GB2262151A

GB2262151A - Suspension control system for vehicles

Info

Publication number: GB2262151A
Application number: GB9301982A
Authority: GB
Inventors: Yasuhiro Tsutsumi
Original assignee: Toyota Motor Corp
Current assignee: Toyota Motor Corp
Priority date: 1989-12-08
Filing date: 1990-11-27
Publication date: 1993-06-09
Anticipated expiration: 2010-11-27
Also published as: GB9301982D0; GB2262151B

Abstract

A suspension control system for controlling a suspension having a shock absorber provided for a wheel of a vehicle includes damping force change rate detecting means for detecting a damping force change rate indicating a rate of change of a damping force of said shock absorber. The system also comprises damping force alteration means, coupled to said shock absorber and said dam ping force change rate detecting means, for determining whether or not said damping force change rate is outside a first range and for altering the setting of said damping force to a first level from a second level greater than said first level when it is determined that said dam ping force change rate is outside said first range. The system further comprises damping force restoration means, coupled to said damping force change rate detecting means, said damping force alteration means and said shock absorber, for determining whether or not said damping force change rate is continuously within a second range narrower than said first range during at least a predetermined period after said damping force alteration means alters the setting of the damping force from said second level to said first level and for restoring the setting of the damping force from said first level to said second level when it is determined that said damping force change rate is continuously within said second range. <IMAGE>

Description

"SUSPENSION CONTROL SYSTEM" The present invention generally relates to a suspension control system, and more particularly to a suspension control system for controlling a damping force of a variable damping force type shock absorber on the basis of a running condition of a vehicle.

There is previously known a suspension control system in which the damping force of a shock absorber is controlled on the basis of the rate of change af the damping force. When the change rate of the damping force exceeds a predetermined value, that is, when the change rate changes abruptly due to a rough road surface or braking, the damping force with respect to a movement of the shock absorber is rapidly changed to a small level, so that the control response characteristic of the shock absorber is improved.

There is also known a suspension control system in which an adjustment reference value provided for the damping force change rate used for changing the setting of the damping force is altered on the basis of a driving condition, such as a vehicle speed, so that ride comfort is improved (see Japanese Laid-Open Patent Application No, 64-67407).

The conventional suspension control system proposed in the above-mentioned Japanese Application has an advantage in that the damping force is rapidly changed in accordance with the road surface condition so that good ride comfort is obtained. However, there is room for improvement in ride comfort in a case where the vehicle is continuously traveling on a rough road surface and thus the damping force change rate frequently changes around the adjustment reference damping force of each shock absorber in response to the road surface condition.

According to Dne aspect of the present invention, there is provided a suspension control system for controlling a suspension having a shock absorber provided for a wheel of a vehicle, the suspension control system comprising: damping force change rate detecting means for detecting a damping force change rate indicating a rate of change of a damping force of the shock absorber; damping force alteration means, coupled to the shock absorber and the damping force change rate detecting means, for determining whether or not the damping force change rate is outside a first range and for altering the setting of the damping force to a first level from a second level greater than the first level when it is determined that the damping force change rate is outside the first range; and damping force restoration means, coupled to the damping force change rate detecting means, the damping force alteration means and the shock absorber, for determining whether or not the damping force change rate is continuously within a second range narrower than the first range during at least a predetermined period after the damping force alteration means alters the setting of the damping force from the second level to the first level and for restoring the setting of the damping force from the first level to the second level when it is determined that the damping force change rate is continuously within the second range.

According to another aspect of the present invention, there is provided a suspension control system for controlling a suspension having a shock absorber provided for a wheel of a vehicle, the suspension control system comprising: damping force change rate detecting means for detecting a damping force change rate indicating a rate of change of a damping force of the shock absorber; damping force alteration means, coupled to the shock absorber and the damping force change rate detecting means, for determining whether or not the damping force change rate is outside a first range and for altering the setting of the damping force to a first level from a second level greater than the first level when it is determined that the damping force change rate is outside the first range;; first damping force restoration means, coupled to the damping force change rate detecting means, the damping force alteration means and the shock absorber, for determining whether or not the damping force change rate is continuously within a second range narrower than the first range during at least a first period after the damping force alteration means alters the setting of the damping force from the second level to the first level and for restoring the setting of the damping force from the first level to the second level when it is determined that the damping force change rate is continuously within the second range;; holding period calculation means, coupled to the damping force alteration means and the first -damping force restoration means, for calculating a holding period until the setting of the damping force is restored to the second level by the first damping force restoration means after the setting of the damping force is altered to the first level by the damping force alteration means; and second damping force restoration means, coupled to the holding period calculation means and the shock absorber, for determining whether or not the holding period becomes longer than a second period and for restoring the setting of the damping force from the first level to the second level when it is determined that the holding period becomes longer than the second period.

Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: FIG. 1 is a block diagram illustrating a suspension control system according to a first embodiment of the present invention; FIG. 2 is a flowchart illustrating the operation of the suspension control system according to the first embodiment of the present invention; FIG. 3 is a graph illustrating the operation of the suspension control system according to the first embodiment of the present invention; FIG. 4 is a flowchart illustrating the operation of a variation of the suspension control system according to the first embodiment of the present invention; FIG. 5 is a block diagram illustrating a suspension control system according to a second embodiment of the present invention;; FIG. 6 is a diagram illustrating how to combine FIGS. 6A and 6B; FIGS. 6A and 6B are flowcharts illustrating the operation of the suspension control system according to the second embodiment of the present invention; FIGS. 7 and 8 are diagrams illustrating the operation of the suspension control system according to the second embodiment of the present invention; and FIG. 9 is a flowchart illustrating the operation of a variation of the suspension control system according to the second embodiment of the present invention.

A description will now be given of a first embodiment of the present invention.

Referring to FIG 1., there is illustrated the principle of a suspension control system according to the first embodiment of the present invention. In FIG. 1, those parts which are the same as those shown in FIG.1 of our GB-2239506 A are given the same reference numerals.

The suspension control system shown in FIG.1 is composed of the damping force change rate detector M2, and a damping force controller M3a, which is composed of a damping force alteration unit M7 and a damping force restoration unit M8. The damping force alteration unit M7 is supplied with a first reference value S1, and the damping force restoration unit M8 is supplied with a second reference value S2. The damping force alteration unit M7 determines whether or not the damping force change rate V output by the damping force change rate detector M2 is outside a range defined by the first reference value S1. As shown in FIG.8 of GB-2239506 A, the damping force change rate V is represented by a signal which has either a positive or negative polarity.Thus, the range defined by the first reference value S1 is between the negative first reference value S1 and the positive reference value S1. Alternatively, when the signal representing the damping force change rate V is filtered so that the damping force change rate V is represented in the form of absolute value, the first reference value S1 is the positive value.

When it is determined that the damping force change rate V is outside the range defined by the first reference value S1, the damping force alteration unit M7 controls the shock absorber M1 so that the setting of the damping force is altered from the high level to the low level. The damping force restoration unit M8 determines whether or not the damping force change rate V is continuously within a range defined by the second reference value S? during a predetermined period after the damping force is altered to the low level by the damping force alteration unit M4. The range defined by the second reference value S2 is narrower than that defined by the first reference value S1.When it is determined that the damping force change rate V is continuously within the range defined by the second reference value S2 during the predetermined period, the damping force restoration unit M8 controls-the shock absorber M1 so that the setting of the damping force provided thereby is restored to the high level (hard state).

It is possible to carry out the above-mentioned suspension control separately for each wheel, or commonly for all the wheels. It is also possible to carry out the suspension control separately for a set of front wheels and a set of rear wheels.

Referring to FIG.2, there is illustrated a procedure for controlling the setting of the shock absorber 2 being considered. The procedure shown in FIG. 2 commences with step 500, at which step the CPU 61 inputs the damping force change rate V regarding each shock absorber 2 from the damping force change rate detection circuit 70 via the input interface circuit 67. At step 510, the CPU 61 determines whether or not the damping force change rate V is greater than a first reference value Vl (which corresponds to the aforementioned first reference value S1). The first reference value V1 may be a fixed value or varies in accordance with the vehicle speed Sp.Also, the first reference vale Vi is obtained by the learning procedure in the same way as the aforementioned first embodiment of the present invention.

FIG. 3 is a waveform diagram illustrating an example of the damping force change rate V. When the damping force change rate V is less than the first reference value V1, for example, before time tl, the CPU 61 determines whether or not the shock absorber 2 is in the soft state at step 520 by checking a flag FS (which corresponds to the aforementioned flag FHS).

When it is determined, at step 520, that the flag FS is not equal to 1, the CPU 61 controls the shock absorber 2 so that the damping force of the shock absorber 2 is set to the hard state at step 530, and ends the procedure shown in FIG.2..

On the other hand, when it is determined, at step 510, that the damping force change rate V becomes greater than the first reference value V1, for example, at time tl shown in FIG.3 the CPU 61 sets the flag FS corresponding to the aforementioned FHS to 1 at step 540. At step 550, the CPU 61 sets the shock absorber 2 to the soft state in the same way as step 130 shown in FIG.5 of GB-2239506 A.

At time t2 shown in FIG.3, the damping force change rate V becomes equal to less than the first reference value Vl. At this time, the result obtained at step 510 becomes NO. The CPU 61 checks the flag FS at step 520, and determines, at step 560, whether or not the damping force change rate V is less than a second reference value V2, which is less than the first reference value V1. It will be noted that the second reference value V2 corresponds to the aforementioned second reference value S2. During the period between time t2 and t3 shown in FIG. 3, the result obtained at step 560 is NO. The CPU 61 resets a flag FA to zero at step 570. The flag FA indicates the fact that the damping force change rate V becomes less than the second reference value V2 when it is equal to 1. Then, the CPU 61 carries out step 550.

When time t3 has elapsed by repeatedly carrying out the above-mentioned process, the result obtained at step 560 becomes YES. At step 580, the CPU 61 discerns whether or not the flag FA is equal to 0.

The flag FA is set to 1 after the damping force change rate V becomes less than the second reference value V2.

Thus, the flag FA is equal to 0 immediately after the damping force change rate V becomes less than the second reference value V2. Thus, when FA = 0, the CPU 61 sets the flag FA to 1 at step 590, and starts a timer Ta at step 600.

After executing step 600, or when the result obtained at step 580 is NO, the value of the timer variable Ta is incremented by I at step 610. At step 620 subsequent to step 610, the CPU 61 judges whether or not the value of the timer variable Ta is equal to a predetermined reference period TO. When the result at step 620 is NO, the CPU 61 executes step 550 at which step the setting of the damping force is maintained in the soft state. Each time the routine shown in FIG.2 is carried out, the timer variable Ta is incremented by 1. Thus, the result at step 620 is NO until the value of the timer variable Ta becomes equal to the reference period TO.

When the result at step 620 is YES, the CPU 61 resets the flag FS to zero at step 670, and sets the damping force to the hard state. Then, the procedure shown in FIG. 2 is ended.

After time t3 shown in FIG.3, the result obtained at step 560 becomes YES, and the value of the timer variable Ta is compared with the reference period TO at step 620. Since the period between time t3 and t4 is shorter than the reference period TO, the setting of the damping force is continuously maintained in the soft state. After time t4, the result obtained at step 560 becomes NO, and the setting of the damping force is continuously maintained in the soft state. The period between time t5 and time t7 shown in FIG. 12, the setting of the damping force is continuously maintained in the soft state. After time t7, the period during which the damping force change rate V is continuously less than the second reference value V2 exceeds the reference period TO. Thus, the result obtained at step 620 becomes YES, and the setting of the damping force is altered to the hard state at time t8 at which the reference time to has elapsed from time t7. After that, the setting of the damping force is maintained in the hard state until the damping force change rate V exceeds the first reference value Vi.

By repeatedly carrying out the procedure shown in FIG.2, the damping force of each shock absorber 2 is set to the low level immediately after the damping force change rate V exceeds the first reference value V1 (at time tl shown in FIG. 3 ), and maintained at the low level until the reference period TO elapses (time t8) after the damping force change rate V becomes equal to or less than the first reference value V1 in the state where the damping force change rate V is less than the second reference value V2. After that, the damping force is set to the high level.

According to the above-mentioned embodiment of the present invention, it becomes possible to very reliably control the setting of the damping force of each shock absorber 2 in accordance with the road surface condition.

More specifically, this embodiment of the present invention has the following advantages.

When the vehicle is traveling on a substantially flat road surface, the damping force change rate V becomes less than the second reference value V2 soon after the damping force change rate V exceeds the first reference value V1. Thus, the damping force of the shock absorber 2 is maintained at the low level for a short period. Thus, even if the vehicle is riding over a small step portion on a substantially flat road surface, the setting of the damping force is returned to the hard state soon after it is altered to the soft state. As a result, good driving comfort is obtained, and it is thus possible to prevent the setting of the damping force from being in the soft state for a longer time than necessary and to prevent the rpad holding properties from deteriorating.

On the other hand, when the vehicle is traveling on a rough road surface, the damping force change rate V changes greatly, and frequently exceeds not only the first reference V1 but also the second reference V2. Thus, the period during which the damping force change rate V is continuously less than the second reference value V2 becomes short after the damping force of the shock absorber 2 is set to the soft state. Thus, the period during which the setting of the damping force is maintained at the low level becomes long, so that it becomes possible to prevent the setting of the damping force from being unnecessarily altered. Hence, it is possible to obtain improved drive feeling and improved durability of shock absorbers.

A description will be given of a variation of the above-described embodiment of the, present invention with reference to FIG.4. In the variation, the second reference value V2 and the reference period TO are respectively varied in accordance with the vehicle speed Sp. The variation includes a procedure composed of steps 521 - 526, which are arranged between step 520 and 560 shown in FIG.2.

At'step 521, the vehicle speed Sp measured by the speed sensor 51 is input to the CPU 61 via the waveform shaping circuit 73. At step 522, the CPU 61 determines whether or not the vehicle speed Sp is greater than a predetermined reference vehicle speed SO. When Sp > SO, the the CPU 61 writes a value V21 into the second reference value V2 at step 523, and writes a value T01 written into the reference period TO. When S < SO, the CPU 61 writes a value V22 smaller than V21 into the second reference value V2 at step 525, and writes a value T02 greater than TOl into the reference period TO.That is, when the vehicle is traveling at a high speed greater than the reference vehicle speed SO, the second reference value V2 is increased,, and the reference period TO is shortened, as compared with the corresponding values when the vehicle is traveling at a low speed less than the reference vehicle speed SO. It can be seen from the above description when the vehicle is traveling at a high speed greater than S0, the setting of the damping force is controlled so that the damping force is easily altered to the hard state from the soft state, as compared with the case where the vehicle is traveling at a low speed equal to or less than SO. It will be noted that when the vehicle is traveling at a high speed, the damping force change rate V itself increases.According to the above-mentioned procedure of the variation, it becomes possible to prevent the damping force of the shock absorber 2 from being unnecessarily set to the soft state and obtain a hard-oriented suspension characteristic which is required when the vehicle is traveling at a high speed.

It is possible to provide variations and modifications from the second embodiment of the present invention. For example, it is possible to provide the second reference value V2 which consists of a positive-side value and a negative-side value different from the positive value. It is also possible to determine the reference period TO on the basis of the peak value of the damping force change rate V.

A description will now be given of a second embodiment of the present invention with reference to FIG.51 in- which those parts which are the same as those shown in the previous figures are given the same reference numerals. Referring to FIG.5, a suspension control system is composed of the damping force change rate detector M2 and a damping force controller M3b. The damping force controller M3b is composed of a soft-state holding period calculation unit M11 and a second second damping force alteration unit M12 in addition to the damping force alteration unit M7 and the damping force restoration unit M8 shown in FIG. 1.In the following description of the second embodiment of the present invention, the damping force restoration unit M8 is referred to as a first damping force restoration unit M8.

As has been described previously, the damping force alteration unit M7 determines whether or not the damping force change rate V output by the damping force change rate detector M2 is outside of the range defined by the aforementioned first reference value S1. When the damping force change rate V is outside of the range defined by the first reference value S1, the damping force alteration unit M7 controls the shock absorber M1 so that the setting of the damping force is altered to the soft state from the hard state. -The first damping force restoration unit M8 determines whether or not the damping force change rate V is continuously within the range defined by the second reference value S2 (less than the first reference value S1) for at least the aforementioned predetermined period (hereafter referred to as a first predetermined period T1) from the time when the setting of the damping force is altered to the soft state.When the result of this determination is affirmative, the first damping force restoration unit M8 controls the shock absorber M1 so that the setting of the damping force is restored to the hard state.

The soft-state holding period calculation unit Mil calculates a soft-state holding period between the time when the setting of the damping force is altered to the soft state by the damping force alteration unit M7 and the time when the setting of the damping force is restored to the hard state by the first damping force restoration unit M8. The second damping force restoration unit M12 determines whether or not the soft-state holing period calculated by the soft-state holding period calculation unit M 11 is longer than a second predetermined period T2. When the result of this determination is affirmative, the second damping force restoration unit M12 controls the shock absorber M1 so that the setting of the damping force is restored to the hard state from the soft state.The second damping force restoration unit M12 is provided for preventing the setting of the damping force from being maintained in the soft state for a period longer than necessary.

This purpose is also achieved by replacing the second damping force restoration unit M12 by a reference value correction unit M12 or using the reference value correction unit M12 together with the second damping force restoration unit M12. When the soft-state holding period calculated by the soft-state holding period calculation unit Mll is longer than a third predetermined period T3, the reference value correction unit M13 corrects the second reference value S2 in such a way that the second reference value S2 is increased so that a new range defined by an increased reference value S2 becomes wider than before, and controls the first predetermined period T1 so that it is shortened.That is, the condition for restoring the setting of the damping force to the hard state is relaxed so that it facilitates the restoration of the damping force.

Alternatively, it is possible to correct either the second reference value S2 or the first predetermined period T1.

It is also possible to employ an initial value correction unit M14 in addition to the reference value correction unit Ml3. The initial value correction unit M14 increases an initial value of the second reference value S2 and decreases an initial value of the first predetermined period T1 when the reference value correction unit M13 corrects the first predetermined period T1 and the second reference value S2 to a large extent, as compared with a predetermined condition.

That is, the condition for restoring the setting of the damping force from the soft state to the hard state is corrected so that it facilitates the restoration of the damping force. It is possible to design the.initial value correction unit M14 so that it corrects either the first predetermined period T1 or the second reference value S2.

A further description will now be given of the third embodiment of the present invention with reference to FIGS. 6A and 6B, in which those steps which are the same as those shown in FIG.11 are given the same reference numerals. The third embodiment has shock absorbers, each having the configuration shown in FIGS.3A and 3B, and the system shown in FIG.4 of GB-2239506 A. The CPU 61 used in the second embodiment of the present invention executes a damping force control interrupt routine (procedure) shown in FIGS. 6A and 6B.

After turning the power supply ON, various flags including the aforementioned flags FS and FA are reset to zero, and the procedure shown in FIGS. 6A and 6B is repeatedly carried out. The CPU 61 inputs the damping force change rate V regarding the shock absorber 2 being considered at step 500, and determines whether or not V > V1 at step 510. As has been described previously, the first reference V1 is provided for altering the setting of the shock absorber 2 from the hard state to the soft state.

FIG. 7 is a waveform diagram illustrating an example of a change of the damping force change rate V.

When the damping force change rate V is less than the first reference value Vl, for example, before time t1, the CPU 61 determines whether or not the shock absorber 2 is in the soft state at step 520 by checking the flag FS (which corresponds to the aforementioned flag FHS).

When it is determined, at step 520, that the flag FS is not equal to 1, the CPU 61 controls the shock absorber 2 so that the damping force of the shock absorber 2 is set to the hard state at step 530, and ends the zprocedure shown in FIG. 2.

On the other hand, when it is determined, at step 510, that the damping force change rate V becomes greater than the first reference value V1, for example, at time + shown in FIG. 7, the CPU 61 determines, at 1 step 740, whether or not the flag FS is equal to 1.

When FS = 0, that is, when the setting of the damping force at this time is hard, the CPU 61 resets a timer Ts to zero at step 740. The timer Ts is provided for measuring the soft-state holding period. On the other hand, the current setting of the damping force is soft, the CPU 61 increments the timer.Ts by 1 at step 760, which is also executed after step 750. At step 770, the CPU 61 determines whether or not the value of the timer Ts is equal to or less than the second (soft state) reference period T2. When the result at step 770 is YES, the setting of the damping force is maintained in the soft state at step 550 after the flag FS is set to 1 at step 540. The case where it is determined that Ts > T2 will be described later. Until that time, the following description assumes that Ts < T2.

When it is determined, at step 510, that the damping force change rate V becomes equal to or less than the first reference value V1 (at time tl shown in FIG.7) and the flag FS is equal to 1 at step 520, the CPU 61 executes step 560, at which step it is determined whether or not the damping force change rate V is less than the second reference value V2 (less than the first reference value V1). During the period between time t2 and t3 shown in FIG.7, the result at step 560 is successively NO. At step 570, the CPU 61 resets the flag FA to zero, and executes step 740. During the above-mentioned operation, the shock absorber 2 is maintained in the soft state.

The above-mentioned procedure is repeatedly carried out. Then, the result obtained at step 560 becomes YES (at time t3). At step 580, the CPU 61 discerns whether or not the flag FA is equal to 0. The flag FA is set to 1 after the damping force change rate V becomes less than the second reference value V2.

Thus, the flag FA is equal to 0 immediately after the damping force change rate V has become less than the second reference value V2. Thus, when FA = 0, the CPU 61 sets the flag FA to 1 at step 590, and starts the aforementioned timer Ta at step 600.

After executing step 600, or when the result obtained at step 580 is NO, the value of the timer variable Ta is incremented by 1 at step 610. At step 860 subsequent to step 610, the CPU 61 judges whether or not the value of the timer variable Ta is equal to a predetermined reference period Ti. When the result at step 860 is NO, the CPU 61 executes step 740, so that the setting of the damping force is maintained in the soft state. Each time the routine shown in FIGS.6A and B is carried out, the timer variable Ta is incremented by 1. At step 860, the CPU 61 determines whether or not the value of the timer variable is equal to or greater than the first predetermined period T1.When the result at step 860 is NO, the CPU 61 executes step 740, so that the setting of the damping force is continuously maintained in the soft state. The first predetermined period T1 provides the setting restoration condition used when the situation, V < V2, is continuously satisfied.

When it is determined, at step 860, that Ta > T1, the CPU 61 resets the flag FS to zero at step 670, and alters the setting of the shock absorber 2 to the hard state from the soft state at step 530. Then the procedure ends. That is, when the period during which the damping force change rate V is continuously less than the second reference value V2 becomes longer than the first reference period T1, the setting of the damping force is altered to the hard state.

After time t3 shown in FIG. 7, the result obtained at step 560 becomes YES, and the value of the timer variable Ta is compared with the reference period T1 at step 860. Since the period between time t3 and t4 is shorter than the reference period T1, the setting of the damping force is maintained in the soft state. After time to, the result obtained at step 560 becomes NO, and the setting of the damping force is continuously maintained in the soft state. The period between time t5 and time t7 shown in FIG.7, the setting of the damping force is continuously maintained in the soft state. After time t7, the period during which the damping force change rate V is continuously less than the second reference value V2 is longer than the reference period T1.Thus, the result obtained at step 860 becomes YES (Ta > T1), and the setting of the damping force is altered to the hard state at time t8 at which the first reference period T1 has elapsed from time t7.

A description will now be described of a case where the damping force change rate V is continuously less than the second reference value V2 during only a period which is shorter than the first reference period T1. In this case, the result obtained at step 560 or step 860 is NO, and thus the setting of the damping force is maintained in the soft state. When the value of the timer variable Ts, which indicates the period during which the setting of the shock absorber 2 is maintained in the soft state, becomes longer than the second (soft state) reference period T2, the result obtained at stejp, 770 becomes NO, and the CPU 61 executes step 880 (FIG. 6B).

The CPU 61 increments the value of the second reference value V2 by a value x at step 880, and decreases the first reference period T1 by y at step 890. That is, the procedure of steps 880 and 890 functions to change the conditions respectively defined at steps 560 and 860 so that the affirmative results are obtained more easily than before. At step 900, the CPUi 61 increases the second reference period T2 by z (z > Tl). Thereby, the condition defined at step 770 is changed, so that the CPU 61 determines a time at which the procedure starting from the step 880 is to be started.

At step 910, the CPU 61 determines whether or not the increased soft-state reference period T2 is equal to or less than a predetermined upper limit value T21im. When the result at step 910 is YES, the CPU 61 determines, at step 920, whether or not the second reference value V2 is equal to or less than a predetermined upper limit value V2lim. When the result at step 920 is YES, the CPU 61 executes step 540, so that the setting of the damping force is continuously maintained in the soft state. That is, the number of times that the increasing procedures at steps 880 and 900 have not been carried out many times is not so many, the second reference period T2 and the second reference value V2 are still less than the soft-state upper limit value T2lim and the upper limit value V2lim, respectively.

When the result obtained at step 910 or step 920 becomes NO even by repeatedly carrying out the steps 880, 890 and 900 (or at time te or when the second reference value V2 increases to V2e shown in FIG.8 ), the CPU 61 successively executes step 930, 940 and 950.

At step 930, an initial value V20 is written into the second reference value T1. At step 940, an initial value T10 is written into the first reference period Ti. At step 950, an initial value T20 is written into the second (soft state) reference period T2. Then, the CPU 61 executes step 670 and the damping force is altered to the hard state.

By executing the procedure shown in FIGS. 6A and B, the damping force of each shock absorber 2 is set to the soft state immediately after the corresponding damping force change rate V exceeds the first reference value V1 (at time t1 shown in FIG. ), and continuously maintained in the soft state until the damping force change rate V is continuously less than the second reference value until at least the first reference period T1 elapses (time t8 shown in FIG.8 from the time when the damping force change rate becomes less than the first reference value V1. After that, the setting of the damping force is altered to the hard state.

When the damping force change rate V is continuously less than the second reference value V2 during only a period less than the first reference period Ti, the judgment condition for altering the setting of the damping force from the soft state to the hard state is changed so that the setting of the damping force is restored to the hard state more easily than before. Even if the setting of the damping force has not yet been restored to the hard state, the setting of the damping force is once restored to the hard state when the second reference period T2 or the second reference value V2 becomes equal to the soft upper limit value T2lim or the second upper limit value V2lim, respectively. With the above-mentioned control, it is possible to very reliably control the setting of each shock absorber in accordance with the road surface condition.

When the vehicle is traveling on a flat road surface, if the damping force change rate V exceeds the first reference value V1, the damping force change rate V becomes less than the second reference value V2 soon.

Thus, the setting of the damping force of the shock absorber 2 is maintained in the soft state during a short period, and then restored to the hard state.

Thus, if the vehicle is riding over a small stepped portion on a substantially flat road surface, the setting of the damping force is restored to the hard state soon, so that improved riding comfort can be obtained. Thus, it becomes possible to prevent the setting of the damping force from being in the soft state during an unnecessary period and prevent the road holding ability from deteriorating.

On the other hand, when the vehicle is continuously traveling on a rough road surface, the damping force change rate V changes greatly, and will frequently exceed the second reference value V2 after it exceeds the first reference value Vl, Thus, the damping force change rate V is continuously less than the second reference value V2 during only short period after the shock absorber 2 is set to the soft state. Thus, the soft-state holding period T2 is elongated, so that it is possible to prevent the alteration frequency from increasing. Since the upper limit T2lim is provided for the soft-state holding period T2. Thus, it is possible to prevent the setting of the damping force from being maintained in the soft state during a long period.As a result, improved driving feeling and improved durability of the shock absorber 2 can be obtained.

In the second embodiment of the present invention, the setting of the damping force is restored to the hard state on the basis of the soft-state (second) upper limit value T2lim and the upper limit value V2lim respectively provided for the second predetermined period t2 and the second reference value V2. Alternatively, it is possible to employ either the soft-state upper limit value T2lim or the upper limit value V2lim.

A description will now be given of a procedure (initial value correction routine) for determining the initial values V20 and T10 used in the procedure shown in FIGS. 6A and 6B, with reference to FIG.9, The procedure shown in FIG.18 is repeatedly carried out at predetermined intervals which are slightly longer than those for the procedure shown in FIGS.GA and -6B. At step 1000, the CPU 61 calculates frequencies N1, N2 and N3 during the predetermined period during which the procedure shown in FIGS. BA and 6B is repeatedly carried out. Ni denotes a correction frequency which corresponds to the number of times that the second reference value V2 used for defining th range between -V2 and +V2 is corrected during the predetermined period.N2 denotes an upper limit reaching frequency which corresponds to the number of times that the second (soft state) reference period T2 exceeds the soft-state upper limit value T2lim for the .predetermined period.

N3 denotes an upper limit reaching frequency which corresponds to the number of times that the second reference value V2 exceeds the upper limit value V2lim during the predetermined period.

At step 1010 subsequent to step 1000, the CPU 61 determines whether or not the upper limit reaching frequency N2 or N3 is equal to or less than a predetermined upper limit reference frequency NA. When the result obtained at step 1010 is YES, the CPU 61 determines, at step 1020, whether or not the correction frequency N1 is equal to or less than a correction reference frequency NB. When the result obtained at step 1010 or step 1020 is NO, the CPU 61 executes step 1030, at which step the initial value V20 of the second reference value V2 is increased by 'a'. Subsequently, the CPU 61 executes step 1040, at which step the initial value T10 of the first reference period T1 is decreased by 'b'. Then, the procedure shown in FIG.9 ends.

On the other hand, when the result at step 1020 is YES, the CPU 61 determines, at step 1050, whether or not the correction frequency Nl is equal to or greater than a minimum correction frequency NC (NB > NC). When the result obtained at step 1050 is YES, the procedure ends. On the other hand, when the result at step 1050 is NO, the initial value V20 of the second reference value V2 is decreased by 'a' at step 1060, and the initial value T10 of the first reference period T1 is increased by 'b'. The initial values T10 and V10 are arbitrarily selected, for example, on the basis of experimental results. Similarly, values -'a' and 'b' are selected.

When the vehicle is traveling on a rough road surface, the correction frequency N1 is high. In such a case, the number of times that the correction (steps 880-900) is carried out is reduced and thus the soft-state holding period is reduced. Thus, it becomes possible to very reliably control the setting of the damping force in accordance with the road surface condition which is varying momentarily.

On the other hand, when the correction frequency N1 is less than the minimum correction frequency NC1, the initial value V20 is corrected to decrease, and the initial value T10 is corrected to increase. Thus, when the vehicle starts to travel on a rough road surface, there is an increase in the number of times that the correction (steps 880-900)- is carried out until the setting of the damping force is restored to the hard state from the soft state by the procedure shown in FIGS. GA and 63. Thus, the soft-state holding period is elongated. Thus, it becomes possible to very reliably control the setting of the damping force in accordance with the road surface condition and thus improve riding comfort, driving controllability and stability.

It is possible to further modify the above-mentioned second embodiment of the present invention. In the above-mentioned second embodiment of the present invention, the first reference value V1 consists of positive and negative values equal to each other. Alternatively, it is possible to replace the first reference value V1 by a positive. reference value and a negative reference value which is different from the positive reference value. It is also possible to correct either the second reference value V2 or the first reference period T1. It is also possible to correct either the initial value V20 or the initial value T10.

In the specifically disclosed embodiments of the present invention, there are provided shock absorbers, each providing the hard state and the soft state. Alternatively, it is possible to employ different types of shock absorbers having, for example, three or more states, such as hard, sport and soft states. It is also possible to employ shock absorbers which continuously varies the level (setting) of the damping force.

The present invention are not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.

Claims

CLAIMS:

1. A suspension control system for controlling a suspension having a shock absorber provided for a wheel of a vehicle, said suspension control system comprising: damping force change rate detecting means for detecting a damping force change rate indicating a rate of change of a damping force of said shock absorber; damping force alteration means, coupled to said shock absorber and said damping force change rate detecting means, for determining whether or not said damping force change rate is outside a first range and for altering the setting of said damping force to a first level from a second level greater than said first level when it is determined that said damping force change rate is outside said first range; and damping force restoration means, coupled to said damping force change rate detecting means, said damping force alteration means and said shock absorber, for determining whether or not said damping force change rate is continuously within a second range narrower than said first range during at least a predetermined period after said damping force alteration means alters the setting of the damping force from said second level to said first level and for restoring the setting of the damping force from said first level to said second level when it is determined that said damping force change rate is continuously within said second range.

2. A suspension control system as claimed in claim 1,- wherein said damping force restoration means comprises: first means for detecting a time when said damping force change rate goes into said second range; and second means for measuring said predetermined period which starts from a starting time corresponding to said time when said damping force change rate goes into said second range.

3. A suspension control system as claimed in claim 2, further comprising resetting means for resetting said second means to said starting time when said damping force restoration means determines that said damping force change rate goes outside of said second range.

4, A suspension control system as claimed in claim If further comprising: speed detection means for detecting a vehicle speed of said vehicle; and adjustment means, coupled to said speed detection means and said damping force restoration means, for adjusting said second range on the basis of said vehicle speed.

A A suspension control system as claimed in claim 4 wherein said adjustment means comprises: comparing means for comparing said vehicle speed with a predetermined vehicle speed and for outputting a comparison result; and means for increasing said second range whenthe comparison result output by said comparing means indicates that the vehicle speed is greater than said predetermined vehicle speed and for decreasing said second range when the comparison result output by said second comparing means indicates that the vehicle speed is equal to or less than said predetermined vehicle speed.

6. A suspension control system as claimed in claim .1, further comprising: speed detection means for detecting a vehicle speed of said vehicle; and adjustment means, coupled to said speed detection means and said damping force restoration means, for adjusting said predetermined period on the basis of said vehicle speed.

7. A suspension control system as claimed in claim 6, wherein said adjustment means comprises: comparing means for comparing said vehicle speed with a predetermined vehicle speed and for outputting a comparison result; and means for decreasing said predetermined period when the comparison result output by said comparing means indicates that the vehicle speed is greater than said predetermined vehicle speed and for increasing said predetermined period when the comparison result output by said second comparing means indicates that the vehicle speed is equal to or less than said predetermined vehicle speed.

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8. A suspension control system as claimed in claim 3, further comprising: speed detection means for detecting a vehicle speed of said vehicle; first adjustment means, coupled to said speed detection means and said damping force restoration means, for adjusting said second range on the basis of the vehicle speed; and second adjustment means, coupled to said speed detection means and said damping force restoration means, for adjusting said predetermined period on the basis of said vehicle speed.

9. A suspension control system for controlling a suspension having a shock absorber provided for a wheel of a vehicle, said suspension control system comprising: damping force change rate detecting means for detecting a damping force change rate indicating a rate of change of a damping force of said shock absorber; damping force alteration means, coupled to said shock absorber and said damping force change rate detecting means, for determining whether or not said damping force change rate is outside a first range and for altering the setting of said damping force to a first level from a second level greater than said first level when it is determined that said damping force change rate is outside said first range;; first damping force restoration means, coupled to said damping force change rate detecting means, said damping force alteration means and said shock absorber, for determining whether or not said damping force change rate is continuously within a second range narrower than said first range during at least a first period after said damping force alteration means alters the setting of the damping force from said second level to said first level and for restoring the setting of the damping force from said first level to said second level when it is determined that said damping force change rate is continuously within said second range;; holding period calculation means, coupled to said damping force alteration means and said first damping force restoration means, for calculating a holding period from a time when the setting of the damping force is altered to the first level by said damping force alteration means to a time when the setting of said damping force is restored to said second level by said first damping force restoration means; and second damping force restoration means, coupled to said holding period calculation means and said shock absorber, for determining whether Qr not said holding period becomes longer than a second period and for restoring the setting of the damping force from said first level to said second level when it is determined that said holding period has become longer than said second period.

10. A suspension control system as claimed in claim 9, further comprising correction means for determining whether or not said holding period becomes longer than a third period and for correcting said second range so that said first damping force restoration means restores the setting of the damping force to said second level from said first level more easily than before said second range is corrected when it is determined that said holding period has become longer than said third period.

11 A suspension control system as claimed in claim 9, further comprising correction means for determining whether or not said holding period becomes longer than a third period and for correcting said first period so that said first damping force restoration means restores the setting of the damping force from said first level to said second level more easily than before said first period is corrected when it is determined that said holding period has become longer than said third period.

12. A suspension control system as claimed in claim 1Q, further comprising: determining means for determining whether or not said second range corrected by said correction means becomes longer than a limited range; and means, coupled to said shock absorber and said determining means, for restoring the setting of said damping force change rate to said second level when said determining means determines that said second range has become longer than said limited range.

13. A suspension control system as claimed in claim 12; further comprising initialization means for setting said second range to an initial range when said determining means determines that said second range has become greater than said limited range.

14. A suspension control system as claimed in claim 11, further comprising: determining means for determining whether or not said second period corrected by said correction means becomes greater than a limited period; and means, coupled to said shock absorber and said determining means, for restoring the setting of said damping force change rate to said second level when said determining means determines that said second period has become greater than said limited period.

15. A suspension control system as claimed in claim 14, further comprising initialization means for setting said second period to an initial period when said determining means determines that said second range has become greater than said limited range.

16. A suspension control system as claimed in claim 13, further comprising: frequency calculation means for calculating a first frequency which indicates the number of times that said second range becomes greater than said limited range during a predetermined period; first comparing means for comparing said first frequency with a predetermined reference frequency and for outputting a comparison result; and first initial range correction means for correcting said initial range when said comparison result shows that said first frequency is less than said reference frequency, so that said first damping force restoration means alters the setting of the damping force from said first level to said second level more easily than before said initial range is corrected.

17. A suspension control system as claimed in claim d6, further comprising: correction frequency calculation means for calculating a correction frequency which indicates the number of times that said correction means corrects said second range during said predetermined period; second comparing means for comparing said correction frequency with a first reference correction frequency and for outputting a comparison result when said comparison result output by said first comparing means shows said first frequency is equal to or greater than said predetermined reference frequency; and second initial range correction means for correcting said initial range when said comparison result output by said second comparing means shows that said correction frequency is less than said first reference correction frequency, so that said first damping force restoration means alters the setting of the damping force from said first level to said second level more easily than before said initial range is corrected.

A A suspension control system as claimed in claim z 17, further comprising: third comparing means for comparing said correction frequency with a second reference correction frequency.less than said first reference correction frequency and for outputting a comparison result; and third initial range correction means for correcting said initial range when said comparison result output by said third comparing means shows that said correction frequency is less than said second reference correction frequency, so that it is more difficult for said first damping force restoration means to alter the setting of the damping force from said first Level to said second level than before said initial range is corrected.

19. A suspension control system as claimed in claim 15, further comprising: frequency calculation means for calculating a first frequency which indicates the number of times that said second period becomes longer than said limited period during a predetermined period; first comparing means for comparing said first frequency with a predetermined reference frequency and for outputting a comparison result; and first initial period correction means for correcting said initial period when said comparison result shows that said first frequency is less than said reference frequency, so that said first damping force restoration means alters the setting of the damping force from said first level to said second level more easily than before said initial period is corrected.

20. A suspension control system as claimed in claim 19, further comprising: correction frequency calculation means for calculating a correction frequency which indicates the number of times that said correction means corrects said second range for said predetermined period; second comparing means for comparing said correction frequency with a first reference correction frequency and for outputting a comparison result when said comparison result output by said first comparing means shows said first frequency is equal to or greater than said predetermined reference frequency; and second initial period correction means for correcting said initial period when said comparison result output by said second comparing means shows that said correction frequency is less than said first reference correction frequency, so that said first damping force restoration means alters the setting of the damping force from said first level to said second level more easily than before said initial period is corrected.

21. A suspension control system as claimed in claim 20, further comprising: third comparing means for comparing said correction frequency with a second reference correction frequency less than said first reference correction frequency and for outputting a comparison result; and third initial period correction means for correcting said initial period when said comparison result output by said third comparing means shows that said correction frequency is less than said second reference correction frequency, so that it is more difficult for said first damping force restoration means to alter the setting of the damping force from said first level to said second level than before said initial period is corrected.

22. A suspension control system substantially as hereinbefore described with reference to the accompanying drawings.