Allen Bradley's 1756-MO8SE sercos module


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I use (1756-MO8SE) 8-axis sercos module with controllogix PLC. I configure my machine as rotary position servo. I need to change the unwind position programatically. (Normally this has to be done manually with the plc in program mode, by right clicking the axis properties and then writing a constant value.)
There are two ways to handle this. The easiest way to accomplish it is with the SSV instruction. When you want to change the Position unwind value, call as SSV with class name set to AXIS, instance name set to your axis tag, and attribute name set to PositionUnwind (you'll find it in the Configuration folder). Be careful, because the processor has to message down to the MO8SE card, so it's not instantaneous; it might take several scans.

I use a slightly different method. Rockwell seems to really push the rotary axis with unwind, but if your axis doesn't rezero at exactly the same distance every time (which is very realistic given the motor is usually attached to drive something mechanical with chains or belts, etc), then it really sucks. Instead, I set all of my axes up as linear, then execute an MRP set to relative when I want to rezero.

Good Luck!