Types of Mechanisms

Mechanisms can be classified based on several aspects.

Serial, parallel, and hybrid serial-parallel mechanisms

Based on how the kinematic chains look like, mechanisms can be classified into two broad categories: 1) serial mechanisms, 2) parallel mechanisms, and 3) hybrid serial-parallel mechanisms. Serial mechanisms have open-chain kinematics. Parallel mechanisms have closed-chain kinematics. Hybrid serial-parallel mechanisms combine open-chain and closed-chain kinematics.

A serial mechanism consists of: 1) fixed base, 2) links, and 3) end-effector. A serial mechanism can be either a single-chain serial mechanism or a tree-like mechanism. The number of serial mechanism topologies which can be synthesized is rather limited. A parallel mechanism consists of: 1) fixed base, 2) limbs, and 3) moving platform (aka end-effector). A limb in a parallel mechanism can be composed of more than one link. The number of parallel mechanism topologies which can be synthesized is much more than that of serial mechanism topologies.

A hybrid serial-parallel mechanism is a mechanism which combines both serial and parallel kinematics.

Redundant and non-redundant mechanisms

Based on redundancy, mechanisms can be classified into redundant and non-redundant mechanisms. Redundancy itself can be classified into several types:

Kinematic redundancy – A mechanism is kinematically redundant if the number of its degrees of freedom (DOF) is larger than the number of its variables used to describe a given task. This means that a mechanism is kinematically redundant if the dimension of its task space is smaller than the dimension of its joint space.

Functional redundancy – Even if a mechanism has identical dimensions of its task and joint spaces, it is called functionally redundant when not all components of its task space are of concern for a specific task.

Actuation redundancy – A mechanism is redundantly-actuated of the number of its actuators are more than the number of its DOF. Redundancy is usually created to improve one or more performance measure(s) of the mechanism such as workspace, distance to singularity, stiffness, etc. In other words, a redundant mechanism may provide a larger translational workspace or a larger orientational workspace (tilting capability). It may also eliminate singularities. It may also increase the stiffness of the mechanism. On the other hand, redundancy would result in higher cost.