hydraulic servo and related systems
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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. - PowerPoint PPT PresentationTRANSCRIPT
Hydraulic Servo andRelated 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
RPM-Torque
Voltage-Current
orVelocity-Force
orVelocity-Force
Simple open-loop open-center circuitcylinder
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 betterin this case?
Open- orclosed center?
Closed-loop (hydrostatic) system
Variable displacement
reversible pump
Drain or auxiliary line
Check valveMotor
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
xwA
C
pACQ
o
d
od
area flow orifice
coef. discharge flow orifice
2
xwA
C
pACQ
o
d
od
area flow orifice
coef. discharge flow orifice
2
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
21 qq
xppCq
CxppCq s
022
11 constant a ,
2021 ppppps
2;
2 :so
:pressure Load
21
21
ppp
ppp
ppp
ss
xpp
CxppCq ss 211
p0 p0ps
p1, q1p2, q2
x
Dynamic Equations (cont.)
s)order term(high )()(
linearize order tofirst toseriesTaylor ain Expand
1111
pp
p
qxx
x
qqq
ppxx
ppxx
xpp
C
p
qppC
x
q
sppxx
s
ppxx
22
;2
:sderivative partial Taking
11
0;2
0at which ,0;0
commonly point, operating Choose
21
11
1
Kp
qK
pC
x
q
qpx
ppxx
s
ppxx
p0 p0ps
p1, q1p2, q2
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
yAxKpKxKq 1211
Change in volume
Change in density
y
Net area Ap
q1
Compressibility of Fluids and Elasticity of Walls
2Bulk modulus: =
/ /
p N p
V Vm
dp
VMd
dp
d )/(111
For the pure definition, the volume is fixed.
dtqV
dpdtqdM
;
More useful here is an effective bulk modulus that includes expansion of the walls and compression of entrained gasses
p
V
V
M
p
M
Vdp
VMd
eff2
11)/(11
Using this to solve for the change in pressure
dtqkdtMkdMV
dp eff
Choices for modeling the hydraulic actuator
With no compliance or compressibility we get actuator velocity v as
q
1/Av
With compliance and/or compressibility combined into a factor k
And with moving mass m
k dtq p
A /m
dv/dt
Manufacturer’s Data: BD15 Servovalve on HAL
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
sourcep
sourceQ
p
Q
loadp
loadQ
Pressure-Compensated Load-Sensing Circuit
Generated Power
Useful Power
sourcep
p
Q
loadp
load sourceQ Q
EnergySavings
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 loadwith 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.)
References1. Norvelle, F.D. Fluid Power Control Systems,
Prentice Hall, 2000.2. Fitch, E.C. and Hong I.T. Hydraulic Component
Design and Selection, BarDyne, Stillwater, OK, 2001.
3. Cundiff, J.S. Fluid Power Circuits and Controls, CRC Press, Boca Raton, FL, 2002.
4. Merritt, H.E. Hydraulic Control Systems, John Wiley and Sons, New York, 1967.
5. Fluid Power Design Engineers Handbook, Parker Hannifin Company (various editions).