The test effort of modern hybrid drives is significantly higher when compared to conventional powertrains. The main reason for this lies in the higher number and complexity of the components. At the same time, current trends in powertrain development – for example, ”road to rig“ approaches – result in the movement of significant portions of the development process from the vehicle to the test bench and, thus, into earlier project phases. New solutions must be found to manage this increasing test complexity and to accelerate development cycles. Against this background, FEV and the Institute for Combustion Engines of the RWTH Aachen University (VKA) have collaborated in the development of an important new tool called the ”virtual shaft” at the RWTH Center for Mobile Propulsion (CMP). This development was funded by the Deutsche Forschungsgemeinschaft (DFG).
From component testing to system testing
“In conventional development processes complexity increases step-wise as we move from a single component up to system testing in the vehicle“, Dr. Albert Haas, Group Vice President test systems at FEV, explains. “First the dynamometer, the engine and the gear drive are tested in separate test cells. The real interactions between the different components, though, cannot be evaluated until composite system testing can be accomplished on a full powertrain test bench.” This step requires not only a change in the test environment, but also mechanical modifications and software changes. Therefore, this is usually performed in a later development phase, where either a vehicle or a complete powertrain is needed.
Virtual powertrain with real time network at CMP
In order to achieve significant time and cost reduction in the development process, FEV and VKA have implemented the virtual shaft between two test benches at CMP. The dynamometers in both test benches are controlled in a way that achieves the equivalent system behavior of a real mechanical shaft with low mass inertia and high rigidity.
The test environment consists of spatially separated test benches, which are real-time connected by a real-time deterministic EtherCAT connection. This communication protocol ensures low communication latencies at all times. The controller layout of the virtual shaft is realized considering the actual step responses and delays, which are given, based on the standard components that are used. At this point, the target is to minimize transient deviations of speed and torque.
Safely avoid damage
Networking the test cells with the virtual shaft offers some significant advantages: In addition to saving time, these mainly include the provision of a protected test environment and high number of monitoring options for the individual test objects. With this scenario, damage to prototypes can be effectively prevented. In addition, the virtual shaft allows the combination of hybrid powertrains that are mechanically incompatible and would otherwise have to be extensively adapted. “The virtual shaft makes a valuable contribution to handling the increasing complexity of modern hybrid drives and provides the opportunity to make the development process more efficient,” Haas concludes.