movement may be halted and then reversed alternatively the door 24 may travel to the end of its path indicating that an obstruction does not exist in another approach and as described elsewhere in this specification a secondary obstruction detector for instance sensor 46 maybe used in conjunction with a force measurement to determine whether an obstruction exists in the path of the door 24 referring now to fig2 an example of the control unit 200 is described the control unit 200 includes a memory 202 and a controller 204 the controller 204 receives control signals from a current sensor 206 and a speed sensor 208 the current sensor 206 indicates the amount of electrical current that is present in a motor 212 of a moveable barrier operator the speed sensor 208 indicates how quickly a door 214 is moving in a downward direction the controller 204 receives these measurements from the sensors and from these measurements determines a value representing the amount of force being applied to the door 214 as described elsewhere in this specification the controller 204 compares the measured force to a threshold value the measured force may be a value representative of force for instance it may be a speed of the motor or barrier or it may be the amount of current going to the motor sensed by the.