Rotary Scaling

SSI, resolver, and quadrature axes can be configured as rotary axes. To have any useful meaning, the counts from the encoder must be scaled to position units. This topic describes the scaling parameters for a rotary axis and provides instructions on how to scale the axis.

Delta Recommends using the Scale/Offset Wizard for scaling the position. If you need to do it manually, read this topic.

Calculating the Scale Parameters

To calculate the scale parameters, use the Rotational Scale/Offset wizard.

You can also enter the scale parameter manually. Refer to the Scaling Parameter Details section below for a description of each parameter.

Scaling Parameter Details

A rotary axis uses the Count Unwind, Position Unwind, Position Offset, and Count Offset parameters to calculate the position units from the counts:

Count Unwind

The counts for a rotary input are kept within a defined range. When the input goes beyond one end of this range, the counts wrap to the other end of the range. The Count Unwind parameter defines this range. Any control being performed using this input is not interrupted; this is not seen as a position discontinuity.

For absolute rotary axes, this parameter must be a power of 2. If it were not, the wrap point of the counts would not coincide with the wrap point of the encoder and it would not be possible to determine absolute position.

RMC Resolver axes will always give 65,536 counts per revolution.

Example:
Consider a single-turn SSI encoder with 8192 counts per revolution. If the Count Unwind is set to 8192, the counts will go from 0 to 8191 and wrap once per revolution. If the Count Unwind is set to 2048, the counts will go from 0 to 2048 and will wrap four times per revolution.

Position Unwind and Position Offset

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.