Relative Position Breakpoints Algorithm - National Instruments NI-Motion User Manual

Note All breakpoints can be affected by jitter in the motion control system. For example,
if you have a very small breakpoint window, the jitter in the motion control system could
cause the position to change enough to reach the breakpoint when a breakpoint is not
intended. Increase the size of the breakpoint window to compensate for system jitter.

Relative Position Breakpoints Algorithm

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breakpoint is useful in this example because the current position is not
important. The application must move the axis a specific number of counts
from wherever it is, and then generate a breakpoint.
Figure 12-6 shows the basic algorithm for relative breakpoints.
Figure 12-6. Relative Position Breakpoints Algorithm
Notice that relative breakpoints are not ideal for periodic breakpoints.
There is a latency between the time a breakpoint generates and is
re-enabled. If the axis is moving at sufficient velocity, the breakpoint
re-enables only after the axis has moved slightly. Because a relative
breakpoint generates relative to the position the axis was in when the
breakpoint was enabled, the latency between generation and re-enabling
can cause additional counts between breakpoints.
For example, the actual breakpoints might occur at positions 5,000; 10,003;
15,006; and 20,012. In this example, the axis moves three counts between
a breakpoint and the subsequent re-enabling. For exact distances between
breakpoints at high speeds, use
Periodically Occurring
12-13
Configure breakpoint
Configure breakpoint
for relative mode
Load breakpoint position
Relative position where
you want to trigger an event
Enable breakpoint
Wait for breakpoint
to cause a trigger
Buffered Breakpoints (NI 7350 only)
Breakpoints.
Chapter 12 Synchronization
or
NI-Motion User Manual