What is Reliability design, control and management of strain gauge load cell

All load cells and electronic weighing systems composed of load cells will experience performance fluctuations over time or use, fail to complete the specified functions and eventually fail (unstable). The time from the beginning of use to the failure of the load cell is uncertain. The reliability is to study the mean time between failures, the failure rate and the reliable life of the load cell in the specified conditions and time. The reliability of the electronic weighing instrument and electronic weighing system assembled by the sensor is very important. Therefore, reliability has become an important quality indicator of load cells and a major symbol of market competition.

The International Organization for Standardization (ISO8402) defines reliability: "The ability of a unit to complete a specified function under a given environment and operating conditions and within a given time." The term "unit" here refers to the object considered as a whole, which can be a component, a subsystem, or a system. Considering the load cell alone is a component, and considering it together with the weighing system is an important part of the subsystem or system. "Time" should be understood from a broad perspective. In terms of load cells or electronic weighing instruments, it is the number of cycles of stress. For the definition of quality: "The quality of a product reflects the sum of the characteristics of the product's ability to meet explicit and implicit needs." "Need" can include performance, availability, credibility, reliability, etc. by definition. "Performance" here is an aspect of need and a central consideration. Reliability means that the product is completed under specified conditions and within specified time The ability to specify functions.According to the above definition, the state that cannot complete the specified function is called failure or failure. The core of reliability is failure, that is, reliability is caused by product failure. Failure-failure-reliability-reliability, the four are closely related, forming a causal chain. From the above reliability theory, it can be concluded that the reliability of the load cell is the continuous working time without failure under the specified environmental conditions. Reliability is caused by the failure of the load cell. Reliability is a value used to measure the reliability level of the load cell. It is the result of data processing on the basis of counting the working time and failure times of the load cell. To achieve a higher level of reliability, the reliability design, control and management of the load cell must be done well.

Section 1 Reliability of strain gauge load cell

Since the 1990s, my country's load cells and electronic weighing instruments have been widely used in industrial and commercial weighing and measurement. The stability and reliability of load cells have attracted more and more attention from users. Under current conditions, it is not difficult to develop and produce load cells with higher accuracy levels, but it is not easy to develop and produce load cells with higher stability and reliability.

In many international academic conferences and academic exchanges, experts from various countries agreed that in terms of the working principle, structural characteristics, manufacturing process and application conditions of the load cell, it should be a semi-permanent working device with IP67, IP68 protection and sealing levels. The load cell should be able to work stably and reliably for more than 10 years.

According to the definition of product reliability and the actual application of the load cell, its reliability is defined as the ability or probability of the load cell to complete the specified function under the specified use conditions and within a certain period of time. Specifically, it is the ability of the load cell to maintain its various technical performance and work stably under specified conditions of use and within a certain period of time, which is usually measured by trouble-free working time or reliable life. As we all know, load cells are mainly used in various electronic weighing instruments and electronic weighing systems, requiring good and stable overall performance, that is, the sum of all deviations such as nonlinearity, hysteresis error and sensitivity temperature influence, should be within a certain allowable error band . The stability of the zero point and sensitivity directly affects the long-term stability and working reliability of the load cell.

At present, the reliability of sensors at home and abroad is mostly concentrated on some systems with high reliability requirements, such as launch vehicles and missiles, satellites and spacecraft, military and civil aircraft, and large industrial control systems. For the various sensors used in these systems, various tests were carried out according to the four basic reliability functions, and a large amount of data acquisition, analysis and processing were completed. Both the Reliability Analysis Center of the United States and my country’s military scientific research departments have collected a large amount of sensor reliability data. The research on the reliability of load cells is limited to reliability analysis and tracking test stage at home and abroad.

Reliability analysis runs through the entire process of load cell design, manufacture, and use. It is an important part of reliability research. It is mainly the analysis of failure (failure) mechanism. Through macro-to-micro analysis of the failure, the cause of the failure is found, the internal law of the failure is found out, and corresponding countermeasures are taken to improve the reliability of the load cell.

Follow-up testing is an economical and applicable simple method to study the reliability of load cells. Some domestic and foreign companies have carried out two kinds of work in this regard. One is to track the storage life of the load cell under the laboratory environment; the other is to track the load cell's trouble-free working time, that is, the service life, under the use environment. Life test is an important part of reliability test. In addition to storage life test, there are depletion life test and accelerated life test. The reliability test of the load cell requires program design and the establishment of a mathematical model. At the same time, more funds and time are required. Generally, load cell manufacturers have some difficulties. It is recommended that manufacturers and users cooperate closely to exchange information and accumulate reliability data.

According to the traditional reliability analysis method, the failure rate of the product follows the bathtub curve, as shown in Figure 10.1.

Figure 10.1 Product failure rate curve

The left part shows that the early expiration period is decreasing. In the early stage of product use, due to some defects left in the manufacturing process, the failure rate is often high. Generally, the early failure is eliminated through exhaustive testing before the product leaves the factory. The middle part is the accidental failure period, which is close to a constant level. This is because the failure rate of the product stabilizes at a low level after the early failure. The failure rate in this period is often caused by random reasons and is called the accidental failure period. The part on the right shows that the loss and failure period is increasing. Due to the aging and loss of components, the failure rate begins to increase.

The stability and reliability of the load cell fully comply with this law, and it is divided into an initial unstable period, a stable period and a fatigue unstable period. The initial instability period is due to the residual stress generated by the load cell block element through forging, machining, heat treatment, surface polishing, sandblasting, bonding resistance strain gauge, curing and post-curing processes, and continuous relaxation and release, so that zero point and sensitivity occur Changes, causing fluctuations in the performance of the load cell. It can release and eliminate residual stress as much as possible by implementing environmental stress screening on components and performing sophisticated and stable treatment of the load cell. It can pass the initial period of instability during the production process and enter a stable period that can work stably and reliably after leaving the factory. After a long period of trouble-free work, due to the aging of the protective sealing material, the loss of circuit compensation and adjustment components, and the fatigue of the resistance strain gauge, the performance of the load cell fluctuates, and the work is unstable until it fails, which is called fatigue failure. stable period. The reliability theory of load cells is to study and analyze various systematic and random factors that affect their reliability, and scientifically and reasonably put forward qualitative and quantitative requirements for reliability, such as failure mode and impact analysis, trouble-free working time and reliable life. Wait.

Section 2 Reliability Design of Strain Type Load Cell

Introducing the reliability design method of a more complex system into the reliability design of the load cell, adopting a systematic analysis program to analyze the failure mode and severity, identify possible hidden dangers, and take measures to improve the design. There are still great difficulties. Because the reliability of the load cell belongs to the exponential distribution or the Weibull distribution, it remains to be further understood. However, according to the working principle, manufacturing process and application of the load cell, from the analysis of the residual stress, the aging of components and sealing materials, and the fatigue accumulation of resistance strain gauges, a logical treatment is to divide the load cells according to their functions. The interacting and interdependent components are independently studied for their reliability. For example, load cell block component materials and heat treatment, resistance strain gauges and strain adhesives, circuit compensation components and compensation processes, protective sealing materials and sealing processes, etc.After the technical indicators of the load cell are determined, it is not possible to assign the technical indicators to the sub-systems, stand-alone units or components as in the design of a more complex system, so as to provide quantitative targets for reliability design. Because the various components of the load cell interact and depend on each other, it can be said that they are the basis of the load cell. Which link has a high failure rate will have a greater impact on the inherent reliability of the load cell. Therefore, it must Make the basic design for the reliability of the load cell.

1:Reliability design of load cell block components

The structure of the load cell block element is the basis of the load cell and an important factor for stability and reliability. Therefore, the design of the load cell block element and its protective shell, welding sealing diaphragm, upper leading pressure head, and lower bearing pressure pad must ensure that the load cell does not produce performance fluctuations in the structure after being loaded, or reduce the performance fluctuations to The smallest. In order to obtain higher reliability, the design of the load cell block element structure should be done as much as possible: the strain area has a single force and the stress distribution is uniform; through its own structure or the use of a protective shell, it has strong resistance to eccentricity and lateral load. , To overcome the influence of non-measurement load; the installation force is far away from the strain zone, the pressure head introduced by the load and the pressure pad bearing the load are reasonably stressed to avoid the displacement of the load introduction point and the supporting point; the surface of the resistance strain gauge should be flat as much as possible, and There is enough working area to create favorable conditions for patch work and protective sealing.In order to make the structure and force of the load cell block element reasonable, a more complex mechanical model should be established in the design and calculation, and the interactive or high-interaction finite element program package should be used to display the stress field and displacement field with the aid of CAD, and perform dynamic simulation to determine the final size Parameters to get the optimized design results. Due to the advancement of science and technology in recent years and the improvement of structural design level, it is best to adopt three-dimensional digital design to improve the rationality of load cell block element structure and boundary design. Although the load cell is an assembly component, in order to obtain the best performance and ensure its stable and reliable work, the structural design must minimize parts and improve the degree of integration, as much as possible as a whole structure.

2:Selection of load cell block component materials and heat treatment process

load cell block component materials and heat treatment process have a great influence on the comprehensive performance index of the load cell, long-term stability and working reliability, which is a key issue in the reliability design of the load cell. In particular, the solution heat treatment process and the effective method of stainless steel load cell block element materials directly determine the performance and reliability of the load cell. Considering the load cell block modulus, anload cell block effect, residual stress effect and fatigue resistance of the load cell block element material, the load cell block limit and yield limit should be selected as high as possible; the time and temperature stability of the load cell block modulus is good; the load cell block hysteresis and load cell block aftereffect Small; strong resistance to vibration, impact and fatigue; metal materials with good forging, machining and heat treatment process performance and low residual stress. It is difficult to make a certain metal material possess the above-mentioned properties at the same time, but it can be made to meet or approach the above-mentioned requirements as much as possible through certain control methods and treatments.An effective way to improve the reliability of the load cell on the load cell block component material is to strictly require the purity of the components and the uniformity of the material, not highlight a single index, and pursue good overall performance. In particular, the best heat treatment process and aging system must be tested. . For example, 0Cr17Ni4Cu4Nb precipitation hardening stainless steel must be tested in the solution heat treatment process and two aging methods or cooling treatment plus aging methods to determine the heat treatment and aging process system that meets the comprehensive performance and reliability requirements of the load cell.

3: Resistance strain gauge selection

The resistance strain gauge is the core component of the load cell. Its stability and reliability determine the stability and reliability of the load cell to a large extent, so it is an important part of reliability control. Since the resistance strain gauge cannot be used twice after the test, it can only measure the fatigue life under laboratory environmental conditions, which brings great difficulties to the research work. At present, domestic and foreign weighing sensor manufacturers control the reliability of resistance strain gauges, usually around the structure of the sensitive grid, the factors that affect the reliability, and the selection of working characteristics.

3.1. Structure of resistance strain gauge

The structural design of resistance strain gauges for load cells cannot stay at the level of traditional empirical design. It just makes a fuss on the grid length, grid width and resistance value, and focuses on the strain transfer theory, working stability and The fatigue life is studied. Production practice has proved that if resistance strain gauges of different structures are pasted on load cells of the same structure, there are obvious differences in creep index, pass rate and fatigue life of the same accuracy level, which are all strains of resistance strain gauges. It is caused by insufficient research on transfer theory and creep mechanism. Resistance strain gauges for load cells mostly use plane and three-dimensional finite element analysis methods, corresponding to strain transfer, the mechanical effect and optimal thickness of the substrate and cover layer, the end effect of the sensitive grid and the relationship between end shape and creep The influence of substrate thickness on hysteresis is analyzed and selected, and a resistance strain gauge structure that meets the comprehensive performance indicators and reliability requirements of the load cell is designed.

3.2. Factors affecting the reliability of resistance strain gauges

The structure, manufacturing process and strain transfer principle of resistance strain gauges determine various products produced in large quantities, and some process defects will inevitably occur, which directly affect the long-term stability and working reliability of resistance strain gauges. In summary, they affect the stability and The main factors of reliability are:

（1) The influence of sensitive grid foil and heat treatment process

The sensitive grid foil materials of resistance strain gauges for load cells are mostly constantan (copper-nickel alloy) and Karma and Evan alloys (nickel-chromium modified alloy). The factors affecting stability and reliability are the purity of the alloy composition and the uniformity of the structure. In addition to performance, it is mainly the heat treatment process of alloy foil. In the process of multiple rolling and rolling, the crystal lattice produces defects such as dislocations, slippage, and vacancy cracking. The nearby atoms are in a thermodynamically unstable state, which is an important cause of unstable electrical properties. Therefore, it is necessary to carry out a stability treatment, that is, an annealing treatment. When the annealing temperature is reached, these atomic absorption heat energy diffuses, so that the lattice defects migrate and disappear, and the resistivity and the temperature coefficient of resistance tend to stabilize. Annealing temperature, holding time, and cycle times are the three elements of stability treatment. If the foil heat treatment and stability treatment process are not selected properly, the electrical and temperature characteristics of the foil will be unstable, which is the main factor for the instability of the resistance strain gauge.

(2) The influence of substrate, cover layer and strain adhesive

The base, covering layer and strain adhesive of the resistance strain gauge are all organic polymer materials. And all polymer materials are affected by moisture and oxygen in the air, because water can penetrate almost all polymers to produce plasticization, and it will also deteriorate over time, that is, due to physical or chemical changes, its performance is reduced. As a result, the bonding strength between the substrate, the covering layer and the sensitive grid is reduced, which causes the hysteresis, creep, and zero drift of the load cell to increase, and the sensitivity is reduced until it completely fails. The thickness of the base film and the covering layer of the resistance strain gauge has a great influence on strain transmission, creep and hysteresis. Therefore, it is necessary to strengthen the selection of material quality to ensure that the thickness of the base film is 25±5μm, and the thickness of the covering layer is 10±2μm. Experimental tests show that this is the best thickness of the base film and the coating layer of the resistance strain gauge for load cells.

(3) Influence of residual stress

The manufacturing and pasting of resistance strain gauges go through a multi-process production process. Due to the processes of foil cleaning, bonding with frame glue, base glue, chemical etching, adding coating and curing clamping force, mechanical or chemical resistance adjustment, It needs to withstand the action of various media such as acid, alkali, various reagents (acetone, absolute ethanol, butanone) and water, all of which produce residual stress in the sensitive grid, substrate and cover layer. These residual stresses gradually release over time and use, resulting in unstable performance of the resistance strain gauge. In order to overcome the influence of residual stress, a special stability treatment process must be adopted.

(4) The influence of process defects

After the sensitive grid is etched due to factors such as process equipment and manufacturing technology level, large sawtooth, fine grid between sawtooths, over-etched gaps, under-etched "islands", and "bridges" between the grids appear on the grid and between the grids. , Generate bubbles in the substrate, and even entrain defects such as dust and impurities. The fine grid or gap between the serrations on the sensitive grid causes irreversible changes in resistance as the number of stress cycles increases, causing serious consequences such as increased hysteresis and zero instability. The air bubbles in the substrate can produce uneven heat exchange, which increases the zero drift of the resistance strain gauge.

(5) Influence of residual etching solution

In the rough etching and fine etching process of the resistance strain gauge sensitive grid, an etching solution such as ferric chloride is used. If the process control is not strict after etching or adjustment, and the surface is not cleaned thoroughly, a trace amount of etching solution will remain on the surface of the sensitive grid. Continue The sensitive gate is etched to slowly increase the resistance value and produce performance fluctuations.

In order to reduce the above influence and improve the reliability of the load cell, these unreliable factors must be controlled from the source. Since the sensitive grid converts the strain of the load cell block element into resistance change, the substrate supports the sensitive grid to maintain a specified geometric shape, and the covering layer protects the sensitive grid and increases the adhesion between the sensitive grid and the substrate. Therefore, only the sensitive grid, the substrate and the The three covering layers are integrated to ensure the effective transmission of strain and improve the reliability of the resistance strain gauge. To this end, it should be done as much as possible: the purity of the strain resistance alloy foil is high, the structure is uniform and stable; the tempering, aging and stabilization process is perfect, reasonable and strictly controlled; the resistance value of each grid bar should be made as much as possible during plate making and etching Equal; sensitivity coefficient K means good stability; small lateral effect coefficient, high fatigue life; small and stable temperature coefficient of resistance; high shear strength of the base and cover materials, good thermal stability and moisture resistance.

3. Screening of resistance strain gauges

The manufacturing process of the resistance strain gauge determines that its finished product must have some inevitable process defects. As a short-term use of structural experimental stress analysis, it can fully meet the requirements. For long-term use of the load cell, even if it is a class A resistance strain gauge, some indicators still have a certain gap. In order to improve the long-term stability and working reliability of the load cell, the resistance strain gauge must be strictly inspected and screened. The main inspection and screening items are: geometric shape and surface quality, mechanical and electrical properties, resistance value dispersion, etc. Only by pasting a resistance strain gauge with a small and stable resistance temperature coefficient and a small resistance value deviation on the same load cell block element can it be beneficial to various production processes and improve the reliability of the load cell.

4: Circuit compensation and adjustment of the selection of raw materials and components

The raw materials and components used for circuit compensation and adjustment are the same as resistance strain gauges, which are the basic units that affect the reliability of the load cell. In order to improve the reliability of the load cell, the raw materials and components must be carefully selected and subjected to strict environmental stress. Screening should generally follow the following principles:

(1) Compile a preferred catalog of raw materials and components on the basis of extensive research to avoid blind selection and introduction of low-grade or unqualified raw materials and components;

(2) Comprehensive evaluation of raw materials and components, mainly including five indicators of reliability, availability, adaptability, economy and production consistency, to ensure the stability and reliability of various circuit compensation and adjustment indicators of the load cell , It should be decided on the basis of evaluation and test whether to choose;

(3) The quality certification of the manufacturers of raw materials and components must be certified by an authoritative organization, and use this as the basis for selecting raw materials and components;

(4) Strict environmental stress screening should be carried out on key components that have a greater impact on reliability.

5. Simplified design of bridge and compensation circuit

One of the important measures to improve the reliability of the load cell is to simplify the design of the bridge and compensation circuit, compensation and adjustment process. The design principle is to minimize the number of components such as various compensation resistors, wire bonding terminals, wire bonding wiring boards, etc., optimize the manufacturing process, avoid or reduce repeated soldering, and try to make the process work as best as possible. The main contribution of the simplified design of Wheatstone's power circuit and compensation circuit to the reliability of the load cell is:

(1) The number of components is reduced, so that the number of solder joints is also reduced, thereby reducing the chance of failure;

(2) Improve the manufacturability, facilitate the quality control of the production process, and reduce the factors that reduce reliability;

(3) It is easy to realize standardized and generalized high-efficiency intelligent circuit compensation and adjustment technology, which is conducive to multi-variety and mass production, and can greatly reduce human errors.

6: Environment-resistant design

The essential problem of the reliability of the load cell is how to withstand the effects of various environmental conditions that may be encountered, and can work stably and reliably. Therefore, it is necessary to pay attention to and understand various environmental conditions and their potential impacts, and choose design schemes that can resist the impact of these environmental conditions.

According to the nature and characteristics of the environment, general environmental conditions are divided into two types: objective environment and sensing environment. The former is the surrounding natural environment, and the latter is the man-made environment. The environmental conditions that the load cell can withstand are both, mainly temperature, humidity, salt spray, vibration, and shock. Therefore, the measures to withstand environmental conditions are mainly temperature, humidity, vibration, and shock resistance. Due to the structural principle and manufacturing process of the load cell, it can withstand a certain vibration and shock load, and the high temperature and low temperature load cell is a special product specially designed and manufactured. Generally, the use temperature range of the load cell is mostly -10℃ ～+40℃, so the environmental condition design of the load cell is mainly the three-proof design of anti-moisture (including waterproof), anti-salt spray and anti-mold.

Moisture is not entirely H2O, it is usually many complex aqueous solutions, which can penetrate almost all polymers to produce plasticization. The function of the moisture weight sensor is chemical and physical reaction. The chemical reaction corrodes the adhesive surface of the resistance strain gauge in the strain zone of the load cell block element. The physical reaction causes the resistance strain gauge and the strain adhesive to absorb moisture and expand, increasing the dielectric constant and insulating resistance. , The bond strength is reduced, especially the sharp drop in shear strength, which causes irregular changes in the strain of the load cell block element of the load cell, resulting in zero point and sensitivity drift, technical performance fluctuations, and ultimately failure.

Salt spray is an aerosol-like body, the main ingredients are calcium chloride, magnesium chloride, a small amount of calcium sulfate, magnesium sulfate and other impurities. It also has a strong corrosive effect on the strain zone and outer surface of the load cell block element of the load cell.

Molds are fungi, single-celled organisms. When the temperature is between 26°C and 32°C and the relative humidity is above 85%, the organic sealing material is susceptible to mold erosion. The main destructive effect of mold is to absorb water, which leads to a significant drop in insulation resistance. Many anti-moisture and anti-salt spray technologies and processes are also suitable for anti-mold.

The moisture-proof sealing of load cell mainly includes welding sealing, blind hole potting and surface sealing. Welding and sealing mostly use electronic welding, laser welding, weak plasma welding, pulse sub-arc welding, etc., and its sealing level can reach IP68. Blind hole potting is an open type seal, which requires high moisture-proof sealing materials and sealing technology, mainly having excellent moisture-proof and salt spray-proof performance, excellent adhesion; low curing heat, small shrinkage, and soft , Good insulation performance; no corrosive effect on resistance strain gauges and strain adhesives. In addition, there are specific requirements for the electrical and physical and mechanical properties of moisture-proof sealing materials.

Section 3 Reliability control of strain gauge load cell production process

The reliability control of the production process is to adopt design compliance control and process reliability control to eliminate various unreliable factors to the greatest extent and avoid these factors from causing the reliability of the load cell to decrease. Unreliable factors in the production process of load cells are systematic and random. Systematic factors mainly refer to factors that deviate from normal production conditions and process conditions, such as unskilled workers working on shifts, mistakes in raw materials and components, random changes or simplifications of manufacturing processes, and violations of operating process regulations. Once these unreliable factors appear, there will be serious hidden dangers of failure and reduce the reliability of the load cell. Random factors are factors that cannot be accurately predicted, but they exist in large numbers during the production process. For example, changes in the production environment, small differences in raw material composition, and loose heat treatment and tempering process control of load cell block elements, etc., cause the performance of the load cell to fluctuate, directly Affect the stability and reliability of the load cell. Regardless of whether it is systematic or random unreliable factors, quality control and process control must be used to prevent or minimize them.

1: Design compliance control

Design compliance control is to take quality control measures in the whole process of load cell production to make it meet the requirements of the GB/T7551-2008 "Load Cell" national standard and JJG669-2003 "Load Cell" national metrological verification regulations. Various technical performance indicators and accuracy levels, as well as other requirements specified in relevant design documents. The main links of the load cell design compliance control are:

1.1:The process regulations and quality control documents must meet the technical requirements, test conditions, test items, test, calculation, and result determination methods of the weighing sensor national standards and national metrological verification regulations, and the prototype shall be finalized after the test is qualified.

1.2 Out-of-tolerance and substitute material processing must be based on the principle of not introducing factors that reduce the reliability of the load cell. In particular, the metal materials used for the load cell block components of high-accuracy load cells are generally not allowed to substitute materials, because after the load cell block component material is changed, the circuit compensation and adjustment components and compensation process will change, and the comprehensive performance indicators, stability and Reliability has a greater impact, which is the consensus of load cell manufacturers around the world. For example, the metal materials of steel load cell block components such as SAE4340 and H-11 in the United States, 30CrNiMo8 and X45NiCrMo4 in Germany, En24 and En26 in the United Kingdom, SNCM8 in Japan, and 40CrNiMoA in China have remained almost unchanged for a long time.

1.3 The core component of the load cell resistance strain gauge, the raw materials and components used for circuit compensation and adjustment, as well as other purchased parts and outsourcing parts, must be screened, re-inspected and re-tested to ensure that they meet the design requirements. Especially for resistance strain gauges, since it cannot be pasted twice, there must be effective quality inspection and screening methods.

1.4 The process inspection and factory test methods of the load cell must meet the accuracy and reliability requirements of the design to ensure that the national supervision and spot checks meet the requirements of the national metrological verification regulations. In the production process, if the self-prescribed simplified test method is used for quality control, it must be ensured that the various technical performance indicators of the load cell are fully qualified when the test is performed according to the formal test procedure.

1.5 The production process equipment and testing methods must be complete, especially the force standard machine with high and low temperature test chamber compatible with the rated range of the load cell must be equipped, which cannot be simplified. All process equipment and testing instruments must be checked regularly to maintain a 100% pass rate, and the self-made process equipment should be approved by the state-authorized metrological supervision department.

1.6 The production and testing personnel of each process on the production line shall be assessed and appointed to ensure that the operating procedures and product quality meet the requirements of the process and design.

1.7 The production environment meets the requirements, especially the resistance strain gage pasting process must be carried out in a clean room where temperature, humidity, cleanliness and lighting meet the requirements, and the environmental conditions of the moisture-proof sealing process should also be higher.

1.8 The structural design and process design of the load cell should adopt a parallel operation mode, that is, the process design and process preparation should be carried out at the beginning of the load cell design, so that the two coordinate and meet the technical performance requirements. In the production process, it is also advocated to design and cooperate with the process, to implement technical state control, and not to change the process, process and internal control technical indicators at will.

2: Process reliability control

Process reliability control is the fundamental link in the implementation of design reliability control, because improper process methods will introduce systematic factors that reduce the reliability of the load cell, bury hidden troubles, and reduce the reliability of the load cell. The main ways of process reliability control are:

2.1: Optimizing scientific and reasonable manufacturing process methods through research and testing on process reliability. For example, a load cell with 0Cr17Ni4Cu4Nb precipitation hardening stainless steel as an load cell block element, its comprehensive performance indicators, especially hysteresis and creep indicators, depend to a large extent on the solution heat treatment and aging process system. The heating temperature, quenching medium, and cooling rate of solution heat treatment; whether to use secondary aging method or cooling treatment plus aging method; under what kind of heat treatment furnace and furnace atmosphere, etc., it takes many tests to find out the best solution. When the various technical performance indicators of the load cell meet the design requirements, the process method is also finalized.

2.2 Once the process method and process flow are selected, the process stability must be maintained, and it must not be simplified or changed at will. If it is necessary to change, it must be approved in accordance with the procedures, and it must be ensured that this change does not have any impact on the reliability of the load cell.

2.3 Strict process management and process discipline to prevent violations of process regulations, violations of operating regulations and detection errors, and must ensure that the process fulfillment rate reaches 100%. In order to increase output, arbitrarily simplifying test items regardless of quality and reducing the number of tests is very dangerous and must be eliminated.

2.4 Self-inspection is advocated for each process in the whole production process of the load cell, key processes are set up with inspection points, the unqualified rate is strictly controlled, and process defects are minimized. Because a large part of the failure of the load cell is a process problem, the essence of blocking the process defect is to improve the reliability of the load cell.

3: Stability and reliability processing

Due to production conditions, process conditions, raw materials, components and other factors, it is inevitable that there will always be some random fluctuations, so that the produced load cell may have some potential defects and hidden troubles, resulting in an initial period of instability. . Therefore, after the production of the load cell is completed and before leaving the factory, unqualified products are usually screened out, but this screening can only increase the proportion of qualified products in the factory. Since the manufacturing process has not been improved, the batch size cannot be increased. The qualified rate of products produced. According to the problems in the technical performance test after production and before leaving the factory, environmental stress screening of raw materials and components in the production process is carried out, and aging and stability treatments are added to the weighing sensor before leaving the factory, so that the weighing sensor can survive the production process The initial unstable period has entered a stable period when leaving the factory. The stability treatment of the load cell mainly includes two methods: heat treatment and mechanicalHeat treatment stability treatment methods are mostly aimed at load cell block elements after cold and hot processing, mainly high-temperature oil boiling, low-temperature deep cooling, cold-heat cycle, constant temperature aging and other methods. The mechanical stability treatment method is aimed at the products that have completed all the processes, mainly including pulsation fatigue, overload static pressure, vibration aging, resonance aging and other methods.

In the 1980s, the U.S. Navy first introduced hybrid environmental stress screening, known as the high-efficiency stress screening method, which is a new aging method that combines the heat treatment aging method and the mechanical aging method. The core is temperature cycling plus random vibration, which is the most effective for releasing and eliminating various residual stresses. Because the temperature cycle uses a high rate of temperature change, the sensor exposes and eliminates defects as soon as possible under thermal stress. Random vibration is to simultaneously excite each frequency during the entire vibration time, and there is sufficient time to excite resonance, release the residual stress of the sensor and promote defect exposure. This method is currently only used in the screening of military sensors, because civilian load cells have not been adopted due to cost issues. Using this method for reference to the temperature cycle aging, electrical aging and low-frequency pulsation fatigue aging of the load cell has obvious effects on improving the stability of zero point, sensitivity and working reliability.

Section 4 Reliability Management of Strain Type Load Cell

The reliability management of the load cell is a series of technical activities such as planning, organization, coordination, and monitoring of the entire process of design and production, various reliability technical work, and all the development and production personnel to achieve the predetermined reliability target. From the diagram of the various links of reliability engineering shown in Figure 10.2, it is not difficult to see the importance of reliability management, which profoundly illustrates the correctness of the concept of "product reliability is designed, produced, and managed". The status and role of its reliability design, control and management in the development and production of load cells.

The relationship diagram of the various links of reliability engineering basically reflects the reliability control and management of the whole process of weighing sensor development and production. After the load cell has completed various reliability designs and produced products through reliability control in the production process, reliability growth tests and reliability verification tests should be carried out. The purpose of the reliability growth test is to expose problems or understand the potential (reliability margin) for improvement to promote the reliability growth of the load cell. The purpose of the reliability verification test is to evaluate whether the reliability design technology is effective and to verify whether the load cell reliability index requirements are met.

At present, the reliability test of the load cell mostly uses the tracking test method of counting the trouble-free working time. Although it is not perfect, it can also explain the problem. Analyze the problems and failures exposed by the reliability growth test, reliability verification test and follow-up test, find the cause of the failure, find out the internal change law of the failure, and take corresponding countermeasures to improve the reliability design and process reliability control. In these links, reliability management plays a vital and core role, which makes all links of load cell reliability operate effectively.

Section 5 Introduce the concept of reliability to improve the reliability of load cells

Under current conditions, it is not difficult to produce a high-accuracy load cell, but it is difficult to produce a high-stability and high-reliability load cell. The development and production experience of some domestic and foreign companies has proved that the reliability concept must be introduced to produce high accuracy and high reliability load cells. It is clear that the reliability design of the load cell is the load cell design that considers the reliability. Reliability design outside of load cell design. Among the links of reliability, the basic design of reliability and the reliability control of the production process are two important links that determine the reliability of the load cell.

From the definition of reliability, it can be concluded that the reliability of a load cell is the probability that the trouble-free working time exceeds the task time (determined by the type of electronic weighing instrument and working environment conditions). The greater the degree of excess, the higher the reliability. Therefore, the design of the load cell is actually the design of the fault-free working time margin. Of course, this margin is not infinite. It strives for the longest fault-free working time under the conditions of overcoming and coordinating various restrictions. . In summary, the reliability of the load cell can be improved from the following aspects.

1: Introduce the concept of reliability and clarify the reliability requirements. Introduce the reliability design method of the comparative system into the load cell design, and do a good job of the basic design of reliability, including the design of load cell block elements and accessories, the selection of load cell block element materials and heat treatment process, the selection of resistance strain gauges and components, and the resistance to environmental conditions Design, etc., to ensure the smallest fluctuations in the performance of the load cell from the structure, raw materials, and components.

2: Pay attention to process equipment and improve testing methods. In the 21st century, it can be said that the competition of load cells is mainly the competition of manufacturing technology and manufacturing process. The key technological equipment in the production process, such as the force standard machine with high and low temperature test chamber, and the intelligent temperature compensation device cannot be simplified or lacked. The production process must be advanced, scientific and reasonable, and continuously improve the automation and intelligence level of the process equipment, strictly control the production reliability, and try to reduce the influence of human factors on the reliability of the load cell from the process equipment.

3: Strengthen overall quality control and reliability management in the production process, improve the quality awareness and reliability awareness of technical and production personnel, strict process discipline, comply with test and test regulations, eliminate unreliable factors to the greatest extent, and avoid these factors Cause the reliability of the load cell to decrease.

4: Work hard to support the scientific application and integration of technology, basic technology, core technology and special technology. Adopt advanced circuit compensation and adjustment technology to continuously improve the level of compensation technology. In addition to researching and improving basic processes, the research and application of special processes should also be strengthened, especially process technologies and methods that improve stability and reliability.

5: The manufacturer cooperates closely with the user, strengthens the information transmission in the application, and continuously accumulates information such as the trouble-free working time of the load cell. Conditional manufacturers can carry out load cell reliability growth test and reliability verification test to obtain an important basis for the reliable life of the load cell.

6: Fully understand the relationship between the initial unstable period, stable period and fatigue unstable period of load cell stability and reliability. Use effective aging treatment and stability treatment process, try to pass the initial unstable period in the production process, and enter the stable period when put into use. Make the load cell truly become an internationally recognized "semi-permanent device", and the load cell with IP67 and Ip68 protection and sealing grades can work stably for more than 10 years.