Sliding door operator

(smaller retail businesses prefer swing door operators.)

Sensors can be used to detect obstacles in the path of closing doors.

Other sensors such as resistance-sensing motors can be used to reverse the closing action of the door after an obstacle is encountered.

An electric motor, geared down to get a lower speed and a higher torque, drives a pulley at one end of a belt.

Historically, elevator doors were opened using simple harmonic motion by a set of mechanical linkages; the motor, geared down, would rotate linked arms, which in turn would drive the door.