Reliability engineering is an index that characterizes the trouble-free working ability of a product (system, component, device, etc.). It is one of the important internal attributes of the product and one of the important indexes to measure the quality of the product.

Reliability is an emerging discipline that suppresses product failures. It covers a wide range, integrating systems engineering, management engineering, value engineering, ergonomics, electronic computer technology, product testing technology, and probability, statistics, operations research, physics, etc. Applied science of the results of various disciplines.

Reliability engineering originated in the military field. After more than half a century of rapid development, it has now become a comprehensive discipline with a wide range of coverage. With the development of science and technology, users have higher and higher requirements for civilian products, not only requiring low prices, complete functions, but also requiring products to be safe, reliable, and durable. Therefore, products use reliability prediction technology to indicate product reliability indicators, which will help enhance their competitiveness and allow users to buy with confidence. Therefore, reliability research plays an important role for modern enterprises. It can be said that reliability has been extended to all aspects of our lives and production.

The history of reliability

The 1940s is considered to be the budding period of reliability. By the middle of the 20th century, it was an important period for the rise and formation of reliability.

In order to solve the reliability problems of electronic equipment and complex missile systems, the United States has launched an organized reliability study. Meanwhile, the most influential event in the field of reliability was the Electronic Equipment Reliability Advisory Group (AGREE) established in 1952, which was established by the U.S. Department of Defense and was composed of the military, industrial and academic fields. An organization that has promoted the reliability design, testing, and management procedures and methods, and determined the development direction of reliability engineering in the United States.

The AGREE organization began to formulate and implement a comprehensive reliability plan from design, test, production to delivery, storage and use in 1955, and published a research report on "Reliability of Military Electronic Equipment" in 1957, which comprehensively elaborated on 9 aspects. The procedures and methods of reliability design, testing, and management have become the foundation documents for reliability development. The establishment of this organization and the appearance of this report also marked an important milestone in the development of the reliability discipline. At this time, it has become a truly independent discipline.

The early 1940s to the end of the 1960s were the main period for the development of structural reliability theory; the 1960s to the 1980s were an era when structural reliability theory was developed and mature. Research on sexual analysis methods is becoming more active. Many scholars have applied the ideas of artificial intelligence, stochastic simulation, psychology, cognitive engineering, neural networks, information theory, catastrophe theory, fuzzy set theory and other disciplines to the analysis of human reliability. It includes the human reliability psychological model, the comprehensive cognitive model of human reliability analysis, the human fuzzy reliability model, the dynamic reliability technology of the human error rate evaluation of the human-machine system, and the computer-assisted human reliability analysis.

Reliability is also widely used in power systems. Current researches almost involve power system generation, transmission, and distribution. Reliability analysis is gradually becoming an important auxiliary tool for power system planning and decision-making.

In the field of electronics, most of the existing reliability mathematical models and research methods are produced and developed based on electronic products as the initial object. Therefore, the current reliability research on electronic products is no matter from the reliability modeling theory and reliability design. Methods, failure mechanism analysis, reliability test techniques and data statistics methods have all become mature. In addition, reliability also plays an irreplaceable role in the fields of machinery, automobiles, and electricity.

Relevant definitions and concepts of reliability

1. Reliability

It is an index that characterizes the trouble-free performance of a product (system, component, device, etc.), an important internal attribute of the product, and one of the important indicators for measuring product quality. Reliability is the ability of a product to ensure the completion of specified functions throughout its life cycle. It is an important indicator of product quality and a part of product life. According to my country's national standard GB3187-1982 (Reliability Basic Terms and Definitions) The definition of reliability is "the ability of a product to complete a specified function under specified conditions and within a specified time." Reliability is the ability to complete specified functions during the product life cycle. The basic concepts of reliability are: reliability, failure probability, failure (failure) rate, failure probability density, life expectancy and other representations and quantitative indicators, as well as their interrelationships, they represent the basic content of product reliability.

2. Reliability R(t)

It is an index for the product to complete the specified function in the life cycle. According to my country's national standard, the definition of reliability is "the probability of completing the specified work within the specified time under specified conditions." That is, the reliability of the product within a specified time under specified conditions.

The reliability function can be expressed as a function of time t, which can be expressed as

R(t)=P(T>t)

Among them, t is the specified time, and T represents the life of the product.

From the definition of reliability, R(t) describes the probability that the product will be intact within (0, t) time, and R(0)=1, R(+∞)=0.

3. Probability of failure (failure)

F(t)=P(T>t)

F(t)=1-R(t) or F(t)+ R(t)=1

Among them, t is the specified time, and T represents the life of the product

4. Failure probability density function

Equal to the change in the actual failure probability function of the unit.

Ft=[F(t+dt)-F(t)]/dt

5. Failure rate

It refers to the product that has not failed until a certain time, after that time, the probability of failure in a unit of time. Generally denoted as λ, it is also a function of time t, so it is also denoted as λ(t), which is called failure rate function, sometimes also called failure rate function or risk function.

Λ(t)=dr/Nsdt

Where λ(t)——instantaneous failure rate;

dr-the increment of the number of failures

Ns-the number of remaining products

dt-time increment

6. Reliability life

Mean time between failures (MTBF) A basic reliability parameter of repairable products. The measurement method is: the ratio of the total number of product life units to the total number of failures under specified conditions and within a specified period.

Mean time to failures (MTTF) A basic reliability parameter for non-repairable products. The measurement method is: the ratio of the total number of product life units to the total number of faulty products within the specified conditions and specified period.

7. Reliable life

The working time corresponding to the given reliability is called the reliability life. Suppose the reliability of the product is R(t), and the time t when the reliability is equal to the specified value r is the reliable life

8. Median life

The median life refers to the life when the reliability is 0.5, that is, the value of t(0.5), which is expressed as

t(0.5)=0.63915(1/λ)

9. Characteristic life

That is, the corresponding life when R(t)=1/e=0.368, when the failure law obeys the exponential distribution, the characteristic life is the average life θ

t=1/λ=θ

10. Storage life

The length of time the product can still meet the quality requirements when stored under the specified conditions. After the product leaves the factory, it does not work and is stored under specified conditions. The product also has a non-working accidental failure rate, which is generally much smaller than the working failure rate, but the reliability of the stored product is also declining. Therefore, storage life is a measure of product storage reliability.

Reliability design

Reliability requirements are the basis for design, analysis, testing and acceptance. Reliability design requirements can be divided into: qualitative requirements; quantitative requirements.

Qualitative requirements include: compliance with general reliability design and guidelines, etc.

The quantitative requirements include: use and contract parameters and indicators.

Use parameters and indicators: a measurement directly related to product integrity, task success, maintenance manpower and support resource costs. The measurement value is called index (target value and threshold value). Contract parameters and indicators: The requirements of the ordering party are expressed in the contract and are the parameters that the contracting party can control during the development and production process. The measurement value is called contract index (specified value and minimum acceptable value).

Purpose: By formulating and implementing model reliability design criteria, a series of design requirements that help ensure and improve product reliability are designed into products.

Basis for formulating reliability design criteria

(1) Model "Project Demonstration Report", "General Requirements for Development" and the reliability design requirements specified in the development contract (including work instructions);

(2) Relevant content such as reliability design criteria proposed in relevant domestic and foreign codes, standards and manuals;

(3) Relevant clauses in the reliability design criteria formulated and implemented in similar models;

(4) Through research, understand the users' requirements for product reliability in use, and organize and transform them into reliability design guidelines;

(5) The reliability design experience accumulated by the research and development unit and the lessons learned from the failure.

Reliability design criteria by technology classification 1. Simplified design 2. Redundant design 3. Thermal design 4. Environmental protection design 5. Anti-shock, vibration and noise design 6. Stability design 7. Installation design 8. Hydraulic parts design 9. Selection of raw materials, parts and components 10. Packaging, storage, handling and transportation design.

Reliability test

1. Reliability growth test:

The reliability growth test is a separately arranged reliability work item in the product development stage, as a component of the engineering development stage. Ensure that the reliability of the product before entering batch production reaches the expected goal. The reliability growth test is usually arranged after the engineering development is basically completed and before the reliability qualification test. Reliability growth test is a special test with objectives, plans and growth models. The reliability growth test consumes a huge amount of resources. The total test time is usually 5 to 25 times the expected MTBF target value of the product. Therefore, not every product is suitable for the growth test.

The general method of reliability growth test is to formulate growth targets, determine the growth model, discover product failures through experiments, and improve the design based on failure analysis. Such a continuous trial and error improvement process

2. Reliability Development Test (RDT):

(1) Purpose By imposing appropriate environmental stress and working load on the product, looking for design defects in the product, so as to improve the design and increase the inherent reliability level of the product.

(2) Method testing, analysis and improvement.

(3) Type reliability enhancement test (RET), highly accelerated life test (HALT), reliability growth test.

(4) Applicable timing Reliability research and development test should be carried out as early as possible in the development stage of the product, and the inherent reliability level of the product should be improved through the process of testing, analysis and improvement.

The purpose of the reliability verification test is to confirm whether the product meets the quantitative reliability requirements specified in the contract.

3. Reliability qualification test:

In order to verify whether the equipment design meets the specified reliability requirements, the application can represent the equipment with the approved technical status, and perform the reliability qualification test under the specified environmental test conditions. If there are no other regulations, at least two devices shall be used to carry out the qualification test of the timed censoring test program. The statistical test plan should be the plan in GB5080 specified or agreed by the ordering party

4. Reliability acceptance test:

In order to determine whether the produced equipment meets the specified reliability requirements, the equipment should be selected from each production batch according to the specified sampling principle, and the reliability acceptance test should be carried out under the same environmental test conditions as the reliability qualification test. Should be able to represent the characteristics of the batch to which it belongs. The statistical test plan of the acceptance test should be the sequential censoring test plan or the timed censoring test plan given in GB5080.

Classification of verification test (statistical test) schemes

Exponential life type: timing truncation, sequential truncation, fixed number, etc.

Success-or-failure: one-time sampling, sequential censoring, etc.

EMC test

As electronic products increasingly use low-power, high-speed, and highly integrated LSI circuits, these systems are more susceptible to electromagnetic interference than ever before. At the same time, the wide application of high-power equipment, mobile communications and wireless paging, etc., has greatly increased the source of electromagnetic disturbance. Therefore, we should improve the anti-interference ability of the product itself, that is, the product must have a certain electromagnetic environment The ability to work normally. The EMC test of certain products is mandatory by the state. Under normal circumstances, EMC needs to test the following items:

Conducted emission

Radiation emission

Electrostatic immunity test;

Electrical fast pulse train immunity test;

Surge immunity test;

RF radiation immunity;

Conduction immunity:

Power drop immunity;

Power frequency magnetic field immunity;

Power line contact

Power line induction;

Reliability engineering technology has gone through 45 years from its birth to the present. Through the above introduction, we can see that its development trajectory is: born in the field of defense science and technology in the United States, and then radiated to its industrial fields; advanced capitalist countries such as Britain, France, Germany, and Japan first introduced their own defense and industrial fields; my country In the mid-1980s, it began with the translation of US military standards and first applied to weapon systems. In the mid to late 1990s, my country's rapidly developing private enterprises began to gradually introduce environmental testing work and made great progress.

Reliability is a very important supporting application subject. We can expect that with the continuous improvement of the level of science and technology, reliability will have greater development.

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