Abstract: A monitoring circuit for a door operating D.C. electric motor, the circuit comprises a first means for detecting a back EMF of the motor and producing a load signal indicative of the magnitude of the back EMF, a second means for detecting the time rate of change of the load signal and issuing an obstruction signal if the magnitude of the time rate of change of the load signal exceeds a predetermined value.
Abstract: An improved roller door drive assembly comprising a drum, upon which a curtain is rolled, and a gear coaxially mounted to and engaged with the drum. The means of engagement consists of a plurality of forks projecting from the gear adapted to engage drum spokes. This arrangement prevents the ends of the drum while running in a swashplate manner from dragging the gear out of alignment with its driving pinion. In so doing this engagement means prevents excessive wear.