Inner workings
Pneumatic muscles were not powerful enough to move larger objects, like an artificial human arm, so hydraulics were used for large figures. On/off movement would cause an arm to be either up over the artificial man's head (on switch), or down (off switch), but no movement in between. To create realistic in-between movement in large figures, an analog system was used. This gave the figure's limbs/parts a full range of in-between motion, rather than only two positions. The digital system was used with small pneumatic moving limbs (eyelids, beaks, fingers), and the analog system was used for large hydraulic human or animal (arms, heads) moving limbs.
To permit a high degree of freedom, the control cylinders resemble typical miniature pneumatic or hydraulic cylinders, but mount the back of the cylinder on a ball joint and threaded rod. This ball joint permits the cylinders to float freely inside the frame, such as when the wrist joint rotates and flexes.
Disney's technology is not infallible however; the oil-filled cylinders do occasionally drip or leak. It is sometimes necessary to do makeup touch-up work, or to strip the clothing off a figure due to leaking fluids inside. The Tiki Room remains a pneumatic theatrical set, primarily due to the leakage concerns — Disney does not want hydraulic fluids dripping down onto the audience during a show.
Because each individual cylinder requires its own control/data channel, the original audio-animatronic figures were relatively simple in design to reduce the number of necessary channels. For example, the first human designs (referred to internally by Disney as series A-1) included all four fingers of the hand as one actuator. With modern digital computers and vast data storage, the number of channels is virtually unlimited. The current versions (series A-100) now have individual actuators for each finger, and similar improvements have spread throughout the figures.
Compliance is a new technology that allows faster, more realistic movements without sacrificing control. In the older figures, a fast limb movement would cause the entire figure to shake in a strange way. The Imagineers thus had to program slower movements, sacrificing speed in order to gain control. This was frustrating for the animators, who wanted faster movements in many cases. Compliance improves the situation by allowing limbs to continue past the points where they are programmed to stop; they then return quickly to the "intended" position, much as real organic body parts do. The various elements also slow to a stop at their various positions, instead of using the immediate stops that caused the unwanted shaking. This absorbs shock, much like the shock absorbers on a car or the natural shock absorption in a living body.
The skin of an Audio-Animatronics (shorthand AA) is made from silicone rubber. Because the neck is so much narrower than the rest of the skull, the skull skin cover has a zipper up the back to permit easy removal. The facial appearance is painted onto the rubber, and standard cosmetic makeup is also used. Over time the flexing causes the paint to loosen and fall off, so occasional makeup work and repainting is required.
Generally as the rubber skin flexes, the stress causes it to dry and begin to crack. Figures that do not have a high-degree of motion flexibility (such as the older A-1 series Lincoln) may only need the skin to be replaced every 10 years. The most recent A-100 series human AA's (such as for Barack Obama) also include flexion actuators that move the cheeks and eyebrows to permit more realistic expressions, but the skin wears out more quickly and needs replacement at least every five years.
The wig on each human AA is made from natural human hair for the highest degree of realism, but using real hair creates its own problems since the changing humidity and constant rapid motions of the moving AA carriage hardware throughout the day cause the hair to slowly lose its styling, requiring touch-ups before each day's showing.