Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 32
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This is what a urethane track does. There are no pinch pointsat all; the track is a continuous loop with or without treads. Molding thetreads into the urethane works for most surface types. It is very tough,relatively high friction compared to steel, and inexpensive. It also doesnot damage prepared roads. Ironically, if higher traction is needed, steelcleats can be bolted to the urethane.
Just like urethane road pads on steeltracks, the steel links are usually designed to be removable.Urethane by itself is too stretchy for most track applications. Thisweakness is overcome by molding the urethane over steel cables. Thesteel is completely covered by the urethane so there is no corrosion prob-170Chapter 5Tracked Vehicle Suspensions and DrivetrainsFigure 5-3 Urethane pads forhard surface roadsFigure 5-4 Cross section of urethane molded track withstrengthening bars and internalcablesChapter 5Tracked Vehicle Suspensions and Drivetrainslem. The steel eliminates stretching, and adds little weight to the system.For even greater strength, hardened steel crossbars are molded into thetrack.
These bars are shaped and located so that the teeth on the drivesprocket can push directly on them. This gives the urethane track muchgreater tension strength, and extends its life. Yet another modification tothis system is to extend these bars towards the outer side of the track,where they reinforce the treads. This is the most common layout for urethane tracks on industrial vehicles.
Figure 5-4 shows a cross section ofthis layout.TRACK SHAPESThe basic track formed by a drive sprocket, idler, and road wheels workswell in many applications, but there are simple things that can be done tomodify this oblong shape to increase its mobility and robustness.Mobility can be increased by raising the front of the track, which aids ingetting over taller obstacles. Robustness can be augmented by movingvulnerable components, like the drive sprocket, away from possiblyharmful locations.
These improvements can be applied to any trackdesign, but are unnecessary on variable or reconfigurable tracks.The simplest way to increase negotiable obstacle height is to make thefront wheel of the system larger. This method does not increase the complexity of the system at all, and in fact can simplify it by eliminating theneed for support rollers along the return path of the track. This layout,when combined with locating the drive sprocket on the front axle, alsoraises up the drive system.
This reduces the chance of damaging thedrive sprocket and related parts. Many early tanks of WWI used thistrack shape.Another way to raise the ends of the track is to make them into ramps.Adding ramps can increase the number of road wheels and therefore thenumber of moving parts, but they can greatly increase mobility. Rampingthe front is common and has obvious advantages, but ramping the backcan aid mobility when running in tight spaces that require backing upover obstacles. As shown in Figure 5-5 (a–d), ramps are created by raising the drive and/or idler sprocket higher than the road wheels. Some ofthese designs increase the volume inside the track system, but this volume can potentially be used by other components of the robot.More than one company has designed and built track systems that canchange shape. These variable geometry track systems use a track that ismore flexible than most, which allows it to bend around smaller sprockets and idler wheels, and to bend in both directions.
The road wheels are171172Chapter 5Tracked Vehicle Suspensions and DrivetrainsFigure 5-5a–d Various trackshapes to improve mobility androbustnessFigure 5-5bFigure 5-5cChapter 5Tracked Vehicle Suspensions and Drivetrains173Figure 5-5dusually mounted directly to the chassis through some common suspension system, but the idler wheel is mounted on an arm that can movethrough an arc that changes the shape of the front ramp. A second tensioning idler must be incorporated into the track system to maintain tension for all positions of the main arm.This variability produces very good mobility when system height isincluded in the equation because the stowed height is relatively smallcompared to the negotiable obstacle height. The effectively longer track,in addition to a cg shifting mechanism, gives the vehicle the ability tocross wider crevasses. With simple implementations of this concept, thevariable geometry track system is a good choice for a drive system formobile robots.
Figure 5-6 (a–b) shows one layout for a variable geometry track system. Many others are possible.Figure 5-6a–b Variable tracksystem174Chapter 5Tracked Vehicle Suspensions and DrivetrainsFigure 5-6bSince it carries both the tension in the track and the drive torque,the drive sprocket (and associated drive mechanism) is the most vulnerable moving part of a track system. They can be located at eitherthe front or rear of the track, though they are usually in the rear tokeep them away from the inevitable bumps the front of an autonomousvehicle takes. Raising the sprocket up off the ground removes thesprocket from possible damage when hitting something on the roadsurface.
These modifications result in a common track shape, shownin Figure 5-5c.A simple method that extends the mobility of a tracked vehicle is toincorporate a ramp into the chassis or body of the vehicle. The staticramp extends in front and above the tracks and slides up obstacles thatare taller than the track.
This gives the vehicle the ability to negotiateobstacles that are taller than the mobility system using a non-movingpart, a neat trick.TRACK SUSPENSION SYSTEMSThe space between the drive sprocket and idler wheel needs to be uniformly supported on the ground to achieve the maximum benefit oftracks. This can be done in one of several ways. The main differencesbetween these methods is drive efficiency, complexity, and ride characteristics.
For especially long tracks, the top must also be supported, butChapter 5Tracked Vehicle Suspensions and DrivetrainsFigure 5-7this is usually a simple passive roller or two evenly spaced between thedrive sprocket and idler.The main types of ground support methods are••••Guide bladesFixed road wheelsRocker road wheel pairsRoad wheels mounted on sprung axlesGuide blades are simple rails that are usually designed to ride in the Vshaped guide receivers on the track’s links.
They can extend continuously from one end to the other, and therefore are the most effective atsupporting the track along its whole length. Unfortunately, they are alsoquite inefficient since there is the long sliding surface that cannot bepractically lubricated. They also produce a jarring ride for the rest of thevehicle.One step up from guide blades is fixed road wheels (Figure 5-7).These are wheels on short stub-shafts solidly mounted to the robot’schassis. The wheels can be small relative to the track, since the thing theyroll on is always just the smooth inner surface of the track. They too produce a jarring ride, more so than guide blades, but they are far more efficient. Fixed rollers are a good choice for a robust track system on a robotsince ride comfort is not as important, at lower speeds, as on a vehiclecarrying a person.The bumpy ride does hamper track efficiency, however, because thechassis is being moved up and down by the rough terrain.
Reducing this175Fixed road wheels176Chapter 5Figure 5-8rockersTracked Vehicle Suspensions and DrivetrainsRoad wheels onmotion is especially beneficial at higher speeds, and the rocker layoutused on wheeled vehicles is almost as effective on tracks. The rollers aremounted in pairs on rockers between the drive sprocket and the idlerwheel.
The rockers (Figure 5-8) allow the track to give a little when traversing bumpy terrain, which reduces vertical motion of the robot chassis.Careful tensioning of the track is essential with movable road wheels.The most complex, efficient, and smooth ride is produced by mounting the road wheels on sprung axles. There are three main types of suspension systems in common use.• Trailing arm on torsion spring• Trailing arm with coil spring• Leaf spring rockerThe trailing arm on a torsion spring is pictured in Figure 5-9.
It is asimple device that relies on twisting a bunch of steel rods, to which thetrailing arm is attached at one end. It gets its name because the arms thatsupport the wheel trail behind the point where they attach, through thetorsion springs, to the chassis. The road wheels mount to the end of thetrailing arms and forces on the road wheel push up on the arm, twistingthe steel rods. This system was quite popular in the 1940s and 1950s andwas used on the venerable Volkswagen beetle to support the frontwheels. It was also used on the Alvis Stalwart, described in more detailin Chapter Four.You can also support the end of the trailing arm with a coil spring, oreven a coil over-shock suspension system that can probably produce thesmoothest ride of any track system (Figure 5-10).
The shock can also beadded to the torsion arm suspension system. The advantage of the coilChapter 5Tracked Vehicle Suspensions and DrivetrainsFigure 5-9177Trailing armFigure 5-10 Trailing arm andcoil springsspring over the torsion suspension is that the load is supported by thespring very close to the load point, reducing forces and moments in thetrailing arm. This can reduce the weight of the suspension system, andputs the system more inside the track’s volume rather than inside thechassis.178Chapter 5Tracked Vehicle Suspensions and DrivetrainsFigure 5-11 Leaf spring rockersA simple variation of the rocker system is to replace the rockers withleaf springs (Figure 5-11). This eases the shock to the rocker and produces a smoother ride. The springs are usually very stiff since the rockerarm’s swinging motion still allows the wheels to make large motions.This system can be retrofitted to rocker arm suspension systems if thecurrent rocker arm does not smooth the ride enough.
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