Published on April 9, 2013
Sclater FM 5/3/01 9:50 AM Page iii MECHANISMS & MECHANICAL DEVICES SOURCEBOOK Third Edition NEIL SCLATER NICHOLAS P. CHIRONIS McGraw-Hill New York • Chicago • San Francisco • Lisbon • London • Madrid Mexico City • Milan • New Delhi • San Juan • Seoul Singapore • Sydney • Toronto
Sclater FM 5/3/01 9:50 AM Page iv Library of Congress Cataloging-in-Publication Data Sclater, Neil. Mechanisms and mechanical devices sourcebook / Neil Sclater, Nicholas P. Chironis.— 3rd ed. p. cm. Rev. ed of: Mechanisms & mechanical devices sourcebook / [edited by] Nicholas P. Chironis, Neil Sclater. 2nd ed. 1996. ISBN 0-07-136169-3 1. Mechanical movements. I. Chironis, Nicholas P. II. Mechanisms & mechanical devices sourcebook. III. Title. TJ181.S28 2001 621.8—dc21 2001030297 Copyright © 2001, 1996, 1991 by The McGraw-Hill Companies, Inc. All rights reserved, Printed in the United States of America. Except as permitted under the United States Copyright Act of 1976, no part of this publication may be reproduced or distrib- uted in any form or by any means, or stored in a data base or retrieval system, without the prior written permission of the publisher. 1 2 3 4 5 6 7 8 9 0 KGP/KGP 0 7 6 5 4 3 2 1 ISBN 0-07-136169-3 The sponsoring editor for this book was Larry S. Hager and the production supervisor was Pamela A. Pelton. It was set in Times Roman by TopDesk Publishers’ Group. Printed and bound by Quebecor/Kingsport. McGraw-Hill books are available at special quantity discounts to use as premiums and sales promotions, or for use in corporate training programs. For more information, please write to the Director of Special Sales, Professional Publishing, McGraw-Hill, Two Penn Plaza, New York, NY 10121-2298. Or contact your local bookstore. This book is printed on acid-free paper. Information contained in this work has been obtained by The McGraw-Hill Companies, Inc. (“McGraw-Hill”) from sources believed to be reliable. However, neither McGraw-Hil1 nor its authors guarantee the accuracy or completeness of any information published herein and neither McGraw-Hill nor its authors shall be responsible for any errors, omissions, or damages arising out of use of this informa- tion. This work is published with the understanding that McGraw-Hill and its authors are supplying information but are not attempting to render engineering or other professional services. If such services are required, the assistance of an appro- priate professional should be sought.
Sclater FM 5/3/01 9:50 AM Page v ABOUT THE EDITORS Neil Sclater began his career as an engineer in the military/aerospace industry and a Boston engineering consulting firm before changing his career path to writing and edit- ing on electronics and electromechanical subjects. He was a staff editor for engineering publications in electronic design, instrumentation, and product engineering, including McGraw-Hill’s Product Engineering magazine, before starting his own business as a consultant and contributing editor in technical communication. For the next 25 years, Mr. Sclater served a diversified list of industrial clients by writing marketing studies, technical articles, brochures, and new product releases. During this period, he also directly served a wide list of publishers by writing hundreds of by-lined articles for many different magazines and newspapers on various topics in engineering and industrial marketing. Mr. Sclater holds degrees from Brown University and Northeastern University, and he has completed graduate courses in industrial management. He is the author or coau- thor of seven books on engineering subjects; six of these were published by McGraw- Hill’s Professional Book Group. He previously revised and edited the Second Edition of Mechanisms and Mechanical Devices Sourcebook after the death of Mr. Chironis. The late Nicholas P. Chironis developed the concept for Mechanisms and Mechanical Devices Sourcebook, and was the author/editor of the First Edition. He was a mechani- cal engineer and consultant in industry before joining the staff of Product Engineering magazine as its mechanical design editor. Later in his career, he was an editor for other McGraw-Hill engineering publications. He had previously been a mechanical engineer for International Business Machines and Mergenthaler Linotype Corporation, and he was an instructor in product design at the Cooper Union School of Engineering in New York City. Mr. Chironis earned both his bachelor’s and master’s degrees in mechanical engineering from Polytechnic University, Brooklyn, NY.
Sclater FM 5/3/01 9:50 AM Page xv ACKNOWLEDGMENTS This author gratefully acknowledges the permission granted by the publisher of NASA Tech Briefs (Associated Business Publications, New York, NY) for reprinting four of its recent articles. They were selected because of their potential applications beyond NASA’s immediate objectives in space science and requirements for specialized equip- ment. The names of the scientist/inventors and the NASA facilities where the work was performed have been included. For more information on those subjects, readers can write directly to the NASA centers and request technical support packages (TSPs), or they can contact the scientists directly through the NASA Tech Briefs Web site, www.nasatech.com. I also wish to thank the following companies and organizations for granting me per- mission to use selected copyrighted illustrations, sending me catalogs, and providing other valuable technical information, all useful in the preparation of this edition: • Anorad Corporation, Hauppauge, NY • Bayside Motion Group, Port Washington, NY • BEI Industrial Encoder Division, Goleta CA • FANUC Robotics North America, Inc. Rochester Hills, MI • Kollmorgen Motion Technologies Group, Radford, VA • Ledex Actuation Products, TRW Control Systems, Vandalia OH • Sandia National Laboratories, Sandia Corporation, Albuquerque, NM • SolidWorks Corporation, Concord, MA • Stratasys Inc., Eden Prairie, MN • Thomson Industries, Inc., Port Washington, NY xv
Sclater FM 5/3/01 9:50 AM Page xiii PREFACE This is the third edition of Mechanisms & Mechanical Devices Sourcebook, a well illus- trated reference book containing a wide range of information on both classical and mod- ern mechanisms and mechanical devices. This edition retains a large core of the con- tents from both the first and second editions, (published in 1991 and 1996, respectively), that has been supplemented by new and revised articles reflecting present and future trends in mechanical engineering and machine design. The new articles in this edition address topics that are covered regularly in mechani- cal engineering and science magazines as well as being the subjects of technical papers presented at engineering conferences. Among these new articles is an overview of motion control systems, highlighting the influence of programmable computer and digi- tal technology on those systems. Other articles discuss servomotors, actuators, sole- noids, and feedback sensors—important electromechanical, and electronic components used in motion control systems. Also included are articles on gearheads, single-axis motion guides, and X-Y motion systems assembled from stock mechanical components. Other articles in this edition describe commercially available 2D and 3D CAD (com- puter-aided design) software and update previous articles on industrial robots and rapid prototyping (RP) systems. Another article reviews recent research in MEMS (micro- electromechanical systems) and recent spinoffs of that technology. All of these subjects are continuing to influence the direction of mechanical engineering, and they are having a profound impact on engineering education and practice. Since the publication of the second edition, the term mechatronics has gained wider acceptance as a word that identifies an ongoing trend in mechanical engineering—the merging of mechanics, electronics, and computer science. Coined in Japan in the 1970s, mechatronics describes the synergistic blend of technologies that has led to the creation of many new functional and adaptable products that could not have been produced with a purely mechanical approach. While there is no formal definition of mechatronics, most mechanical engineers agree on its meaning. The concept of mechatronics has been illustrated as a Venn diagram showing four overlapping circles representing the fields of mechanics, electronics, computers, and controls. Over the years, this convergence has spawned the more specialized disciplines of electromechanics, computer-aided design, control electronics, and digital control sys- tems, all considered to be within the purview of mechatronics. These specialties have, in turn, fostered the creation of the even more focused technologies of system analysis, transducers, simulation, and microcontrollers. Some of the important consumer products that have been identified as resulting from the practice of mechatronics are the computer hard-disk drive, the inkjet printer, the dig- ital video disk (DVD) player, and the camcorder. Examination of these products reveals that they are eclectic assemblies of different kinds of mechanical devices, motors, elec- tronic circuits, and in some of them, optics. The inclusion of such classical mechanical elements as gears, levers, clutches, cams, leadscrews, springs, and motors in those advanced products is evidence that they still perform valuable functions, making it quite likely that they will continue to be included in the new and different products to be developed in this century. A major attraction of the earlier editions of this book has been their cores of illustra- tions and descriptions of basic mechanisms and mechanical devices, accompanied by useful applications information. This material has been culled from a wide range of books and magazines that were published during the last half century. In an era of rap- idly changing technology, most of this hardware has retained its universal value. As a result, this book has become recognized worldwide as a unique repository of historical engineering drawings and data not available in other more formal books. These earlier editions have served as a convenient technical reference and even as inspirational "mind-joggers" for seasoned professional machine designers as well as learning aids for engineering students. Readers trying to arrive at new and different solutions for machine design problems can thumb through these pages, study their many illustrations, and consider adapting some of the successful mechanical inventions of the past to their new applications. Thus, proven solutions from the past can be recycled to perform new duties in the pres- ent. An old invention might be transferred without modification, or perhaps it could be improved if made from newer materials by newer manufacturing methods. What is old can be new again! For those unable to find instant solutions, this book contains a chap- ter of tutorial text and formulas for the design of certain basic mechanisms from scratch. It is assumed that the reader is familiar with the basics of mechanics gained from formal education, practical experience, or both. This book is expected to be of most value to practicing machine designers and mechanical engineers, but its contents should xiii
Sclater FM 5/3/01 9:50 AM Page xiv also be of use to machine design instructors at the college and vocational school level, amateur and professional inventors, and students of all of the engineering disciplines and physical sciences. Last but not least, it is hoped that the book will be attractive to those who simply enjoy looking at illustrations of machines and figuring out how they work. The drawings in this edition have stood the test of time. Certain material published in the previous editions has been deleted because reader feedback suggested that impor- tant design details were missing or unclear. Also, some material considered to be obso- lete and unsuitable for new designs was deleted. For example, clockwork mechanisms for timing, control, and display have almost universally been replaced in contemporary designs by more cost-effective electronic modules that perform the same functions. References to manufacturers or publications that no longer exist were deleted so that readers will not waste time trying to contact them for further information. However, the names of the inventors, where previously given, have been retained to help the reader who may want to do further research on any patents now or once held by those individuals. Many of the mechanisms illustrated in this book were invented by anonymous arti- sans, millwrights, instrument makers, and mechanics over the past centuries. They left behind the sketches, formal drawings, and even the working models on which many of the illustrations in this book are based. It is also worth noting that many of the most influential machines from the water pump, steam engine, and chronometer, to the cotton gin, and airplane were invented by self-trained engineers, scientists, and technicians. By themselves, many of the mechanisms and devices described in this book are just mechanical curiosities, but when integrated by creative minds with others, they can per- form new and different functions. One need only consider the role of basic mechanisms in the crucial inventions of the past century—the airplane, the helicopter, the jet engine, the programmable robot, and most of our familiar home appliances. Have you noticed how the size of objects is both increasing and decreasing? There is now a 142,000-ton cruise ship that can accommodate more than 5000 passengers, and plans have been announced for building jumbo jet aircraft capable of carrying more than 500 passengers. Moreover, laptop computers now have more computing power than mainframe computers that filled a room a quarter century ago. Work is progressing in efforts to combine the functions of computer, cellular telephone, personal digital assistant (PDA), and Internet-access terminal in a single wireless handheld device. MEMS are expected to evolve beyond their current prime roles as sensors to become security locks for computers, optical switches, and practical micromachines. Meanwhile, scientists are studying the feasibility of microminiature, self-propelled cap- sules made with even smaller-scale nanotechnology that could navigate through the human body and seek out, diagnose, and treat diseases at their source. —Neil Sclater xiv
Sclater FM 5/3/01 9:50 AM Page vii CONTENTS PREFACE xiii ACKNOWLEDGMENTS xv CHAPTER 1 MOTION CONTROL SYSTEMS 1 Motion Control Systems Overview 2 Glossary of Motion Control Terms 9 High-Speed Gearheads Improve Small Servo Performance 10 Modular Single-Axis Motion Systems 12 Mechanical Components Form Specialized Motion-Control Systems 13 Servomotors, Stepper Motors, and Actuators for Motion Control 14 Servosystem Feedback Sensors 22 Solenoids and Their Applications 29 CHAPTER 2 ROBOT MECHANISMS 33 Industrial Robots 34 FANUC Robot Specifications 38 Mechanism for Planar Manipulation With Simplified Kinematics 43 Tool-Changing Mechanism for Robot 44 Piezoelectric Motor in Robot Finger Joint 45 Six-Degree-of-Freedom Parallel Minimanipulator 46 Self-Reconfigurable, Two-Arm Manipulator With Bracing 47 Improved Roller and Gear Drives for Robots and Vehicles 48 All-Terrain Vehicle With Self-Righting and Pose Control 49 CHAPTER 3 PARTS-HANDLING MECHANISMS 51 Mechanisms That Sort, Feed, or Weigh 52 Cutting Mechanisms 56 Flipping Mechanisms 58 Vibrating Mechanism 58 Seven Basic Parts Selectors 59 Eleven Parts-Handling Mechanisms 60 Seven Automatic-Feed Mechanisms 62 Seven Linkages for Transport Mechanisms 65 Conveyor Systems for Production Machines 68 Traversing Mechanisms for Winding Machines 73 Vacuum Pickup Positions Pills 75 Machine Applies Labels from Stacks or Rollers 75 High-Speed Machines for Adhesive Applications 76 Automatic Stopping Mechanisms for Faulty Machine Operation 82 Electrical Automatic Stopping Mechanisms 88 Automatic Safety Mechanisms for Operating Machines 90 CHAPTER 4 RECIPROCATING AND GENERAL-PURPOSE MECHANISM 93 Gears and Eccentric Disk Combine in Quick Indexing 94 Timung Belts, Four-Bar Linkage Team Up for Smooth Indexing 95 Modified Ratchet Drive 96 Odd Shapes in Planetary Give Smooth Stop and Go 97 Cycloid Gear Mechanism Controls Stroke of Pump 99 Converting Rotary-to-Linear Motion 100 New Star Wheels Challenge Geneva Drives for Indexing 100 vii
Sclater FM 5/3/01 9:50 AM Page viii Geneva Mechanisms 103 Modified Geneva Drives 106 Indexing and Intermittent Mechanisms 108 Rotary-to-Reciprocating Motion and Dwell Mechanisms 116 Friction Devices for Intermittent Rotary Motion 122 No Teeth on These Ratchets 124 Cam-Controlled Planetary Gear System 125 CHAPTER 5 SPECIAL-PURPOSE MECHANISMS 127 Nine Different Ball Slides for Linear Motion 128 Ball-Bearing Screws Convert Rotary to Linear Motion 130 Three-Point Gear/Leadscrew Positioning 131 Unique Linkage Produces Precise Straight-Line Motion 132 Twelve Expanding and Contracting Devices 134 Five Linkages for Straight-Line Motion 136 Linkage Ratios for Straight-Line Mechanisms 138 Linkages for Other Motions 139 Five Cardan-Gear Mechanisms 140 Ten Ways to Change Straight-Line Direction 142 Nine More Ways to Change Straight-Line Direction 144 Linkages for Accelerating and Decelerating Linear Strokes 146 Linkages for Multiplying Short Motions 148 Parallel-Link Mechanisms 150 Stroke Multiplier 150 Force and Stroke Multipliers 152 Stroke-Amplifying Mechanisms 154 Adjustable-Stroke Mechanisms 155 Adjustable-Output Mechanisms 156 Reversing Mechanisms 158 Computing Mechanisms 159 Eighteen Variations of Differential Linkage 163 Space Mechanisms 165 Seven Popular Types of Three-Dimensional Drives 167 Inchworm Actuator 172 CHAPTER 6 SPRING, BELLOW, FLEXURE, SCREW, AND BALL DEVICES 173 Flat Springs in Mechanisms 174 Pop-Up Springs Get New Backbone 176 Twelve Ways to Put Springs to Work 177 Overriding Spring Mechanisms for Low-Torque Drives 179 Spring Motors and Typical Associated Mechanisms 181 Flexures Accurately Support Pivoting Mechanisms and Instruments 183 Taut Bands and Leadscrew Provide Accurate Rotary Motion 185 Air Spring Mechanisms 186 Obtaining Variable Rates from Springs 188 Belleville Springs 189 Spring-Type Linkage for Vibration Control 190 Twenty Screw Devices 191 Ten Ways to Employ Screw Mechanisms 194 Seven Special Screw Arrangements 195 Fourteen Adjusting Devices 196 Linear Roller Bearings Are Suited for High-Load, Heavy-Duty Tasks 197 CHAPTER 7 CAM, TOGGLE, CHAIN, AND BELT MECHANISMS 199 Cam Basics 200 Cam-Curve Generating Mechanisms 201 viii
Sclater FM 5/3/01 9:50 AM Page ix Fifteen Ideas for Cam Mechanisms 207 Special-Function Cams 209 Cam Drives for Machine Tools 210 Toggle Linkage Applications in Different Mechanisms 211 Sixteen Latch, Toggle, and Trigger Devices 213 Six Snap-Action Mechanisms 215 Eight Snap-Action Devices 217 Applications of the Differential Winch to Control Systems 219 Six Applications for mechanical Power Amplifiers 221 Variable-Speed Belt and Chain Drives 224 Getting in Step With Hybrid Belts 227 Change Center Distance Without Affecting Speed Ratio 231 Motor Mount Pivots for Controlled Tension 231 Bushed Roller Chains and Their Adaptations 232 Six Ingenious Jobs for Roller Chain 234 Six More Jobs for Roller Chain 236 Mechanisms for Reducing Pulsations in Chain Drives 238 Smoother Drive Without Gears 240 CHAPTER 8 GEARED SYSTEMS AND VARIABLE-SPEED MECHANISMS 241 Gears and Gearing 242 Nutating-Plate Drive 243 Cone Drive Needs No Gears or Pulleys 244 Variable-Speed Mechanical Drives 245 Unidirectional Drive 253 More Variable-Speed Drives 254 Variable-Speed Friction Drives 256 Variable-Speed Drives and Transmissions 258 Precision Ball Bearings Replace Gears in Tiny Speed Reducers 260 Multifunction Flywheel Smoothes Friction in Tape Cassette Drive 261 Controlled Differential Drives 262 Twin-Motor Planetary Gears Provide Safety Plus Dual-Speed 263 Harmonic-Drive Speed Reducers 263 Flexible Face-Gears Make Efficient High-Reduction Drives 266 Compact Rotary Sequencer 267 Planetary Gear Systems 268 Noncircular Gears 275 Sheet-Metal Gears, Sprockets, Worms, and Ratchets 279 How to Prevent Reverse Rotation 281 Gear-Shift Arrangements 282 Shifting Mechanisms for Gears and Clutches 284 Fine-Focus Adjustments 286 Ratchet-Tooth Speed-Change Drive 287 Twinworm Gear Drive 287 Compliant Gearing for Redundant Torque Drive 289 Lighter, More-Efficient Helicopter Transmissions 290 Worm Gear With Hydrostatic Engagement 290 Straddle Design of Spiral Bevel and Hypoid Gears 292 CHAPTER 9 COUPLING, CLUTCHING, AND BRAKING DEVICES 293 Coupling of Parallel Shafts 294 Novel Linkage Couples Offset Shafts 295 Disk-and-Link Coupling Simplifies Transmissions 296 Interlocking Space-Frames Flex as They Transmit Shaft Torque 297 Off-Center Pins Cancel Misalignment of Shafts 299 Hinged Links and Torsion Bushings Give Drives a Soft Start 300 ix
Sclater FM 5/3/01 9:50 AM Page x Universal Joint Relays Power 45° at Constant Speeds 301 Basic Mechanical Clutches 302 Spring-Wrapped Slip Clutches 304 Controlled-Slip Concept Adds New Uses for Spring Clutches 306 Spring Bands Grip Tightly to Drive Overrunning Clutch 307 Slip and Bidirectional Clutches Combine to Control Torque 308 Walking Pressure Plate Delivers Constant Torque 309 Conical-Rotor Motor Provides Instant Clutching or Braking 310 Fast-Reversal Reel Drive 310 Seven Overrunning Clutches 311 Spring-Loaded Pins aid Sprags in One-Way Clutch 312 Roller-Type Clutch 312 One-Way Output From Speed Reducers 313 Springs, Shuttle Pinion, and Sliding Ball Perform in One-Way Drives 314 Details of Overriding Clutches 316 Ten Ways to Apply Overrunning Clutches 318 Applications for Sprag-Type Clutches 320 Small Mechanical Clutches for Precise Service 322 Mechanisms for Station Clutches 324 Twelve Applications for Electromagnetic Clutches and Brakes 326 Trip Roller Clutch 328 Geared Electromechanical Rotary Joint 329 Ten Universal Shaft Couplings 330 Methods for Coupling Rotating Shafts 332 Linkages for Band Clutches and Brakes 336 Special Coupling Mechanisms 337 Link Coupling Mechanisms 338 CHAPTER 10 TORQUE-LIMITING, TENSIONING, AND GOVERNING DEVICES 339 Caliper Brakes Help Maintain Proper Tension in Press Feed 340 Sensors Aid Clutch/Brakes 340 Warning Device Prevents Overloading of Boom 341 Constant Watch on Cable Tension 341 Torque-Limiters Protect Light-Duty Drives 342 Limiters Prevent Overloading 343 Seven Ways to Limit Shaft Rotation 346 Mechanical Systems for Controlling Tension and Speed 348 Drives for Controlling Tension 352 Switch Prevents Overloading of a Hoist 355 Mechanical, Geared, and Cammed Limit Switches 356 Limit Switches in Machinery 358 Automatic Speed Governors 362 Centrifugal, Pneumatic, Hydraulic, and Electric Governors 364 Speed Control Devices for Mechanisms 366 Floating-Pinion Torque Splitter 367 CHAPTER 11 PNEUMATIC AND HYDRAULIC MACHINE AND MECHANISM CONTROL 369 Designs and Operating Principles of Typical Pumps 370 Rotary-Pump Mechanisms 374 Mechanisms Actuated by Pneumatic or Hydraulic Cylinders 376 Foot-Controlled Braking System 378 Linkages Actuate Steering in a Tractor 378 Fifteen Jobs for Pneumatic Power 379 Ten Ways to Use Metal Diaphragms and Capsules 380 Differential Transformer Sensing Devices 382 High-Speed Counters 384 Designing With Permanent Magnets 385 x
Sclater FM 5/3/01 9:50 AM Page xi Permanent Magnet Mechanisms 387 Electrically Driven Hammer Mechanisms 390 Thermostatic Mechanisms 392 Temperature-Regulating Mechanisms 396 Photoelectric Controls 398 Liquid Level Indicators and Controllers 400 Instant Muscle With Pyrotechnic Power 402 CHAPTER 12 FASTENING, LATCHING, CLAMPING, AND CHUCKING DEVICES 405 Remotely Controlled Latch 406 Toggle Fastener Inserts, Locks, and Releases Easily 407 Grapple Frees Loads Automatically 407 Quick-Release Lock Pin Has a Ball Detent 408 Automatic Brake Locks Hoist When Driving Torque Ceases 408 Lift-Tong Mechanism Firmly Grips Objects 409 Perpendicular-Force Latch 409 Quick-Release Mechanisms 410 Ring Springs Clamp Platform Elevator Into Position 411 Quick-Acting Clamps for Machines and Fixtures 412 Friction Clamping Devices 414 Detents for Stopping Mechanical Movements 416 Ten Different Splined Connections 418 Fourteen Ways to Fasten Hubs to Shafts 420 Clamping Devices for Accurately Aligning Adjustable Parts 422 Spring-Loaded Chucks and Holding Fixtures 424 Short In-Line Turnbuckle 424 Actuator Exerts Tensile or Compressive Axial Load 425 Gripping System for Mechanical Testing of Composites 426 Passive Capture Joint With Three Degrees of Freedom 427 Probe-and-Socket Fasteners for Robotic Assembly 428 CHAPTER 13 KEY EQUATIONS AND CHARTS FOR DESIGNING MECHANISMS 429 Four-Bar Linkages and Typical Industrial Applications 430 Designing Geared Five-Bar Mechanisms 432 Kinematics of Intermittent Mechanisms—The External Geneva Wheel 436 Kinematics of Intermittent Mechanisms—The Internal Geneva Wheel 439 Equations for Designing Cycloid Mechanisms 442 Designing Crank-and-Rocker Links With Optimum Force Transmission 445 Design Curves and Equations for Gear-Slider Mechanisms 448 Designing Snap-Action Toggles 452 Feeder Mechanisms for Angular Motions 455 Feeder Mechanisms for Curvilinear Motions 456 Roberts’ Law Helps to Find Alternate Four-Bar Linkages 459 Ratchet Layout Analyzed 460 Slider-Crank Mechanism 461 CHAPTER 14 NEW DIRECTIONS IN MACHINE DESIGN 463 Software Improvements Expand CAD Capabilities 464 New Processes Expand Choices for Rapid Prototyping 468 Micromachines Open a New Frontier for Machine Design 475 Multilevel Fabrication Permits More Complex and Functional MEMS 478 Miniature Multispeed Transmissions for Small Motors 481 MEMS Chips Become Integrated Microcontrol Systems 482 LIGA: An Alternative Method for Making Microminiature Parts 484 INDEX 487 xi
Sclater Chapter 1 5/3/01 9:52 AM Page 1 CHAPTER 1 MOTION CONTROL SYSTEMS
Sclater Chapter 1 5/3/01 9:52 AM Page 2 MOTION CONTROL SYSTEMS OVERVIEW Motion control systems today can be found in such diverse applications as materials handling equipment, machine tool cen- ters, chemical and pharmaceutical process lines, inspection sta- tions, robots, and injection molding machines. Merits of Electric Systems Most motion control systems today are powered by electric motors rather than hydraulic or pneumatic motors or actuators because of the many benefits they offer: • More precise load or tool positioning, resulting in fewer product or process defects and lower material costs. • Quicker changeovers for higher flexibility and easier product customizing. • Increased throughput for higher efficiency and capacity. • Simpler system design for easier installation, programming, Fig. 1 This multiaxis X-Y-Z motion platform is an example of a and training. motion control system. • Lower downtime and maintenance costs. • Cleaner, quieter operation without oil or air leakage. Introduction Electric-powered motion control systems do not require A modern motion control system typically consists of a motion pumps or air compressors, and they do not have hoses or piping controller, a motor drive or amplifier, an electric motor, and feed- that can leak hydraulic fluids or air. This discussion of motion back sensors. The system might also contain other components control is limited to electric-powered systems. such as one or more belt-, ballscrew-, or leadscrew-driven linear guides or axis stages. A motion controller today can be a stand- Motion Control Classification alone programmable controller, a personal computer containing a motion control card, or a programmable logic controller (PLC). Motion control systems can be classified as open-loop or closed- All of the components of a motion control system must work loop. An open-loop system does not require that measurements together seamlessly to perform their assigned functions. Their of any output variables be made to produce error-correcting sig- selection must be based on both engineering and economic con- nals; by contrast, a closed-loop system requires one or more siderations. Figure 1 illustrates a typical multiaxis X-Y-Z motion feedback sensors that measure and respond to errors in output platform that includes the three linear axes required to move a variables. load, tool, or end effector precisely through three degrees of free- dom. With additional mechanical or electromechanical compo- Closed-Loop System nents on each axis, rotation about the three axes can provide up to six degrees of freedom, as shown in Fig. 2. A closed-loop motion control system, as shown in block diagram Fig. 3, has one or more feedback loops that continuously com- pare the system’s response with input commands or settings to correct errors in motor and/or load speed, load position, or motor torque. Feedback sensors provide the electronic signals for cor- recting deviations from the desired input commands. Closed- loop systems are also called servosystems. Each motor in a servosystem requires its own feedback sen- sors, typically encoders, resolvers, or tachometers that close Fig. 2 The right-handed coordinate system showing six degrees of freedom. Fig. 3 Block diagram of a basic closed-loop control system. 2
Sclater Chapter 1 5/3/01 9:52 AM Page 3 loops around the motor and load. Variations in velocity, position, and torque are typically caused by variations in load conditions, but changes in ambient temperature and humidity can also affect load conditions. Fig. 4 Block diagram of a velocity-control system. A velocity control loop, as shown in block diagram Fig. 4, typi- cally contains a tachometer that is able to detect changes in motor speed. This sensor produces error signals that are proportional to the positive or negative deviations of motor speed from its preset value. These signals are sent to the motion controller so that it can compute a corrective signal for the amplifier to keep motor speed within those preset limits despite load changes. Fig. 7 Examples of position feedback sensors installed on a ballscrew-driven slide mechanism: (a) rotary encoder, (b) linear encoder, and (c) laser interferometer. Fig. 5 Block diagram of a position-control system. The ballscrew slide mechanism, shown in Fig. 6, is an example of a mechanical system that carries a load whose position must be A position-control loop, as shown in block diagram Fig. 5, controlled in a closed-loop servosystem because it is not equipped typically contains either an encoder or resolver capable of direct with position sensors. Three examples of feedback sensors or indirect measurements of load position. These sensors gener- mounted on the ballscrew mechanism that can provide position ate error signals that are sent to the motion controller, which pro- feedback are shown in Fig. 7: (a) is a rotary optical encoder duces a corrective signal for amplifier. The output of the ampli- mounted on the motor housing with its shaft coupled to the motor fier causes the motor to speed up or slow down to correct the shaft; (b) is an optical linear encoder with its graduated scale position of the load. Most position control closed-loop systems mounted on the base of the mechanism; and (c) is the less com- also include a velocity-control loop. monly used but more accurate and expensive laser interferometer. A torque-control loop contains electronic circuitry that meas- ures the input current applied to the motor and compares it with a value proportional to the torque required to perform the desired task. An error signal from the circuit is sent to the motion con- troller, which computes a corrective signal for the motor ampli- fier to keep motor current, and hence torque, constant. Torque- control loops are widely used in machine tools where the load can change due to variations in the density of the material being machined or the sharpness of the cutting tools. Trapezoidal Velocity Profile If a motion control system is to achieve smooth, high-speed motion without overstressing the servomotor, the motion con- troller must command the motor amplifier to ramp up motor velocity gradually until it reaches the desired speed and then ramp it down gradually until it stops after the task is complete. This keeps motor acceleration and deceleration within limits. The trapezoidal profile, shown in Fig. 8, is widely used Fig. 6 Ballscrew-driven single-axis slide mechanism without posi- because it accelerates motor velocity along a positive linear “up- tion feedback sensors. ramp” until t
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