A Stewart Platform is 6 degree of freedom parallel manipulator that uses 6 prismatic actuators and 12 universal joints. The actuators are attached in pairs to three positions on the platform's baseplate, crossing over to three mounting points on a top plate.
The design theory of Stewart platforms is somewhat complicated. The forward kinematics of the system are pretty involved. This means that determining the final position given the actuator positions is relatively difficult. This makes a Stewart platform a poor input device. On the other hand, the inverse kinematics of the system are simple. This means that determining the necessary actuator positions to support a given final position is easy. This makes a Stewart platform an excellent output device.
The prismatic actuators can be replaced by rotary actuators using a crank arm and push rod configuration with ball joints as in the Racing Simulator project. The constraints of this system has the same result. The rotation of the motor causes a lengthening and shortening of an imaginary line between the center of the motor shaft and the upper ball joint. Linear actuators are typically much more expensive for a similar power and resolution compared to a motor. This “rotary” Stewart platform is a compromise between more complex calculations and cheaper hardware.
Stewart platforms are what is known as parallel robots. This means that all of the actuators are fixed in place and move an end effector. In contrast, the actuators in serial robots may be moved by other actuators. One of the key advantages of parallel robots is that they tend to be much stronger than serial robots. In the case of a stewart platform, all six actuators are supporting the load. In serial robot with the same degree of freedom each actuator would not only have to carry the entire load, but also the load of the next actuator in the chain. The most common disadvantage of parallel robots is that they have more limited range