Abstract:
The present invention provides a system and method for actively controlling the suspension of a vehicle. The system includes struts for providing an adjustable suspension to the vehicle, sensors to measure the strut relative displacement, and a controller configured to determine the frequency amplitude for the heave, pitch, or roll of the vehicle based on the strut relative displacement and manipulate the struts in response thereto.

Description:
BACKGROUND  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to a system and method for controlling a vehicle suspension.  
         [0003]     2. Description of Related Art  
         [0004]     Generally, people all over the world drive their automobiles to various destinations. In order for these people to enjoy the ride to their destinations the suspensions systems in the automobiles must be stable and as comfortable as possible. Different types of automobiles have various suspension systems, which control the ride and handling performance of the vehicle. For example, some vehicles may have a sport or stiff suspension system that limits movement of its vehicle chassis with respect to the road wheels, but provides less isolation from rough road surfaces. In contrast to the stiff suspension system, some vehicles may have a luxury or soft suspension system that provides a more comfortable ride by isolating the vehicle occupied from the rough road surface, but allowing increased vehicle chassis movement causing a decrease in the handling performance.  
         [0005]     Recently, low-bandwidth active suspension control systems have been developed employing compressible fluid struts and digital displacement pump motors. One key enabling technology of these systems are efficient and effective control algorithms to fully utilize the actuation systems, while avoiding various difficulties of control algorithm implementation. One such difficulty includes developing frequency domain vibration control methods to achieve desired dynamic performance for a specific working frequency range. This frequency range, between zero and up to 30 Hz, provides two significant frequency modes, a body mode around 1 Hz and a wheel-hop mode around 11 Hz each requiring different suspension control strategies. To implement the control strategies, the control system utilizes the frequency amplitude of the vehicle heave, pitch, and roll to calculate the suspension system adjustment.  
         [0006]     Generally, heave, pitch, and roll frequency information is determined using three body accelerometers. However, it would be advantageous to calculate heave, pitch, and roll frequency information using existing sensors thereby eliminating the need for the three body accelerometers. In view of the above, it is apparent there exists a need for an improved system and method for controlling a suspension system that does not require three body accelerometers.  
       SUMMARY  
       [0007]     In satisfying the above need, as well as overcoming the enumerated drawbacks and other limitations of the related art, an embodiment of the present invention provides a system for controlling the suspension of a vehicle. The system includes compressible fluid struts as components of vehicle suspension, sensors to measure a strut relative displacement, and a controller configured to determine the frequency amplitude for the heave, pitch, or roll of the vehicle based on the strut relative displacement.  
         [0008]     In another aspect of the present invention, the controller includes a derivative filter to generate a strut relative velocity based on the strut relative displacement. Further, the strut relative velocity is used to calculate a body relative velocity. A first and second frequency amplitude are extracted from the body relative velocity to generate an effective frequency of the suspension. In addition, a desired strut pressure is calculated based on the effective frequency, the strut relative velocity, and the strut relative displacement. The struts are adjusted in accordance with the desired strut pressure to improve vehicle suspension performance.  
         [0009]     Further objects, features and advantages of this invention will become readily apparent to persons skilled in the art after a review of the following description, with reference to the drawings and claims that are appended to and form a part of this specification. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a schematic view of a vehicle having a system for controlling a suspension system in accordance with the present invention;  
         [0011]      FIG. 2  is a block diagram of an algorithm for controlling a suspension system in accordance with the present invention;  
         [0012]      FIG. 3  is a block diagram of an algorithm for deriving three body relative velocities from four strut relative displacements in accordance with the present invention;  
         [0013]      FIG. 4  is a block diagram of a frequency decoding algorithm in accordance with the present invention;  
         [0014]      FIG. 5  is a block diagram of an algorithm for extracting frequency amplitude in a frequency decoding algorithm in accordance with the present invention;  
         [0015]      FIG. 6  is a plot of a sample control strategy for heave stiffness control; and  
         [0016]      FIG. 7  is a block diagram of an algorithm for determining desired strut pressure in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0017]     Referring now to  FIG. 1 , a system  12  for controlling the suspension of the vehicle  10  and embodying the principles of the present invention is provided. The system  12  includes an electronic control unit  16 , a digital displacement pump motor (DDPM)  18 , compressible fluid struts (CFS)  14 , and displacement sensors  15 . A suspension system of this general type is generally disclosed in U.S. patent application Ser. No. 10/688,095, filed on Oct. 17, 2003, which is hereby incorporated by reference.  
         [0018]     Electronic control unit  16  interfaces with the displacement sensors  15  to collect strut relative displacement information. The strut displacement sensors are of the type well known in the industry and therefore need not be discussed in greater detail herein. Utilizing the strut relative displacement information, the electronic control unit  16  selects a control strategy to optimize the suspension performance and calculates the desired strut pressure information to implement the control strategy. The desired strut pressure is utilized to operate the DDPM  18  thereby tuning the stiffness and damping characteristics of each compressible fluid strut  14  in accordance with the control strategy.  
         [0019]     Now referring to  FIG. 2 , the displacement sensors  15  provide the strut relative displacement  22  to a control algorithm  20  contained in the electronic control unit  16 . Block  24  receives the strut relative displacement signals and converts the strut relative displacement signals to body relative velocities  26 . In addition, block  24  also generates strut relative velocities  25  to be used in calculating the desired strut pressure  34 ,  36 ,  38 . Block  28  receives the body relative velocities  26  and performs a frequency decoding algorithm to generate the effective frequencies  30 . Block  32  then generates the desired strut pressure  34 ,  36 ,  38  based on the strut relative displacement  22 , the strut relative velocity  25 , and the effective frequencies  30 . The desired strut pressure  34 ,  36 ,  38  is received by block  40  to calculate the combined desired strut pressure  42  for each strut  14 . The combined desired strut pressure  42  is provided to the digital displacement pump motor  18  to effectuate a desired control strategy by adjusting the pressure in each strut  14 . Various portions of the control algorithm  20  will be discussed in more detail below.  
         [0020]     Now referring to  FIG. 3 , the details of block  24  are provided. The strut relative displacement signals  22  (D if , D ir , D rf  and D rr ) are received by the derivative filter  50 , and the derivative filter  50  generates the strut relative velocities  25  (Vs if , Vs ir , Vs rf , and Vs rr ). The strut relative velocities  25  are independently used to calculate the desired strut pressure as discussed later. Further, the strut relative velocities  25  are received by block  53  to generate the body relative velocities  26 , or more specifically the body relative heave, pitch, and roll velocity (V h , V p  and V r ). For a specific vehicle, wheelbase (L) and tread (t) are known and used to calculate the body relative heave, pitch and roll velocities according to the relationship V h =(V lf +V lr +V rf +V rr )/4, V p =(V lf −V lr +V rf −V rr )/(2*L), and V r =(V lf +V lr −V rf −V rr )/(2*t).  
         [0021]     After the body relative velocity V i  (i=h, p and r) is calculated, each signal can be used to extract the effective frequency ω ie1  (i=h,p,r) for ride control. Now referring to  FIG. 4 , the frequency decoding algorithm 28 is applied at the vehicle body mode frequency range. Accordingly, the body relative velocity  26  is provided to a high-pass filter  60  and a low-pass filter  62 . The vehicle body mode frequency is ω 1  (=2πf 1 ), therefore, a lower frequency ω 0  (about two or three times less than ω 1 ) can be selected, along with an intermediate frequency ω 01  between ω 0  and ω 1 . These frequencies can be used as break frequencies for the high-pass filter  60  and the low-pass filter  62 . The high-pass filtered body relative velocity  61  is used to extract a first frequency amplitude  65  (A 1 ) at the selected frequency ω 1 , as denoted by block  64 . Similarly, the low-pass filtered body relative velocity  63  is used to extract a second frequency amplitude  67  (A 0 ) at the selected frequency ω 0 , as denoted by block  66 . In block  68 , the first frequency amplitude  65  in the second frequency amplitude  67  are combined according to the relationship A 1 /A 0  to generate the effective frequency  30 .  
         [0022]     Now referring to  FIG. 5 , a description of the algorithm to extract the frequency amplitude at the selected frequency such as in blocks  64  and  66 , is provided in reference to selection of the first frequency amplitude  65  (A 1 ). The high-pass filtered body relative velocity  61  is provided to a washout filter in block  70 . The washout filter modifies the high-pass filtered body relative velocity  61  according to certain washout factors  76 . The selected frequency  80  (ω 1 ), along with the result of the washout filter  70 , is provided to a band-pass filter in block  72 . The results from the band-pass filter  72  and the washout filter  70  are provided to an integrator  74 . The result of the integrator  74  is provided, along with the result of the band-pass filter  72  and the selected frequency  80 , to a modal generator in block  78 . Utilizing the selected frequency information  80  the modal generator result is provided to a smoothing filter  82 , which results in the frequency amplitude  65  (A 1 ).  
         [0023]     Similarly, the above-described algorithm to extract the frequency amplitude at a selected frequency may be applied to the second frequency amplitude  67  (A 0 ) in the same manner.  
         [0024]     Referring again to  FIG. 4 , the effective frequency ω ie1  (i=h,p,r) is used for integrating different control strategies required for different frequency ranges. Similarly, the above procedure can be applied to the frequency range around the wheel-hop mode frequency ω ie2  (i=h,p,r). For illustrative purposes the control algorithm for the low-band-width active suspension system is provided.  
         [0025]     For the low bandwidth active suspension, a bandwidth of 5 to 7 Hz is targeted due to the limited capability of the DDPM with a limited power supply. Therefore, if the suspension dynamics dominate in the frequency range beyond the bandwidth, the control algorithm will set the DDPM to idle to save power and let the CFS work in a passive state. If the effective frequencies of the suspension dynamics are less than the bandwidth, the control algorithm can select different strategies to better isolate the vehicle body from the subjected vibrations. Those strategies can be stiff stiffness, soft stiffness, soft rebound damping, hard compression damping or variations thereon. In addition, a traditional passive shock absorber damping capability exists in the CFS, such as, hard damping for rebound and soft damping for compression.  
         [0026]     Based on the effective frequencies ωie 1  and ω ie2  (i=h,p,r), strategy mappings can be determined for stiffness control and damping tuning with different effective frequencies as described in Tablel below. For example, if the heave body mode is 1.4 Hz, then the ω he1 -based strategy mapping can be −1 (representing stiff stiffness) for ω he1  less than 0.9 Hz, 1 for ω he1  near 1.4 Hz (and beyond), and a linearly interpolated value (or other curves) for ω he1  between 0.9 and 1.4 Hz. The control signals may be reduced beyond the given bandwidth by: (1) Directly forcing the ω h31 -based strategy mapping to close to 0 if ω he1  is close to 5 to 7 Hz and  0  beyond the bandwidth, (2) Using ω he2  to identify the high frequencies so that the ω he2 -based strategy mapping is  1  below 5 to 7 Hz and becomes 0 beyond the bandwidth. The product of two strategy mappings, ω he1    84  and ω he2    86 , for the stiffness control are shown in  FIG. 6 . Similarly the strategy mappings for heave damping can be properly derived from Table 1.  
                                         TABLE 1                                   Effective Freq   Adopted           Range   Control Strategy                                    Ride Control   Low   Stiff Stiffness and       (i.e., Ride       Hard Compression Damping       Comfort)   Body Mode   Small Stiffness           &lt;Bandwidth   Small Stiffness and Soft Damping           &gt;Bandwidth   Passive Suspension               (i.e., idle DDPM and no valve control)                  
 
         [0027]     Now referring to  FIG. 7 , the desired strut pressure algorithm  32  is provided in more detail. The strut relative displacements  22  are provided to the transfer function f(D i ) as provided in block  88 . Further, f(D i ) (i=lf, lr, rf and rr) is a function of the strut relative displacements, always no less than zero, and the outputs are desired pressures for each of the CFS. The strategy mapping is also used to decide whether a stiff or soft stiffness should be required for the feedback.  
         [0028]     The effective frequency  30  (ω ie1  and (ω ie2 ) is provided to the strategy mapping for stiffness heave control as denoted by block  90 . In block  92 , the product of the transfer function from block  88  and the strategy mapping for stiffness heave control from block  90  is used to generate the desired strut stiffness heave pressure  93 . The strut relative velocity  22  is provided to the transfer function f(V h ) as provided in block  106 . Effective frequency  30  (ω ie1  and ω ie2 ) is provided to the strategy mapping for heave damping control as denoted by block  108 . In block  110 , the product of the transfer function from block  106  and the strategy mapping for heave damping control from block  108  is used to generate the desired strut heave damping pressure  111 . The desired strut stiffness heave pressure  93  and the desired strut heave damping pressure  111  are combined in block  112  to generate the desired strut heave pressure  34 .  
         [0029]     For pitch control, the strut pitch relative velocity from the strut relative velocity  22  is provided to the transfer function f(V p , L/2), where L is the wheelbase, as provided in block  94 . The effective frequency  30  (ω he1  and ω he2 ) is provided to the strategy mapping for pitch control as denoted by block  96 . In block  98 , the product of the transfer function from block  94  and the strategy mapping for pitch control from block  96  is used to generate the desired strut pitch pressure  36 .  
         [0030]     Similarly, for roll control, the strut roll relative velocity from the strut relative velocity  22  is provided to the transfer function f(V p , t/2), where t is the tread, as provided in block  100 . The effective frequency  30  (ω he1  and ω he2 ) is provided to the strategy mapping for roll control as denoted by block  102 . In block  104 , the product of the transfer function from block  100  and the strategy mapping for roll control from block  102  is used to generate the desired strut roll pressure  38 .  
         [0031]     As a person skilled in the art will readily appreciate, the above description is meant as an illustration of implementation of the principles this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from spirit of this invention, as defined in the following claims.