Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 33
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Having road wheelson both sides of the spring reduces the twisting moment produced byhaving wheels on only one side. Figure 5-11 shows double wheels.TRACK SYSTEM LAYOUTSOne-Track DrivetrainWhat would seem to be the simplest track layout is one that uses onlyone track. This layout is actually in existence in at least one form, andmobility can be quite good. The most common commercially availableform of a one-track vehicle is the snow mobile.
Although these vehicles are designed exclusively for use on snow, replace the skis withwheels, and they can be used on hard surfaces. The track on a snowmobile is quite wide to lower ground pressure as much as practicable, butthere is no reason why a narrower track can’t be used with the wheeledlayout.Mobility is limited somewhat by two factors: The wheels must bepushed over obstacles and the layout is steered by the Ackerman systemChapter 5Tracked Vehicle Suspensions and Drivetrains179Figure 5-12 One track, twofront wheels, Ackerman steerwhich prevents turning in place. The first problem can be reduced bypowering the wheels. There is no known existence of this layout, but itseems worth investigation.
Figure 5-12 shows the typical ramped-fronttrack common on snowmobiles because they normally do not go backwards. A track that is ramped both in front and back would increasemobility. It would be an interesting experiment to build a one-track, twowheel drive, Ackerman steered robot and test its mobility.Two-Tracked DrivetrainsThe two-track layout is by far the most common. In its basic form, it issimple, easy to understand, and relatively easy to construct. Two tracksare attached to either side of the robot’s main chassis, and each are powered by their own motor. Compact designs have the motor mounted substantially inside the track and attached directly to the drive sprocket.Since the drive sprocket must turn at a much lower rpm than the rpm’s atwhich electric motors are most efficient, a speed reduction methodalmost always needs to be part of the drivetrain.
Figure 5-13 shows atwo-track layout, with drive motors, gearboxes, fixed track guide blades,and non-ramped tracks. This represents the simplest layout for a trackedvehicle.180Chapter 5Tracked Vehicle Suspensions and DrivetrainsFigure 5-13 Basic two-tracklayoutTwo-Tracked Drivetrains with Separate Steering SystemsA more complex, but less capable, layout is to have the two tracks driventhrough a differential, and the robot steered by a conventional set ofwheels mounted in front of the tracks. This layout came about whenlarge trucks did not have enough traction on unprepared roads andreplacing the rear wheels with track systems that took up about the samevolume solved that problem.
These trucks (Figure 5-14) were calledhalf-tracks. For a mobile robot, this is a less satisfactory layout since itFigure 5-14 The half-trackChapter 5Tracked Vehicle Suspensions and Drivetrains181Figure 5-15 Two wide tracks,fore-and-aftcan no longer turn in place like the basic two-track layout can, yet hasmore moving parts.An unusual variation of the two-track layout is to place the two tracksinline, one in front of the other (Figure 5-15).
Stability is maintained bymaking the tracks sufficiently wide, and steering is accomplished withan articulated joint between the two tracks. The tracks have to be supported from both sides, like on a snow mobile. Steering power is transmitted through one or two linear or rotary actuators that are part of thearticulated joint.This system also benefits from the trick of making the center of eachtrack a little lower than the ends. Since the tracks are already fore-andaft, the bowed shape on each track does not produce any wobbling.
Thissystem has great mobility, but like the half-track, cannot turn in place. Itwould probably work very well for vehicles intended for use on snow orsand. The two tracks could either each carry their own chassis, or a single chassis could be attached to the universal joint with the outer endsof the track sections suspended to the chassis with a sprung or activesuspension.Four-Tracked DrivetrainsAdding more tracks would seem to increase the mobility of a trackedvehicle, but there are several problems with this approach. Adding moretracks necessarily means more moving parts, but it also usually meansmaking the vehicle longer. The best layout would be one that adds more182Chapter 5Tracked Vehicle Suspensions and DrivetrainsFigure 5-16 iRobot’s Urbie, afour-tracked teleoperated robotlayouttracks with the least number of additional moving parts, and keeps thevehicle the same length.
This last criteria almost exclusively means areconfigurable layout, one where the length is longer when that isneeded, but can reconfigure into a shorter length when that is needed.This concept has been implemented in a couple of different ways, bothof which are patented.Figure 5-16 shows the general layout of iRobot’s Urbie teleroboticplatform.
This layout uses a third actuator to deploy or stow a pair offlipper-like tracks that rotate around the front idler wheels. They arepowered by the same motors that power the main tracks, and always turnwith them. This layout represents the simplest form of a four-track vehicle, and has very high mobility.The center of gravity of Urbie is located ahead of the center of thevehicle so, with the flippers extended, Urbie can cross crevasses that arewider than half the length of the basic vehicle. This clever location of thecg also gives the flippers the ability to flip the robot over if it is overturned. When the flippers are rotated around so they become very largeramps, Urbie can climb over obstacles that are higher than the overallheight of the basic track.
There are also other functions the flippers canperform unrelated to mobility, like the ability to stand up. This makes therobot much taller and allows a strategically placed camera to see overshort walls.Chapter 5Tracked Vehicle Suspensions and Drivetrains183Figure 5-17a Same length flippers, sharing middle axleFigure 5-17bA close relative of Urbie’s layout (Figure 5-17a–b) is one where thetrack pairs are the same size, with the cg located very close to the sharedaxle of the front and rear tracks.
With two actuators to power each of thefour tracks independently and a fifth actuator to power the pivot joint a184Chapter 5Tracked Vehicle Suspensions and Drivetrainsvery capable layout results. The main chassis is geared to the shared axleso it is always at the half-angle between the front and rear tracks, whichallows it to be raised up yet still be level when folding both tracks down.This trick raises the entire chassis, but it also offloads the weight of therobot from the track guide blades, increasing rolling efficiency whenhigh traction is not needed. This reconfigurable layout combines the highmobility of tracks with good smooth-road rolling efficiency.There are two basic layouts for four-tracked vehicles.
They are bothtrain-like in that there are two two-tracked modules connected by somesort of joint. The two modules must be able to move in several directionsrelative to each other. They can pitch up and down, yaw left and right,and, ideally, roll (twist).The simplest connection that allows all three degrees of freedom is theball joint. If the joint is passive, steering is accomplished in the sameway as a two-tracked vehicle, except that now both modules must turn atjust the right time to keep skidding between the modules to a minimum.This turns out to be tricky. The ball joint also limits the range of steeringangle simply because the socket must wrap around the ball enough toadequately capture it.
A universal joint has a greater range of motion, andis easier to use if the joint is to be powered.The articulated joint, an active universal joint, overcomes the steeringproblem by allowing the tracks to rotate at whatever speed limits skidding. This steering method makes this layout very agile.
The HagglundBv206, which uses this layout, is considered nearly unstoppable inalmost any terrain from soft snow to steep hills. It is even amphibious,propelled through the water by the tracks. Because it cannot be skidsteered, it can’t turn in place. Nevertheless, it is a very capable layout.Steering the Hagglund Bv206 is done with a standard steering wheel,which turns the articulated joint and forces the two modules to bend. Thetracks are driven through limited slip differentials, allowing the inner andouter tracks in each module to travel at different speeds just like in anAckerman steered wheeled vehicle.Six-Tracked DrivetrainsThere is at least one track layout (Figure 5-18) incorporated on an existing telerobotic vehicle that uses six tracks.
It is an extension of theUrbie design, but was actually invented before Urbie’s layout. The twotracked layout is augmented by flipper tracks on both the front andback, independently tilted, but whose tracks are driven by the maintrack motors. This layout allows the vehicle to stand up like the oneChapter 5Tracked Vehicle Suspensions and DrivetrainsFigure 5-18 Six-tracked, double flippersshown in Figure 5-17. The double flippers extend the length of the twotracked base unit by almost a factor of two, facilitating crossing widecrevasses and climbing stairs, yet still being able to turn in place in asmall aisle.185This page intentionally left blank.Chapter 6Steering HistoryCopyright © 2003 by The McGraw-Hill Companies, Inc. Click here for Terms of Use.This page intentionally left blank.The Romans extensively used two wheeled carts, pulled by horses.Pull on the right rein and the horse pulls the cart to the right, and viseversa.
The two wheels on the cart were mounted on the same axle, butwere attached in a way that each wheel could rotate at whatever speedwas needed depending on whether the cart was going straight or arounda corner. Carts got bigger and eventually had four wheels, two in frontand two in back.
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