Research On Cloud Model And Its Application In System Reliability Evaluation

Reliability has been paid attention to from last century in 1930's to 1940's. The main reason is that many military products broken down frequently. So people start to think about the reliability of products. From then on, many people start to research the methods which can be used for reliability evaluation. Many countries start to build their own reliability research organization, and reliability evaluation has been used in many fields in our real life. It promotes the quality of products, and ensures the safety of our life.Generally speaking, the development of reliability can be divided into three stages. First stage which is from 1930's to 1940's in last century. Many problems arise from military products. This brings reliability evaluation to our vision. Second stage which is from 1950's to 1960's. The accomplishment in this stage can be classified in to three categories. First, researchers combine the probability theory and mathematical statistics with reliability. They also conducted in-depth research about some reliability evaluation problems. Second, many countries have build their reliability research organizations. It makes the reliability research to be very professional. The reliability research in this stage is focused on the civil products. It brings great profit to our society. Third, reliability has been used in many fields, such as electrical science, chemical engineering, metallurgy, communication, medical treatment, etc. Third stage which is from 1970's, the accomplishment in this stage can be classified in to two categories. First, many countries have published their own criterion of reliability. There are also many other international criterions of reliability. Second, Many reliability evaluation methods appeared in this stage, which include Fault Tree Analysis (FTA), Event Tree Analysis (ETA), Failure Modes Effects and Criticality Analysis (FMECA), etc. These methods have been used in our life successfully.Though these methods presented above are very practical, there still many problems needed to be paid attention to. For instances, these traditional methods work very well in special working conditions, but it is not suitable when the working conditions are described by qualitative language values. Due to this, reliability evaluation method based on cloud model is proposed by Song Yunjun, Li Deyi etc. In our real life, we always use many words which contain uncertainty such as "about 20 kilometers", "young man", etc. In order to better description the uncertainty, Cloud model is proposed by Li Deyi in 1995, which can be used for the transformation between the qualitative language values and quantitative values. Song Yuanjun, Li Deyi, etc. proposed the reliability evaluation method based on cloud model. The basic flow of reliability evaluation using cloud model can be divided into four steps. First, construct the working condition cloud and suitability cloud. Second, take different working conditions which are produced by working condition cloud as parameters input into the suitability cloud model, and then compute the suitability degree and MTTF of products under different working conditions. Third, compute the reliability of products under different working conditions. Forth, Many times of stochastic implements reflect the reliability of products. There are still two points need further research:(1) The reliability evaluation method presented above is only used for single product and component, what about some other complex systems, such as series systems, parallel systems?(2) According to our experiences, different environmental factors may have different effect on reliability; also same environmental factors may have different effect on the same products. How to reflect this when using the reliability evaluation method based on cloud model?In order to solve these two problems, we proposed the following two methods:reliability evaluation method for series-parallel systems based on cloud model, weighted cloud model. The main idea is as follows:(1) Reliability evaluation method for series-parallel system. Because traditional reliability evaluation method for series-parallel system works very well under specific working conditions or constant working conditions. Considering that each time of stochastic implement based on cloud model can be viewed as under constant or specific working conditions. So according to the structure of system, the reliability of system can be obtained by many times of stochastic implements which reliability is obtained by the traditional reliability evaluation method. In fact, this method is a hybrid method based on cloud model and traditional reliability evaluation. (2) Weighted cloud model. In order to describe different effects caused by the different environmental factors, different weight values are assigned by to different environmental factors. The environmental factors which have big weighted values may have big effect on the reliability of products; otherwise, the environmental factors may have small effects on the reliability. This makes the reliability evaluation results closer to the real situation.Experimental results prove that these two methods are very practical and useful. It may have border and potential applications in our real life.