing instruments in various industries. It is proposed to use digital weighing systems to break through the limitations of analog weighing systems, and analog weighing sensors are powerless to do so. Because before this, the research of load cells has focused on hardware, such as: innovation of elastomer structure, improvement of manufacturing process, and improvement of circuit compensation and adjustment. The output signals of the analog weighing sensor are small, the anti-interference ability is poor, the transmission distance is short, the weighing display control instrument is complicated, and the group debugging period is long. In order to meet the needs of digital electronic weighing instruments, the United States TOLEDO, STS and CARDINAL companies, Germany HBM companies have developed integrated and separate digital intelligent weighing sensors, and its large output signal, strong anti-interference ability, signal transmission distance It is far away and easy to realize intelligent control and other characteristics, becoming a must-have product for digital electronic weighing instruments and automatic weighing measurement and control systems, forming a development hot spot.
In the 1990s, as the basic technology of load cell design and calculation became more mature, the development of load cells focused on process research and application research, including product standardization, serialization, engineering design and large-scale production technology. Great progress, mainly:
Introduce computer simulation technology and virtual technology in structure and process design;
Incorporate flexible manufacturing technology in elastomer processing;
Use computer network technology in the production process;
New techniques of vibration aging and resonance aging are transplanted in the stable treatment;
In the test verification, automatic rapid detection and dynamic comparison methods were created.
There have also been breakthroughs in applied technology research: on the basis of traditional weighing modules, new weighing modules have been developed. This is a typical product that applies new technology to face new challenges. It is characterized by a modular design, with "plug and play" function, which can reduce the weighing error caused by the bias, thermal effects, accidental overload, etc., and can withstand the bias caused by vibration, impact, stirring or other external forces. In short, two technological breakthroughs in the 1970s, two major changes in the 1980s, and the introduction of high-tech research and development concepts in the 1990s to face new challenges have greatly promoted the development of load cell technology.
Second, the current status and rapid development of foreign load cell technology
The technology and manufacturing process of load cells for industrial and commercial electronic scales, well-known manufacturing companies in industrialized countries such as the United States and Germany are in the leading position in the international market leader, China's load cell manufacturing companies with a certain scale are in the market challenger or market Follower status. The research and development and production center of the weighing sensor for household electronic scales is in China, and in Shenzhen, the manufacturing technology, process level, product quality and annual output are increasing year by year.
The competition of weighing sensor technology in today's international market is concentrated in the competition of accuracy, stability and reliability of products; the competition of manufacturing technology and manufacturing process; the competition of applying high and new technology to develop new products and products with independent intellectual property rights. Each weighing sensor manufacturing enterprise is striving to cultivate its own core competitive technology and create core competitive products.
From the products exhibited at the International Weighing Apparatus Industry Exhibition in recent years and the analysis of the products of a number of market leaders, it can be concluded that the common pursuit of these companies is: more sophisticated elastomer materials; resistance strain gauges, compensation elements The technical requirements of the device and environmental stress screening are stricter; the manufacturing process is more refined; the circuit compensation process is more perfect; the appearance quality is more perfect.
The accuracy, stability and reliability of the load cell are important quality indicators, and are also the issues that users care about most. In response, these companies have conducted many research and test work in structural design, manufacturing process, circuit compensation and adjustment, and stability treatment, etc., and have made great progress. The main results are:
(1) In the process of structural design and calculation, computer simulation technology is introduced for dynamic simulation and dynamic analysis; in the process design process, computer virtual technology is introduced to simulate and test the elastomer production process;
(2) In the processing of elastomers, advanced manufacturing technology is incorporated to make rigid manufacturing flexible. Generally use machining centers, flexible manufacturing units and flexible manufacturing systems;
(3) During the entire production process, try to reduce manual operations and human control, add semi-automatic and automatic control, automatic inspection procedures, and use computer network technology in the production process;
(4) Improve and innovate process equipment, realize efficient intelligent circuit compensation, establish a fully automatic rapid detection system, and improve the success rate of C3 grade products and the sampling inspection pass rate of mass production products;
(5) Transplant advanced stabilization processing technology and equipment, implement new technology of vibration aging or resonance aging to improve the long-term stability and working reliability of the load cell;
(6) Apply high and new technology to develop new products and products with independent intellectual property rights to enhance core competitiveness. Companies in a leading position in the international market have their own core competitive technologies, processes and products, such as: "O creep" load cells of positive and negative creep bridges; beryllium bronze dynamic load cells; integral and separate digital Intelligent load cell; high-accuracy stainless steel 3-column and 4-column high-temperature load cell; component-designed "plug and play" type new weighing module.
The characteristics of foreign load cell technology development and the reasons for rapid development:
(1) Pay attention to the research of basic technology, basic technology and common key technology, make basic research and advance research parallel; common key technology research and applied technology research parallel; typical product development and product engineering parallel. Ensure that the basic technology and basic technology (resistance strain gauges, strain adhesives, compensation components, protection and sealing materials, etc.) have always been in the world's leading position.
(2) Pay attention to the research and application of infrastructure construction and manufacturing technology and manufacturing process. Equipped with excellent process equipment and testing instruments, especially intelligent process equipment, the most advanced process equipment;
(3) Describe the development trend and strategic frontier of the world's load cell technology, and determine the research topics and product development directions. Pay attention to the development of new products and products with independent intellectual property rights to enhance core competitiveness. The standards for its technological innovation and new product development are: high technical leadership, advanced technology, market diffusion, and benefit proliferation. Keep technology and craftsmanship in the leading position in the world.
(4) Pay attention to the reliability design, control and management of load cells, strictly design compliance control and process reliability control, and strive to make the process realization rate reach 100%.
(5) Pay attention to market competition, strengthen market investigation and analysis, and respond quickly to the market. The market competition in the 21st century is based on the speed of market response and on the basis of improved and innovative products.
(6) Pay attention to the study of relevant laws and regulations, fully understand and implement it carefully, and ensure that every product produced meets the requirements.
Because of this, there are many varieties of load cells abroad, complete specifications, all kinds of alloy steel, aluminum alloy, stainless steel, beryllium bronze products; underwater, downhole measurement, pressure and explosion-proof, anti-radiation, corrosion-resistant products; small and ultra-large range ; Multiple weighing and dynamic weighing; integrated and modular structure for users to choose. In addition, the internal and external quality of the product are equally important. The improvement and improvement of the external appearance quality in recent years are very obvious. There are basically no painted products, and almost all are bright or matt chemical nickel plating.
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