Patent ID: 8862328

Claim:
A system for determining an absolute rotational position of a rotatable shaft of a motor in an electric power steering system of a vehicle, comprising: a microprocessor configured to be periodically activated to wake up from a low power sleep mode during an ignition off state of the vehicle by a timer circuit according to an activation duty cycle defined by a desired quiescent current draw of the microprocessor and a desired maximum speed of the rotatable shaft, the microprocessor further configured to energize first and second position sensors at a first time when the microprocessor is activated; the first and second position sensors configured to generate first and second signals, respectively, indicative of a relative rotational position of the rotatable shaft at the first time; the microprocessor further configured to measure the first and second signals and to determine a first relative position value indicating the relative rotational position of the rotatable shaft at the first time, and then to be deactivated, the microprocessor further configured to energize the first and second position sensors at a second time when the microprocessor is activated, the second time being after the first time and after the microprocessor was deactivated; the first and second position sensors further configured to generate third and fourth signals, respectively, indicative of a relative rotational position of the rotatable shaft at the second time; the microprocessor further configured to measure the third and fourth signals and to determine a second relative position value indicating the relative rotational position of the rotatable shaft at the second time, the microprocessor further configured to determine an amount of relative rotation of the rotatable shaft during the ignition off state based on the first and second relative position values, and the microprocessor further configured to determine a current absolute position value indicating a current absolute rotational position of the rotatable shaft based on a previously stored absolute position value and the amount of relative rotation of the rotatable shaft.