Snake attack!

Ty Boyce aims the remote control at the VCR and slows the video to half speed, hoping to reveal subtle details in the scene unfolding on the large-format TV. On screen, the gigantic Queen snake, star of the movie Anaconda, clings to a tree 30 ft in the air, and thrusts its head effortlessly through a cascading waterfall, all the while remaining unaffected by the tons of liquid pounding down on it.

With a click, Boyce advances the tape to another scene where a second anaconda, in a pique, whips its tail across the deck of a river barge, tossing tables and chairs 20 ft into the water. A mere 25 ft in length, this "Warrior" snake, the smaller of the two slithering antagonists, scatters the furniture like an angry child sweeping away doll-house miniatures.

Boyce gestures to the screen. "You see that? The snake actually does that on its own; it's not a special effect," he stresses, sounding more like an animal trainer who teaches anacondas to act rather than an engineer who helped build them.

You can't blame him. The snakes are mechanical--animatronic, technically--and therefore really special effects. However, after observing them up close and in action, you almost acquire more respect for them than for a real anaconda. In fact, the term "special effect" seems completely inadequate in describing the magnitude of the engineering work involved. As possibly some of the most complex robots ever designed, the snakes are no more a "special effect" than the Taj Mahal is a "building."

Snake stats. Mere hyperbole meant to grab your attention? Judge for yourself.

The Queen measures more than 40-ft-long, 18 inches in diameter, and weighs 5,500 lb. Its spine contains 60 two-axis joints controlled in three dimensions by 120 hydraulic actuators (two per joint). Each joint is special--unique valves, unique cylinders, and even unique hose fittings--custom designed for a particular position on the snake.

The result: a pair of snakes that couple flexibility with smooth, life-like, methodical, slithering, and terrifying high-end speed and power. The spine provides enough flexibility for each snake to coil back on itself twice. Completely waterproof and corrosion resistant, the snakes operate for extensive periods completely submerged. It's enough to trigger an outbreak of ophidiophobia in the staunchest snake lover.

"We've never clocked it, but we guess that the head of the Queen snake could move at 30 to 40 mph," says Boyce, hydraulic and electrical systems engineer at Edge Innovations, the firm that designed and built the snakes. "It can move so fast it's scary."

Mechanical menagerie. Edge Innovations lies tucked away in a nondescript industrial park in Mountain View, CA, identified by a happy-face sign out front. The inside, however, is anything but nondescript. It looks likes God's workshop, with room after room of mechanical porpoises, snakes, alien creatures, and other animals in various states of disassembly. The menagerie is propped on stands in hallways and offices or, in the case of the 24-ft-long, 14,000-lb. killer whale from the Free Willy series of movies, resting in a sling attached to the ceiling of the two-story model shop. Edge's film and TV credits include Terminator 2, The Abyss, SeaQuest, Flipper, Zeus & Roxanne, Maverick, and White Squall.

Columbia Pictures approached Walt Conti, CEO of Edge Innovations, in early 1995 about Anaconda. From those initial meetings the design specifications and a list of capabilities for the snakes quickly formed. Engineers and artists spent weeks studying real anacondas. The final shape, however, involves a montage of viper, rattler, anaconda, and other reptiles. And though a real anaconda has about 300 vertebrae, engineers determined through a series of structural mockups that 60 joints would provide the snake with sufficient flexibility to look realistic.

Sleek means unique. From eight full-size, foam-rubber prototypes, the movie staff selected a moderately slim shape that measured (for the Queen) about 1.25 ft in diameter. For the engineers--who had to create the internal structure and mechanisms--this set the bar extremely high. "It's not like a dinosaur where you have this torso you can stuff all the mechanism in," says Conti. "It's this slender, extremely limited package that forced us to go custom on all our parts."

From the get go, engineers knew that the only technology that could power such a creation was hydraulics. "You could never get the necessary power density with anything else," says Boyce. But the design pushed well beyond the limits of ordinary fluid power. To obtain the required performance, engineers specified a 5,000-psi system powered by 200-hp pumps (600-hp peak capability with accumulators) built as mobile HPUs by B & T Hydraulics (Sacramento, CA). The units feature full remote monitoring and control capability--a must, since for noise reasons they were placed more than 250 ft from the movie set.

Filters strain the hydraulic fluid down to 3 microns. The snakes had to operate in dirt and water, and any contamination could easily clog the high-frequency response servo valves and necessitate removing a snake's skin for maintenance.

The skin is a proprietary formulation of thick, low-durometer urethane rubber. Some 40,000 scales cover the surface, sculpted by hand into clay mold forms by more than a dozen people working for several weeks.

Each snake mounts at its midpoint to a pedestal through which the hydraulic lines and electrical cables enter the body. The pedestals rotate 120 degrees, driven by a 17/8-inchbore cylinder with a 16-inch stroke. They are attached to poured concrete foundations around the set in order to counteract the more than five tons of reaction force the snakes can produce.

A total of 30 two-axis, stainless-steel universal joints (60 in all) extend forward and backward from the pedestal to form the snake's spine. Each joint, or "link," contains more than 250 components and operates through plus or minus 30 degrees. They are driven by two hydraulic actuators--one for the X-axis and another for the Y-axis--controlled by servo valves developed in conjunction with Dynamic Valve, Inc. (Palo Alto, CA).

Amazingly, no two links are alike. Instead, each is custom designed for its position on the snake's body, and use unique servos and cylinders ranging in bore from 13/4 to 1/2 inches that provide strokes from less than an inch to 3 or 4 inches. Engineers even designed their own fasteners and a line of custom hose fittings when no off-the-shelf items would do.

"The form factor drove everything," says Joss Geiduschek, general manager at Edge. "We were sweating a tenth of an inch in packaging, because even bumps that small would show through the skin and destroy the illusion."

The only stock items they chose were hydraulic hoses from Parker Hannifin (Cleveland, OH) and Furon (Laguna Niguel, CA), and O-ring seals from Apple Rubber Products (Lancaster, NY). "Apple had an incredibly thorough selection of the odd seal sizes we needed," says Boyce.

An elaborate system of sensors provides information, such as joint angle and various loading values--though engineers consider the force-feedback system proprietary and would not elaborate. Signals travel on the more than 40 miles of 30-gauge, medical-grade wire supplied by Cooner Wire Co. (Chatsworth, CA). The wires also carry command and control information for the 120 servo valves.

Routing the hoses and wires through the maze of joints proved so challenging that Edge hired one technician to focus only on this task. Engineers also created full-scale, geometrically precise mechanical mockups out of aluminum to help determine plumbing paths that wouldn't bind. "It was a completely Zen thing," says Geiduschek. "You had to visualize the empty spaces between the links and understand how that space could change."

Heads and tails. The design of the head posed special challenges for mechanical engineer Tom Hsiu. He needed to stuff mechanisms for the jaw, eyes, tongue, and lips in the same space artistic designers wanted to place important cosmetic features, like the teeth and epiglottis. In addition, for both cosmetic and maintenance reasons, every fastener, actuator, wire, hose, and connector had to be accessible from inside the mouth.

Early on, engineers opted to drive all the head mechanisms off the primary 5,000-psi hydraulics. This solution avoided the complication of a second, low-pressure hydraulic circuit or electric motors, which would have required waterproofing and larger wires routed the length of the snake. But it also presented problems. With far more power available than needed, Hsiu had to create mechanical fuses for many components that, should a mechanism become jammed, would fail before more delicate components would break.

Due to space limits, the eyes run off a single actuator and only have the ability to focus. The cylinder has a 3/8-inch bore--providing up to 500 lbs of force--and attaches to the actuator via quick disconnect links found in model airplanes. "These are the most powerful eyes in the world," jokes Hsiu. "Nothing will prevent these eyes from focusing."

The mouth opens 90 degrees using an over-center, four-bar linkage that, to look natural, drops the lower jaw first before the top begins moving. Inside the mouth is the head's most interesting device, the tongue. It not only extends eight inches and rapidly retracts, but also "waggles" up and down in a convincing snake-like manner.

Hsiu accomplishes the extension/retraction with a rack and pinion mechanism. A single cylinder drives the rack, turning the pinion, which, in turn, winds a small stainless-steel chain to move the tongue. To get the waggle, he made the core of the tongue from two thin metal strips connected only at the tongue's tip. At the limit of travel, a cam drives the two strips differentially in pulses, causing the tongue to flutter up and down. "With this design, we get the extension, retraction, and excitation with just one cylinder," he explains.

A similar mechanism controls 36 small joints that taper to a point at the tip of the snake's tail. It uses two hydraulic actuators to drive rack-and-pinion assemblies connected to a pair of pulleys. The pulleys, in turn, carry cables that move the tail either horizontally or vertically.

Input meets output. Building a 120-axis hydraulic snake is only half of the equation. The other half: controlling it.

"Above all else, it had to move realistically," Conti explains. "A snake's entire personality comes from it's movement. It was paramount that we get it right."

Engineers developed, in house, what Boyce describes as "the most sophisticated control system any of us has ever seen." It features 480 analog channels and more than 70 microprocessors--a combination of DSPs and Intel Pentium Pros--that can crunch 2.4 gigaflops of floating-point processing power.

No fewer than six computer monitors can display the performance of any joint in real time. The operator can set the control loop parameters individually for each axis. Numerous filters and damping characteristics can be applied to alter the snake's "personality" for special situations. "It has the capability for a 24th order control-loop closure," Boyce explains.

The control system feeds the snake preprogrammed moves--called "key frames" -- which consist of a series of snake positions placed at desired time intervals. To get from one key frame to the next, the system interpolates the position of each joint at every point along the path.

During a shot, the rate at which the snake moves along the preprogrammed path is determined by a puppeteer turning a knob attached to a rotary potentiometer. Turn the knob clockwise, and the snake advances from one key frame to the next at a speed proportional to the pot's rate of rotation. Turn it counterclockwise, and the snake retreats along the same path.

Select functions--such as the tongue, eyes, jaw, and neck--are also manually controlled by puppeteers with joysticks. This allows the snake to react in a realistic way to the natural flow of scenes during a shot.

After all, the ultimate goal of building such a sophisticated hydraulic robot was to produce the largest, most realistic man-eating anaconda possible. "We go to all this trouble for one reason," says Dave Caldwell, Edge's model shop supervisor. "It's for the moment when people see it for the very first time and say, 'whoa, that's really fantastic.'"

Animation software directs snake's performance

To program sequences of moves into the snake, electrical engineer John Williams created a custom animation program. It applies the "key frame" animation concept in which a user orients an image of the snake on a computer screen in a discrete number of positions--as few as two--and then the computer calculates the path the snake must take to travel from one key frame to the next. Move sequences can then be saved to a file and transferred to the snake's hydraulic control system for playback later.

Initially, engineers used Alias Animator from Alias/Wavefront (Toronto) to do the same task. But the general-purpose nature of that software program made work out of animating move sequences. As an alternative, Williams worked up his Snake Animator with Delphi and Object Pascal from Borland running on Microsoft Windows NT.

Snake Animator can display one, two, or more views of a scene, complete with a three-dimensional snake model mounted to a pedestal. Specific movie sets can be shown, as can such limits as water levels and ceilings.

To position the snake, an operator manipulates on-screen slider controls that correspond to actuators in the snake's body. He or she can also input key-frame positions using a "waldo," a three-ft-long miniature replica of the snake outfitted with pairs of potentiometers at each of the 60 joints. In a way, the waldo acts like an extremely complex computer mouse for controlling the on-screen snake image.

During filming, Williams worked 15-hr days tweaking ongoing shots and programming the next day's action. Linked to the snake control system, his software also functions as a positional feedback tool. "Without it, you wouldn't know where the snake was when it was under water," he says.