The Orbiter Ride Case Study

Steve Rundell:

So the case was presented initially as an amusement park ride case, like an orbiter type ride. What had happened is a young girl rode the ride twice and some point afterwards, spinal cord itself had lost blood supply for some period of time. This young person ended up with pretty significant injury being paralyzed from the neck down, so becoming a quadriplegic. The question was, was there something about the ride caused it, or in general, were the forces of such a nature to cause this kind of condition or this injury? The final part, I guess, I was asked to look at is, could this cause resonant in the cervical spine?

And the best example I give to explain resonance is being on a swing. So when you're seated on a swing, you and the swing have a natural frequency, and what happens if you kick your feet back and forth on the swing at the right frequency, which is the natural frequency of the system, you get resonance. Which is you go further in one direction, you go further in the other, further back and forth, and so you have resonance. So, that's how the case was initially presented to me.

We got to the ride, we were able to instrument it with I think six different accelerometers. We used GoPro to even just look at the GPS data and to get onboard video. I personally rode it a few times. I also flew a drone up above it. It was a steady enough few that I could use that to characterize the motion along with off board video from a few different locations as well. So pretty much whole bunch testing and a whole bunch of data so that I could say, all right, here's exactly what this ride does when a person is on it.

At Explico, the primary software we use to evaluate forces on a person is MADYMO, a dynamic software, so 3D dynamic solver that comes built in with essentially virtual occupants, virtual tests, devices, test dummies at different sizes that can be scaled to whoever you're doing an analysis for. We use the MADYMO software. We build out the geometry of the car. We tell the MADYMO software, okay, this car is rotating and moving through space that these specific velocities, rotational velocities, and then put one of these validated virtual occupants in the car, scale them appropriately, run a simulation.

And then the outputs from that show us, okay, what were the forces on the neck? How hard did it bend the neck forward, bend it backwards to the side? All of those things can be taken from the simulation and then compared with, okay, well what sort of forces cause damage to the cervical spine? What kind of forces do we typically see when we do a jumping jack or when we flop in a chair in order to answer the question in general, is this ride injurious or dangerous for the neck? And then more specifically, okay, well if we change the occupant to someone like me, an average 50th percentile male or a very tall, lightweight female occupant.

We were able to look at those variations and show that essentially the dynamics experienced by the occupant didn't vary much at all based on height and weight, because we had a full understanding of the 3D motion as a function of time.

The conclusions and the opinions that I came to were one, that in general, the forces and motions acting on a person in this ride specifically are less than the forces in motions you typically experience in everyday activity, especially for a young person. And then finally, the idea that this could cause resonance in the cervical spine was not remotely close to being accurate. The natural frequency of a system is based on the stiffness. The ride's input frequency is basically the same as it goes around, but you can adjust your stiffness and therefore adjust your natural frequency as you go around.

So there's no way that this ride could cause resonance in the cervical spine and there certainly wasn't any indication of that in anyone who had ever ridden this ride for the years it had been in service. To develop a start to finish system to track and characterize the 3D motion of a ride to build that out, pulling all of that together into MADYMO mode to fully simulate end to end what the dynamics of an occupant are on an amusement park ride. I don't know of anyone else that's done that. And now we've done that for roller coasters and we've done that for other amusement park rides and it wasn't an easy thing to figure out, but we have figured that out now.

So we have that end to end workflow and we could even take that one step further and create a full animation.