Patent Publication Number: US-6219602-B1

Title: Vehicle suspension control with stability in turn enhancement

Description:
This application is a continuation-in-part of U.S. Ser. No. 09/283,789, filed Apr. 1, 1999 now abandoned and assigned to the assignee of this application. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The technical field of this invention is a vehicle suspension control system. 
     BACKGROUND OF THE INVENTION 
     When a motor vehicle is steered into an extended turn, the vehicle body weight tends to shift toward the outside wheels, and the best handling in the turn is obtained by maximizing the contact of the outside wheels with the road surface, which contact can be affected by road surface roughness. 
     Many automotive vehicles have suspensions that vary damping force in response to control commands determined by a computer controller, in order to improve overall vehicle ride comfort and handling. One such system is responsive to absolute body modal velocities derived from relative body/wheel position or velocity sensors and acts through controllable dampers to provide control of sensed body motions and reduce ride harshness. This control also provides a measure of wheel control; but it is generally designed for a balanced approach between comfort and handling. 
     SUMMARY OF THE INVENTION 
     The suspension control of this invention includes a body control responsive to relative body/wheel velocity at the corners of the vehicle body to derive a demand force command for each of the dampers for vehicle body control and, as in the prior art, applies each of the derived demand force commands to its respective damper only when a comparison of the direction of the demand force command with the sensed relative velocity of the damper indicates that a force corresponding to the demand force command can be effectively exerted by the damper. But the suspension control of this invention adds to such a body control a vehicle stability control that is responsive to indicated lateral acceleration of the vehicle in a turn to determine, independently of the vehicle body control, a stability compression damping command for the suspension dampers on the side of the vehicle opposite the direction of the lateral acceleration and a stability rebound damping command for the suspension dampers on the side of the vehicle in the direction of the lateral acceleration. While the lateral acceleration is sensed, the vehicle stability control applies the stability compression damping command to the suspension dampers on the side of the vehicle opposite the direction of the lateral acceleration and the stability rebound damping command to the suspension dampers on the side of the vehicle in the direction of the lateral acceleration, without regard for the direction of demand force for any of the suspension dampers. The resulting stiffening of the suspension on the outside of the turning vehicle in compression and on the inside of the turning vehicle during rebound helps keep the tires of the vehicle firmly in contact with the road surface in spite of road surface irregularities. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described by way of example with reference to the following figures, in which: 
     FIG. 1 illustrates a vehicle with a suspension control system according to this invention; 
     FIG. 2 is a block diagram of a suspension controller for use in the suspension control system of FIG.  1 . 
     FIG. 3 illustrates a vehicle stability control for use in the suspension controller of FIG.  2 . 
     FIG. 4 illustrates a signal processing block for use in the vehicle stability control of FIG.  3 . 
     FIG. 5 illustrates a control during turning block for use in the vehicle stability control of FIG.  3 . 
     FIG. 6 illustrates a turning direction corner control for use in the vehicle stability control of FIG.  3 . 
     FIG. 7-9 show graphs illustrating aspects of the operation of the vehicle stability control of FIG.  3 . 
     FIG. 10-13 show flow charts illustrating the operation of the vehicle stability control of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, an example apparatus implementing this invention comprises a vehicle body  10  supported on four wheels  11  by four suspensions including springs of a known type (not shown). Each suspension includes a variable-force, real time, controllable damper  12  connected to exert a vertical force between wheel  11  and body  10  at that suspension point. Although many such suspension arrangements are known and appropriate to this invention, actuator  12  of the preferred embodiment comprises an electrically controllable, variable force damper in parallel with a weight bearing coil spring in a parallel spring/shock absorber or McPherson strut arrangement. A description of a variable force damper suitable for use as actuator  12  is the continuously variable damper described in U.S. Pat. No. 5,282,645. 
     Each corner of the vehicle includes a position sensor  13  that provides an output signal indicative of the relative vertical distance between the vehicle wheel and the suspended vehicle body at that corner of the vehicle. The outputs of position sensors  13  may be differentiated to produce relative body-wheel vertical velocity signals for each corner of the vehicle and may be used, for example, as described in U.S. Pat. No. 5,606,503, to determine the body modal velocities of body heave velocity, body roll velocity and body pitch velocity. The relative body-wheel vertical velocity signals are an example of what is referred to herein as a set of parameters indicative of motion of a body of the vehicle and of motion of wheels of the vehicle. 
     An example position sensor  13  includes a resistive device mounted to the vehicle body and a link pivotally coupled between the vehicle wheel and a pivot arm on the resistive device such that the resistive device provides an impedance output that varies with the relative vertical position between wheel  11  and the corner of body  10 . Each position sensor  13  may further include an internal circuit board with a buffer circuit for buffering the output signal of the resistive device and providing the buffered signal to a suspension controller  15 . Suitable position sensors  13  of this are known to, or can be constructed by, those skilled in the art. Any alternative type of position sensor, including transformer type sensors, may be used as position sensors  13 . 
     The outputs of relative position sensors  13  are provided to suspension controller  15  which processes the signals, for example as described in U.S. Pat. No. 5,606,503, to determine the states of vehicle body  10  and wheels  11  and generates an output actuator control signal for each variable actuator  12 . Suspension controller  15  sends these signals through suitable output apparatus to control actuators  12  in real time. Other signals that suspension controller  15  may use include a lift/dive signal from a sensor  17 , a vehicle speed signal from a sensor  18 , a steering wheel angular position from a sensor  19  and a measured lateral acceleration signal from a sensor  20 . Obtaining such signals may be achieved through the use of known types of sensors or vehicle control signals available to those skilled in the art. 
     Suspension controller  15 , shown in more detail in FIG. 2, may be a digital microcomputer  22  programmed to process a plurality of input signals in a stored algorithm and generate output control signals for actuators  12 . Analog signal processing is provided for some of the input signals. For example, signals from relative position sensors  13  are low-pass filtered through four analog low-pass filters  24  and differentiated through four analog differentiators  26  to provide four relative velocity signals. An exemplary combination of such a low pass filter and differentiator is shown in U.S. Pat. No. 5,255,191, issued Oct. 19, 1993. The resulting relative velocity signals represent the relative vertical velocities between each of wheels  11  and the corresponding corner of the body. Each of these relative velocity signals is input to microcomputer  22 , which includes an input A/D converter  28  with multiplexed inputs. Reference  50  represents the four corner suspension relative vertical velocities input into the microprocessor  22  through A/D converter  28 . In an alternative example implementation, relative position sensors  13  are replaced with relative velocity sensors of a type known to those killed in the art capable of outputting signals indicative of the relative velocity between each wheel and corner of the vehicle body. In this alternative, there is no need for the differentiators  26 . 
     Various other digital/discrete signals are provided to microcomputer  22  through I/O apparatus  67 . Line  32  provides a measured lateral acceleration signal from sensor  20 , which is a standard lateral acceleration sensor, and is output on line  34 . Line  52  provides vehicle speed signal from sensor  18 , which signal is preferably buffered in a known manner in block  67  to remove unwanted noise; and the buffered signal is output on line  71 . This signal, which may be the same as that used for the vehicle speedometer and/or other vehicle systems, may comprise a pulse train having pulse timing varying with vehicle speed, a signal the decoding of which is well known in the art. Line  53  provides a steering angle signal to block  67  from sensor  19  and is output on line  73 . This signal may be obtained from a rotational sensor in the steering gear, with a number of sensors and designs known in the art. Line  60  provides a signal that indicates when the vehicle is in a dive (front end dip) or lift (front end rise) tendency situation such as occurs during hard braking or hard acceleration of the vehicle. Lift/dive sensor  17  may be part of a powertrain controller that determines a vehicle dive tending situation if a decrease in vehicle speed over a predetermined time period is greater than a predetermined limit and determines a lift tending situation if an increase in throttle angle over a predetermined time period is greater than a predetermined threshold. The signal from lift/dive sensor  17  is generally a discrete, binary signal that has a first value when there is either a detected lift or dive, and is otherwise inactive. Line  62  provides a discrete, binary ignition state signal indicative of vehicle operation; and line  66  provides a discrete override signal useful for in-plant testing or service of the system. 
     A lateral acceleration calculator  55  is effective to derive a calculated vehicle lateral acceleration signal in a known manner from the vehicle speed signal on line  71  and the vehicle steering angle signal on line  73  and output the derived vehicle lateral acceleration signal on line  54 . In particular, the signal may be the computed lateral acceleration derived as described in block  194  later in this description. Alternatively, the signal provided on line  54  may be the measured lateral acceleration from lateral acceleration sensor  20  or the combined lateral acceleration derived in block  196  described at a later point in this description. A diagnostic routine is responsive to various signals in I/O apparatus  67  to perform known functions such as checking for open circuits and short circuits from any of the sensors, input lines or actuators or any of the other lines (represented in general as bus  61 ) and is capable of generating a system failure command on an output line  56 . 
     The digital outputs of A/D converter  28  are provided to signal conditioning block  68 , in which each is digitally high-pass filtered to remove any DC offset introduced by the digitization of A/D converter  28 . Block  68  derives from these filtered signals a set of relative velocity signals for the four corners on bus  76 , a set of estimated average wheel velocity signals for the four wheels on bus  80  and a set of body modal (heave, pitch and roll) velocity signals on lines  70 ,  72  and  74 , respectively for use in automatic control algorithm  82 , to derive actuator control signals representing the demand force commands for each of actuators  12  and outputs these commands on lines  84 ,  86 ,  88  and  90 . The demand force commands generated by automatic control algorithm  82  are preferably PWM duty cycle commands. However, actuators of another type not based on PWM control can be substituted as an alternative; and it will be recognized that variable force controls other than those with PWM control are equivalents to the PWM control example set forth herein. 
     The PWM duty cycle commands from automatic control algorithm  82  on lines  84 ,  86 ,  88  and  90  are provided to environmental compensation block  92 . A set of four vehicle stability PWM duty cycle commands derived in accordance with this invention for the same wheels in a vehicle stability control  75  is also provided to environmental compensation block  92  on a bus  79 . Environmental compensation block  92  derives a combined PWM duty cycle command for each wheel from the PWM duty cycle command from automatic control algorithm  82  and the vehicle stability PWM duty cycle command from control  75  corresponding to the same wheel. Preferably, the method of combination is to select the larger of the PWM duty cycle command from automatic control algorithm  82  and the vehicle stability PWM duty cycle command from control  75  for the same wheel. Vehicle stability control  75  will be described in detail below. 
     Environmental compensation block  92  then scales the four combined PWM duty cycle commands based on a scaling factor derived from the vehicle battery voltage VBAT, which is input to microcomputer  22  through an A/D converter  28 . The scaled combined PWM duty cycle commands for the four wheels are then output on lines  94 ,  96 ,  98  and  100 . 
     Damper output control  110  receives the scaled combined PWM duty cycle commands and determines when to output these signals on output lines  112 ,  114 ,  116 ,  118  and  120  and when to override these signals for some specific purpose. For example, damper output control  110  may be responsive to a diagnostic failure command from diagnostic block  59  to output predetermined “failure mode” PWM duty cycle commands: for example, a default PWM command that is scaled simply in response to vehicle speed. Control  110  may be responsive to the override signal from line  66  to actuate all dampers in a predetermined manner for in-plant or service testing. Control  110  may be responsive to the lift/dive signal, debounced in signal conditioning block  102 , to set minimum values for the PWM duty cycle commands, as described in greater detail in the aforementioned U.S. Patent No.  5 , 606 , 503 . Control  110  is responsive to an enable signal on line  108  from a mode control apparatus  106  to enable the output of commands from block  110 . The enable signal is generated by mode control apparatus  106  in response to an active ignition state signal on line  62 . Without an enable signal on line  108 , any commands determined will not be output on lines  112 ,  114 ,  116 ,  118  and  120  and the controller is allowed to enter a standard “sleep” state of the type used in automotive controllers when the vehicle ignition is off. An enable signal on line  108  does not force any output command levels, but simply enables output of the commands from block  110 . 
     The resultant control outputs from block  110  are provided to an output interface  111  on lines  112 ,  114 ,  116  and  118  and comprise the duty cycle commands for the four actuators  12  in the suspension system. The damper low side control command is provided on line  120 . The duty cycle commands on lines  112 ,  114 ,  116  and  118  are converted in a known manner to pulse width modulated signals having the duty cycles commanded by the signals on lines  112 ,  114 ,  116  and  118 . Output interface  111  includes a PWM control comprising standard signal processing and power electronic circuitry, possibly including another microcomputer, such as a Motorola™ 68HC11 KA4, which is adapted for providing PWM output control commands. The interface between the microcomputer controller and the variable force dampers may include standard power electronic switches and protective circuitry as required for controlling current in a valve activating solenoid coil such as is shown in U.S. Pat. No. 5,282,645, issued Feb. 1, 1994. The valve responds to a pulse width modulated signal and provides a continuously variable range of decrease in flow restriction of a bypass passage to the reservoir of the damper between maximum restricted flow when the valve is closed in response to a 0% duty cycle command and a minimum restricted flow when the valve is open and responsive to 100% duty cycle command, or vice versa. Those skilled in the art will understand that any suitable microprocessor-based controller capable of providing the appropriate actuator command and performing the required control routine can be used in place of the example set forth herein and are equivalents thereof. 
     Referring now to FIG. 3, a general block diagram of the vehicle stability control  75  is shown. Signal processing block  160  receives the measured lateral acceleration LA M , the steering wheel angle θ and the vehicle speed signal V V  on lines  34 ,  73  and  71 , respectively. Block  160  uses these signals to determine a signal, DLA, on line  164  indicative of the rate of change in vehicle lateral acceleration and a combined lateral acceleration signal LA CM  on line  162  that is the greater in magnitude of (a) measured lateral acceleration LA M  and (b) computed lateral acceleration LA C  and has the direction of measured lateral acceleration. Block  160  is described in more detail below with reference to FIG.  4 . 
     Control During Turning block  166  responds to the signal DLA on line  164 , as well as signal LA M  on line  34  and the vehicle speed signal V V  on line  71  to control the status of a flag on line  170  that indicates whether the vehicle is in a turning maneuver. Block  166  also determines a turning PWM command on line  168  responsive to the vehicle speed V V  and combined lateral acceleration LA CM  signals on lines  71  and  162 , respectively. Block  166  is described in more detail below with reference to FIG.  5 . 
     The signals on lines  168  and  170 , along with the combined lateral acceleration signal on line  162  are provided to block  172 , the turning direction corner control. Block  172  determines to which corners (e.g., front left, front right, rear left and rear right) the turning PWM command based on the combined lateral acceleration signal will be provided and whether such corners are in compression or rebound. Block  172  provides the resultant corner STAB PWM commands on bus  79 . Block  172  is described in more detail below with reference to FIG.  6 . 
     Signal processing block is shown in more detail in FIG.  4 . The steering wheel velocity V θ  is determined at block  190  by differentiating the steering wheel angle signal on line  73 . For example, a second order digital differentiating filter may be implemented according to the following function: 
     
       
         H(z)= g   1 (1− z   −1 )/(1− c   1   z   −1   +c   2   z   −2 ),  
       
     
     where g 1  is the filter gain and c 1  and c 2  are the filter coefficients selected to provide the desired differentiator operation at the applicable frequency and loop time. For example, at a one millisecond sampling interval (1 kHz sampling frequency) and loop time, the following coefficients may provide the desired response: g 1 =11.1, c 1 =1.8705 and c 2 =0.8816. The system designer can adjust these factors to tune the phase and frequency response of the filter as desired. The steering wheel velocity signal determined by block  190  is provided to block  192 , described below. 
     Block  188  performs a partial calculation of lateral acceleration based on vehicle speed LAVS, for example, according to: 
     
       
           LAVS=V   V   2 /( g   VS   V   V   2   +g   WB ),  
       
     
     where V V  is the vehicle speed, g VS  is the steering gear ratio times an understeer coefficient of the vehicle and g WB  is the steering gear ratio times the vehicle wheel base. The signal LAVS is provided to blocks  192  and  194 . Block  192  then determines the rate of change of lateral acceleration signal DLA according to: 
     
       
           DLA=|LAVS*V   θ |,  
       
     
     where V θ  is the steering wheel velocity signal from block  190  and the vertical lines indicate the absolute value of the product. According to the above equation, DLA is directly proportional to steering wheel velocity and, if the steering wheel is not moving, i.e., V θ =0, then DLA equals zero. The signal DLA is provided on line  164 . 
     Block  194  computes the vehicle lateral acceleration, LA C  as follows: 
     
       
         
           LA 
           C 
           =LAVS*θ.  
         
       
     
     The computed lateral acceleration LA C  is provided to block  196  along with the measured lateral acceleration signal LA M  on line  34 . Block  196  outputs a combined lateral acceleration signal LA CM  on line  162  with a magnitude equal to the greater of the measured and computed lateral acceleration signal magnitudes but a direction always equal to the direction of the measured lateral acceleration signal. Thus the combined lateral acceleration signal has the advantage of fast response, since the computed value is derived from steering wheel velocity, which precedes the actual vehicle body acceleration, but is always referenced to the measured value for direction, since the steering angle can be momentarily incorrect for this purpose on low friction road surfaces. 
     The control during turning block  166 , shown in more detail in FIG. 5, has a raw PWM command generator block  208  that responds to the absolute value of the combined lateral acceleration signal on line  162  to derive a raw PWM command according to the function depicted, for example, in FIG.  7 . The raw PWM is an inverse linear function between upper limit OSP1 and lower limit OSP2, corresponding to lateral acceleration values LA1 and LA2, respectively. A scale factor block  210  is responsive to vehicle speed to generate a scale factor as shown, for example, in FIG.  8 . The vehicle speed scale factor VSSF is a direct linear function of vehicle speed between the limits VSSF2 and VSSF1, corresponding to vehicle speed values VS1A and VS2A, respectively. The raw PWM and vehicle speed scale factor are provided to scaled PWM command generator block  212 , which derives the scaled PWM command by multiplying the raw PWM value from block  208  by scale factor VSSF from block  210 . The product is limited to a predetermined maximum value and then provided on line  168 . 
     The vehicle speed signal on line  71  is provided to the switch point generator  218 . Block  218  determines a first switch point LAOSP as a function of vehicle speed as shown, for example, in FIG. 9, wherein switch point LAOSP is an inverse linear function of vehicle speed between a maximum value OSP1 corresponding to vehicle speed VSB1 and a minimum value OSP2 corresponding to vehicle speed VSB2. A second switch point LAISP is derived as a scaled fraction of switch point LAOSP. 
     Switch points LAOSP and LAISP are provided on lines  220  and  222 , respectively, along with the measured lateral acceleration on line  34  and signal DLA on line  164 , to event detector block  224 . Block  224  provides a signal STAB FLAG on line  170  that indicates when the vehicle is in a turning maneuver and requires the stability enhancement. The STAB FLAG is determined as shown in the flow chart of FIG.  10 . The subroutine first determines at  300  if DLA, the rate of change of lateral acceleration, exceeds a predetermined constant value DLAOSP. If it is, at  302  a timer is loaded with a predetermined time HOLD; and STAB FLAG is set to 1. If not, the subroutine determines at  304  if the measured lateral acceleration exceeds the value of LAOSP received from block  220 . If so, the timer is loaded with HOLD and STAB FLAG is set to 1 at  302 . If neither of the signals DLA or the measured lateral acceleration exceeds its switch point, the timer is checked at  306 . If it is zero (expired), the STAB FLAG is set to 0 at  316 . If it is greater than zero (unexpired), the measured lateral acceleration is compared at  308  to the smaller scaled value LAISP from block  220 . If it exceeds LAISP, TIMER is set to HOLD at  310 ; if it does not, TIMER is decremented at  312 . In either case, the STAB FLAG is set to 1 at  314 . 
     Referring now to FIG. 6, the turning direction corner control block  172  includes a mask selector block  228 , a table  226 , a corner direction command generator  232  and an apply command generator  236 . The mask selector block  228  receives the STAB FLAG on line  170  and combined lateral acceleration LA CM  on line  162  and uses those signals to select which of two data masks stored in table  226  are used by block  232  as the selected data mask. 
     Each data mask is coded to define a unique relationship to the four corner suspensions for both compression and rebound modes, and the two data masks provide enhanced stability for turns in opposite directions. An example table stored in block  226  is as follows, wherein “1” indicates that stability enhancement is applied and “0” indicates it is not applied at the indicated corner and compression/rebound state: 
     
       
         
           
               
               
               
            
               
                   
                   
               
               
                   
                 COMPRESSION 
                 REBOUND 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 LF 
                 RF 
                 LR 
                 RR 
                 LF 
                 RF 
                 LR 
                 RR 
               
               
                   
                 (bit 7) 
                 (bit 6) 
                 (bit 5) 
                 (bit 4) 
                 (bit 3) 
                 (bit 2) 
                 (bit 1) 
                 (bit 0) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 STAB L MASK 
                 0 
                 1 
                 0 
                 1 
                 1 
                 0 
                 1 
                 1 
               
               
                 STAB R MASK 
                 1 
                 0 
                 1 
                 0 
                 0 
                 1 
                 1 
                 1 
               
               
                   
               
            
           
         
       
     
     One of the stability data masks is chosen only when the stability flag indicates that stability enhancement is required: 
     
       
         STAB FLAG=1.  
       
     
     If the previous condition is not met, the data mask is set to all zeros. But if the condition is met, STAB R MASK is chosen if combined lateral acceleration is positive: 
     
       
         LA CM &gt;0.  
       
     
     Otherwise, STAB L MASK is chosen. 
     The chosen data mask is provided along with the value of SCALED PWM to corner direction command generator block  232 , which determines and outputs corner PWM values for each of the four corners of the vehicle body in compression and rebound modes on bus  234 . Essentially, corner direction command generator block  232  determines, as directed by the chosen mask from block  228 , which corner(s) will receive the SCALED PWM command generated in block  212  in compression, in rebound or both. The operation of block  232  is described with reference to the flow chart of FIG.  11 . Subroutine DETERMINE CORNER COMMANDS is a repetitive loop that is run eight times, once for each corner in each damper direction. At  320 , the parameters of the loop are declared: DO for each value of bit XY of the stated mask, where the values of X indicate the four corners (LF, RF, LR, RR) and the values of Y indicate the compression/rebound state (CMP, REB), resulting in eight possible combinations. The loop begins by determining at  322  if the appropriate bit of the selected mask (STAB L MASK or STAB R MASK) is equal to 1. If so, it sets the corresponding value of CORNER PWM (XY) to the received value of SCALED PWM at  324 ; if not, it sets the corresponding value of CORNER PWM (XY) to zero at  326 . The loop is then repeated for the next value of XY determined at  328 . When the loop has completed its eight cycles, the result is an array of eight values of CORNER PWM (XY), one for each corner of the vehicle in each of the compression and rebound modes. 
     The eight corner direction commands CORNER PWM (XY) on bus  234  are provided along with the signals on bus  76  to the apply command generator block  236 . Block  236  uses the high pass filtered relative velocity signals on bus  76  to determine whether each corner is in a compression or a rebound state and select the corresponding compression or rebound CORNER PWM (XY) command for that corner for the STAB APPLY PWM (X) commands on bus  79 . With reference to the flow chart of FIG. 12, the process SELECT STAB APPLY PWM first declares parameters of a DO loop at  330  for the four corners: X=LF, RF, LR, RR. For each corner, the compression/rebound state of the damper is determined at  332  by examining the high pass filtered relative velocity signal for the corner. If that signal is greater than or equal to zero, indicating that the corner is in rebound, then Y=REB; and the rebound CORNER PWM (XREB) command for that corner is selected at  334  as the STAB APPLY PWM command for that corner on bus  79 . Otherwise, Y=CMP; and the compression CORNER PWM (XCMP) command for that corner is selected at  336  as the corner STAB APPLY PWM command on bus  79 . The next value of X is then chosen at  338  to repeat the loop, thus determining the corner STAB APPLY PWM commands for the left front, left rear and right rear corners for output on bus  79  in a similar manner. 
     As previously stated, the STAB APPLY commands serve as minimum PWM values for each corner. This can be accomplished for each corner as shown in the flow chart of FIG.  13 . The subroutine DETERMINE COMBINED PWM (X) proceeds at  340  in a DO loop for X=LF, RF, LR, RR. The maximum of STAB APPLY PWM (X) and SUSP PWM (X) is selected at  342  for the value of COMBINED PWM (X). The next value of X is then chosen at  344  until all four corners have determined values of COMBINED PWM (X). It may be noted that the previously mentioned U.S. Pat. No. 5,606,503, which describes automatic control algorithm  82  in greater detail, shows a process block  220  titled “automatic mode PWM duty cycle floor,” which provides an opportunity to set a minimum PWM value for each corner. That block could be modified to receive the STAB APPLY PWM commands from vehicle stability control  75  in this apparatus and determine the maximum of the values at each corner as described above, as an alternative to performing the same function in environmental compensation block  92  as described herein. 
     This invention is also applicable to suspension systems using bi-state, real time controlled dampers. Such dampers differ from the continuously variable dampers used in the preferred embodiment described above in that, although they can be switched between two different valve conditions (low damping or high damping, as by opening or closing a bypass valve that supplements the main damper valving) sufficiently fast for real time suspension control, they cannot be switched sufficiently fast for pulse width modulated continuously variable valving. With no pulse width modulation possible, there is no point in deriving a PWM value. Blocks  208 ,  210  and  212  of FIG.  5  and block  232  of FIG. 6 can be eliminated from the control. The data mask is selected in mask detector  228  in response to the stability flag from event detector  224 , which determines when stability control is required, and a vehicle lateral acceleration indicating signal, which determines which side of the vehicle will be controlled in compression and which in rebound. The selected data mask is provided directly to apply command generator  236 . 
     The vehicle lateral acceleration indicating signal used in the bi-state damper system is preferably the measured lateral acceleration LA M , since it always indicates the most accurate direction of vehicle lateral acceleration. But the vehicle steering angle signal could alternatively be used, since it will indicate the direction of vehicle lateral acceleration in all but extreme vehicle handling situations. Of course, if either is available, the combined lateral acceleration or the calculated lateral acceleration could be used, with the former providing the same indicated direction of vehicle lateral acceleration as the measured lateral acceleration and the latter providing the same direction as the calculated lateral acceleration. Similarly, in the system described above in detail for continuously variable dampers, the calculated lateral acceleration, though not preferred, could serve as a substitute for combined lateral acceleration if a lateral acceleration sensor was not available. 
     Through the use of the mask data elements the controller controls the left and right suspensions during extended turning maneuvers. The purpose is not to control vehicle roll during turning, which is best handled by a faster acting transient roll control that responds to steering wheel motion and prevents roll from occurring. Rather, the control of this invention is intended to provide vehicle handling stability during an extended turn, such as the steady turn required on a highway entrance or exit ramp or a long turn in a road. The purpose is to help the vehicle tires maintain contact with the road entirely through the turn. The dampers are stiffened in compression on the outside of the turning vehicle to keep the wheels from bouncing up, off the road; and the dampers are stiffened in rebound on the inside of the turning vehicle for the same reason. 
     A basic difference should be noted in the application of damping commands by vehicle stability control  75  of this invention and the prior art semi-active suspension control described as automatic control algorithm  82 . The prior art system modified by this invention applies damping in the classic semi-active “sky hook” manner. The control is primarily body control oriented for occupant comfort; and damping is increased only when so doing would provide a force on the body in the correct direction to retard vertical movement of the body. This is determined by comparing the vertical direction of demand force with the direction of the damper (compression or rebound), as described in the referenced U.S. Pat. No. 5,606,503 with reference to the quadrant power check of block  316  in that patent. When demand force on the body results from upward body movement and the damper is in a rebound state (extension), the demand force can be applied by a damper (by resisting extension, the damper is able to resist upward body movement). This is also true when demand force results from downward movement of the body and the damper is in a compression state. Thus, the demand force command is provided to each damper only in the two quadrants wherein the direction of demand force (or body movement) matches the damper state. In the other two quadrants, the damper is not activated. 
     In contrast, the goal of the control of this invention is primarily vehicle handling; and the damping commands produced by vehicle stability control  75  of this invention are applied in response to the compression/rebound state of the damper as mapped by the data mask, without regard to the direction of demand force or vertical body motion. Thus, the damping commands for a corner produced by the two controls are determined independently of each other and will not always provide zero and non-zero values simultaneously. This is expected, since the objectives of the two controls are different.