ORIGINAL: marc s
You need to detect a change in RPM instantly and brake the motor with a pulse of reverse
Easier said than done..... often the gear will encounter 'resistance' at different stages of its travel from cycle to cycle, maybe retracting gear at different speeds following take off etc so 'when' exactly do you tell the motor to stop as there is a strong chance it will stop mid cycle?
With current sensing which is still not ideal I admit you can dial in some 'allowance' and therefore avoid some cases of hung gear.
In an ideal world what is actually required is a limit switch at each end of the retract unit, positioned so it trips the electronics and allows the unit to avoid the 'crunch zone' - on big retracts this I doubt would be a problem if they were designed/made to accommodate this, smaller units pose problems simply due to being small.
marcs
Actually its not difficult to do. I did software for that exact application back in 1998.
The current is irrelevant. The speed of the motor is the only reliable no cost sensing option.
The best solution is to change the thread and pitch so that there is no locking.
Generally a helix pitch angle of more than 22 deg is enough.
Any capable CNC operator can cut you such a thread and nut/camblock.