Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 43
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Industrialrobots can be floor-standing, benchtop, or mobile.Industrial robots are classified in ways that relate to the characteristicsof their control systems, manipulator or arm geometry, and modes ofoperation. There is no common agreement on or standardizations ofthese designations in the literature or among industrial robot specialistsaround the world.A basic industrial robot classification relates to overall performanceand distinguishes between limited and unlimited sequence control. Fourclasses are generally recognized: limited sequence and three forms ofunlimited sequence—point-to-point, continuous path, and controlledpath.
These designations refer to the path taken by the end effector, ortool, at the end of the industrial robot arm as it moves between operations.Another classification related to control is nonservoed versus servoed.Nonservoed implies open-loop control, or no closed-loop feedback, inthe system. By contrast, servoed means that some form of closed-loopfeedback is used in the system, typically based on sensing velocity, position, or both.
Limited sequence also implies nonservoed control whileunlimited sequence can be achieved with point-to-point, continuouspath, or controlled-path modes of operation.Industrial robots are powered by electric, hydraulic, or pneumaticmotors or actuators. Electric motor power is most popular for the majoraxes of floor-standing industrial industrial robots today.
Hydraulic-driveindustrial robots are generally assigned to heavy-duty lifting applications. Some electric and hydraulic industrial robots are equipped withpneumatic-controlled tools or end effectors.The number of degrees of freedom is equal to the number of axes ofan industrial robot, and is an important indicator of its capability.Limited-sequence industrial robots typically have only two or threedegrees of freedom, but point-to-point, continuous-path, and controlledpath industrial robots typically have five or six. Two or three of thosemay be in the wrist or end effector.Most heavy-duty industrial robots are floor-standing.
Others in thesame size range are powered by hydraulic motors. The console containsa digital computer that has been programmed with an operating systemand applications software so that it can perform the tasks assigned to it.Chapter 10Manipulator GeometriesSome industrial robot systems also include training pendants—handheldpushbutton panels connected by cable to the console that permit directcontrol of the industrial robot .The operator or programmer can control the movements of the industrial robot arm or manipulator with pushbuttons or other data inputdevices so that it is run manually through its complete task sequence toprogram it. At this time adjustments can be made to prevent any part ofthe industrial robot from colliding with nearby objects.There are also many different kinds of light-duty assembly or pickand-place industrial robots that can be located on a bench.
Some of theseare programmed with electromechanical relays, and others are programmed by setting mechanical stops on pneumatic motors.Industrial Robot AdvantagesThe industrial robot can be programmed to perform a wider range of tasksthan dedicated automatic machines, even those that can accept a wideselection of different tools. However, the full benefits of an industrial robotcan be realized only if it is properly integrated with the other machineshuman operators, and processes. It must be evaluated in terms of costeffectiveness of the performance or arduous, repetitious, or dangeroustasks, particularly in hostile environments.
These might include high temperatures, high humidity, the presence of noxious or toxic fumes, andproximity to molten metals, welding arcs, flames, or high-voltage sources.The modern industrial robot is the product of developments made inmany different engineering and scientific disciplines, with an emphasison mechanical, electrical, and electronic technology as well as computerscience. Other technical specialties that have contributed to industrialrobot development include servomechanisms, hydraulics, and machinedesign. The latest and most advanced industrial robots include dedicateddigital computers.The largest number of industrial robots in the world are limitedsequence machines, but the trend has been toward the electric-motorpowered, servo-controlled industrial robots that typically are floorstanding machines. Those industrial robots have proved to be the mostcost-effective because they are the most versatile.Trends in Industrial RobotsThere is evidence that the worldwide demand for industrial robots hasyet to reach the numbers predicted by industrial experts and visionaries259260Chapter 10Manipulator Geometriessome twenty years ago.
The early industrial robots were expensive andtemperamental, and they required a lot of maintenance. Moreover, thesoftware was frequently inadequate for the assigned tasks, and manyindustrial robots were ill-suited to the tasks assigned them.Many early industrial customers in the 1970s and 1980s were disappointed because their expectations had been unrealistic; they hadunderestimated the costs involved in operator training, the preparationof applications software, and the integration of the industrial robotswith other machines and processes in the workplace.By the late 1980s, the decline in orders for industrial robots drovemost American companies producing them to go out of business, leaving only a few small, generally unrecognized manufacturers.
Suchindustrial giants as General Motors, Cincinnati Milacron, GeneralElectric, International Business Machines, and Westinghouse enteredand left the field. However, the Japanese electrical equipment manufacturer Fanuc Robotics North America and the Swedish-Swiss corporation Asea Brown Boveri (ABB) remain active in the U.S. robotics market today.However, sales are now booming for less expensive industrial robotsthat are stronger, faster, and smarter than their predecessors. Industrialrobots are now spot-welding car bodies, installing windshields, anddoing spray painting on automobile assembly lines.
They also place andremove parts from annealing furnaces and punch presses, and theyassemble and test electrical and mechanical products. Benchtop industrial robots pick and place electronic components on circuit boards inelectronics plants, while mobile industrial robots on tracks store andretrieve merchandise in warehouses.The dire predictions that industrial robots would replace workers inrecord numbers have never been realized. It turns out that the most costeffective industrial robots are those that have replaced human beings indangerous, monotonous, or strenuous tasks that humans do not want todo.
These activities frequently take place in spaces that are poorly ventilated, poorly lighted, or filled with noxious or toxic fumes. They mightalso take place in areas with high relative humidity or temperatures thatare either excessively hot or cold. Such places would include mines,foundries, chemical processing plants, or paint-spray facilities.Management in factories where industrial robots were purchased andinstalled for the first time gave many reasons why they did this despitethe disappointments of the past twenty years. The most frequent reasonswere the decreasing cost of powerful computers as well as the simplification of both the controls and methods for programming the computers.This has been due, in large measure, to the declining costs of more pow-Chapter 10Manipulator Geometrieserful microprocessors, solid-state and disk memory, and applicationssoftware.However, overall system costs have not declined, and there have beenno significant changes in the mechanical design of industrial robots during the industrial robot’s twenty-year “learning curve” and maturationperiod.The shakeout of American industrial robot manufacturers has led tothe near domination of the world market for industrial robots by theJapanese manufacturers who have been in the market for most of the pasttwenty years.
However, this has led to de facto standardization in industrial robot geometry and philosophy along the lines established by theJapanese manufacturers. Nevertheless, industrial robots are still available in the same configurations that were available fifteen to twentyyears ago, and there have been few changes in the design of the end-usetools that mount on the industrial robot’s “hand” for the performance ofspecific tasks (e.g., parts handling, welding, painting).Industrial Robot CharacteristicsLoad-handling capability is one of the most important factors in anindustrial robot purchasing decision. Some can now handle payloads ofas much as 200 pounds. However, most applications do not require thehandling of parts that are as heavy as 200 pounds.
High on the list ofother requirements are “stiffness”—the ability of the industrial robot toperform the task without flexing or shifting; accuracy—the ability toperform repetitive tasks without deviating from the programmed dimensional tolerances; and high rates of acceleration and deceleration.The size of the manipulator or arm influences accessibility to theassigned floor space.
Movement is a key consideration in choosing anindustrial robot. The industrial robot must be able to reach all the parts ortools needed for its application. Thus the industrial robot’s workingrange or envelope is a critical factor in determining industrial robot size.Most versatile industrial robots are capable of moving in at least fivedegrees of freedom, which means they have five axes. Although mosttasks suitable for industrial robots today can be performed by industrialrobots with at least five axes, industrial robots with six axes (or degreesof freedom) are quite common. Rotary base movement and both radialand vertical arm movement are universal. Rotary wrist movement andwrist bend are also widely available. These movements have been designated as roll and pitch by some industrial robot manufacturers.
Wristyaw is another available degree of freedom.261262Chapter 10Manipulator GeometriesMore degrees of freedom or axes can be added externally by installingparts-handling equipment or mounting the industrial robot on tracks orrails so that it can move from place to place. To be most effective, allaxes should be servo-driven and controlled by the industrial robot’s computer system.Chapter 11 Proprioceptive andEnvironmental SensingMechanisms and DevicesCopyright © 2003 by The McGraw-Hill Companies, Inc. Click here for Terms of Use.This page intentionally left blank.Mechanical limit switches are devices that sense objects by beingeither directly or indirectly touched by the object.
Most use abutton, lever, whisker, or slide as their local sensor. Two other typesthat warrant their own categories are the magnetic reed switch and themembrane switch, which is much like a long button actuated switch.On a robot, the switch alone can be the whole sensor, but in most casesthe switch makes up only a part of a sensor package.The limit switch can be thought of as a device that has at least oneinput and one output. The input is the button, lever, whisker, or slide(or for the magnetic type, anything ferrous nearby). The output isalmost always closing or opening an electric circuit. There are severalother types of limit switches whose inputs and outputs are differentthan those discussed above, but only those that sense by direct contactor use magnets will be included here. Other types are not strictlymechanical and are more complex and beyond the scope of this book.In a robot, there are two general categories of things that the robot’smicroprocessor needs to know about, many of which can be sensed bymechanical limit switches.
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