Abstract:
The invention provides a method and apparatus for controlling dampers in a suspension system of a vehicle body. A heave velocity of a vehicle body is derived from sensed dynamic variables of the vehicle body. A slew rate limit for a damping control command is derived in response to the heave velocity of the vehicle body. The damping control command for at least one dampers is generated in accordance with the slew rate limit. The limited damping control command is applied to the at least one damper

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to vehicle coefficients of force being offset through suspension damping, and, in particular, to a method and system for providing slew rate limiting parameters for use with suspension damping control outputs. 
     BACKGROUND OF THE INVENTION 
     Known variable force suspension systems include variable force shock absorbers and/or struts that provide suspension damping forces at a magnitude controllable in response to commands provided by a suspension system controller. Some systems provide control between two damping states and others provide continuously variable control of damping force. 
     In a known manner of control of a variable force suspension, the demand force for each variable force damper is determined responsive to a set of gains, the wheel vertical velocity and the body heave, roll and pitch velocities. An example system determines the demand force as follows: DF b =G h H′+G,R′+G p P′+G w ν, where DF b  is the demand force, G h  is the heave gain, G r  is the roll gain, G p  is the pitch gain, G w  is the wheel velocity gain, H′ is the body heave velocity, R′ is the body roll velocity, P′ is the body pitch velocity and ν is the wheel vertical velocity. The portion of the demand force computation G h H′+G,R′+G p P′, represents the body component determined responsive to the body heave, roll, and pitch velocities. The portion of the demand force computation G w ν represents the wheel component determined responsive to the difference between the computed body corner velocity and the body-wheel relative velocity. 
     A control signal representing the determined demand force is output to control the variable force damper responsive to the demand force. Example systems are described in U.S. Pat. Nos. 5,606,503; 5,235,529; 5,096,219; 5,071,157; 5,062,657; and 5,062,658. Current damping algorithms tend to change the damper output signals abruptly during zero crossings of the associated damper relative velocity signal. Over large inputs, which cause significant body motions, it is sometimes more desirable to retain an elevated damping state. Damper output stewing is a means known in the art for performing this function, however its slew rate parameters cause excess damping control to be present during normal driving. The excess damping causes unwanted forces to be dissipated throughout the vehicle. 
     Therefore, it would be desirable to have an algorithm that would mode or switch the slew rate limiting parameters in order to allow longer slew times during large inputs and smaller slew times during smaller inputs. Such an algorithm would provide for smoother, more complete control over large inputs caused by rough terrain, while not adversely affecting ride comfort over other road surfaces. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention provides a method for controlling dampers in a suspension system of a vehicle body. A heave velocity of a vehicle body is derived from sensed dynamic variables of the vehicle body. A slew rate limit for a damping control command is derived in response to the heave velocity of the vehicle body. The damping control command for at least one dampers is generated in accordance with the slew rate limit. The limited damping control command is applied to the at least one damper. 
     Another aspect of the invention provides a method of determining a slew rate limit for a suspension damping control system. An average heave velocity is determined. The slew rate limit is then determined as a function of the average heave velocity. 
     Another aspect of the invention provides a control module for a suspension damping control system comprising a means for deriving a heave velocity of a vehicle body from sensed dynamic variables of the vehicle body, a means for deriving a slew rate limit for a damping control command in response to the heave velocity of the vehicle body, a means for generating the damping control command for at least one dampers in accordance with the slew rate limit, and a means for applying the limited damping control command to the at least one damper. 
     Another aspect of the invention provides a control module for a suspension damping control system comprising means for determining an average heave velocity; and means for determining a slew rate limit as a function of the average heave velocity. Another aspect of the invention provides a computer readable medium storing a computer program comprising computer readable code for deriving a heave velocity of a vehicle body from sensed dynamic variables of the vehicle body, computer readable code for deriving a slew rate limit for a damping control command in response to the heave velocity of the vehicle body, and computer readable code for generating the damping control command for at least one dampers in accordance with the slew rate limit. 
     Another aspect of the invention provides a computer readable medium storing a computer program comprising computer readable code for determining an average heave velocity, and computer readable code for determining a slew rate limit as a function of the average heave velocity. 
     The invention provides the foregoing and other features, and the advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention and do not limit the scope of the invention, which is defined by the appended claims and equivalents thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of one embodiment of an apparatus in accordance with the invention; 
     FIG. 2 is a block diagram of a control module for one embodiment implementing the invention; 
     FIG. 3 is a block diagram of signal computational components of a control module for use with a control module of one embodiment of the invention; 
     FIG. 4 is a block diagram of one embodiment for determining demand force commands according to the invention; and 
     FIG. 5 is a block diagram of one embodiment for determining slew rate limitation values in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, an example apparatus for implementation of this invention is shown and, in general, comprises a vehicle body  10  supported by four wheels  11  and 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 this embodiment comprises an electrically controllable, variable force damper in parallel with a weight bearing coil spring in a parallel shock absorber/spring or for example, a McPherson strut arrangement. U.S. Pat. No. 5,282,645 describes a variable force damper suitable for use as actuator  12 . 
     Each corner of the vehicle can include a linear position sensor  13  that provides an output signal indicative of the relative distance between the vehicle wheel and the suspended vehicle body at that corner of the vehicle. Suitable position sensors  13  can be easily constructed by those skilled in the art. The outputs of the position sensors  13  may be differentiated to produce relative body-wheel vertical velocity signals for each corner of the vehicle. The vertical velocity signals may be used to determine the body modal velocities of body heave velocity, body roll velocity and body pitch velocity. One embodiment defines the heave velocity as the vertical velocity of the center of gravity of a vehicle. 
     One embodiment of a position sensor  13  includes a rotary resistive device mounted to the vehicle body and a link pivotally coupled between both the vehicle wheel and a pivot arm on the rotary resistive device. This allows the rotary resistive device to provide an impedance output that varies with the relative position between the wheel  11  and the corner of the body  10 . Each position sensor  13  may further include an internal circuit board with a buffer circuit for buffering the output signal of the rotary resistive device and providing the buffered signal to the controller  15 . Suitable position sensors  13  can be easily constructed by those skilled in the art. Any alternative type of position sensor, including transformer type sensors, may be used as position sensor  13 . 
     The outputs of relative position sensors  13  can be provided to a controller  15  which processes the signals to determine the states of vehicle body  10  and wheels  11  and generates an output actuator control signal for each variable actuator  12 . These signals are applied from controller  15  through suitable output apparatus to control actuators  12  in real-time. Input signals for the determination of the output actuator control signals may also be provided to a microcomputer to provide anticipation of vehicle pitch (lift/dive)  17 . Alternatively, the input signals may be used by a vehicle speed sensor  18  and a steering wheel angular position sensor  19  to provide anticipation of vehicle roll. Obtaining such signals is easily achieved using known types of sensors available to those skilled in the art. Processing of the sensor input can be conducted by a digital microcomputer  22 , a component of the controller  15 . 
     One embodiment of control module  15  is shown in more detail in FIG.  2 . Signals from relative position sensors  13  can be low-pass filtered through, as shown, four analog low-pass filters  24  and differentiated through four analog differentiators  26 , to provide four relative velocity signals. One embodiment of such a low-pass filter and differentiator is shown in U.S. Pat. No. 5,255,191. The resulting signals represent the relative velocity between the front left wheel and the front left corner of the body rν 1 , the rear left wheel and the rear left corner of the body rν 2 , the front right wheel and the front right corner of the body rν 3 , and the rear right wheel and the rear right corner of the body rν 4 . Each of these relative velocity signals is input to the digital microcomputer  22 , which includes input A/D converter  28  with multiplexed inputs. Each input is digitally high-pass filtered within microcomputer  22  to remove any DC offset introduced by the digitization of A/D converter  28 . 
     In the described embodiment, the actuators are controlled by a PWM (Pulse Width Modulation) signal. However, actuators of another type not based on PWM signals can be substituted in alternative embodiments. It will be recognized that variable force controls other than those with PWM are equivalent to the PWM controlled signal example set forth herein. 
     The actuator control is performed by a microprocessor suitable for providing PWM output. Such microprocessors are known and readily available to those skilled in the art. The output of the PWM duty cycle within control module  15  is represented as lines  112 ,  114 ,  116  and  118 . This output may be in the form of signals representing duty cycles that standard microprocessors readily convert to the proper duty cycle PWM output, for example as with a standard PWM output interface  111 . 
     Reference  50  represents four input signals of the relative velocities of the four corner suspensions of the vehicle. The relative velocities are determined from the position sensors  13 , by the low pass filtering of the outputs of sensors  13 , through four analog low pass filters  24 . The filtered outputs continue through four analog differentiators  26  to produce four relative velocity signals. These signals represent the relative velocities rν 1 , rν 2 , rν 3 , and rν 4 . Each of these relative velocity signals is input into the microprocessor  22  through an A/D converter  28 . 
     In an alternative embodiment, relative position sensors  13  are replaced with relative velocity sensors of a type known to those skilled in the art. These sensors are capable of outputting a signal indicative of the relative velocity between each wheel and corner of the vehicle body. With these sensors, there is no need for the differentiators  26  described above used to convert the signals from sensors  13  to relative velocity signals. 
     Various discrete signals are provided to the microprocessor input/output port  67 . The vehicle speed input  52  is preferably buffered in a known manner to remove unwanted noise. Input of lateral acceleration  54  of the vehicle is computed  55  in a manner based on the steer angle of the front wheels  53  and the vehicle speed  52 . A standard diagnostic routine  59  performs known diagnostic functions such as checking for open circuits and short circuits from any of the input lines represented by bus  50  and  61 . In response to a diagnostic  59  failure command  56 , the damper output control  110  forces a default output on lines  112 ,  114 ,  116  and  118  to control the actuators in a default mode. 
     A digital signal representing the battery voltage is input  58  through the microprocessors A/D converter  28  and is used at environmental compensation  92  to scale the duty cycle commands responsive to the battery voltage. The lift/dive signal  60  indicates 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. The dive signal may be provided by a power train controller that determines a vehicle dive situation if a decrease in vehicle speed over a predetermined period is greater than a predetermined limit. The lift signal may be determined if an increase in throttle angle over a predetermined period is greater than a predetermined threshold. In general, the lift/dive signal  60  is active when there is either a detected lift or dive, and is otherwise inactive. Signal Conditioning  102  receives the lift/dive signal  60  and performs a de-bounce function of a known type. The de-bounced lift/dive signal  104  is provided to the damper output control  110 . The damper output control  110  further buffers the lift/dive signal. It also determines whether a base profile output override is active and if so, determines what PWM duty cycle to use as the base (minimum duty cycle). Additionally the damper output control  110  applies the time-varying lift/dive PWM duty cycle base as the minimum PWM duty cycle. 
     The damper output control  110  limits to 10 seconds (as an example of this embodiment) the amount of time that the control module  15  will follow a lift/dive signal. Thus, if a short circuit causes an erroneous lift/dive signal, the system is only affected for 10 seconds. 
     The ignition state signal  62  is representative of the ignition voltage available when the vehicle is keyed on, and is de-bounced in a known manner. The moving determination Block  69  provides a signal  64  indicating whether or not the vehicle is moving. An override line  66  is available to be used for in-plant testing of the system. 
     The corner relative velocity signals  50  that are input to the signal conditioning Block  68  are computed to provide velocity information to the automatic control algorithm  82 . The information includes the relative velocity signals provided on buses  76 ,  78 , and  80  along with heave velocity signal provided on bus  70 , roll velocity signals on bus  72 , and pitch velocity signals on bus  74 . These signals are used as a set of inputs to a control algorithm  82  to help determine the output actuator control signals for the vehicle suspension system. 
     Descriptions of signal conditioning Block  68  computational components are illustrated in FIG.  3 . The relative velocity input signals  50  from the microprocessor&#39;s A/D converters are offset  122  so that the scaled value represents wheel motion. The scaling is achieved by simply multiplying the offset results by a predetermined scaling factor. The results are the offset, scaled relative velocity signals  124 . The signals  124  are provided to a relative velocity high-pass filter  126 , which performs a digital high pass filtering to further remove any DC offsets introduced by the digitization of the A/D converter. The filtered relative velocity signals for the four corner suspension are provided on lines  128 ,  130 ,  132  and  134  to the average wheel velocity determination  135 , the modal velocity estimation  136 , and the relative velocity lead-lag filter  138 . 
     The average vertical wheel velocity is determined  135  by first high pass or band pass filtering each of the signals on lines  128 - 134  separately to isolate the wheel content of each relative velocity signal from the body content. That is, the high pass or band pass filter eliminates the 1 Hz. component while passing the 10 Hz. component of the signal. The absolute value of the output of the high pass filter is taken in order to obtain the magnitude information of the output signal. The magnitude information is averaged and a low pass filter is applied to each of the rectified signals  80 . The signals  80  are then indicative of the average wheel velocity of one of the four wheels. 
     An example high pass filter for isolating the wheel component of each signal  128 - 134  can be implemented using the equation V av (k)=rv x (k)−rv x (k−1)+a*V av (k−1), where V av  is the average vertical wheel velocity, k represents the current sampling period, rv x  is the suspension relative velocity for corner x of the vehicle, and a is a predetermined constant. An example low pass filter may be implemented according to the equation V av (k)=b. rvx (k)+(1−b)V av (k−1), where b is a constant. 
     The modal velocity estimation  136  receives the relative velocity signals  128 - 134  and determines the body heave roll and pitch velocities. This is done by performing a set of geometric transforms to obtain the relative states of relative heave velocity, relative roll velocity and relative pitch velocity between the vehicle body and wheels. The geometric transforms used to obtain the relative heave velocity (rv H ), relative roll velocity (rv R ), and relative pitch velocity (rv P ) are implemented respectively as follows: rv H =(rv 1 +rv 2 +rv 3 +rv 4 )/4, rv R =(−rv 1 +rv 3 )/tw, rv p =(−rv 1 +rv 2 −rv 3 +rv 4 )/(2*wb), where tw is the average track width or wheel span, and wb is the wheel base. In the determination of rv R , only rv 1  and rv 3  are used in vehicles in which flexing or noise of the rear suspension affects the quality of the relative roll velocity determined using rv 2  and rv 4 . If rear suspension flexing or noise does not affect the relative roll velocity determination, then the determination of rv R  can be set according to the equation rv R =(rv 1 −rv 2 +rv 3 +rv 4 )/(2*tw). 
     Once each of the transforms is completed, a digital low pass filter filters each of the relative heave, roll and pitch velocities to derive accurate estimates of the body heave, roll and pitch velocity. The low-pass filter provides a significant amplitude roll-off above the 1 Hz signal typical of resonant body modal vibrations. This is to suppress the 10 Hz signals typical of resonant vertical wheel vibrations and thus yield a signal with information about the amplitude of vehicle body motion in the heave, roll and pitch modes. However, for such signals to be usefully applied in a real time control system, their phase is critical. Low pass filters tend to produce a phase lag in the signal and this phase lag increases across the frequency spectrum through a range that increases with the number of filter poles. In order to produce effective control in real time, of rapidly moving suspension components, the control signal must be applied in correct phase relationship thereto. To emulate the integrated output of an absolute accelerometer, a phase lead of approximately 90 degrees is required. To achieve this, a two pole low pass filter is applied to the relative body modal velocity signal, producing a signal with a 90 degree phase lag, which is inverted to provide the required 90 degree phase lead. A suitable second order low pass filter for use in the estimation is equated as H Q (s)=K Q [ω o   2 /(s 2 +(ω o /Q)s+ω o   2 )], where K Q  is the filter gain, ω o  describes the filter pole locations in radians and Q is the filter quality factor. 
     Each low pass filter may be adjusted independently to tune the modal velocity estimates to match in magnitude. With an additional phase inversion, signals achieved by the use of an accelerometer or any similar device may be obtained. The values of the filter gains, pole locations and quality factor tend to change from vehicle type to vehicle type due to differences in the natural body and wheel frequencies of different vehicle models. The heave, roll and pitch velocities obtained are estimations since they are derived from relative measurements and not from absolute measurements. The results of the modal velocity estimation  136  are heave velocity  70 , roll velocity  72 , and pitch velocity  74 . 
     The lead-lag filter  138  receives the relative velocity signals  128 - 134 , filters the signals and adds an approximate lead to the signal. This compensates for phase lag that may be introduced within the system, including that of the differentiator circuit, at the expense of a certain degree of magnitude distortion. 
     A single pole infinite impulse response high-pass filter is used to provide a desired amount of phase lead at wheel hop frequencies. Specifically, the filter may be implemented using the transfer function H(z)=(LLA−LLBz −1 )/(1−LLCz −1 ), where H(z) is a discrete, or “z” domain transfer function relating the z-transform of the filter output to that of the filter input. LLA, LLB and LLC are calibration constants stored in memory. The outputs of the relative velocity lead-lag filter  138  are the filtered relative velocity signals  78  for each corner of the vehicle. 
     All of the outputs from signal conditioner  68  as well as additional outputs from FIG. 2 are provided as inputs to the automatic control algorithm  82 , which is shown in greater detain in FIG.  4 . The automatic control algorithm  82  includes body control algorithm  142 , wheel control algorithm  168 , total demand force determination  184 , demand force limiting  190 , raw PWM duty cycle determination  200 , PWM slew rate limitation  208 , and automatic mode PWM duty cycle determination  220 . 
     The body control algorithm  142  receives the body modal velocities  70 - 74 , the vehicle speed signal  52 , and the computed lateral acceleration  54 . The body control algorithm outputs a set of body demand forces on bus  164  (four lines). 
     The wheel control algorithm  168  responds to the buffered vehicle speed signal  52 , the lateral acceleration  54 , the four average wheel velocity signals  80 , and the four lead-lag filtered relative velocity signals  78 . The wheel control algorithm  168  provides the wheel demand force signals  170  to the total demand force determination Block  184 . The wheel demand force signals include one signal for each corner of the vehicle. 
     The total demand force determination  184  receives the body demand forces on bus  164 , the wheel demand forces on bus  170  and the lead-lag filtered relative velocity signals on lines  78 . The total demand force determination  184  output is the total demand force for each corner of the vehicle responsive to each of the input values. While the total demand force for this embodiment is determined responsive to the lead-lag filtered relative velocity signal, if a lead-lag filter is not implemented, a non-lead-lag filtered relative velocity signal may be implemented instead. 
     More specifically, the total demand force determination  184  determines the total demand force DF tot , for each actuator by combining the wheel and body demand forces and DF w  and DF b  respectively, based upon the phase of the demand forces. If the body and wheel demand forces are in-phase, the demand force is determined according to a wheel force as a minimum approach. That is, the demand force is set equal to the body demand force unless the body demand force is less than the wheel demand force, in which case the total demand force is set equal to the wheel demand force. If the wheel and body demand forces are out of phase, the forces are summed to generate the total demand force. 
     The wheel demand force of this embodiment will always have the same sign as the relative velocity and will always be passive. The body demand force may be active or passive. If the body and wheel demand forces are both passive, the demand forces are in-phase. If the body demand force is active, the demand forces are out-of-phase. Examples of a passive system are damper systems that do not provide energy from another source, such as a compressor storing high-pressure hydraulic fluid. These systems can only generate force that is the same sign (direction) as the relative velocity of the damper. Active dampers that have external supplies of energy can supply force independent of the relative velocity of the damper. On a force-versus-relative velocity plot, the passive quadrants are the first and third quadrants, that is, the quadrant in which relative velocity and force are both positive and the quadrant in which relative velocity and force are both negative. In the second and fourth quadrants, a passive damper cannot provide a commanded force. 
     The resulting four total demand force signals  188  of the total demand force determination  184  are passed to demand force limiting Block  190 . Demand force limiting  190  performs a limiting function on the total demand force signals  188  responsive to the lead-lag filtered relative velocity signals  78 , the vehicle speed signal  52  and the diagnostics signal  140 . Demand force limiting  190  dynamically determines a maximum slope for the demand force curve. Additionally, it determines the maximum demand force responsive to that slope and limits the total demand force to that computed maximum. Once the demand forces are limited at Block  190 , the resultant demand forces  191  are provided to the raw PWM duty cycle determination Block  200 . 
     Raw PWM duty cycle determination provides damping control commands on bus  201  responsive to the signals on bus  191 , the lead-lag filtered signals on lines  78  and the high pass relative velocity signals on line  76 . Raw PWM duty cycle determination  200  compares the lead-lag filtered relative velocity signal and the total demand force signal for a particular corner, to determine if that corner suspension is in compression or rebound. If both the lead-lag filtered signal and demand force signal are positive, then the system is in rebound mode. If both the lead-lag filtered signal and the demand force signal are negative, then the system is in compression mode. A damper force table is applied to determine a raw PWM duty cycle command. The damper force table is implemented using table values  202  stored in memory, PWM duty cycles  204  stored in memory, and a damper force axis intercept scale factors  206 , also stored in memory. The inputs to the table are the total demand force and the high pass filtered relative velocity for each corner. A total of up to eight tables are required in this embodiment because different tables are used for compression and rebound. Numerous other approaches are possible for alternative embodiments. The process will remain however to derive damping control commands from demand forces. 
     In total, four signals are provided by raw PWM duty cycle determination  200 , representing the damping control commands for the four corners of the vehicle. These are output  201  to the PWM slew rate limitation Block  208 . PWM slew rate limitation  208  is responsive to the damping control command signals  201 , and the heave velocity signal  70 . The PWM slew rate limitation  208  has separate up and down limits on the rate of change of the PWM signals used to control the variable force actuators. Separate limits are used to limit noise created by sudden changes in damper force, limit high frequency cycling of the damper valves, and attenuate high frequency components of the damper valve command. The slew factors in the downward direction may be a function of body demand forces. The PWM slew rate limitation Block  208  outputs the four slew filtered PWM signals  218 . 
     The automatic mode PWM duty cycle determination  220  received signals  218  and may be implemented if desired to calibrate a minimum PWM command for each actuator. This minimum command on line  222  is used as a limit or base, which the duty cycle of the PWM command cannot be lower. The resultant output signals on lines  84 ,  86 ,  88  and  90  are the filtered PWM duty cycle commands. 
     Details of the PWM slew rate limitation Block  208  are illustrated in FIG.  5 . The damping control commands  201  for each corner are independently subjected to the PWM slew rate limitation algorithm in order to generate the slew filtered PWM signals  218  for each corner. For this embodiment, slew rate limitation is defined as a control placed on the amount by which each damping control command is allowed to change from one control loop to the next. The maximum change that is allowed will generally be different for upward versus downward changes in the damping control command input signals  201 . The PWM slew rate limitation Block  208  is broken down into the slew rate limit determination  535 , and the slew rate limit application  565 . At all times, the control software is running the slew rate limitation algorithm. As a function of the control algorithm status, the parameters for the slew rate limitation algorithm will change. The parameters for the slew rate limit determination  535  are summarized in the table below: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 Inputs(s) 
                 Heave Velocity 
               
               
                 Outputs(s) 
                 Front Increasing Delta 
               
               
                   
                 Front Decreasing Delta 
               
               
                   
                 Rear Increasing Delta 
               
               
                   
                 Rear Decreasing Delta 
               
               
                 Calibrations(s) 
                 Front Increasing Delta 1 &amp; 2 (FI1, FI2) 
               
               
                   
                 Rear Increasing Delta 1 &amp; 2 (RI1, RI2) 
               
               
                   
                 Front Decreasing Delta 1 &amp; 2 (FD1, FD2) 
               
               
                   
                 Rear Decreasing Delta 1 &amp; 2 (RD1, RD2) 
               
               
                   
                 Average Heave Velocity 1 &amp; 2 (AHV1, AHV2) 
               
               
                   
                 Increasing low pass filter (LPF) Coefficient (INC_A) 
               
               
                   
                 Decreasing low pass filter (LPF) Coefficient (DEC_A) 
               
               
                 Execution Rate 
                 10 ms or faster 
               
               
                   
               
             
          
         
       
     
     The first step of PWM slew rate limitation block  208  is to compute a signal referred to as the average heave velocity  525 . The average heave velocity is not a true mathematical average and is referenced herein as the output of the average heave velocity determination block  510 . In one embodiment, the average heave velocity  525  is calculated according to the following equations and logic, or equivalent: 
     Rectified heave velocity (RHV) 520 =|heave velocity|  515   
     If rectified heave velocity (RHV)&gt;average heave velocity (AHV) then  510   
     
       
         
               
               
             
               
             
               
               
             
               
             
           
               
                   
                   
               
             
             
               
                   
                 AHV = INC_A * RHV + (1 − INC_A) * AHV 
               
             
          
           
               
                 Else 
               
             
          
           
               
                   
                 AHV = DEC_A * RHV + (1 − DEC_A) * AHV 
               
             
          
           
               
                 Endif 
               
               
                   
               
             
          
         
       
     
     Once the average heave velocity signal  525  has been computed as indicated above, the active set of slew rate limiting parameters  550 - 559  are determined from the above calibrations according to the following equations and logic, or equivalent  535 - 545 : 
     If average heave velocity (AHV)&lt;average heave velocity 1 (AHV 1 ) then 
     
       
         
               
             
               
               
             
               
             
               
               
             
               
             
               
               
             
               
             
               
               
             
               
             
               
               
             
               
             
               
               
             
               
             
           
               
                   
               
             
             
               
                 Front increasing delta = front increasing delta 1 (FI1) 
               
             
          
           
               
                   
                 Front decreasing delta = front decreasing delta 1 (FD1) 
               
               
                   
                 Rear increasing delta = rear increasing delta 1 (RI1) 
               
               
                   
                 Rear decreasing delta = rear decreasing delta 1 (RD1) 
               
             
          
           
               
                 Elseif average heave velocity (AHV) ≧ average heave velocity 2 (AHV2) 
               
               
                 then 
               
             
          
           
               
                   
                 Front increasing delta = front increasing delta 2 (FI2) 
               
               
                   
                 Front decreasing delta = front decreasing delta 2 (FD2) 
               
               
                   
                 Rear increasing delta = rear increasing delta 2 (RI2) 
               
               
                   
                 Rear decreasing delta = rear decreasing delta 2 (RD2) 
               
             
          
           
               
                 Else 
               
             
          
           
               
                   
                 Front increasing delta = FI1 + (AHV − AHV1)*(FI2 − FI1)/ 
               
             
          
           
               
                 (AHV2 − AHV1) 
               
             
          
           
               
                   
                 Front decreasing delta = FD1 + (AHV − AHV1)*(FD2 − FD1)/ 
               
             
          
           
               
                 (AHV2 − AHV1) 
               
             
          
           
               
                   
                 Rear increasing delta = RI1 + (AHV − AHV1)*(RI2 − RI1)/ 
               
             
          
           
               
                 (AHV2 − AHV1) 
               
             
          
           
               
                   
                 Rear decreasing delta = RD1 + (AHV − AHV1)*(RD2 − RD1)/ 
               
             
          
           
               
                 (AHV2 − AHV1) 
               
               
                 Endif 
               
               
                   
               
             
          
         
       
     
     It is allowable and preferred from a loop-time standpoint, to perform the division operations in the above equations during calibration upload in order to store and use multiplicative equivalents during normal algorithm execution. 
     The parameters for the slew rate limit application  565  are summarized in the table below: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Inputs(s) 
                 Scaled Body PWM (4) 
               
               
                   
                   
                 Front Increasing Delta 
               
               
                   
                   
                 Front Decreasing Delta 
               
               
                   
                   
                 Rear Increasing Delta 
               
               
                   
                   
                 Rear Decreasing Delta 
               
               
                   
                 Outputs(s) 
                 Filtered Body PWM (4) 
               
               
                   
                   
                 (carried forward as simply Body PWM (4)) 
               
               
                   
                 Calibrations(s) 
                 None 
               
               
                   
                 Execution Rate 
                 @Primary Loop Time 
               
               
                   
                   
               
             
          
         
       
     
     The slew rate limit application  565  is executed as the following equations and logic, or equivalent: 
     
       
         
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
             
           
               
                   
               
             
             
               
                 Do for all corners (xx = lf through rr) 
               
             
          
           
               
                   
                 If front damper (xx = lf or xx = rf) then 
               
             
          
           
               
                   
                 Increasing delta = front increasing delta 
               
               
                   
                 Decreasing delta = front increasing delta 
               
             
          
           
               
                   
                 Else 
               
             
          
           
               
                   
                 Increasing delta = rear increasing delta 
               
               
                   
                 Decreasing delta = rear increasing delta 
               
             
          
           
               
                   
                 Endif 
               
               
                   
                 Delta (temp) = xx scaled body PWM - xx filtered body PWM 
               
               
                   
                 If (delta &lt;= 0) then 
               
             
          
           
               
                   
                 If (|delta| &gt; decreasing slew rate limit) then 
               
             
          
           
               
                   
                 Delta = −decreasing slew rate limit 
               
             
          
           
               
                   
                 Endif 
               
             
          
           
               
                   
                 Else 
               
             
          
           
               
                   
                 If(delta &gt; increasing slew rate limit) then 
               
             
          
           
               
                   
                 Delta = increasing slew rate limit 
               
             
          
           
               
                   
                 Endif 
               
             
          
           
               
                   
                 Endif 
               
               
                   
                 xx filtered body PWM = xx filtered body PWM + delta 
               
             
          
           
               
                 End do 
               
               
                   
               
             
          
         
       
     
     As with all pseudo-code representations included in this application, the above is one of many possible embodiments. Any approach may be implemented provided the functionality is equivalent to that shown above. The resulting properties are those of the slew filtered PWM signals  218 . 
     While the embodiments of the invention disclosed herein are presently considered to be preferred, various changes and modifications can be made without departing from the spirit and scope of the invention. The scope of the invention is indicated in the appended claims, and all changes that come within the meaning and range of equivalents are intended to be embraced therein