Electric steering gear

Abstract

An electronic control system for a steering assembly having an electric motor coupled to the steering assembly for providing power assist. The electric power assist motor is drivably connected to a steering member for assisting movement thereof in response to applied input torque to the vehicle steering wheel. The electronic control unit monitors an output signal from a torsion sensor which measures the amount of applied input torque. The electronic control unit generates a pulse width modulated signal having a duty cycle proportional to the measured amount of applied input torque. Solid state switches provide connection between the vehicle battery and the motor, the connection time being controlled by the duty cycle of the pulse width modulated signal. A speed decoder determines which of a plurality of speed ranges the vehicle is moving. A plurality of look-up tables are provided, each table includes a value of power assist for a given amount of applied input torque. A look-up table is selected depending upon the decoded vehicle speed. Soft start circuitry is provided to control the maximum amount of assist that is permitted upon initial application of power to the electronic control unit. The amount to assist permitted increases from a minimum to a maximum within a predetermined amount of time. Temperature foldback circuitry is also provided to limit the amount of current through the electric power assist motor dependent upon sensed temperature of the solid state switches. The amount of current permitted through the motor is also limited to an absolute maximum. A connection cable is provided to reduce radiated electromagnetic emissions and voltage transients.