Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 37
Текст из файла (страница 37)
There are nofigures included here, but the reader is urged to investigate these websites:http://mozu.mes.titech.ac.jp/http://www.aist.go.jp/MEL/mainlab/rob/rob08e.htmlFLEXIBLE LEGSA trick taken from animals and being tested in mobility labs is the use offlexible-leg elements. A compliant member can sometimes be used togreat advantage by reducing the requirement for exact leg placement.They are simple, extremely robust mobility systems that use independentleg-walking techniques. A simple version of this concept is closer to awheeled robot than a walker. The tires are replaced with several longflexible arms, like whiskers, extending out from the wheel. Thisincreases their ability to deal with large perturbations in the environ-Chapter 7Walkers215Figure 7-15 Whisker-wheeledroller walkerment, but decreases efficiency.
They have very high mobility, able toclimb steps nearly as high as the legs are long. Robotics researchers areworking on small four- and six-wheel leg robots that use this conceptwith very good results. Figure 7-15 shows the basic concept. A variationof this design extends the whisker legs more axially than radially.
Thisidea is taken from studying cockroaches whose legs act like paddleswhen scrambling over bumpy terrain.If walking is being considered as the mobility system for anautonomous robot, there are several things to remember.• Using a statically-stable design requires far less expertise in severalfields of engineering and will therefore dramatically increase thechances of success.• Frame walking is easier to implement than wave- or independent-legwalking.• Studies have shown six legs are optimal for most applications.• Rotary joints are usually more robust.216Chapter 7WalkersWalkers have inherently more degrees of freedom, which increasescomplexity and debug time. As will be investigated in the chapter onmobility, walkers deal with rugged terrain very well, but may not actually be the best choice for a mobility system. Roller walkers offer theadvantages of both walking and rolling and in a well thought out designmay prove to be very effective.Walkers have been built in many varieties.
Some are variations onwhat has been presented here. Some are totally different. In general, withthe possible exception of the various roller walkers, they share two common problems, they are complicated and slow. Nature has figured outhow to make high-density actuators and control many of them at a timeat very high speed. Humans have figured out how to make the wheel andits close cousin, the track. The fastest land animal, the cheetah, has beenclocked at close to 100km/hr.
The fastest land vehicle has hit more thanseven times that speed. Contrarily, a mountain goat can literally runalong the face of a steep cliff and a cockroach can scramble over terrainthat has obstacles higher than itself, and can do so at high speed. Thereare no human-made locomotion devices that can even come close to agoat’s or cockroach’s combined speed and agility.Nature has produced what is necessary for survival, but nothing more.Her most intelligent product has not yet been able to produce anythingthat can match the mobility of several of her most agile products. Perhapssomeday we will. For the person just getting started in robotics, or forsomeone planning to use a robot to do a practical task, it is suggested tostart with a wheeled or tracked vehicle because of their greater simplicity.For a mechanical engineer interested in designing a complex mechanismto learn about statics, dynamics, strength of materials, actuators, kinematics, and control systems, a walking robot is an excellent tool.Chapter 8Pipe Crawlersand OtherSpecial CasesCopyright © 2003 by The McGraw-Hill Companies, Inc.
Click here for Terms of Use.This page intentionally left blank.There are many less obvious applications for mobile robots. One particularly interesting problem is inspecting and repairing pipelinesfrom the inside. Placing a robot inside a pipe reduces and, sometimes,removes the need to dig up a section of street or other obstruction blocking access to the pipe. The robot can be placed inside the pipe at a convenient location by simply separating the pipe at an existing joint orvalve.
These pipe robots, commonly called pipe crawlers, are very special designs due to the unique environment they must work in. Pipecrawlers already exist that inspect, clean, and/or repair pipes in nuclearreactors, water mains under city streets, and even down five-mile longoil wells.Though the shape of the environment may be round and predictable,there are many problems facing the locomotion system of a pipe crawler.The vehicle might be required to go around very sharp bends, throughwelded, sweated, or glued joints. Some pipes are very strong and thecrawlers can push hard against the walls for traction, some are very softlike heating ducts requiring the crawler to be both light and gentle.
Somepipes transport slippery oil or very hot water. Some pipes, like watermains and oil pipelines, can be as large as several meters in diameter;other pipes are as small as a few centimeters. Some pipes change sizealong their length or have sections with odd shapes.All these pipe types have a need for autonomous robots. In fact, pipecrawling robots are frequently completely autonomous because of thedistance they must travel, which can be so far that it is nearly impossibleto drag a tether or communicate by radio to the robot when it is inside thepipe.
Other pipe crawlers do drag a tether which can place a large load onthe crawler, forcing it to be designed to pull very hard, especially whilegoing straight up a vertical pipe. All of these problems place unusual anddifficult demands on the crawler’s mechanical components and locomotion system.End effectors on these types of robots are usually inspection tools thatmeasure wall thickness or cameras to visually inspect surface conditions.Sometimes mechanical tools are employed to scrape off surface rust orother corrosion, plug holes in the pipe wall, or, in the case of oil wells,blow holes in the walls.
These effectors are not complex mechanically219220Chapter 8Pipe Crawlers and Other Special Casesand this chapter will focus on the mobility systems required for unusualenvironments and unusual methods for propulsion including externalpipe walking and snakes.The pipe crawler mechanisms shown in the following figures give anoverview of the wide variety of methods of locomoting inside a pipe.Choosing between one and the other must be based on the specific attributes of the pipe and the material it transports, and if the robot has to workin-situ or in a dry pipe. In addition to those shown in this book, there aremany other techniques and layouts for robots designed to move about inpipes or tanks.HORIZONTAL CRAWLERSMoving along horizontal pipes is very similar to driving on level ground.The crawler must still be able to steer to some degree because it mustnegotiate corners in the pipes, but also because it must stay on the bottom of the pipe or it may swerve up the walls and tip over.
There aremany horizontal pipe crawlers on the market that use the four-wheeledskid-steer principle, but tracked drives are also common. The wheels ofwheeled pipe crawlers are specially shaped to conform to the roundshape of the pipe walls, on tracked crawlers the treads are tilted for thesame reason. These vehicles’ suspension and locomotion systems arefrequently quite simple. Figures 8-1 and 8-2 show two examples.Figure 8-1 Four-wheeledhorizontal pipe crawlerChapter 8Pipe Crawlers and Other Special Cases221Figure 8-2 Two-track horizontalpipe crawlerVERTICAL CRAWLERSRobotic vehicles designed to travel up vertical pipe must have some wayto push against the pipe’s walls to generate enough friction.
There aretwo ways to do this, reaching across the pipe to push out against thepipe’s walls, or putting magnets in the tires or track treads. Some slippery nonferrous pipes require a combination of pushing hard against thewalls and special tread materials or shapes. Some pipes are too soft towithstand the forces of tires or treads and must use a system that spreadsthe load out over a large area of pipe.There is another problem to consider for tethered vertical pipecrawlers. Going straight up a vertical pipe would at first glance seemsimple, but as the crawler travels through the pipe, it tends to corkscrewbecause of slight misalignment of the locomotors or deformities on thepipe’s surface.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
