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of the. subassemblies m=&bed Using the pradicate
CURRENT-FIXTURE.
Similarly, the specification of tools can be described by the following
predicate
ifrlisonassMlMymachineam
ifrl is not on apsanbly machinc om CURRENT-TOOL ( I1 , am ) =
where am and rl are two variables repnSenting assembly mechim and
tools. respectively,
if tl is not on any assembly machine
if rl is on an assembly machine.
.-=I"". name of the assembly machine,
and
if am has no tool mounted on it
'1= Kif the tod. if am has a tod mound on it.
It is assumed drat in a formula CURREW-TOOL( tl , am ), 11 and am can-
not be NULL at the Same time.
As an example, consider the state tqnesentation of the assembly in Fig-
ure 1. The state of the assembly isrcprcsented in both relational form and
component form. Subassembly C1 is fixed by the fixture F1 while com-
ponent co is a single component and has no fix- assigned to it. It is
assumed that there are only two typts of took in this system, a GRIPPER
and a SCREW_DRIVER. currently. the GRIFTER is mounted on the
assembly machine ROBOT and the SCREW_DRIVER is available for use,
but not mounted on any assembly machine.
2.3. Knowledge Acqulalon Mechanlsm
The lolowledge acquisition mechanisn consists of three modules. l'k
first module is the preoedence knowledge acquisition mechanism. It utilizes
the CAD description of aproduct from the world database and produces the
precedence knowledge about mating tasks that is. which mating task
should go 6rst and which should go next, etc. Based on the prrcedence
knowledge, the second module. the fixture specification mechanism, finds
those subassemblies of the product which BIC suitable for applying fixtures.
Integer programming techniques are used to spec@ the faces on which the
fixture elements can be applied to ensure successful assemblying. ?he third
module is the tool selection mechanism. Based on the tvpes of mating tasks
in the CAD model and the took provided in the assembly cell, this mechan-
ism specifies which mating task should be performed by what tool accord-
ing to availabk manuals or experience. In this papa, we assume that the
tool selection is simply a direct mapping between tasks and took. thus only
the first two mechanisms will be discussed.
2.3.1. Precedence Knowledge Acqulsftlon Mechanism
The ordering of mating tasks of a feusible assembly plan has to satisfy
certain constraints. The knowledge about the ordering wnstraints among
mating tasb is called the precedence knowledge of an assembly. It has
precondition and post-condition l3.51. ?he prc-condition of a mating task
(T,) is the set of states such that it is guaranteed that task (Ti) can be suc-
cessfully pexfonned once any one of these states appears. The posr-