Testing the bus for auto safety
17 March 2008
Manfred Schneider takes a look at how a PXI-based system was used to test CAN and LIN network characteristics of control units to assure the reliability of bus systems in motor vehicles
Electronic control units (ECUs) in modern vehicles are all linked together via various communication interfaces. The information which each ECU needs to function correctly is received from other system components. For example, an instrument cluster can read the current speed or engine RPM from gearbox and engine ECUs. In addition, the ECU puts data about its own status on the network for reception and evaluation by other control units.
The CAN bus, in its high speed and low speed varieties, is the most common data network in modern vehicles. However, as the trend moves towards an increasing number of ECUs per vehicle the LIN bus has found its way into new vehicle designs. The LIN bus is a subsystem of the CAN network and is used to relieve the bus load of the supervising CAN network and to reduce overall cost.
During each stage of development and production (eg. prototyping, manufacturing, burn-in / run-in and reliability testing) ECUs are subjected to various functional tests. To test the functions of an ECU, it must be installed in a realistic vehicle environment to provide the input and output information it needs to perform its control tasks.
Modern vehicle ECU’s also support overall vehicle safety features on a system level. It is therefore of paramount importance that the ECU works faultlessly under all operating conditions. It is furthermore required that an ECU will not affect the overall communication in the vehicle environment or goes out of control in the event of a communications breakdown if the unit is connected to the vehicle bus.
System architecture requirements
The test system was originally developed and implemented around the specifications for high speed and low speed CAN buses, CAN transport protocols and CAN diagnosis (KWP 2000) requirements. Further developments have added the ability to test K-line functions and LIN bus systems.
Basic test needs were drawn from car manufacturers design specifications which define the interface conditions of the CAN and LIN networks in the vehicle. Detailed analysis resulted in a number of mandatory core test components and so the base configuration of the tester for checking CAN network characteristics was derived:
Oscilloscope and multimeter functions for electrical measurements
CAN interfaces for high and low speed CAN. The software must support measuring functions and also be able to generate fault protocols
CAN module for “rest bus” simulation with 32 independent real bus nodes
Programmable and expandable unit under test (UUT) power supplies
The ability to simulate line faults
Diagnostic interface to access the UUT’s internal fault memory
Hardware configuration
The system architecture is centred around a PXI stimulus and measurement module which accommodates all required instruments except the UUT power supply, CAN rest bus simulation and CAN stress assemblies for fault simulation. The system was based on the PXI bus because of its compact structure and the fact that a wide range of test instruments are already available from an existing product line (TESSY – a modular functional test system for vehicle control units). These include signal generators and I/O assemblies which are especially designed to simulate typical signals found in ECUs used for chassis, drive and equipment controllers.
The CAN instruments used in the test system are also taken from this product line. They can be fitted with up to four CAN interfaces, each with its own dedicated microprocessor. The CAN assemblies are fitted with three bus nodes, one for the comfort CAN and two for the drive CAN in the present application. The firmware of the CAN instruments is an extended version of the “standard” CAN controller firmware with additional functions to vary and measure bus communication timing and to simulate faults on the bus .
An arbitrary signal generator, multimeter and oscilloscope are provided for stimulus and measurement in the basic configuration of the test system. The drive and comfort CAN networks must be available simultaneously (to test control units with gateway functions) so it is necessary to switch the measuring instruments onto various test points using relays. The relay matrix switches terminals 15 (ignition) and 30 (Vbatt+) on the UUT and connects additional CAN nodes for the simulation of the rest bus (or bus load).
System software
The overall software architecture is designed to accommodate the user’s desire to execute individual tests in an interactive manner as well as to automatically run complex test program sequences. The GUI and test run controls are based on test sequencing software which are part of an existing product based on LabVIEW. It has been used successfully for several years in test systems for both R&D and production environments.
Practical experience gained from using the system for testing network characteristics
Since introducing the automated test bench the users have noted the following improvements regarding CAN network characteristic testing:
Reducing the test time to less than one third of that required for laboratory testing before.
The test system enables the user to record and evaluate time-synchronous runs in a timescale of under 10 ms.
By multiple vehicle bus and diagnostic functions within a single test system special measurements can be carried out which were not possible before.
Since the functional tester’s first installation in spring 2000, its functional capability has been extended to meet the technical demands of communication networks in current vehicle designs. In particular, special test routines for the CAN transport protocol and for diagnostics via CAN - based on Keyword 2000 - have been implemented. As a result of the real test benefits of the system users have decided to migrate the test philosophy even to other vehicle communication networks. This includes diagnostics via K-line.
Nowadays the K-line as defined by ISO 9141 is only used for external diagnosis rather than for ECU communication, and there is a strong trend towards replacing it with CAN bus diagnostics. However, the K-line is still present in current control units. It is used for programming parameter sets in production often at the end-of-line vehicle manufacturing.
The LIN bus communication has become an integral part of vehicle safety. The LIN controller assembly with three LIN interfaces was developed to expand the capabilites of the test system into this area too. The system’s macro library has been supplemented with a range of programmable test routines to support the LIN controller assembly.
As requirements change, the modular architecture of the network tester opens up the possibility for system upgrades covering a variety of communication interfaces. In addition to the obvious benefit of cost saving, the main thrust of this approach is in testing gateway functions between different communication networks. The ability of the PXI bus to trigger and synchronise its instruments is invaluable in establishing a common timing basis for these tests.
And last but not least, the most important conclusion drawn from using the network tester should be grounds for some serious thought among the designers and manufacturers of vehicles and control units:
Nearly 100% of all tested ECUs show defects in the implementation of bus specifications!
This alarming finding indicates an urgent need to establish the network test system as a key part of the quality management system in addition to its existing presence in R&D departments. Communication faults in the comfort area such as radio or GPS systems may only lead to annoying malfunctions. However, considering more critical applications such as the advent of drive-by-wire systems the introduction of effective test strategies becomes absolutely mandatory.
The author is Managing Director of GOEPEL electronic GmbH, in Germany
Contact Details and Archive...
Related Articles...
Most Viewed Articles...