4
Appendix F
More Feedback/ Precision
There
are
three
ways
to
improve
precision
and
get
more
information
from
the
VXM.
1.
Use
the
limit
switch
to
establish
a
home
position.
2.
Turn
on
backlash
compensation
feature
if
high
accuracy
is
needed
when
moving
both
directions.
3.
Set
the
limit
switch
output
feature
to
let
the
host
computer
know
that
a
positioner
has
inadvertently
hit
a
limit
switch.
Use Limit Switch for Home
The
limit
switch
on
a
typical
Velmex
slide
assembly
can
provide
a
repeatability
of
better
than
0.0004”
(0.010
mm.)
Using
a
limit
switch
on
initial
power-up
to
set
a
home
is
a
good
way
to
insure
consistent
performance
from
a
precision
positioning
system.
See
Appendix
O
for
setting
program
4
to
do
homing
routines
at
power-up.
end
4.*.*.*".3
start
®
¬
STOPPED
BY
LIMIT
SWITCH
Backlash Compensation Improves
Accuracy
Mechanical
devices
have
some
clearances
between
mating
parts.
Whenever
such
a
device
is
commanded
to
reverse
direction
there
can
be
some
lost
motion.
The
VXM
has
the
following
command
to
overcome
this
situation.
B
x
%
DFNODVK&RPSHQVDWLRQFRPSHQVDWLRQLVVWHSVZKHQ RIIZKHQ
(default).
Desired
number
of
“comp”
steps
can
be
input
directly
(
x
=
2
to
255)*
The
VXM
can
compensate
for
mechanical
backlash
by
ending
every
index
in
the
positive
direction.
When
backlash
compensation
is
on,
and
a
motor
makes
a
negative
Index,
“comp”
number
of
steps
will
be
added
to
the
Index.
The
Motor
will
then
immediately
reverse,
indexing
positive
“comp”
number
of
steps.
The
VXM
will
not
do
the
ending
positive
“comp”
step
move
if
the
index
is
the
I
m
M-0
(Index
until
negative
limit
encountered.)
Memory
usage
=
0
bytes
(this
command
is
immediate,
and
uses
a
reserved
memory
location)
B
<cr>
Read
acklash
Compensation
setting,
off
when
value
returned=0(default)
The
number
of
“comp”
steps
will
be
returned
when
set**
B
**Only
on
VXM
firmware
versions
1.25
&
up,
older
versions
return
“1”
to
indicate
on
for
20
steps
of
compensation
