A flexible member sensor for detecting motion and tension of a flexible member includes a rigid member pivotally connected to a base, in contact with the flexible member. A sensor detects pivotal motion of the rigid member in response to movement of the flexible member. The sensor may provide a corresponding signal to a motion control system that adjusts the tension of the flexible member accordingly. In one implementation, the flexible member sensor is integrated with a feedback control system for an electric seat belt retractor that includes a feedback control loop. The flexible member sensor measures tension in seat belt webbing retained by the retractor. A motor coupled to a servo amplifier rotates a spool of the electric retractor to extract or retract webbing in accordance with the output of the flexible member sensor. The seat belt tension may thus be continuously maintained at a desired level.