pdf download - Software and Computer Technology - TU Delft

State-of-the-Practice
Fault diagnosis at PMS
2.4 Optimal Fault Diagnosis
where a system is located, and start the search for the malfunctioning FRU. Some service engineers
are familiar with checking the log, and start their search there. If the service engineer recognizes
the appropriate messages, the TBCB, CRCB and the Collimator are the starting point of the search.
From experience the service engineer knows the fuses of these components are the first suspects. So,
these are the first components that are checked. Then the TBCB, CRCB and Collimator themselves
are examined. This can be done, because of various status LEDs, that give an indication of their
health. If none of these seems to be the cause, all other components are sequentially checked
(looking at the LEDs, cabling, etc.) for inconsistencies. Eventually, the service engineer will find
out that CableB is not connected well or is broken. The latter can be detected by measurement.
2.3.1 Drawbacks
The described process above shows that finding a simple broken cable takes much time. In the real
case, there are even more cables, fuses and components. All of them must be checked, and this is
time consuming. Interviews with experts and service engineers show that it takes approximately one
hour to diagnose the system. If one of the more complex components (the components depicted on
the right of Figure 2.2) are broken, and LEDs do not indicate malfunctioning, the identification of
these as the wrongdoers takes much more time. In these situations, developers of the power supply
example need to help, because they know more about the system.
2.4 Optimal Fault Diagnosis
This section defines and clarifies the problem that this thesis addresses precisely. The former described
the current approach to fault diagnosis at PMS. A better diagnostic approach can only be
suggested if it is known which items make a fault diagnosis process good. If these items are known,
it is possible to evaluate the current approach, and evaluate alternative approaches. Section 2.4.1
introduces the items that an ideal fault diagnosis would have. Section 2.4.2 presents the evaluation
using the items of an ideal fault diagnosis process as criteria. The final subsection shows what items
could possibly be introduced, or improved, in a new approach to fault diagnosis in order to achieve
higher dependability.
2.4.1 Ideal Fault Diagnosis
The ideal approach to fault diagnosis utilizes as much information as possible in the search for root
causes of failures, in a way that optimizes dependable operation at minimum costs and risks. This
applies to PMS as well as to all companies constructing embedded systems. In order to address
this ideal approach as much as possible, items are identified that make a fault diagnosis approach
’perfect’. These items can be used to evaluate diagnostic approaches. Dash and Venkatasubramanian
list some characteristics that a diagnostic system should ideally posses [9]. With this list as
a starting point, and by interviews with PMS employees, it is derived that the following item are
present in an ideal approach.
1. Accuracy. Accuracy is the extent to which a diagnosis produced by the diagnostic process
agrees with reality. If a diagnosis is not accurate (inaccurate) there can be two situations:
15