Engineers Take CAD Tools for a Wild Ride

When the team at Dynamic Structures designed and built the large Canada-France-Hawaii Telescope enclosure on Mauna Kea, a dormant volcano, no one foresaw the company's future expansion to design, fabricate and assemble amusement rides. "Think of an amusement ride not as a structure but as a large complete machine and you'll understand the connection," says senior designer Craig Breckenridge.

Dynamic Structures designs roller coasters and theme-type rides. A roller-coaster vehicle rides along a track and nothing else occurs, except some screaming and white knuckles. The passengers just go along for the ride. In a theme ride, though, the vehicle travels along a track and interacts with things along the way. "For a theme ride, we work with a large team from the buying company," says Breckenridge. "So we regularly review and exchange many drawings and models. And we must design and fabricate rides with small tolerances. In a 1.5-mile ride, for example, we have many sections and we must keep the gap between them to within 30 thousandths of an inch."

"Most of the time we use Autodesk's Inventor for part of a ride and AutoCAD for the remainder," says Breckenridge. "The programs function differently and offer different capabilities. Here's an example: Most rides and roller coasters include mechanical track switches that let operators remove a vehicle or route a ride onto a different path. We create the switches in Inventor because they represent compact machines compared with the structure and mechanics of the rest of the ride."

"Often we create a track in AutoCAD and use Inventor to design a vehicle," continues Breckenridge. "Then we can put them together in 3D Studio (3ds) and produce an animation that shows what a ride will look like. We use Navisworks for visualization and dynamic simulation. Then we can play the video animation for clients so they can see what the vehicle looks like as seen from a spot on the ground. Or, we can show them what the ride looks like from the front seat."

"Navisworks also lets us check the reach envelope, or the space a passenger can reach from a moving vehicle," says Breckenridge. "We must ensure they can't reach out during the ride and touch something as they go past. We also run dynamic simulations that tell us the forces a passenger will experience during a ride. This analysis gives us the maximum g-force experienced on the center line of the rails. But we also must know the g-force on the passenger's heart line, or roughly just above the center of a human torso. When someone sits in a seat, their body can still move, much like a lever, and experience greater g forces than we would measure along the track's center line. We want to ensure a ride doesn't put too much strain on the rider."

Breckenridge and his colleagues also use Autodesk tools to help control the stack-up of tolerances. "We had a ride where we thought we knew how the tolerances would accumulate, but when we installed the track, it grew slightly in length. Our method to maintain the proper length of track involved two fixed survey points, but one had floated a bit. We didn't think about it at first, but when we got to the site and started to erect the track we realized we had a problem."

"The reference point moved slightly during track fabrication," continues Breckenridge. "One point we surveyed to was fine, but the other end moved by about 1/8th of an inch. That doesn't seem like much, but when the track comprises 70 pieces, the final assembly can increase by several inches. When you attach the track to fixed columns, you don't have that leeway." The Dynamic Structures engineers now include expansion joints between the track and the column in all rides to let the track move slightly in a prescribed direction.

"We also run thermal analyses for each ride because parts could have to operate indoors or outdoors," says Breckenridge. "Thus the track behaves differently based on ambient temperatures. Our designs have used the Autodesk tools to let the track moved in tightly controlled ways to take out the expansion due to solar heating. That's not a trade secret, but it might be something engineers haven't thought about."

Some structural-analysis and dynamic-analysis tools used at Dynamic Structures come from companies that work closely with Autodesk. But the company still relies on an Excel spreadsheet to "control" those two programs and provide all the geometrical design information. "Right now it's easier to communicate that way and have a single source of data," says Breckenridge. "As the interoperability between software packages gets better - and it happens in each new release - we'll get to the point where we don't use the spreadsheet. Now we can take the Inventor model and give it to the dynamic-analysis add-on package and get the results back out of Inventor. Our last dynamic analysis for a ride vehicle occurred entirely within Inventor. We didn't export information to any other software."

Often, Dynamic Structures delivers a complete ride that includes the electronic controls. "We work with a company that creates all the controls for us," explains Breckenridge. "They use the AutoCAD Electrical package that integrates with Inventor, so we take advantage of good interoperability again."

Design software works well and many recent engineering graduates can create amazing designs. According to Breckenridge, however, many students lack practical design skills. "On one ride we have a complicated switch that we assigned to a structural and a mechanical engineer. When their design got to a peer review, we found the structure was a bit on the light side and the stiffeners were a bit heavy. When the engineers realized their structure wasn't quite strong enough, they kept adding heavy stiffeners. So they created a tiny frame and many big stiffeners. The creation was fine structurally, but impractical to build. It just didn't look right, either. One of the other engineers and I say, ‘If it looks like something you might have designed in high school it's probably better.' In an amusement ride, aesthetics play a role, too. We want a nice-looking structure and Inventor helps us create it."

Breckenridge says new engineering graduates need to learn how to communicate better with their colleagues. "When they start a design or project, they should talk with fellow engineers and find people in the company who have a lot of experience. Talk with engineers who have worked on similar projects and talk with the people in manufacturing and on the shop floor. Then the new engineers will learn whether the mechanical shop people can produce that they designed. Can the fabricators get a weld into the space provided? Can they drill a hole in a given position?

"The Inventor software lets us create a 3-D model to see how it will go together. So we make sure we can erect what we build. We really like the animation tools."