Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 30
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Imagine the vehicle in Figure 4-30, butwith the pivot axis on its side. This layout is similar to the double rockerlayout, with similar mobility and fewer moving parts.The two halves of an eight-wheeled layout can also be coupledtogether with a ball joint. The ball joint allows pitch, roll, and yawbetween the two parts which facilitates keeping all eight wheels on theground most of the time. The ball joint is a simple joint and can be maderobust. It has a limited range of motion around two of the axis, but thethird axis can rotate three hundred sixty degrees.
Aligning this axis vertically aligns it in the steering axis. This allows the vehicle to have atighter steering radius, but it cannot turn in place. Figure 4-31 shows thefour-wheeled sections connected through a vertical axis ball joint. Theball joint is difficult to use with a four-wheeled vehicle because theChapter 4Wheeled Vehicle Suspensions and Drivetrains157Figure 4-28 Eight wheels, double bogiewheel torque would try to spin the section around the wheels. This problem can be reduced if the wheels are coupled together so their torquesare always nearly indentical.Figure 4-29 Two part, eightwheeled, vertical center pivot158Chapter 4Wheeled Vehicle Suspensions and DrivetrainsFigure 4-30 Two part, eightwheeled, vertical and roll jointsFor a truly complicated wheeled drive mechanism, the Tri-star LandMaster from the movie Damnation Alley is probably the most impressive.
This vehicle, of which only one was built, is a two-section, centerpivot steered layout with a Tri-star wheel at each corner. The Tri-starwheels consist of three wheels, all driven together, arranged in a three-Figure 4-31 Two part, eightwheeled, vertical ball jointChapter 4Wheeled Vehicle Suspensions and Drivetrainspointed star on a shared hub that is also driven by the same shaft thatdrives the wheels. When a bump or ditch is encountered that the wheelsalone cannot traverse, the whole three-wheeled system rotates around thecenter hub and the wheels essentially become very large cleats. The Tristar wheels are driven through differentials on the Land-Master, but powering each with its own motor would increase mobility even further.159This page intentionally left blank.Chapter 5Tracked VehicleSuspensionsand DrivetrainsCopyright © 2003 by The McGraw-Hill Companies, Inc.
Click here for Terms of Use.This page intentionally left blank.There has long been a belief that tracks have inherently better mobility than wheels and anyone intending to design a high mobility vehicle should use tracks. While tracks can breeze through situations wherewheels would struggle, there are only a few obstacles and terrains whichwould stop a six wheeled rocker bogie vehicle, but not stop a similarsized tracked vehicle.
They are• very soft terrain: loose sand, deep mud, and soft powder snow• obstacles of a size that can get jammed between wheels• crevassesThey get this higher mobility at a cost of greater complexity and lowerdrive efficiency, so tracks are better for these situations, but not inherently better overall.Tracked vehicles first started to appear in the early 1900s and wereused extensively in WWI. The basic layout used then is still in use todayon heavy construction equipment; a drive sprocket at one end, an idlerwheel at the other that usually serves as a tensioner, and something inbetween to support the tracks on the ground.
This basic, simple layout isrobust and easy to control. Even in its most simple form, this track layouthas all of the improvements over wheels previously listed.The continuous surface in contact with the ground is what producesthe benefits of tracks.
The long surface combined with widths similar towheels puts a large surface on the ground. This lowers ground pressure,allowing traveling on softer surfaces. It also provides more area fortreads, increasing the number of teeth on the ground.The continuous surface eliminates a wheeled vehicle’s problem ofbecoming high centered between the wheels on one side. A correctlysized obstacle can get caught between the wheels on one side, but thetrack stays on top. The wheeled vehicle can get stuck in these situations,where the track would simply roll over the obstacle.Perhaps the most important capability the continuous surface facilitates that a wheeled suspension cannot match (without undue complexity) is the ability to cross crevasses.
Clever suspension components canbe added to a six-wheeled or eight-wheeled vehicle to increase its nego163164Chapter 5Tracked Vehicle Suspensions and Drivetrainstiable crevasse width, but these add complexity to the wheeled vehicle’sinherent simplicity. Tracks, however, have the ability to cross crevassesbuilt in to their design. Add a mechanism for shifting the center of gravity, and a tracked vehicle can cross crevasses that are wider than half thelength of the vehicle.Most types have many more moving parts than a wheeled layout, allof which tend to increase rolling friction, but a well-designed track canactually be more efficient than a wheeled vehicle on very soft surfaces.The greater number of moving parts also increase complexity, and oneof the major problems of track design is preventing the track from beingthrown off the suspension system.
Loosing a track stops the vehiclecompletely.Track systems are made up of track, drive sprocket, idler/tensionwheel, suspension system, and, sometimes, support rollers. There areseveral variations of the track system, each with its own set of bothmobility and robustness pros and cons.• The design of the track itself (steel links with hinges, continuous rubber, tread shapes)• Method of keeping the tracks on the vehicle (pin-in-hole, guideknives, V-groove)• Suspension system that supports the track on the ground (sprung andunsprung road wheels, fixed guides)• Shape of the one end or both ends of the track system (round orramped)• Relative size of the idler and/or drive sprocketVariations of most of these system layouts have already been tried,some with great success, others with apparently no improvements inmobility.There are also many varieties of track layouts and layouts with different numbers of tracks.
These various layouts have certain advantages anddisadvantages over each other.•••••One track with a separate method for steeringThe basic two track side-by-sideTwo tracks and a separate method for steeringTwo track fore-and-aftSeveral designs that use four tracksChapter 5Tracked Vehicle Suspensions and Drivetrains• A six-tracked layout consisting of two main tracks and two sets offlipper tracks and each endThe six-track layout may be overkill because there is a patented tracklayout that has truly impressive mobility that has four tracks and usesonly three actuators.Robots are slowly coming into common use in the home and onetough requirement in the otherwise benign indoor environment is climbing stairs.
It is just plain difficult to climb stairs with any rolling drivesystem, even one with tracks. Tracks simplify the problem somewhat andcan climb stairs more smoothly than wheeled drivetrains, allowinghigher speeds, but they have difficulty staying aligned with the stairs.They can quickly become tilted over, requiring steering corrections thatare tricky even for a human operator. At the time of this writing there isno known autonomous vehicle that can climb a full flight of stairs without human input.This chapter covers all known track layouts that have been or arebeing used on production vehicles ranging in size from thirty centimeterslong (about a foot) to over forty five meters (a city block).
Tracks can beused with good effectiveness on small vehicles, but problems candevelop due to the stiffness of the track material. Toys only ten centimeters long have used tracks, and at least one robotics researcher has constructed tiny robots with tracks about twenty-five millimeters long.These fully autonomous robots were about the size of a quarter. Inuktun(www.inuktun.com) makes track units for use in pipe crawling robotsthat are about twenty-five centimeters longThe opposite extreme is large construction equipment and militarytanks like the M1A2 Abrams. The M1A2’s tracks are .635 meters wide(the width of a comfortable chair) and 4.75 meters long (longer thanmost cars) and together, including the suspension components, make upnearly a quarter of the total weight of the tank.
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