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In this second edition of Industrial Servo Control Systems, the chapters have been updated and expanded and a new chapter has been added on servo compensating techniques.. Discussion of

Marcel Dekker, Inc New York•Basel

INDUSTRIAL SERVO CONTROL SYSTEMS

Fundamentals and

Applications

GEORGE W YOUNKIN

Industrial Controls Consulting, Inc.

Fond du Lac, Wisconsin, U.S.A.

Second Edition, Revised and Expanded

Copyright 2003 by Marcel Dekker, Inc All Rights Reserved

Library of Congress Cataloging-in-Publication Data

A catalog record for this book is available from the Library of Congress

ISBN 0-8247-0836-9 This book is printed on acid-free paper

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Current printing (last digit):

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PRINTED IN THE UNITED STATES OF AMERICA

FLUID POWER AND CONTROL

A Series of Textbooks and Reference Books

FOUNDING EDITOR

Frank Yeaple

President TEF Engineering Allendale, New Jersey

1 Hydraulic Pumps and Motors: Selection and Application for Hydraulic Power

Control Systems, Raymond P Lambeck

2 Designing Pneumatic Control Circuits: Efficient Techniques for Practical

Application, Bruce E McCord

3 Fluid Power Troubleshooting, Anton H Hehn

4 Hydraulic Valves and Controls: Selection and Application, John J Pippenger

5 Fluid Power Design Handbook, Frank Yeaple

6 Industrial Pneumatic Control, Z J Lansky and Lawrence F Schrader, Jr.

7 Controlling Electrohydraulic Systems, Wayne Anderson

8 Noise Control of Hydraulic Machinery, Stan Skaistis

9 Interfacing Microprocessors in Hydraulic Systems, Alan Kleman

10 Fluid Power Design Handbook: Second Edition, Revised and Expanded, Frank

Yeaple

11 Fluid Power Troubleshooting: Second Edition, Revised and Expanded, Anton H.

Hehn

12 Fluid Power Design Handbook: Third Edition, Revised and Expanded, Frank

Yeaple

13 Industrial Servo Control Systems: Fundamentals and Applications, George W.

Younkin

14 Fluid Power Maintenance Basics and Troubleshooting, Richard J Mitchell and

John J Pippenger

ADDITIONAL VOLUMES IN PREPARATION

In this second edition of Industrial Servo Control Systems, the chapters have been updated and expanded and a new chapter has been added on servo compensating techniques The book continues to be dedicated to the practicing engineer making the transition from academic theory to the real-world solution of engineering problems related to the application of servo drives to industrial machines

Part I focuses on the evolution and classification of servos, with descriptions of servo drive actuators, amplifiers, feedback transducers, performance, and troubleshooting techniques Part II discusses system analogs and vectors, with a review of differential equations The concept of transfer functions for the representation of differential equations is discussed in preparation for block diagram concepts Discussion of the mathematical equations for electric servo motors has been expanded to include both DC and brushless DC servo motors The equations for mechanical and electrical time constants are derived with additional analysis

on the effects of temperature on these time constants

The representation of servo drive components by their transfer function is followed by the combination of these servo drive building blocks into system block diagrams Frequency response techniques are introduced because of their usefulness in determining proper servo compensation Also included are practical formulas for calculating inertia and examples that show how machine components reflect inertia to the

servo drive motor Torque-to-inertia ratios for electric servomotors are discussed along with calculations of the reflected inertias for various machine configurations These reflected inertias are critical when sizing the drive and motor and when determining time constants for a stability analysis Servo drive analysis and servo compensation techniques include the use of lag and lead compensation for analog-type servo drive amplifiers This discussion is followed by a description of proportional, integral, and differential compensation, which is very popular with digital-type amplifiers

To classify practical servo performance criteria, the use of servo indexes of performance is discussed for both electrical and hydraulic servo drives The chapter on servo plant compensation techniques addresses the continuing problems of structural dynamics and resonances in industrial machinery This type of compensation is different from feedback control compensation in that it uses special electrical devices and software algorithms to correct for machine nonlinearities and mechanical resonance problems Nonlinearities of dead zone and change-in-gain to overcome such machine problems as stiction and cogging of the servo drive at low velocities are covered The topic of simple and practical notch networks used to compensate for machine mechanical resonances has been expanded to include frequency selective feedback with suggestions for software implementation This edition also includes a discussion on the use of a control technique known as feedforward control to minimize the position errors of machine axes with widely varying dynamics Chapters on servo drive stiffness and drive resolution are an integral part of the analysis of industrial servo drives Coverage of speed and acceleration has been broadened to include the derivation and application of ‘‘S’’ type acceleration Machine considerations for ball screw resonances have been expanded to include ball screw critical speed, axial, and torsional resonance calculations, and drive ratio considerations have been expanded to include worm/wheel gear boxes Friction considerations include the three types of friction and their relation to servo drives on industrial machines

One important aspect of selecting a servo for a given application is sizing the drive, because the servo must be large enough to meet the load requirements Manual drive sizing forms for both electric and hydraulic servo drives are included in this book The electric drive sizing forms have been expanded to include both DC and brushless DC drives

Chapter 14is entirely new and reviews the process of compensating an electric servo drive It is assumed that this industrial servo drive has a brushless DC motor with a current loop An example is given for the case in which the motor and current loop are closed with a position loop, and also with a velocity and a position loop The application of proportional plus

integral compensation is used throughout Chapter 14 and detailed in the inclusion of machine structural resonances in the position servo loop Once a servo drive has been properly sized for a given machine, it is necessary to determine whether the machine and servo drive will be stable There are many classical engineering methods that can be used, but it is also possible to use high-speed computer iterative techniques to simulate the control–servo drive–machine combination Transient step responses in input are used in this simulation since it is readily possible to duplicate them on an actual machine and compare the results Machine simulation is used to predict performance before the machine is built Machine simulation includes the significant parameters of the servo drive: the nonlinearities of feedback; coulomb, static, and viscous friction; machine structural resonance; and machine mass A number of examples are presented with case histories for comparing simulated response with actual response

My contemporaries encouraged me to pull together a lifetime of servo drive experience and write the first edition of this book The second edition updates, expands, and increases the usefulness of this book to the practicing engineer

I am sincerely grateful to Dr Thomas Higgins for providing me, as a student, with the academic tools needed in the field of feedback control I

am also grateful to Dr John Bollinger and Dr Robert Lorenz from the University of Wisconsin, for their help over the last 45 years in researching, teaching, and applying principles that have been critical in improving the performance of servo drives Tom Rehm, my friend of 40 years and a fellow software ‘‘hacker,’’ has been a great help in software technology In addition, I would like to thank the Giddings and Lewis Machine Tool Company for providing the atmosphere for study and research in the field of industrial servo drives, and my family for their long years of patience as my career progressed

George W Younkin

Preface

1 The What and Why of a Machine Servo

Just What Are Some of the Benefits of a Servo System?

2 Types of Servos 2.1 Evolution of Servo Drives 2.2 Classification of Drives

3 Components of Servos 3.1 Hydraulic/Electric Circuit Equations 3.2 Actuators—Electric

3.6 Transducers (Feedback)

4 Machine Servo Drives 4.1 Types of Drives 4.2 Feed Drive Performance

5 Troubleshooting Techniques 5.1 Techniques by Drive 5.2 Problems: Their Causes and Cures

6 Conclusion: Machine Feed Drives—An Integral Part of a Machine Control System

6.1 Advances in Technology 6.2 Parameters for Making Application Choices

SERVO DRIVES

7.1 Introduction 7.2 Physical System Analogs, Quantities, and Vectors 7.3 Differential Equations for Physical Systems 7.4 Electric Servo Motor Transfer Functions and Time Constants

7.5 Transport Lag Transfer Function 7.6 Servo Valve Transfer Function 7.7 Hydraulic Servo Motor Characteristics 7.8 General Transfer Characteristics

8 Generalized Control Theory

8.2 Frequency-Response Characteristics and Construction

of Approximate (Bode) Frequency Charts 8.3 Nichols Charts

8.4 Servo Analysis Techniques

9 Indexes of Performance 9.1 Definition of Indexes of Performance for Servo Drives 9.2 Indexes of Performance for Electric and Hydraulic Drives

Summary

10 Performance Criteria 10.1 Percent Regulation

11 Servo Plant Compensation Techniques 11.1 Dead-Zone Nonlinearity 11.2 Change-in-Gain Nonlinearity 11.3 Structural Resonances 11.4 Frequency Selective Feedback 11.5 Feedforward Control

12 Machine Considerations 12.1 Machine Feedback Drive Considerations 12.2 Ball Screw Mechanical Resonances and Reflected Inertias for Machine Drives

12.3 Drive Stiffness 12.4 Drive Resolution 12.5 Drive Acceleration 12.6 Drive Speed Considerations 12.7 Drive Ratio Considerations 12.8 Drive Thrust/Torque and Friction Considerations 12.9 Drive Duty Cycles

13 Drive Sizing Considerations 13.1 Introduction

13.2 Hydraulic Drive Sizing 13.3 Electric Drive Sizing

14 Adjusting Servo Drive Compensation 14.1 Motor and Current Loop 14.2 Motor/Current Loop and Position Loop 14.3 Motor/Current Loop with a Velocity Loop 14.4 PI Compensation

14.5 Position Servo Loop Compensation

15 Machine Simulation 15.1 Introduction 15.2 Simulation Fundamentals 15.3 Machine Simulation Techniques to Predict Performance

15.4 Other Simulation Software

16 Conclusion

Glossary

Key Constants and Variables

Contents

Appendix: Hydraulic Drive and Electric Drive-Sizing Forms Bibliography