When designing an algorithm that is to be implemented in a product that already has existing diagnostic routines, such as the Scania engine, the new algorithm has to be compatible with what already exists. Therefore, it is essential to analyse the existing diagnostic routine to identify errors that are not detected today.
Generally, all of the existing diagnostics follow the same approach. The algorithms monitor a signal, and when it detects what it has been designed to find, it stores the value in the short-term memory. The algorithm is active over a predetermined time, and then it starts over. The predetermined time is called the loop time. After every loop, the detected entity is treated and compared with a threshold. The treatment varies from algorithm to algorithm, but usually, it is the mean, standard deviation, percentage, or total sum of the detected entity.
If the treated detected entity is above the threshold, a DTC is triggered. If the treated
detected entity is below the threshold, multiple courses of action are standard. The course of action can be to disregard the number that is compared with the threshold (i.e. the number is deleted / not used in any function / not saved), save the number in the long-term memory (hereafter referred to as operational data), or use it for calibration.
If a DTC is triggered, it comes with a severity tag. [23] Some errors do not pose a hazard risk. Therefore, the truck can continue operation even though there is a fault present. In those cases, the fault is repaired during the next workshop visit. Some errors require immediate attention. It is because the EMS cannot guarantee that legislative requirements are met, or that engine damage is possible when these types of DTC are active. In those cases, the truck operator is informed to visit the workshop as soon as possible. The last kind
of DTCs is the most severe. If those errors are detected, the errors pose a risk to the safety of the truck operator. This can be the case if sensors within the steering wheel or brake system, for example, are broken. In those cases, the truck is put under a speed limit or has to request towing.
In the cases where data is stored as operational data, the information can at all times be accessed by workshop personnel. Furthermore, during workshop visits, some of the
operational data is uploaded to a Scania server which enables the R&D engineers at Scania to research the data for development purposes. The information from the operational data also fulfils another role, namely the data can be examined by the workshop personnel if there is a truck with an unspecified DTC for fault isolation.
2.5.1 Exhaust Back Pressure Sensor
There is one already existing diagnosis identified as relevant to the exhaust back pressure sensor study. The diagnosis algorithm triggers a DTC if any sample, in ’H’ different ‘C’ ms loops contains any values out of the voltage range (i.e., out of the voltage range, as explained in 2.1.3) within ‘I’ sec. However, if a DTC is triggered, but no sample is out of range for ’J’ sec, then the DTC is invalidated. When invalidating the DTC, the information is saved in the operational data.
If the number of past DTCs in the operational data is high, it indicates the occurrence of IEs.
However, the current diagnoses leave a window for specific IE data to go undetected.
Furthermore, the current diagnosis does not capture the magnitude of the signal deviance.
Consequently, the subsystem impact due to IEs is unknown. See Figure 10 for a visualization of the IEs whose full extent is not captured. [24]
Figure 10. Illustration of the type of IE who’s full extent is not captured with today’s existing diagnostic routine for the exhaust manifold pressure sensor.
2.5.2 High Temperature Sensor
For the HTS, there is one already existing diagnosis identified. The current diagnosis is based on the sensor's self-diagnostics. The self-diagnostics in the sensor can detect multiple types of internal errors such as overflow of data processing, the common-mode voltage of probe out of range, and insulation resistance lower than the limit. If any errors are detected, the SENT signal going from the sensor to the ECU contains the error code 'K' instead of the temperature. If 'K' is sent consecutively for ’L’ sec, a DTC is triggered. Furthermore, if no 'K'
code has been sent from the sensor for ‘L’ sec, the DTC is invalidated. See Figure 11 for a visualization of the IEs that are not detected.
Figure 11. Illustration of the type of IE who’s full extent is not captured with today’s existing diagnostic routine for the HTS.
The sensor's internal software can differentiate different types of errors, even though the same fault code is transmitted for all errors (in the fast channel). The kind of error causing the fault code is communicated through the slow channel; the fast and slow channel is explained in 2.2.2.
However, in this master thesis, only the information in the fast channel will be used by the developed algorithms. Only the fast channel is used because only the sensor's operational status is of interest because the action taken at the workshop is the same for all
malfunctioning sensors; the workshop operators exchange the entire sensor. Moreover, if all types of errors were treated separately, it would require extensive, unnecessary fault code management and memory space. Furthermore, suppose that the root cause of the sensor malfunction is of interest, for example, for development purposes, in that case, Scania can examine the returned malfunctioning sensors in a lab at Scania or the supplier. [25]
Moreover, the information in the slow channel is not always reliable. The slow channel is not reliable because of the informational broadcasting priority order when two errors co-occur.
For example, suppose that probes ‘X’ and ‘Y’ are broken simultaneously. In that case, only the error that probe ‘X’ is suffering from is transmitted in the slow channel because probe ‘X’
is prioritized over probe ‘Y’. However, in the fast channel, it is communicated that both are indeed broken. It is best not to involve the information in the slow channel because it can result in double and confusing DTCs.
When a SENT message is missing, different values are used instead depending on which probe whose value is missing. Sometimes the measured values from other probes are used.
Sometimes a modelled value is used.
Usually, the modelled value is based on the previous, fault-free sample. However, to not overcomplicate things, in this master thesis, it is assumed that the value used in the EMS when a SENT message is missing is equal to the last fault-free sample. If the SENT messages are missing for more than ‘L’ sec, a DTC is triggered. [26]
2.5.3 NOx Sensor
The NOx sensors used in Scania have extensive self-diagnostics capabilities. For example, if any internal parameters, such as power supply, various currents, voltages, are out of range, they are handled by the NOx sensor's internal diagnostics. If an error is persistent enough, an FMI is triggered. When Scania's software detects an FMI, a DTC is triggered. If a fault is detected but is not severe enough, OBD can degrade the signal status instead of triggering an FMI.
If the CAN bus messages from the NOx sensor are absent, a DTC is triggered within ‘M’ ms.
If there is a signal status degradation because of an electrical error, a DTC is activated within
‘M’ ms. If the measured NOx and O2content are out of range, a DTC is triggered within ‘N’
sec. If there are plausibility errors, the signal status is lowered depending on the type of error and how long it takes to find the error. Once the error is encountered, a DTC is triggered within ‘M’ ms. Whenever a deviation resulting in a DTC is found, the measured values from the NOx sensor are not used in any EMS functions. In those cases, the measured values from the NOx sensor are only used to invalidate the DTC.
Moreover, if the signal status is degraded, there are two modes: not perfect and not OK.
During not perfect, the upper layer diagnostics is inhibited, and some functions are not run or run a bit differently, resulting in subsystem impact. During not OK, the upper layer
diagnostics is hindered, and no functions using the NOx sensor signal are used, resulting in subsystem impact.
The signal status of the NOx sensor can also be degraded because of natural reasons, not only because of errors within the sensor. For example, rapid changes in engine load may cause a considerable pressure change in the exhaust gas after-treatment system because of the abrupt change in the exhaust gas's mass flow. The pressure changes may cause the currents in the amperometric NOx sensor elements to go from static to oscillation, which results in degradation of the signal status.