Chapter 2. Prehension

Chapter 4 - Planning of Prehension 65
the motion, kinetic analyses of muscle/joint forces and torques, or
muscular innervation). The other concerns the movement product or
goal of the action. Using the terminology of Schmidt (1988), the en-
vironmentally defined goal (e.g., placing a mug in a dishwasher,
throwing a basketball through a hoop, grasping a hammer) is the task.
In movement sciences, the distinction between movement process and
movement product has been a valuable one (Gentile, 1972; Schmidt,
1975, 1988).
Parallel to movement process and movement product, a distinction
is made in robotics between trajectory planning and task planning.
Traiectorv Dlanning is primarily concerned with transporting the hand
from a starting position to a final location in space. The paths through
which the arm and fingers move have to be determined over time.
This can be done under the direct control of an active controller, or it
can be done in a passive sense. Involved in this latter idea is the no-
tion that the controller sets up key parameters, and lets the ‘physics
acting on the system’ do the control. Task planning, on the other
hand, is used to achieve some goal. A task plan consists of desired
subgoals without regard to the details of the actions necessary to ac-
complish them.
Since these two types of planning are fundamentally different, task
planning is addressed in this chapter, and trajectory planning and
movement execution are detailed in the next chapter.
4.2 Task Plans
A task plan is a scheme for achieving a goal. A goal to grasp a
hammer in order to place it on a shelf must be broken down into a
method for achieving that goal. In this section, task plans and their
components are evaluated at three different levels: as a plan one might
construct for a robot, as a plan distributed over many processes, and
as a plan as it might look in the nervous system.
4.2.1 Robot task plans
In the robotics literature, task plans are constructed in order to
achieve some goal (see Figure 4.1). For example, Lozano-Pert5z and
Winston (1977) list five phases involved in a task such as ‘insert peg
in hole’. First, there is a gross motion to get the arm to the peg. Once
this is achieved, the gripper opens and grasps the peg. There is then a
gross motion to move the arm to the hole while the hand is holding the
peg. Then there is a fine motion of the arm during the time the peg is