Ideas for a DIY ramp

["Bill Stock" <me7@xxxxxxxxxxx>]

I'd like to build a folding ramp for an exterior door. This is not a wheel
chair ramp, so it does not have to be wheelchair specific.

My design criteria are as follows:

It would be nice if the extended ramp did not land more than 4' from the
wall.
Step height is about 18"
This is a nice to have, so cheap is good.
Automatic lowering would be nice when the door is opened
It would be nice if it did not kill anyone/anything when lowering. (inside
only)
Exterior door opens in.

My initial thought was some diamond plate, with an electric hoist and some
kind of pulleys from the rafters (garage).
Plan B, would be some kind of hydraulic cylinders.
Plan C, some kind of spring loaded manual system like a garage door.

The hoist would be kind of messy with the cables in the way.
The hydraulics are likely more dangerous in that they have crushing power.

Any thoughts or reasonable commercial alternatives.