Delta RMC151 - Manual (Page 41)

2 Starting Up the RMC
For the range of counts described above, the positions must be kept within a
corresponding range. The Position Unwind and Position Offset parameters define this
range.
The Position Unwind defines how many position units the range will span. The Position
Offset adjusts the modulo position range up or down.
For absolute rotary axes, the Counts are in the opposite direction if the Position Unwind
is negative.
The Counts formula for a positive Position Unwind is:
Counts = (RawCounts + CountOffset) MOD MaxCounts
The Counts formula for a positive Negative Unwind is:
Counts = ([MaxCounts-1] - RawCounts + CountOffset) MOD MaxCounts
Example:
Consider an SSI rotary axis with a Count Unwind of 1024. If the Position Unwind is set
to 10, the counts range from 0 to 1024 will span 10 position units. If the Count Offset is
set to 0, then the range will go from 0 up to, but not including, 10. If the Count Offset
is set to 3, then the range will go from 3 up to, but not including, 13.
• Count Offset
The count offset is important for absolute rotary axes. The position range—as defined
by the Count Unwind, Position Unwind and Position Offset parameters—will start at the
same point where the counts are 0. The Count Offset adjusts the point where the
counts are zero, and thereby adjusts the point where the position range will start.
This parameter is not used by incremental rotary axes, such as quadrature axes.
Example:
Consider a single-turn SSI absolute rotary encoder with 2048 counts. The axis is set up
with a Count Unwind of 2048, a Position Unwind of 360 and a Position Offset of -180.
Therefore, the position range will go from -180 to 180 in one revolution of the encoder.
Negative 180 will occur at the point where the transducer counts are 0. Now, assume
the encoder was mounted on the machine such that -180 pointed straight up, but you
want -180 to be pointing down. To fix this, rotate the encoder so that it is pointing
straight down. Record the counts, and enter the negative of that value in the Count
Offset parameter. -180 will now be pointing down.
2.2.9. Quadrature Scaling
To have any useful meaning, the counts from the transducer must be scaled to position units.
For a quadrature axis, the Position Scale parameter defines the position units as a function of
transducer counts. This topic describes how to correctly calculate this parameter for an
Quadrature encoder.
If your motion is linear, read this topic. If your motion is purely rotary, see the Rotary Scaling
topic.
Delta Recommends using the Scale/Offset Wizard for scaling the position. If you need to do it
manually, read this topic.
Scaling Counts to Position Units
The RMC calculates the quadrature axis Actual Position every control-loop time using the
following formula:
Actual Position [pu] = (Change in Counts [cnt] x Position Scale [pu/cnt]) + Last
Position
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