Instruction manual

7
Scales-Incremental Vs. Absolute
Simply stated, an Incremental system measures the distance it has
traveled relative to a starting point.
An absolute system (ABS) measures its actual position by reading a
pattern which is unique at any given location over its length. The
maximum length of this pattern is 430mm (16.932 in.). The pattern must
then repeat itself.
Consider the illustration above to represent a ProScale ABS. There are
three absolute patterns (each pattern is 430mm long), joined together
end to end. Within each pattern the system is totally absolute. However,
crossing over a pattern joint now presents the readhead with information
identical to what it read in the previous pattern. At this point the system
must be able to recognize that it has crossed over a pattern joint and
therefore must add or subtract the value of 1 pattern offset. In fact,
each time the readhead passes over a pattern joint it must keep track of
how many patterns it has passed, and in which direction. This process is
accomplished by the software in the display.
What does all this mean? If the readhead remains on the same absolute
pattern, it can have power removed, its position changed and power
restored without loss of position information. However, if power is
removed and the readhead passes over a pattern joint, the
transition will not be recognized. When power is restored the
system knows its absolute position on the new pattern, but does
not know how many patterns it has passed, or in which direction!
Displays used in absolute systems provide the operator with a method
to adjust the pattern offset in the display memory so the system displays
the correct reading at all times without loss of accuracy. See Section 3
for information about the different ABS displays and how to correct for
pattern offsets.