Inverse Kinematics relies on hierarchical relationships between the components of a model and the IK system to pose and animate the components. Before using Inverse Kinematics it is important to understand hierarchies in Maya.
A hierarchy is composed of a series of nodes that are combined for some purposeful relationship. Individual nodes may represent the surfaces of a model, a network of textures that describe a shading material, or series of joints in a skeleton. Hierarchies are structured in a top-down manner, with one node at the top (the parent node or root node) and other nodes (child nodes or leaf nodes) attached and interconnected beneath the top node.
Selecting the parent node at the top of the hierarchy also selects the items contained in the hierarchy below. Selecting a child node lower in the hierarchy selects any child nodes that are lower in the hierarchy.
Hierarchies allow you to create complex structures with relationships between components. For example, when you animate a hierarchical model, you simply need to select and move the parent node of the hierarchy and the rest of the model (child nodes) also moves.
Regardless of whether you decide to animate the mechanical arm using FK or IK, or a combination of both methods on the same model, the best approach involves making the mechanical arm model into a hierarchical structure.