Variable ratio steering system

A variable ratio steering system of a vehicle comprising:

• a variable steering ratio mechanism that varies a steering ratio of a steering angle of a manually-operated steering mechanism to a road wheel steer angle at a steered road wheel;
• a vehicle speed sensor that detects a vehicle speed;
• a steering angle sensor that detects the steering angle;
• a steering ratio control unit configured to be electronically connected to the vehicle speed sensor, the steering angle sensor and the variable steering ratio mechanism, for controlling the variable steering ratio mechanism responsively to the vehicle speed and the steering angle and for switching between a fixed steering ratio control mode and a vehicle-speed dependent steering ratio control mode responsively to the steering angle;
• and
• the steering ratio control unit having a processor programmed to perform the following:
• (a) executing the fixed steering ratio control mode in which the steering ratio is kept constant irrespective of the vehicle speed under a condition where the steering angle is less than or equal to a predetermined angle substantially corresponding to a neutral position of the manually-operated steering mechanism, for stabilizing a steering-wheel angular position, required to maintain a running state of the vehicle at straight ahead, at the neutral position, in presence of offsetting of a steering angle versus road wheel steer angle characteristic from a predetermined characteristic;
• and
• (b) executing the vehicle-speed dependent steering ratio control mode in which a steering angle versus road wheel steer angle characteristic varies depending on the vehicle speed, when the steering angle is greater than the predetermined angle substantially corresponding to the neutral position of the manually-operated steering mechanism.

In a computer-controlled variable ratio steering system for an automotive vehicle with an electronically-controlled variable steering ratio mechanism varying a steering ratio of a steering angle of a manually-operated steering mechanism to a road wheel steer angle at a steered road wheel, a vehicle speed sensor detecting a vehicle speed, a steering angle sensor detecting the steering angle, and a steering ratio control unit configured to be electronically connected to the vehicle speed sensor, the steering angle sensor and the variable steering ratio mechanism, for controlling the variable steering ratio mechanism responsively to the vehicle speed and the steering angle and for switching between a fixed steering ratio control mode and a vehicle-speed dependent steering ratio control mode responsively to the steering angle, the system comprising:

• (a) a first variable ratio steering means for executing the fixed steering ratio control mode in which the steering ratio is kept constant irrespective of the vehicle speed under a condition where the steering angle is less than or equal to a predetermined angle substantially corresponding to a neutral position of the manually-operated steering mechanism, for stabilizing a steering-wheel angular position, required to maintain a running state of the vehicle at straight ahead, at the neutral position, in presence of offsetting of a steering angle versus road wheel steer angle characteristic from a predetermined characteristic;
• and
• (b) a second variable ratio steering means for executing the vehicle-speed dependent steering ratio control mode in which a steering angle versus road wheel steer angle characteristic varies depending on the vehicle speed, when the steering angle is greater than the predetermined angle substantially corresponding to the neutral position of the manually-operated steering mechanism.

A method for executing variable ratio steering functions for a computer-controlled variable ratio steering system of a vehicle with an electronically-controlled variable steering ratio mechanism varying a steering ratio of a steering angle of a manually-operated steering mechanism to a road wheel steer angle at a steered road wheel, a vehicle speed sensor detecting a vehicle speed, a steering angle sensor detecting the steering angle, and a steering ratio control unit configured to be electronically connected to the vehicle speed sensor, the steering angle sensor and the variable steering ratio mechanism, for controlling the variable steering ratio mechanism responsively to the vehicle speed and the steering angle and for switching between a fixed steering ratio control mode and a vehicle-speed dependent steering ratio control mode responsively to the steering angle, the method comprising:

• executing the fixed steering ratio control mode by keeping the steering ratio constant irrespective of the vehicle speed under a condition where the steering angle is less than or equal to a predetermined angle substantially corresponding to a neutral position of the manually-operated steering mechanism, for stabilizing a steering-wheel annular position, required to maintain a running state of the vehicle at straight ahead, at the neutral position, in presence of offsetting of a steering angle versus road wheel steer angle characteristic from a predetermined characteristic;
• and
• executing the vehicle-speed dependent steering ratio control mode by varying a steering angle versus road wheel steer angle characteristic depending on the vehicle speed, when the steering angle is greater than the predetermined angle substantially corresponding to the neutral position of the manually-operated steering mechanism.

A method for executing variable ratio steering functions for a computer-controlled variable ratio steering system of a vehicle with an electronically-controlled variable steering ratio mechanism varying a steering ratio of a steering angle of a manually-operated steering mechanism to a road wheel steer angle at a steered road wheel, a vehicle speed sensor detecting a vehicle speed, a steering angle sensor detecting the steering angle, and a steering ratio control unit configured to be electronically connected to the vehicle speed sensor, the steering angle sensor and the variable steering ratio mechanism, for controlling the variable steering ratio mechanism responsively to the vehicle speed and the steering angle and for switching between a fixed steering ratio control mode and a vehicle-speed dependent steering ratio control mode responsively to the steering angle, the method comprising:

• comparing the steering angle to a predetermined angle substantially corresponding to a neutral position of the manually-operated steering mechanism;
• fixing the steering ratio to a predetermined steering ratio suited to a high speed range for execution of the fixed steering ratio control mode under a condition where the steering angle is less than or equal to the predetermined angle substantially corresponding to the neutral position of the manually-operated steering mechanism, for stabilizing a steering-wheel angular position, required to maintain a running state of the vehicle at straight ahead, at the neutral position, in presence of offsetting of a steering angle versus road wheel steer angle characteristic from a predetermined characteristic;
• and
• executing the vehicle-speed dependent steering ratio control mode in which a steering angle versus road wheel steer angle characteristic varies depending on the vehicle speed, when the steering angle is greater than the predetermined angle substantially corresponding to the neutral position of the manually-operated steering mechanism.

A method for executing variable ratio steering functions for a computer-controlled variable ratio steering system of a vehicle with an electronically-controlled variable steering ratio mechanism varying a steering ratio of a steering angle of a manually-operated steering mechanism to a road wheel steer angle at a steered road wheel, a vehicle speed sensor detecting a vehicle speed, a steering angle sensor detecting the steering angle, and a steering ratio control unit configured to be electronically connected to the vehicle speed sensor, the steering angle sensor and the variable steering ratio mechanism, for controlling the variable steering ratio mechanism responsively to the vehicle speed and the steering angle and for switching between a fixed steering ratio control mode and a vehicle-speed dependent steering ratio control mode responsively to the steering angle, the method comprising:

• comparing the steering angle to a predetermined angle substantially corresponding to a neutral position of the manually-operated steering mechanism;
• fixing the steering ratio to a predetermined minimum steering ratio for execution of the fixed steering ratio control mode under a condition where the steering angle is less than or equal to the predetermined angle substantially corresponding to the neutral position of the manually-operated steering mechanism, for stabilizing a steering-wheel angular position, required to maintain a running state of the vehicle at straight ahead, at the neutral position, in presence of offsetting of a steering angle versus road wheel steer angle characteristic from a predetermined characteristic;
• and
• executing the vehicle-speed dependent steering ratio control mode in which a steering angle versus road wheel steer angle characteristic varies depending on the vehicle speed, when the steering angle is greater than the predetermined angle substantially corresponding to the neutral position of the manually-operated steering mechanism.