HEIGHT CONTROL IN ACTIVE SUSPENSION SYSTEM

Abstract: An active suspension system derives a target pressure to adjust fluid pressure in a working chamber (24L) of an active cylinder (24), on the basis of a difference between an actual height level and a desired or target vehicular height level. Pressure supply for the working chamber (24L) is so controlled as to establish the target height. The system detects a vehicular parking state wherein the vehicle speed is zero and a road wheel (12) is locked to limit the variation range of the fluid pressure in the working chamber at the target pressure.

Description: The present invention relates to a hollow cylinder disposed between a vehicular body and a suspension member for rotatably supporting a road wheel, a piston being disposed within the hollow interior space of said hollow cylinder for thrusting movement, said hollow cylinder and said piston being cooperative for defining a working chamber therein, a fluid circuit system including a pressurized fluid source for supplying and draining pressurized fluid to and from said working chamber, a pressure control valve disposed in said fluid circuit system for adjusting fluid pressure within said working chamber, first sensor means for monitoring vehicular height to produce a first vehicular body height indicative signal, second sensor means for detecting the vehicle speed and producing a second signal, and a controlling means for controlling said pressure control valve position to adjust the fluid pressure in said working chamber for adjusting height level of the vehicular body within a predetermined height range. An active suspension system of this kind in known in various shapes, e.g. from the EP-A-0 143 984. In general, active suspension systems form a current trend in the automotive technologies for greater capability of adjustment of suspension characteristics, such as vehicular attitude change suppressive capability, height regulating capability, shock absorbing capability and so forth. Japanese Patent First (unexamined) Publication (Tokkai) Showa 61-263818 discloses a height adjustment technology in the active suspension system. The disclosed system utilizes a fluid pressure in an active cylinder disposed between a vehicular body and a suspension member, such as suspension arm, suspension link and so forth, for adjustment of the vehicular height level. In general, height adjustment for the vehicle body is performed so as to maintain a relative distance between the vehicular body and the suspension member within a desired height range. In general, height adjustment toward the desired height range is performed by monitoring an actual vehicular height from the road surface or from the axle of the road wheel in feedback manner. The fluid pressure in a working chamber of the active cylinder is continuously or cyclically adjusted for establishing the vehicular height within the desired height range. Therefore, height adjustment can be continued until the desired range of vehicular height is established. As can be appreciated, the suspension member is pivotably connected to the vehicular body for permitting movement of the vehicular body and the suspension member relative to the other in bounding and rebounding motion. Pivotal movement of the suspension member relative to the vehicle body for varying vertical positional relationship between the vehicular body and a road wheel rotatably mounted on the suspension member, necessarily causes horizontal or axial displacement of the road wheel relative to the pivoting point on the vehicular body. For instance, when vehicular height level is to be risen, respective suspension member pivotally moves to cause displacement of the vehicular body and/or the road wheel away from each other. This movement necessarily leads axial displacement of the road wheels toward the pivot points of the associated suspension members. In other words, unless axial displacement of the road wheel relative to the pivot point of the suspension member on the vehicle body is permitted, relative vertical distance between the vehicular body and the road wheel will never change irrespective of the pressure level in the working chamber of the active cylinder. It sometimes causes necessity of height adjustment at parking state of the vehicle, in such an occasion that vehicular load is changed due to boarding or unboarding of passengers or loading or unloading of lagguages. Since the active suspension system is normally maintained active state as long as the main power switch, i.e. an ignition switch, is held ON. Therefore, when the vehicular height is changed out of the desired height range, height adjustment function of the active suspension system is triggered to perform height adjustment operation. If the height adjustment operation is taken place while the all road wheels are locked by application of brake, fluid pressure in the working chamber of the active cylinder is adjusted for causing height variation toward the desired height range, without causing no significant or noticeable change of actual vehicular body height. Non achievement of the desired height range causing continuous adjustment of the fluid pressure in the working chamber so that the fluid pressure becomes excessively high or low. At this position, if the brake is released for running, the road wheel becomes free from brake and thus is permitted to move in axial direction for causing axial displacement relative to the pivot point of the suspension member. At this time, since the fluid pressure in the working chamber is overshot, unexpected rapid change of the vehicular height is caused upon releasing of the brake. This can surprise the driver and passengers to cause degradation of the riding comfort. SUMMARY Of THE INVENTION Therefore, it is an object of the present invention to provide an active suspension system which can solve the aforementioned drawback in the prior art. In order to accomplish the aforementioned object, a suspension system according to the invention is characterized in that a third sensor means is provided for monitoring a vehicular braking condition in which said road wheel is in a locked non-rotating state, to produce a third vehicular brake active state indicative signal, whereby when said road wheel is detected to be in a locked braking condition and the vehicle speed is substantially zero, said controlling means only adjusts the fluid pressure in said working chamber within a limited pressure adjustment range the magnitude of which is derived on the basis of the height error between the actual and a target height level set within said predetermined height range. Thus, the system according to the invention detects a vehicular parking state wherein the speed is substantially zero and a road wheel is locked to limit the variation range of the fluid pressure in the wording chamber at the target pressure. The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment of the invention, which, however, should not be taken to limit the invention to the specific embodiment but are for explanation and understanding only. In the drawings: Fig. 1 is a block diagram of the preferred embodiment of an active suspension system according to the present invention; Fig. 2 is a chart showing relationship between a control current and control pressure to be supplied to a working chamber in an active cylinder; Fig. 3 is a flowchart showing process of height control to be executed by a control unit in the active suspension system; and Fig. 4 is a timing chart showing example of height control operation performed in the active suspension system. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, particularly to Fig. 1, the preferred embodiment of an active suspension system, according to the present invention, is generally represented by the reference numeral "10". The suspension system comprises a road wheel 12 rotatably supported on a suspension member 14, and a vehicle body 16. An active cylinder 24 is disposed between the suspension member 14 and the vehicle body 16. The active cylinder 24 defines a working chamber 24L, in which working fluid pressure is adjusted for controlling suspension characteristics. A piston 24c is disposed in the interior space of a cylinder tube 24a of the active cylinder for defining the working chamber 24L set forth above. The piston 24c is connected to the vehicle body 16 via a piston rod 24b. A pressure accumulator 40 is connected the working chamber 24L via a flow restrictive orifice 38. The working chamber 24L is connected to a fluid pressure source unit 18 via a supply line and a drain line. A pressure accumulator 20 is connected to the supply line for accumulating fluid pressure and thus regulates the line pressure. A pressure control valve 22 is disposed in the hydraulic circuit for controlling introduction and drain into and from the working chamber 24L of the active cylinder. The pressure control valve 22 is associated with a proportioning solenoid 22a for controlling valve operation. The pressure control valve 22 can be constructed in various fashion. For example, any of the pressure control valves disclosed in the following prior proposed constructions of the valves will be applicable for the shown embodiment of the active suspension system. A suspension coil 30 is disposed between the vehicle body 16 and the suspension member 14 in parallel relationship with the active cylinder 24. The suspension coil 30 is designed to receive static load. In order to control the proportioning solenoid 22a, a control unit 36 which comprises a microprocessor, is provided. The control unit 36 performs various suspension control operations, such as attitude change suppressive control, e.g. anti-rolling, anti-pitching control, vehicular height regulating control, and so forth. For performing suspension control operation, the control unit 36 is connected to various sensors monitoring suspension control parameters. Though the active suspension system according to the invention may perform attitude change suppressive control and other various control, the following discussion will be concentrated to height control. In the shown embodiment, a vehicle speed sensor 33 for monitoring a vehicle speed to produce a vehicle height indicative signal V, a brake switch 34 which detects active condition of vehicular brake to produce a vehicular brake active state indicative signal B, and a vehicular height sensor 32 which monitors vehicular height level to produce a height indicative signal H. In the shown embodiment, the brake switch 24 is designed to detect active state of foot brake and parking brake to produce HIGH level vehicular brake active state indicative signal when either one of the foot brake and the parking brake is active. The control unit 36 processes the input signals from the sensors to derive control signal for controlling operation of the proportioning solenoid for adjusting the vehicular height level within a predetermined target height range defined by an upper height limit H U and a lower height limit H L . The control signal is a current signal commanding pressure to be established in the working chamber 24L. Relationship between the magnitude of current of the control signal and the fluid pressure in the working fluid is illustrated in Fig. 2. As can be seen from Fig. 2, the current level of the control signal varies between a maximum level Is MAX and a minimum level Is MIN across an intermediate neutral level Is N . The maximum current level Is MAX corresponds to a maximum pressure Pc MAX of the control pressure and thus corresponds to the maximum pressure in the working chamber 24L. The minimum current level Is MIN corresponds to a minimum pressure Pc MIN and thus corresponds to the minimum pressure Pc MIN . The fluid pressure in the working chamber 24L is thus varies across a neutral pressure Pc N . Process of height control operation performed by the shown embodiment of the active suspension system set forth above will be discussed herebelow with reference to Fig. 3. The shown process of height control operation will be initiated in response to starting of power supply, i.e. turning ON of the ignition switch and executed cyclically or periodically with a predetermined interval, e.g. 20 msec. Immediately after starting execution, the vehicular height indicative signal H of the vehicular height sensor 32 is read out at a step 1002. Then, the vehicular height indicative signal value H is compared with the upper limit value H U and the lower limit value H L which define the predetermined target height range, to check whether the vehicle body is within the predetermined height range, at a step 1004. When the vehicle height indicative signal value H as checked at the step 1004, is smaller than or equal to the upper limit value H U and is greater than or equal to the lower limit value H L , judgement can be made that the vehicle body is within the target height range. Then, the process directly goes END and returns to a background job which governs various control routines. On the other hand, if the vehicular height indicative signal value H is greater than the upper limit value H U or is smaller than the lower limit value H L , as checked at the step 1004, the vehicle speed indicative signal value V and the braking state indicative signal B are read out at a step 1006. Then, the vehicle speed indicative signal value V is checked whether the vehicle speed is zero or not at a step 1008. When the vehicle speed indicative signal value V as checked at the step 1008 is zero, the braking state indicative signal level is checked at a step 1010. If the braking state indicative signal is HIGH level and thus judgement can be made that the road wheel is locked by application of the brake, a timer is reset at a step 1012. On the other hand, if the braking state indicative signal level is LOW as checked at the step 1010, the timer is started to start measurement of an elapsed time at a step 1014. Thereafter, at a step 1016, the timer value is checked whether the elapsed time indicated by the timer value is longer than a predetermined timer criterion, e.g. 2 sec. This step is provided for avoiding hunting of control during intermittent braking, such as pumping brake. After process in the step 1016, process goes to a step 1018. On the other hand, when the vehicle speed indicative signal value V is not zero as checked at the step 1008, process directly goes to the step 1018. If the elapsed time is longer than or equal to the timer criterion or after the process in the step 1008, check is performed whether the vehicular height indicative signal value H is greater than the lower limit value H L , at the step 1018. When the elapsed time is longer than or equal to the timer criterion, an UP command is output to increase the fluid pressure in the working chamber 24L in a magnitude of ΔP at a step 1020. On the other hand, if the height indicative signal value H is greater than the lower limit value H L , which means that the height level of the vehicular body is too higher than the upper limit, a DOWN command is output to decrease the fluid pressure in the working chamber 24L in a magnitude of ΔP at a step 1022. After the process one of the step 1020 and 1022, process goes to a step 1024. In the step 1024, a height error dependent control pressure P B and an accumulated pressure ΔP* which will be discussed later, is cleared. Then, a flag F function of which will be discussed latter, is reset at a step 1026. After the step 1026, process goes END and returns to the background job. On the other hand, if the braking state indicative signal level as checked at the step 1010 and the timer is reset at the step 1012, or when the timer value as checked at the step 1016 is smaller than the timer criterion, the flag F is checked at a step 1028. If the flag F is not set as checked at the step 1028, a height error Δx between the actual vehicular body height as represented by the vehicular height indicative signal H, and a target height H N which is set at a center value in the target height range, is derived by: Δx = H N - H at a step 1030. Thereafter, a height error dependent control pressure ΔP B which represents a magnitude of fluid pressure to be increased or decreased in the working chamber 24L of the active cylinder 24, is derived on the basis of the height error Δx by the following equation at a step 1032. ΔP B = (k x Δx )/(A x L) wherein k is a spring coefficient of the suspension coil spring 30; A is an effective area in the active cylinder 24; and L is a lever ratio. After the process at the step 1032, the flag F is set at a step 1034. Therefore, the flag F indicates states of control operation, in which the error dependent control pressure ΔP B is already derived. When the flag F as checked at the step 1028 is set or after the process through the steps 1030 through 1034, the accumulated pressure ΔP* is derived at a step 1036. For deriving the accumulated pressure ΔP* , the following equation is used: ΔP* = ΔP* (old) + ΔP As can be appreciated, the accumulated pressure ΔP* is increased by the predetermined magnitude ΔP intermittently. The accumulated pressure ΔP* represents assumed or predicted pressure in the working chamber 24L after pressure adjustment. The accumulated pressure ΔP* is then compared with the absolute value |ΔP B | of the error dependent control pressure, at a step 1038. If the accumulated pressure ΔP* is smaller than or equal to the absolute value |ΔP B | as checked at the step 1038, fluid pressure adjustment is permitted. On the other hand, if the accumulated pressure ΔP* is greater than the absolute value |ΔP B ,| fluid pressure adjustment is disabled. Therefore, in the latter case, the process directly goes END after the step 1038. On the other hand, when fluid pressure adjustment is permitted at the step 1038, the height error value Δx is checked at a step 1040 whether the height error value is greater than zero. If the answer at the step 1040 is positive, then the UP command is output for increasing the control pressure by ΔP. On the other hand, if the answer at the step 1040 is negative, the DOWN command is output for decreasing the control pressure by ΔP. Practical control operation to be performed by the active suspension system set forth above will be discussed herebelow with reference to Fig. 4. In the timing chart shown in Fig. 4, variation of the vehicle speed V, the actual vehicular height H, the braking state indicative signal level are illustrated. It is assumed that the vehicle is in parking state and the vehicular height is held lower than the lower limit, the vehicular height indicative signal H representative of the excessively low level of the vehicle body is output by the vehicular height sensor 32 upon tuning ON of the ignition switch. Then, the height control program is started to be executed for starting adjustment of the fluid pressure in the working chamber 24L of the active cylinder 24. Here, assuming the vehicular brake is not applied during the period between a time t₀ to t₁, then the answer at the step 1010 becomes positive. Therefore, the timer is started at the step 1012 at the time t₀. After expiration of the predetermined time, e.g. 2 sec., the fluid pressure in the working chamber 24L is increased in a rate of ΔP by the UP command output at the step 1020. At this time, since the road wheel is not locked, the actual vehicular height is risen according to increasing of the fluid pressure in the working chamber. At the time t₁, the vehicular brake is applied. By application of the brake at the time t₁, braking state indicative signal level turns into HIGH level at the step t₁. Therefore, the answer at the step 1010 becomes negative. Then, process through the steps 1028 to 1034 is performed for deriving the error dependent control pressure ΔP B based on the height error Δx at the time t₁. After the time t₁, the fluid pressure is maintained to be increased in a rate of ΔP until the fluid pressure is increased in a magnitude of ΔP B . At a time t₃, the fluid pressure in the working chamber reaches the maximum level. Then, the answer at the step 1038 becomes negative. Therefore, after the time t₃, height adjustment is disabled. At times t₄ and t₅, the vehicular brake is temporarily released in a short period, e.g. shorter than 2 sec. In such case, the answer at the step 1016 becomes negative. Therefore, in such case, process through the steps 1028 to 1044 is performed to permit height adjustment in the limited height adjustment range ΔP B . On the other hand, at a time t₆, the brake is released to trigger timer. By releasing the brake, the wheel becomes free to rotate. As a result, the vehicular height is raised by the fluid pressure in the active cylinder which is increased during the period from t₀ to t₆. After expiration of the given period, e.g. 2 sec., the answer at the step 1048 becomes positive to permit height adjustment in feedback manner. Therefore, the the vehicular height is raised toward the target height H N . When the vehicular height is raised across the lower limit H L , the answer at the step 1004 becomes positive. Therefore, height adjustment operation is terminated. As can be appreciated herefrom, according to the shown embodiment, since the pressure adjustment range is limited to a magnitude derived on the basis of the height error between the actual height and the target height, while the road wheel is locked by application of brake, overshooting of height adjustment which leads abrupt or sudden change of the vehicular height upon releasing the road wheel from locking state, can be successfully prevented. Therefore, the present invention fulfills all of the objects and advantages sought therefor. While the present invention has been disclosed in terms of the preferred embodiment in order to facilitate better understanding of the invention, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments which can be embodied without departing from the principle of the invention set out in the appended claims. For instance, the active suspension system can be constructed in various fashion and control of the active suspension can be done in various manner. For example, reference is mode to the following co-pending application, publication and patents. U. S. Patent Application Serial No. 052,934, filed on May 22, 1989, which has now been issued as U. S. Patent No. 4,903,983, on February 27, 1990: U. S. Patent Application Serial No. 059,888, filed on June 9, 1987, corresponding European Patent Application has been published as First Publication No. 02 49 209: U. S. Patent Application Serial No. 060,856, filed on June 12, 1987, corresponding European Patent Application has been published as First Publication No. 02 49 227: U. S. Patent Application Serial No. 060,909, filed on June 12, 1987: U. S. Patent Application Serial No. 060,911, filed on June 12, 1987, which has now been issued as U. S. Patent No. 4,801,115, on January 31, 1989: U. S. Patent Application Serial No. 176,246, filed on March 31, 1988, the corresponding European Patent Application has been published as First Publication No. 02 85 153: U. S. Patent Application Serial No. 178,066, filed on April 5, 1988, which has now been issued as U. S. Patent No. 4,848,790, on July 18, 1989, and the corresponding European Patent Application has been published as First Publication No. 02 86 072: U. S. Patent Application Serial No. 167,835, filed on March 4, 1988, which has now been issued as U. S. Patent No. 4,865,348, on September 12, 1989: U. S. Patent Application Serial No. 244,008, filed on September 14, 1988: U. S. Patent Application Serial No. 255,560, filed on October 11, 1988: U. S. Patent Application Serial No. 266,763, filed on November 3, 1988, corresponding European Patent Application has been published under First Publication No. 03 18 721: U. S. Patent Application Serial No. 261,870, filed on October 25, 1988: U. S. Patent Application Serial No. 263,764, filed on October 28, 1988, corresponding European Patent Application has been published under First Publication No. 03 14 164: U. S. Patent Application Serial No. 277,376, filed on November 29, 1988, corresponding European Patent Application has been published under First Publication No. 03 18 932: U. S. Patent Application Serial No. 303,338, filed on January 26, 1989, corresponding German Patent Application has been published under First Publication No. 39 02 312: U. S. Patent Application Serial No. 302,252, filed on January 27, 1989: U. S. Patent Application Serial No. 310,130, filed on March 22, 1989, corresponding German Patent Application has been published under First Publication No. 39 04 922: U. S. Patent Application Serial No. 327,460, filed on March 22, 1989, corresponding German Patent Application has been published under First Publication No. 39 10 030: U. S. Patent Application Serial No. 303,339, filed on January 26, 1989: U. S. Patent Application Serial No. 331,602, filed on March 31, 1989: U. S. Patent Application Serial No. 331,653, filed March 31, 1989, corresponding German Patent Application has been published under First Publication No. 39 10 445: U. S. Patent Application Serial No. 364,477, filed on June 12, 1989, corresponding European Patent Application has been published under First Publication No. 03 45 816: U. S. Patent Application Serial No. 365,468, filed on June 12, 1989, corresponding European Patent Application has been published under First Publication No. 03 45 817: U. S. Patent Application Serial No. 422,813, filed on October 18, 1989: U. S. Patent Application Serial No. 454,785, filed on December 26, 1989: The disclosures of the hereabove listed prior applications, publications and patents are herein incorporated by reference. Furthermore, any two or more prior proposed inventions may be combined in practical implementation of an active suspension system. Therefore, any combination of the above mentioned prior proposed inventions are to be deemed as disclosed due to incorporation by reference as a part of the present invention.

Claims: An active suspension system comprising: a hollow cylinder (24) disposed between a vehicular body (16) and a suspension member (14) for rotatably supporting a road wheel (12), a piston (24C) being disposed within the hollow interior space of said hollow cylinder for thrusting movement, said hollow cylinder and said piston being cooperative for defining a working chamber (24L) therein, a fluid circuit system including a pressurized fluid source (18) for supplying and draining pressurized fluid to and from said working chamber, a pressure control valve (22) disposed in said fluid circuit system for adjusting fluid pressure whithin said working chamber, first sensor means (32) for monitoring vehicular height to produce a first vehicular body height indicative signal, second sensor means (33) for detecting the vehicle speed and producing a second signal, and a controlling means (36) for controlling said pressure control valve position to adjust the fluid pressure in said working chamber for adjusting height level of the vehicular body within a predetermined height range, characterized in that a third sensor means (34) is provided for monitoring a vehicular braking condition in which said road wheel (12) is in a locked non-rotating state, to produce a third vehicular brake active state indicative signal, whereby when said road wheel is detected to be in a locked braking condition and the vehicle speed is substantially zero, said controlling means (36) only adjusts the fluid pressure in said working chamber within a limited pressure adjustment range (ΔP β ) the magnitude of which is derived on the basis of the height error (Δx) between the actual and a target height level set within said predetermined height range. An active suspension system as claimed in claim 1, characterized in that said controlling means (36) is responsive to termination of said third signal to control said pressure control valve (22) for adjusting said vehicular height level within said predetermined height range after expiration of a predetermined period of time.

Inventors: KAWABATA, KAZUNOBU
Applicants: NISSAN MOTOR CO., LTD.
Application year: 1990
Main classification: B60G17
IPC codes: B60G17