Control of a moveable barrier

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 obstacle. 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 requirements (and further compensated, with respect to other conditions such as temperature and motor runtime .This characteristic force value is then utilized to determine the excess force threshold value.