A rigid robot is translated by using two parameters, . Using definitions from Section 3.2.1, , and is defined as
Now consider a solid representation of , defined in terms of primitives. Each primitive of the form
The translated robot is denoted as . Translation by is the identity transformation, which results in , if it is assumed that (recall that does not necessarily have to be initially embedded in ). It will be convenient to use the term degrees of freedom to refer to the maximum number of independent parameters that are needed to completely characterize the transformation applied to the robot. If the set of allowable values for and forms a two-dimensional subset of , then the degrees of freedom is two.
Suppose that is defined directly in with translation. As shown in Figure 3.7, there are two interpretations of a rigid-body transformation applied to : 1) The world frame remains fixed and the robot is transformed; 2) the robot remains fixed and the world frame is translated. The first one characterizes the effect of the transformation from a fixed world frame, and the second one indicates how the transformation appears from the robot's perspective. Unless stated otherwise, the first interpretation will be used when we refer to motion planning problems because it often models a robot moving in a physical world. Numerous books cover coordinate transformations under the second interpretation. This has been known to cause confusion because the transformations may sometimes appear ``backward'' from what is desired in motion planning.
Steven M LaValle 2020-08-14