Hydraulic Servo and Related Systems Chris Paredis / Wayne J. Book G.W. Woodruff School of Mechanical Engineering Georgia Institute of Technology

Lecture Overview Why fluid power? Basic fluid-power circuits Simple dynamic model Efficiency considerations Advanced metering methods

Hydraulics is Especially critical to the Mobile Equipment Industry

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 flexible 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

Difficulties with Fluid Power Possible leakage Noise generated by pumps and transmitted by lines Energy loss 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

Flow Press. System Overview The system consists 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 Load RPM Torque Flow Press. RPM-Torque Voltage- Current or Velocity- Force

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

Simple open-loop closed-center circuit Which is better in this case? Open- or closed center?

Closed-loop (hydrostatic) system Variable displacement reversible pump Drain or auxiliary line Check valve Motor

Axial Piston Pump

Proportional Valve

Basic Operation of the Servo Valve (single stage) Torque motor moves spool left Positive motor rotation Flow enters Flow exits

Basic Operation of the Servo Valve (single stage) Torque motor moves spool right Negative motor rotation Flow enters Flow exits

Orifice Model

4 Way Proportional Spool Valve Model Spool assumptions –No leakage, equal actuator areas –Sharp edged, steady flow –Opening area proportional to x –Symmetrical –Return pressure is zero –Zero overlap Fluid assumptions –Incompressible –Mass density  p0p0 p0p0 psps p 1, q 1 p 2, q 2 x

Dynamic Equations (cont.) p0p0 p0p0 psps p 1, q 1 p 2, q 2 x

Dynamic Equations: the Actuator If truly incompressible: Specification of flow without a response in pressure brings 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 Change in volume Change in density y Net area A p q1q1

Compressibility of Fluids and Elasticity of Walls For the pure definition, 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

Choices for modeling the hydraulic actuator With no compliance or compressibility we get actuator velocity v as q 1/A  v With compliance and/or compressibility combined into a factor k And with moving mass m k  dt qp A /m dv/dt

Manufacturer’s Data: BD15 Servovalve on HAL

Controls Issues: Summary Nonlinearities Good velocity control –Velocity approximately proportional to valve position –Bandwidth determined by compressibility and spool dynamics How about position control? How about force control?

Lecture Overview Why fluid power? Basic fluid-power circuits Simple dynamic model Efficiency considerations Advanced metering methods

Open-loop open-center circuit – Revisited Energy efficiency? Generated Power Useful Power p Q

Pressure-Compensated Load-Sensing Circuit Generated Power Useful Power p Q Energy Savings

Independent Metering: Introduction F Kat A B Ksa Kbt Ksb Tank Pump Check Valve x Independent Metering Configuration

Advantages of Independent Metering: Metering Modes Energy saving potential: Regenerative flow. F Kat A B Ksa Kbt Ksb Tank Pump Check Valve x Powered Extension Mode

F Kat A B Ksa Kbt Ksb Tank Pump Check Valve x High Side Regeneration Extension Advantages of Independent Metering: Metering Modes Energy saving potential: Regenerative flow.

F Kat A B Ksa Kbt Ksb Tank Pump Check Valve x Low Side Regeneration Extension Advantages of Independent Metering: Metering Modes Energy saving potential: Regenerative flow.

F Kat A B Ksa Kbt Ksb Tank Check Valve Pump x Powered Retraction Advantages of Independent Metering: Metering Modes Energy saving potential: Regenerative flow.

F Kat A B Ksa Kbt Ksb Tank Pump Check Valve Low Side Regeneration Retraction Advantages of Independent Metering: Metering Modes Energy saving potential: Regenerative flow.

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.

Summary Main advantage of fluid power: –Very high forces/torques at moderate speed –Very high power density at point of action Challenges: –Energy efficiency – hot research topic –Compactness (including prime mover) –User friendliness (leakage, noise, etc.)

References 1.Norvelle, F.D. Fluid Power Control Systems, Prentice Hall, Fitch, E.C. and Hong I.T. Hydraulic Component Design and Selection, BarDyne, Stillwater, OK, Cundiff, J.S. Fluid Power Circuits and Controls, CRC Press, Boca Raton, FL, Merritt, H.E. Hydraulic Control Systems, John Wiley and Sons, New York, Fluid Power Design Engineers Handbook, Parker Hannifin Company (various editions).