Tilt and spin
The stage must also tilt from 0° (horizontal) to 100°, where the stage appears to be a great wall in front of the audience. As the stage is tilting, it can also spin infinitely at up to 4 rpm. The speed an artist may see while spinning at the end of the stage is 25 ft/sec. The spin axis is accomplished utilizing a 100-hp vector motor on a 800:1 gearhead. A vector motor is a closed-loop, high-hp electric motor that can run at very slow speeds.
The stage platform that lifts, tilts, and spins is 6-ft thick and contains over 100 axes of its own automation. Inside the stage, the controls all around make a person feel more like an astronaut than a stage technician. During the tilt and spin of the stage, the on-board technicians must hold on tight, as they can be spun at 10 rpm.
The stage is tilted with four Parker 10-in. bore, 110-in. stroke cylinders. The control manifold contains two D81FH Parker zerooverlap proportional valves, which pass about 80 gpm each during a full-speed tilt. The stage will tilt 100° in less than 40 sec. During one scene, the tilt will dump a beach set, sand and all, into the depths of the theatre. In another scene, the tilted stage becomes a battlefield of acrobats while images are projected onto it.
Artist safety is of utmost importance to the MGM Grand Hotel and Cirque Du Soleil. Two retractable 50-ft diameter safety nets sit below the stage as it lifts, tilts, and spins. Due to space and noise constraints, 14 hydraulically driven winches were chosen to set and strike the nets. Hydraulic winches use two-speed (dual displacement) motors for a wide speed range and are controlled by a proportional directional valve for fine speed control. A solenoid valve on the motors will shift the motor's piston swashplate from hi-torque/low speed to low-torque/high speed. Eight of the winches use a mooring control circuit, which allows the net to move as a whole while under constant tension. An on-board encoder and pressure transducers provide feedback to the controls — the net position as well as the net's tension. Perfect tension on the net must be maintained for optimal artist safety. The nets are set and struck over a dozen times during each performance.
Any time you lift 150 tons at 2 ft/sec to within 0.10-in., controls must be extremely fast and reliable. A Delta Tau fiber optic servo control loop was employed for this task. The stage lift and stage tilt servo loop contains five nodes. The node in the control room contains the system CPU where the motion profiles are stored and the command signals are generated; the other four nodes (called macro nodes), located throughout the theatre, process positional feedback and control the servovalves. Hundreds of I/O, including load cells providing feedback on each cylinder for both lifting and tilting axes, are also fed into the macro nodes. The fiber optic servo system operates at 2.2 kHz and also controls the hydraulic net winches.
Position feedback is via four string encoders on the stage lift and four string encoders on the tilt axis. One string encoder is used on each side of the stage. As the stage is lifting, following error, rod pressure, and load cells are monitored to ensure synchronous motion of the stage. Any excessive difference in following error, or load cell reading between the four lifting cylinders or the four tilting cylinders, will result in a fault condition and a controlled stop of motion.
Synchronous motion of the lifting cylinders is achieved by simultaneously giving the same command and the same trajectory to each cylinder. Each cylinder is controlled by two servovalves, one 700-gpm proportional 3-way, one 20 gpm servo solenoid for fine tuning, a total of eight servo axes to lift the stage. The servo loop containing each of the eight axes is closed over fiber optic cable.