I have considered automatic gates but I never got around to it.

The gates must be safe in that if they are obstructed they stop without damage, the gates are quite long and heavy (usually) which requires a tremendous torque applied by the actuator.
So I came up with an idea...
Usually, the gate swings on a vertical pivot but I think it would be easy to make an automatic opening gate by moving the pivot point slightly. When the gate is closed the top pivot is further out from the post than the lower pivot so the gate naturally 'hangs' in the closed position. Now if the top pivot is moved to one side the gate will lean slightly and also rise a little at the closing end, the result is that the gate will swing open until it hits a stopper post. To close the gate move the pivot to the original position.
I think you could do this by incorporating an eccentric mechanism in one of the hinges. The gate would also need some sort of positive latch to keep it closed.
My thoughts for what they may be worth...
