Reliability is a developing science. The main driving force of the development of the industry is the urgent application demand. Nowadays, the reliability engineering design is in full swing in the understanding, just like the "Dragon", in the "Tianlong Ba Bu", the sweeping old for Xiao Yuanshan / Murong rehabilitation Swearing, "The two donors are keen on all kinds of exercises, but they turn a blind eye to the Buddhist scriptures next to them. But when martial arts reach the realm, they must solve "[K1] in the Dharma. The progress of product reliability is also the same, and the practice and Dharma need to be parallel. The Dharma of reliability is the basis of thought and mathematics, and the method of work is reliability design technology. The topic of this paper focuses on the basic mathematics of reliability and the application of mathematics to promote the design and management of product reliability.
MTBF / failure rate / reliabilityThe most common indicator of reliability is MTBF (mean time between failures). Its popular expression can use an ideal test to accurately determine the MTBF value of a batch of products, that is, the batch of products will be put into use, when all the products fail. (If the failure time of the first product is t1, the failure time of the second product is t2, and the failure time of the nth product is tn), calculate the average time of failure MTBF=(t1+t2+...+tn ) /n. For non-repairable products, use MTTF (Mean Time To Fail) instead of MTBF (Mean Time Between Failure). Since it is not repairable, such as bullets, missiles, etc., it is impossible to repair it if it is impossible to start. There is no essential difference between the two parameters calculation method, except that the MTBF has to count multiple data for one product, and the MTTF only has one product for one product. The larger the MTBF, the higher the reliability of the product.
It is also common to see the formula MTBF==1/λ, but there is an additional condition. When the life of the device under test or device is exponentially distributed, this formula is applicable. The life of the electronic component is exponentially distributed. The online article is in other It is rarely mentioned that the life of systems, mechanical structures, plastic parts, etc. is not in line with the exponential distribution. At this time MTBF≠1/λ, how to determine? The following gives a universal basic formula and a reliability failure rate relationship formula,
Where R(t) is the reliability of the device and λ(t) is the failure rate, which is a function of time. This is the two most basic formulas used in reliability assessment. If we do military products and do foreign trade with large foreign companies (especially large equipment contracts or engineering contracts), these two formulas are almost inevitable. We often think that it is useless, but the customer's request is a wooden bridge that can't be bypassed. Just like the atomic bomb/hydrogen bomb, whoever thinks it will be related to our daily life, nuclear power will be the cleanest energy that is most hopeful in the new century. The reliability indicator is also true, it is not useless, it is too much effect. We must have the courage to try to understand it and use it. It will become the star of tomorrow for product reliability evaluation and improvement.
Reliability test metricIn order to measure the reliability of the product (that is, to measure the MTBF of the product), it is necessary to take out certain samples, perform long-term running tests, find the failure time of each sample, and calculate the MTBF according to the above formula. The more the number, the more accurate the results. But the ideal test is actually impossible, because for this test, until the last sample fails, the test time required is unimaginable, all the samples are faulty, and the cost is unimaginably high. .
Therefore, in order to test the reliability, the reliability test method of HALT has appeared, and the defect is quickly manifested by strengthening the test stress; that is, the product is subjected to large environmental stress (such as high temperature 80 degrees + rocking vibration composite test condition), after the expert A lot of long-term statistics, found some ways to increase the stress, into some test items. If the product has passed the test of these items, there are still no obvious defects, which means that the reliability of the product can reach at least a certain level. After conversion, the MTBF can be calculated (because the product can pass these tests, there is no obvious defect, indicating that it has not been reached. The ultimate capacity of the product, so the corresponding MTBF is the minimum value of the product). Let the product go through a process of "one day above the sky, the ground has been a thousand years". One day under bad conditions is equivalent to January or even more in a normal environment. This way, the fault reproduction time can be greatly shortened, according to the product. Specific stress indicators are analyzed and designed for experimentation.
Reliability design process comparisonThere are two procedures for the reliability design management process. One is top-down reliability estimation and allocation method, and the other is bottom-up reliability design method. For small enterprises, the latter is generally applicable. The project is generally used by the former. If the company is developing at a rapid pace, it can gradually rely on the expectation and distribution, just like the family expects the child, each family is going to college for the child to save money, although he is only in elementary school now, although we counsel him only every day. To the low-level knowledge of the one-dimensional equation. In the same way, it is irresponsible to abandon the idea of ​​predicting and distributing purely advocating design methods. Now some scholars in other industries often make words that are "not surprisingly endless" and advocate particularly extreme thoughts, aiming at arousing the world. Note that it is their way of managing themselves, but as users and audiences, it is rational, the world is moderate, always swinging around the balance, the reliability of the past stage is biased and allocated, and today it is not necessary to overcorrect, completely negated It is also necessary.
Reliability quantitative indexIn order to quantitatively allocate, estimate, and evaluate product reliability, establishing a product reliability model is an intuitive and effective method. The reliability model includes a reliability block diagram and a reliability mathematical model. Typical reliability models for products include tandem models and parallel models (this topic is more commonly used in my EE Times blog post "Reliability Review" and rdcoo.chinardm.com blog post "Reliability Design in Medical Instruments There are some complicated models in the application, but they are all based on the basis of series and parallel. Note: The reliability diagram of the product represents the functional logic relationship between the units in the product. The product schematic diagram shows the physical relationship of each unit of the product. The two cannot be confused. For example, an oscillator consists of an inductor and a capacitor. The parallel relationship, but from the reliability relationship, as long as one of them fails, the oscillator can not work, so it is a series model.
The estimation formula of the whole machine failure rate: λs=λ0 * K1 * K2 * K3 * K4 * K5 * N; λs machine failure rate; λ0 component average failure rate (1~3)*10-5; K1 derating factor (1~10)*10-2; K2 sophisticated screening effect factor 0.1~0.5; K3 environmental factor (laboratory 0.5~1, indoor 1.1~10, land fixed 5~10, vehicle 13~30, ship 10~ 22, airborne 50~80); K4 mechanical structure factor 1.5~2.5; K5 manufacturing process factor 1.5~3.5; N components.
There must be indicators for carrying out reliability work. With specific quantitative indicators, there will be a direction for design improvement and research. There will be a phased reference guide, which is a management issue. There are two kinds of indicators, one is based on reliability standards and norms, so international cooperation and procurement of enterprises have common reliability professional language; one indicator is a self-made technical assessment index, which can also be classified into quality. Management indicators, such as the average unloaded running time of the first post-installation machine is not less than ** hours, the first-time inspection pass rate of parts processing, and so on. Everything is practical, and it has a reference value for the steady quality improvement of products. For example, the voltage load rate of the capacitor and the derating margin of the power resistor.
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