Tire contact load control system

An active actuator is interposed between the unsprung mass and the sprung mass of a vehicle, and a controller selectively extends and retracts the actuator at a prescribed acceleration so as to selectively apply an additional contact load to the wheel by making use of the inertial force of the sprung mass and/or the unsprung mass of the vehicle. A particularly advantageous result can be achieved by increasing the tire contact load when the wheel is about to lock up when braking. The present invention can thus reduce the braking distance for the given road condition.

TECHNICAL FIELD
 The present invention relates to a tire contact load control system for
 increasing the road wheel contact pressure on the road surface as needed
 by actuating an actuator interposed between the sprung mass and the
 unsprung mass of the vehicle at a certain acceleration.
 BACKGROUND OF THE INVENTION
 The gripping force F of a tire can be given by the product of the
 frictional coefficient .mu. between the tire and the road surface and the
 vertical contact load W acting on the tire contact surface (F=.mu.W). In
 other words, the tire gripping force which plays an important role in the
 handling of the vehicle is proportional to the magnitude of the tire
 contact load for a given road condition.
 It is well known that the handling of a vehicle is severely impaired when a
 road wheel thereof is locked up when braking. In particular, on a road
 surface such as a frozen road surface and a gravel road surface which
 could significantly reduce the gripping force of the tire, it is important
 to apply as large a braking force to the wheel as possible, but, without
 locking the wheel. The antilock brake system (ABS) makes use of this
 principle, and is widely used in road vehicles now on the market.
 According to this system, by noting the relationship between the tire slip
 ratio (.lambda.=(V.sub.v -V.sub.w)/V.sub.v : the ratio of the difference
 between the vehicle speed V.sub.v and the tire speed V.sub.w to the
 vehicle speed V.sub.v) and the tire grip force F, the slip ratio is kept
 within a limit typically by intermittently releasing the brake so as to
 avoid excessive slipping and to provide a high gripping force at all
 times. However, the ABS system is not able to change the gripping force of
 the tire. In other words, the capability of the ABS system to reduce the
 braking distance of the vehicle is limited by the given road gripping
 force. It is therefore desirable if the gripping force itself can be
 increased in view of further reducing the braking distance.
 The road gripping force of a tire is important also when accelerating a
 vehicle. When a vehicle is excessively accelerated for a given road
 condition, the tires start slipping, and not only a desired acceleration
 is prevented from being achieved, but also the lateral stability of the
 vehicle may be lost. By noting this problem, it has been proposed to
 control the traction force of each driven wheel so that the slip ratio of
 the wheel may be kept within a limit, and a maximum available traction may
 be obtained at all times. The traction control system is designed to carry
 out such a control action. However, the conventional traction control
 system was not able to increase the magnitude of the available traction,
 and simply reduces the torque transmitted to the wheels so as to prevent
 the slip ratio from exceeding a prescribed limit.
 When a linear actuator interposed between a wheel and a vehicle body is
 either extended or retracted at a certain acceleration, a corresponding
 inertia force is produced in the sprung mass and the unsprung mass. The
 reaction of such an inertia force may be used to increase the contact load
 of the road wheel or the gripping force of the tire. Therefore, when this
 concept is applied to a brake control system or a traction control system,
 it is possible to reduce the braking distance or to increase the magnitude
 of the available traction. In particular, the inventors have recognized
 that an optimum result can be achieved if the contact load of the wheel is
 increased when the slip ratio of the wheel is about to exceed a threshold
 level beyond which the tire grip force starts diminishing.
 BRIEF SUMMARY OF THE INVENTION
 In view of such problems of the prior art and the recognition by the
 inventors, a primary object of the present invention is to provide a
 vehicle tire contact load control system which can selectively increase
 the tire contact load of a road wheel.
 A second object of the present invention is to provide a vehicle tire
 contact load control system which can increase the gripping force of a
 road wheel for a given road condition.
 A third object of the present invention is to provide a vehicle tire
 contact load control system which can maximize the braking or the traction
 force of a vehicle on a given road surface.
 A fourth object of the present invention is to provide a vehicle tire
 contact load control system which can allow a vehicle to accelerate or
 decelerating on a low-.mu. road surface in a stable manner.
 According to the present invention, such objects can be accomplished by
 providing a vehicle tire contact load control system, comprising: a wheel
 suspension system for supporting an unsprung mass including a wheel to a
 sprung mass including a vehicle body; an active actuator interposed
 between the unsprung mass and the sprung mass; means for computing a slip
 ratio of a wheel; and a controller for extending the actuator at a
 prescribed acceleration so as to selectively apply an additional contact
 load to the wheel when a slip ratio computed by the computing means has
 exceeded a prescribed value. The extended actuator may be retracted at
 such a time when the tire contact load is not so critical.
 Thus, the tire grip force can be increased for the given road condition,
 and the braking force or the traction force available to the wheel can be
 increased. The prescribed value may be selected at a level which is
 preferably below the level from which the tire grip force starts
 diminishing. In particular, it is preferable to select the prescribed
 value at the level at which the tire grip force takes a maximum value. In
 case of a tire contact load control during braking in a vehicle equipped
 with an antilock brake system, it is preferable to set the prescribed
 value lower than the reference value at which the antilock brake system is
 activated. Thus, the additional brake force which is made available by
 increasing the tire contact load can be fully utilized, and the antilock
 brake system can take over the brake control only after the maximum
 braking force has been used so that the overall braking distance may be
 minimized.
 Because the road condition could significantly change the tire grip force
 available to the tire, it is preferable to determine the road condition,
 for instance by analyzing the tire noise or the road noise, and to
 properly estimate the critical slip ratio at which the contract load
 control should be started for an optimum result.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 1 schematically illustrates an essential part of an active wheel
 suspension system to which the present invention has been applied. A tire
 wheel 1 is supported by upper and lower suspension arms 2 and 3 so as to
 be moveable vertically with respect to a vehicle body 4. A linear actuator
 5 consisting of a hydraulic cylinder is connected between the lower
 suspension arm 3 and the vehicle body 4. The linear actuator 5 includes a
 piston 6 received in a cylinder defining an upper oil chamber 7 and a
 lower oil chamber 8, and a thrust force is produced in a piston rod 11 by
 controlling the hydraulic pressures transmitted from a variable capacity
 oil pump 9 to these oil chambers 7 and 8 via a servo valve 10. Thereby,
 the relative distance between the center (axle) of the wheel 1 and the
 vehicle body 4 can be controlled at will.
 The oil delivered from the pump 9 is initially stored in an accumulator 12
 to remove the pulsating ripples in the oil pressure, and avoid shortage of
 the oil flow during a transient process. The pressure is then transmitted
 to each of the wheels 1 via the corresponding servo valve 10. This oil
 circuit further comprises an unloading valve 13, an oil filter 14, a check
 valve 15, an oil pressure regulating valve 16, and an oil cooler 17, but
 these components are conventional and are therefore not described here in
 any great detail.
 The servo valve 10 controls the magnitude and direction of the hydraulic
 pressure applied to the hydraulic actuator 5 in a continuous manner
 according to a control signal supplied from an electronic control unit
 (ECU) 18 to a solenoid 10a of the servo valve 10 via a servo valve driver
 19. The ECU 18 produces the control signal by processing output signals
 from a load sensor 20 interposed between the vehicle body 4 and the piston
 rod 11, a stroke sensor 21 interposed between the vehicle body 4 and the
 lower suspension arm 3, a sprung mass acceleration sensor 22 for detecting
 the vertical acceleration of the vehicle body 4, and an unsprung mass
 acceleration sensor 23 for detecting the vertical acceleration of each
 wheel 1 according to a control algorithm illustrated in FIG. 2.
 The output signals from a vehicle speed sensor 27 and a wheel speed sensor
 28 are supplied to a slip ratio computing unit 29 to compute the slip
 ratio .lambda. when braking (step 1). A map 30 (FIG. 3) defining a
 relationship between the grip force F and the slip ratio .lambda. is
 looked up to determine if the slip ratio has reached a prescribed
 reference value .lambda..sub.w at which the grip force F takes a maximum
 value (step 2). In other words, the grip force starts diminishing once the
 slip ratio exceeds this reference value .lambda..sub.w. This allows the
 determination if the tire is about to be locked up or not. If any
 particular tire is determined to be about to lock up, a target load
 computing unit 24 internally defines a target load while referring to the
 output signals of the sprung mass acceleration sensor 22 and the unsprung
 mass acceleration sensor 23 (step 3). A stabilizing computing unit 25 then
 computes a deviation between the actual tire load obtained from the load
 sensor and the internally defined target tire load (step 4), and processes
 the deviation to produce a command signal for the servo valve driver 19 in
 cooperation with a stroke limit computing unit 26 which adjusts the
 command signal so as to keep the stroke of the actuator 5 within a limit
 (step 5). The adjusted command signal drives the servo valve 10 to produce
 a prescribed stroke from the actuator 5, and produces a vertical
 acceleration in at least one of the sprung mass and the unsprung mass so
 as to increase the tire contact load (step 6). As a result, the tire grip
 force is temporarily increased as shown in FIG. 4, and the limit of
 locking up the wheel is raised so as to reduce the braking distance.
 FIG. 4 schematically illustrates the distribution of the tire contact load
 (=gripping force) among the four wheels of the vehicle during braking. The
 static contact load is indicated by solid circles, and the dynamic contact
 load or the contact load produced by the stroke of the actuator 5 is
 indicated by the double chain dot line. In the example shown in FIG. 4,
 the contact load of the rear wheels is increased, but the actuator for
 each individual wheel can be individually controlled so that the actuator
 for any one of the wheels which is about to lock up may be actuated.
 In particular, it is preferable to set the threshold slip ratio
 .lambda..sub.w for initiating the contact load increase control lower than
 the threshold slip ratio .lambda..sub.a for initiating the antilock brake
 control of the ABS system (see FIG. 3). After the tire contact load has
 been increased by extending the actuator 5, it is necessary for the
 actuator 5 to retract before it becomes capable of increasing the tire
 contact load again. During this intermission, the slip ratio may increase
 again. In view of such a possibility, it is desirable to first apply a
 sufficient braking force by extending the actuator 5 to increase the tire
 contact load, and then to activate the ABS to prevent the wheel from
 locking up.
 The above description was directed to the embodiment for increasing the
 fore-and-aft force available as the braking force by increasing the tire
 load, but it is obvious for a person skilled in the art that the same
 principle may be applied to increase the fore-and-aft force available as
 the traction force. The description given above can be readily modified
 into such an embodiment by interchanging the terms "braking" and
 "traction".
 When the vehicle travels over an irregular road surface and the tires
 bounce repeatedly on the road surface, the contact load of each tire tends
 to be lower than normal. In other words, the relationship between the tire
 grip force F and the slip ratio .lambda. is somewhat different depending
 on the condition of the road surface. Normally, a sufficient braking force
 may not be obtained if the above described control is carried out when the
 vehicle is traveling over an irregular road surface by using a threshold
 slip ratio suitable for a smooth road surface. This problem can be
 eliminated as described in the following with reference to the flow chart
 of FIG. 5.
 First of all, road noises are picked up by using a microphone provided near
 one of the tires (step 11). The obtained noises are processed by a
 frequency analyzing circuit and a band pass filter to extract a sound
 pressure value of a certain frequency band (step 12). By looking up a
 database or a table map which defines the relationship between the sound
 pressure value, the vehicle speed and the road condition, the current road
 condition is determined (step 13). According to the result of this
 determination process, an optimum slip ratio map is selected from a
 plurality of F-.lambda. maps (see FIG. 6) which are prepared in advance so
 as to cover a conceivable range of road conditions (step 14). By thus
 evaluating the tendency of each road wheel to lock up according to the
 obtained slip ratio, it is possible to carry out the tire contact load
 control at an optimum timing for each current road condition.
 The working principle of this invention is described in the following with
 reference to FIG. 7 in which the following notations are used.
 M.sub.2 : sprung mass
 M.sub.1 : unsprung mass
 Z.sub.2 : position of the sprung mass
 Z.sub.1 : position of the unsprung mass
 Kt: spring constant of the tire
 Fz: thrust force of the actuator
 Suppose that the downward direction corresponds to a positive direction.
 Then, the equations of motion for the sprung mass M.sub.2 and the unsprung
 mass M.sub.1 are given as follows.
EQU M.sub.2.multidot.(d.sup.2 Z.sub.2 /dt.sup.2)=-Fz
 M.sub.1.multidot.(d.sup.2 Z.sub.1 /dt.sup.2)+Kt.multidot.Z.sub.1 =Fz
 Therefore, the tire contact load W can be given by the following equation.
 ##EQU1##
 In other words, the tire contact load W can be given as a sum of the
 inertia forces of the sprung mass and the unsprung mass. Therefore the
 tire contact load W can be controlled by controlling the acceleration of
 extending and retracting the actuator, and thereby changing the inertia
 force of at least one of the sprung and unsprung masses. In particular, by
 controlling the individual actuator 5 for each of the wheels, it is
 possible to increase the contact load W of each tire at a desired timing.
 For instance, when the suspension stroke is 200 mm, and the actuator 5 can
 produce a thrust force of one ton or an acceleration of approximately 1 G,
 the maximum time duration of this inertia force will be approximately 0.2
 seconds.
 Typically, with the aim of minimizing the energy consumption of each
 actuator, the proposed active wheel suspension system uses a suspension
 spring for supporting the weight of the vehicle body, and a damper for
 producing a damping force (see FIG. 8). In this case, if Ks is the spring
 constant of the suspension spring, and C is the damping coefficient of the
 damper, the equations of motion for the sprung mass M.sub.2 and the
 unsprung mass M.sub.1 are given as follows.
EQU M.sub.2.multidot.(d.sup.2 Z.sub.2 /dt.sup.2)+C.multidot.(dZ.sub.2
 /dt-dZ.sub.1 /dt)+Ks.multidot.(Z.sub.2 -Z.sub.1)=-Fz
EQU M.sub.1.multidot.(d.sup.2 Z.sub.1 /dt.sup.2)+C.multidot.(dZ.sub.1
 /dt-dZ.sub.2 /dt)+Ks.multidot.(Z.sub.1 -Z.sub.2)+Kt.multidot.Z.sub.1 =Fz
 Therefore, the tire contact load W can be given by the following equation.
 ##EQU2##
 In other words, the tire contact load W can be likewise controlled by
 controlling the acceleration of extending and retracting the actuator.
 Hydraulic cylinders were used for the actuators in the above described
 embodiment, but other actuators may also be used. Such actuators include,
 not exclusively, electric motors such as linear motors and moving coils,
 and mechanical arrangements such as cam mechanisms and spring members.
 Also, the various sensors may be simplified without departing from the
 spirit of the invention. For instance, the stroke sensor 21 may be omitted
 because a stroke can be computed by integrating the difference between the
 outputs from the acceleration sensors for the sprung mass and the unsprung
 mass 22 and 23. The load sensor 20 may also be omitted, because the output
 force of the actuator 5 may be computed from the actual values of the
 sprung mass and the unsprung mass, and the outputs from the acceleration
 sensors for the sprung mass and the unsprung mass 22 and 23. Also, the
 accelerations of the sprung mass and the unsprung mass may be indirectly
 computed from the outputs of the load sensor and the displacement sensor
 by defining a state estimating unit. The ECU 18 may consist of a digital
 computer, an analog computer or a hybrid computer.
 Thus, according to the present invention, the limit of the grip force of
 each tire can be raised so that the locking of the tire can be avoided
 without reducing the braking force, and the braking distance can be
 significantly reduced. In other words, the tire is prevented from being
 lock up by increasing its contact load when the tire is about to lock up.
 Also, by detecting the road condition, for instance by analyzing the road
 noise, the threshold slip ratio for controlling the tire load control can
 be optimized for each particular road condition.
 It is known that the vector sum of the lateral force and the fore-and-aft
 force of a tire is fixed or each given road condition so that the
 fore-and-aft force which is available as the traction force or the braking
 force is reduced when the lateral force is used for the turning motion of
 the vehicle. However, according to the present invention which allows the
 tire load to be increased, it is possible to improve the braking
 capability of a vehicle making a turn by increasing the tire load when the
 fore-and-aft force available for the braking force becomes inadequate.
 Although the present invention has been described in terms of a preferred
 embodiment thereof, it is obvious to a person skilled in the art that
 various alterations and modifications are possible without departing from
 the scope of the present invention which is set forth in the appended
 claims.