Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 27
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Powering only oneof the three wheels is the major cause of this problem. Nevertheless,there have been many successful indoor test platforms that use this layout precisely because of its simplicity.In order to improve the mobility and stability of motorcycles, the threewheeled All Terrain Cycle (ATC) was developed. This vehicle demonstrates the next step up in the mobility of three wheeled vehicles.
Therear two wheels are powered through a differential, and the front steers.This design is still simple, but although ATCs seemed to have highmobility, they did not do well in forest environments filled with rocksand logs, etc. The ATC was eventually outlawed because of its majorflaw, very poor stability. Putting the single wheel in front lead to reducedresistance to tipping over the front wheel. This is also the most commonform of accident with a child’s tricycle.Increasing the stability of a tricycle can be easily accomplished byreversing the layout, putting the two wheels in front. This layout worksfine for relatively low speeds, but the geometry is difficult to controlwhen turning at higher speeds as the forces on the rear steering wheeltend to make the vehicle turn more sharply until eventually it is out ofcontrol.
This can be minimized by careful placement of the vehicle’scenter of gravity, moving it forward just the right amount without goingso far that a hard stop flips the vehicle end over end. A clever version ofthis tail dragger-like layout gets around the problem of flipping over byvirtue of its ability to flip itself back upright simply by accelerating rapidly. The vehicle flips over because there is no lever arm to resist thetorque in the wheels. Theoretically, this could be done with a tricyclealso. At low speeds, this layout has similar mobility to a tail dragger and,in fact, they are very similar vehicles.Steering with the front wheels on a reversed tricycle removes thesteering problem, but adds the complexity of steering and driving bothwheels.
This layout does allow placing more weight on the passiverear wheel, significantly reducing the flipping over tendencies, andmobility is moderately good. The layout is still dragging around a passive wheel, however, and mobility is further enhanced if this wheel ispowered.137138Chapter 4Wheeled Vehicle Suspensions and DrivetrainsFigure 4-6 Reversed tricycle,differential steerFigure 4-7front steerReversed tricycle,Chapter 4Wheeled Vehicle Suspensions and Drivetrains139Figure 4-8 Reversed tricycle, alldrive, all steerThe most complicated and highest mobility three-wheeled layout isone where all wheels are powered and steered. This layout is extremelyversatile, providing motion in any direction without the need to be moving; it can turn in place.
This ability is called holonomic motion and isvery useful for mobile robots because it can significantly improve mobility in cluttered terrain. Of the vehicles discussed so far, all, except thefront steer reversed tricycle, can be made holonomic if the third wheellies on the circumference of the circle whose center is midway betweenthe two opposing wheels, and the steering or passive wheel can swingthrough 180 degrees.
To be truly holonomic, even in situations where thevehicle is enclosed on three sides, like in a dead-end hallway, the vehicleitself must be round. This enables it to turn at any time to find a path outof its trap. See Figure 4-8.Before we investigate four-wheeled vehicles, there is a mechanismthat must be, at least basically, understood—the differential. The differential (Figure 4-9) gets its name from the fact that it differentiates therotational velocity of two wheels driven from one drive shaft. The mostbasic differential uses a set of gears mounted inside a larger gear, but onan axis that lies along a radius of the larger gear. These gears rotate withthe large gear, and are coupled to the axles through crown gears on theends of the axles.
When both wheels are rolling on relatively high fric-140Chapter 4Wheeled Vehicle Suspensions and DrivetrainsFigure 4-9 The common andunpredictable differentialtion surfaces, and the vehicle is going straight, the wheels rotate at thesame rpm. If the vehicle turns a corner, the outside wheel is traversing alonger path and therefore must be turning faster than the inside wheel.The differential facilitates this through the internal gears, which rotateinside the large gear, allowing one axle to rotate relative to the other.
Thissystem, or something very much like it, is what is inside virtually everycar and truck on the road today. It obviously works well.The simple differential has one drawback. If one wheel is rolling on asurface with significantly less friction, it can slip and spin much fasterthan the other wheel. As soon as it starts to slip, the friction goes downfurther, exacerbating the problem. This is almost never noticed by ahuman operator, but can cause mobility problems for vehicles that frequently drive on slippery surfaces like mud, ice, and snow.There are a couple of solutions. One is to add clutches between theaxles that slide on each other when one wheel rotates faster than theother.
This works well, but is inefficient because the clutches absorbpower whenever the vehicle goes around a corner. The other solution isthe wonderfully complicated Torsen differential, manufactured by Zexel.The Torsen differential uses specially shaped worm gears to tie thetwo axles together. These gears allow the required differentiationbetween the two wheels when turning, but do not allow one wheel to spinas it looses traction. A vehicle equipped with a Torsen differential caneffectively drive with one wheel on ice and the other on hard dry pavement! This differential uses very complex gear geometries. The bestexplanation of how it works can be found on Zexel’s web site:www.torsen.com.Chapter 4Wheeled Vehicle Suspensions and Drivetrains141FOUR-WHEELED LAYOUTSThe most basic four-wheeled vehicle actually doesn’t even use a differential.
It has two wheels on each side that are coupled together and issteered just like differential steered tricycles. Since the wheels are in lineon each side and do not turn when a corner is commanded, they slide asthe vehicle turns. This sliding action gives this steering method itsname—Skid Steer. Notice that this layout does not use differentials, eventhough it is also called differential steering.Skid steered vehicles are a robust, simple design with good mobility,in spite of the inefficiency of the sliding wheels.
Because the wheelsdon’t turn, it is easy to attach them to the chassis, and they don’t take upthe space required to turn. There are many industrial off-road skidsteered vehicles in use, popularly called Bobcats. Figure 4-10 shows thata skid steered vehicle is indeed very simple.The problem with skid steered, non-suspended drivetrains is that as thevehicle goes over bumps, one wheel necessarily comes off the ground.This problem doesn’t exist in two or three wheeled vehicles, but is amajor thing to deal with on vehicles with more than three wheels. Thoughnot a requirement for good mobility, it is better to use some mechanismthat keeps all the wheels on the ground. There are many ways to accomplish this, starting with a design that splits the chassis in two.Figure 4-10 All four fixed, skidsteered142Chapter 4Wheeled Vehicle Suspensions and DrivetrainsFigure 4-11 Simple longitudinalrockerThe longitudinal rocker design divides the entire vehicle right downthe middle and places a passive pivot joint in between the two halves.This joint is connected on each end to a rocker arm, which in turn carry awheel at each of their ends.
This layout allows the rocker arms to pivotwhen any wheel tries to go higher or lower than the rest. This passivepivoting action keeps the load on all four wheels almost equal, increasing mobility simply by maintaining driving and braking action on allwheels at all times. Longitudinal rocker designs are skid steered, withthe wheels on each side usually mechanically tied together like a simpleskid steer, but sometimes, to increase mobility even further, the wheelsare independently powered.
Figure 4-11 shows the basic layout, developed by Sandia Labs for a vehicle named Ratler.The well-known forklift industrial truck uses a sideways version ofthe rocker system. Since its front wheels carry most of all loads lifted bythe vehicle, structurally tying the wheels together is a more robust layout. These vehicles have four wheels without any suspension, and, therefore, require some method of keeping all the wheels on the ground.
Themost common layout has the front wheels tied together and a rockerinstalled transversely and coupled to the rear wheels, which are usuallythe steering wheels. Figure 4-12 shows this layout.The weakness of the forklift is that it is usually only two-wheel drive.This works well for its application, and because so much of the weight ofthe vehicle is over the front wheels. In general, though, powering all fourwheels provides much higher mobility. In a two-wheel drive vehicle, thedriven wheels must provide traction not only for whatever they are tryingto get over, but also must push or pull the non-driven wheels. Many ofthe wheeled layouts are complex enough that they require a motor forChapter 4Wheeled Vehicle Suspensions and Drivetrains143Figure 4-12 Rear transverserocker, rear steerevery wheel.
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