Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 38
Текст из файла (страница 38)
This corkscrewing winds up the tether, eventually twisting and damaging it. One solution to this problem is to attach the tetherto the chassis through a rotary joint, but this introduces another degree offreedom that is both complex and expensive. For multi-section crawlers,a better solution is to make one of the locomotor sections steerable by asmall amount.222Chapter 8Pipe Crawlers and Other Special CasesTraction Techniques for Vertical Pipe CrawlersThere are at least four tread treatments designed to deal with the tractionproblem.• spikes, studs, or teeth• magnets• abrasives or nonskid coating• high-friction material like neopreneEach type has its own pros and cons, and each should be studied carefully before deploying a robot inside a pipe because getting a stuck robotout of a pipe can be very difficult. The surface conditions of the pipewalls and any active or residual material in the pipe should also be investigated and understood well to assure the treatment or material is notchemically attacked.Spiked, studded, or toothed wheels or treads can only be used wheredamage to the interior of the pipe can be tolerated.
Galvanized pipewould be scratched leading to corrosion, and some hard plastic pipematerial might stress crack along a scratch. Their advantage is that theycan generate very high traction. Spiked wheels do find use in oil wells,which can stand the abuse. They require the crawler to span the inside ofthe pipe so they can push against opposing walls.The advantage of magnetic wheels is that the wheels pull themselvesagainst the pipe walls; the disadvantage is that the pipe must be made ofa ferrous metal.
Magnets remove the need to have the locomotion systemprovide the force on the walls, which reduces strain on the pipe. Theyalso have the advantage that the crawler can be smaller since it no longermust reach across the whole of a large pipe. Use of magnetic wheels isnot limited to pipe crawlers and should be considered for any robot thatwill spend most of its life driving on a ferrous surface.Tires made of abrasive impregnated rubber hold well to iron and plastic pipe, but these types loose effectiveness if the abrasive is loaded withgunk or worn off. Certain types of abrasives can grip the surface of cleandry pipes nearly as well as toothed treads, and cause less damage.High-friction rubber treads work in many applications, but care mustbe taken to use the right rubber compound.
Some rubbers maintain muchof their stickiness even when wet, but others become very slippery. Somecompounds may also corrode rapidly in fluids that might be found inpipes. They cause no damage to pipe walls and are a simple and effectivetraction technique.Chapter 8Pipe Crawlers and Other Special CasesFigure 8-3Wheeled Vertical Pipe CrawlersWheeled pipe crawlers, like their land-basedcousins, are the simplest type of vertical pipecrawlers.
Although these types use wheels andnot tracks, they are still referred to as pipecrawlers. Practical layouts range from three tosix or more wheels, usually all driven for maximum traction on frequently very slippery pipewalls.Theoretically, crawling up a pipe can be donewith as little as one actuator and one passivesprung joint. Figure 8-3 shows the simplest layout required for moving up vertical pipe. Thisdesign can easily get trapped or be unable to passthrough joints in the pipe and can even bestopped by large deformities on the pipe walls.The next best layout adds a fourth wheel. Thislayout is more capable, but there are situations incertain types of pipes and pipe fittings in which ittoo can become trapped, see Figure 8-4.
The center linear degree of freedom can be actuated tokeep the vehicle aligned in a pipe.Figure 8-4Four-wheeled, center steer223Basic three-wheeled224Chapter 8Pipe Crawlers and Other Special CasesFigure 8-5 Three locomotors,spaced 120º apartTRACKED CRAWLERSWheeled crawlers work well in many cases, but tracks do offer certainadvantages. They exert much less pressure on any given spot due to theirlarger footprint. This lower pressure tends to scratch the pipe less.Spreading out the force of the mechanism that pushes the locomotor sections against the walls also means that the radial force itself can behigher, greatly increasing the slip resistance of the vehicle.
Figure 8-5shows the very common three-locomotor tracked pipe crawler.OTHER PIPE CRAWLERSFor pipes that cannot stand high internal forces, another method must beused that further spreads the forces of the crawler over a larger area.There are at least two concepts that have been developed. One uses balloons, the other linear extending legs.The first is a unique concept that uses bladders (balloons) on eitherend of a linear actuator, that are filled with air or liquid and expand topush out against the pipe walls. The rubber bladders cover a very largesection of the pipe and only low pressure inside the bladder is required toChapter 8Pipe Crawlers and Other Special CasesFigure 8-6 Inchwormmulti-section roller walkerget high forces on the pipe walls, generating high-friction forces.Steering, if needed, is accomplished by rotating the coupling betweenthe two sections.This coupling is also the inchworm section, and forward motion of theentire vehicle is done by retracting the front bladder, pushing it forward,expanding it, retracting the rear section, pulling it towards the front section, expanding it, then repeating the whole process.
Travel is slow, andthis concept does not deal well with obstructions or sharp corners, butthe advantage of very low pressures on the pipe walls may necessitateusing this design. A concept that uses this design was proposed for moving around in the flexible Kevlar pipes of the Space Shuttle.Another inchworm style pipe crawler has a seemingly complex shape,but this shape has certain unusual advantages. The large pipes insidenuclear reactor steam pipes have sensors built into the pipes that extend infrom the inner walls nearly to the center of the pipe. These sensor wellsare made of the same material as the pipe, usually a high-grade stainlesssteel, but cannot be scraped by the robot.
The robot has to have a shapethat can get around these protrusions. An inchworm locomotion vehicleconsisting of three sections, each with extendable legs, provides greatmobility and variable geometry to negotiate these obstacles. Figure 8-6shows a minimum layout of this concept.225226Chapter 8Pipe Crawlers and Other Special CasesEXTERNAL PIPE VEHICLESThere are some applications that require a vehicle to move along the outside of a pipe, to remove unwanted or dangerous insulation, or to movefrom one pipe to another in a process facility cluttered with pipes.CMU’s asbestos removing external pipe walker, BOA, is just such avehicle.
Though not a robot according to this book’s definition, it is stillworth including because it shows the wide range of mobility systems thattrue robots might eventually have to have to move in unexpected environments. BOA is a frame walker. Locomotion is accomplished by moving and clamping one set of grippers on a pipe, extending another setahead on the pipe, and grasping the pipe with a second set of grippers.RedZone Robotics’ Tarzan, an in-tank vertical pipe walking arm, is anexample of a very unusual concept proposed to move around inside atank filled with pipes.
This vehicle is similar to the International SpaceStation’s maintenance arm in that it moves from one pipe to another, onthe outside of the pipes. Unlike the ISS arm, Tarzan must work againstthe force of gravity. Since Tarzan is not autonomous, it uses a tether toget power and control signals from outside the tank.
The arm is allhydraulic, using both rotary actuators and cylinders. All together, thereare 18 actuators. Imagine the complexity of controlling 18 actuators andmanaging a tether all on an arm that is walking completely out of viewinside a tank filled with a forest of pipes!SNAKESIn nature, there is a whole class of animals that move around by squirming. This has been applied to robots with a little success, especially thoseintended to move in all three dimensions. Almost by definition, squirming requires many actuators, flexible members, and/or clever mechanisms to couple the segments. The advantage is that the robot is verysmall in cross section, allowing it to fit into very complex environments,propelling itself by pushing on things.
The disadvantage is that the number of actuators and high moving parts count.There are many other unusual locomotion methods, and many moreare being developed in the rapidly growing field of mobile robots. Thereader is encouraged to search the web to learn more of these varied andsometimes strange solutions to the problem of moving around in uncommon environments like inside and outside pipes, inside undergroundstorage tanks, even, eventually, inside the human body.Chapter 9ComparingLocomotionMethodsCopyright © 2003 by The McGraw-Hill Companies, Inc. Click here for Terms of Use.This page intentionally left blank.WHAT IS MOBILITY?Now that we have seen many methods, mechanisms, and mechanicallinkages for moving around in the environment, let’s discuss how tocompare them.
A standardized set of parameters will be required, but thiscomparison implies that we must first answer the question: What ismobility? Is it defined by how big an obstacle the mobility system can getover, or is it how steep a slope it can climb? Perhaps it is how well, oreven if, it climbs stairs? What about how deep a swamp it can get throughor how wide a crevasse it can traverse? Is speed part of the equation?The answer would seem to be all of these things, but how can we compare the mobility of an autonomous diesel powered 40-ton bulldozer to adouble “A” battery powered throwable two-wheeled tail-dragger robotthe size of a soda can? That seems inherently impossible.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
