Motion Control Resources
by Kristin Lewotsky, Contributing Editor
Motion Control & Motor Association Posted 11/30/2011
By using innovative control schemes and robust components, integrators can develop flexible, powerful motion systems for a range of entertainment applications.
A roller coaster train running through a 360° loop. Opera singers, and camels, standing on a temple wall as it rises from the floor in a staging of Aida. Although we more commonly associate motion control with industrial automation and other technical applications, the systems find ever broader application in the entertainment industry. Forget about manufacturing glass, beverages, pharmaceuticals, furniture, and consumer goods. Offering flexibility, performance, and robustness, motion has become a key enabling technology in the production of fun.
Roller coaster trains used to start out clacking their way slowly up an initial hill, drawn by a chain drive, to amass potential energy - after that, gravity took over. Today’s roller coasters skip the chain drive in favor of powerful motors that send the train rocketing forward from the first second.
Rather than working with a rotary motor/linear actuator combination, roller coasters typically generate motion directly via linear motors. We can think of linear motor as being essentially a rotary motor unrolled. Instead of a ring configuration, the magnets form two facing rows, applying thrust to a forcer that runs through the air gap between them. The lack of mechanical contact reduces the effects of friction and increases reliability. That's particularly important in amusement parks, where if a ride goes out of service, the majority of guests in the park will not be around in a day or a week when it is operating again.
As you might imagine, accelerating a multicar train full of people to speeds as high as 100 miles an hour requires a fair bit of muscle. Unlike rotary motors, which must increase diameter in order to raise output power, boosting the horsepower of a linear motor simply involves lengthening the magnet track. In low duty-cycle applications, a linear induction motor produces roughly 1 lb/in2 of thrust - to generate 10,000 pounds of thrust, the motor needs to have an active area of 10,000 in². For some attractions, that might be just fine, but for others, size matters. Change the motor to a brushless linear design with neodymium magnets, and the force density jumps up to 7.5 lb/in2, allowing a motor one seventh the size to do the job.
There are, of course, trade-offs. Glitches in the supply chain for the rare-earth oxides that go into making neodymium magnets have sent magnet costs soaring. Although using a permanent-magnet linear motor rather than an induction motor might cut power consumption, in a comparison of capital outlay versus operating cost, the price of the motor track may outweigh the savings. Typically, the launch motor only runs for a few seconds at the start of the ride, so choosing a permanent-magnet option may not conserve an enormous amount of power. That said, roller coasters are robust, long-term installations, allowing asset owners to amortize the initial cost outlay over decades.
In addition to launch, linear motors can be used to accelerate the train mid ride, perhaps during a coasting segment when it drops down into a dip. “You have to be able to track the velocity it's coming in at and shoot it out at a higher velocity," says Mark Wilson, President of H2W, which supplies motion control solutions for the amusement park market. “If you got one person on the ride, or a dozen people, there's a different acceleration. The train will be coming in at a different velocity, and based on that information you would apply more voltage or less voltage to get the speed you want.” The system involves a pair of position sensors connected back to the controller, which calculates the velocity and adjusts the drive parameters to suit.
From the controls standpoint, running a roller coaster is far simpler than, say, operating a form-fill-seal machine at 300 parts per minute. With the induction motors, the systems typically use variable-frequency drives, with a PLC running both the machine and the motion axes. In the case of brushless motors running closed loop, systems typically incorporate a programmable motion controller and a servo amplifier. And while the roller coaster might set people screaming, the motion control axes won't.
The actors in a stage show aren't the only ones with a role to play in the production. Motion comes into play, as well. At Hudson Scenic, which provides motion solutions for a variety of entertainment events, the tasks break down into two basic categories: rolling a set across the stage or flight effects. Set positioning can range from moving something as modest as a table atop a small platform to a set the size of a small house. Flying effects can be as basic as moving a simple curtain up and down to flying performers around on cables. Although Hudson Scenic does not work with applications that involve aerial effects with performers, they do get involved with motion tasks positioning stage sets with performers atop them.
In this application, the challenges are less about the specifics of the motion itself than about developing an extremely robust, reliable, flexible solution. “What we do in terms of motion is fairly simple: get something from point A to point B in the same amount of time each night," says Senior Engineer Chuck Adomanis. “The challenge is that we frequently don't know how big or how small that piece is going to be.”
The development time for a project can run as short as five weeks, from first discussion to on-site implementation. To simplify the process, Hudson Scenic team starts with a basic platform that they customize for a given project. “We try to give ourselves as much flexibility as we can,” Adomanis says. "Everything we do gets some sort of variable speed drive on it. Even if the client says, ‘All I ever need this thing to do is open and close,’ we still put on a little bare-bones variable-speed drive because when they get there and they see it and they say, ‘I wish we could open it more quickly,’ we can do it. What we've learned over the years is that the more flexibility we can possibly give ourselves without it costing a fortune, the better. These are creative, artistic people. They come up with new ideas. We need to be ready to adapt to them as much as we can.”
Although the company explored a variety of standard controls solutions based on commercial off-the-shelf (COTS) components, they ultimately could get the results that they needed. Although motion sequences for a given show may be pre-programmed, the details tend to be a typical, at least from an industrial perspective. An object may start abruptly, then slow down gradually, then speed up again. COTS solutions couldn’t necessarily meet the requirements. "We found a lot of preprogrammed motion routines where either you could change the speed on the fly but you couldn't change the motion, or you could change acceleration but not deceleration because that was the basic math that the controller had," he says.
After trying a wide range of products, then debating PLC versus motion controller versus PC, they settled on a hybrid PC/PLC solution. "We have the PC and then we have a PLC running in a real-time kernel inside that machine, which also has real motion capability," he says. "If we need a near-CNC motion axis, we have that. If we need something with a little more flexibility, or a simple on/off control axis that we feel like we can handle with PLC, we have that, too, and yet they’re very easily integrated because ultimately they're all running on the same machine. That's really where we see the future of our equipment going, into industrial PC controllers where you can embed those different components and have them tightly integrated.
Above all, the equipment needs to be reliable. “Right now we have over 200 or 300 deployed positioning axes out there in shows," he says. "Some of those axes get used once a day. But they'd better work.”
That can be particularly challenging in the case of a touring show that sets up and dismantles the set dozens of times. Cabling is the single biggest point of failure in the motion world, even for machines installed on a factory floor. Because of the constraints of the application, Hudson Scenic works with soft cables rather than hardwiring or conduit. They need to depend on the stagehands to be knowledgeable enough to keep sensor cables away from high-frequency power cables, for example, to make sure wiring crosses at 90° angles. They also need to depend on the quality of the cable, which can be a challenge. “We do a lot to solve those problems with shielding and grounding, and addressing it in our rack and cabling design," Adomanis says.
Of course, even the toughest structured cable may not be able to keep up with the punishment of repeated setup and takedown operations. "We've done quite a few shows that a traveled around the world and just the abuse of being connected and disconnected, and stored and packed, over and over again, without a doubt it's hands-down the biggest weakness of the whole system, says Adomanis. “When people call and say the system is not working, the first thing I do is have them check the cables. Very few of those components were ever designed for that much abuse. We use lots of Ethernet in our systems and one of the things we have been [waiting for] is a more robust RJ-45 Ethernet connector, because the generic plastic office duty stuff might be cheap but it doesn't hold up. It cost you more in the long run.”
And most of all it may cost in terms of downtime of the production which has an unforgiving. It's enough to keep an engineer up at night - or in the evenings. “If the phone rings between 6:30 and 8 pm, I cringe, because inevitably it’s somebody calling with a problem," he says. "It's usually the silly things: the cord that gets unplugged, the cable that gets cut. Those are the things that end up worrying you from day to day, though, because it happens in front of an audience. Not that anybody wants to see their factory line shut down, but there aren't usually 1800 people standing there watching it.”
Solutions like distributed control, smart components, and networked design can help minimize the stress and strain on cabling. Components get tougher all the time. With the help of robust, flexible solutions and smart engineering, motion control continues to rise to the occasion, powering entertainment across the globe.