Abstract:
Direct sensing of rough road conditions are used to modify operation of a wheel slip control system. At least one suspension sensor ( 139 ) senses an operating parameter of the suspension system. A road surface classifier is responsive to the suspension sensor ( 139 ) for generating a road surface signal representing a roughness of a road surface over which the vehicle travels. A braking system includes a wheel speed sensor and a brake actuator. An active braking control detector wheel slip in response to the wheel speed sensor ( 108 ) during at least one of braking or accelerating of the vehicle and modulates the brake actuator in response to the detected wheel slip. The active braking control is responsive to the road surface signal for modifying modulation f the brake actuator as a function of the road surface signal.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates in general to active vehicular braking and suspension systems. In particular, this invention is concerned with detection of rough road conditions using suspension information and then adjusting active braking control for improved performance for the current road surface conditions.  
           [0002]    Electronically-controlled active vehicular braking systems can include anti-lock braking (ABS), traction control (TC), and yaw stability control (YSC) functions. In such braking systems, sensors deliver input signals to an electronic control unit (ECU). The ECU sends output signals to electrically activated devices to apply, hold, and dump (relieve) pressure at wheel brakes of a vehicle. Electrically activated valves and pumps are used to control fluid pressure at the wheel brakes. Such valves and pumps can be mounted in a hydraulic control unit (HCU). The valves typically include two-state (on/off or off/on) solenoid valves and proportional valves.  
           [0003]    A basic function of active braking systems is to detect wheel slip (e.g., skidding or loss of traction) and actuate the brakes (or reduce torque from the engine) in a manner to reduce or control wheel slip. An individual wheel speed is measured and wheel slip is detected by comparing the individual wheel speed to a target speed determined for that wheel. Various control parameters of the active braking system are chosen to provide satisfactory performance over all conditions that are encountered during operation. For example, activation of the active control (e.g., abs or TC) to being to control slip does not occur until the difference between actual wheel speed and target wheel speed exceeds a slip threshold. A base threshold is chosen that achieves best overall performance for all conditions.  
           [0004]    Certain assumptions or tradeoffs are made in selecting a base threshold. For example, the flatness or roughness of the road surface influences the amount of slip that will achieve the highest overall vehicle acceleration or deceleration. Thus, to achieve a shortest stopping distance, there is an optimum slip threshold. Since characterization of road surface condition is not available to prior art systems, the base threshold is chosen for achieving best overall stopping distances.  
           [0005]    It is known to dynamically vary this slip threshold in response to certain characteristics of the wheel speed signals (e.g., acceleration changes) to either increase or decrease the amount of slip that is controlled. For example, wheel speed signals have been analyzed in attempts to detect wheel hop, but this has not led to accurate road surface classification.  
           [0006]    Electronically-controlled suspension systems typically include semi-active suspension systems and active suspension systems to provide active damping for a vehicle. In such suspension systems, sensors deliver input signals to an electronic control unit (ECU). The ECU sends output signals to electrically activated devices to control the damping rate of the vehicle. Such devices include actuators to control fluid flow and pressure. The actuators typically include electrically activated valves such as two-state digital valves and proportional valves.  
         SUMMARY OF THE INVENTION  
         [0007]    This invention employs information from a suspension sensor to classify a road surface condition (i.e., a rough road index) and modifies activation of an active braking control system in response thereto, achieving advantages in the performance of slip control.  
           [0008]    In one aspect of the invention, an apparatus for a vehicle comprises a suspension system for connecting a vehicle body and vehicle wheels. The suspension system includes at least one suspension sensor for sensing an operating parameter of the suspension system and at least one suspension actuator for modifying a performance characteristic of the suspension system. An active suspension control controls the performance characteristic in response to the suspension sensor. A road surface classifier is responsive to the suspension sensor for generating a road surface signal representing a roughness of a road surface over which the vehicle travels. A braking system includes a wheel speed sensor and a brake actuator. An active braking control is coupled to the braking system and the road surface classifier for detecting wheel slip in response to the wheel speed sensor during at least one of braking or accelerating of the vehicle. The active braking control modulates actuation of the brake actuator in response to the detected wheel slip and is responsive to the road surface signal for modifying modulation of the brake actuator as a function of the road surface signal.  
           [0009]    Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a schematic diagram of a first embodiment of an integrated vehicular control system according to the present invention illustrating input signals delivered to electronic control units, transfer of signals between the electronic control units, and output signals delivered from the electronic control units to electrically activated braking and suspension devices.  
         [0011]    [0011]FIG. 2 is a schematic diagram of a second embodiment of an integrated vehicular control system according to the present invention for controlling braking and suspension devices wherein an anti-lock braking/traction control algorithm and a vehicular stability control algorithm are provided.  
         [0012]    [0012]FIG. 3 is a schematic diagram of a third embodiment of an integrated vehicular control system according to the present invention for controlling braking and suspension devices wherein a single electronic control unit is utilized.  
         [0013]    [0013]FIG. 4 is 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0014]    A first embodiment of a vehicular control system according to the present invention is indicated generally at  100  in FIG. 1. The control system  100  is particularly adapted to control fluid pressure in an electronically-controlled vehicular braking system and an electronically-controlled vehicular suspension system. The braking system can include anti-lock braking, traction control, and yaw stability control functions. The suspension system can include active damping functions.  
         [0015]    The control system  100  includes a first electronic control unit (ECU)  102 . The first ECU  102  includes a signal processor  104  and a braking algorithm  106 . Various sensors  108  strategically placed in a vehicle deliver input signals  110  to the signal processor  104 . Specifically, a lateral acceleration sensor  112  delivers an input signal  114  to the signal processor  104 . A longitudinal acceleration sensor  115  delivers an input signal  116  to the signal processor  104 . A steering wheel sensor  117  delivers an input signal  118  to the signal processor  104 . A yaw rate sensor  120  delivers an input signal  122  to the signal processor  104 . Depending upon the braking functions of the braking system, some of the above-listed sensors and their associated input signals may be deleted and others may be added. For example, a braking system that provides only ABS and TC functions may not require some of the above-listed sensors.  
         [0016]    The signal processor  104  delivers transfer signals  124  to the braking algorithm  106 . The braking algorithm  106  delivers output signals  126  to a hydraulic control unit (HCU)  128 . The HCU  128  can include electromechanical components such as digital and/or proportional valves and pumps (not illustrated). The HCU  128  is hydraulically connected to wheel brakes and a source of brake fluid, neither of which is illustrated.  
         [0017]    The control system  100  also includes a second ECU  130 . The second ECU  130  includes a signal processor  132  and a suspension algorithm  134 . Various sensors  135  strategically placed in a vehicle deliver input signals  136  to the signal processor  132 . Specifically, a suspension state sensor  137  delivers an input signal  138  to the signal processor  132 . A suspension displacement sensor  139  delivers an input signal  140  to the signal processor  132 . A relative velocity sensor  141  delivers an input signal  142  to the signal processor  132 . An upsprung mass acceleration sensor  143  delivers an input signal  144  to the signal processor  132 . Depending upon the performance requirements of suspension system, some of the above-listed sensors may be deleted and others may be included.  
         [0018]    The second signal processor  132  delivers transfer signals  145  to the suspension algorithm  134 . The first signal processor  104  delivers transfer signals  146  to the suspension algorithm  134 . The suspension algorithm  134  delivers output signals  148  to suspension actuators  150 , only one of which is illustrated. The actuators  150  are electrically controlled devices such as dampers that vary and control a damping rate of a vehicle. An actuator  150  can include electromechanical components such as digital and proportional valves.  
         [0019]    Information from the vehicular braking system can be shared with the vehicular suspension system. For example, ECU  102  can direct information to ECU  130 . One example of transferred information from the braking system to the suspension system is the transfer signal  146  from signal processor  104  to suspension algorithm  134 . A second example of transferred information from the braking system to the suspension system is indicated by transfer signal  152 , wherein information from the braking algorithm  106  is directed to the suspension algorithm  134 .  
         [0020]    Information from the suspension system can also be shared with the braking system. For example, ECU  130  can direct information to ECU  102 . One example of transferred information from the suspension system to the braking system is a transfer signal  154  to a load and load transfer detector  155 . Another example is a transfer signal  156  to a turning detector  157 . Yet another example is a transfer signal  158  for surface and mismatch tire detector  159 .  
         [0021]    The control system  100  can be configured in various manners to share information from ECU  102  to ECU  130 , and vice versa. In one example, an ECU  102  for the braking system that receives inputs signals  114 ,  116 ,  118  and  122 , for lateral acceleration, longitudinal acceleration, steering wheel angle, and yaw rate, respectively, can transfer these input signals to ECU  130  for the suspension system. The signal processor  104  of ECU  102  can send transfer signal  146  to the suspension algorithm  134 .  
         [0022]    In another example, if lateral acceleration and steering wheel angle signals  114  and  122  are not available to the braking system, a turning detector signal can be generated by ECU  130  and transmitted to ECU  102  to improve braking performance. If an electronically controlled suspension system is integrated with an electronically controlled ABS/TC braking system, turning of the vehicle can be detected by the suspension system, thereby generating a turning detector signal that is transmitted to a braking system that does not receive signals from lateral acceleration and steering wheel angle sensors. A turn detection signal to the braking system via ECU  102  can enhance braking performance, particularly during braking-in-turn and accelerating-in-turn.  
         [0023]    A second embodiment of a control system for controlling vehicular braking and suspension functions is indicated generally at  200  in FIG. 2. Elements of control system  200  that are similar to elements of control system  100  are labeled with like reference numerals in the  200  series.  
         [0024]    Control system  200  also includes an ABS/TC algorithm  206 A and a YSC algorithm  206 B in place of the braking algorithm  106  of control system  100 . Signal processors  204  and  232  may be placed separately from their respective algorithms  206 A,  206 B, and  230 , or they may be located in common ECU&#39;s (not illustrated in FIG. 2). Transfer signal  270  between ABS/TC algorithm  206 A and VSC algorithm  206 B is provided. Transfer signal  272  for load and load transfer is provided to the VSC algorithm  206 B. Transfer signal  273  from the signal processor  204  is provided to the VSC algorithm  206 B. Transfer signal  274  for the surface and mismatch tire detector is provided to the YSC algorithm  206 B. Transfer signal  275  is provided from the YSC algorithm  206 B to the suspension algorithm  234 . Output signal  276  is sent from the YSC algorithm  206 B to the HCU  228 .  
         [0025]    Various calculations can be made for the suspension system. For example, relative velocity can be calculated from suspension displacement if it is not directly measured. A vehicle load and load transfer signal  154 ,  254  can also be calculated or enhanced from a lateral acceleration signal  114 , a longitudinal acceleration signal  118 , and a steering wheel angle signal  122  when these are available.  
         [0026]    A load and load transfer signal  154 ,  254  is used by the braking algorithms to enhance braking torque proportioning and apply and dump pulse calculations.  
         [0027]    A turning detector signal  156 ,  256  (roll moment distribution) can be used to optimize vehicle handling before YSC activation and enhance brake torque distribution calculation during YSC activation.  
         [0028]    A road surface roughness and tire mismatching signal  158 ,  258  can be detected from suspension states and used by ABS/TC and YSC systems.  
         [0029]    Braking/traction status information from the wheels can also be used to enhance braking algorithms by predicting pitch and roll motion in advance.  
         [0030]    Suspension algorithms and braking algorithms can be embodied in separate ECU&#39;s  102  and  130  as illustrated in FIG. 1. In other embodiments, the suspension and braking algorithms can be integrated into a single electronic control unit.  
         [0031]    If steering wheel angle signal  122 ,  222  and/or a lateral acceleration signal  114 ,  214  are available, then split mu detection in ABS and TC algorithms (for stand alone ABS and TC systems) can be improved.  
         [0032]    In other examples, ECU  102  can only receive information from ECU  130 . Thus, various input signals from the suspension system can be transferred to the braking system, but no signals are transferred from the braking system to the suspension system.  
         [0033]    In yet other examples, ECU  130  can only receive information from ECU  102 . Thus, various input signals from the braking system can be transferred to the suspension system, but no signals are transferred from the suspension system to the braking system.  
         [0034]    A third embodiment of a control system for controlling vehicular braking and suspension functions is indicated generally at  300  in FIG. 3. In control system  300 , a single ECU  302  receives inputs signals  304  from various sensors  306  strategically placed in a vehicle. A signal processor  308  may be incorporated in the ECU  302  that delivers transfer signals  310  to an algorithm  312 . The algorithm  312  delivers output signals  314  to a HCU  328  to provide a desired brake response. The algorithm  312  also delivers output signals  316  to actuators  350  to provide a desired suspension response. Control system  300  may be referred to as a totally integrated system for controlling vehicular braking and suspension.  
         [0035]    The present invention employs a rough road index as a classification of the road surface for the purpose of enhancing ABS, TC and YSC functions. The generation of the rough road index will be described with reference to FIG. 4. The intent of the rough road identification algorithm is to create a signal indicative of a rough surface terrain from suspension travel information. The signal is then used in ABS/TCS/YSC to modify activation thresholds and control targets.  
         [0036]    The method of FIG. 4 uses a relative suspension travel signal X d  from a suspension sensor  400 . The relative travel is differentiated in derivative block  401  to give a relative velocity signal which is then filtered in a bandpass filter (BPF)  402 . The direct detection of wheel hop is employed to classify the road surface in terms of roughness. In general, wheel hop frequency caused by rough road conditions is approximately 10 Hz. Thus, BPF  402  has a passband of about 10 Hz to 15 Hz to determine the amount of surface roughness being transmitted through the suspension. A 4 th  order Butterworth bandpass filter design can be used as follows:  
         G        (   s   )       =         b   2          s   2           s   4     +       a   1          s   3       +       a   2          s   2       +       a   3        s     +     a   4                               
 
         b 2 =1421  
         a 1 =53.31  
         a 2 =6415  
         
       a 
       3=1.331×10 
       5  
     
         
       a 
       4=6.235×10 
       6  
     
         [0037]    The output of the bandpass filter represents the signal content of interest that is used to define the roughness of the road. If an active damping system is being used to control the relative wheel and body velocity, then the signal content in the wheel hop frequency range will be attenuated as measured through the relative suspension deflection, however, the road information is not removed by the damping change. Therefore, a gain  403  is inserted to change the signal content as a function of damping. The nominal value for the gain is one.  
         [0038]    In block  404 , the absolute value of the signal is taken to give a more energy-oriented parameter. The signal is then saturated in saturation block  405  to keep the peak detection from artificially being pulled too high and then taking several seconds to decay. A peak detector  406  implements a peak detection algorithm to capture the peak of |{dot over (X)} d | and to decay the index between peaks. Peak detector  406  generates the rough road index as an indication of the magnitude of the roughness of the road surface. The decay rate must be designed in accordance with the bandpass frequency. It is desired to exponentially decay (i.e., e −t/τ ) between peaks. λ k  is the discrete implementation of e −t/τ , therefore, one must choose X such that the desired decay rate (τ) is achieved. The following is a formulation for computing the appropriate λ:  
         [0039]    Letƒ avg =average frequency of the bandpass filter  
         τ=1/f avg    
         λ k   =e   −t/τ , at τ=         k=τ/ T   s    
         λ k=e   −1    
           kln (λ)=−1  
         λ= e   −1/k   =e   −Ts/τ   
         [0040]    Choose actual τ=100/f avg    
         [0041]    The actual peak detection is realized by the following:  
         [0042]    If |{dot over (X)} d |&gt;λ·Peak Then  
         [0043]    Peak=|{dot over (X)} d | 
         [0044]    Else  
         [0045]    Peak=λ·Peak(z −1 )  
         [0046]    Endif  
         [0047]    The output of the peak detect circuit can be appropriately scaled for use in the ABS, TCS, or YSC algorithms. The rough road index signal can be a continuous signal or can be quantized to provide a discrete level indication. Thus, there would be a maximum peak velocity from the peak detect circuit which would be assigned to a maximum magnitude of the rough road index signal and a lower or minimum peak velocity which would be assigned to a zero value of the rough road index (i.e., a smooth road). The lower peak velocity is preferably greater than zero in order to reject noise. Thus, one preferred formula for the rough road index is:  
           RRID=C ·(Peak−Peak_Min)÷(Peak_Max−Peak_Min),  
         [0048]    where RRID is the rough road index, C is a scaling factor for the maximum value of the RRID, Peak_Min is the minimum peak velocity below which RRID is zero, and Peak_Max is the maximum peak velocity corresponding to the roughest road.  
         [0049]    The trimming of the algorithm takes into account the physical properties of the suspension. For example, suspension properties such as spring stiffness, nominal damping rate, and sprung and unsprung masses help determine the specific implementations of the derivative and bandpass filters.  
         [0050]    Using the rough road index from FIG. 4, the performance of ABS, TCS, and YSC functions are enhanced during maneuvers where wheel hop due to surface irregularities generally degrades performance. The enhancement in the manner in which the slip control systems modify their modulation of brake actuation preferably comprises permitting an increased amount of wheel slip. Controlling to a greater amount of wheel slip generally improves performance in the case of a deformable surface such as snow or loose gravel where less tire rotation can promote digging into or plowing into the deformable surface to shorten stopping distance, for example.  
         [0051]    Preferred methods of increasing the amount of wheel slip will be described with reference to FIG. 5. This description is in the context of an ABS system where wheels are decelerating, although the concepts also apply in an analogous manner to a traction control system where wheels are accelerating.  
         [0052]    During braking, a vehicle generally decelerates. Curve  410  shows the slowing deceleration of the vehicle. A curve  411  is an actual wheel speed as measured at a wheel as the vehicle is braking. As the wheel begins to slip or skid, the wheel speed drops faster that the vehicle speed. In order to maximize brake performance, the wheel speed should be controlled to a target wheel speed  412  which corresponds to an amount of wheel slip where maximum braking force is obtained. Assuming the wheel is slipping, then the actual wheel speed cannot be used to establish the target speed. Instead, a target speed is maintained by decaying a previous value of the wheel speed according to a predetermined gradient. The gradient can be determined in response to overall vehicle deceleration and/or deceleration of the wheel prior to the onset of slipping, for example.  
         [0053]    The difference between target speed  412  and actual speed  411  is monitored. When the difference equals a predetermined threshold, then an ABS activation decision is made and the ABS system begins to modulate the braking to control the slip. A nominal threshold Δ 1  corresponds to a base threshold as used in the prior art. The difference exceeds threshold Δ 1  at a time t 1  resulting in an ABS activation event. In order to increase the amount of slip permitted when a rough road is indicated, one preferred embodiment of the present invention uses an increased slip threshold Δ 2 . This delays an activation decision until t 2  when the difference between target speed  412  and actual speed  411  exceeds Δ 2.    
         [0054]    [0054]FIG. 6 shows actual wheel speed  411  after the onset of slip. A target wheel speed is determined based on a predetermined gradient or decay  414  (which would instead be an increase during acceleration in a traction control system). Based on following the predetermined gradient from the previous target wheel speed value, a current target wheel speed value  415  is generated. In a second preferred embodiment of the present invention, the increased slip desired when the rough road index is high is obtained using an increased gradient  416 . Following increased gradient  416  generates a current target wheel speed value  417  which is less than target speed  415 .  
         [0055]    Referring to FIG. 7, using an increased gradient results in a target wheel speed curve  417  which decays more quickly than prior art target curve  412 . Consequently, at time t 2  the difference between the target wheel speed and the actual wheel speed is less than the nominal threshold Δ 1 . Due to the faster decay of the target wheel speed, the difference does not exceed nominal threshold Al until time t 2 . Slip is thereafter controlled to a lower target wheel speed curve  417  so that an increased slip level is maintained.  
         [0056]    The rough road index signal can be generated in either the active braking control or the active suspension control system. When generated in the active suspension system, the value of the rough road index signal can be transmitted to the active braking control system via a multiplex communication network, such as CAN, for example.  
         [0057]    [0057]FIG. 8 shows apparatus with several separate improvements for making the modified activation decision according to FIGS. 5 and 7. A base threshold  500  is typically determined as a fixed percentage of current vehicle speed (e.g., 10%). The base threshold is coupled to one input of a summer  501 . The prior art has included various additions to and subtractions from the base threshold. For example, U.S. Pat. No. 5,627,755 shows a desensitizer computation  502  based on acceleration and slip duration which increases the final threshold. U.S. Pat. No. 5,627,755 is hereby incorporated by reference. This desensitizer addition may be added to the base threshold in summer  501 . The final threshold is multiplied by actual wheel speed in a multiplier  505  and the product is compared to a target wheel speed in a comparator  506  which generates an activation signal.  
         [0058]    [0058]FIG. 8 shows modifications in both the determination of the final threshold value and the determination of decay rate for determining target wheel speed, although both modifications would not usually be used together.  
         [0059]    To adjust the activation threshold, the rough road index is coupled to a scaling block  504  to provide a desired transfer function as appropriate for the relative values used in the control system. Scaling takes into account any differences in relative magnitude for maximum roughness, and matches the general phasing of the signal (i.e., the circuit providing the rough road index signal may have more lead depending on the equations used). The scaling block may also provide filtering to smooth out fast changes in the rough road index so that signal dynamics do not cause significant digital noise downstream. This filtering works as follows:  
                                                                                                                                   If road_id_in &gt;= ABS_road_id_filt                ABS_road_id_filt = road_id_in           road_id_timer + 0                Else                road_id_timer = road_id_timer = 1                Endif           If road_id_timer &gt;= 200 msec                road_id_timer = 0           If ABS_road_id_filt &gt; 0                ABS_road_id_filt = ABS_road_id_filt − 1                Endif                Endif                      
 
         [0060]    Where road_id_in is the rough road signal from FIG. 4 and ABS_road_id_filt is the filtered rough road signal. This filter allows positive changes in the road ID to pass through and then requires 200 milliseconds to pass before allowing the signal to reduce.  
         [0061]    In a preferred embodiment, the scaled/filtered rough road index is provided to one input of a multiplier  503 , the other input of which receives the desensitizer factor from desensitizer computation  502 . The rough road index is scaled such that increasing surface roughness increases the amount of desensitization by preselected proportions. This preferred embodiment is particularly advantageous in the interplay with the prior art desensitization computation. Increased slip is primarily beneficial when a deformable road condition is present. It has been found that instances when both the prior art desensitization and the present rough road index are relatively large is a good indicator of deformable road conditions. Thus, using the product of the two results in enhanced performance.  
         [0062]    In an alternative embodiment, the rough road index is scaled for additive affect upon the final threshold value. Thus, the scaled rough road index is provided to an input of summer  501 . This input to the summer is an alternative to the use of multiplier  503 .  
         [0063]    In another alternative embodiment, the decay rate used in determining target wheel speed is adjusted in response to the rough road index. Thus, the rough road index signal is provided to a decay rate generator  510 . The selected decay rate is provided to a decay block  511  that receives the previous target wheel speed from a unit delay block  513 . The decayed target wheel speed is provided from decay block  511  to one input of a maximum selector block  512  which also receives the current actual wheel speed measurement. Maximum selector block provides the greater of the current wheel speed or the decayed previous target speed to the non-inverting input of comparator  506  and to the input of unit delay block  513 . the general phasing of the signal (i.e. one design may have more lead than another depending on the equations used).  
         [0064]    A more specific example of the “reference decay increase” modification will now be described. The rough road ID signal is quantized to values of 0, 1, 2, or 3 for each wheel and depending on the overall vehicle average. The reference gradient for updating target wheel speed is decayed for ABS and increased for TCS.  
         [0065]    Definition of variables is as follows:  
                                           Name   Description   Units   Resolution                   ABS_road_id_filt   Filtered road ID input for use   —   1           in ABS and TCS functions       Ax   Estimated vehicle acceleration   m/sec 2     1/256           input       Temp   Temporary value that is added   km/h/   1/256           to the previous reference   loop           value in order to decay or           increase the control reference                  
 
         [0066]    The following pseudo code illustrates a preferred implementation.  
                                                                                                                                                                           If sum(ABS_road_id_filt(1:4))/4 = 0                Temp = max(-ax,REF_DECAY_RATE_MIN)·REF_OVER_DK/                ABS_LOOPS_PER_SEC/16384                Endif           If sum(ABS_road_id_filt(1:4))/4 = 1                Temp = max(-ax,REF_DECAY_RATE_MIN)·REF_OVER_DK01/                ABS_LOOPS_PER_SEC/16384                Endif           If sum(ABS_road_id_filt(1:4))/4 = 2                Temp = max(-ax,REF_DECAY_RATE_MIN)·REF_OVER_DK02/                ABS_LOOPS_PER_SEC/16384                Endif           If sum(ABS_road_id_filt(1:4))/4 = 3                Temp = max(-ax,REF_DECAY_RATE_MIN)·REF_OVER_DK03/                ABS_LOOPS_PER_SEC/16384                Endif                      
 
         [0067]    REF_DECAY_RATE_MIN is the minimum of the reference gradient. Temp modifies the reference gradient used in the ABS algorithm. One of four different gain values (i.e., REF_OVER_DK) are selected in response to the average level of the road identification signals in order to modify Temp. A similar algorithm is used for traction control gradient modifications, however, the incremental change is increasing instead of decreasing.  
         [0068]    A more specific example of the “slip threshold increase” modification of the present invention will now be described. The rough road ID signal is used to increase the slip threshold by multiple integers of 5% of vehicle speed. An additional variable for this pseudo-code implementation is ABS_sthr_final_abslt which is the ABS slip threshold for each wheel in km/h with a resolution of 1/256:  
         Surface_id_rear =min(max(front_road_id[1 :2]), rear_road_id[1:2])  
         [0069]    [0069]                                                                                           /* Select lowest value between maximum of front and smallest of rears as the           modifier */           ABS_sthr_final_abslt *= (1 + ABS_road_id_filt(           Surface_id_rears)*0.12 (0.08 for rears))           /* Increasing the final slip threshold by multiple integers of 12% (8% for rears) */           T = 5*ABS_road_id_filt*filtered wheel speed/100           /* Adding integer values of 5% of vehicle speed to slip threshold */           If T &lt; 3 km/h                T = 3 km/h                Endif           /* Minimize to 3 km/h unless road_id = 0 */           If ABS_road_id_filt (Surface_id_rear[1:2]) = 0                T = 0                Endif           ABS_sthr_final_abslt += T                        
         [0070]    The ABS slip threshold is then used for activation detection and cyclical wheel control modes. The increase in the threshold for activation inherently will increase the level of slip to which the wheel is being controlled. An analogous implementation is performed for the slip thresholds for TC, thus increasing the amount of spin on the driven wheels.  
         [0071]    In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.