Hydraulic Control Systems
Herbert Merritt
HYDRAULIC CONTROL
SYSTEMS
Herbert E. Merritt
Section Head
Hydraulic Components Section
Product Development Department
Cincinnati Milling Machine Company
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Copyright © 1967 by John Wtley & Sons, Iik .
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Preface
Although hydraulic control dates from the water regulating devices of
ancient times, the branch of this field concerning the hydraulic control of
machinery has made the greatest progress in this century, particularly
since World W ar II. The growth of hydraulic control has paralleled
developments in transportation, farm and earth moving equipment,
industrial machinery, machine tools, ship control, fire control, aircraft,
missiles, and numerous other applications. Government and industry
supported research at several universities—the Dynamic Analysis and
Controls Laboratory at the Massachussetts Institute of Technology is
especially noteworthy—has accelerated hydraulic control technology.
Increased usage of hydraulic control has brought demands for rational
design techniques to replace effective but costly and time-consuming
cut-and-try procedures and for a classification of the knowledge for
instruction.
This book should be useful to both practicing engineers and students
and is at a level attained after a basic college course in feedback control
theory. Its purpose is to present a rational and well-balanced treatment
of hydraulic control components and systems. A course in fluid mechanics
would be helpful but not essential. The book is particularly well suited
as a text for a college-level course in hydraulic control. Selected topics
could be used to supplement feedback control theory courses with some
instruction on components.
The analyses of many hydraulic components—electrohydraulic servovalves in particular—are involved and tedious. However, in every case I
have tried to wring conclusive design relations from these analyses rather
than leave a mess of equations for the reader to untangle. This has
sometimes necessitated making judgm ents and rules of thumb with which
the reader may not agree.
The arrangement of the book follows in a fairly logical sequence.
After some introductory remarks in Chapter 1, the physical and chemical
properties o f the working fluid are discussed in Chapter 2. Fluid flow
VI
PRE FACE
through various passages and basic hydraulic equations are covered in
C hapter 3. Hence these first chapters are basically a review of applicable
topics in fluid mechanics.
The next four chapters are devoted to components encountered in
hydraulic servo controlled systems. The characteristics of hydraulic
actuators are discussed in Chapter 4. Hydraulic control valves, chiefly
spool and flapper types, are covered in C hapter 5. The combination formed
by a valve or pum p controlling an actuator is the basic power element in
hydraulic control servos, and the various combinations are discussed
quite thoroughly in Chapter 6. Chapter 7 is devoted to the principal types
of electrohydraulic servovalve and includes a static and dynamic analysis
o f torque motors.
The remaining five chapters treat systems oriented topics. C hapter 8
covers the m ajor types of electrohydraulic servo. Hydromechanical
servos are touched briefly in Chapter 9 because many comments in the
previous chapter are applicable. Systems often perform somewhat
differently than anticipated because of nonlinearities, and C hapter 10
discusses the efl’ect of these on performance. Practical suggestions
concerning testing and limit cycle oscillation problems are also given.
C hapter 11 covers some common control valves useful in power generation,
and C hapter 12 treats hydraulic power supplies and their interaction with
the control.
Material for this book was taken from a set of notes used to teach a
course in hydraulic control to engineers in industry. Much new informa
tion has been included, and I have tried to improve older treatments.
Experience and the available literature also were sources. F or the latter,
I am indebted to the many original contributors, too numerous to
mention.
I am particularly grateful to my good friend Mr. George L. Stocking of
the General Electric Company for contributions to Sections 5-6 and 5-7.
Finally, I would like to express appreciation to my fellow associates
at the “ Mill,” especially to Mr. James T. Gavin, for their help and cncouragment.
H e r b e r t . E. M e r r i t t
Cincinnati, Ohio
December 1966
Contents
1
2
INTRODUCTION
1
1-1
1-2
1
3
HYDRAULIC FLUIDS
2-1
2-2
2-3
2-4
2-5
2-6
2-7
2-8
3
6
Density and Related Quantities
Equation of State for a Liquid
Viscosity and Related Quantities
Thermal Properties
Effective Bulk Modulus
Chemical and Related Properties
Types of Hydraulic Fluids
Selection of the Hydraulic Fluid
FLUID FLOW FUNDAMENTALS
3-1
3-2
3-3
3-4
3-5
3-6
3-7
3-8
4
Advantages and Disadvantages of Hydraulic C ontrol
Genera! Comments on Design
General Equations
Types of Fluid Flow
Flow Through Conduit?
Flow Through Orifices
Minor Losses
Power Loss and Temperature Rise
Pressure Transients in Hydraulic Conduits
Summary
HYDRAULIC PUM PS AND M OTORS
4-1
4-2
4-3
4-4
Basic Types and Constructions
Ideal Pump and M otor Analysis
Practical Pump and M otor Analysis
Performance Curves and Parameters
vii
6
7
9
13
14
18
20
23
25
25
29
30
39
46
48
49
52
54
54
64
65
72
Vlll
5
CONTENTS
HYDRAULIC CONTROL VALVES
5-1
5-2
5-3
5-4
5-5
5-6
5-7
5-8
5-9
6
132
6-1
6-2
6-3
6-4
6-5
133
545
150
152
6-7
Valve Controlled M otor
Valve Controlled Piston
Three-Way Valve Controlled Piston
Pump Controlled M otor
Valve Controlled M otor with Load Having Many
Degrees of Freedom
Pressure Transients in Power Elements
Nonlinear Analysis of Valve Controlled Actuators
157
162
170
ELECTROHYDRAULIC SERVOVALVES
174
7-1
7-2
7-3
7-4
175
177
193
7-5
7-6
8
76
79
84
94
99
301
108
112
118
HYDRAULIC POW ER ELEM ENTS
6-6
7
Valve Configurations
General Valve Analysis
Critical Center Spool Valve Analysis
Open Center Spool Valve Analysis
Three-Way Spool Valve Analysis
Flow Forces on Spool Valves
Lateral Forces on Spool Valves
Spool Valve Design
Flapper Valve Analysis and Design
76
Types of Electrohydraulic Servovalves
Permanent Magnet Torque M otors
Single-Stage Electrohydraulic Servovalves
Two-Stage Electrohydraulic Servovalve with
Direct Feedback
Two-Stage Electrohydraulic Servovalve with
Force Feedback
Specification, Selection, and Use of Servovalves
202
212
217
ELECTROHYDRAULIC SER VO M EC H ANISM S
224
8-1
8-2
8-3
225
234
Supply Pressure and Power Element Selection
Electrohydraulic Position C ontrol Servos
Lag Compensated Electrohydraulic Position
C ontrol Servos
8-4 Electrohydraulic Velocity Control Servos
8-5 Servo Design Considerations
246
258
261
CONTENTS
IX
9
HYDROM ECHANICAL SERVOM ECHANISMS
266
10
NONLINEARITIES IN CONTROL SYSTEMS
271
lO-l
Typical Nonlinear Phenomena and Input-Output
Characteristics
10-2 Describing Function Analysis
10-3 Saturation
10-4 Deadband
10-5 Nonlinear Gain Characteristics
10-6 Backlash and Hysteresis
10-7 Relay Type Nonlinearities
10-8 Friction Nonlinearities
10-9 Use of Describing Function Concept in Sinusoidal
Testing
10-10 Troubleshooting Limit Cycle Oscillations
11
12
272
273
277
280
282
285
290
294
310
312
PRESSU RE AND FLOW CONTROL VALVES
319
11-1
11-2
11-3
11-4
319
321
331
332
Functional Classification of Valves
Single-Stage Pressure Control Valves
Two-Stage Pressure Control Valves
Flow Control Valves
HYDRAULIC POW ER SUPPLIES
334
12-1
12-2
12-3
12-4
335
337
339
12-5
12-6
12-7
In dex
Basic Configurations of Hydraulic Power Supplies
Bypass Regulated Hydraulic Power Supplies
Stroke Regulated Hydraulic Power Supplies
Interaction of Hydraulic Power Supply and
Servo Loop
Reservoirs of Hydraulic Systems
Heat Generation and Dissipation in Hydraulic Systems
Contamination and Filtration
341
343
344
348
355
Introduction
The increasing amount of power available to man that requires control
and the stringent demands of modern control systems have focused
attention on the theory, design, and application of control systems.
Hydraulics—the science of liquid flow—is a very old discipline which has
commanded new interest in recent years, especially in the area of hydraulic
control, and fills a substantial portion of the field of control. Hydraulic
control components and systems are found in many mobile, airborne, and
stationary applications.
1-1 ADVANTAGES AND DISADVANTAGES OF HYDRAULIC
CONTROL
There are many unique features of hydraulic control compared to other
types o f control. These are fundamental and account for the wide use of
hydraulic control. Some of the advantages are the following:
1. Heat generated by internal losses is a basic limitation of any machine.
Lubricants deteriorate, mechanical parts seize, and insulation breaks down
as temperature increases. Hydraulic components are superior to others in
this respect since the fluid carries away the heat generated to a convenient
heat exchanger. This feature permits smaller and lighter components.
Hydraulic pumps and motors are currently available with horsepower to
weight ratios greater than 2 hp/lb. Small compact systems are attractive
in mobile and airborne installations.
2. The hydraulic fluid also acts as a lubricant and makes possible long
com ponent life.
3. There is no phenomenon in hydraulic components comparable to the
saturation and losses in magnetic materials of electrical machines. The
torque developed by an electric m otor is proportional to current and is
limited by magnetic saturation. The torque developed by hydraulic
actuators (i.e., motors and pistons) is proportional to pressure difference
2
INTRODUCTION
and is limited only by safe stress levels. Therefore hydraulic actuators
develop relatively large torques for comparatively small devices.
4. Electrical motors are basically a simple lag device from applied
voltage to speed. Hydraulic actuators are basically a quadratic reson.amcc
from flow to speed with a high natural frequency. Therefore hydrauliic
actuators have a higher speed of response with fast starts, stops, and spee:d
reversals possible. Torque to inertia ratios are large with resulting high
acceleration capability. On the whole, higher loop gains and bandwudths
are possible with hydraulic actuators in servo loops.
5. Hydraulic actuators may be operated under continuous, intermit tenit,
reversing, and stalled conditions without damage. With relief valwe
protection, hydraulic actuators may be used for dynamic breaking. Larg
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