turning now to the figures , fig1 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 . 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 . turning to fig2 , 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 . 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 . it can be seen in fig2 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 . the process or algorithm 80 employed by the mean pressure estimator 70 will now be described in detail with reference to fig3 – 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 . 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 . 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 ). with reference to fig3 , 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 : where mpr is mean pressure rate , mpv is mean pressure variation and t is the period . 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 : where smp c is the current strut mean pressure and smp p is the prior strut mean pressure . 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 . 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 fig2 . in accordance with another embodiment of the present invention , the strut mean pressure can be updated according to the following equations : 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 . fig4 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 . similarly , fig5 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 . 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 . 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 .