Virtual driving test at BMW using active steering
Hardware-in-the-loop-(HIL) test stands for the virtual testing of both subsystems and complete vehicles have become standard tools in automotive development. BMW AG uses this test procedure to optimise active steering applications for series production.
One of the latest 3 series BMWs is driving along the motorway at 120km/h. When changing lanes, the steering angle and the steering torque are progressively increased until the vehicle turns in too soon and too fast and is close to oversteering. Before the rear starts swerving, the active steering steps in to correct this driving error, stabilizing the vehicle and taking it back to the ideal line. This time, however, there is no prototype vehicle driving along a real test track. The BMW is driving the "lane change" functional test as a virtual vehicle model in real time simulation. This is one of many tests carried out on the HIL test stand for active steering systems at BMW's FIZ Research and Innovation Centre.
BMW took this test stand into operation more than four years ago, before the active steering system went into series production for the first time with the BMW 5 series. "It was generally felt that, due to the complexity of the active steering, there was no guarantee that all of the specifications could be met in series production. The development effort and the extent of tests with actual prototypes required for such a sophisticated Mechatronics system was simply too high. The HIL test stand helped us save a great deal of time and money," says Stefan Schott, development engineer for "active steering systems" at the FIZ.
Testing as if the vehicle was real
The setup of the HIL test stand accurately reflects the situation in the series vehicle. The complete steering systems of the respective BMW series are installed on the test stand, along with the complete and vehicle-compliant steering columns and steering gearboxes as well as the power-assisted pumps. The active steering itself is installed in a climatic chamber with a control range between minus 40 and plus 130° Celcius for a functional test under changing weather conditions. The steering rod power is generated by hydropulse cylinders. The electronic control is analogue to that in a real series vehicle.
The heart of the HIL test stand is a unit developed by a joint venture between IPG Automotive GmbH and IABG. While IABG was responsible for the design and the control of the test stand, IPG Automotive supplied the software for the simulation system CarMaker/HIL, a comprehensive driving dynamics simulation solution for real time applications. All vehicle and system components which are not actually integrated into the HIL test stand have been simulated as software modules.
The HIL-version of CarMaker used by BMW has an insertion slot for further vehicle control devices in addition to the active steering device, whose function can also be controlled in the virtual vehicle. Stefan Schott comments: "In a real vehicle, the active steering control device communicates with the control device for the electronic stability programme DSC. The huge advantage for virtual vehicle testing is that CarMaker/HIL enables both control devices to be integrated into the hardware; otherwise the control parameters and algorithms of the control devices would have to be entered anew for each test vehicle model."
CarMaker/HIL for a wide range of applications
The main task of the HIL test stand is to ensure that the software and electronics control for the active steering and its feedback during driving operations harmonize with the overall vehicle. A multitude of possible driving errors and functional tests - from ramp steer input to lane change tests, consumption and energy measurements to µ-split brake test - are simulated before an active steering application fulfils these requirements. The effects of the active steering on the body and the engines can be simulated, as can be the haptic feedback on the driver.
The test stand also allows to simulate complete, so-called operating stress fatigue tests. The vehicle data are entered into the CarMaker/HIL software to enable virtual driving along handling tracks or even consumption measuring tracks on the test stand. Individual parameters can be adjusted in the course of the test. Schott: "This allows us to iterate the optimum which will then only have to be verified with the actual vehicle."
The virtual test programme does not just aim at the development of active steering systems for completely new BMW vehicle models: on the way towards the next facelift, the potential for improvement of the existing active steering systems of the BMW 5 series, the 3 series, the 1 series and the 6 series are being investigated. Schott: "This is done by virtual measurements of the energy saving pumps of both conventional and active steering systems. The software can be used to adjust the volume flow and the energy absorption; and this can be easily done on the test stand."
The feedback from the active steering to the on-board electrical system can also be tested on the HIL test stand. Voltage changes caused by battery discharge or generators can cause problems. According to Schott, the objectives in this area are clearly defined: "We want to test such feedback increasingly on the test stand instead of on the actual vehicle to solve current problems and questions."
Saving on prototypes
The current and future conditions in automotive development are widely known: in response to the rise in the number of vehicle models, shorter product development cycles and, of course, price pressure, the automotive industry strives to make product development more efficient. Saving on expensive physical prototypes in all development steps is one important area with cost saving potential.
The use of HIL test stands with innovative software pays a significant contribution to this. The development of active steering applications at BMW has been able to save on a lot of hardware. Stefan Schott explains: "Thanks to our test stand and - above all - thanks to the versatile CarMaker/HIL software we have been able to test around 20 percent of the developments for the active steering system of the E60 series and thus have done without a large number of driving tests with actual prototypes. The intelligent use of the HIL test stand can certainly help to save on prototypes."
