Parallel manipulator

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A parallel manipulator consists of a fixed "base" platform, connected to an end effector platform by means of a number of "legs". These legs often consist of an actuated prismatic joint, connected to the platforms through passive (i.e. not actuated) spherical and/or universal joints. Hence, the links feel only traction or compression, not bending, which increases their position accuracy and allows a lighter construction. The actuators for the prismatic joints can be placed in the motionless base platform, so that their mass does not have to be moved, which again makes the construction lighter.
Parallel manipulators have (in principle) high structural stiffness, since the end effector is supported in several places at the same time. All these features result in manipulators with a high bandwidth motion capability. Their major drawback is their limited workspace, because the legs can collide and, in addition, each leg has five passive joints that each have their own mechanical limits. Another drawback of Parallel Robots is that they loose stiffness in singular positions completely (The robot gains finite or infinite degrees of freedom which are uncontrolable; it becomes shaky or mobile). This means that the Jacobian matrix, which is the mapping from joint space to euclidian space, becomes singular (the rank decreases from six).

[edit] Applications

Major industrial applications of these devices are

airplane simulators
automobile simulators
in work processes
photonics / fiber alignment http://www.pb.izm.fhg.de/p2sa/020_Technologies/Optik/FundE_Faserjustage.html

They also become more popular

in high speed, high-accuracy positioning with limited workspaces, such as in assembly of PCB's
as micro manipulators mounted on the end effector of larger but slower serial manipulators
as high speed/high-precision milling machines