Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 29
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Incorporating the Ackerman steering layout removes the abilityof the robot to turn in place. This can be a real handicap in tight places.Figure 4-24 shows the basic layout. Remember that the relative sizes ofwheels and the spacing between them can be varied to produce differentmobility characteristics.The epitome of complexity in a once commercially available sixwheeled vehicle, not recommended to be copied for autonomous robotuse, is the Alvis Stalwart.
This vehicle was designed with the goal ofgoing anywhere in any conditions. It was a six-wheeled (all independently suspended on parallel links with torsion arms) vehicle whose frontfour wheels steered. Each bank of three wheels was driven togetherthrough bevel gears off half-shafts. It had offset wheel hub reductiongear boxes, a lockable central differential power transfer box with integral reversing gears, and twin water jet drives for amphibious propulsion. All six wheels could be locked together for ultimate straight-aheadtraction. No sketch is included for obvious reasons, but a website withgood information and pictures of this fantastically complicated machineis www.4wdonline.com/Mil/alvis/stalwart.html.The main problem with these simple layouts is that when one wheel isup on a bump, the lack of suspension lifts the other wheels up, drasticallyreducing traction and mobility.
The ideal suspension would keep the loadChapter 4Wheeled Vehicle Suspensions and Drivetrains153Figure 4-25 Six wheels, threesections, one DOF between eachsection, skid steeron each wheel the same no matter at what height any one wheel is. Thefollowing suspension systems even out the load on each wheel—somemore than others.An interesting layout that does a good job of maintaining an even loaddistribution divides the robot into three sections connected by a singledegree of freedom joint between each section (Figure 4-25). The centersection has longitudinal joints on its front and back that attach to thecross pieces of the front and rear sections. These joints allow each section to roll independently.
This movement keeps all six wheels on theground. The roll axes are passive, requiring no actuators, but the separation of the wheeled sections usually forces putting a motor at each wheel,and the vehicle is skid steered. This layout has been experimented withby researchers and has very high mobility. The only drawback is that theroll joints must be sized to handle the large forces generated when skidsteering.The rocker bogie suspension system shown in Figure 4-26 uses anextension of the basic four-wheel rocker layout. By adding a bogie toone end of the rocker arm, two wheels can be suspended from one endand one from the other end.
Although this layout looks like it would produce asymmetrical loads on the wheels, if the length of the bogie is halfthat of the rocker, and the rocker is attached to the chassis one third of itslength from the bogie end, the load on each wheel is actually identical.The proportions can be varied to produce uneven loads, which canimprove mobility incrementally for one travel direction, but the basiclayout has very good mobility. The rocker bogie’s big advantage is that it154Chapter 4Wheeled Vehicle Suspensions and DrivetrainsFigure 4-26 Rocker bogiecan negotiate obstacles that are twice the wheel height. This figure showsonly the basic parts of the mobility layout. The part labeled “chassis” isthe backbone or main support piece for the main body, which is notshown.The very fact that each wheel is passively loaded by the rocker bogiesuspension reduces its negotiable chasm width.
Lockable pivots on thebogie can extend the negotiable chasm width by making the centerwheels able to support the weight of the entire vehicle. This adds yetanother actuator to this already complicated layout. This actuator can bea simple band or disc brake.The rocker bogie suspension can be skid steered, but the side forceson the wheels produce moments in the rockers for which the rockersmust be designed. Since the wheels are at the end of arms that move relative to each other, the most common layout puts a motor in each wheel.Steering is done by turning both the front and the rear wheels with theirown steering motors. This means that this layout uses 10 motors toachieve its very high mobility.
In this design, the large number of actuators reduces the number of moving parts and over all complexity.The steering geometry allows turning in place with no skidding at all.This is the layout used on Sojourner, the robot that is now sitting on Marsafter completing an entirely successful exploration mission on the RedPlanet. Mobility experts claim this layout has the highest mobility possi-Chapter 4Wheeled Vehicle Suspensions and Drivetrainsble in a wheeled vehicle, but this high mobility comes at the cost of thoseten actuators and all their associated control electronics and debug time.There is a layout that is basically six-wheeled, but with an extra pairof wheels mounted on flippers at the front. These wheels are poweredwith the three on each side and the vehicle is skid steered, but the frontset of wheels are only placed on the ground for extra traction and stairclimbing.
This layout is in the same category as several layouts oftracked vehicles, as are several of the eight-wheeled layouts.The next logical progression, already commercially available fromRemotec in a slight variation, is to put the four center wheels on theground, and put both end pairs on flippers.
The center pair, instead ofwheels, could be tracks, as it is on Remotec’s Andros. The flipperscarry either wheels or short tracks. This vehicle is rather complicated,but has great mobility since it can reconfigure itself into a long stairclimbing or crevasse-crossing layout, or fold up into a short vehicleabout half as long.EIGHT-WHEELED LAYOUTSIf six wheels are good then eight wheels are better, right? For a certainset of requirements, eight wheels can be better than six.
There is, theoretically, more surface area simply because there are more wheels, butthis is true mostly if there is a height limitation on the robot. If the robotneeds to be particularly low for its size, then eight wheels may be theanswer.The most common layout for eight wheels, since inherently there aremore moving parts already, is to skid-steer with fixed wheels. Loweringthe center two pairs aids in skid steering just like on a six wheeled skidsteer, but the four wheels on the ground means there is less wobblingwhen stopping and starting.
Figure 4-27 shows this basic layout with thecenter wheels lowered slightly.With all the wheels fixed there are many times when several of thewheels will be lifted off the ground, reducing traction greatly. A simplestep to reduce this problem is to put the wheels on rockers, in pairs oneach side. A set of wheels may still leave the ground in some terrains, butthe other six wheels should remain mostly in contact with the ground togive some traction.
Adding steering motors at the attachment point ofeach rocker would produce four-corner steering with minimal skidding.Since the bogie is a fairly simple arm connecting only a pair of wheels, asingle motor could potentially be mounted near the center of the bogieand through a power transfer system, drive both wheels.
This would155156Chapter 4Wheeled Vehicle Suspensions and DrivetrainsFigure 4-27 Eight wheels, all fixed, center axles offsetreduce the number of actuators, even with four corner steering, to eight.No known instances of this layout, shown in Figure 4-28, have been builtfor testing, though it seems like an effective layout.With eight wheels, there is the possibility of dividing the vehicle intotwo sections, each with four wheels.
The two parts are then either connected through a passive joint and individually skid steered, or the jointis articulated and steering is done by bending the vehicle in the middle.This is identical to the four tracked layouts discussed and shown in chapter five. This can be a very effective layout for obstacle negotiation andcrevasse crossing, but cannot turn in place. Figure 4-29 shows an example of a two-part passive joint eight-wheeled layout. Figure 4-30 adds aroll joint to aid in keeping more wheels on the ground.Another eight-wheeled layout, also applicable to a four-tracked vehicle, uses a transverse pivot, which allows the two halves to pitch up anddown. It is skid steered, and is suited for bumpy terrain, but which hasfew obstacles it must go around.
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