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There is feedback from the motors: electric motors draw a lot more current when they stall. This is generally how factory auto-down systems know when to stop. Telling apart stalling because you've reached the top vs something is blocking requires at least one more sensor or timer, but is doable. To rig a system like you're talking is very much possible and without much expense either, I think, but you'll probably require a fair amount of time - more if you have to pick up the skills as you go.
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