You may want to check out Wayne-Dalton's I-Drive an innovative design
that came out about 2 years ago, have it on mine 2 years and have had
no trouble. It installs above the garage door, and pulls the garage up
using wires that are wound around drums at the end of a rod. It does
away with the installation of a unit in the center of your garage and
looks a lot neater. However it does not use sensors, the motors are
designed to reverse if the doors encounter some resistance