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
VoltageCurrent
System Overview
Electric RPM
Trans- Flow
Flow
or IC
mission
Motor or
Pump
prime
line &
cylinder
Press.
mover Torque
valve Press.
RPM-Torque
or
VelocityForce
Coupling
mechanism
• 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
RPMTorque
Load
or
VelocityForce
Simple open-loop open-center circuit
cylinder
Actuating
solenoid
Pressure relief
valve
Spring return
4-way, 3 position
valve
filter
Fluid tank or reservoir
Fixed displacement
pump
Simple open-loop closed-center circuit
Which is better
in this case?
Open- or
closed center?
Closed-loop (hydrostatic) system
Motor
Check valve
Variable
displacement
reversible pump
Drain or auxiliary line
Axial Piston Pump
Proportional Valve
Basic Operation of the Servo Valve
(single stage)
Flow
enters
Torque motor
moves spool
left
Flow
exits
Positive motor
rotation
Basic Operation of the Servo Valve
(single stage)
Flow
enters
Flow
exits
Torque motor
moves spool
right
Negative motor
rotation
Orifice Model
Q  Cd Ao
Q  Cd Ao
2

p

p
Cd  orificeflow discharge coef.
Ao  orificeflow area  w x
Cd  orificeflow discharge coef.
Ao  orificeflow area  w x
2
4 Way Proportional Spool Valve Model
• Spool assumptions
q1  q2
– No leakage,
q1  C ps  p1 x, C  a const ant
equal actuator areas
q2  C p2  p0 x
– Sharp edged, steady flow
ps  p1  p2  p0  p2
– Opening area proportional to x
Load pressure: p  p1  p2
– Symmetrical
p  p
p  p
– Return pressure is zero
so : p1  s
; p2  s
2
2
– Zero overlap
ps  p
x
ps
p0 q1  C ps  p1 x  C
• Fluid assumptions p0
2
– Incompressible
x
– Mass density 
p2, q2
p1, q1
Dynamic Equations (cont.)
Expandin a T aylorseries to first order tolinearize
 q
q1  q1   1
 x
 q


 (p  p )   (high order terms)
( x  x )   1
x x
 p x  x 

p  p 
p  p 

T akingpartialderivatives :
q1
x
x x
p  p
ps  p
C
;
2
q1
p
x x
p  p
C

x
2 2 ps  p
p0
Chooseoperatingpoint,commonly
ps
p0
x  0; p  0, at whichq1  0
q1
x
x x
p  p
ps
q1
C
 K1 ;
2
p
x x
p  p
x
 K2  0
p2, q2
p1, q1
Dynamic Equations: the Actuator
q1
Change in
volume
Change
in density
y
q1  K1 x  K2 p  K1 x  Ay
Net area Ap
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
Compressibility of Fluids and Elasticity of Walls
p
p
N
Bulk modulus:  
=
2

 /   m  V / V
1 1 d 1 d ( M / V )


  dp 
dp
For the pure definition, the volume is fixed.
  
q dt
dM  q dt; dp  
 V 
More useful here is an effective bulk modulus that includes
expansion of the walls and compression of entrained gasses
1
 eff

1 d (M / V ) 1  1 M M V 
 
 2

dp
 V p V p 
Using this to solve for the change in pressure
  eff
dp  
 V

dM  k M dt  k q dt

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
q
k
 dt
p
dv/dt
A
/m
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?
p
psource
pload
Qload
Qsource
Generated Power
Useful Power
Q
Pressure-Compensated Load-Sensing Circuit
p
Energy
Savings
psource
pload
Qload  Qsource
Generated Power
Useful Power
Q
Independent Metering: Introduction
Independent Metering Configuration
Pump
F
x
Ksb
Ksa
B
A
Kbt
Kat
Tank
Check Valve
Advantages of Independent
Metering: Metering Modes
• Energy saving potential: Regenerative flow.
Pump
F
x
Ksb
Ksa
B
A
Kbt
Kat
Tank
Check Valve
Powered Extension Mode
Advantages of Independent
Metering: Metering Modes
• Energy saving potential: Regenerative flow.
Pump
F
x
Ksb
Ksa
B
A
Kbt
Kat
Tank
Check Valve
High Side Regeneration Extension
Advantages of Independent
Metering: Metering Modes
• Energy saving potential: Regenerative flow.
Pump
F
x
Ksb
Ksa
B
A
Kbt
Kat
Tank
Check Valve
Low Side Regeneration Extension
Advantages of Independent
Metering: Metering Modes
• Energy saving potential: Regenerative flow.
Pump
F
x
Ksb
Ksa
B
A
Kbt
Kat
Tank
Check Valve
Powered Retraction
Advantages of Independent
Metering: Metering Modes
• Energy saving potential: Regenerative flow.
Pump
F
Ksb
Ksa
B
A
Kbt
Kat
Tank
Check Valve
Low Side Regeneration Retraction
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, 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).