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CAN bus parameter converter of vehicles with analogue signals for tests on roller benches

"Thanks to the NI Single-Board RIO embedded system, an autonomous device has been created that is able to identify the type of CAN traffic present on the bus."

- Stefano Vianelli, EURINS

The challenge:

Provide the data from the CAN bus of the vehicles in the test phase to the roller magazine acquisition system in the absence of a dedicated hardware and software interface.

The solution:

Thanks to the NI Single-Board RIO embedded system, an autonomous device has been created that is able to identify the type of CAN traffic present on the bus, intercept and decode the chosen parameters and generate the corresponding analog signals.

A roller bench is a system through which a vehicle can be subjected to tests and measurements to define its performance, reliability, polluting emissions, vibrations and all the characteristics related to the operation, simulating more or less realistic conditions with respect to its use in the street. Some measures require a mathematical analysis to derive the contributions of the various elements in the kinematic chain, while others depend on the interaction of the vehicle with the bench measurement sensors.

The motor starting phase, very important in the dynamic analysis of the parameters linked to the combustion, is difficult to perceive using vibrational measures and in any case requires specific calibrations that at most are able to identify the already ignited engine moment.

At the JRC (Joint Research Center) of Ispra (VA), the Joint Research Center of the European Union where, among others, the VELA laboratories are present for the study of all types of vehicles on the roads. EURINS has been commissioned to provide a solution to overcome this "historical" approach in three already operating roller tables; drawing on its experience in acquiring data from electronic control units in vehicle-based applications, EURINS has developed a device that allows the bank to "measure" directly the parameters available in the CAN bus or in the network connecting the control units in the vehicle, with the advantage to have in real time the same quantities that the vehicle is using for its operation.

Two advantages are immediately obvious: it is not necessary to replicate sensors already present in the vehicle and it is sufficient to power the CAN bus (ie "key") to have the data available even before the engine is started and therefore also during the "cranking" phase".

It is intuitive to extend the tests also for vehicles that do not base their operation exclusively on the thermal engine, such as for example the hybrid and electric ones, with which the "standard" roller bench would be struggling to give the right dimension to the measurements made without using information on how the control units are operating.

Not being equipped with hardware and software interfaces dedicated to the management of messages and protocols on CAN bus, the systems of acquisition of roller benches have been connected each to a stand-alone device, developed by EURINS, based on the NI Single-Board RIO appropriately installed inside a container equipped with power supply, wiring and connectors; this card, programmable with NI LabVIEW Real Time and NI LabVIEWFPGA, is distinguished by the availability, among others, of the following resources:

- a CAN High Speed port

- four analog outputs ± 10 V 16 bit

- an RS232 serial port.

The software has been structured in processes able to autonomously manage the data flow and the generation of the related analog signals with the possibility to choose between the following protocols and structures to be decoded on CAN bus:

- OBD (On Board Diagnosis), typically for cars and light trucks

- FMS (Fleet Management System), typically for trucks, buses and coaches

- DBC (message description file) for CAN flows not bound to any protocol.

For the OBD and FMS protocols the parameters (engine revolutions, percentage of accelerator, wheel speed and many others) are identifiable according to standard rules within the CAN data flows and therefore automatically combinable with the analog outputs that have been chosen.

In the case of the DBC description file, on the other hand, it is necessary to specify the details of the parameters to be decoded and combined with the analog outputs.

When the system is powered starts the search for a possible message traffic on the CAN bus that is consistent with the structure of one of the protocols, trying to "stimulate" the CAN bus with messages of commencement of communication alternating the different formats and the potentially usable baud rates , then analyzing any answers to define the "rules" with which the specific vehicle can communicate.

Once the communication has been established, the selected parameters are decoded and assigned to the dedicated analogue outputs, for example the motor revolutions acquired between 0 and 10000 rpm drive the generation of a signal between 0 and 10 V.

To verify the operation of the system it is possible to connect the RS232 serial port to the computer where, thanks to a program like Hyper Terminal, it is possible to see the sequence of diagnostic messages that indicate the current configuration and the values both decoded and generated.

The system is able to recognize the presence in the same CAN bus of another active node in the management of the same protocol, typically another independent data acquisition program. In this case the system automatically switches to passive mode that allows the other program to work by intercepting and using the same stream of messages where it is present, and then returning to autonomous mode.

The analogue outputs of the system are connected to the same number of analogue inputs of the roller table, which can be configured according to scales corresponding to the measured analogue signals and the quantities required. The bank can thus acquire in real time a selection of the parameters available on the CAN bus of the vehicle in sync with the measurements made using its own sensors.

RESULTS

The three roller banks at the JRC can acquire a selection of the parameters present in the CAN bus quickly and reliably. Relatively at the motor revolutions, no calibration phase is required and measurements are possible both before and during start-up of the engine.

About the author:
Stefano Vianelli
EURINS