Top-Down Design in Motion
by Jim Buchanan

Until recently, trying to combine motion dynamics with the skeletons used in top-down design has been tricky.

"Typically, you'd have to apply a two stage process to run your motion analysis," says PTC Product Manager Vincent Pihlstrom.

"If you wanted to do a top-down design, you'd start by defining the top-level skeletons and then reference the skeletons when placing or creating your models. But then, moving the assembly would be restricted to skeleton editing and regeneration, so you wouldn't get a smooth movement,” Pihlstrom says.

“For true motion analysis you'd have to create separate motion assemblies bottom-up, using the same models. In the motion assemblies you would use motion connections placing the models relative to each other allowing for motors, friction, and so forth to be added. Either way, you'd be defeating the purpose of top-down design or true motion.”

Pro/ENGINEER Wildfire 3.0 lets you set the motion definition within the skeleton itself. This means that the model structure you're used to working with, the one that uses skeletons, now also contains your motion skeleton definitions as well.

Skeletons on wheels. The new motion-skeleton functionality translates into several new on-screen capabilities. Now you can create 2D mechanisms, and use a drag handle to create movements and explore the range of motion. Parts can also be based on the skeleton using data sharing features. This means that the parts size, location and even motion connections will be derived from the skeleton.

You can also build 3D mechanisms by drawing multiple 2D mechanisms with the sketcher, and then defining the bodies for each moving element. Here, the motion-skeleton function automatically detects and defines your connections among the different sketches.

Finally, you can assign dynamic properties to the skeleton bodies, reducing the need for solid models in the early design stage. You can then use properties such as position, static, and force balance to run sophisticated kinematic and dynamic analysis.

Let’s say you’re designing a motorcycle. You’d like to see how the shock absorbers, fenders and steering mechanism will move in relation to the frame. But your frame design will also determine the sizes and types of shocks, fenders, and so on, that you’ll be able to use. So you’ll save significant time if you can try some different shock, fender and steering types with several frame designs. To do all this in assemblies could take days or weeks. With motion skeletons you could have your frame design by the end of the day.

“You’re looking at what materials the frame is using, the weight of the driver, that kind of thing,” Pihlstrom says. “And you’ve got constraints; the size, placement and weight of the engine, for example. All of that can be taken into account early in the design if you use motion skeletons.”

Pihlstrom notes that the new motion skeleton functionality lets you apply both kinematic and dynamic analysis to your top-down models.

“Kinematic analysis just looks at the movement itself,” he says. “For instance, you might want to see how the front wheel turns when the driver is steering the bike, or see the clearance of the suspension system. For this, you’d want the 3D motion skeletons only. Dynamics adds mass properties — the forces associated with gravity. Now you’re adding density and friction, and you can start trying different springs and dampers.

“For instance, there is a huge difference between using a ball bearing and a bushing,” Pihlstrom says. “So you can try both, just by changing the friction parameters you’re applying to the joints of the skeletons. You don’t have to stop and model both types of assemblies first.”

Building in motion expertise. In a production setting, motion skeletons give the design team a way of setting parameters early in the design cycle, well before the model, or its subassemblies, may be outsourced to other groups of companies. The design team might include one or more motion experts to help the team set the new mechanism’s range of motion. Once the parameters are set, the design engineers would build them into the motion skeletons, and then proceed with the top-down design of the model.

Later, the model complete, the designers can email the subassembly models to their suppliers, confident that the design-team’s original range-of-motion intent will be faithfully executed in the suppliers’ own models.

“For the design engineer, the motion skeleton functionality makes it easier to think — and design — in a top-down manner,” says Pihlstrom. “This is a huge benefit, because it removes mental clutter at a time when the designer should be thinking at a high level. But it pays dividends for the design team as well. It reduces the risks of mistakes that might be made by suppliers, and that might come back to haunt you at a time when the process is well along, when changes become very expensive and time-consuming.”