Motion Control Resources
Motion Plays Around
by Kristin Lewotsky, Contributing Editor
Motion Control & Motor Association Posted 04/22/2009
All work and no play makes Jack a dull boy, so the saying goes. It is fitting, then, that in addition to being an ubiquitous industrial technology, motion control plays a vital role in entertainment. From whirling circus performers high overhead to bringing motion picture camera lenses into perfect focus, motion control makes our leisure hours ever more enjoyable.
Although entertainment applications may not involve packaging 400 bottles per minute, they can be surprisingly demanding. “Clients will say, ‘We want to be able to fly a person around this room in all three dimensions at 30 ft/s and we want it to be safe and we need to be able to do this repeatedly night after night, for 10 or 15 years,’” says Nathan Wells, Project Manager at Stage Technologies Inc. (Las Vegas, Nevada), which engineers motion systems for everything from stage shows like Cirque de Soleil to the Princess cruise lines. Part of the challenge can simply be the sheer speed of development. Forget modifications to specs; often, schedules force StageTech designers to begin work on a project before the specs are even resolved. They didn’t receive specs until last fall, for example, for a project that must be designed, built, commissioned, and handed over by summer.
For StageTech, a 50-axis system is small; more common are systems in excess of 100 axes, with the largest currently at 250 axes. To control and coordinate motion, they turn to PLCs, ganged together in a master/slave configuration and run by multiple human-machine interfaces (HMIs). “We’re generally pushing network topology,” says Wells. “It's not that uncommon for us to have two or three, or even six HMIs located throughout the building, all operating at the same time. If you have six people moving things around, you can very easily bog down a network.”
Of course, shows involve human performers, which add excitement but also significantly increases the level of responsiveness required from the system. A typical show might involve a trap door that slides open to reveal a lift that rises up to fill the hole, while panels slide in from the side of the stage, chandeliers descend from overhead, and a backdrop slides into place to create a particular effect in concert with lighting and sound. Timing is critical in such a complex scenario, but the performers are human, not animatronic, which adds a degree of uncertainty to the process. If a performer bounded across the stage early, before the lift filled the hole in the stage, they could suffer injury or worse.
The solution is to pre-program the sequence and run it with an HMI that features both execute buttons and sliders. During the performance, the operator triggers the sequence with the execute button, but has the freedom to speed it up or slow it down using the slider. That level of control also allows shows the flexibility to move acts around out of the original sequences if problems arise.
Reliability is key. In any application, downtime is a problem. In the case of stage shows, where tickets can run over $200 per seat, the cost of failure rises to a whole new level. By its very nature, the application is selectively unforgiving. If the system is down for 8 or even 12 hours between 6 am and 6 pm, for example, it’s merely inconvenient. If it’s down during show time and the show has to be cancelled, the cost could easily rise as high as half a million dollars.
Keeping operators, machinery, and product undamaged - safety - is a critical concern in industrial applications but StageTech’s customers take it to a whole new level. “Plant owners are very concerned about having something go ‘bang’ but you have maybe only a couple of people in the entire building, whereas we are literally hanging people off of our equipment and flying performers out over the audience,” says Wells. “Technically, flying the performer over the audience works for the theater, but the innocent person that the performer could fall upon creates a whole new set of issues. It has all sorts of huge liability impacts. We can't have things go ‘bang,’ we can't have runaway motors or brakes fail.”
It’s an interesting challenge: How, exactly, do you get a performer flying around at 30 ft/s into safe-torque-off mode? Too rapid a stop could lead to injuries, so StageTech focuses primarily on NFPA Category 1 stops, which call for powered deceleration. In addition to the PLCs that control the basic motion, the company adds an additional network of safety PLCs, basing their strategy on redundant error checking and independent processing to avoid common-mode failures. Their winches incorporate redundant, full-load dynamic brakes, for example. When the units are taking up or playing out rope, the software includes positional and speed limits that are monitored by encoder position. Because the encoders provide positional awareness, the StageTech engineers can program in a controlled deceleration as the machine approaches its functional limits. They also incorporate hard-struck mechanical limits as an additional protective device in the case of encoder failure or fault.
In the early part of the decade, StageTech worked with drives controlled by PLCs networked via Profibus. An additional, manually controlled network provided backup and allowed them to commission system submodules before the entire system was integrated or the theatre construction complete. “[With this architecture], we can be running machines and moving them before we have the master control setup,” says Wells. “It also gives us a level of redundancy. If you're in the middle of the show and someone cuts the main network trunk going from the basement of the building to the controllers - which could be a couple of floors away - then you can have someone run down and grab a manual controller. They may not be able to complete the show but they can at least still move things and maybe get the performers off the platform or out of the air. We are also getting more and more into redundant network paths for wired systems and rapid roaming technology with wireless devices.”
In more recent designs, StageTech has been moving away from smart drives, putting more intelligence back into the control unit. The approach provides appreciable cost savings, Wells says, given the high axes counts of their systems.
Keeping things quiet
In addition to all of the other challenges, another arises that is seldom an issue in the industrial environment: acoustic noise. In a show filled with carefully orchestrated effects, the whine and whirr of motors can be disastrous. Typical noise limits for a StageTech installation are 55 dB; for comparison, normal conversational levels are around 65 dB.
Meeting spec requires careful attention to brake and motor selection, as well as servo tuning. The engineering team favors the use of AC permanent-magnet servo motors with absolute encoders reading off the motor shaft. “We do that because of the efficiency of the motors,” says Wells. “On induction motors, you lose something like 30% of your amperage just for coil excitation. With permanent magnet servo motors, every amp that you send out to it goes to motion. We get much higher efficiency so we can use smaller drives and less current; less current means we’re inducing less motor noise. Also, servo motors tend be fully enclosed and don’t have cooling fans like induction motors.”
Noise is also an issue for Preston Cinema Systems (Santa Monica, California), which makes automatic lens-adjustment systems for motion picture cameras. For every scene with explosions and car crashes there are others that hinge entirely on nuance and murmurs. “We can’t have any noise from the lens motors intruding into the sound recording, so that means we need special bearings and special balancing of the rotors to keep them quiet and minimize vibration,” says president and founder Howard Preston.
Operated by wireless handset, the company’s three-axis design controls focus, iris, and zoom. Preston’s engineering team focused on making the units compatible with as many different cameras and lenses as possible, which presented a significant challenge - lenses for motion picture cameras can range from 0.5 to 25 lbs in weight, with widely varying diameters. Finding quiet motors wasn’t enough; the team needed components with enough torque density to adjust even the biggest lenses. In addition, the system needed to be as small and as light as possible, since much of today’s filming takes place using cameras carried by the operator.
“In that market in general, people are very concerned about the weight of the product and the performance,” says Gus Geil of MicroMo Electronics Inc. (Clearwater, Florida). An easy way to increase torque is by gearing down, but directors want fast response during filming; adding a gearhead would reduce the speed of the response to an unacceptable level. The answer was motor design. “Power density comes from the magnet and the coil system,” says Geil. “When you combine the right coil and the right magnet, you get quite a bit of power density.”
Preston’s systems have been such a success that “the Preston” has become industry shorthand for an automatic lens adjuster. In 2007, Preston and colleague Mirko Kovacevic received an Oscar for their contributions to the film industry.
In the theatre
Show control and multimedia specialist MediaMation Inc. (Torrance, California) operates on the other side of the lens. The company started out building animatronics like the dinosaurs in the Jurassic Park movies. More recently, they’ve broadened into motion simulation, producing special-effects seats for so-called 4-D theatres. Powered by servo-valve controlled pneumatics, the seats offer motion over three degrees of freedom, in addition to more nuanced effects like vibration, air blasts, and water spray. Audiences watching 3-D films in the seats get a true multimedia experience.
The company takes a kinematic approach to controls. “With a large animatronics like the dinosaur, it takes about 12 movements just to make his head go out,” says cofounder and CTO Dan Jamele. “We built what we call universal kinematics into our control system, which allows us to map these complex movements into single-slider activation that you can do in real time.” While prepping for a show, for example, they can watch the film and easily adjust the preprogrammed routine to add a bump here, a greater tilt there. “Editing the motion through degrees of freedom rather than the individual axes is a huge thing for us,” he adds.
The company has developed its own control systems. They work with a combined centralized and distributed control architecture, running the overall show from a master controller with an HMI, but operating each bank of four seats as a unit. The approach reduces cabling, minimizes points of failure, and makes the theater more modular so whether there are 20 or 200 seats, the control system is essentially the same.
The applications may vary, but fast, accurate, repeatable, and reliable performance lies at the heart of it all. Day and night, on every continent, motion control works at play.