In the first 30 years, the load sensors of column, tube, ring and beam structures with normal stress (tensile, compressive and bending stress) dominated the world. During this period, Jackson, a British scholar, developed a metal foil resistance strain gauge, which provided an ideal conversion element for the load sensor, and initiated a new process of sticking the resistance strain gauge with thermosetting adhesive. After years of practice, BLH Company and Revere Company in the United States created the compensation and adjustment technology of load sensor circuit, which improved the accuracy and stability of load sensor, and increased the accuracy from a few percent in the 1940s to 0.05 in the early 1970s. However, the problems in the application process are also very prominent, mainly as follows: (1) the change of application point will cause a relatively large sensitivity change; When tension and pressure cyclic loading are carried out at the same time, the sensitivity deviation is large; Poor ability to resist eccentric and lateral loads; Small load measurement is not possible. The above shortcomings seriously restrict the development of load sensors.
After more than 30 years, it experienced two technical breakthroughs: shear stress load sensor and aluminum alloy small range load sensor in the 1970 s; In 1980s, the weighing sensor was completely separated from the weighing sensor, and two major changes were made, namely, making R60 international recommendation and developing digital intelligent weighing sensor. In 1990s, high and new technologies were continuously integrated into the structural design and manufacturing process to meet new challenges, which accelerated the development of load cell technology.
In 1973, Hogg Sturm, an American scholar, put forward the theory that the shear stress is independent of the bending moment rather than the normal stress to design the load sensor, and designed the cantilever shear beam load sensor with circular I-section. It breaks the unification of conventional stress load sensors and forms a new development trend. This is a major breakthrough in the structural design of load sensor.
Around 1974, American scholar Stein and German scholar Edom proposed to establish a more complex elastic mechanical model, and analyze the strength, stiffness, stress field and displacement field of elastic body by finite element calculation method to get the optimal design. It opens up a new way for designing and calculating load sensors by using modern analysis means and calculation methods.
In the middle and early 1970s, weighing instrument manufacturing companies in the United States, Japan and other countries began to develop commercial electronic pricing scales, which urgently needed small-range load sensors. Neither the traditional normal stress sensor nor the newly developed shear stress load sensor can measure in the range of several kilograms to dozens of kilograms. Charters, an American scholar, put forward that aluminum alloy with low elastic modulus should be used as elastomer and multi-beam structure should be used to solve the contradiction between sensitivity and stiffness. A small aluminum alloy parallel beam load sensor is designed, and it is pointed out that the parallel beam load sensor is based on the principle of constant bending moment, which makes the normal stress structure using the surface bending stress of parallel beam have the characteristics of shear stress load sensor, which lays a theoretical foundation for the design and calculation of parallel beam structure load sensor and forms another development trend.
Creep is a key problem that resistance strain gage and aluminum alloy load sensors often encounter and must be solved. From 65438 to 0978, Kolokova, a scholar from the former Soviet Union, put forward the theory that resistance strain gauges with different creep values can be manufactured by controlling the ratio of the width of the grating head to the width of the grating line, and successfully developed a series of creep compensation resistance strain gauges. It plays an important role in reducing creep error and improving the accuracy of low-capacity aluminum alloy load sensors, making it possible to produce aluminum alloy load sensors for electronic pricing scales in various varieties and in large quantities.
Due to the rapid development of electronic weighing technology, the evaluation method of load sensor performance can no longer meet the needs of evaluating the accuracy level of electronic weighing instrument with step tolerance zone, and there is an urgent need for measurement regulations suitable for the accuracy evaluation method of electronic weighing instrument. In the early 1980s, the OIML Quality Metrology Guidance Secretariat decided to completely separate the sensor used for electronic weighing from the sensor used for force measurement, and the Eighth Report Secretariat, which was in charge of the United States, drafted the Measurement Rules for Weighing Sensors. After written voting by OIML member countries, it was formally adopted at the 7th General Meeting of Legal Metrology in June 1984+00, and was promulgated together with the international recommendation of OIML and R60 in June 1985 and distributed to all member countries. Countries are implementing the 2000 version of R60. It can be said that R60 "Measurement Rules for Load Cells" is a "passport" for load cells from all 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 system to break through the limitations of analog weighing system, and analog weighing sensors are powerless. Because before this, the research of load cell mainly focused on hardware, such as: innovating elastic structure, improving manufacturing technology, perfecting circuit compensation and adjustment. The disadvantages of analog weighing sensor, such as small output signal, poor anti-interference ability, short transmission distance, complex weighing display control instrument and long debugging period of scale components, still exist. In order to meet the needs of digital electronic scales, Toledo, STS, CARDINAL and HBM in Germany have successively developed integrated and separated digital intelligent weighing sensors. Because of their large output signal, strong anti-interference ability, long signal transmission distance and easy intelligent control, they have become the necessary products of digital electronic scales and automatic weighing measurement and control systems, and become the focus of development.
In the 1990s, due to the maturity of basic technologies such as design and calculation of load cells, the development of load cells focused on process research and application research, and made great progress in product standardization, serialization, engineering design and large-scale production technology, mainly as follows:
Introduce computer virtual reality technology and virtual technology into structural and technological design; Combining flexible manufacturing technology into elastomer processing; Adopt computer network technology in the production process; In the stabilization treatment, new technologies of vibration aging and vibration aging are transplanted; In the experiment and verification, the methods of automatic rapid detection and dynamic comparison are created. There is also a breakthrough in the research of application technology: based on the traditional weighing module, a new weighing module is developed. This is a typical product that applies new technology to face new challenges. It is characterized by modular design and plug-and-play function, which can reduce the weighing error caused by overweight, thermal effect and accidental overload, and can bear the overweight caused by vibration, impact, stirring or other external forces. In short, two technological breakthroughs in 1970s, two major changes in 1980s, and the proposal of R&D high-tech concept facing new challenges in 1990s greatly promoted the development of load cell technology.
Strain gauge load cell -II. The present situation of foreign weighing sensor technology and the reasons for its rapid development. The technology and manufacturing process of weighing sensors for industrial and commercial electronic scales, famous manufacturing companies in developed countries such as the United States and Germany are in the leading position in the international market, and manufacturing companies with a certain scale in China are in the position of market challengers or market followers. The R&D and production centers of household electronic scales are located in China and Shenzhen, and the manufacturing technology, process level, product quality and annual output are improved year by year.
The competition of weighing sensor technology in the international market today is mainly manifested in the competition of product accuracy, stability and reliability; Competition between manufacturing technology and manufacturing process; The application of high-tech research and development of new products and the competition of independent intellectual property products. All weighing sensor manufacturers are striving to cultivate their own core competitive technologies and build core competitive products.
From the analysis of the products exhibited in the international weighing instrument industry exhibition in recent years and the products of many enterprises in the market leader position, it can be concluded that these enterprises have the same pursuit: better elastomer materials; The technical requirements of resistance strain gauge, compensation element 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 weighing sensor are important quality indicators, and are also the most concerned issues for users. In this regard, these enterprises have done a lot of research and test work in structural design, manufacturing technology, circuit compensation and adjustment, stability treatment and so on, and made great progress. The main results are as follows:
(1) In the process of structural design and calculation, computer virtual reality technology is introduced for dynamic simulation and dynamic analysis; Computer virtual technology is introduced into process design to simulate and test the production process of elastomer.
(2) Advanced manufacturing technology is integrated into the elastomer processing process, and rigid manufacturing is changed into flexible manufacturing. Machining center, flexible manufacturing unit and flexible manufacturing system are widely used;
(3) In the whole production process, reduce manual operation and manual control as much as possible, increase semi-automatic and automatic control and automatic inspection procedures, and adopt computer network technology in the production process;
(4) Improve innovative process equipment, realize high-efficiency intelligent circuit compensation, establish a fully automatic rapid detection system, and improve the success rate of C3 products and the sampling qualification rate of mass-produced products;
(5) Transplanting advanced stabilization technology and equipment, and implementing vibration aging or new vibration aging technology to improve the long-term stability and working reliability of the weighing sensor;
(6) Apply high technology to develop new products and products with independent intellectual property rights to improve core competitiveness. Enterprises in the leading position in the international market all have their own core competitive technologies, processes and products, such as: "O Creep" load cell with positive and negative creep bridge; Beryllium bronze dynamic weighing sensors: integral and split digital intelligent weighing sensors: high-precision stainless steel 3-column and 4-column high-temperature weighing sensors; Modular design of "plug and play" new weighing module, etc.