Hydraulic Control Systems, Merritt

Hydraulic Control Systems, Merritt

(Parte 1 de 6)

Hydraulic Control Systems Herbert Merritt

Herbert E. Merritt

Section Head Hydraulic Components Section

Product Development Department Cincinnati Milling Machine Company

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JOHN WILEY & SONS, New York • Chichester • Brisbane •Toronto • Singapore

A NOfTE TO TOE READER: Thii book has been electronically teproduced from digiiil inforaiMion itoted at John Wiley it Sou. Inc. We are pleated that the of ihii new technolofy will enable in to keep worki of endnring Kholarty value in print u long a* there i a reatooable demand for them. The content of this book is identicai to previous printings.

Copyright © 1967 by John Wtley & Sons, Iik.

All Rights Reserved

Reproduction or translation of any part of tNs work beyond that permitted by Sections 10)7' or 108 of the 1976 United States Copyright Act without the permission of the copyright owner is unlawful. Requests for permission or further information should t>e addressed to> the Permissions Department, John Wiley & Sons, Inc.

Library of Congress Catalog Card Number: M-28759

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 War I. 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 servo- valves 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 judgments 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 of the working fluid are discussed in Chapter 2. Fluid flow through various passages and basic hydraulic equations are covered in Chapter 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 Chapter 5. The combination formed by a valve or pump 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 of torque motors.

The remaining five chapters treat systems oriented topics. Chapter 8 covers the major 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 Chapter 10 discusses the efl’ect of these on performance. Practical suggestions concerning testing and limit cycle oscillation problems are also given. Chapter 1 covers some common control valves useful in power generation, and Chapter 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. For 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 cncour- agment. Herbert. E. Merritt

Cincinnati, Ohio December 1966

Contents

1 INTRODUCTION 1

1-1 Advantages and Disadvantages of Hydraulic Control 1 1-2 Genera! Comments on Design 3

2 HYDRAULIC FLUIDS 6

2-1 Density and Related Quantities 6 2-2 Equation of State for a Liquid 7 2-3 Viscosity and Related Quantities 9 2-4 Thermal Properties 13

2-5 Effective Bulk Modulus 14 2-6 Chemical and Related Properties 18

2-7 Types of Hydraulic Fluids 20 2-8 Selection of the Hydraulic Fluid 23

3 FLUID FLOW FUNDAMENTALS 25

3-1 General Equations 25 3-2 Types of Fluid Flow 29 3-3 Flow Through Conduit? 30

3-4 Flow Through Orifices 39 3-5 Minor Losses 46

3-6 Power Loss and Temperature Rise 48 3-7 Pressure Transients in Hydraulic Conduits 49 3-8 Summary 52

4 HYDRAULIC PUMPS AND MOTORS 54

4-1 Basic Types and Constructions 54 4-2 Ideal Pump and Motor Analysis 64 4-3 Practical Pump and Motor Analysis 65 4-4 Performance Curves and Parameters 72 vii

5 HYDRAULIC CONTROL VALVES 76

5-1 Valve Configurations 76

5-2 General Valve Analysis 79 5-3 Critical Center Spool Valve Analysis 84 5-4 Open Center Spool Valve Analysis 94

5-5 Three-Way Spool Valve Analysis 9 5-6 Flow Forces on Spool Valves 301 5-7 Lateral Forces on Spool Valves 108 5-8 Spool Valve Design 112 5-9 Flapper Valve Analysis and Design 118

6 HYDRAULIC POWER ELEMENTS 132

6-1 Valve Controlled Motor 133 6-2 Valve Controlled Piston 545 6-3 Three-Way Valve Controlled Piston 150 6-4 Pump Controlled Motor 152 6-5 Valve Controlled Motor with Load Having Many

Degrees of Freedom 157 6-6 Pressure Transients in Power Elements 162 6-7 Nonlinear Analysis of Valve Controlled Actuators 170

7 ELECTROHYDRAULIC SERVOVALVES 174

7-1 Types of Electrohydraulic Servovalves 175

7-2 Permanent Magnet Torque Motors 177 7-3 Single-Stage Electrohydraulic Servovalves 193 7-4 Two-Stage Electrohydraulic Servovalve with

Direct Feedback 202 7-5 Two-Stage Electrohydraulic Servovalve with

Force Feedback 212 7-6 Specification, Selection, and Use of Servovalves 217

8 ELECTROHYDRAULIC SERVOMECHANISMS 224

8-1 Supply Pressure and Power Element Selection 225 8-2 Electrohydraulic Position Control Servos 234 8-3 Lag Compensated Electrohydraulic Position

Control Servos 246

8-4 Electrohydraulic Velocity Control Servos 258 8-5 Servo Design Considerations 261

Vlll CONTENTS

10 NONLINEARITIES IN CONTROL SYSTEMS 271 lO-l Typical Nonlinear Phenomena and Input-Output

Characteristics 272 10-2 Describing Function Analysis 273 10-3 Saturation 277 10-4 Deadband 280 10-5 Nonlinear Gain Characteristics 282

10-6 Backlash and Hysteresis 285 10-7 Relay Type Nonlinearities 290 10-8 Friction Nonlinearities 294 10-9 Use of Describing Function Concept in Sinusoidal

Testing 310 10-10 Troubleshooting Limit Cycle Oscillations 312

1 PRESSURE AND FLOW CONTROL VALVES 319

1-1 Functional Classification of Valves 319 1-2 Single-Stage Pressure Control Valves 321 1-3 Two-Stage Pressure Control Valves 331 1-4 Flow Control Valves 332

12 HYDRAULIC POWER SUPPLIES 334

12-1 Basic Configurations of Hydraulic Power Supplies 335 12-2 Bypass Regulated Hydraulic Power Supplies 337 12-3 Stroke Regulated Hydraulic Power Supplies 339 12-4 Interaction of Hydraulic Power Supply and Servo Loop 341

12-5 Reservoirs of Hydraulic Systems 343 12-6 Heat Generation and Dissipation in Hydraulic Systems 344 12-7 Contamination and Filtration 348

CONTENTS IX 9 HYDROMECHANICAL SERVOMECHANISMS 266

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 of 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 component 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 motor 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 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<er speed ranges are possible with hydraulic actuators. Both linear and rotairy actuators are available and add to the flexibility of hydraulic pow(er elements.

6. Hydraulic actuators have higher stifl'ness, that is, inverse of slope <of speed-torque curves, compared to other drive devices since leakages aire low. Hence there is little drop in speed as loads are applied. In close;d loop systems this results in greater positional stifl'ness and less position error.

7. Open and closed loop control of hydraulic actuators is relatively simple using valves and pumps. 8. Other aspects compare less favorably with those of eIectromechanic:al control components but are not so serious that they deter wide use anid acceptance of hydraulic control. The transmission of power is moderatoly easy with hydraulic lines. Energy storage is relatively simple wiith accumulators.

Although hydraulic controls ofl'er many distinct advantages, several disadvantages tend to limit their use. Major disadvantages are thie following:

1. Hydraulic power is not so readily available as that of electrical powe:r

This is not a serious threat to mobile and airborne applications but moist certainly afi'ects stationary applications. 2. Small allowable tolerances results in high costs of hydraulic comi- ponents. 3. The hydraulic fluid imposes an upper temperature limit. Fire am<i explosion hazards exist if a hydraulic system is used near a source of ignii- tion. However, these situations have improved with the availability (of high temperature and fire resistant fluids. Hydraulic systems are messs) because it is diflicult to maintain a system free from leaks, and there 1»

2 INTRODUCTION always the possibility of complete loss of fluid if a break in the system occurs. 4. It is impossible to maintain the fluid free of dirt and contamination.

Contaminated oil can clog valves and actuators and, if the contaminant is abrasive, cause a permanent loss in performance and/or failure. Con taminated oil is the chief source of hydraulic control failures. Clean oil and reliability are synonymous terms in hydraulic control. 5. Basic design procedures are lacking and difficult to obtain because of the complexity of hydraulic control analysis. For example, the current flow through a resistor is described by a simple law—Ohm’s law. In contrast, no single law exists which describes the hydraulic resistance of passages to flow. For this seemingly simple problem there are almost endless details of Reynolds number, laminar or turbulent flow, passage geometry, friction factors, and discharge coeflicients to cope with. This factor limits the degree of sophistication of hydraulic control devices. 6. Hydraulics are not so flexible, linear, accurate, and inexpensive as electronic and/or electromechanical devices in the manipulation of low power signals for purposes of mathematical computation, error detection, amplification, instrumentation, and compensation. Therefore, hydraulic devices are generally not desirable in the low power portions of control systems.

The outstanding characteristics of hydraulic power elements have com bined with their comparative inflexibility at low power levels to make hydraulic controls attractive primarily in power portions of circuits and systems. The low power portions of systems are usually accomplished by mechanical and/or electromechanical means.

1-2 GENERAL COMMENTS ON DESIGN

The term “design” has a broad meaning. It is often associated with the creativity required to produce sketches and rough layouts of possible mechanisms that will accomplish an objective. As a second meaning, it is sometimes associated with the engineering calculations and analyses necessary in the selection and sizing of hardware to form a component or system. Design is also associated with the many details of material selec tion, minor calculations, and making of complete engineering drawings. This book is directed toward the analysis and design (by paper and pencil) of control systems whose power elements are hydraulic. The term design is used in the sense of specifying proper size. Although considerations such as material, stress level, and seals are equally important to a finished device, they do not relate directly to the dynamic performance of a system and are treated with more authority elsewhere.

(Parte 1 de 6)

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