Reliability Demonstration Testing (RDT)
The aim of RDT testing is to confirm that the product’s expected life meets or exceeds the target, and it is generally carried out as part of R&D.
Factors that must be known or determined in order to design ALT/RDT tests are the product’s expected reliability at end of life, the mission profile during its life, required confidence level, and the allowed stress levels identified in HALT tests.
The tests expose the product to stress it will experience over its entire lifetime. Most common stresses for drives are temperature and temperature cycling. To reduce the time needed for testing, stress levels that exceed specified operating conditions are used.
Depending on the confidence level required and the reliability levels expected, 7-20 samples are typically needed for RDT testing. The product can be launched on the market following successful RDT testing, but testing nevertheless continues after the launch in the form of ALT testing. This follow-on ALT testing will confirm the validity of the model and may provide opportunities for total life cycle cost reduction if the tested life is too long.
At ABB complete drives can be RDT tested in a special Reliability Container where they are exposed to drastic stresses. Various reliability models are used to determine the testing time that corresponds to the stresses that the drive will see during its whole life. Typically 10 years life can be tested in a few months.
Accelerated Life Testing (ALT)
ALT testing is done to determine the product’s expected lifetime. It is very similar to RDT except that various stress levels are used and the tests are continued until failure.
The difference between the RDT test and ALT test is that after RDT we don’t know what the actual expected life is because the units are not supposed to fail. We just know that it survives x years or longer. ALT tests will give us estimate of the real life. RDT tests also assumes certain failure models and material constants. ALT testing provides us with the model and material constants.
Depending on the expected confidence and reliability levels, ALT testing typically requires 7-60 samples. Large sample sizes and a significant test time are often needed, especially if the activation energy or other coefficients used by the model are not known.