I built a home-brew autofocuser for my 600mm f/4 Nikon lens using a stepper motor and a timing belt. It is able to move the focus barrel accurately enough to focus when I operate it manually. However, it doesn't work with SGP autofocus because the belt requires a fair amount of tension, and hence a barrel position is not the same depending on the direction it is approached from. SGP autofocus typically finds one side of the "V" perfectly, but then uses all of the points for the other side of the "V" tensioning the belt and leaves the lens out of focus.
Would it be possible to add an autofocus mode where SGP walks the focus in one direction - say "in" - until a minimum HFR is found, then backs off in the "out" direction and walks "in" again until within 10% of the previous minimum?