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27.07.2005 - Paul E. Sandin. Robot Mechanisms and Mechanical Devices Illustrated


РобоКлуб/Книжная полка/Научно-техническая литература/Главы из книг/27.07.2005 - Paul E. Sandin. Robot Mechanisms and Mechanical Devices Illustrated




Paul E. Sandin. Robot Mechanisms and Mechanical Devices Illustrated

This book is meant to be interesting, helpful, and educational to hobbyists, students, educators, and midlevel engineers studying or designing mobile robots that do real work. It is primarily focused on mechanisms and devices that relate to vehicles that move around by themselves and actually do things autonomously, i.e. a robot. Making a vehicle that can autonomously drive around, both indoors and out, seems, at first, like a simple thing. Build a chassis, add drive wheels, steering wheels, a power source (usually batteries), some control code that includes some navigation and obstacle avoidance routines or some other way to control it, throw some bump sensors on it, and presto! a robot.

Unfortunately, soon after these first attempts, the designer will find the robot getting stuck on what seem to be innocuous objects or bumps, held captive under a chair or fallen tree trunk, incapable of doing anything useful, or with a manipulator that crushes every beer can it tries to pick up. Knowledge of the mechanics of sensors, manipulators, and the concept of mobility will help reduce these problems. This book provides that knowledge with the aid of hundreds of sketches showing drive layouts and manipulator geometries and their work envelope. It discusses what mobility really is and how to increase it without increasing the size of the robot, and how the shape of the robot can have a dramatic effect on its performance. Interspersed throughout the book are unusual mechanisms and devices, included to entice the reader to think outside the box. It is my sincere hope that this book will decrease the time it takes to produce a working robot, reduce the struggles and effort required to achieve that goal, and, therefore, increase the likelihood that your project will be a success.

Building, designing, and working with practical mobile robots requires knowledge in three major engineering fields: mechanical, electrical, and software. Many books have been written on robots, some focusing on the complete robot system, others giving a cookbook approach allowing a novice to take segments of chapters and put together a unique robot. While there are books describing the electric circuits used in robots, and books that teach the software and control code for robots, there are few that are focused entirely on the mechanisms and mechanical devices used in mobile robots.

This book intends to fill the gap in the literature of mobile robots by containing, in a single reference, complete graphically presented information on the mechanics of a mobile robot. It is written in laymen’s language and filled with sketches so novices and those not trained in mechanical engineering can acquire some understanding of this interesting field. It also includes clever schemes and mechanisms that mid-level mechanical engineers should find new and useful. Since mobile robots are being called on to perform more and more complex and practical tasks, and many are now carrying one or even two manipulators, this book has a section on manipulators and grippers for mobile robots. It shows why a manipulator used on a robot is different in several ways from a manipulator used in industry.

Autonomous robots place special demands on their mobility system because of the unstructured and highly varied environment the robot might drive through, and the fact that even the best sensors are poor in comparison to a human’s ability to see, feel, and balance. This means the mobility system of a robot that relies on those sensors will have much less information about the environment and will encounter obstacles that it must deal with on its own. In many cases, the microprocessor controlling the robot will only be telling the mobility system “go over there” without regard to what lays directly in that path. This forces the mobility system to be able to handle anything that comes along.

In contrast, a human driver has very acute sensors: eyes for seeing things and ranging distances, force sensors to sense acceleration, and balance to sense levelness. A human expects certain things of an automobile’s (car, truck, jeep, HumVee, etc.) mobility system (wheels, suspension, and steering) and uses those many and powerful sensors to guide that mobility system’s efforts to traverse difficult terrain. The robot’s mobility system must be passively very capable, the car’s mobility system must feel right to a human.

For these reasons, mobility systems on mobile robots can be both simpler and more complex than those found in automobiles. For example, the Ackerman steering system in automobiles is not actually suited for high mobility. It feels right to a human, and it is well suited to higher speed travel, but a robot doesn’t care about feeling right, not yet, at least!

The best mobility system for a robot to have is one that effectively accomplishes the required task, without regard to how well a human could use it. There are a few terms specific to mobile robots that must be defined to avoid confusion. First, the term robot itself has unfortunately come to have at least three different meanings. In this book, the word robot means an autonomous or semi-autonomous mobile land vehicle that may or may not have a manipulator or other device for affecting its environment.

Colin Angle, CEO of iRobot Corp. defines a robot as a mobile thing with sensors that looks at those sensors and decides on its own what actions to take.

In the manufacturing industry, however, the word robot means a reprogrammable stationary manipulator with few, if any sensors, commonly found in large industrial manufacturing plants. The third common meaning of robot is a teleoperated vehicle similar to but more sophisticated than a radio controlled toy car or truck. This form of robot usually has a microprocessor on it to aid in controlling the vehicle itself, perform some autonomous or automatic tasks, and aid in controlling the manipulator if one is onboard.

This book mainly uses the first meaning of robot and focuses on things useful to making robots, but it also includes several references to mechanisms useful to both of the other types of robots. Robot and mobile robot are used interchangeably throughout the book.

Autonomous, in this book, means acting completely independent of any human input. Therefore, autonomous robot means a self-controlled, selfpowered, mobile vehicle that makes its own decisions based on inputs from sensors. There are very few truly autonomous robots, and no known autonomous robots with manipulators on them whose manipulators are also autonomous. The more common form of mobile robot today is semiautonomous, where the robot has some sensors and acts partially on its own, but there is always a human in the control loop through a radio link or tether. Another name for this type of control structure is telerobotic, as opposed to a teleoperated robot, where there are no, or very few, sensors on the vehicle that it uses to make decisions. Specific vehicles in this book that do not use sensors to make decisions are labeled telerobotic or teleoperated to differentiate them from autonomous robots. It is important to note that the mechanisms and mechanical devices that are shown in this book can be applied, in their appropriate category, to almost any vehicle or manipulator whether autonomous or not.

Another word, which gets a lot of use in the robot world, is mobility. Mobility is defined in this book as a drive system’s ability to deal with the effects of heat and ice, ground cover, slopes or staircases, and to negotiate obstacles. Chapter Nine focuses entirely on comparing drive systems’ mobility based on a wide range of common obstacles found in outdoor and indoor environments, some of which can be any size (like rocks), others that cannot (like stair cases).

I intentionally left out the whole world of hydraulics. While hydraulic power can be the answer when very compact actuators or high power density motors are required, it is potentially more dangerous, and definitely messier, to work with than electrically powered devices. It is also much less efficient—a real problem for battery powered robots. There are many texts on hydraulic power and its uses. If hydraulics is being considered in a design, the reader is referred to Industrial Fluid Power (4 volumes) 3rd ed., published by Womack Education Publications.

The designer has powerful tools to aid in the design process beyond the many tricks, mechanical devices, and techniques shown in this book. These tools include 3D design tools like SolidWorks and Pro-Engineer and also new ways to produce prototypes of the mechanisms themselves.
This is commonly called Rapid Prototyping (RP).

Contents
Introduction xi
Acknowledgments xxxv
Chapter 1 Motor and Motion Control Systems 1
Introduction 3
Merits of Electric Systems 4
Motion Control Classification 5
Closed-Loop System 5
Trapezoidal Velocity Profile 7
Closed-Loop Control Techniques 8
Open-Loop Motion Control Systems 9
Kinds of Controlled Motion 9
Motion Interpolation 10
Computer-Aided Emulation 10
Mechanical Components 11
Electronic System Components 15
Motor Selection 16
Motor Drivers (Amplifiers) 18
Feedback Sensors 19
Installation and Operation of the System 20
Servomotors, Stepper Motors, and Actuators for
Motion Control 20
Permanent-Magnet DC Servomotors 21
Brush-Type PM DC Servomotors 22
Disk-Type PM DC Motors 23
Cup- or Shell-Type PM DC Motors 24
Position Sensing in Brushless Motors 29
Brushless Motor Advantages 30
Brushless DC Motor Disadvantages 31
Characteristics of Brushless Rotary Servomotors 31
Linear Servomotors 31
Commutation 34
Installation of Linear Motors 35
Advantages of Linear vs. Rotary Servomotors 36
Coil Assembly Heat Dissipation 37
Stepper Motors 37
Permanent-Magnet (PM) Stepper Motors 38
Variable Reluctance Stepper Motors 38
Hybrid Stepper Motors 38
Stepper Motor Applications 40
DC and AC Motor Linear Actuators 41
Stepper-Motor Based Linear Actuators 42
Servosystem Feedback Sensors 43
Rotary Encoders 43
Incremental Encoders 44
Absolute Encoders 46
Linear Encoders 47
Magnetic Encoders 48
Resolvers 49
Tachometers 51
Linear Variable Differential Transformers (LVDTs) 53
Linear Velocity Transducers (LVTs) 55
Angular Displacement Transducers (ATDs) 55
Inductosyns 57
Laser Interferometers 57
Precision Multiturn Potentiometers 59
Solenoids and Their Applications 60
Solenoids: An Economical Choice for Linear or Rotary Motion 60
Technical Considerations 62
Open-Frame Solenoids 63
C-Frame Solenoids 63
Box-Frame Solenoids 63
Tubular Solenoids 64
Rotary Solenoids 64
Rotary Actuators 66
Actuator Count 67
Debugging 67
Reliability 68
Cost 68
Chapter 2 Indirect Power Transfer Devices 69
Belts 72
Flat Belts 73
O-Ring Belts 73
V-Belts 73
Timing Belts 75
Smoother Drive Without Gears 76
Plastic-and-Cable Chain 77
Chain 79
Ladder Chain 80
Roller Chain 80
Rack and Pinion Chain Drive 82
Timing or Silent Chain 82
Friction Drives 83
Cone Drive Needs No Gears Or Pulleys 84
Gears 85
Gear Terminology 87
Gear Dynamics Terminology 88
Gear Classification 88
Worm Gears 90
Worm Gear with Hydrostatic Engagement 90
Controlled Differential Drives 93
Twin-Motor Planetary Gears Provide Safety Plus Dual-Speed 95
Harmonic-Drive Speed Reducers 96
Advantages and Disadvantages 99
Flexible Face-Gears Make Efficient High-Reduction Drives 100
High-Speed Gearheads Improve Small Servo Performance 102
Simplify the Mounting 102
Cost-Effective Addition 104
Chapter 3 Direct Power Transfer Devices 107
Couplings 109
Methods for Coupling Rotating Shafts 110
Ten Universal Shaft Couplings 114
Hooke’s Joints 114
Constant-Velocity Couplings 115
Coupling of Parallel Shafts 117
Ten Different Splined Connections 118
Cylindrical Splines 118
Face Splines 120
Torque Limiters 121
Ten Torque-Limiters 121
One Time Use Torque Limiting 125
Chapter 4 Wheeled Vehicle Suspensions and Drivetrains 127
Wheeled Mobility Systems 130
Why Not Springs? 130
Shifting the Center of Gravity 131
Wheel Size 134
Three-Wheeled Layouts 136
Four-Wheeled Layouts 141
All-Terrain Vehicle with Self-Righting and Pose Control 144
Six-Wheeled Layouts 150
Eight-Wheeled Layouts 155
Chapter 5 Tracked Vehicle Suspensions and Drive Trains 161
Steering Tracked Vehicles 167
Various Track Construction Methods 168
Track Shapes 171
Track Suspension Systems 174
Track System Layouts 178
One-Track Drive Train 178
Two-Tracked Drive Trains 179
Two-Tracked Drive Trains with Separate Steering Systems 180
Four-Tracked Drive Trains 181
Six-Tracked Drive Trains 184
Chapter 6 Steering History 187
Steering Basics 190
The Next Step Up 193
Chapter 7 Walkers 199
Leg Actuators 202
Leg Geometries 203
Walking Techniques 208
Wave Walking 208
Independent Leg Walking 208
Frame Walking 211
Roller-Walkers 214
Flexible Legs 214
Chapter 8 Pipe Crawlers and Other Special Cases 217
Horizontal Crawlers 220
Vertical Crawlers 221
Traction Techniques for Vertical Pipe Crawlers 222
Wheeled Vertical Pipe Crawlers 223
Tracked Crawlers 224
Other Pipe Crawlers 224
External Pipe Vehicles 226
Snakes 226
Chapter 9 Comparing Locomotion Methods 227
What Is Mobility? 229
The Mobility System 229
Size 230
Efficiency 231
The Environment 232
Thermal 232
Ground Cover 233
Topography 233
Obstacles 234
Complexity 235
Speed and Cost 235
The Mobility Index Comparison Method 236
The Practical Method 236
Explain All This Using the Algebraic Method 237
Chapter 10 Manipulator Geometries 239
Positioning, Orienting, How Many Degrees of Freedom? 241
E-Chain 243
Slider Crank 243
Arm Geometries 245
Cartesian or Rectangular 246
Cylindrical 247
Polar or Spherical 248
The Wrist 250
Grippers 252
Passive Parallel Jaw Using Cross Tie 255
Passive Capture Joint with Three Degrees of Freedom 256
Industrial Robots 258
Industrial Robot Advantages 259
Trends in Industrial Robots 259
Industrial Robot Characteristics 261
Chapter 11 Proprioceptive and Environmental Sensing
Mechanisms and Devices 263
Industrial Limit Switches 270
Layouts 276
Combination Trip (Sense) and Hard Stop 277
By-Pass Layouts 278
Reversed Bump 279
Bumper Geometries and Suspensions 280
Simple Bumper Suspension Devices 282
Three Link Planar 283
Tension Spring Star 284
Torsion Swing Arm 284
Horizontal Loose Footed Leaf Spring 285
Sliding Front Pivot 286
Suspension Devices to Detect Motions in All Three Planes 287
Conclusion 289
Index 291


Copyright © 2003 by The McGrawHill Companies, Inc.

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