Vehicular power steering having steering effort control system

Claim 1

A vehicular power steering system comprising: a steering mechanism including a steering wheel (4), hydraulic actuator means having a power cylinder (6) for transmitting mechanical work converted from hydraulic power to said steering mechanism for providing power assistance, hydraulic fluid supply means having a fluid pump (1) , servo valve means (3), connected with said fluid pump (1) through a supply fluid conduit (2) and a return fluid conduit (5), for controlling a fluid flow from said pump and introducing a fluid pressure to said .power cylinder (6) in accordance with movement of the steering wheel (4), characterized by bypass control valve means (8) disposed in a bypass fluid conduit (7) connected between said supply conduit (2) and return conduit (5) to bypass said servo valve means (3), for controlling the rate (Q b ) of a fluid flow through said bypass conduit (7) thereby to control the degree of power assistance by controlling the fluid supply to said servo valve means (3), speed sensing means (11) for sensing the speed of the vehicle, angular velocity sensing means (14) sor sensing the angular velocity of the steering wheel, and control means (16,17) connected with said vehicle speed sensing means (11) and said angular velocity sensing means (14), for producing a control signal which is sent to said bypass control means (8) to control the bypass flow rate (Q b ) in accordance with the sensed vehicle speed (V) and the sensed angular velocity ω of the steering wheel in such a manner that the bypass flow rate (Q b ) is controlled to be equal to a basic quantity (Q bv ) which increases as the sensed vehicle speed increases, while the bypass flow rate (Q b ) is decreased from said basis quantity (Q bv ) to a modified quantity in accordance with the sensed angular velocity ω of the steering wheel (4) so as to prevent the fluid supply to said servo valve means (3) from decreasing below a required quantity which is required by a volume increase of a power chamber of the power cylinder due to a movement of a power piston of the power cylinder, said control means (16,17) further controlling said bypass control valve means (8) in such a manner that the bypass flow rate (Q b ) is increased by a correction quantity which is determined in accordance with the sensed vehicle speed and the sensed angular velocity of the steering wheel.