Abstract:
To prevent slip-out or slide-out of a vehicle operating in a curve, the lateral stability or, respectively, instability of the vehicle is determined and, if instability or incipient instability is sensed, the vehicle is automatically braked, and/or engine torque of the vehicle engine is reduced. A reference parameter K, which may be variable in accordance with vehicle operating and operation conditions, is compared with a quotient of change of lateral forces applied to the front (V) and hind or rear (H) axles with respect to the change of the slip angle (δ V , δ H ), and if the change of lateral force (Δ S  /Δ.sub.δ) at the respective axles, and passes the reference K. The respective axles can then be together or selectively braked in accordance with the extent of deviation from said comparison reference K and/or engine torque also reduced.

Description:
The present invention relates to a control system to control the speed of a motor vehicle which is operating in a curved path, and more particularly to automatically reduce drive torque applied by the wheels of the vehicle to the road surface if critical parameters of vehicle operation in the curved path occur. 
     BACKGROUND 
     If a vehicle operates in a curved path, a centrifugal force F ZF  will act thereon in accordance with the relationship ##EQU1## wherein m is the mass of the vehicle, v is the speed of the vehicle and r is the radius of the curved path in which the vehicle operates. A force acting laterally on the vehicle is transferred via the tires of the vehicle on the road surface. &#34;Road surface&#34; as used herein may mean the actual road surface or any other operating surface, for example snow or surface deposits on a road. Additional force components besides the centrifugal force are transferred between wheel and road surface, for example laterally applicable force components due to wind, inclination of the vehicle, and the like. The force components at the forward and rear or hind axle are usually different, depending on the distribution of axle loading of the specific vehicle. 
     If the lateral force transferred by a wheel to the road surface exceeds a limiting value, instability of vehicle operation will result. The vehicle slips or slides out laterally and the operator loses control over the vehicle. 
     THE INVENTION 
     It is an object to improve the stability or operation of a vehicle operating in a curved path and to prevent instability of operation and loss of control by the operator. 
     Briefly, means are provided on the vehicle to sense lateral forces, both as applied to the front axle and to the rear or hind axle. Change in the application of lateral forces, that is, the derivative of the forces with respect to time, is derived or sensed. The angle of lateral wheel slip, or or turning slip, δ, more specifically δ V  and δ H  for, respectively, the front and hind wheels, as well as the change in the lateral wheel slip or turn slip angle with respect to a commanded path are determined. 
     A quotient is then calculated based on the change of lateral forces and change of angle of lateral slip for at least one of the front or rear wheels, respectively, and the so-derived at least one quotient is compared with a predetermined value, based, for example, empirically on vehicle characteristics. If the predetermined value is exceeded, negatively or positively, or, for short, is passed, e.g. below its value, the speed of the vehicle is reduced. Reduction of vehicle speed can be obtained by for example reducing the amount of fuel being supplied so that engine braking will take effect and/or applying brakes to one or all of the wheels, for example under control of an anti-wheel brake locking system (ABS), which may be of any standard and suitable construction. In other words, if an ABS is present, it can be energized to provide, in accordance with its characteristics, braking effort on one or all of the wheels. 
     The system, thus, tends to reduce vehicle speed if instability of the wheels of at least one of the axles is sensed or a tendency to instability is sensed. The tendency to instability can be determined by suitably setting the comparison value, for example at a level related to incipient instability. Stable and controlled operation of the vehicle in a curve is thus obtained. Reduction of speed is obtained selectively or in combination by reducing, for example, engine torque by reduction of fuel, change of ignition timing or the like, and/or application of brakes. Of course, combination effects may be used. The degree of braking can be controlled directly in relation to the degree of difference of the calculated quotient from the predetermined reference value. It is readily possible, also, to apply brakes only to one of the axles, for example the one which does not show instability, so that optimum control over the vehicle is retained. Preferably, braking is combined with an ABS control, that is, braking is carried out under controlled conditions which inhibit slippage or wheel blocking. 
     The predetermined reference value, hereinafter referred to as reference K, may be a variable value, depending on conditions of the vehicle and characteristics thereof, as well as, for example, on external, for example road conditions. Further, the reference value K can be changed in dependence on the operating mode selected to reduce vehicle speed. For example, if only engine braking is used, that is, reduction of motor torque, the reference threshold level can be selected to be lower than when brakes are applied or, especially, when motor torque is reduced and the brakes are applied. 
    
    
     DRAWINGS 
     FIG. 1 is a block diagram of the system controlling vehicle speed when operating in a curve; 
     FIG. 2 is a schematic diagram showing the wheels on one side of a vehicle, and including force and speed vectors; 
     FIG. 3 is a schematic diagram of the wheels of vehicle illustrating the derivation of values used in deriving values explained in the specification; 
     FIG. 4 illustrates lateral forces as a function of slip angle; 
     FIG. 5 is a diagram illustrating lateral force Sy as a function of longitudinal slippage λ x  ; and 
     FIG. 6 illustrates a lateral force Sy as a function of wheel loading F Z  in kiloponds (kilogram-mass). 
    
    
     DETAILED DESCRIPTION 
     The control system of FIG. 1 is installed in a vehicle, schematically shown by block 1. The vehicle is equipped with an anti-brake lock system (ABS) 2. Braking pressure, for example hydraulic pressure derived from a source of pressurized hydraulic supply, under control of the vehicle operator, is applied over a line 3, to place an operator controlled braking pressure P B  on the ABS. The ABS receives wheel speed, vehicle speed and similar signals--as well known. It the controlled braking pressure P B  and, in accordance with standard operating mode of an ABS, provides controlled braking pressures P V1 , P V2 , P H1  and P H2  for the brakes of the four wheels of the vehicle. In the discussion hereinafter, the subscript  V   will be used for the forward wheels of the vehicle and the subscript  H  for the rear or hind wheels of the vehicle. The vehicle, further, is equipped with an electrical operator controller, in short, a fuel control 4, which converts a deflection position, α GP  commanded by an operator, into an electrical signal. This signal controls fuel supply to the vehicle, for example by providing a deflected position α DK  representative of a specific deflection of the vehicle throttle plate, and corresponding to a then pertaining deflection position α GP  of the gas pedal. The gas pedal position is signalled to the electrical operator controller 4 via a linkage or a control line 5. 
     A plurality of sensors or transducers are located on the vehicle as follows: 
     
         ______________________________________Transducer    Sensed Parameter______________________________________1a       commanded angle of deflection β by turning of the    steering wheel1b       vehicle speed V.sub.F1c       turning or rotary speed α of the vehicle about    its vertical axis1d       lateral acceleration b.sub.V at the front axle1e       lateral acceleration b.sub.H at the rear axle1f       axle loading at the front axle1g       axle loading at the rear axle.______________________________________ 
    
     Vehicle speed can be determined, for example, as well known, by processing signals derived from wheel speed sensors associated with the ABS 2. Alternatively, a transducer can be used to measure vehicle speed, for example an acceleration sensor with an integrator, Doppler radar or the like. The turning speed α can be measured by use of a gyroscope. Lateral acceleration can be measured by associating acceleration sensors with the respective axles. The vehicle mass can be determined or sensed by sensing the deflection path of the springs of the vehicle axles or, for example, if the vehicle is fitted with an automatic leveling system, typically a hydraulic leveling system, from the respective leveling piston-cylinder arrangements for the respective wheels or axles. The rotary or turning speed α need not be measured directly; it is also possible to measure the turning or lateral acceleration and, by integrating, to derive the turning speed α. The transduced values, in form of signals, are applied to a block 6 which is a signal processing stage, to derive the lateral forces F V  and F H  and the angles of or later slip δ V  and δ H  at the respective front (V) and hind (H) axles. 
     The lateral forces are determined by the following relationships ##EQU2## 
     The angles of inclination slip δ V  and δ H  are determined as follows (see FIG. 2): ##EQU3## wherein α B  is the angle of the vector of the curve, or angle, in which the vehicle operates 
     β is the commanded angle of deflection of the wheel 
     α is the rotation angle of the vehicle about a vertical axis 
     The relationships of formula (2) can be readily derived when considering FIG. 2, in which the wheels of one side of the vehicle are indicated at A and B for, respectively, the front and hind wheels. As can be seen, the wheel A has been deflected by the angle β, as measured, for example, in the sensor or transducer 1a. The wheels are subjected to lateral forces S V  and S H . FIG. 2 shows the center of gravity SPKT of the vehicle, and speed vectors V BV  and V BH . Additionally, further speed vectors V RV  and V RH  are shown. These vectors represent: 
     V RV  and V RH  resulting wheel speeds at the front and hind wheels respectively 
     V BV  and V BH  : vehicle speed vectors at the front or rear wheel, in a path curve, without superposition of rotation or turning about a vertical axis. 
     FIG. 3 shows the position of the wheels in subsequent time intervals t 1  and t 2 . The elapsed time between the intervals t 1  and t 2  also forms the clock time of the evaluation, mathematically (t 2  -t 1  =T) wherein T is the evaluation clock time. 
     The wheel base is shown as 1 A , and the radius of the curved path is shown at r. The following relations can then be derived: ##EQU4## wherein V F  is the vehicle speed vector; 
     From which is derived ##EQU5## 
     Substituting in relationship (2): ##EQU6## 
     For further consideration, only the change in the lateral step angle is of interest, and one can then formulate: ##EQU7## 
     The difference Δα between two measuring instants will be: ##EQU8## 
     At the last derivation, the trapezoidal conversion rule was used; and t represents any one instant of time, e.g. t 1 , t 2  . . . ; and  T  is the clock time interval. 
     It can be assumed that, within the clock interval T, the values β and V F  are constant. Then: ##EQU9## 
     The values S V , S H  and Δδ V  and Δδ H  are then applied to a stage 7, which is a computing stage. Stage 7, in accordance with well known arithmetic calculation, and considering the lateral forces with respect to a predetermined time instant, that is, before the time T, calculates the quotients ##EQU10## 
     These quotients are formed for the two axles V and H, independently. 
     The quotients determined in accordance with the relationship (9) are then applied to comparators 8 and 9. The comparators have the value K applied. The value K may be considered a constant value, but it may be variable, for example depending on vehicle operation or operating conditions or parameters. 
     FIG. 4 illustrates the relationship between lateral forces S with respect to the lateral slip angle of, or the angle δ of the actual path of the wheel with the commanded or rolling path. As can be seen, the lateral forces rise, starting from zero, and then drop after a value of δ x . This relationship also clearly shows that the regions beyond δ x  and -δ x  are unstable. Thus, control must be carried out in the range between -δ x  and +δ x . Consequently, ##EQU11## 
     If it is considered that, as the result of the control to be effected, the vehicle should be retarded or decelerated, a braking force is also applied in the longitudinal direction on the road surface. The longitudinal axis of the vehicle is shown in FIG. 2, schematically, by the line passing through the center of gravity SPKT and connecting the turn centers of the front and rear wheels A and B. In accordance with the well known course of the stability limit upon transfer of longitudinal and transverse forces, the relationship (10) above may not be zero, or close to zero; it may have a value of K&gt;0, that is, a value to be determined. 
     If K is passed, for example by being passed below its value, threshold circuits 10, 11 will respond and generate a switching signal B V  and B H , respectively. These switching signals are applied to a vehicle braking controller 12. Vehicle braking controller 12 is connected to the ABS 2, which permits application of braking pressure to the respective wheels or axles--as will appear below--from a pressurized source of braking fluid, schematically shown by arrow P S . Thus, the vehicle will be braked. In addition, the vehicle braking controller is connected via terminal G to the electrical operator controller 4, for example independently or additionally, selectively decreasing fuel supply and thus decreasing engine torque--or otherwise controlling the engine to decrease output torque. The vehicle braking controller 12 includes a timing unit, schematically shown as TU in order to, for example, sequentially control various effects. The following illustrates possibilities which, independently or together, can be used to retard or decelerate the vehicle, in accordance with a simple control program retained, for example, within the vehicle braking controller 12: 
     (a) Controlled braking is effected by applying vehicle brakes. Upon braking, the driven axle will be subjected to partial blocking of the differential or differential effect. No output signal is applied to terminal G, that is, engine output torque is not affected. 
     (b) First, and at a first timing interval, as determined by the timing unit TU, the engine torque is reduced; thereafter, the ABS 2 is caused to become operative by energizing the output terminals H, V, to provide for controlled braking. The engine torque can be reduced, under controlled conditions, that is gradually or faster, in accordance with the degree of deviation of the quotient derived from the computing stage 7 with respect to the constant K. 
     (c) Both axles are braked under controlled conditions by the ABS 2 when either a signal B V  or B H  is present. The ABS prevents wheel blocking and excessive braking. 
     (d) The wheels at the front or forward axle and the wheels at the hind or rear axle are separately braked. The axle, at which the wheels are in non-critical condition, are braked with greate braking effort than the axle which has reached or is about to reach critical conditions. Different values of B V  and B H  must then be evaluated in the vehicle braking controller; this can be instrumented very simply, for example by a comparator or, additionally, comparison with the level of deviation from the value K as determined in the comparators 8, 9. 
     (e) The ABS provides braking effort only to the wheels at the axle which is not under critical or incipiently critical condition. 
     Various combinations of the conditions (a), (b) with (c), (d), (e) are possible. One suitable combination is (b)+(e). Additional ways to influence the respective wheels are given in the table attached hereto. In the table, B V  and B H  are representative of the signals applied to the vehicle braking controller 12. α G  is the signal generated by the electrical operator controller, that is, by the gas pedal. V, H and G are the limiting effects or limiting values to which the braking pressures P at the forward and rear wheels, and the deflection of the throttle G are, respectively, subjected. 
     FIG. 5 illustrates the relationship of longitudinal slippage λ x  to the ratio of lateral force S Y  with respect the longitudinal force S X  related to the inclination angle or turn or inclination running angle δ. As can be clearly seen, as the angle δ increases, the lateral forces, with a given longitudinal force, will increase. FIG. 6 also shows this relationship. Further, the lateral force S Y  increases with wheel loading F Z  within the range of interest, as also clearly seen in FIG. 6. 
     The value K can depend on operating or operation conditions of the vehicle and on the vehicle characteristics themselves. If only engine torque is to be controlled, it is desirable to use in the relationship (10) a lower value for K than if braking, for example by the ABS 2, also is controlled. 
     In the description and in the claims of this application the expression &#34;lateral slip angle&#34; is used for the angles δ V  and δ H . The meaning of these angles can be taken from formula (2) and from FIG. 2. The anle δ H  is the angle between the direction of the base of the wheel and the vector of the actual wheel-speed V RH  of the rear wheel. The angle δ V  is the angle between a line being normal to the axle of the steered wheels and the vector of actual wheel-speed V RV  of the steered wheels. 
     
                       TABLE______________________________________Conditions        Limiting EffectsB.sub.V  B.sub.H    α.sub.G                     V       H   G______________________________________0      0          0       0       0   α.sub.max0      0          ≠0                     0       0   α.sub.max0      1          0       1       0   00      1          ≠0                     0       0   01      0          0       0       1   01      0          ≠0                     0       0   01      1          0       1       1   01      1          ≠0                     0       0   0______________________________________