Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 2
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This book providesthat knowledge with the aid of hundreds of sketches showing drive layouts and manipulator geometries and their work envelope. It discusseswhat mobility really is and how to increase it without increasing the sizeof the robot, and how the shape of the robot can have a dramatic effect onits 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 achievethat goal, and, therefore, increase the likelihood that your project will bea success.Building, designing, and working with practical mobile robotsrequires knowledge in three major engineering fields: mechanical, electrical, and software.
Many books have been written on robots, somefocusing on the complete robot system, others giving a cookbookapproach allowing a novice to take segments of chapters and put togetherxiCopyright © 2003 by The McGraw-Hill Companies, Inc. Click here for Terms of Use.xiiIntroductiona unique robot. While there are books describing the electric circuitsused in robots, and books that teach the software and control code forrobots, there are few that are focused entirely on the mechanisms andmechanical devices used in mobile robots.This book intends to fill the gap in the literature of mobile robots bycontaining, 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 inmechanical engineering can acquire some understanding of this interesting field.
It also includes clever schemes and mechanisms that mid-levelmechanical engineers should find new and useful. Since mobile robotsare being called on to perform more and more complex and practicaltasks, and many are now carrying one or even two manipulators, thisbook has a section on manipulators and grippers for mobile robots. Itshows why a manipulator used on a robot is different in several waysfrom a manipulator used in industry.Autonomous robots place special demands on their mobility systembecause of the unstructured and highly varied environment the robotmight drive through, and the fact that even the best sensors are poor incomparison to a human’s ability to see, feel, and balance. This means themobility system of a robot that relies on those sensors will have muchless information about the environment and will encounter obstacles thatit 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 mobilitysystem to be able to handle anything that comes along.In contrast, a human driver has very acute sensors: eyes for seeingthings and ranging distances, force sensors to sense acceleration, andbalance 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 toguide that mobility system’s efforts to traverse difficult terrain. Therobot’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 forhigh mobility. It feels right to a human, and it is well suited to higherspeed 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 effectivelyaccomplishes the required task, without regard to how well a humancould use it.IntroductionThere are a few terms specific to mobile robots that must be defined toavoid confusion. First, the term robot itself has unfortunately come tohave at least three different meanings. In this book, the word robotmeans an autonomous or semi-autonomous mobile land vehicle that mayor may not have a manipulator or other device for affecting its environment. Colin Angle, CEO of iRobot Corp.
defines a robot as a mobilething with sensors that looks at those sensors and decides on its ownwhat actions to take.In the manufacturing industry, however, the word robot means areprogrammable stationary manipulator with few, if any sensors, commonly found in large industrial manufacturing plants.
The third commonmeaning of robot is a teleoperated vehicle similar to but more sophisticated than a radio controlled toy car or truck. This form of robot usuallyhas a microprocessor on it to aid in controlling the vehicle itself, performsome 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 onthings useful to making robots, but it also includes several references tomechanisms useful to both of the other types of robots.
Robot andmobile robot are used interchangeably throughout the book.Autonomous, in this book, means acting completely independent of anyhuman input. Therefore, autonomous robot means a self-controlled, selfpowered, mobile vehicle that makes its own decisions based on inputsfrom sensors. There are very few truly autonomous robots, and noknown autonomous robots with manipulators on them whose manipulators are also autonomous.
The more common form of mobile robot todayis semiautonomous, where the robot has some sensors and acts partiallyon its own, but there is always a human in the control loop through aradio link or tether. Another name for this type of control structure istelerobotic, as opposed to a teleoperated robot, where there are no, orvery few, sensors on the vehicle that it uses to make decisions. Specificvehicles in this book that do not use sensors to make decisions arelabeled telerobotic or teleoperated to differentiate them fromautonomous robots.
It is important to note that the mechanisms andmechanical devices that are shown in this book can be applied, in theirappropriate category, to almost any vehicle or manipulator whetherautonomous 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 withthe effects of heat and ice, ground cover, slopes or staircases, and tonegotiate obstacles.
Chapter Nine focuses entirely on comparing drivesystems’ mobility based on a wide range of common obstacles found inxiiixivIntroductionoutdoor and indoor environments, some of which can be any size (likerocks), others that cannot (like stair cases).I intentionally left out the whole world of hydraulics. Whilehydraulic power can be the answer when very compact actuators orhigh power density motors are required, it is potentially more dangerous, and definitely messier, to work with than electrically powereddevices. It is also much less efficient—a real problem for battery powered robots.
There are many texts on hydraulic power and its uses. Ifhydraulics is being considered in a design, the reader is referred to IndustrialFluid Power (4 volumes) 3rd ed., published by Womack EducationPublications.The designer has powerful tools to aid in the design process beyondthe many tricks, mechanical devices, and techniques shown in this book.These tools include 3D design tools like SolidWorks and Pro-Engineerand also new ways to produce prototypes of the mechanisms themselves.This is commonly called Rapid Prototyping (RP).NEW PROCESSES EXPAND CHOICESFOR RAPID PROTOTYPINGNew concepts in rapid prototyping (RP) have made it possible to buildmany different kinds of 3D prototype models faster and cheaper than bytraditional methods. The 3D models are fashioned automatically fromsuch materials as plastic or paper, and they can be full size or scaleddown versions of larger objects. Rapid-prototyping techniques make useof computer programs derived from computer-aided design (CAD)drawings of the object.
The completed models, like those made bymachines and manual wood carving, make it easier for people to visualize a new or redesigned product. They can be passed around a conferencetable and will be especially valuable during discussions among productdesign team members, manufacturing managers, prospective suppliers,and customers.At least nine different RP techniques are now available commercially,and others are still in the development stage. Rapid prototyping modelscan be made by the owners of proprietary equipment, or the work can becontracted out to various RP centers, some of which are owned by the RPequipment manufacturers.
The selection of the most appropriate RPmethod for any given modeling application usually depends on theurgency of the design project, the relative costs of each RP process, andIntroductionthe anticipated time and cost savings RP will offer over conventionalmodel-making practice. New and improved RP methods are being introduced regularly, so the RP field is in a state of change, expanding therange of designer choices.Three-dimensional models can be made accurately enough by RPmethods to evaluate the design process and eliminate interference fits ordimensioning errors before production tooling is ordered. If design flawsor omissions are discovered, changes can be made in the source CADprogram and a replacement model can be produced quickly to verify thatthe corrections or improvements have been made.
Finished models areuseful in evaluations of the form, fit, and function of the product designand for organizing the necessary tooling, manufacturing, or even castingprocesses.Most of the RP technologies are additive; that is, the model is madeautomatically by building up contoured laminations sequentially frommaterials such as photopolymers, extruded or beaded plastic, and evenpaper until they reach the desired height. These processes can be used toform internal cavities, overhangs, and complex convoluted geometries aswell as simple planar or curved shapes. By contrast, a subtractive RPprocess involves milling the model from a block of soft material, typically plastic or aluminum, on a computer-controlled milling machinewith commands from a CAD-derived program.In the additive RP processes, photopolymer systems are based on successively depositing thin layers of a liquid resin, which are then solidified by exposure to a specific wavelengths of light.
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