Ask engineers to identify cutting-edge industries in the use of fluid
power, and they're likely to name aerospace or factory automation. The truth is
that today's most innovative fluid-power applications are taking place in the
There, fluid power simulates realistic movement in Broadway plays, Hollywood movies, and theme parks. On Broadway, for example, "Phantom of the Opera" and "Sunset Boulevard" use hydraulics to quickly move heavy stage sets. Movies, from "Free Willy" to the soon-to-be-released "Executive Decision," employ hydraulics to pitch and roll a 747, shoot water from a whale's artificial blowhole, and blink a mechanical eyeball. At theme parks, fluid power's role is almost limitless. Universal Studios technicians use it to simulate motion on such rides as "Back To The Future." They also enlist it to power heavy-duty monsters, such as King Kong and the great white shark from "Jaws."
From a technical standpoint, entertainment applications are consistently among the most complex. Most use some measure of proportional control and closed-loop feedback. Many also require high-speed operation. "In entertainment applications, engineers are trying to realistically simulate motion, and that requires very fast response times," notes Ken Buda, vice president of sales and marketing for Parker Hannifin Corp., Cleveland.
Many of the applications also require high force. "Motion bases," or platforms on theme-park rides, often involve tremendous loads. What's more, those loads typically are accelerated at extraordinary rates, and the impact has to be absorbed by the power medium. For that reason, hydraulics have emerged as the system of choice for such applications.
The following examples show why. They were chosen not only for their obvious high-force requirements, but also to demonstrate the combination of advantages afforded by fluid power--speed, power, precision, and quiet operation.
Whatever the requirements, engineers say that such applications head the list of innovative uses of fluid power. As Buda puts it: "In terms of proportional controls, servos, and feedback devices, the entertainment industry is at the cutting edge."
Hydraulics shake, rattle, and roll Boeing 747
Imagine that you've been assigned to move a 75,000-pound structure at peak velocities of eight feet per second. What's more, you've got to accelerate and decelerate it in less than three-quarters of a second in a safe, controlled manner.
How do you do it?
That was the problem facing special effects engineers assigned to the new Warner Brothers film "Executive Decision." The movie, in which a terrorist hijacks a passenger airliner, includes scenes in which a Boeing 747 mock-up is rocked Ů20 degrees from side to side and nose to tail.
To achieve the special effects, Warner Brothers purchased the interior of a 747 airliner and attached it to a 46-foot-long by 20-foot-wide platform supported by steel beams. To enable the platform to move in two directions, technicians mounted its center on a large pedestal containing a universal joint-style bearing. Total weight of the aircraft interior, steel platform, camera riggings, and 150 live actors seated on board: 75,000 pounds.
To pitch and roll the "plane," engineers from Mayo Hydraulics, Bakersfield, CA, designed a massive, hydraulically powered, closed-loop motion system. A custom-built, 500 hp hydraulic unit powers the motion package. It includes:
Eight Parker Hannifin PAVC100, low-noise, high-efficiency, 3,600-psi variable volume piston pumps.
Located 10 feet from the platform's giant universal joint, the cylinders work in harmony to provide the desired motion profile. As they retract and extend to the full length of their 78-inch strokes, the cylinders pitch and roll the platform. Residing 15 feet off the floor, the platform can be tilted as high as 26 feet at one end and as low as 4 feet at the other, depending on the position of the cylinders.
To move the 75,000-pound load, engineers employed two cylinders with 5-inch bore diameters and 3-inch-diameter rods, and two cylinders with 7-inch bore diameters and 4-inch-diameter rods. Operating at 2,500 psi, a single 7-inch cylinder can generate 65,000 pounds-force while retracting.
One of the key technical challenges for engineers was to quickly move the platform's enormous mass. To reach the 8 ft/sec velocities they needed, the engineers supplemented the pump flow with the nitrogen gas bottles and the accumulator to help the motion system attain flow rates of 600 gpm. Visitors to the film site say that the 8 ft/sec movement of the giant platform generates a 747-like whoosh of air throughout the studio.
To control the platform's motion, engineers incorporated a PC-based system. The controller generates a motion profile and communicates that to a Parker BD99 amplifier card. The amplifier card compares the analog signal from "string-pot" potentiometer sensors on the cylinders to the motion profile command. It then sends a signal to two 482 gpm Parker D111FH proportional directional valves, which control the velocity and the position of the actuator.
By using the hydraulic control system, Mayo and Parker engineers enabled the movie's director to choreograph the motion to his liking. It also enabled him to replay the sequence of movements as many times as necessary to obtain the desired film footage for a given scene.
Engineers say that the magnitude of the project eliminated alternative types of motion systems. "With electrics you couldn't achieve the speeds or forces that were required," notes John Rothas of Mayo Hydraulics. "For this kind of application, it absolutely had to be hydraulics."
Fluid power shines on 'Sunset Boulevard' stage
When Norma Desmond, the tragic heroine of Andrew Lloyd Webber's hit musical "Sunset Boulevard," is lowered onto the stage on a 37,000-pound set, the audience at New York's Minskoff Theater never hears the whine of a hydraulic pump. Nor do they hear the hum of a cooling fan.
The reason Norma sings without the accompaniment of machine noise is that the movement of the set is controlled by a hydraulic system designed specifically for quiet operation. The heart of the system consists of an integrated motor-pump that dramatically reduces noise. The secret: it has no cooling fan.
During the play, the motor-pump acts in conjunction with a steel-cabled winch system to lower the set into place. The winch's cables connect to a hydraulic brake, digitally regulated by proportional control valves. A microprocessor-based controller, made by Feller Precision Inc., Tappan, NY, regulates the hydraulic system.
Quiet pump the solution. Key to the system: a 30 hp IMP 22 Integrated Motor Pump with a PVH 74 piston pump running at 1,300 psi. The unit, manufactured by Vickers, Inc., Maumee, OH, can deliver flow rates up to 33 gpm at 1,000 psi. Vickers engineers claim that the 33 gpm motor-pump is so quiet that it operates with less noise than a 3 gpm oil recirculation pump that runs adjacent to it.
Although previous productions of "Sunset Boulevard" have solved the noise problem, they have done so at considerably more expense, says Dan Hoffman, president of Birchstreet Design, the system's designer. In the show's West Coast version, Hoffman used a separate pump and motor, which was cooled by a fan. The hydraulic equipment, contained in a basement room at Los Angeles' Schubert Theater, was housed in a soundproofed acoustical enclosure. The enclosure retained the heat generated by the pump and motor and, as a result, had to be air conditioned.
In contrast, the Vickers Integrated Motor Pump doesn't require a fan. Instead, cooling is provided by oil flowing through the motor and into the pump. This enables the motor and pump to be enclosed within a shroud, reducing the noise level even further. Overall, Vickers engineers say their unit runs about 3.5 times quieter than a conventional motor and pump combination.
Because of the integrated design, the new motor-pump also reduces the potential for leaks. Oil flows at low velocity (roughly 2 ft/sec) and at low turbulence as it travels to the center of the pump. The oil absorbs heat by circulating through the bearings and around the rotor, stator poles, and windings before flowing into the pump.
Moreover, the new system is 30 to 45% smaller than a conventional system, yet it still produces the same pressures and flow rates. Because oil absorbs heat more effectively than air, the smaller motor can operate at greater speeds and torque levels.
By using the Integrated Motor Pump, the system needed no extra sound-proofing or air conditioning. This reportedly saved the producer of the show $6,000 when compared to the cost of installing a conventional hydraulic system.
Zoo hydraulics make elephant's trunk more user friendly
Engineers typically cite high force as a prime reason for selecting hydraulics as a power medium. Often forgotten is the fact that hydraulics can offer pinpoint control and luxurious feel for low cost.
That was the case for a design team at the Tulsa (OK) Zoo in the construction of a joystick-based "edu-tainment" system. Located in the zoo's pachyderm house, the system electrohydraulically simulates movement of an elephant's prehensile trunk. Modeled after the Indian elephant, the cloth skin replica even mimics the movement of the elephant's "finger" at the end of the trunk.
Children operate the trunk with joysticks that tip it, move it up and down, or slide it left and right. A single-axis joystick provides up-down and left-right movement. A dual-axis joystick controls trunk tip and finger movement.
To accomplish all that, the trunk uses cable-and-pulley systems mounted over flexible leaf springs. Four hydraulic cylinders apply tension to each of four cables. An electrohydraulic valve stack, made by Apitech, Butler, WI, meters flow to each of the four cylinders. Two Apitech MJ Series electric joysticks command the Apitech VPL Series electrohydraulic proportional valve assembly. An electric motor, a 15 gpm hydraulic pump, and a reservoir provide power to move the trunk.
Valves the key. The VPL electrohydraulic proportional valves form the heart of the system. They offer pressure-compensated flow that enables each of the four cylinders to operate independently. Pressure compensation alleviates potential problems that can occur when one channel in the valve assembly operates at different pressures and flow rates than another channel. Without it, a pressure increase to one cylinder could result in the same increase to a neighboring cylinder.
To prevent that, the VPL valve employs a reducing valve scheme. This enables the valve to maintain a constant pressure drop across the main metering element.
In the elephant-trunk application, the valve results in a more consistent feel for the operator. "The feel of each function is independent of the operation of the other functions," notes Scott Nagro, southeast regional manager for Apitech. "This way, we maintain the integrity of each function."
The system's designers claim they could have used an electro-mechanical system to power the trunk, but not at the same cost. They estimate that one servomotor-driven axis would have cost the same as all three hydraulically driven axes. "The open-loop, load-sensing hydraulic system is more energy efficient and cost effective for this application's duty cycle than electrics or pneumatics," adds Robert Cook, special projects designer for the zoo.
The pachyderm application aptly reflects the advantages of hydraulically powered motion, Nagro contends. "This isn't a case of the brute force advantage of hydraulics," he says. "It's a matter of hydraulics offering a less expensive, more user-friendly package."