Test bench concept cuts changeover time
Some of today’s luxury cars are fitted with more than 100 electronic control units (ECUs). They communicate via data buses with one another and with the numerous sensors distributed throughout the vehicle. The increasing complexity of the components also makes greater demands on product testing.
Before it can be released, every ECU has to pass a comprehensive series of functional, electrical and mechanical tests. For example, the devices are subjected to thermal cycling and vibration in an endurance test lasting several months, as well as undergoing spray testing with salt and water. These environmental and life tests can only produce meaningful results if the test bench provides full simulation of the electrical and electronic environment of the control units in the vehicle. Simulation of all analog and digital signals is essential, as is the so-called residual bus simulation of messages from other control units in the vehicle network.
The problem is that no two vehicle environments are alike. Test bench changeover is not only required when control units are to be tested for different applications, but also if a particular unit is to be used in another vehicle model. The time needed for adapting the test systems has increased enormously in recent years on account of the ever more complex nature of vehicle electronics. The time frames available have however remained more or less the same. There is only one possible answer to this dilemma: Namely to shorten the test bench changeover times.
From eight months down to just a few weeks
This has been achieved by the Active & Passive Safety Technology division of tier one supplier in cooperation with Smart Testsolutions. Whereas the adaptation of test benches used to take up to eight months, it can now be completed in three to four weeks. The tremendous time saving was made possible by the development of a new generation of test benches offering the necessary flexibility and easy configuration thanks to the use of a lightweight scripting language (Lua) and special drivers.
One of the factors making the test bench so flexible is its modular design. A test rig is made up of six subracks with one channel per subrack. This means that six control units can be tested at the same time. Other than sharing the PC for starting the test and for data visualisation and having the same power supply, the six units operate completely independently of one another.
Each subrack is a separate functional unit with an internal ARM9-based process computer, running a Linux operating system with a realtime patch. This has proven to be extremely reliable, even in endurance tests lasting more than 5,000 hours. The Smart “MCM Process Realtime” application with integrated Lua script interpreter runs on the realtime-compatible operating system. It is therefore possible to create test sequences with the relatively simple Lua scripting language – a platform-independent interpreter language -, which does not require a compiler but is rather translated into machine language at runtime.
The process computer is also provided with an integrated buffer memory for storage of all measurement data. Once the test parameters have been set, the test procedure can thus be performed independently of an external computer.
Independent subracks offer several advantages. For example, a problem with one subrack does not lead to the loss of a complete test run for all six control units. “All we then have to do is to replace the one subrack, and there is no disruption of the other tests in progress,” says Stefan Siefert-Gäde, who is responsible for the Test Equipment of the Active & Passive Safety Technology division at ZF.
Subrack configuration away from the test bench
The fact that the individual subracks can be simply removed from the test bench and then configured saves yet more time. For this purpose, the modules are fitted in a mobile desktop enclosure which can be connected to a PC separate from the test bench. This enables the test engineers to configure the subracks for the next planned test while the current test is still in progress on the test bench. Opon completion of this test, all that has to be done is to exchange the subracks and connect up the new units under test.
Being in effect a separate, independent test bench, each subrack can be configured and also operated away from the test rig. They can therefore be used to accompany the development process, and the scripts written at this stage can later be directly employed in testing. When actually working on the test bench, this independent operation permits the parallel and asynchronous execution of different test scripts for each unit under test.
Separation of communication and measuring equipment adds to flexibility
A further step towards greater flexibility is the strict separation of the communication level from the test controller together with the associated measurement hardware. “The test sequences for a particular control unit and the measuring equipment employed tend not to differ much from one vehicle manufacturer to another,” explains Siefert-Gäde. “But communication certainly does.”
Upon request from ZF, Smart Testsolutions implemented separation by integrating a VN8900 modular communication interface from Vector Informatik with a VN8970 module into each subrack. The network interface supports bus systems such as CAN, FlexRay, LIN and K-Line and, like the measurement equipment, is an embedded system which can be configured from a PC via USB or Ethernet. It handles all communication with the control unit and communicates with the test controller from Smart Testsolutions via LAN. “The reason for choosing the Vector solution was that our Development departments work with the Vector simulation tool CANoe,” says Siefert-Gäde. Thus, test developers can incorporate the CANoe residual bus simulations from the Development departments into their tests with very little modification.
Originally, the LAN connection between the communication box and process computer used the Ethernet protocol UDP. This protocol transmits data packets but does not check that they have been transmitted in full and is therefore somewhat unreliable. Messages not transmitted on account of an electrical fault or EMC interference are irretrievably lost. ZF therefore requested Smart Testsolutions to further develop their test controller software MCM Process Realtime to support the reliable TCP/IP protocol.
The latest version of the software also features a whole range of extra new functions, many of which are the result of the close cooperation between the two partners in developing the test benches. These include a further CAN interface which permits the connection of additional measurement or simulation assemblies without having to give up the separation of communication and measuring equipment. Like the existing simulation and measurement assemblies from Smart Testsolutions, the hardware extensions are controlled by the test controller as the central instance for the entire system.
The advantage of separating communication from the measuring equipment is that it allows a high degree of standardisation at the measuring equipment level. The test scripts running on the process computer only require minimal modification, if at all, on changing over from one OEM to another. Most of the adaptation work required takes place at the communication interface. That not only saves time but also provides additional security. After all, the testing and diagnostic scripts have successfully withstood month-long endurance tests and every change means a potential risk.
Standardised scripting API simplifies configuration
A further major factor contributing to the efficiency of the test rig is the integration of the Lua scripting language into the MCM Process Realtime test controller software. Lua has been specially developed for resource-limited controllers. The scripting language is relatively lightweight and so does not take up much computing time and resources. With regard to the aim of shortening changeover times, the great advantage of Lua is the fact that the personnel concerned in the test departments require only a short period of familiarisation before being able to create test programs. Lua is very similar to the C programming language generally learnt by engineers in the course of their studies.
To speed up the configuration process still further, Smart Testsolutions has implemented a number of functional modules which can be accessed by the test case developers via Lua and can be used in various combinations. Very little time is then required to define the test sequences for different control units and to configure the measurement hardware accordingly. Another advantage is that the test departments can access more Lua functional modules through the internet, where they are freely accessible in libraries.
This enables them to react swiftly if the requirements suddenly change. There is no need for any complex tools or development environments. It is only necessary to write a comparatively simple script in a standard text editor and transfer it in file form to the test controller, which Smart Testsolutions also refer to as the process module.
Nearly all the ZF scripts were created in-house, but the company also benefits from the fact that, once programmed, modules can be re-used and test sequences can be defined independently of the operating software of the test benches. The flexibility of the test benches, of which ZF now has more than 30 in use around the world, has led to a drastic reduction in changeover times. The test benches were first employed for testing control units for an electronic traction control system. Siefert-Gäde’s test equipment team only needed four weeks to convert them for testing a camera system. “In the past it would have taken us eight months,” says the test bench expert.
The test bench already included most of the measuring equipment required, as the traction control system with integrated parking brake for which it was originally designed is one of the most complex modules in the portfolio of the ZF division in terms of the electronic interfaces. As the test systems have a special evaluation assembly (EVA board) for each unit under test, any necessary electrical adaptation to the I/O channels of the units under test can be performed in-house by ZF. According to Siefert-Gäde, use of the EVA board and Lua scripting permitted quick adaptation of the I/O interfaces to the camera system. He further explained that the main differences between testing camera systems and traction control systems are the complexity of communication and the difference in loads. For the camera system use is made of CAN and Automotive Ethernet, for which the company employs an additional Vector module (VN5610), whereas the ESP only involves CAN, FlexRay and LIN.”
Outlook: Lightweight commands for Industry 4.0
The Lua scripting capability does not just shorten changeover times however. Thanks to the integrated Lua script interpreter the new test bench generation is also well equipped to deal with the Industry 4.0 scenarios of the future. Requiring only minimal resources, the Lua scripting language is ideal for use in small networked devices and control systems. This is why the first manufacturers are already starting to implement Lua as a basis for networked machine intelligence. The progress of industrial automation will also extend to test departments. One feasible scenario could be: The unit under test communicates with the test bench and identifies itself. The test bench then performs configuration automatically on the basis of this information.
In a nutshell:
The increasing complexity of automotive electronics also makes greater demands on product testing for electronic components. To nevertheless minimise the amount of work involved in configuring the test benches and programming test sequences, ZF got together with Smart Testsolutions in Stuttgart to develop a new generation of test benches. The modern test benches offer outstanding flexibility based on three main factors:
1. A test bench consists of six subracks, each of which can also be independently configured and operated away from the test bench.
2. The communication level and measuring equipment are kept strictly separate in each subrack.
3. The integration of a Lua script interpreter into the test controller allows the test developers to access ready-made functional modules via Lua.
These features enabled ZF to shorten changeover times from a period of several months to just a few weeks.
About the Author:
Norbert Witteczek is Business Manager Test Systems & Applications at SMART TESTSOLUTIONS GmbH in Stuttgart (Germany).