Fault diagnosis and model debugging

When building and testing models of physical systems that interact with controllers, it is important to build in verification / diagnostics systems that check the communication between the controllers and the plant is valid.

For example: in the case of an engine model, the response of the actuation of any device, whether it be a valve or solenoid, etc. should be compared to the expected response, to confirm the correct operation of the part or flag up communication errors including unit issues. At the end of the day, one rarely uses open loop control in the real world.

So here’s the recommendation: if you are driving a plant model via a controller of any sort, provide a feedback signal that measures the response of the system and compares it to the control signal that is sent to the actuator. Discrepancies between the supplied signal and the measured response could point to physical and/or controller issues including parameterisation and signal routing.

Example:

Figure 1: Throttle actuation system.
Figure 1: Throttle actuation system.

The system shown in Figure 1 is that of a throttle demand/control system as found on drive-by-wire vehicles. This one in particular uses a coil and magnet actuator, as found on Magneti Marelli solutions fitted to a range of Volvo, Maserati and Lamborghini vehicles.

How this model works:

The model functions as follows: The combiTimeTable (1) outputs a desired accelerator pedal position which is transformed to a physical quantity via the pedalPosition actuator (2). The position of the accelerator pedal is then converted to a rotational position, which is sensed by a twin-circuit sliding-contact potentiometer (4). From here two independent position signals are sent to the throttle controller (5). These are normally voltages that are mapped to an equivalent pedal position. These two signals are continuously compared to make sure that there are no anomalies between the two, which is indicative of a system or sensor malfunction.

The throttle controller (5) then sends a signal to the throttle body actuator (6) and this will then actuate the throttle spindle and plate (7). The rotational range of the spindle and plate are constrained by the return spring and stop mechanism (8). These determine the minimum and maximum plate angle, as well as the no-power/fast idle setting for the throttle plate.

The spindle position is sensed by the throttle position sensors (TPS) (9). Again, these are two independent sensors that produce two position signals that are sent to the throttle controller (5). These two signals are then compared to the mapped pedal demand to make sure that the desired position has been reached and that there are no electrical anomalies.

The whole system is powered by a simplified 5V power source (10). Open circuit faults are triggered by the boolean tables (12) which create open circuits within the electrical network. These are used for fault injection and controller response observations.

A simpler version of what is shown in figure 1 could be entirely signal based. However, it is still worth comparing the actuator and sensed signals to make sure they are in agreement. Signals may be put onto busses with incorrect units and bus to sub-bus connections also made incorrectly without a meticulous approach to the modelling.

The end product of this approach is a model that more closely resembles the real system and an extra help, built into our models, that aids debugging of them.

Written by: Alessandro Picarelli – Engineering Director

Please get in touch if you have any questions or have got a topic in mind that you would like us to write about. You can submit your questions / topics via: Tech Blog Questions / Topic Suggestion.

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