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More Feedback/ Precision
Appendix F
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
S1M500,I1M0,I1M-200,IA1M-0,R S1M500,I1M0,I1M-200,IA1M-0,R
start
®
¬
STOPPED BY
LIMIT SWITCH
Backlash Compensation Improves Accuracey
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.
Bx Backlash Compensation, compensation is on when x=1, off when x=0 (default).
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, 20 steps will be added to the Index. The Motor will then immediately
reverse, indexing positive 20 steps. The VXM will not do the ending positive 20 step
move if the index is the ImM-0 (Index until negative limit encountered.)
Memory usage = 0 bytes (this command is immediate, and uses a reserved
memory location)
Indicate Limit Switch Encounter to Detect Faults
A properly connected/ configured limit switch will always stop a motor immediately.
Normally the host computer would not know if a limit switch stopped a move.
With the “O” command described below, the VXM can be set to notify the host computer
that it has hit a limit switch and stop the entire program too.
Ox Indicate Limit Switch Over-travel to the host.
The VXM sends "O" when x=1 and a limit switch is encountered. This command
is useful when the host needs to know if a positioner's travel has been exceeded
due to a motor stall or an index(es) that are too long. When Indicate-Limit-
Switch-Over-travel = 1, the VXM transmits the single character "O" to the host
when an indexing motor activates it's limit switch input.
Memory usage = 0 bytes (this command is immediate, and uses a reserved
memory location)
NOTE: limit switches also stop motor motion immediately.
B<cr> Read Backlash Compensation setting, compensation is on when x=1, off when
x=0(default).
x Value Function
0 Disabled (default)
1 Send O to host
when hit Limit
2 Stop program if hit
Limit
3 Send O to host
and Stop program
when hit Limit
O<cr> Read current setting for Indicate Limit Switch Over-travel to the host.
See Also
V, ?, ImM0, ImM-0, Example #7
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