Virtual test driving at GME - Performance simulation of modern chassis control systems
Due to the increasing complexity of control systems and the variety of different vehicle variants, virtual development methods become more and more important. For years, GME Engineering has been aiming to increase the application of simulation methods to systematically support the driving test on the road concerning the integration of chassis control systems. IPG Automotive's vehicle dynamics software CarMaker supports this objective through high-precision simulation models for Software-in-the-Loop and Hardware-in-the-Loop simulation.
Over the last few years, there has been a steady increase in the use of chassis control systems to improve vehicle safety and ride comfort. ABS and ESC became standard features in modern vehicles; from September 1st, 2011 it will be a legal requirement in the United States to equip every new vehicle with an Electronic Stability Control. Further systems, such as active steering, active damping and controlled all-wheel-drive systems, will also be part of the chassis network.
The vehicle manufacturer is responsible for the complete vehicle and has to ensure that the individual control systems, which usually come from different suppliers, work together smoothly. Every additional control system makes this task even more complex. A further difficulty is the growing number of different vehicle variants. For reasons of time and cost it is not possible to build prototypes of all variants, which means that virtual development methods will have to complement test drives on the road.
Controlling the variety of vehicle variants through simulation
This is a challenge which engineers at GME are also confronted with. The development and application of control systems for several global GM-vehicle platforms are to be carried out at the Rüsselsheim site. This includes the integration of ABS/ESC control systems, the damping control system CDC (Continuous Damping Control), a controlled all-wheel-drive system (AWD) and AFS (active steering) as well as the linking of all theses systems. A large number of different vehicle variants are created on these platforms, each with different combinations of bodystyle, drivetrain and control systems. "This clearly shows that we can no longer really work without simulation and without mature long-term test automation, which is able to run for a weekend or even longer. Simulation is necessary in order to cover the wide variety of future variants" said Dr. Henning Holzmann, Head of Driving Performance Simulation at GME Engineering.
For this purpose, GME established the so-called "Road-Lab-Math" strategy, which aims to cover a large part of the development work through Hardware-in-the-Loop tests (Lab) and Software-in-the-Loop simulations (Math). In SIL simulation, the software for the electronic control unit can be tested at a very early stage, before a real control unit is available in hardware form. This allows high-throughput tests of whether the control algorithm produces the desired driving characteristics. In HIL simulation on the other hand, the actual control unit has to prove that it works according to specifications. For this, the ECU is connected to a simulator, which imitates dynamic driving effects within in a virtual test drive environment.
From virtual development to series production
For many years, GME Engineering has been using IPG Automotive's CarMaker software in the simulation of driving dynamics for SIL and HIL. This comprises of a realistic, virtual vehicle environment based on a real-time capable vehicle model, which reaches the limits of driving dynamics.
CarMaker was most recently successfully used for the integration of ABS and ESC into the latest Corsa by GME. In co-ordinating the overall system performance, the road test was viably supported by virtual methods, because simulation was used for a large portion of the development work. Pre-requisites for this were the high quality of the model for the simulation environment and an extensive validation of the model's parameters, in order to guarantee correspondence between reality and simulation. "This allowed us to release vehicles based on release recommendations both from the driving test as well as - for the first time - software-based using the CarMaker model. As far as I know, this hasn't been achieved by any other vehicle dynamic software tool in the market," explains Holzmann.
The HIL simulator: CarMaker from the dSPACE hardware platform
In the current development projects, GME CarMaker is for the first time not using a CarMaker/HIL test bench, whose hardware is based on established standard technology, but a real-time system from dSPACE. The reason for this is a global decision by General Motors, for all HIL applications within the whole group to use the platforms from the hardware specialist dSPACE.
IPG has ported the CarMaker driving dynamics software onto the dSPACE platform, to allow GME to continue from the preceding development work as smoothly as possible. Substantial differences in the architecture of both platforms made some expansions of the runtime environment of the dSPACE real-time system necessary. However, the expansions have no effect on other applications, which run on the real-time system at the same time as CarMaker is running. They merely guarantee that CarMaker can be used as interactively on the dSPACE hardware as on the CarMaker/HIL test bench.
Direct file access of the dSPACE real-time computer to the work station guarantees that the driving dynamics model can be completely parameterized via the CarMaker user interface. With the dSPACE system, lengthy compile-link-load cycles, which typically appear if a model is parameterized on Simulink, are a thing of the past. In addition multithreading was established on the dSPACE platform in order to guarantee continuous real-time operation. Just like with CarMaker/HIL, communication with the control units is never interrupted - neither during an alteration to the model parameters nor during data backup. An interruption to communication would cause a change in the behavior of the control units, which would lead to the simulation results no longer being reproducible. A CarMaker-specific communication service takes care of the data exchange with the CarMaker front-end tools (3D-Animation, graphical analysis etc.), which is available in its usual functionality.
The actual real-time operation that is so important for HIL simulation can also be guaranteed. "Up to now, we have not observed any performance losses on the dSPACE hardware with CarMaker," says Holzmann.
Virtual road test
The main task of this test bench is to ensure that the integrated control systems guarantee optimal functioning of the vehicle on the road. Firstly, the control units are individually examined on the CarMaker/HIL test bench and then successively linked to the other systems before being tested together.
The definition and specification of the simulation maneuvers strongly depends on the driving test. According to Holzmann, "The simulation replicates the driving test. By intensive co-operation with testing colleagues, we have identified representative driving maneuvers, which would normally be executed in reality, to parameterize, co-ordinate and test an ABS/ESC system, for example. We have integrated these driving maneuvers including the relevant target values into the CarMaker simulation."
As part of the virtual control system development, the generated driving maneuver catalog is simulated using numerous different parameter variations. For this, the engineers take into account different speeds, load conditions or road surfaces. These parameter variations lead to a number of simulated driving maneuvers for the complete validation of this development step. To control this number, GME Engineering used their own test automation tool which, thanks to the unchanged CarMaker interfaces, also functions on the new hardware platform.
Universal virtual tool chain
The HIL test does not replace the real test drive. SIL, HIL and test drives mutually complement each other and only the results from all three areas lead to a dependable release of the complete control system network for series production. A simulation environment, which is supported by SIL to HIL all along the way and reproduces the test drive realistically, provides an important base. Dr. Henning Holzmann believes that with CarMaker he is well prepared for simulation-supported development: "I believe CarMaker is the most well-rounded, best performing system in the market. At GME we have established such a complete chain of tools, which functions excellently and is highly validated, so we can fully rely on the results of our simulation."
