Motor-driven power steering apparatus for automobiles

A motor-driven power steering apparatus for a motor vehicle, comprising:

• a torque sensor for detecting a steering torque of a steering wheel;
• a vehicle speed sensor for detecting a speed of said motor vehicle;
• an electric motor for assisting manipulation of said steering wheel;
• two pairs of switching elements connected in the form of a bridge circuit together with said electric motor for allowing said electric motor to rotate selectively in either one of forward and backward directions;
• motor current detecting means for detecting the current flowing through said electric motor;
• and
• a signal processing unit for driving said switching elements on the basis of said steering torque, said vehicle speed and said motor current;
• wherein said signal processing unit includes:
• steering wheel return decision means for deciding whether the steering wheel of said motor vehicle is in a return state in which said steering wheel is to be returned to a center position;
• target current value arithmetic means for arithmetically determining a target current value of said motor current on the basis of said steering torque and said vehicle speed;
• control quantity arithmetic means for arithmetically determining a control quantity for said electric motor in dependence on deviation of said motor current from said target current value;
• selecting means for generating first and second control quantities corresponding to first and second driving modes, respectively, on the basis of said control quantity and selecting one of said first and second control quantities in dependence on the return state of said steering wheel;
• conversion means for converting said first and second control quantities into first and second PWM duty ratios, respectively, for said switching elements;
• and
• a driving circuit for driving said switching elements on the basis of said first or second PWM duty ratio;
• wherein said selecting means generates said first control quantity corresponding to said first driving mode when said steering wheel is judged as not being in said return state while generating said second control quantity corresponding to said second driving mode when said steering wheel is judged as being in said return state;
• and
• wherein said driving circuit responds to said first PWM duty ratio to thereby hold constantly one switching element in each pair in conducting state while turning on and off the other switching element in each pair according to said first PWM duty ratio, while said driving circuit responds to said second PWM duty ratio by turning on and off simultaneously both of said switching elements in each pair according to said second PWM duty ratio.