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- 自动装配规划系统英文文献和中文翻译(5):http://www.751com.cn/fanyi/lunwen_6791.html