DCX uses simulation for chassis development. (WIP).

Combining physical and virtual data acquisition has enabled DaimlerChrysler (DCX) to reduce the time needed to obtain road loads an individual chassis components from months to weeks. The traditional approach to determining road loads is to instrument all of the individual components on mules or prototype vehicles. This is time-consuming. And expensive. And when vehicle designs change, there must be reinstrumentation. The potential for error in physical data acquisition means that redundant data must be taken. Another problem with this approach is that multiple vehicle development groups, such as Body and Powertrain all need many channels of load data and have to go through much the same process on their own.

Historically, differences in the prototype vehicle and data acquisition methods often meant that these groups were working with inconsistent models, possibly reaching conclusions based on incompatible data or test results. While it is possible to collect the data entirely from a virtual prototype, simulation of the tire-road interface is not currently as accurate as desired and extensive testing is required to validate the model. DCX engineers led by Sudhakar Medepalli of Loads Analysis Group, based in Auburn Hills, Michigan, have developed a new method that provides the best of both the experimental and virtual worlds by physically measuring a minimum set of loads and using them as input to a virtual prototype that calculates the rest. "This new hybrid approach provides dramatic time and cost savings while providing accuracy as good as, if not better than, the previous physical testing methods," Medepalli said.

In recent years, the automobile industry has been making increasing use of virtual prototyping. For instance, designers can obtain body loads to test their components at the same time analysts obtain the chassis loads they need to set up their bench tests. And a single, common virtual prototype enables all team members to be sure they're using the same data. Virtual prototyping software can be used with real road profiles obtained from over the road testing data to generate component-level load histories. These load histories can be used to perform durability testing in the Fatigue laboratory or as input for fatigue analysis software. And these durability tests then generate component life predictions without requiring physical durability testing. This approach has many obvious advantages such as substantially reducing the time and cost involved in obtaining component loads and allowing for design changes to be quickly implemented. But, the correlation of this method has not yet been established to the comple te satisfaction of the design community.

DCX engineers developed an innovative hybrid approach that requires experimental measurements of a minimum set of loads at key locations, and then uses these measured loads as input to a virtual prototype test that calculates the loads on all other components of interest. In order to avoid the challenge of validating the road-tire interface, the engineers felt that the best place to acquire loads would be at the wheel spindles. With spindle loads, the virtual prototype can be used to very quickly and accurately calculate the loads on all other components. When they first had the idea, however, they recognized with the technology available at that moment, measuring spindle loads would be a very difficult and expensive process. But just about that time, MTS Systems Corp. [Eden Prairie, MN) introduced the spinning wheel integrated force transducer (SWIFT) that simply bolts onto an adapted wheel, replacing tedious hand placement of transducers. The recorded loads From these transducers are used as input at the sp indles of a virtual prototype developed with ADAMS software from Mechanical Dynamics (Ann Arbor, Ml]. ADAMS simulates the Full motion behavior of the vehicle and calculates the load time-histories of the individual components. Fatigue software from nCode International (Southfield, MI] is used to generate analytical fatigue life predictions from the load time-histories

The tire-road interface is the most difficult obstacle and modeling of hysteretic elements such as shock absorbers, bumpers, and elastomers also presents a problem. Because of these challenges, validation of the computer model might require more testing than a physical road load data acquisition. A virtual prototype that included the entire vehicle and road-tire interface could also become very computationally intensive, placing a strain on data processing resources.

Using this new approach, the number of input channels acquired is reduced approximately by a factor of two and the time for instrumentation is reduced from months to weeks. The cost of one round of data acquisition is also reduced greatly. Overall, it requires about one-fourth the time and one-third the cost of the traditional approach. The time savings result, in part, from the fact that DCX has developed templates to automate their virtual-prototype creation and testing processes. They can now create full-vehicle models in a few days versus several weeks required in the past. Like physical road load data acquisition, this new approach eliminates the uncertainties of tire and road modeling and the extensive physical testing required for validation. Comparisons to physical testing have shown better than 90% accuracy for a number of different suspensions, including McPherson struts, multi-link independent rear suspensions, short-long arm (SLA) suspensions, and multi-link live axles. DCX's analytical fatigue li fe predictions are within physical test-based fatigue failure intervals. In many cases, the virtual prototype identified problems with measured loads that were later confirmed by the people who had acquired the road load data. The result is that in some programs, design engineers have started to rely more on the calculated loads than on the measured loads. Like virtual road load data acquisition, minimal time is required for data acquisition and the impact of design changes can often be determined without additional testing. Moreover, body, chassis, and powertrain loads can be collected simultaneously on the same vehicle. So the hybrid approach effectively integrates physical testing with virtual testing to yield large savings in time and money. "Management at DaimlerChrysler has gotten strongly behind the hybrid physical/virtual approach to prototyping because they recognize the time and cost savings that it can provide," Medepalli concluded.