tailieunhanh - Assessing Product Reliability_15

Tham khảo tài liệu 'assessing product reliability_15', kỹ thuật - công nghệ, điện - điện tử phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | . Constant repair rate HPP exponential model These bounds are exact for the case of one or more repairable systems on test for a fixed time. They are also exact when non repairable units are on test for a fixed time and failures are replaced with new units during the course of the test. For other situations they are approximate. When there are zero failures during the test or operation time only a one-sided MTBF lower bound exists and this is given by MTBFlower T -ln The interpretation of this bound is the following if the true MTBF were any lower than MTBFlower we would have seen at least one failure during T hours of test with probability at least 1- f . Therefore we are 100 1- confident that the true MTBF is not lower than MTBFlower. Dataplot EXCEL Calculation of Confidence Intervals Dataplot A lower 100 1- Ỉ 2 confidence bound for the MTBF is given by and EXCEL 1 . calculation LET LOWER T 2 CHSPPF 1 72 2 r l of where T is the total unit or system test time and r is the total number of confidence failures. limits The upper 100 1 2 confidence bound is LET UPPER T 2 CHSPPF fỈ 2 2 r and LOWER UPPER is a 100 1- confidence interval for the true mtBf. The same calculations can be performed with EXCEL built-in functions with the commands T 2 CHIINV f 2 2 r 1 for the lower bound and T 2 CHIINV 1- 2 2 r for the upper bound. Note that the Dataplot CHSPPF function requires left tail probability inputs . f 2 for the lower bound and 1-f 2 for the upper bound while the EXCEL CHIINV function requires right tail inputs . 1-c 2 for the lower bound and f 2 for the upper bound . Example http div898 handbook apr section4 5 of 6 5 1 2006 10 42 32 AM . Constant repair rate HPP exponential model Example showing how to calculate confidence limits A system was observed for two calendar months of operation during which time it was in operation for 800 hours and had 2 failures. The MTBF estimate is 800 2 400 hours. A 90 confidence interval .