Hollywood goes ape over hydraulics

Standard valves modified for special control

Robinson got what he needed with innovations that show promise of finding their way into a variety of industrial hydraulic systems. To control the cylinder's position accurately along its stroke, his team attached a small rack on the cylinder rod, Figure 2. The rack engages a pinion gear to position the sweep of a feedback potentiometer as the rod extends or retracts.

Figure 2. Cylinder stroke was controlled by a proportional valve, which received signals from a command and feedback pot.

Voltage from the feedback pot passes to the proportional valve, as does voltage from the command pot in the manual controlled. When the voltage of the command pot in the control box equals the voltage on the driven pot, the proportional valve shifts and blocks fluid flow to the cylinder.

The problem of deadband lag was resolved by Ernie Escobar, of SLI Industries, by removing 0.030 in. from the width of the spool lands at the input and output ports, Figure 3. Reworking the valve spool was the key: it reduced the deadband to zero and eliminated any time lag between remote control commands and Kong's responses. When the control operators moved levers to order Kong to caress Dwan, he did so instantly, creating the illusion of a living creature on screen.

Figure 3. Lands of standard proportional valve were machined to eliminate 30% deadband in valve. The colored portion indicates where lands were machined.

Aside from these modifications, Kong's hydraulic system was made with off-the-shelf components, put together in a hurry with products from nearby distributors and manufacturers.

All hydraulic components were rated for 3000 psi because Robinson and his staff wanted to be sure they had all the power they needed. As it turned out, they over-specified, because Kong behaved perfectly operating at pressures to only 1200 psi supplied by pumps delivering flows from 15 to 35 gpm.

"Our Kong is full functional", Robinson said, "He wiggles his arms, turns his head, twitches his ears, rolls his eyes, bends both legs, pulls his mouth back to show his gums, rotates on his hips, thrusts out his legs, and, when he has to, smiles.” All hydraulically.

"He really frightens you, he is so thoroughly alive," reminisces Robinson, "even when you realize he is only eight tons of hydraulic animation, latex skin, horsetail hair hide (1012 lb of it), and 3½ tons of aluminum framework."

Kong preformed with minimal time out to repair frame breakage and fluid leaks. The leaks were often caused by a technician in a hurry, who had not properly tightened a hydraulic connection.