Delta RMC151 User Manual download pdf (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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