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1、1,Hydraulic Servo andRelated Systems,Chris Paredis / Wayne J. Book G.W. Woodruff School of Mechanical Engineering Georgia Institute of Technology,2,Lecture Overview,Why fluid power? Basic fluid-power circuits Simple dynamic model Efficiency considerations Advanced metering methods,3,Hydraulics is Es
2、pecially critical to the Mobile Equipment Industry,4,The Strengths of Fluid Power(Hydraulic, to a lesser extent pneumatic),High force at moderate speed High power density at point of action Fluid removes waste heat Prime mover is removed from point of action Conditioned power can be routed in flexib
3、le a fashion Potentially “Stiff” position control Controllable either electrically or manually Resulting high bandwidth motion control at high forces NO SUBSTITUTE FOR MANY HEAVY APPLICATIONS,5,Difficulties with Fluid Power,Possible leakage Noise generated by pumps and transmitted by lines Energy lo
4、ss due to fluid flows Expensive in some applications Susceptibility of working fluid to contamination Lack of understanding of recently graduated practicing engineers Multidisciplinary Cost of laboratories Displaced in curriculum by more recent technologies,6,Flow Press.,System Overview,The system c
5、onsists of a series of transformation of power variables Power is either converted to another useful form or waste heat Impedance is modified(unit force/unit flow) Power is controlled Function is achieved,Electric or IC prime mover,Pump,Trans-mission line & valve,Motor or cylinder,Coupling mechanism
6、,Load,RPM Torque,Flow Press.,RPM-Torque,RPM-Torque,Voltage-Current,orVelocity-Force,orVelocity-Force,7,Simple open-loop open-center circuit,cylinder,4-way, 3 position valve,Pressure relief valve,Fixed displacement pump,filter,Fluid tank or reservoir,Actuating solenoid,Spring return,8,Simple open-loo
7、p closed-center circuit,Which is betterin this case? Open- orclosed center?,9,Closed-loop (hydrostatic) system,Variable displacement reversible pump,Drain or auxiliary line,Check valve,Motor,10,Axial Piston Pump,11,Proportional Valve,12,Basic Operation of the Servo Valve(single stage),Torque motor m
8、oves spool left,Positive motor rotation,Flow enters,Flow exits,13,Basic Operation of the Servo Valve(single stage),Torque motor moves spool right,Negative motor rotation,Flow enters,Flow exits,14,Orifice Model,15,4 Way Proportional Spool Valve Model,Spool assumptions No leakage,equal actuator areas
9、Sharp edged, steady flow Opening area proportional to x Symmetrical Return pressure is zero Zero overlap Fluid assumptions Incompressible Mass density ,16,Dynamic Equations (cont.),17,Dynamic Equations: the Actuator,If truly incompressible: Specification of flow without a response in pressure brings
10、 a causality problem For example, if the piston has mass, and flow can change instantaneously, infinite force is required for infinite acceleration Need to account for change of density and compliance of walls of cylinder and tubes,18,Compressibility of Fluids and Elasticity of Walls,For the pure de
11、finition, the volume is fixed.,More useful here is an effective bulk modulus that includes expansion of the walls and compression of entrained gasses,Using this to solve for the change in pressure,19,Choices for modeling the hydraulic actuator,With no compliance or compressibility we get actuator ve
12、locity v as,With compliance and/or compressibility combined into a factor k And with moving mass m,20,Manufacturers Data: BD15 Servovalve on HAL,21,Manufacturers Data: BD15 Servovalve on HAL,22,Controls Issues: Summary,Nonlinearities Good velocity control Velocity approximately proportional to valve
13、 position Bandwidth determined by compressibility and spool dynamics How about position control? How about force control?,23,Lecture Overview,Why fluid power? Basic fluid-power circuits Simple dynamic model Efficiency considerations Advanced metering methods,24,Open-loop open-center circuit Revisite
14、d,Energy efficiency?,25,Pressure-Compensated Load-Sensing Circuit,Generated Power,Useful Power,p,Q,EnergySavings,26,Independent Metering: Introduction,Independent Metering Configuration,27,Advantages of Independent Metering: Metering Modes,Energy saving potential: Regenerative flow.,28,Advantages of
15、 Independent Metering: Metering Modes,Energy saving potential: Regenerative flow.,29,Advantages of Independent Metering: Metering Modes,Energy saving potential: Regenerative flow.,30,Advantages of Independent Metering: Metering Modes,Energy saving potential: Regenerative flow.,31,Advantages of Indep
16、endent Metering: Metering Modes,Energy saving potential: Regenerative flow.,32,Advantages of Independent Metering: Metering Modes,Energy saving potential: Regenerative flow.,Regeneration flow can be defined as pumping the fluid from one chamber to the other to achieve motion control of the load with using no or minimum flow from the pump.,33,Summary,Main advantage of fluid power: Very high forces/torques at moderate speed Very high power density at point of ac
