Abstract:
A method of controlling a vehicle suspension system including the steps of detecting, computing and altering the center of gravity of the vehicle. The detecting step includes the detecting of a center of gravity of the vehicle thereby defining a detected center of gravity. The computing step includes computing what alteration of a plurality of actuators is needed to substantially reduce a value of a difference between the detected center of gravity and a preferred center of gravity using a fuzzy relations control strategy. The computing step is carried out in a controller. The altering step includes altering the center of gravity of the vehicle by way of the controller actuating at least one of the actuators connected to suspension elements of the vehicle dependent upon the alteration arrived at in the computing step.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an active suspension system, and, more particularly, to an active suspension system that optimizes the location of the center of gravity of a vehicle. 
     BACKGROUND OF THE INVENTION 
     Active center of gravity management techniques involve the tilting of the vehicle chassis relative to the surface on which it rests in order to position the center of gravity with respect to the support points on the terrain. The prior art focus has been on the effect that regular and irregular surfaces of the ground have on the vehicle as well as the positioning of the vehicle system alignment relative to the ground and to reduced abrupt changes in the position of the vehicle relative to passengers. Some prior art systems include control systems that measure movement and timing of the suspension devices and after contact with an irregular surface the system calculates the reaction to take. 
     The term “center of gravity” (CG) is often interchangeably used with the term “center of mass.” Although they can be different concepts, they do coincide when the object under consideration is in a uniform gravitational field. The center of mass of a system of particles is defined as the average of their physically weighted masses. For a continuous distribution of mass the CG is the same as the centroid of the shape. 
     The concept of center of mass or CG was first introduced by the ancient Greek physicist, mathematician and engineer Archimedes of Syracuse. Archimedes showed that the torque exerted on the lever by weights resting at various points along the lever is the same as what it would be if all the weights were moved to a single point, or their center of mass. Working with floating bodies Archimedes demonstrated that the orientation of a floating object is the one that makes its center of mass as low as possible. He even developed mathematical techniques for finding the center of gravity of objects of uniform density of various well-defined shapes such as the triangle, hemisphere and a frustum. 
     What is needed in the art is an active suspension that changes the center of gravity in an efficient economical manner. 
     SUMMARY 
     The invention in one form is directed to a method of controlling a vehicle suspension system including the steps of detecting, computing and altering the location of the center of gravity of the vehicle with respect to the support points on the terrain. The detecting step includes the detecting of a center of gravity of the vehicle thereby defining a detected center of gravity. The computing step includes computing what alteration of a plurality of actuators is needed to substantially reduce a value of a difference between the detected center of gravity and a preferred center of gravity using a fuzzy relations control strategy. The computing step is carried out in a controller. The altering step includes altering the center of gravity of the vehicle by way of the controller actuating at least one of the actuators connected to suspension elements of the vehicle dependent upon the alteration arrived at in the computing step. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a schematical top view of a vehicle utilizing an embodiment of a center of gravity altering suspension system of the present invention; 
         FIG. 2  is a block diagram of a control system utilized by the center of gravity altering suspension system of  FIG. 1 ; and 
         FIG. 3  is a flowchart of a method utilized by the control system of  FIGS. 1 and 2 . 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one embodiment of the invention and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION 
     Referring now to the drawings, and more particularly to  FIGS. 1 and 2 , there is illustrated a vehicle  10  having a chassis  12  with a center of gravity  14  and a desired center of gravity  16 . Chassis  12  has ground engaging traction devices  18 , suspension elements  20  and actuators  22 . Sensors  24  are fixed about chassis  12  as well as on suspension elements  20  to determine the position, attitude and orientation of vehicle  10 . Controller  26  is connected to sensors  24  as well as actuators  22  and based upon the input received from sensors  24 , controller  26  processes the information and actuates actuators  22  to thereby carry out the method of the present invention. 
     An actuating element  28  may be connected to chassis  12  with an acquired load  30  being coupled thereto. This may be in the form of articulated arm or other device that is configured to engage load  30  and move load  30  relative to chassis  12  and/or the ground. For example, actuating element:  28  may be the arm of a backhoe or a forestry vehicle. Although  FIG. 1  illustrates a top view of vehicle  10 , it is also to be understood that center of gravity  14  and  16  is a three-dimensional calculated position relative to the support points of vehicle  10  on the terrain. The desired center of gravity  16  is illustrated as being in a different position than center of gravity  14  to illustrate the method carried out in controller  26 . The desired center of gravity  16  may be a predetermined center of gravity, or it may be a dynamically determined desired center of gravity  16 . For example, as load  30  is acquired the desired center of gravity  16  may shift away from load  30  to compensate for an additional potential load. In reaction to changes in the orientation of vehicle  10  and loads applied thereto, center of gravity  14  is calculated and the method of the present invention is carried out on controller  26  by the actuation of actuators  22  which affect suspension  20  so as to alter the calculated center of gravity  14  such that it substantially corresponds with the desired center of gravity  16 . 
     Now, additionally referring to  FIG. 3 , there is illustrated an embodiment of the present invention in the form of a method  100 , which includes steps  102  through  108 . At step  102  method  100  determines the desired center of gravity  16 , which may be predetermined or a dynamically determine position. At step  104 , method  100  detects the current center of gravity of vehicle  10 . Again this is a three-dimensional determination and the results are compared to the desired center of gravity  16  in the actions to be undertaken in step  106 . In step  106 , controller  26  computes what the needed movements of actuators  22  are in order to minimize the difference between desired center of gravity  16  and the calculated center of gravity  14 . At step  108 , method  100  directs actuators  22  to activate and interact with elements of suspension  20  in order to shift the center of gravity  14  toward the desired serum gravity  16 . 
     Method  100  illustrates an advanced suspension system with an active CG management component. Active CG management involves tilting the chassis relative to the surface on which it is rested to optimally position the CG with respect to the support points, which in this case are ground engaging traction devices  18 , which may be in the form of wheels, on the terrain. In simple cases, with a properly balanced vehicle, this is essentially a self-leveling scheme. In more complex cases, the vehicle is able to use its own weight to counterbalance the terrain changes and to offset loads that may be placed upon chassis  12 . 
     The suspension  20  is either semi-active or fully active with passive elements to support the base load. The linkages of suspension  20  and actuators  22  have sufficient travel to permit moderate degrees of CG management while still isolating vehicle  10  from terrain roughness. The present invention may shift between fully active suspension and semi-active suspension based on a trade-off between performance and energy requirements. A regenerative fully active system is contemplated with the regeneration occurring so that energy can be recaptured. Energy may be drawn from the power system, omitted for the sake of clarity, of vehicle  10  to provide power and to enhance mobility of vehicle  10 . Vehicle  10  maybe in an unmanned guided vehicle (UGV) that actively controls the center of gravity. 
     Active center of gravity management entails lateral and longitudinal leveling of chassis  12  by retracting high side suspension units  20  while simultaneously extending low side suspension units  20 . Controller  26  interacts with the plurality of ground engaging traction devices  18  so as to conform with terrain curvature in both directions while simultaneously equalizing wheel loads to the extent allowed by the actual loading of vehicle  10 . Since total suspension travel is finite, the maximum leveling results in a relatively rigid suspension while operating on level train allows all of the suspension travel to be used to isolate the movements of chassis  12  and the payload thereon from terrain-induced vibration. 
     When in fully active mode the suspension is most efficient when the dead load is supported by passive devices. The passive devices of the present invention allow for both static position adjustment and continuously variable spring rate. The advantages of these features are provided by the positioning of actuators  22  and may include passive damping in a semi-active system using a minimum frequency response to allow for active CG management and a fully active version. The frequency response of the fully active suspension is adequate for center of gravity management and allows the static suspension adjustments to be made with a low-frequency system thereby reduce total energy requirements. 
     In contrast to prior art systems, the present invention, particularly for an unmanned vehicle, allows for the removal of human operator considerations with a corresponding loosening of limitations in the control the center of gravity. The use of semi-autonomous and autonomous vehicles allow many of the vehicle states to be measured and implemented, to also be used to perform vehicle guidance control as well. Exploiting the autonomous nature of the present invention and the subsequent knowledge of the vehicle states, the operational envelope of the vehicle&#39;s capability are enhanced by using the nonlinear suspension controller of the present invention. Controller  26  has access to a memory device that contains constraints that are based on the material stress limits of vehicle  10  and the electronics reliability factors such as those that are related to vibration. 
     The inputs to controller  26  include feedback from sensors  24  which include load sensors on each axle, wheel slip sensors, a vehicle speed sensor, a roll sensor, a pitch sensor and a yaw sensor. The sensors provide angle, angular rates and acceleration information to controller  26 , with the information coming from an onboard inertial measurement unit (IMU). Additional inputs include vehicle vibration amplitude and frequency that is reported to controller  26  by way of sensors  24 . Based on the input information received by controller  26  an optimal CG location  16  is determined at step  102 , with kinematic and dynamic constraints, and the errors between the current center of gravity  14  and the desired center of gravity  16  provide inputs to the nonlinear suspension control system of the present invention. 
     Further, positional detecting lasers such as scanning 3D SICK lasers, provide terrain mapping and perform predictive control of the center of gravity position. This allows for predictive control of the center of gravity based upon anticipated changes in the terrain. 
     The nonlinear suspension system used by controller  26  may be in the form of an optimal Mamdani type fuzzy system with some modifications to the rule base and the defuzzification method. The present invention controls the fluid flow to actuators  22  and act as a rate controller to remove saturation nonlinearity. As a rate controller, method  100  does not require rate of error changes as inputs. As there is no previous memory required by the present invention each controlled actuation will be a discrete event based upon the current state of the vehicle, thereby precluding a prior state input. 
     Due to the large number of inputs to controller  26  and the need for a solution with a high degree of fidelity, an approach known as a fuzzy relations control strategy (FRCS) is employed to reduce the size of the rule base, while maintaining fidelity when beneficial. The FRCS method is a hierarchical method employed by the multi-objective nonlinear controller method implemented in controller  26 . Method  100  applies the control strategy in the following way: The linguistic variables for each membership function are termed the fuzzy relations control variables (FRCV). A multistage control strategy is defined using the fuzzy membership functions and their linguistic variables and is based on reducing the system error state or states to zero or as close to zero as possible. The related fuzzy relations control variables are used as classifiers within the rule-based structure, defining the relative importance of the other membership functions and maintaining the control strategy. Each fuzzy relations control variable has a corresponding degree of importance assigned thereto, based on the error state. 
     An issue avoided by the present invention is one that is reoccurring in prior art systems, relative to fuzzy controllers known as a “bang-bang” control effect. The definition of an acceptable region linguistic variables in all of the membership functions alleviates the bang-bang control effects as there is a specified state region on the system manifold where the vehicle state is acceptable (having reasonable error), with a corresponding crisp output of zero from controller  26 . Advantageously system response granularity is also reduced. 
     Whereas the system operates in real-time a center of maximum defuzzification method is utilized to provide crisp values to the valves associated with actuators  22 , to control the fluid flow to the suspension actuators  22 . Advantageously, the manipulating of the center of gravity placement allows for an operational envelope of the vehicle to be enhanced as well as extending the ability of the vehicle to operate in the most complex terrain. Movement of actuators  22  can also be utilized to regenerate energy for storage in either pressurized hydraulic system or the charging of batteries. 
     While this invention has been described with respect to at least one embodiment, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.