Absolute feedback compensation

Summit servo drives include the ability to compensate absolute feedback inconsistencies caused by mechanical imperfections of the feedback and mechanical components.

Absolute feedback compensation is a LUT (Look-up table) that uses feedback position as a selector of the index. The values from the table are applied as an offset in the output of the selected absolute feedback (primary or secondary) by the user.

The values of the compensation table of the primary and secondary absolute encoders are applied as an offset in the output value of each encoder:

Example of an Absolute Feedback Compensation Table

The number of encoder counts that covers every value of the table is the result of dividing the resolution of the absolute encoder by the length of the compensation table. The compensation table holds 2048 positions from index 0 to 2047. E.g. With a resolution of 22 bits and a length of 2048 values, each value of the table covers:

That means that for index equal to 0, the range of encoder counts where the first value of the table applies go from 0 to 2048 counts, having full impact in the middle of the range. The rest of the counts are linearly interpolated. See the next table and graph for an example:

To control this table multiple registers are defined:

· 0x2300 - Abs feedback 1 compensation table- Enable: Allows enabling/disabling the compensation table compensation for the Primary absolute encoder slave 1.

· 0x2301 - Abs feedback 1 compensation table - Index: Allows selecting the index at which the data will be written / read by the user in the Compensation table for the Primary absolute encoder slave 1.

· 0x2302 - Abs feedback 1 compensation table - Value: Shows the data available in the compensation table for the Primary absolute encoder slave 1 at the index specified. Writing to this parameter also replaces the value in the table with a newer one.

· 0x2303 - Abs feedback 2 compensation table - Enable: Allows enabling/disabling the compensation table compensation for the Secondary absolute encoder slave 1.

· 0x2304 - Abs feedback 2 compensation table - Index: Allows selecting the index at which the data will be written / read by the user in the Compensation table for the Secondary absolute encoder slave 1.

· 0x2305 - Abs feedback 2 compensation table - Value: Shows the data available in the compensation table for the Secondary absolute encoder slave 1 at the index specified. Writing to this parameter also replaces the value in the table with a newer one.

The values of the absolute Primary absolute encoder slave 1 and Secondary absolute encoder slave 1 after the effect of the compensation table offset can be monitored in the following registers:

· 0x204C - Primary Absolute Slave 1 - Compensated Position

· 0x2056 - Secondary Absolute Slave 1 - Compensated Position

To store the values of the compensation table to the NVM a store all process must be performed.

Absolute feedback compensation tables can only be applied to encoders that are hardwired into the drive. These are defined as registers starting with Primary Absolute Encoder 1 or Secondary Absolute Encoder 1. These tables will not work with an encoder in daisy chain.