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Lesson31 Suspension Dynamics-II.ppt

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SUSPENSION DYNAMICS
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Automotive Vehicle Dynamics,Suspension Dynamics-IISean BrennanDept. of Mechanical and Nuclear EngineeringPenn State Universitysbrennan@psu.edu? 2002 PSU, 2000 UIUC BOT, All Rights Reserved,1/4 car vehicle with 2 Degrees of freedom,2 DOF State Space System,road displacement,unsprung mass displacement,sprung mass displacement,Displacements measured from equilibrium,2 DOF State Space System,The system can be represented in state space form using the following statesx1 = zs – zu suspension deflectionx2 = zs ?sprung mass velocity ? ? ?x3 = zu – zrtire deflectionx4 = zuunsprung mass velocity,?,?,The state equations are,2 DOF State Space System,The poles of the transfer function ? ? ? ? ? ? ? ? ? ? ? and the eigenvalues of the above A matrix are the same and will dictate the passive system response.,2 DOF State Space System,disturbance,,System Response,bodymode,tire mode“wheel hop.”,?1 HZ,?10 HZ,System Response,System Response,If we add an “active” element to the above system, it looks like:,,,,,,,,,,,,,,ms,,,,,,,,,,,mu,zs,ks,bs,kt,zu,zr,,,,,,,,F,,Active System Response,The equations of motion become:or,Active System Response,We notice something if we add the two equations of motion:Notice that this equation is independent of the suspension inputs (ks, bs, F)Taking the Laplace transform (selectively),Active System Response,At the natural frequency of the unsprung mass riding on the tires,Therefore, at the unsprung natural frequency, very little can be done to influence the effect of the tire hop on sprung mass acceleration.,Active System Response,Active System Layout,Det.Front actuatorAccumulatorControl ?PanelHydraulic pumpSteerangle transducerOiltankYaw gyroLong. lat. AccelerometerRear actuatorHub accelerometerComputer unitServovalve,Active System Layout,Active System Layout,plant,Controller,Determination of F,In linear control theory, there are two main divisions ‘Classical and Modern’Classical: ?Transfer function based,,,,,,,,,,,+,-,reference,error,input,output,Determination of F,The most popular type of classical controller is the PID controller.Modern: ?State space based,,,,,,,,,Linear plant,Feedback ?matrix,states,input,Here we assume we know all the states.Choose K to place the eigenvalues of (A – BK) to give you a desired response. (Note: this is for regulation; for tracking redefine states),Determination of F,Determination of F,Therefore, the actual force put out by the active strut is a linear combination of the suspension system states.You can get most of the benefits just by feeding back sprung mass velocity.,skyhook damping,Skyhook Damping,Inertial damper,,,,,,,,,,,,,,ms,,,,,,,,,,,mu,zs,ks,bs,kt,zu,zr,,,,,F,,,,,,,,Active Performance Benefits,Ref. Butsuen & Hedrick,“Alternative Control Laws for Automotive Active Suspension,”ASME JDSMC, Vol. 111, pp. 286.,Lower accelerations with active…in the low frequency range,Active Performance Benefits,Ref. Butsuen & Hedrick,“Alternative Control Laws for Automotive Active Suspension,”ASME JDSMC, Vol. 111, pp. 286.,Larger strokes to compensate for disturbances in the low frequency range,Active Performance Benefits,Ref. Butsuen & Hedrick,“Alternative Control Laws for Automotive Active Suspension,”ASME JDSMC, Vol. 111, pp. 286.,Semi-Active Suspension Control,Problems with active suspensions that have limited its industrial use are:- ?cost- ?complexity- ?weight- ?powerAn alternative approach is semi-active suspension control. ?This is basically a variable rate of passive damping.,Semi-Active Suspension Control,Usually achieved by a variable orifice damper.Initial attempts used two state dampers.More refined attempts used multi-state dampers as well as continuously variable dampers.,Semi-Active Suspension Control,These systems can dissipate energy but cannot supply energy the way an active system can.For an active system:,,,,,,PowerRequired,PowerRequired,PowerDissipated,PowerDissipated,Force,Suspension ?velocity,Semi-Active Suspension Control,For a corresponding semi-active system:,,,,Force,Suspension ?velocity,,,,,Semi-Active Suspension Control,The skyhook damper objective is the same as an active system. ?When a dissipation of power is required, the semi-active provides the same force as an active system. When a generation of power is required, the semi-active damping becomes as low as possible.,Semi Active Suspension Control,For the purpose of ride quality, the semi-active performs nearly as well as the active system. ?However, For handling it does not perform as well.In any case, the industry has moved to semi-active for actual implementation.,ME 452Automotive Vehicle Dynamics,Produced by theCollege of EngineeringPenn State University? 2004 PSU, 2000 UIUC BOT, All Rights Reserved,

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