GB2233939A - Actively-controlled vehicle suspension giving adjustable cornering characteristics - Google Patents

Abstract

In a system for controlling active suspensions of a motor vehicle, having fluid suspensions (1a, 1b, 1c, 1d) for the respective wheels, control valves (2a, 2b, 2c, 2d) for the respective suspensions, and a controller (3) for adjusting the control valves to charge and discharge a fluid into and out of the fluid suspensions so as to control the vehicle attitude, lateral acceleration of the vehicle is detected during a turning thereof, and the control valves (2a, 2b, 2c, 2d) are adjusted responsive to the detected lateral acceleration for changing the vehicle heights at the respective fluid suspensions (1a, 1b, 1c, 1d) such that the ground-contact load of one wheel of the front wheel pair is increased while the ground-contact load of the other wheel is decreased and such that the ground-contact load of one wheel of the rear wheel pair, diagonally opposite the other wheel of the front wheel is increased while the ground-contact load of the other wheel of the rear wheel pair is decreased. The turning or steering characteristic of the vehicle is thus changed to have a strong or a weak understeering degree. The steering characteristic may be changed dependent upon the vehicle speed. <IMAGE>

Description

:2 171 _3 ',- '-:3:1 '-D 1 METHOD AND SYSTEM FOR CONTROLLING ACTIVE

SUSPENSIONS OF A VEHICLE The present invention relates to a method and a system for controlling an active suspension of a vehicle.

Active suspension systems of various forms have been developed and disclosed as in, for example, Japanese Pat. Appln. Laid-Open Publn. No-. 62-139709. A typical example of a known active suspension systeT comprises the following essential components. An individual suspension is provided for each wheel for supporting the vehicle by fluid pressure. Charging and discharging fluid into and out of each suspension are controlled independently by 1-9 operations' of respective control valves. The operation of each control valve is controlled by opening and closing control signals generated by a controller responsive to information such as vertical displacement of the suspension. And the controller calculates command quantities of charging and discharging fluid for each suspe.nsion. Thus charging and discharging the fluid into and out of each suspension are controlled.

In a vehicle having the active suspension system as described above, the vehicle attitude is controlled so as to constantly take a desired attitude by detecting deviation of the vehicle height for each wheel from a reference vehicle height and by adjusting each suspension so as to eliminate the deviation.

The object of the present invention is to provide a method and a system for controlling active suspensions of a vehicle so as to vary turning characteristics of a vehicle.

According to the present invention, in' an aspect thereof, the a.-jo;e 's 7 atta--.-i,-.d by a method for controlling active suspensions of the vehicle,having fluid suspensions provided for respective wheels, means 2 f or charging and discharging fluid into and out of the respective fluid suspensions to extend and contract the suspensions independently, and a controller for adjusting the charging and discharging means to control the vehicle heights at the respective wheels, said method comprising the steps of: detecting lateral acceleration of the vehicle, and adjusting said charging and discharging means responsive to a magnitude of the detected lateral acceleration, for changing the vehicle heights and hence the difference between loads adapted to inner and outer wheels during the turning, in such a manner that the difference is larger for one of the front wheel pair and the rear wheel pair and the difference is smaller for-the other pair, thereby to vary.a turning characteristic of the vehicle.

According to the present invention, in another aspect thereof, the above object is attained by a system for controlling active suspensions of a vehicle, having fluid suspensions provided for respective wheels, means for charging and discharging a fluid into and out of the respective fluid suspensions to extend and contract the suspensions independently, and a controller for adjusting said charging and discharging means to control the vehicle heights at the respective wheels, said system comprising sensor -means f o r detecting lateral acceleration of the vehicle, and calculating means, - responsive to the lateral acceleration detected by the sensor means, for calculating 'a stroke control quantity used to adjust said means for charging and discharging the fluid so as to change the vehicle heights and hence the difference between loads adapted to inner and outer wheels during the turning in such a manner that the difference is larger for one of the front wheel pair and the rear wheel pair and is smaller for the other pair, thereby to vary turning characteristics of the vehicle.

When the difference between the ground-contact loads of the inner and outer wheels is adjusted such that the k is 3 difference becomes large at the front wheels and becomes small at the rear wheels, then the de gree of understeering becomes strong. On the other hand, when the difference between the loads of the inner and outer wheels is adjusted such that the difference becomes small at the front wheels and becomes large at the rear wheels, then the degree of understeering becomes weak. Thus, the turning or steering characteristics of the vehicle can be controlled as desired according to a driver's preference.

Furthermore, the steering characteristics can be changed according to the vehicle speed.

Preferred embodiments of the present invention will become understood from the following detailed description referring to the accompanying drawings.

FIG. 1 is a schematic perspective view of a motor vehicle showing a control system according to the present invention; FIG. 2 is a diagram showing a hydraulic system for the suspension units according to the present invention; FIG. 3 is a block diagram showing a system according to the present invention; FIG. 4 is a f low chart showing an operation of the system of FIG. 3; FIG. 5 is a graph showing a relation between ground contact load and cornering power; FIG. 6 is another flow chart showing the operation of the system of FIG. 3; FIG. 7 is a block diagram showing another system according to the present invention; and FIG. 8 is a f low chart showing the operation of the system of FIG. 7.

FIGGES. 1 and 2 show an active suspensioa systlen. to which the present inventionmay be applied. In FIG. 2, reference characters la and lb indicate suspensions of left and right front wheels of a motor vehicle, and lc 1 1 4 and ld indicate suspensions of left and right rear wheels. Each of the suspensions la, lb, lc and id is provided with a pneumatic spring portion D and a hydraulic cylinder E. The spring portion D has an oil chamber A and an air chamber B which are divided by a diaphragm C. The oil chamber A of the spring portion D communicates with an oil chamber F of the hydraulic cylinder E through an orifice G. As shown in FIG. 1, one end of the hydraulic cylinder E (such as a bottom portion of the cylinder) is connected to a suspension arm member 14 on the vehicle wheel W, and the other end (a piston rod) of the hydraulic cylinder E is connected to a member 15 of a vehicle chassis. In accordance with load on the cylinder E, hydraulic oil in the oil chamber F flows into is and out of the oil chamber A through the orifice G so as to generate an appropriate damping force and at the same time to' produce a spring action by volumetric elasticity of air sealed in the air chamber B. The system described above is a known hydro-pneumatic suspension system. 20 There are provided control valves 2a, 2b, 2c and 2d that charge and discharge oil to and from the oil chamber F of the hydraulic cylinders E. The control valves 2a, 2b, 2c and 2d are operated independently by a valve drive signal from a controller 3 to be described later. In 25 FIG. 1, the control valves 2a, 2b, -2c and 2d are installed separately in two groups for the front and rear suspensions. An oil pump 5 is driven by an engine 6 to pump up oil from an oil reservoir 4.to the system. In the system shown, an oil pump S' for power steering and the oil pump 5 described above are driven in tandem by the engine 6.

The oil discharged from the oil pump 5 passes through a check valve 7 and is stored in a high-pressure accumulator 8. In FIG. 1, the accumulator 8 is shown to 3 5 be divide d into two sections for_ the front and rear suspensions. When, the control-valves 2a, 2b, 2c and 2d are switched to the charging side, high-pressure oil is- supplied through the control valves to the oil chamber F of the suspensions la, lb, lc and ld. When the control valves 2a, 2b, 2c and 2d are switched to the discharging side, oil is discharged f rom the oil chambers F of the suspensions la, 2b, lc and ld through an oil cooler 9 into the oil reservoir 4.

FIG. 2 shows a relief valve 10 and a valve 11 which is switched to the unload state indicated in the figure, when signals, generated by the controller 3 responsive to signals from a pressure sensor 81, indicate that the high-pressure accumulator 8 has attained a predetermined pressure. When the valve 11 is switched to the unload side, the oil discharged from the oil pump 5 flows to the oil cooler 9 and then into the oil reservoir 4.

The suspensions la, lb, lc and ld are provided with suspension stroke sensors 13 as shown in FIGS. 1 and 2. The sensor 13 detects vertical relative displacement for each suspension provided between the wheel, and the vehicle body and -input the information of the relative displacement for each of the suspensions la, lb, lc and ld to the controller 3.

In order to detect behaviors of the vehicle, there is provided a lateral G-sensor 12 to detect vehicle lateral acceleration (lateral G). The position where the G-sensor 12 is installed is indicated in FIG. 1. There is further provided a vehicle speed sensor S to detect the speed of the vehicle. The lateral acceleration may be computed from the vehicle speed detected by the sensor S and steering angle detected by a steering angle sensor, or from steering torque and steering assisting force instead of the lateral G-sensor 12. Signals of the sensors 12, 13 and S are inputted to the controller 3. Responsive to the input, the controller 3 determines control quantity for charging and discharging oil for each suspension and sends valve drive signals representing the control quantity to the respective 6 control valves 2a, 2b, 2c and 2d to control each of the suspensions, as will be described below.

As shown in FIG. 3, there is provided a vehicle height adjusting switch 16 which is a changeover switch between a normal vehicle height adapted for plain roads and a high vehicle height adapted for rough roads. In accordance with the selection of the vehicle height by the switch 16, a reference vehicle height signal generating circuit H generates a reference vehicle height signal H2. The suspension stroke sensor 13 for each wheel generates a str- oke signal H1. The signal H2 is subtracted from the stroke signal H,, and an actual relative stroke signal is obtained. The actual relative stroke signal is. passed through -a dead-zone circuit I vhex-e a signal fraction within a set zone in the vicinity of zero is removed. The resulting signal is passed thr,o-ugh a gain circu it G to become a control command quantity which, is delivered t o a valve drive signal geneTating circuit 18. In accordance with a signal from the circuit 18, each suspension is controlled for extension and contraction to attain the reference vehicle height.

In a vehicle having the vehicle height adjusting system as described above, the adjusting system operates in the following manner. It will be assumed that the vehicle is in a normal attitude and the vehicle is controlled, for example, to increase the vehicle height to certain reference vehicle height, only for any one wheel. Then, in the case wherein, -for example, the' suspension stroke sensor 13 carries out measurement or detection without including the height of the gas chamberg (FIG. 1), variation of an actual vehicle height is smaller than variation of the suspension stroke detected by the sensor 13 because of a deflection of both the gas chamber and the tire. In the case wherein the sensor 13 measures with the inclusion of the height of the gas chamber B, the variation of the actual vehicle height is 1 smaller than the variation of the suspension because of a deflection of the tire. As a consequence, the vehicle height becomes lower by the amount of deflection than the increased reference vehicle height. Thus, load adapted to a tire for that wheel increases. Conversely, when control is carried out to decrease the vehicle height of only one wheel, the decreased reference vehicle height is not attained, and the load to the tire decreases.

It will be understood from the foregoing that by the adjustment of the vehicle height, the ground-contact load of the tire can be controlled.

A possibility to control the ground-contact load is utilized in the present invention to variably control the turning characteristics of the vehicle at the time of its turning in accordance with the lateral acceleration in general, rolling rigidity of the vehicle is set at a specific value by the elastic members of all suspension units and the torsion springs of the stabilizers connecting the left and right wheels. A ratio (front-rear ratio) -of rolling rigidity on the front-wheel side and rolling 'rigidity of the rear-wheel side is also set at a specific value.

Therefore, a front-rear ratio of load shift quantity at the time of turning is also at a specific value. In order to change the turning characteristics, it has been necessary to vary essential factors such as tires, suspension springs, and stabilizers in the case of the prior art.

According to the present invention, the turning characteristics are readily controlled, as shown in FIG. 3, by providing, in the controller 3, a circuit 20 for calculating a stroke control quantity, and a characteristic setting circuit 21. The circuit 20 is supplied with a signal from the lateral G-sensor 12. The circuit 21. receives a vehiclespeed signal from the vehicle speed sensor S, and a signal indicative of a set - characteristic is delivered from the circuit 21 to the 1 the (a) is 8 calculating circuit 20. The circuit 20 delivers a stroke control quantity signal Ah which is subtracted f rom the signal Hi from the suspension stroke sensor 13. The operation of the controller 3 as above is shown in the 5 flow chart of FIG. 4.

When the lateral O-sensor 12 detects a left turning of the vehicle, the calculating circuit 20 calculates the stroke control quantity, responsive to the signal from lateral G-sensor 12, and delivers a signal to cause:

the vehicle height of the right-front wheel (front outer wheel) to be raised by Ah; (b) the vehicle height of the left-front wheel (front inner wheel) to be lowered by Ah; (c) the vehicle height of the right-rear wheel (rear outer wheel) to be lowered by Ah; and (d) the vehicle height of the left-rear wheel (rear inner wheel) to be raised by Ah.

Here, the quantity Ah is as follows.

Ah 2: 0, Ah a lateral G When the above controls (a), (b), (c),_ and (d) are carried out, the tire load to the front outer wheel increases, while the ground-contact load of the rear outer wheel decreases. For this reason, the load shift quantity of the front wheels increases, and the cornering power of the front wheel side decreases for a reason set forth later. Conversely, the load shift quantity of the rear wheel side decreases, and the cornering power of the rear wheel side increases for a reason set forth later. As a result, the degree of understeering increases. -30 To the contraryt control is carried out reversely to that described above, that is, the calculating circuit 20 delivers a signal to cause: (a') the vehicle height of the right-front wheel (front outer wheel) to be lowered by Ahl, (b') the vehicle height of the left-front wheel (f ront inner wheel) to be raised by Ah; 1 1 9 tel) the vehicle height of the right-rear wheel (rear outer wheel) to be raised by Ah; and (d') the vehicle height of the left-rear wheel (rear inner wheel) to be lowered by Ah.

Here, the quantity Ah is as follows.

Ah k Of Ah a lateral G When the above controls (a'), (b'), (cl). and (d') are carried out, the resultant variations are respectively the reverse of those of the controls (a), (b), (c), and (d). More specifically, the load to the front outer wheel decreases, and the load shift quantity of the front wheel side decreases. Thus, the cornering power on the front wheel side increases. At the same time, the load to the rear outer wheel increasesf and the load shift quantity -on the rear wheel side increases. Thus, the cornering power on the rear wheel side decreases. As a result, the degree of understeering decreases.

In the case of a right turning of a vehicle, the relation of the inner and outer wheels is reversed so that the raising and the lowering of the vehicle height in (a)(b)(c)ld) and (a')(b')(c')(d') above are also reversed.

The above controls and results will now be described in detail.

The relation of the cornering power to the load to the tire is indicated graphically, as in FIG. 5, by smooth convex curve as viewed from above with progressively decreasing slope.

In terms of representing the lateral acceleration and AW representing the load shift quantity both at turning of the vehicle, the following equation is obtained.

AW MYH/2 tr wherein:

M is vehicle gross weight; (1) H is difference between height of a vehicle center of gravity above ground and height of a rolling center above ground; and tris tread of the vehicle.

Therefore, the load shift quantity AW can be considered to be proportional to the lateral acceleration i.

In a conventional vehicle, vehicle height control as described above is not carried out at the time of vehicle turning. In such a known vehicle, a ratio r of the front-wheel-side load shift quantity AWfo and the rear wheel-side load shift quantity AWro is constant. The sum of AW and AW is AW. Therefore, the load shift fo ro quantities AWfo and AWro are respectively given by the following equations.

AWfo = AW x r/(l+r) AWro = AW x r/(l+r) (2) The tire ground-contact load % of the inner and 20 outer wheels on the front-wheel side and the groundcontact load Wr of the inner and outer wheels on the rear-wheel side both at the time of turning can be expressed as follows in terms of the tire loads Wf, and Wro respectively to the front and rear wheels.

% W fo Awfo (3 Wr Wro AWro} Under the conditions expressed by the above Equations (1), (2), and (3), the cornering power for each of the wheels can be calculated from the graph of FIG. 5.

Here, when vehicle height is controlled as set forth in (a), (b), (c), and (d) above at the time of vehicle turning, the load to the outer wheel on the f ront-wheel side increases, while the ground-contact load of the inner wheel decreases. Thus, the difference between the two loadsincreases. On the rear-wheel side, conversely, the load to the outer wheel decreases while the load to 1 11 the inner wheel increases. Thus, the difference between the two loads decreases.

As mentioned hereinbefore, the curve shown in the graph of FIG. 5 is of smooth convex shape as viewed from above with a progressively decreasing slope. With reference to FIG. 4, a consideration will be given to the case wherein, for example, the load to an outer wheel increases, and the load to an inner wheel decreases. In this case, the characteristic of the curve in FIG. 5 indicates that the decrease in the cornering power of the inner wheel is larger than the increase in cornering power of the outer wheel. Thereforer a resultant total value of the cornering power of the inner and outer wheels decreases. Converselyr when the load to the outer wheel decreases and the load to the inner wheel increases, the increase in the cornering power of the inner wheel is larger than the decrease in the cornering power of the outer wheel. Therefore a resultant total value of the cornering power of the inner and outer wheels increases.

That is, when the difference between the tire ground-contact loads of the inner and lower wheels becomes large, the resultant total value of the cornering power decreases. Conversely, when the difference becomes small, the resultant total value of the cornering power increases.

Therefore, as stated hereinabove, when the controls (a), (b), (c), and (d) are carried out, the cornering power on the front-wheel side decreases, while the cornering power oA the rear-wheel side increases. As a resultp the degree of understeering becomes strong. When the controls (a'), (bl), (c'), and (d') are carried out, the cornering power on the front-wheel side increases, while the cornering power on the rear wheel side decreases. Thus, trie degree of understeei:ing becomes weak.

12 In the manner described above, the vehicle height is varied by the circuit 20 for calculating the stroke control quantity in accordance with the magnitude of the lateral acceleration developed during turning of the vehicle. This can be expressed by an equation Ah = k where k is a constant. The turning characteristic of the vehicle thereby can be freely controlled.

more specifically, some drivers have personal preference for a tendency toward weak understeering.

Such a preference is set in the characteristic setting circuit 21 (FIG. 3). In such a case of weak understeering, the setting circuit 21 is so set that the controls of (a), (b'), (c'), and (d') described above will be carried out. Conversely, in the case of a driver is preferring a. strong understeering tendency,, the setting circuit 21 is so -set that the controls of (a) - (b), (c), and (d) described above will be carried out. Moreover, in both' modes of control, the value of- the proportionality constant - k in the equation Ah = k is made large or small depending on respective preferences. By this measure, the degree of weak understeering and strong understeering ca n be set as desired. Furthermore, it is also possible to control the steering characteristics in accordance with the vehicle speed as detected by the vehicle speed sensor S. For example, at low vehicle speed, the setting circuit 21 is set at weak understeering, responsive to the signal from the sensor S, thereby to attain improvement in turning. characteristic of the vehicle. At high speed, the setting circuit 21 is set at strong understeering thereby to attain improvement in stability.

The operation stated above is shown in the flow chart of FIG. 6.

The present invention can also be applied to a vehicle suspension control system as shown in FIG. 7, in which -the control is carried out in consideration of 13 variations of a load shift quantity for each wheel responsive to the rolling moment of the vehicle.

in FIG. 7, the portion enclosed by chain line is a control block diagram showing one of four suspensions la, lb, Ic and ld for the four wheels, for example, the suspension la of the left front wheel. While not shown in FIG. 7, a total of four sets of the same control logic are provided for independently controlling the respective suspensions la, lb, lc and ld.

In each suspension unit, a vertical acceleration of a vehicle body and a vertical relative displacement suspension fstroke) are respectively -detected by a vertical G-sensor 32 and a suspension stroke sensor 13. A vertical-acceleration signal from the vertical G- sensor 32 is passed through a low-pass filter LPF to reduce its high- frequency component. The signal is then passed through a dead-zone circuit I, to remove a signal of a set range in the neighborhood of zero. The resulting signal is subjected to multiplication by a gain circuit G1. Thus a control command quantity Q, matched to the characteristics of the corresponding control valve 2a, 2b, 2c or 2d is obtained.

A vertical relative displacement or stroke signal from the stroke sensor 13 is inputted to a differentiating circuit Dc and a dead-zone circuit 13. The signal passing through the differentiating circuit Dc is converted into a vertical relative displacement or stroke speed signal. The speed signal passes through a dead-zone circuit 12F.hich removes therefrom a signal.

fraction within a set zone in the vicinity of zero. The resulting signal is passed through a gain circuit G2 to become a control command quantity Q2 matched to the corresponding control valve characteristics.

By a setting of a vehicle height adjusting switch 16, a re ference vehicle height signal is generated from a reference vehic le height generating circuit H. The reference vehicle height signal is subtracted from the 4 14 vertical relative displacement signal to be inputted to the dead-zone circuit IV and an actual relative displacement signal is obtained. The actual relative displacement signal is passed through the dead-zone circuit IV where a signal fraction within a set zone in the vicinity of zero is removed therefrom. The resulting signal is passed through a gain circuit G3 to become a control command quantity Q3 matched to the corresponding control valve char-acteristic..

The control command quantity (QIF Q2, and Q3) matched to the characteristics of the corresponding control valve is as follows. In the case where the control valve is, for example, a flow rate control valve, the control -command quantity is the length of opening 1.5 time of the valve, necessary to obtain a required quantity of Sydraulic oil to be charged or discharged.

The length of the valve opening time is determined with consideration of the valve opening-closing characteristics.

The three control command quantities Q1, Q2f and Q3 are added as shown. The'resulting sum of the quantities is passed through a control quantity correction circuit R where it is converted into a corrected command quantity Q corrected by such an environmental condition such as temperature and pressure loss due to length of piping. The corrected quantity Q is passed through a valve driving signal generating circuit W, which thereupon generates a control valve opening/closing signal. Thus, the control valve 2a is switched to the oil charging side or the discharging side. As a resultr charging or discharging the oil of the command quantity into or out of the suspension la is accomplished.

In the control operation described above, when vertical acceleration is detected, oil within the -35 suspension la, for example, is discharged in case of upward acceleration. For downward accelerationr the oil is 'charged into the suspension la. By such a control operationr with respect to forces from below such as a bump or thrust from the road surface, soft and highattenuation suspension characteristics are created. With respect to forces from above (i.e., from the vehicle body), hard suspension characteristics are created so as to maintain the vehicle height at the reference vehicle height due to the control responsive to the vertical stroke speed and the vertical stroke, by controlling the charge and discharge of the oil.

Furthermore, by passing the -vertical acceleration signal through the lowpass filter LPF, the control system does not react to vibrations in the high-frequency region as in rsonance of the mass below the suspensions, but responds to vibrations of low-frequency region as in resonance of the mass above the suspensions.

Accordingly, the control system can avoid bouncing so as. to improve the driveability, thus preventing waste of energy for the control.

The lateral acceleration' detected by a lateral G- sensor 12 is passed through a hysteresis circuit 31 and a dead-zone circuit 32. Thus, response to minute lateral G-fluctuations occurring during normal driving is prevented. In this manner, only a signal of a value above a predetermined value is inputted to a circuit 33 for calculating the rolling moment. From the input signal, the calculating circuit 33 calculates the rolling moment, on the basis of the vehicle specification previously stored and information on the height of the vehicle body center of gravity determined by the vehicle height adjusting switch 16. The calculation result is transmitted to a circuit 34 for calculating lateral load shift quantity.

Separately, a vehicle speed signal generated by a vehicle speed sensor S is transmitted to a circuit 35 for setting the rolliug mc-ment front-rear distribut-'.-), z-iLio. From the vehicle speed information thus received, the circuit 35 operates to determine the rolling moment i 16 front-rear distribution ratio, on the basis of a characteristic of a previously set vehicle speed. The determined rolling moment front-rear distribution ratio is transmitted to the calculating circuit 34.

The calculating circuit 34 operates to distribute the generated rolling moment inputted from the rolling moment calculating circuit 33 to the front and rear wheels and to calculate the lateral load shift quantity between the f ront and rear wheels on the basis of the 3.0 rolling moment front-rear distribution ratio determined-. by the distribution ratio setting circuit 35.

The resulting output of the distributing circuit 34 - is fed to a circuit.36 for calculating the variation of the suspension reaction force. In the calculating circuit 36, the total lateral force acting on the wheels corresponding to the generated lateral G is distributed to the f ront and rear wheels based on the yaw moment equilibrium equation, with the position of the vehicle center' of gravity and the distance between the front and rear axles. Then, the variation of the suspension reaction force is calculated separately f or each of the front and rear suspensions in accordance with the lateral load shif t quantity between the f ront and rear wheels calculated by the calculating circuit 34, the lateral forces on the front and rear wheels, 'the vehicle height, and the type of the suspensions.

The variation of the suspension reaction forces calculated by the calculating circuit 36 is delivered to a circuit 37 for calculating control quantity. - The variation of the suspension reaction force is determined for each suspension. Furthermor e, calculation is made. for each suspension to obtain the control quantity of charging and discharging the oil matching the total variation of the suspension reaction force for maintaining the internal pressure of each suspension. The resultingcontrol quantity is converted into a control command quantity matching the 'valve specification -1 is, 17 or characteristic in a circuit 38 for converting control quantity. The converted control command quantity is added to the control command quantities Q,, Q2. and Q3. The resulting quantity is fed into the circuit R for correcting control quantity.

In the control system of the type described above, there is provided a circuit 20 for calculating a stroke control quantity and a characteristic setting circuit 21. As in the case of FIG. 3. the calculating circuit 20 10 receives a lateral acceleration signal from the lateral G-sensor 12. The calculating circuit 20 further receives a signal from the circuit 34. The calculating circuit 20 calculates the stroke control quantity which is subtracted from the signal from the sensor 13.

The opeeration of the control system of FIG. 7 is shown in the flow chart of FIG. 8. It will be noted that in the cDntrol system of FIG. 7, the load shift quantity of each wheel is calculated on the basis of the information of the lateral acceleration i and the vehicle speed V, and'-the stroke control quantity for each wheel is calculated on the basis of the oad shift quantity. The setting circuit 21 sets a coefficient K in response to the vehicle speed so as to change a steering characteristic.

The present invention is not limited to the embodiments described above. That is, the present invention is applicable to all vehicles with vehicle height adjusting systems of the type having means for detecting the suspension strokes of all four front, rear and right, left wheels, and a controller operating on he basis of each suspension stroke to control the vehicle height of the respective suspension independently so as to maintain a reference datum vehicle height..

The present invention as described- above is advantageous in that the steering characteristic can be freely controlledf and therefore a steering characteristic conforming to the preference of the driver 18 1 - can be attaine d. Furthermore, the steering characteristic can be readily varied to suit the vehicle speed. It will be observed that all of these features are highly effective and of great utility in practical use.

While the presently preferred embodiments of the present invention have been shown and described, it is to be understood that these disclosures are for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention as set forth in the appended claims.

1 19

Claims (10)

CLAIMS:

1. A method f or controlling active suspensions of a vehicle, having fluid suspensions provided for respective wheels (W), means for charging and discharging a fluid into and out of the respective. f luid suspensions to extend and contract the suspensions independently, and a controller for adjusting said charging and discharging means to control the vehicle heights at the respective wheels, characterized by the steps of:

detecting lateral acceleation of the vehicle; and adjusting said charging and discharging means for introducing and discharging the fluid, responsive to the magnitude of the detected lateral acceleration (); and changing the vehicle heights and hence the difference between ground-contact loads of inner and outer wheels during -the turning in such a manner that the difference is larger for one of the front wheel pair. and the rear wheel pair and the difference is smaller for the other pair, thereby to vary a turning characteristic of the vehicle.

2. The method according to claim 1, wherein the difference between the ground-contact loads is changed by increasing the ground-contact load to one wheel of the front wheel pair and decreasing the ground-contact load of the other wheel of the front wheel pair, and by increasing the groundcontact load to one wheel of the rear wheel pair, diagonally opposite said other wheel of the front wheel pair, and decreasing the ground-contact load to the other wheel of the rear wheel pair.

3. The method according to claim.1, further c omprising the step of: detecting the speed of the vehicle; and i modifying the turning characteristic responsive to the detected speed.

4. The method according to claim 3, wherein the turning characteristic is modified such that at low speed of the vehicle the degree of understeering is weak while at high speed the degree of understeering is strong.

5. The method according to claim 1, wherein said means for introducing and discharging the fluid is adjusted responsive also to a dIstibution of a frontrear wheel lateral load shift quantity.

6. A system for controlling active suspensions of a vehicle, having fluid suspensions provided for respective wheels,- means for charging and discharging a fluid into and out of the respective fluid suspensions to extend and contract the suspensions independently, and a controller for adjusting said charging and discharging means to control the vehicle heights at the respective wheels, characterized by: - sensor means for acceleration of- the vehicle; and calculating means, responsive to the lateral acceleration detected by the sensor means) for calculating a stroke control quantity used to adjust said charging and discharging means so as to change the vehicle heights and hence the difference between ground-contact loads adapted to inner and outer wheels during the turning in such a manner that the difference is larger for one of the front wheel pair and the rear wheel pair and is smaller for the other pair, thereby to vary a turning characteristic of the vehicle.

detecting lateral

7. - The" system according comprising:

j to claim 6, further 21 a speed sensor (S) f or detecting the speed of the vehicle; and characteristic setting means responsive to the detected speed of the vehicle for modifying the stroke control quantity such that at low speed of the vehicle the degree of understeering becomes weak and at high speed the degree of understeering becomes strong.

8. The system according to claim 6, further comprising:

a speed sensor for detecting.vehicle speed; and load shift quantity calculating means.

responsive to lateral acceleration detected by the sensor means, vehicle speed and vehicle height, for calculating lateral load shift quantity, said stroke control quantity calculating means being responsive also to a signal from the load shift quantity calculating means.

9. A system for controlling active suspensions o f a vehicle substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

10. A method for controlling active suspensions of a vehicle substantially as hereinbefore described with reference to the accompanying drawings.

Published 1991 at The Patent Office. State House, 66/71 High Holborn. London WC I R47P. Further copier, may be obtained C ss - St Mary Cray. Kent.

Sales Branch. Unit 6. Nine Mile Point, Cwinfelinfach. ro Keys, Newport, NPI 7HZ. Printed bY Multiplex techniques lid from