Usually when you go to the theater, the only stars you see are the ones on stage. Not so at Rock Valley College’s Starlight Theatre in Rockford, Ill., where theater-goers sit under the stars in the sky.
The open theater has been a landmark in the community for 30 years, but weather had a way of raining on the plays. The theater’s director wanted to keep the under-the-stars experience, minus the rainouts. Using the theater’s name for inspiration, architects designed a roof of six triangular panels that open to the sky, forming a six-point star audience members can see from their seats. When it came to actually moving the panels, Minneapolis-based Uni-Systems LLC was called in. The company is no stranger to moving large structures — it designed the retractable roofs on Houston’s Minute Maid Park and Reliant Stadium — but this particular project required motion unlike any roof they had designed in the past.
Act one: Motion
When viewed from below, the roof panels form a hexagon perimeter. They rotate up, one after the other in a clockwise motion, 54° about the perimeter to form the sixpoint star shape the audience sees.
“Our goal was to maintain the architect’s and theater director’s vision of having the roof open, but make the activating mechanism invisible to the people in the audience,” says Alan Wilcox, mechanical engineer at Uni-Systems. “One of the biggest challenges was getting all the components to fit inside the shape, while still making the area where the mechanisms are enclosed easy to access and maintain. Significant geometry changes go on inside as the panels rotate 54°, so we had to make sure there was clearance for all the components.”
The solution was found in simple mechanics. A 20-in.-diameter stationary torque tube spans the inside cavity of the panels, which are 36 ft wide, 42 ft long, and weigh about 30,000 lb. Two supports, or load plates, connect the torque tube to a ring truss structure below. Four large, engineered-plastic-lined radial bearings connect the panel and torque tube and provide circular arc motion. The torque tube is completely hidden inside the panel. Only the 2-in.-wide load plates are visible holding up the cantilevered structure. Audience members can’t see any gears or motors. In fact, they can’t see anything that indicates how the roof moves.
“The resulting panel architecture integrates the mechanical and structural systems so seamlessly, the panels appear to float open,” says Frank Worms, Uni-Systems’ architect and Starlight Theatre project manager.
So, how does it move? A 50-ton keyed machine screw jack from Dayton-based Joyce/Dayton sits between the roof panel frame and torque tube in a double-clevis arrangement to power each panel. As the machine screw jack extends out, the panel slowly rises to its fully open position. The jack’s inherent design, as opposed to a ballscrew jack, doesn’t let it backdrive under the weight of the panel alone.
Spherical bearings at each end of the jack allow for any misalignment between the moving panel and torque tube. A torque-arm assembly supports and balances the weight of the jack and the off-center weight from the gearmotor and motor mount assembly. Thus, all components stay level throughout the entire motion while giving the jack freedom to float on spherical bearings.
Gearmotors from Nord Gear in Waunakee, Wis., activate each machine screw jack. They consist of a helical-bevel gearbox with a 5-hp, 480-Vac, three-phase motor and spring-set safety brake. A reversing contactor lets the gearmotors run in both directions to raise or lower the panels. When the motor is de-energized, the brake automatically sets. An added safety device, the brake holds the panel in position and further prevents the screw jack from retracting.
“The term ‘non-backdrivable’ does not take vibration into account,” says Wilcox. “Even though our system is not technically in a vibration environment, the whole structure is subject to some oscillation while in a static condition because of wind and other factors. The brake is for these situations.”
A Browning torque clutch, made by Emerson Power Transmission, Maysville, Ky., couples the gearmotor to the screw jack and serves as a torque-limiting, automatic-resetting, protection device. In normal operation, the torque clutch engages to provide a rigid coupling between the gearmotor and screw jack. If an internal jack failure prevents the jack from extending or retracting, or a sensor malfunction causes the jack to drive into the rigid safety stops, the torque clutch will disengage or decouple.
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Act two: Safety
Along with the drive mechanism, inside each panel is a safety mechanism that serves as a redundant load path for the weight of the panel. This mechanism consists of a nonpowered hydraulic pack and two hydraulic actuators.
The hydraulic pack, built by Innovative Fluid Power, Minneapolis, contains a custom accumulator, manifold, pressure relief valve, flow control, and pressure switch. The cylinders, which mount between the panel frame and torque tube, have a twofold purpose. One, they serve as rigid safety stops for the jack. If it goes beyond normal operating range when extending or retracting, the cylinders will bottom out to stop the panel. This means the hydraulic cylinders become a rigid member in a fully extended or fully retracted position. The cylinders were custom designed and built specifically for this kind of loading, so they are considerably stronger than an equivalent cylinder needed to support the roof only. The second purpose of the cylinders is, in the unlikely event of machine screw jack failure, to support the weight of the panel with hydraulic pressure, essentially “freezing” the panel in its failed position. When the cylinders retract (closing the panel), fluid exiting the cylinders is forced over a relief valve on the hydraulic pack. The relief valve pressure is set high enough to hold the weight of the roof, plus a factor of safety.
One of the reasons the hydraulic system was designed this way was the roof’s gutter system. The panels have a gutter on each side, so when they are down, the roof has a full gutter system. The panels raise and lower one by one in a specific order so the gutters can interlock and create a trough in between each panel.
“Freezing a panel in place, versus allowing a safety device to let the panel drift closed at a slow rate and stop, is safer for many reasons,” Wilcox says. “In this case, moving the panels out of order can result in gutter collisions. In the event of a malfunction, the panel in question will remain fixed while the remaining panels are lowered in order. With the other panels lowered, the roof can be safely accessed and troubleshooting can begin.”
Act three: Control
Controlling the brute-strength components of the roof requires extreme finesse, the kind available only through electronics. Inside each panel is a linear displacement transducer from Minneapolis-based Turck Inc. As the panels open or close, the LDT sends positional data via DeviceNet to a PLC, located in the theater’s control room. The PLC, from GE Fanuc in Charlottesville, Va., runs the Cimplicity Machine Edition HMI program. When the LDT displays a predetermined value of when the panel should stop, the PLC does exactly that — with surprising accuracy. The tip of the operable panels can be positioned to within 1⁄16 in.
Operators enter all roof commands and view data coming back from the PLC on a PC inside the control room. A graphic screen interface displays status of the PLC and remote I/O blocks, as well as their respective input and output elements. Positional data shows the direction each panel is moving and how far each has progressed along its cycle. The program also provides feedback on distances between the panels, indicated by green, yellow, or red. The PLC will stop a panel briefly if a gutter is going to contact another gutter on an adjacent panel.
“The software program is unique in that in can be very hands-off. It can take care of a lot of its own faults,” says Uni-Systems’ Alex Krueger, electrical engineer. For each panel, Uni-Systems figured out the exact point where the panels can’t get any closer or the gutters will collide. In monitoring the panels, the control software focuses on space, rather than time.
“One of our constraints was getting the panels open within 12.5 min. To do so, we constantly measure distance, instead of calculating it, because it’s absolute,” says Krueger. “That said, the spacing between the panels is best described by time (about 45 sec). But physically that corresponds to about 6° of rotation, or about 3 ft apart at the tip.”
In addition to safety features, the panel control program offers wind data, failure diagnostics, Internet access for weather reports, and requires biometric thumbprint identification to access. It also plays any sort of sound bit desired. Krueger chose the theme song from the movie 2001: A Space Odyssey to test the system, and it stuck. Now it plays over the speaker system each time the roof opens and closes.