This HSRC application combines a full vehicle under physical test with a set of virtual tires, all of which is "driven" over a digital, three-dimensional test track.

MTS HSRC Waveform
HSRC-generated loads closely correlated to fatigue-critical loads from a physical road measurement.

Introducing HSRC

The integration of physical tests and virtual models, or hybrid simulation, holds great potential for accelerating and enhancing vehicle development processes. Developing and implementing it successfully, however, presents considerable challenges. Among of the greatest of these are situations where simulations can’t be executed in real-time due to the control requirements of the physical test system or the complexity of the virtual model (for example, models that involve elastomeric components and/or tires that exhibit nonlinear behavior). To enable hybrid simulation in these situations, MTS developed Hybrid System Response Convergence (HSRC), an iterative technique that makes hybrid simulation practical for more complex applications, such as ride comfort and durability.

An example of a typical HSRC application combines a physical test vehicle with a set of virtual tires, all of which is “driven” over a digital, three-dimensional test track. The test vehicle is installed on a road simulator and can be excited at the vehicle spindles, which act as the interface between the vehicle and virtual tires. The road simulator is equipped with force and motion sensors to measure spindle dynamic behavior during each iteration. On the virtual side, the tires are maneuvered by a virtual “driver” over the road.

HSRC systems generate results through the sequential execution of physical and virtual simulations. An initial excitation sequence is played into the physical vehicle and the resulting spindle forces and motions are captured. For each control axis, one of the spindle responses (force or motion) is used to control tire behavior in the virtual simulation. 

For example, the vertical force measured on the physical rig can be used to control the vertical loading of the virtual tire as it is maneuvered over the virtual road. This creates a unique vertical motion of the virtual tire center, based on the tire dynamics and the road profile. The system then compares the rig motion that caused the vehicle force with the tire motion measured when the same force was applied to the virtual tire. If the motions are not equal, the two halves of the hybrid system are not yet operating with dynamic compatibility.

By using the difference in motion, or convergence error, the HSRC control model changes the dynamics of the physical rig, resulting in new force and motion measurements. This creates a new virtual test condition and new convergence error results. Through successive iterations, HSRC ultimately creates a road simulator drive file in which the measured vertical force, when applied to the virtual tire, results in the same motion that generated the measured force on the rig. At this point, the two halves of the simulation have converged and behave exactly as they would if coupled together as a real-time dynamic system.

The simplified example above describes the convergence of vertical motion at a single spindle. In a real HSRC application, convergence occurs for all degrees of freedom simultaneously with the final drive file representing converged dynamic behavior at all four vehicle spindles.

MTS had to overcome both technical and operational challenges to make HSRC a viable simulation technique. 

A significant key technical challenge was determining where to start simulations. To solve this problem, MTS developed a proprietary strategy for initiating the iterative process from a valid starting point, significantly reducing the number of iterations required for convergence. A valid starting point is also critical for avoiding inappropriate initial loads, which could easily damage the physical test article. Other technical challenges included devising a way to “drive” four “disembodied” (virtual) wheels and tires on a virtual test track in a coordinated way, and developing a compensation function to convert error into directionally correct drive file updates that advance simulations toward convergence.

Making HSRC operationally practical involved integrating solution components to minimize complexity for users. A number of activities, including coordinate transformations, DOF matching, polarity matching, virtual driving and process initiation, were designed to be managed “under the hood,” minimizing operator errors that could compromise simulation quality and test schedules. MTS also developed a menu-driven approach to HSRC familiar to any engineer using RPC® Pro software.

To date, the HSRC technique has been validated in six vehicle programs at five major OEM sites. Convergence was successful in all cases.

Initial proof of concept occurred with Audi in 2010. An Audi A5 Coupé was installed on a spindle-coupled road simulator combined with virtual vehicle tires (modeled using FTire™) and digital road sections running in an ADAMs environment. Working with Audi, MTS developed a complete set of 20-channel control signals for the road simulator for three rough-road durability test profiles that represented select Audi proving ground surfaces.

For this first evaluation, physical spindle loads developed using HSRC were compared to vehicle spindle loads obtained by three other methods: road load measurement for the A5; road load measurement for a similar vehicle; and predicted loads from a virtual vehicle model. Comparisons indicated that the loads created with HSRC closely correlated to fatigue-critical loads from a physical road measurement, resulting in more appropriate loads than those predicted through analysis.

Since then, MTS has pursued enhancements to HSRC to improve its efficiency and broaden its scope of applications. MTS worked directly with FTire to integrate tire simulations in a stand-alone application (outside the ADAMs environment), which improved iterative processing efficiency by 85 percent. Successful support of a second industry-standard tire model from TNO proved that HSRC could work with multiple tire models. Another enhancement allowed the simulation of an entire proving ground track instead of individual sections.

HSRC innovation continues today. Currently, MTS is creating an HSRC technique for using fixed-body axle test configurations instead of the typical floating body. MTS is also entertaining the pursuit of HSRC techniques for other spindle-coupled road simulators, as well as other system applications such as engine mounts, steering systems, exhaust systems and more.

Contact MTS today to learn how the iterative Hybrid System Response Convergence (HSRC) technique can bring the benefits of hybrid simulation to more complex applications, such as ride comfort and durability.


MTS Systems

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Eden Prairie, MN USA

Tel: 952.937.4000
Tel: 800.328.2255
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