Customer story BMW Group

SimulationX helps BMW to visualize the specific characteristics of Bowden cables through a simulation model with realistic physical behavior in a quick and easy way.

As part of today’s product development at BMW, numerous technical questions are discussed on the basis of virtual vehicles.

The task at hand

Analyzing the functionality of flexible components early on to save on real parts

Whether you want to release the front hood or door locks, operate the window lift inside the door or actuate the shift cable to the gearbox – Bowden cables are all around in today’s vehicles, but usually go unnoticed by the customer. Due to their physical behavior, Bowden cables pose a particular challenge to simulation techniques. In the context of today’s product development at BMW, numerous technical questions are discussed on the basis of virtual vehicles. Flexible parts like hoses, cables and Bowden cables are usually represented as rigid parts. Their shapes, however, often deviate from the physically correct ones. A joint project between BMW and ITI tried to find ways of simulating Bowden cables that would reflect a realistic physical behavior.

A Bowden cable is a type of flexible cable used to transmit mechanical force or energy by the movement of an inner cable (most commonly of steel or stainless steel) relative to a hollow outer cable housing. The housing is generally of composite construction, consisting of a helical steel wire, often lined with plastic, and with a plastic outer sheath. 

[1]Regardless of a particular application, the behavior of Bowden cables is mostly a matter of the effective direction. Despite fundamental similarities between various applications, there is, however, a multitude of varying parameters concerning the Bowden cable itself as well as its structural and functional boundary conditions.

In order to be able to deal with certain issues at a very early stage without the need to have real parts available, BMW has chosen to go with virtual product development. However, BMW’s experience was shaped by finite elements simulation and other time-consuming methods which often required advanced hardware resources. The field of virtual system simulation was new, and the expectations for the simulation software were high.

The solution

Virtual system simulation with SimulationX

The simulation had to show not only the quasi-static shape and rest position of the balanced Bowden cable in the coordinate system of the vehicle, but also the forces applied by the customer when the corresponding mechanism is operated. These forces cause changes in shape and position of the Bowden cable. For an accurate representation, several aspects must be taken into account. The model computes the position of the Bowden cable depending on its given length, the clip positions for the fixation to the body as well as the required forces. Also friction between the inner cable and the covering sheath is part of the consideration with respect to tension, coefficient of friction and curvature radius. The location of the maximum curvature of the Bowden cable and its corresponding wrap angle are calculated as well together with the forces that are necessary to release the lock or actuate any other mechanism.

During the project it became apparent that the simulation time can be reduced to a minimum when the computation is split into two phases. That means that the results from the pre-simulation are used as input parameters for the main simulation. The pre-simulation determines the static starting position of the Bowden cables and is used for multiple main simulation runs. During the pre-simulation, the shape and position of the sheath is computed without the inner cable taken into account. Key input parameters for this part are the dimensions of the Bowden cable, such as length and diameter, the clip positions that represent the fixation to the body and the coefficient of friction between inner cable and sheath. The main simulation delivers the actuation forces, the changes in the Bowden cable’s shape which these forces cause and the time-dependent behavior of the involved system components. The simulated changes in shape and position can be visualized not only in the SimulationX 3D view, but also transferred to a 3D CAD system.

The benefits

Evaluating and safeguarding usability on a virtual basis

 

By and large, virtual product development depends on the quality of simulation results, while the acceptance of simulation results relies heavily on the comparison against measured data taken from real hardware. This was also the case during the project “Bowden Cables” when suitable scenarios were used for evaluation purposes like releasing the front hood or a door lock via inner and outer door handles. In each scenario, simulation results matched the measured data to a great extent. SimulationX offers the possibility to map specific features of Bowden cables to a simulation model which comes close to reality. One of the major benefits of SimulationX is that all calculations can be run on a laptop with a 32 bit operating system. The developed models have enabled BMW to predict a system’s behavior at a very early stage of the design phase and to show the impact of parameter variations on the usability straightaway. This is already of great importance in today’s virtual prototype development and will become even more relevant in the future.

Source: [1] Andrew Alder and others: Bowden Cable. http://en.wikipedia.org/wiki/Bowden_Cable

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