Motor Circuit for Electric Power Assisted Steering and Method

A motor circuit for use in an electric power assisted steering system of the kind in which a measurement of torque carried by a part of the steering system is used by the motor circuit to produce a torque demand signal indicative of a torque to be applied to the steering system by the motor, the motor circuit comprising:

• a switching circuit comprising a plurality of electrical switches,
• a motor controller that receives a motor torque demand signal and produces control signals for a motor drive circuit that in turn drives the switches of the switching circuit;
• whereby under certain fault mode conditions of the motor the motor is incapable of generating an output torque when a motor rotor is located in at least one known angular position, in which
• the motor circuit is operable under a defined set of motor conditions to reduce the risk of the motor becoming stuck in the at least one known angular position,
• the circuit including:
• means for identifying the presence of a condition in which the rotor is within a predefined angular range of the at least one known angular position, the rotor is moving towards that position due to a driver turning the steering wheel, and a torque is demanded from the motor which would tend to move the rotor closer to that angular position as the wheel continues to move in the identified direction of movement;
• and
• torque demand varying means arranged in use to vary the torque demanded from the motor to cause the motor rotor to move away from the known angular position, and further arranged in use to subsequently vary the torque demanded from the motor to cause the motor rotor to move back in a direction towards the known angular position.