Control of a Movable Barrier with Obstacle Detection based upon a Runtime Compensated Force Threshold Value

A movable barrier operator having a motor controller 10 and motor 11 that control movement of a movable barrier 12, and also has an obstacle detector 14 that utilizes an automatically determined excess force threshold value to permit reliable detection of an obstruction. A wide variety of operational circumstances such as, changing physical circumstances, aging components, temperature variations, and motor runtime may be detected by the obstacle detector. Preferably, a characteristic force value for the system is frequently updated as a function of actual measured force required to move the barrier determined from motor current and compensated with respect to temperature and other conditions such as motor runtime. This characteristic force value is then utilized to determine the excess force threshold value. The duration of motor operation is sensed and either on-time or off-time may be used to determine the motor operation compensation value, particularly at low temperature.