<s>
A	O
virtual	B-General_Concept
fixture	I-General_Concept
is	O
an	O
overlay	O
of	O
augmented	O
sensory	O
information	O
upon	O
a	O
user	O
's	O
perception	O
of	O
a	O
real	O
environment	O
in	O
order	O
to	O
improve	O
human	O
performance	O
in	O
both	O
direct	O
and	O
remotely	O
manipulated	O
tasks	O
.	O
</s>
<s>
Developed	O
in	O
the	O
early	O
1990s	O
by	O
Louis	O
Rosenberg	O
at	O
the	O
U.S.	O
Air	O
Force	O
Research	O
Laboratory	O
(	O
AFRL	O
)	O
,	O
Virtual	B-General_Concept
Fixtures	I-General_Concept
was	O
a	O
pioneering	O
platform	O
in	O
virtual	B-Application
reality	I-Application
and	O
augmented	B-General_Concept
reality	I-General_Concept
technologies	O
.	O
</s>
<s>
Virtual	B-General_Concept
Fixtures	I-General_Concept
was	O
first	O
developed	O
by	O
Louis	O
Rosenberg	O
in	O
1992	O
at	O
the	O
USAF	O
Armstrong	O
Labs	O
,	O
resulting	O
in	O
the	O
first	O
immersive	O
augmented	B-General_Concept
reality	I-General_Concept
system	O
ever	O
built	O
.	O
</s>
<s>
Because	O
3D	O
graphics	O
were	O
too	O
slow	O
in	O
the	O
early	O
1990s	O
to	O
present	O
a	O
photorealistic	O
and	O
spatially-registered	O
augmented	B-General_Concept
reality	I-General_Concept
,	O
Virtual	B-General_Concept
Fixtures	I-General_Concept
used	O
two	O
real	O
physical	O
robots	O
,	O
controlled	O
by	O
a	O
full	O
upper-body	O
exoskeleton	O
worn	O
by	O
the	O
user	O
.	O
</s>
<s>
Fitts	B-General_Concept
Law	I-General_Concept
performance	O
testing	O
was	O
conducted	O
on	O
batteries	O
of	O
human	O
test	O
subjects	O
,	O
demonstrating	O
for	O
the	O
first	O
time	O
,	O
that	O
a	O
significant	O
enhancement	O
in	O
human	O
performance	O
of	O
real-world	O
dexterous	O
tasks	O
could	O
be	O
achieved	O
by	O
providing	O
immersive	O
augmented	B-General_Concept
reality	I-General_Concept
overlays	O
to	O
users	O
.	O
</s>
<s>
The	O
concept	O
of	O
virtual	B-General_Concept
fixtures	I-General_Concept
was	O
first	O
introduced	O
as	O
an	O
overlay	O
of	O
virtual	O
sensory	O
information	O
on	O
a	O
workspace	O
in	O
order	O
to	O
improve	O
human	O
performance	O
in	O
direct	O
and	O
remotely	O
manipulated	O
tasks	O
.	O
</s>
<s>
The	O
concept	O
of	O
sensory	O
overlays	O
is	O
difficult	O
to	O
visualize	O
and	O
talk	O
about	O
,	O
as	O
a	O
consequence	O
the	O
virtual	B-General_Concept
fixture	I-General_Concept
metaphor	O
was	O
introduced	O
.	O
</s>
<s>
To	O
understand	O
what	O
a	O
virtual	B-General_Concept
fixture	I-General_Concept
is	O
an	O
analogy	O
with	O
a	O
real	O
physical	O
fixture	O
such	O
as	O
a	O
ruler	O
is	O
often	O
used	O
.	O
</s>
<s>
When	O
the	O
Virtual	B-General_Concept
Fixture	I-General_Concept
concept	O
was	O
proposed	O
to	O
the	O
U.S.	O
Air	O
Force	O
in	O
1991	O
,	O
augmented	O
surgery	O
was	O
an	O
example	O
use	O
case	O
,	O
expanding	O
the	O
idea	O
from	O
a	O
virtual	O
ruler	O
guiding	O
a	O
real	O
pencil	O
,	O
to	O
a	O
virtual	O
medical	O
fixture	O
guiding	O
a	O
real	O
physical	O
scalpel	O
manipulated	O
by	O
a	O
real	O
surgeon	O
.	O
</s>
<s>
The	O
definition	O
of	O
virtual	B-General_Concept
fixtures	I-General_Concept
is	O
much	O
broader	O
than	O
simply	O
providing	O
guidance	O
of	O
the	O
end-effector	O
.	O
</s>
<s>
For	O
example	O
,	O
auditory	O
virtual	B-General_Concept
fixtures	I-General_Concept
are	O
used	O
to	O
increase	O
the	O
user	O
awareness	O
by	O
providing	O
audio	O
clues	O
that	O
helps	O
the	O
user	O
by	O
providing	O
multi	O
modal	O
cues	O
for	O
localization	O
of	O
the	O
end-effector	O
.	O
</s>
<s>
However	O
,	O
in	O
the	O
context	O
of	O
human-machine	O
collaborative	O
systems	O
,	O
the	O
term	O
virtual	B-General_Concept
fixtures	I-General_Concept
is	O
often	O
used	O
to	O
refer	O
to	O
a	O
task	O
dependent	O
virtual	O
aid	O
that	O
is	O
overlaid	O
upon	O
a	O
real	O
environment	O
and	O
guides	O
the	O
user	O
's	O
motion	O
along	O
desired	O
directions	O
while	O
preventing	O
motion	O
in	O
undesired	O
directions	O
or	O
regions	O
of	O
the	O
workspace	O
.	O
</s>
<s>
Virtual	B-General_Concept
fixtures	I-General_Concept
can	O
be	O
either	O
guiding	O
virtual	B-General_Concept
fixtures	I-General_Concept
or	O
forbidden	O
regions	O
virtual	B-General_Concept
fixtures	I-General_Concept
.	O
</s>
<s>
A	O
forbidden	O
regions	O
virtual	B-General_Concept
fixture	I-General_Concept
could	O
be	O
used	O
,	O
for	O
example	O
,	O
in	O
a	O
teleoperated	O
setting	O
where	O
the	O
operator	O
has	O
to	O
drive	O
a	O
vehicle	O
at	O
a	O
remote	O
site	O
to	O
accomplish	O
an	O
objective	O
.	O
</s>
<s>
Such	O
illegal	O
commands	O
could	O
easily	O
be	O
sent	O
by	O
an	O
operator	O
because	O
of	O
,	O
for	O
instance	O
,	O
delays	O
in	O
the	O
teleoperation	O
loop	O
,	O
poor	O
telepresence	B-Application
or	O
a	O
number	O
of	O
other	O
reasons	O
.	O
</s>
<s>
An	O
example	O
of	O
a	O
guiding	O
virtual	B-General_Concept
fixture	I-General_Concept
could	O
be	O
when	O
the	O
vehicle	O
must	O
follow	O
a	O
certain	O
trajectory	O
,	O
</s>
<s>
With	O
both	O
forbidden	O
regions	O
and	O
guiding	O
virtual	B-General_Concept
fixtures	I-General_Concept
the	O
stiffness	O
,	O
or	O
its	O
inverse	O
the	O
compliance	O
,	O
of	O
the	O
fixture	O
can	O
be	O
adjusted	O
.	O
</s>
<s>
This	O
section	O
describes	O
how	O
a	O
control	O
law	O
that	O
implements	O
virtual	B-General_Concept
fixtures	I-General_Concept
can	O
be	O
derived	O
.	O
</s>
