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Hydraulics Safely and Smoothly Moves Broadway Stage

Oct. 5, 2009
Hydraulic servoactuators control special effects for the Broadway show Billy Elliot.
Delta Computer Systems,

One example is the raising and lowering of a whole scenery room as a live audience looks on during the Tony award-winning Broadway show Billy Elliot. The show is about a coal-miner’s son who wants to be a ballet dancer, so graceful and fluid motion is essential for the scenery.

The set requires two hydraulic linear actuators, explains Chuck Adomanis, automation department head of Hudson Scenic Studio, Yonkers, N. Y. One is a large lift at center stage that raises and lowers Billy’s house. The second cylinder mounts to the house and moves a bed. At times, the house lowers into the floor while the bed extends at precisely the same rate and appears to hover motionless in the air.

Because such movements involve close synchronization, the Hudson team selected an RMC75 two-axis motion controller with Ethernet communications and special capabilities for precisely controlling hydraulics. It’s built by Delta Computer Systems of Battle Ground, Wash. String potentiometers that unfurl as the piston rods extend connect to quadrature encoders that send cylinder-position information to the controller. And to ensure accurate closed-loop positioning, the RMC75 interfaces directly with the unit’s servovalves.

In addition to the motion controller, the system includes a PLC for supervisory control and a PC the scenery operator uses for programming and executing motion, all communicating over Ethernet using Modbus TCP/IP. Motion profiles are called for via cues programmed using a custom Visual Basic program that the Hudson engineers developed for the PC. Each cue involves setting the target position, speed, acceleration, and deceleration as programmable parameters. This also let engineers program and test basic moves before installing hardware in the theater.

The Delta controller runs different commands depending on the cue. Simple point-to-point motion of the house or bed platform uses a Move Absolute command, with parameters sent by the Visual Basic front end.

Delta’s controller also can link the motion of multiple axes together in a mode called “gearing.” With electronic gearing, slave-axis motion is programmed to be a function of the master-axis movements. When the bed must hover, for instance, the bed lift becomes a slave axis controlled by the house lift using an Advanced Gear Move or Gear Position (Clutch by Time) command. “These commands let us accelerate and sync the bed to the house lift in a half second, or the time the house lift has moved a quarter inch,” said Erik Nelson, an engineer with Hudson.

Nelson used Delta’s autotuning software to set initial gains for the closed-loop motion-control algorithm and fine-tune with the plot manager that comes with the company’s RMCTools software package. The development and tuning process took about two weeks. Repeatable gearing, easy tuning, and the ability to make quick changes to motion parameters are strengths of the Delta controller, he says.

Proper positioning of hydraulic components was also an issue. For example, the Hudson team generally uses counterbalance valves on lifting axes as safety mechanisms to keep loads from dropping if the system loses hydraulic pressure. Depending on where it is located in the hydraulic circuit, however, a counterbalance control valve can compete with the servocontrol system, causing the cylinder to chatter as it extends or retracts. Delta engineers solved the problem with custom manifolds bolted right to the cylinders.

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