Apparatus and method for producing a desired return torque in a vehicle

Abstract

A vehicle power steering system has a steering shaft with a rotational position sensor providing signal rollover so that each sensor output indicates a plurality of potential absolute steering positions separated from each other by multiples of a rollover angle. Upon each initiation of vehicle operation, the system selects an absolute steering positions corresponding to an initial sensor output as an unadjusted steering position and repeatedly updates the unadjusted steering position in response to sensor output changes, including sensed rollover events. The system stores data dividing a two dimensional map of possible vehicle steering operating points defined by total steering torque and steering position into a first region of likely vehicle steering system operating points, a second region of unlikely steering system operating points indicating a revolution offset in a first direction, and a third region of unlikely steering system operating points indicating a revolution offset in an opposite direction. While a minimum vehicle velocity and, optionally, other enabling conditions exist, a vehicle steering operating point derived from sensed total steering torque, the updated unadjusted steering position and a revolution offset compared with the stored data; and the revolution offset is updated as indicated if the derived vehicle steering system operating point is within the second or third stored regions of steering system operating points. A second return torque command is derived from the updated unadjusted steering position and updated revolution offset and provided to the actuator to produce a second return torque, preferably after the revolution offset is walked incrementally to the updated value, with return torque provided in a single, correct direction only during the walk.