Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 31
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A much larger trackedvehicle is NASA’s Crawler Transporter used to move the Mobile LaunchPad of the Space Shuttle program. A single link of the CrawlerTransporter’s tracks is nearly 2m long and weighs nearly eight thousandnewtons (about the same as a mid-sized car). There are 57 links per trackand eight tracks mounted in pairs at each corner of what is the largestvehicle in the world. Although mobility of this behemoth is limited, it isdesigned to climb the five-percent grade up to the launch site while holding the Space Shuttle exactly vertical on a controllable pitch platform. Itblazes along at a slow walk for the whole trip. Most large vehicles likethese use metal link tracks because of the very large forces on the track.165166Chapter 5Tracked Vehicle Suspensions and DrivetrainsOn a more practical scale for mobile robots, urethane belts withmolded-in steel bars for the drive sprocket and molded-in steel teeth fortraction are increasingly replacing all-metal tracks.
The smaller sizes canuse solid urethane belts with no steel at all. Urethane belts are lighter andsurprisingly more durable if sized correctly. They also cause far lessdamage to hard surface roads in larger sizes. If properly designed andsized, they can be quite efficient, though not like the mechanical efficiency of a wheeled vehicle. They do not stretch, rust, or require anymaintenance like a metal-link track.The much larger surface area in contact with the ground allows aheavier vehicle of the same size without increasing ground pressure,which facilitates a heavier payload or more batteries. Even the veryheavy M1A2 has a ground pressure of about eighty-two kilo pascals(roughly the same pressure as a large person standing on one foot).
Atthe opposite end of the scale the Bv206 four-tracked vehicle has aground pressure of only ten kilo pascals. This low ground pressureallows the Bv206 to drive over and through swamps, bogs, or soft snowthat even humans would have trouble getting through. Nevertheless, theBv206 does not have the lowest pressure. That is reserved for vehiclesdesigned specifically for use on powdery snow. These vehicles havepressures as low as five kilo-pascals. This is a little more than the pressure exerted on a table by a one-liter bottle of Coke.When compared to wheeled drivetrains, the track drive unit canappear to be a relatively large part of the vehicle.
The sprockets, idlers,and road wheels inside the track leave little volume for anything else.This is a little misleading, though, because a wheeled vehicle with a drivetrain scaled to negotiate the same size obstacles as a tracked unit wouldhave suspension components that take up nearly the same volume. Infact, the volume of a six wheeled rocker bogie suspension is about thesame as that of a track unit when the negotiable obstacle height is thebaseline parameter.The last advantage of tracks over wheels is negotiable crevasse width.In this situation, tracks are clearly better.
The long contact surface allowsthe vehicle to extend out over the edge of a crevasse until the front of thetrack touches the opposite side. A wheeled vehicle, even with eightwheels, would simply fall into the crevasse as the gap between thewheels cannot support the middle of the vehicle at the crevasse’s edge.The clever mechanism incorporated into a six-wheeled rocker bogie suspension shown in Chapter Four is one solution to this problem, butrequires more moving parts and another actuator.To simplify building a tracked robot, there are companies that manufacture the undercarriages of construction equipment. These all-in-onedrive units require only power and control systems to be added.
They areChapter 5Tracked Vehicle Suspensions and Drivetrainsextremely robust and come in a large variety of styles and are made forboth steel and rubber tracks. Nearly all are hydraulic powered, but a fewhave inputs for a rotating shaft that could be powered by an electricmotor. They are not manufactured in sizes smaller than about 1m long,but for larger robots, they should be given consideration in a designbecause they are designed by companies that understand tracks andundercarriages, they are robust, and they constitute a bolt-on solution toone of the more complex systems of a tracked mobile robot.STEERING TRACKED VEHICLESSteering of tracked vehicles is basically a simple concept, drive one trackfaster than the other and the vehicle turns.
This is exactly the same as askid-steer wheeled vehicle. It is also called differential steering. Theskidding power requirements on a tracked vehicle are about the same, orperhaps a little higher, as on a four-wheel skid steer layout. Since brakeswere required on early versions of tracked vehicles, the simplest way tosteer by slowing one track was to apply the brake on that side.Several novel layouts improve on this drive-and-brake steering system. Controlling the speed of each track directly adds a second majordrive source, but gives fine steering and speed control.
A secondimprovement to drive-and-brake steering uses a fantastically complicated second differential powered by its own motor. One output of thisdifferential is directly connected to one output of the main differential;the other is cross connected to the other output axle of the main differential. Varying the speed of the steering motor varies the relative speed ofthe two tracks. This also gives fine steering control, but is quite complex.Another method for steering tracked vehicles is to use some externalsteerable device. The most familiar vehicle that uses this type of systemis the common snow mobile.
This is a one-tracked separately steered layout. For use on surfaces other than snow, the skis can be replaced withwheels.A steering method that can improve mobility is one called articulatedsteering. This layout has two major sections, both with tracks, which areconnected through a joint that allows controlled motion in at least onedirection. This joint bends the vehicle in the middle, making it turn a corner.
This is the same system as used on wheeled front-end loaders. Thesesystems can aid mobility further if a second degree of freedom is addedwhich allows controlled or passive motion about a transverse pivot jointat nearly the same location as the steering joint.167168Chapter 5Tracked Vehicle Suspensions and DrivetrainsThe same trick that reduces steering power on skid steered wheeledvehicles can be applied to tracks, i.e., lowering the suspension a little atthe middle of the track. This has the effect of raising the ends, reducingthe power required to skid them around when turning.
Since this reducesthe main benefit of tracks, having more ground contact surface area, it isnot incorporated into tracked vehicles very often.VARIOUS TRACK CONSTRUCTION METHODSTracks are constructed in many different ways. Early tracks were nearlyall steel because that was all that was available that was strong enough.Since the advent of Urethane and other very tough rubbers, tracks havemoved away from steel. All-steel tracks are very heavy and on smallervehicles, this can be a substantial problem.
On larger vehicles or vehiclesdesigned to carry high loads, steel linked tracks may be the best solution.There are at least six different general construction techniques for tracks.•••••All steel hinged linksHinged steel links with removable urethane road padsSolid urethaneUrethane with embedded steel tension membersUrethane with embedded steel tension members and external steelshoes (sometimes called cleats)• Urethane with embedded steel tension members and embedded steeltransverse drive rungs with integral guide teethAll-steel hinged linked track (Figure 5-1) would seem to be the toughest design for something that gets beat on as much as tracks do, but thereare several drawbacks to this design. Debris can get caught in the spacesbetween the moving links and can jamb the track.
A solution to this problem is to mount the hinge point as far out on the track as possible. Thisreduces the amount that the external surface of the track opens andcloses, reducing the size of the pinch volume. This is a subtle but important part of steel track design. This lowered pivot is shown in Figure 5-2.Tracked vehicles, even autonomous robots, will drive on finishedroads at some point in their life, and all-steel tracks tear up macadam.The solution to this problem has been to install urethane pads in the linksof the track. These pads are designed to be easily replaceable. The padsare bolted or attached with adhesive to pockets in special links on thetrack. This allows them to be removed and replaced as they wear out.Chapter 5Tracked Vehicle Suspensions and Drivetrains169Figure 5-1 Basic steel link layoutshowing pinch pointFigure 5-2 Effective hinge location of all-steel trackFigure 5-3 shows the lowered pivot link with an added pocket for theurethane road wheel.The way to completely remove the pinch point is to make the track allone piece.
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