A quick google search of "robot distance sensor" turns up quite a few choices. Can you arrange to drop the bag from (let's say) 50mm above where it thinks the top of the stack is, then have it measure the distance to the new top of the stack (having dropped off the previous bag), and remember that for the next cycle?

Quote (Tablet)

Im looking for suggestions on how to automatically detect and offset for this situation without operator intervention

Easy peasy. Couple of paths to success:

1) Your pallet is always in the same place, yes? Hang a laser distance sensor above the pallet. You could hang it from the ceiling if you want (and pick the correct sensor). Have it measure height after each pick/place and offset the next z height for the place operation.

2) Are all bags in a given lot the same thickness? Measure bag height on the conveyor feeding the robot, and use that thickness to offset each place operation. More complicated from a logic standpoint if they aren't consistent (because you'll need to measure Bag A, place Bag A on the pallet, then use bag A's measured thickness to offset the place operation for Bag B and so on) but not terribly difficult

3) Don't have enough overhead clearance for a laser? Install a profiling light curtain either side of the pallet, which will measure the current height. Offset the next placement cycle accordingly.

We use laser distance sensors located on the EOAT.
We put it on the EOAT so that it is directly linked to the PLC in the controller, and makes dynamic decisions if required. We know little about your cycle time, pallet load, pattern or reach study. Any laser mounted outside of the EOAT should still be connected to the robot PLC.

ABB will have recommendation for you, and should be your first stop.