Abstract:
A method and apparatus is provided for estimating the mean pressure in a compressible fluid strut. A database is employed containing values for mean pressure variation corresponding to a specific combination of motor speed and flow demand, and may also account for strut temperature. The flow demand and the speed of the motor are determined, and the mean variation corresponding to the determined combination of motor speed and flow demand is selected. The estimation of strut mean pressure is updated with the selected mean pressure variation. In this way, costly pressure sensors are eliminated as well as the complicated control algorithms which are used therewith.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method and apparatus for estimating the mean pressure in a compressible fluid strut, forming a portion of an active suspension system in a motor vehicle.  
       BACKGROUND OF THE INVENTION  
       [0002]     An active suspension system for a motor vehicle utilizes actuable struts at each wheel of the vehicle whereby the pressure within the struts may be controlled to actively regulate the damping and spring effect of the suspension system. One key component of such an active suspension system is a pressure detector or sensor that provides a reading of the strut mean pressure for each strut. As used herein, the strut mean pressure is the static pressure variation a strut can have after executing flow demands.  
         [0003]     Typically, a high level vehicle dynamics controller creates a desired pressure for a particular strut, and based on a comparison between the detected strut mean pressure and the desired pressure, an actuator increases or decreases the pressure within the strut to meet the desired pressure level. It can therefore be seen that the pressure sensor is a very important component of the active suspension system.  
         [0004]     When a strut is exposed to payload, vibration and the execution of flow demands, the strut pressure is composed of payload-dependent pressure (i.e. precharged pressure), vibration-dependent pressure, and pulsation-dependent pressure. Additionally, the strut load also includes friction due to vibration. Unfortunately, all of these pressure components are not desirable from the standpoint of controlling the pressure within the strut. Specifically, if a pressure sensor is used, the control algorithm needs to include a complicated estimation algorithm to figure out the achieved controllable pressure when a flow demand is executed. The complicated estimation algorithm must factor out certain pressure components such as those previously mentioned.  
         [0005]     Accordingly, there exist a need to provide a method and apparatus for estimating the strut mean pressure in a strut forming a portion of an active suspension system, the method and apparatus eliminating the need for a costly pressure sensor and the complicated estimation algorithm which are required to determine the achieved controllable pressure.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006]     The present invention provides a method and apparatus for estimating the mean pressure in a compressible fluid strut without the use of a pressure sensor or complicated estimation algorithms. The strut forms a portion of an active suspension system for a vehicle, the system further including a motor having a crankshaft driving a cylinder, the cylinder being responsive to flow demands to deliver or remove fluid from the strut.  
         [0007]     One embodiment of the method includes the steps of providing a database of values for mean pressure variation corresponding to a specific combination of motor speed and flow demand. The flow demand and the speed of the motor are determined, and the mean variation corresponding to the determined combination of motor speed and flow demand is selected. The estimation of strut mean pressure is updated with the selected mean pressure variation. In this way, costly pressure sensors are eliminated as well as the complicated control algorithms which are used therewith.  
         [0008]     According to more detailed aspects, the method further includes the step of determining the period of the mean pressure variation based on the motor speed. A mean pressure rate may then be determined based on the mean pressure variation and the period. The mean pressure rate equals the mean pressure variation divided by the period. The updating step preferably includes updating the estimation of strut mean pressure with a mean pressure rate over a length of time equal to the period. The estimation of strut mean pressure is preferably updated according to the equation SMP c =SMP p +λ*MPR where SMP c  is current estimated strut mean pressure SMP p  is prior estimated strut mean pressure, λ is the efficiency of the motor (including electric and hydraulic sub-systems), and MPR is mean pressure rate. The quantity expressed by λ*MPR may also be adjusted by a factor (1+a) for the first half of the period and the factor (1−a) for the second half of the period.  
         [0009]     The method also preferably adjusts for temperature variation of the strut. That is, the database may include values for mean pressure variation corresponding to a specific combination of motor speed, temperature and flow demand. Further, the updating step may be delayed by a period of time corresponding to the travel time of fluid flow from the cylinder to the strut.  
         [0010]     An active suspension system constructed in accordance with an embodiment of the present invention includes a motor, a cylinder and a compressible fluid strut. The motor has a crankshaft and the cylinder is driven by the crankshaft. The cylinder has high pressure and low pressure valves for supplying and removing fluid from the strut. The strut is fluidically connected to the cylinder for increasing or decreasing the pressure in the strut. A vehicle dynamics controller generates a requested pressure for the strut. A device control is provided for regulating the pressure in the strut. The device control includes a valve controller, a mean pressure estimator, and flow demand creator. The valve controller regulates the high and low pressure valves of the cylinder. The mean pressure estimator provides an estimation of the mean pressure in the strut. The flow demand creator sends flow demand signals to the valve controller based on the difference between the requested pressure and the estimation of current mean pressure. The mean pressure estimator receives data on the speed of the motor and the flow demand signals, and based thereon determines a mean pressure variation. The estimation of strut mean pressure is updated with the mean pressure variation.  
         [0011]     According to more detailed aspects, a database is provided having mean pressure variation values corresponding to specific combinations of motor speed and flow demand. When temperature of the strut is accounted for, the database has mean pressure variation values corresponding to specific combinations of motor speed, temperature and flow demand. The mean pressure estimator determines the period of the mean pressure variation based on the motor speed. Then, the mean pressure estimator determines a mean pressure rate based on the mean pressure variation divided by the period.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description serve to explain the principles of the invention. In the drawings:  
         [0013]      FIG. 1  is a schematic illustration of an embodiment of an active suspension system constructed in accordance with the teachings of the present invention;  
         [0014]      FIG. 2  is a schematic diagram showing a device controller forming a portion of the active suspension system which is in communication with the vehicle dynamics controller;  
         [0015]      FIG. 3  is a schematic flow diagram showing an algorithm for updating the estimation of strut mean pressure in accordance with the teachings of the present invention;  
         [0016]      FIG. 4  is graph showing a comparison of the pressure as detected through a pressure sensor versus the estimation generated in accordance with the present invention; and  
         [0017]      FIG. 5  is also a graph showing a comparison of the pressure as detected through a pressure sensor versus the estimation generated in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     Turning now to the figures,  FIG. 1  depicts a schematic illustration of an active suspension system  20  constructed in accordance with the teachings of the present invention. The active suspension system  20  includes, among other components not listed or shown here, a motor  22  driving a cylinder  28 , which in turn supplies and returns pressurized fluid to a compressible fluid strut  40 . The motor  22  is preferably a digital displacement pump motor which allows execution of discrete flow demands. The motor  22  includes a shaft  24  which in turn drives a crankshaft that  26  translates the rotational motion of the motor  22  and shaft  24  into an axial motion for driving the cylinder  28 .  
         [0019]     The cylinder  28  generally includes a piston rod  30  connected to a piston  32 . The piston rod  30  is driven by the crankshaft  26 , and the piston  32  reciprocates within the cylinder  28  to pressurize fluid contained therein. The cylinder  28  includes a high pressure valve  34  and a low pressure valve  36 . The high pressure valve  34  is fluidically connected via a conduit  38  to the compressible fluid strut  40 . The low pressure valve  36  is fluidically connected to an accumulator  50  via a conduit  48 . The accumulator  50  is utilized to store a charge of fluid which may be provided to the strut  40 , or alternatively which may have been removed from the strut  40 . The strut  40  generally includes a cylinder  42  having a piston  44  fitted therein to divide the cylinder  42  into upper and lower portions which are filled with a fluid  46  such as a composition of liquid silicone as is known in the art. It will be recognized that numerous fluid mediums  46  may be utilized in conjunction with the present invention.  
         [0020]     Turning to  FIG. 2 , the cylinder  28  and its valves  34 ,  36  are regulated by a low level device controller  60  in order to supply or remove fluid to or from the strut  40 . The device controller  60  generally includes a flow demand creator  62 , a valve controller  66 , and a mean pressure estimator  70 . The valve controller  66  is the actuator responsible for controlling the valves  34 ,  36  of the cylinder  28 , and hence the flow of fluid to or from the strut  40 . The valve controller  66  receives a command  64  from the flow demand creator  62  which opens or closes the valves  34 ,  36  in order to achieve the desired pressure within the strut  40 .  
         [0021]     The vehicle dynamics controller.  56  sends a signal  58  to the device controller  60  that is indicative of a desired or requested pressure in the strut  40 . The mean pressure estimator  70  outputs a signal  72  indicative of the current estimated mean pressure in the strut  40  which is compared to the requested pressure  58  at subtractor  74 . Based on the difference between the requested pressure  58  and the current estimated mean pressure  72 , the flow demand creator  62  generates a signal  64  which is used by the valve controller  66  to operate the valves  34 ,  36  of the cylinder  28  to adjust the pressure within the strut  40 . In this way, the device controller  60  makes the actuation system a smart actuator for active suspension control.  
         [0022]     It can be seen in  FIG. 2  that the mean pressure estimator  70  also receives the signal  64  from the flow demand creator  62 . Using this data  64 , as well as other data such as the speed of the motor  22  and the temperature of the strut  40 , the mean pressure estimator  70  utilizes a database  76  having stored values of mean pressure variation  78  corresponding to the particular combination of flow demand, motor speed and temperature. Using the mean pressure variation  78  from the database  76 , the mean pressure estimator  70  updates the current estimation of mean pressure  72  for continued use by the device controller  60 .  
         [0023]     The process or algorithm  80  employed by the mean pressure estimator  70  will now be described in detail with reference to  FIGS. 3-5 . The algorithm  80  used by the mean pressure estimator  70  receives several pieces of information including the flow demand  64  as previously discussed. The algorithm  80  also receives information on motor speed  82 , strut temperature  84  and the shaft trigonometry  86  which is representative of the positioning of the crankshaft  26  in thus the cylinder  28 .  
         [0024]     Generally, there are five flow demands to control each cylinder  28 . The five flow demands are full pumping (FP), partial pumping (PP), partial motoring (PM), full motoring (FM), and idle. Each one of these flow demands represents a particular combination of high pressure valve  34  position and low pressure valve  36  position. Pumping generally refers to providing pressurized fluid to the strut  40 , while motoring generally refers to removing pressurized fluid from the strut  40 , thus driving the motor  22  as a generator. Each strut  40  generally includes two cylinders  28  linked thereto. Accordingly, there are  14  combined flow demands available for each strut.  
         [0025]     The database  76  may be constructed by testing a particular vehicle by setting up the compressible fluid strut  40  and the active suspension system  20  to represent an on vehicle installation. The motor  22  is then run at a certain nominal speed that is specified for production system requirements. During the testing, a series of FP, PP, PM, FM, IDLE or their combination are sent to the device controller  60 , and in particular the valve controller  66 . At the same time, the motor speed, strut pressure, shaft trigonometry and strut temperature are monitored to provide collected testing data which characterizes the variation of the strut mean pressure for each one command corresponding to the different flow demands. For example, at a certain motor speed and temperature, the least mean square method can be applied to determine the mean pressure variation with respect to a single flow demand (FP, PP, PM, and FM).  
         [0026]     With reference to  FIG. 3 , the algorithm  80  utilizes the data on motor speed  82  to determine the period (T, ms) related to the mean pressure variation, since the motor speed  82  can be changed according to the flow demand for each cylinder  28 . The algorithm  80  utilizes the database  76  to look up the mean pressure variation  78  corresponding to the particular combination of motor speed  82 , strut temperature  84  and the flow demand  64 . The algorithm determines the period T as indicated by block  88 . As indicated at block  90 , the mean pressure rate is determined according to the equation: 
 
 MPR=MPV/T    (1) 
 
 where MPR is mean pressure rate, MPV is mean pressure variation and T is the period. 
 
         [0027]     When the flow demand is IDLE, the mean pressure rate=0. The mean pressure rate is computed for each cylinder  28  and each flow demand thereon in order to update the strut mean pressure (SMP). As indicated at step  92 , the strut mean pressure is updated every millisecond for a length of time equal to the period T according to the equation: 
 
 SMP   c   =SMP   p   +λ*MPR.    (2) 
 
 where SMP c  is the current strut mean pressure and SMP p  is the prior strut mean pressure. 
 
         [0028]     The λ represents a variable which is set to approximate the efficiency of the digital displacement pump motor  22  (including the combined electric and hydraulic sub-systems), and hence λ usually falls between 0.9 and 1.1. In most cases, λ=1. The efficiency for different flow demand combinations can be decided by using the testing data through an optimization process to reduce the estimation error.  
         [0029]     A time delay is calculated as indicated at block  94 , the time delay being predetermined to represent the travel time of the flow demand execution through the pipe lines from the motor  22  to the compressible fluid strut  40 . Finally, the algorithm  80  sends a current estimate  96  of the strut mean pressure, which is utilized by the mean pressure estimator  70  and the device controller  60  in order to generate future flow demands as previously discussed with reference to  FIG. 2 .  
         [0030]     In accordance with another embodiment of the present invention, the strut mean pressure can be updated according to the following equations: 
 
 SMP   c   =SMP   p   +λ*MPR* (1+a).   (3) 
 
 SMP   c   =SMP   p   +λ*MPR* (1−a).   (4) 
 
         [0031]     In this case, a is a value between 0 and 1, in the mean pressure estimator  70  will utilize equation 3 for the first half of the period (T), and then use the equation 4 for the second half of the period (T). Accordingly, based on the testing data, the equations for determining the strut mean pressure may be adjusted between the first half of the period and the second half of the period to more accurately reflect the change in pressure within the compressible fluid strut  40 .  
         [0032]      FIG. 4  depicts a graph showing the change in pressure (shown on the Y axis) over time (shown on X axis). The first line  100  represents actual testing data that was directly detected for a single strut  40  being controlled by a first cylinder having a flow demand of full pumping (FP) and a flow demand of IDLE for the other cylinder  28 . The second line  102  represents the current estimation of strut mean pressure (SMP c ) estimated by the device controller  60  and the mean pressure estimator  70  as previously discussed. It can be seen that the estimation of mean pressure in accordance with the present invention eliminates much of the undesired fluctuations in the detected pressure  100 .  
         [0033]     Similarly,  FIG. 5  depicts a graph of pressure versus time for one cylinder having a flow demand of full motoring (FM) and the other cylinder having a flow demand of partial pumping (PP). As in the previous figure, the line  104  represents the testing data, While line  106  represents the data generated from the device controller  60  and the mean pressure estimator  70  of the present invention.  
         [0034]     Accordingly, the present invention provides a method to continuously update the mean pressure in a compressible fluid strut in correspondence with the flow demands executed by a digital displacement pump motor. The present invention excludes pressure sensors in the pulsations induced by executing the discrete flow demands. In the estimation, motor speed and strut temperature are included to improve the accuracy in all environments and operating conditions.  
         [0035]     The foregoing description of various embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Numerous modifications or variations are possible in light of the above teachings. The embodiments discussed were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.