Emerson FM-3 - Manual (Page 34)

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FM-3 Programming Module Reference Manual
Absolute vs. Incremental
The difference between absolute and incremental indexes is that absolute indexes move to a
specific absolute position and incremental indexes move the motor a specific distance. The
figures and explanations below demonstrate this concept.
Absolute Indexes
Absolute indexes are used in applications where the motor must travel to a specific position,
regardless of where the motor is when the index is initiated.
The FM-3 module calculates the distance required to move to the specified position from the
current position.
Absolute Index
Figure 23: Absolute Index Example 1
In the example above, the current position is 1 rev. If this index is initiated, the motor will
travel to a position of 5 revs no matter where it is sitting before the move. From 3 revs, it will
travel 2 revs to finish at 5 revs. If the absolute index to 5 revs is initiated a second time
immediately after the index, no motion will occur because the motor will already be at a
position of 5 revs.
The direction of an Absolute Index is determined by the starting position and the absolute
index position. If the starting position for the above index is 9 revs, then the motor will rotate
in the negative direction to end up at 5 revs. The diagram below shows this.
Figure 24: Absolute Index Example 2
Absolute Index
Start Position = 1 Rev
Index Position = 5 Revs
Absolute Index
Start Position = 9 Revs
Index Position = 5 Revs