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Control Systems Keep The Rockettes in Motion

For most people, the Rockettes Christmas Spectacular is all about the leggy dancers. Daniel Sandoval has a much different take on the show.

A senior systems design engineer with Festo's Customer Solutions group, Sandoval designed the motion control system for the show's largest moving set element: a full-scale bus that carries 35 Rockettes plus one very lucky stagehand.

"At one point during the development process, the choreographer was rehearsing the Rockettes and there I was geeking out on my laptop, oblivious to the dancers," he says.

Question Sandoval's priorities if you will, but it's a good thing he takes his job seriously. The control system gives the 30-ft bus all its stage directions. It has to keep nearby performers safe while positioning the bus within a quarter inch on the stage and closely syncing a series of linear and rotational movements to the show's video effects.

Designed and fabricated by Beyond Imagination Inc., a firm specializing in scenery for Broadway shows and other entertainment applications, the bus actually comes in two versions. One of them is a fully autonomous DC version designed last year for use in Radio City Music Hall. The other is a new tethered AC version for use in the Rockettes' 2008 traveling show. Festo Customer Solutions, the mechatronics consulting arm of Festo USA, served as the controls system integrator.

Both buses offer design lessons that will strike a chord with engineers who build industrial machines that will no doubt run way off Broadway. "The motion control challenges we encountered are ones that pop up in industrial applications too," says Sandoval. These challenges boil down to finding accurate, reliable and safe ways to position high-inertia loads.

Bus Drivers

Despite the differences in their drive technology, the two buses play an identical role in the show. They carry the dancers across the stage and then make a series of rotational and linear moves in sync with video scenes that play in the background. According to Donald Wright, owner of Beyond Imagination, "The idea is to create the impression that the Rockettes are on a bus tour through Manhattan."

Wright devised a system of motor-driven friction wheels that move the bus around the stage. Mounted on carriages that deploy from the bus' sled-like chassis, the soft rubber friction wheels make just enough contact with the stage to move the bus along. One set of wheels propels the bus forward and backward. Another set, mounted in a different orientation, handles rotational moves during which the bus pivots on a large center bearing. "It's something like a tank turret bearing," Wright says. The drive train uses air bags to deploy the friction wheels, while pneumatic cylinders actuate the bearing, pushing it down against the surface of the stage.

Sandoval's control systems coordinate all the movements, as well as auxiliary tasks such as opening and closing the bus doors and spinning a set of cosmetic wheels that reinforce the illusion that the bus is moving within the video scenery.

To sync the bus movements with what's going on in the show, both bus control systems take their cues from a Medialon show control computer that Sandoval describes as "an alarm clock on steroids." The show computer reads a SMPTE time code that serves as a the timeline for all the show's audio and visual elements and allows them to be synchronized within a thirtieth of a second. "The Medialon pushes a cue to a supervisory PLC," Sandoval explains. "Another set of PLCs then execute a fairly simple motion profile." In all, the bus control systems use four Festo PLCs – two FEC-660 mini units and two CPX-FEC modular units – to handle the motion and auxiliary tasks.
Sandoval interfaced the show control computer and the PLCs using Festo's OPC server.

Beyond their reliance on a shared time code and show computer, the drive and control systems for the two busses differ substantially. Each had its own engineering challenges, but "the first bus was in many ways more innovative," says Sandoval.

Because Radio City Music Hall is a historic building, no holes could be cut in its stage for a bus guidance system or for an electrical tether. "We had to make the bus completely self-contained," says Sandoval. "And that meant using a lot of big batteries and motors to drive the bus." It also meant putting all of the control electronics, an HMI console, an air compressor for the pneumatics and a hydraulic steering system on board. Sandoval also created a wireless communications link from the bus to the show controller, using a system that Control Chief originally developed for switch-yard locomotives.

The bus' motors lineup consists of six 5-hp golf cart motors. Series wound traction motors of this type usually run in open-loop mode and have contactor that reverse the polarity the field windings to change the motor's direction. Four motors, two fore and two aft, handle the linear propulsion movements while two additional motors take care of the rotational moves. Three banks of batteries provide the 48-volt juice for the motors. Two of banks handle the motion, while the third acts as an auxiliary power unit, running a set of inverters and powering the spinning wheels and other non-motion tasks.

This arrangement created a couple of design headaches for Sandoval. One was related to achieving the desired positioning accuracy. The 14,000-pound bus, with its high inertial loads, has to hit linear positioning targets down to 1/4 inch and and rotational positions to a 1/10 degree. At first, Sandoval tried to control the golf cart motors with simple open-loop commands, which is how they run out on the links. This control scheme, however, couldn't compensate in the difference in loads between an empty and full bus. 

So Sandoval went back the drawing board and redesigned the controls. His second system added a tachometer to the golf cart motors and implemented PID control of both velocity and position with the position feedback coming from a set of quadrature encoders mounted on idler wheels beneath the chassis. "We took an ugly golf car motor and made it act like a crude but effective servo," he says.

As part of the move to closed loop control, Sandoval also tweaked the bus' trapezoidal motion profile a bit to account for the fact that his makeshift servo won't hold position the way a true servo system will. For the bus to hit its marks on stage, Sandoval's control algorithms incorporate a calculated "coasting" period as well as the acceleration, target velocity and deceleration periods that make up a traditional trapezoidal motion profile.

The second big engineering challenge relates to the distributing analog power command to the motor drives. In this system, motors sets that need to work together are powered by separate, galvanically isolated 48-volt battery packs located at opposite ends of the bus. Yet they all have share a single current loop from the PLC. "Maintaining the analog signal integrity in this environment was a huge problem," Sandoval says, adding that series wound motors have "gigantic current" requirements and grounding issues that made matters worse.

He found a simple, inexpensive solution to the problem in the form of two DC selectable signal conditioners with 3-way isolation. Available for $119 from Automation Direct, the signal conditioners take in a 0 to 20 milliamp signal from the PLC and output identical 0-5 volt power commands for PWM control of two independently powered motor drives with isolated grounds."It's a nifty little box that solved all our isolation problems," Sandoval says.

As for the second bus, its drive design was in many ways more straightforward. It plays a role in the traveling show whose stage design permits an electrical tether. The drive system for this bus works as a position servo. Its friction wheels are direct-driven by SEW-Eurodrive AC induction gearmotors configured for voltage flux control. "I was extremely impressed by their rock-solid velocity regulation," Sandoval says. In all, there are four of these drives working in master-slave configuration – a pair of 7.5 hp models for linear propulsion and a pair of 5-hp motors for rotational moves. Encoders on their own idler wheel assemblies provide true position feedback even if the drives happen to slip a bit.

Here, too, Sandoval controls the motion with what he describes as a "fairly simple trapezoidal profile" to which he has added a coasting period. The PLC calculates that profile. It then outputs the appropriate instantaneous velocity for the drives, which close their own velocity loops with the help of a resolver on each motor. 

"The main difference is that the AC bus closes the velocity in the drive while the DC bus closes velocity in the PLC," Sandoval says.

Riding Safe

Moving the bus was only one part of Sandoval's control design mission. Given the proximity of the performers to the moving bus, the project also had a strong safety focus. Sandoval had his work cut out for him here too. "I've found that theater people know a lot about the traditional hazards found on a stage, but they don't necessarily have a thorough understanding of the safety implications of modern automation systems," he says.

Fortunately, Festo Customer Solutions deals with those safety implications in industrial settings day in and day out. And both buses have E-stop provisions along the lines of what you would find in a factory. On the AC-bus, for example, E-stops that quickly cut the power to the drives are located on the bus itself and off-stage at an operator station. Its safety system further features wireless communication provisions that kick in only if the bus loses communications via its tether. And all its pneumatically-actuated devices are on springs, so they return to their safe position in the event of power loss.

The safety system does, however, make concessions to stagecraft. Both buses have doors that open automatically, but a stagehand masquerading as a driver has to close the bus doors manually. The stagehand also has access to a manual override via a joystick on each bus' onboard console. Finally, the system employs a classic "dead mans' switch," a pedal that the stagehand has to depress whenever the bus is in motion.

To get the bus up and running quickly after any loss of power-for safety or any other reason-Sandoval also added an uninterruptible power supply to the AC bus. "It keeps the CPUs in the PLCs and drives from going to sleep, saving us 30 seconds or more in reboot time," he says.

And while 30 seconds may not sound like a lot of time, it really can be a show stopper. "That's entertainment," Sandoval says. "The show really does have to go on."

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