Chapter 2. Prehension

66 THE PHASES OF PREHENSION
being pushed into the hole, while the controller complies with the
forces generated by such activity. Finally, an ungrasp occurs where
the hand opens, thus releasing the peg. Using the language, LAMA,
Lozano-Per6z and Winston construct these steps as a task plan
containing five commands. However, this represents only a general
scheme for what to accomplish. In order to actually perform the task,
this 'skeleton' plan is filled out by LAMA. Extra commands are added
describing the sensory information necessary for performing the sub-
task. For example, the fine motion command is expanded into the
following guarded motion commands:
MOVE ALONG Z-AXIS
STOP WHEN FORCE ALONG Z-AXIS = 1 0 NEWTONS
This specifies both the direction of the movement in the hand's
Cartesian coordinate frame and also the force sensor to test for sub-
task completion. As specified, there is still no guarantee that the peg
will be inserted in the hole, for example, if the gross motion moves to
an incorrect location and the hand is now pressing against the table
surface.
"Pick up peg and insert in hole"
Figure 4.1 Task plan for robot manipulator and end-effector.
Lozano-PerCz & Winston (1977) identify five phases for this task.
In LAMA, a task plan is a skeleton version of a plan that is filled
in for a feedback controller.
What can be observed about this task plan? The goal of 'insert peg
in hole' is subdivided into sub-tasks occurring in serial order'. As in
lIn an analagous framework within the human movement literature, Lashley
(1951) was concerned with the representation of serial behaviour in order to achieve
a goal.
I