Even though cylinders often are used for their linear motion capability, we also call on hydraulics for rotational motion on much of our equipment. In the past, we may have driven a cable and drum assembly with a variable-speed DC motor through a gearbox. But we’re changing over more and more to low-speed/high-torque motors to drive drums directly. We started using these LSHT motors for hoisting loads of 5000 lb or greater. LSHT motors eliminate the need for gearboxes, which reduces installation time and cost.

Siefried

Costs are reduced further just by taking advantage of the hydraulic power unit. As long as we have to have a power unit installed to run the cylinders, we can distribute the cost of the power unit over every hydraulic function. Because cost of installing and operating the power unit is relatively fixed, the more hydraulic actuators we use, the more cost-effective hydraulics becomes because we can more widely distribute the cost of the power unit.

Safety is another advantage of hydraulics. It’s easy to set up stall conditions by using crossport relief cartridge valves or other standard components. If something locks up, instead of having a DC motor burn itself out or shear the teeth off a gear, it’s very easy to control pressure applied to any component, and without a lot of fancy electronics or torque limiters.

System hardware

Much of the hardware used in shows might, at first, seem identical to that used in theme parks. While this is sometimes true, duty cycles of components used in theater productions come nowhere near those of theme parks. Attractions in theme parks must cycle hundreds of times a day, seven days a week. In these applications, hardware is designed, constructed, and assembled almost as if it were industrial material-handling equipment.

Equipment for the theater must be just as reliable, but duty cycles, in general, run only a few times a day. Of greater importance is the ability to assemble equipment quickly and get it operating, regardless of how great loads and speeds deviate from original specifications. In fact, if a show really flops, equipment may cycle only a few times in front of an actual audience. On the other hand, equipment in successful shows may operate for months on end.

If a show does run for only a short time, it would seem that parts could be recycled for future productions. Power units almost always can be applied over and over. This again illustrates the versatility of hydraulics because a power unit can be put on line for a production and pump displacement scaled down to the flow required.

For example, the power unit used in Guys and Dolls had formerly been used in the Teenage Mutant Ninja Turtles musical show that toured the continent. This power unit is much larger than what was required for Guys and Dolls, but using it saved us from having to make a new one, which, ultimately, helped us land the contract of a very successful show.

Other components, such as long sections of hose and cartridge valves can be recycled for more than one show. We have standardized our hose assemblies by installing female fittings at both ends. By doing so, we can re-use a hose assembly and not have to delay assembly of the set because we need an adaptor.

Custom hardware

We custom build some of our equipment as a cost-effective alternative to commercially available equipment. This is because we use 10-turn pots almost exclusively as input devices. Commercial controls must accept a variety of input signals, so, naturally, they have to have more complex cicuitry, which, of course, adds to its cost. In a few cases, we even sell some of our equipment to outside industries, if, for example, they also use ten-turn pots as their primary input device.

Moreover, much of what we need would not be available off-the-shelf from most manufacturers. For example, 120 ft of linear travel is not unusual for us. We may need only 1/8 in. repeatability over that 120-ft stroke, so it’s hard to justify paying for something with capabilities beyond that required unless it will be used repeatedly.

We also generally don’t close the velocity loop. We control velocity through software, so it wouldn’t be practical to purchase an industrial control with velocity control as standard equipment. We could control position with respect to time, but having a time-based control costs more. Besides, we usually don’t know how fast something should run because everything is being built for the first time.

We did build a time-based system for controlling the motion of a dragon used in the Sigfried and Roy show in Las Vegas. Among the myriad of moving platforms and sets, a dragon with a 60-ft wingspan and over 30 axes of hydraulically actuated motion appears to engage in battle with the show’s stars. Motion of the dragon is controlled by a computer that was programmed from ten dual-axis joysticks. A trained operator would impart movement to the dragon through the joysticks. Variable-voltage output of these joysticks was then recorded in EPROM and played back. Any segment of the program could then be edited any number of times until achieving the desired effect. The computer integrated ramps into the command signals to smooth out any abrupt changes that may have been introduced by editing.

An advantage of our control system over, say, an industrial programmable logic controller (PLC) is that we change command signals simply by rerecording a particular segment. Because PLCs use ladder-diagram logic, any changes in the program must be made in the ladder-logic program.

For example, in Miss Saigon, we made a Cadillac whose rear half could fold up to a vertical position because backstage room was so limited. As the Cadillac moved forward, the rear half would begin unfolding downward. We originally had only two limit switches for the motion — fully folded and fully unfolded. Later, we had to add an intermediate limit switch. Because the show’s main program was controlled by a PLC, a programmer had to search for the exact switch closure in the ladder logic program before the change could be added there.

Dan Hoffman is vice president and director, electronic and motion control division, Showtech, Inc., South Norwalk, Conn.