The present invention relates to a suspension control apparatus utilizing a PWM type proportional solenoid valve, by use of which damping of vibration of a sprung mass is effected, to thereby improve ride quality in a vehicle.
As a conventional suspension control apparatus, there can be mentioned a suspension control apparatus proposed by the assignee of the present application in Unexamined Japanese Patent Application Public Disclosure (Kokai) No. 8-303628. The suspension control apparatus disclosed in Kokai No. 8-303628 comprises: a proportional solenoid valve having a movable member (spool) for controlling an amount of hydraulic fluid passing therethrough according to a position of the movable member, the movable member being adapted to be displaced according to an intensity of a current applied to a solenoid (hereinafter frequently referred to simply as "applied current" for the sake of convenience); a variable damping force generating shock absorber provided between a vehicle body and an axle to generate a damping force which varies depending on the intensity of the applied current, that is, depending on the position of the movable member; and a vehicle height sensor or an acceleration sensor. In this apparatus, the intensity of the applied current is determined, based on the detection value of the vehicle height sensor or of the acceleration sensor, to thereby generate a desired damping force (for an extension stroke and a compression stroke). Thus, damping of vibration of a sprung mass is effected, thereby improving ride quality in a vehicle.
In the above-mentioned suspension control apparatus, the applied current is obtained in a form such that an oscillating current (dithering current, which is hereinafter frequently referred to simply as "dither") is superimposed on a target current having a desired intensity (equal to an average intensity of the applied current) which is determined from the detection value of the vehicle height sensor or of the acceleration sensor. The dithering current is a high-frequency current which is obtained on the basis of a PWM (pulse width modulation) signal. Due to the dithering current, the movable member oscillates slightly (dithers) about a predetermined position corresponding to the intensity of the target current, so that displacement of the movable member can be easily performed, thereby improving responsivity in controlling a damping force. Illustratively stated, the dithering current is obtained by switching the duty ratio of the PWM signal between an increase duty ratio and a decrease duty ratio at predetermined time intervals T.sub.0, each being equal to 1/2 of a cycle time of the dithering current. The term "increase duty ratio" herein means the duty ratio when the intensity of the applied current increases at each cycle of the PWM signal, and the term "decrease duty ratio" herein means the duty ratio when the intensity of the applied current decreases at each cycle of the PWM signal.
However, in the above-mentioned conventional suspension control apparatus, when the intensity of the target current (the average intensity of the applied current) falls below the lower limit of a predetermined range or exceeds the upper limit of the predetermined range, the amplitude of the dithering current (hereinafter, frequently referred to simply as "dither amplitude") becomes small, so that displacement of the movable member cannot be smoothly performed.