I brought this question up to the group I work with here at automationdirect, this is what came up:
-- the interference is caused by the elctro-mechanical contact of the shoes to the rings. (wear spots, oxide build-ups, conductive lubricants, material, etc)
-- it was worse when it ran intermittently versus continuously
-- 4-20 ma or 0-10 vdc analog was consistant when isolated from the plc
-- thermocouples just didn't work very well at all.
This is all I could get for you.
Carl