Automotive communication networks
Part 3: FlexRay Networks
Published: 02 September, 2021
Damien concludes his three-part series with an look at FlexRay
In recent years to meet the demands of Drive wire systems as well as the addition of advanced driver assistance systems (ADAS), vehicle manufacturers in association with electronic component manufacturers investigated possible solutions for high-speed data transfer. In the previous article, we looked at CAN (Controller Area Network) Bus communication. However, with safety-critical drive-by-wire systems, a higher transfer data rate and improved error detection is required. With this in mind, a number of vehicle manufacturers, namely Volkswagen, BMW, Daimler and General Motors, became core members of the FlexRay consortium.
Physical layer
FlexRay is similar to CAN Bus as the data is transmitted over two twisted wires, which act to reduce the effects of external interference on the differential voltage between both wires. Data transfer speeds can be up to 10 times faster than CAN bus, operating at speeds up to 1 Mbits per second. Although the normal data transfer speed for CAN is 500 kbits per second, it has the capability to transmit at speeds of 1 Mbits per second.
FlexRay can support network redundancy or dual channels (see Fig.1). This ensures greater fault tolerance as well as increased bandwidth for additional data transfer. However, this additional network is rarely implemented.
Signal conditioning
To match the impedance of the network cabling, a terminating resistance of 90 to 110 Ω within the modules at either end of the network is required. This ensures signal reflections are reduced, which is an issue with such high data transfer speeds.
Bus access
CAN Bus messages are subject to arbitration to ensure messages of higher priority have access to the network first. FlexRay supports both event-triggered messaging and deterministic messaging which allows for high data rates and guarantees message delivery when required. Each node or module has a predetermined time slot to transmit its data. This is referred to as time division multiple access or TDMA. For sporadic messaging (event-driven) TDMA is not optimal so flexible time division multiple access or FTDMA is used.
FlexRay voltages
FlexRay bus voltage is similar to CAN Bus. With the bus idle or in a recessive state, the voltage present on both wires, Bus (+) and Bus (–), is 2.5 volts.
Approximate voltage levels
When a logic bit of 1 is required the Bus (+) voltage increases to 3.1 volts and the Bus (–) voltage reduces to 1.9V. This creates a differential voltage of +1.2 volts. When a logic bit of 0 is required the Bus (+) voltage reduces to 1.9 volts and the Bus (–) voltage increases to 3.1 volts. This creates a differential voltage of -1.2 volts.
Two-channel network voltage
To see the two-channel network voltage, please refer to Fig.2.
Differential voltage
To see differential voltage, please refer to Fig.3.
BMW X5 vertical dynamics control module
The E70 BMW X5 was the first production vehicle to have a FlexRay network fitted. It is implemented for the vertical dynamics control module, which controls the suspension height for each individual suspension strut.
The network is configured in a Star bus (see Fig.4). This ensures the system can still operate if a satellite unit fails. It is also an advantageous setup when long lengths of wiring are required, as normal electrical interference will only affect one leg of the network due to this configuration. A satellite unit is located in each corner of the vehicle.
- VDM – Vehicle dynamics control module.
- S1 – Satellite sensor left front.
- S2 – Satellite sensor right front.
- S3 – Satellite sensor left rear.
- S4 – Satellite sensor right rear.
- PT CAN – Powertrain CAN Bus.
A terminating resistor of 90 – 110 Ohms is fitted in each of the satellite units.
Waveforms
Due to the incredibly high data transfer speed of FlexRay, in-depth waveform analysis is difficult. However, the oscilloscope can be used to validate that a signal is present on the network and to test for an open or short with in the circuit.
The waveform as seen in Fig.5 shows a trace captured from a FlexRay system. The zero lines are set up to show the transition between a Logic 1 and Logic 0 bit due to the voltage switch for Bus (+) and Bus (–). The next waveform displayed the differential voltage between
Bus (+) and Bus (–). This is observed using a single channel test with channel 1 connected to Bus (+) and the oscilloscope scope ground connected to Bus (–).
The final waveform shows Bus (+) and
Bus (–) separated to show the mirror image of both traces.
- Automotive communication networks
Local Interconnection (LIN) Bus is a low-speed, cost-effective alternative to CAN bus. LIN gets its name from being a ‘local’ sub-system. Data transfer speeds can be up to 20,000 bits per second. It is designed for sensor/actuator level.
Data transfer speed
The waveform seen in Fig.1 shows a typical LIN trace, the time for the transmission of 1 bit is 0.11ms, this equates to a data transfer speed of 9,480 bits per second.
Network topology
The bus can contain up to 16 modules or subscribers, one master (module) and fifteen slaves (intelligent components). LIN is a linear bus topology, with communication over a single wire. Fig.2 shows a basic network topology.
The central electronics module (CEM) is the master module and communicates with the gateway module using controller area network (CAN) bus. Each of the LIN components is a sensor or actuator, with the ability to transmit and receive a LIN message. The master module is responsible for controlling the timing of the message data packet. Fig.3 shows a CEM and front wiper motor. The purple wire is the LIN bus wire and is used to control the speed and intermittent delay for the wipers.
Message structure
The waveform seen in Fig.4 shows a LIN message with the defined message structure. The ‘sync break’ is used to signal the beginning of the message. The delimiter indicates the completion of the sync break and serves to show the LIN wire is not shorted to ground. The sync field is to ensure the timing of the message is set prior to the data field being transmitted. This is not required for CAN bus, as each module on the network has its own time clock for message timing.
The message is transmitted by manipulating the voltage on the LIN data circuit. See Fig.5. 1 equals recessive bit. 2 equals dominant bit.
For accuracy of data transfer, the following conditions must be met. For a recessive bit, the LIN voltage must be greater than 80% of the battery voltage. For a dominant bit, the LIN voltage must be less than 20% of the battery voltage. See Fig.6. The slew rate or transition time between a high bit and a low bit is critical too for data transfer.
Vehicle charging systems
One particular system which has utilised LIN bus communication is the vehicle charging system. Fig.7 shows a conventional charging system layout fitted to an older vehicle.
The D+ terminal is used as an exciter current for the rotor field winding. The rotor field winding is wired in series with the charge warning light. The W terminal was used for measuring engine speed by tapping off a stator phase.
A layout of a modern system using LIN bus is shown in Fig.8. The internals of the alternator are fundamentally the same:
- Part two The good and THE GREAT
In part one, we looked at the start of the ‘diagnostic process.’ The first steps were customer questioning, confirming the fault and knowing the system and its function. These help the technician to build the ‘big picture’ necessary to repair the vehicle correctly.
In this article we will look at the next four steps.
Step 4: Gather evidence
It is easy to overlook this step as many technicians think of it as the overall ‘diagnosis.’ However, once the technician understands the system, gathering evidence will provide key information. This step is normally best carried out with the use of test equipment that does not mean the dismantling of systems and components.
Many technicians have their own favourite tools and equipment but this list can include (but not limited to)
the following:
Scan tool – It is always best practice to record the fault codes present, erase the codes, and then recheck. This means codes which reappear are still current. Remember that a fault code will only indicate a fault with a circuit or its function. It is not always the component listed in the fault code that is at fault
Oscilloscope – An oscilloscope can be used for a multitude of testing/initial measuring without being intrusive. Some oscilloscope equipment suppliers are looking at systems within high voltages hybrid/electric vehicle technology. The waveforms produced by the test equipment can be used when analysing the evidence and may indicate that a fault exists within a system. An understanding of the system being tested will be necessary to understand the information. This may even include performing sums so all those missed maths lessons at school may come back to haunt you. It may take time to become confident analysing the waveforms, so be patient
Temperature measuring equipment – This can include the use of thermal imaging cameras. Most systems that produce energy/work will also produce some heat. The temperatures produced vary from system to system. Examples include everything from engine misfires to electrical components, as well as air conditioning system components and mechanical components such as brake and hub assemblies. The possibilities are endless and results can be thought provoking.
Emission equipment – By measuring the end result, an exhaust gas analyser can show you if the engine is functioning correctly. The incorrect emissions emitted from the exhaust help indicate a system fault or a mechanical fault with the engine
Technical service bulletins – Many vehicle manufacturers produce technical service bulletins (TSBs) that are generated by a central point (usually a technical department) from the information that is gathered from their network of dealers. Some of these may be available to the independent sector either through the VM or through a third party – It’s always worth checking if these exist. They may indicate a common fault that has been reported similar to that the technician is facing. Some test equipment suppliers may provide TSBs as part of a diagnostic tool package
Software updates – Many vehicle systems are controlled by a ECU. Most vehicle manufacturers are constantly updating system software to overcome various faults/ customer concerns. Simply by updating the software can fix the vehicles problem without any other intervention of repairing a possible fault. This is where having a link to a vehicle manufacturer is vital in repairing the vehicle
Hints & tips – Most technicians will have a link or access to a vehicle repair forum where they can ask various questions on vehicle faults and may get some indication of which system components are likely to cause a vehicle fault
Functional checks – Vehicle systems are interlinked and typically share information using a vehicle network. The fault may cause another system to function incorrectly, so it is vitally important that the technician carries out a functional check to see if the reported fault has an effect on another system. By carrying out this check the technician again is building the big picture
Actuator checks – Most systems today are capable of performing actuator tests. The technician can perform various checks to components to check its operation and if the system ECU can control the component, often reducing the time to the diagnosis, by performing this task the technician can identify whether it is the control signal, wiring or component or it is sensor wiring. This function can be used in conjunction with serial data to see how the system reacts as the component functions
Serial (live) data – The technician can typically review a vehicle system serial data through a scan tool. Having live data readings to refer to can help you review the data captured. Using actuator checks and viewing the serial data can also help the technician to identify a system fault
Remember to record all the evidence gathered so it can be analysed during the next step in the diagnosis. We can’t remember everything. If the technician needs to contact a technical helpline they will ask for the actual readings obtained recoding the data gathered will help.
Step 5: Analyse the evidence
Analysing evidence gathered during the previous steps can take time. The technician needs to build the big picture from all the evidence gathered during the first few steps. You need to analyse the information gathered, and decide on what information is right and wrong.
This step may rely on experience as well as knowledge on the product. You should take your time – don’t be hurried. Time spent in the thinking stages of the diagnosis can save time later. Putting pressure on the technician can lead to errors being made. It may be necessary to ask the opinion of other technicians. If the evidence is documented it may be easier to analyse or share between others.
Step 6: Plan the test routine
After analysing the evidence gathered it’s now time to start to ‘plan’ the best way to approach to the task or tasks in hand.
The technician should plan their test routine, decide on what test equipment should they use, what results are they expecting, if the result is good or bad and which component should they test next.
Document the plan – this enables you to review decisions made at this stage in the next step. The technician may not always get it right as there may be various routes to test systems/components. The test routine may have to be revisited depending on the results gathered during testing. Documenting the test routine will provide a map. Also, don’t forget to list the stages, as this is something that could be incorporated into an invoicing structure later.
The technician should indicate on the routine what readings they expect when they carry out the system testing. This can be generated by their own knowledge/skill or the expected readings may come from vehicle information which they have already sourced. If the information is not known at the time the test routine is planned, then the test routine may highlight what information is required and what test equipment is needed. You shouldn’t be afraid to revisit the plan at any time and ask further questions on which direction the tests should take. If the plan is well documented and the technician becomes stuck at any point, they can pause the process and revisit later. Also the information can then be shared with various helplines that support workshop networks.
Step 7: System testing
The technician then follows their pre-determined plan, if it is documented they can record the results of the test(s) as they follow the routine.
Many technicians tend to go a little off-piste when they get frustrated. Having the routine documented can keep the technician on track and focused on the result. If the routine is followed and the fault cannot be found the technician may have to go back to the analysing the evidence or planning the test routine. The technician shouldn’t be scared of going back a few steps, as I said previously analysing the evidence takes practice and can be time consuming, not to be rushed.
Summing up
Remember to follow the process. It is easy to be led off track by various distractions but don’t try to short circuit the process. Some steps may take longer than first thought to accomplish than others. Some distractions may be outside of your control, and it may be necessary to educate others. Practice, practice, practice. Refine the process to fit in with your business and its practices, the business could align its estimating/cost modelling to the process, being able to charge effectively and keeping the customer informed at each stage of the process.
Coming up...
In the next article I will be looking at the next four steps which are; Step 8: Conclusion (the root cause), Step 9: Rectify the fault and Step 10: Recheck the system(s). The last article in this series will indicate the final three steps and how to fit them all together in order to become a great technician and perhaps succeed in Top Technician or Top Garage in 2018.
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