Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 28
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Although this seems like a complicated solution from anelectrical and control standpoint, it is simpler mechanically.Steering with the rear wheels is effective for a human controlled vehicle, especially in an environment with few obstacles that must be drivenaround. The transverse rocker layout can also be used with a front steeredlayout (Figure 4-13) which makes it very much like an automobile.Couple this layout with all wheel drive, and this is a good performer.Figure 4-13 Rear transverserocker, front steer144Chapter 4Wheeled Vehicle Suspensions and DrivetrainsALL-TERRAIN VEHICLE WITHSELF-RIGHTING AND POSE CONTROLWheels dr iven by gearmotors are mounted on pivoting struts.NASA’s Jet Propulsion Laboratory, Pasadena, CaliforniaA small prototype robotic all-terrain vehicle features a unique drive andsuspension system that affords capabilities for self righting, pose control,and enhanced maneuverability for passing over obstacles.
The vehicle isdesigned for exploration of planets and asteroids, and could just as wellbe used on Earth to carry scientific instruments to remote, hostile, or otherwise inaccessible locations on the ground. The drive and suspensionsystem enable the vehicle to perform such diverse maneuvers as flippingitself over, traveling normal side up or upside down, orienting the mainvehicle body in a specified direction in all three dimensions, or settingthe main vehicle body down onto the ground, to name a few.
Anothermaneuver enables the vehicle to overcome a common weakness of traditional all-terrain vehicles—a limitation on traction and drive force thatmakes it difficult or impossible to push wheels over some obstacles: Thisvehicle can simply lift a wheel onto the top of an obstacle.The basic mode of operation of the vehicle can be characterized asfour-wheel drive with skid steering. Each wheel is driven individually bya dedicated gearmotor. Each wheel and its gearmotor are mounted at thefree end of a strut that pivots about a lateral axis through the center ofFigure 4-14 Each wheel Is driven by a dedicated gearmotor and is coupled to the idlerpulley. The pivot assembly imposes a constant frictional torque T, so that it is possible to(a) turn both wheels in unison while both struts remain locked, (b) pivot one strut, or (c)pivot both struts in opposite directions by energizing the gearmotors to apply variouscombinations of torques T/2 or T.Chapter 4Wheeled Vehicle Suspensions and Drivetrainsgravity of the vehicle (see figure).
Through pulleys or other mechanismattached to their wheels, both gearmotors on each side of the vehicledrive a single idler disk or pulley that turns about the pivot axis.The design of the pivot assembly is crucial to the unique capabilitiesof this system. The idler pulley and the pivot disks of the struts are madeof suitably chosen materials and spring-loaded together along the pivotaxis in such a way as to resist turning with a static frictional torque T; inother words, it is necessary to apply a torque of T to rotate the idler pulley or either strut with respect to each other or the vehicle body.During ordinary backward or forward motion along the ground, bothwheels are turned in unison by their gearmotors, and the belt couplingsmake the idler pulley turn along with the wheels.
In this operationalmode, each gearmotor contributes a torque T/2 so that together, both gearmotors provide torque T to overcome the locking friction on the idler pulley. Each strut remains locked at its preset angle because the torque T/2supplied by its motor is not sufficient to overcome its locking friction T.If it is desired to change the angle between one strut and the mainvehicle body, then the gearmotor on that strut only is energized. In general, a gearmotor acts as a brake when not energized. Since the gearmotor on the other strut is not energized and since it is coupled to the idlerpulley, a torque greater than T would be needed to turn the idler pulley.However, as soon as the gearmotor on the strut that one desires to turn isenergized, it develops enough torque (T) to begin pivoting the strut withrespect to the vehicle body.It is also possible to pivot both struts simultaneously in opposite directions to change the angle between them.
To accomplish this, one energizes the gearmotors to apply equal and opposite torques of magnitudeT: The net torque on the idler pulley balances out to zero, so that the idlerpulley and body remain locked, while the applied torques are just sufficient to turn the struts against locking friction. If it is desired to pivot thestruts through unequal angles, then the gearmotor speeds are adjustedaccordingly.The prototype vehicle has performed successfully in tests.
Currentand future work is focused on designing a simple hub mechanism, whichis not sensitive to dust or other contamination, and on active controltechniques to allow autonomous planetary rovers to take advantage ofthe flexibility of the mechanism.This work was done by Brian H. Wilcox and Annette K. Nasif ofCaltech for NASA’s Jet Propulsion Laboratory.If a differential is installed between the halves of a longitudinal rockerlayout, with the axles of the differential attached to each longitudinalrocker, and interesting effect happens to the differential input gear as the145146Chapter 4Wheeled Vehicle Suspensions and DrivetrainsFigure 4-15 Pitch averagingmechanismvehicle traverses bumpy terrain.
If you attach the chassis to this gear, thepitch angle of the chassis is half the pitch angle of either side rocker. Thispitch averaging effectively reduces the pitching motion of the chassis,maintaining it at a more level pose as either side of the suspension system travels over bumps. This can be advantageous in vehicles undercamera control, and even a fully autonomous sensor driven robot canbenefit from less rocking motion of the main chassis.
This mechanismalso tends to distribute the weight more evenly on all four wheels,increasing traction, and, therefore, mobility. Figure 4-15 shows the basicmechanism and Figure 4-16 shows it installed in a vehicle.Another mechanical linkage gives the same result as the differentialbased chassis pitch-averaging system.
See figure 4-17. This design usesa third rocker tied at each end to a point on the side rockers. The middleof the third rocker is then tied to the middle of the rear (or front) of thechassis, and, therefore, travels up and down only half the distance eachend of a given rocker travels. The third rocker design can be more volumetrically efficient and perhaps lighter than the differential layout.Another layout that can commonly be found in large industrial vehicles is one where the vehicle is divided into two sections, front and rear,each with its own pair of wheels.
The two sections are connected throughChapter 4Wheeled Vehicle Suspensions and Drivetrains147Figure 4-16 Chassis pitch averaging mechanism usingdifferentialan articulated (powered) vertical-axis joint. In the industrial truck version, the front and rear sections’ wheels are driven through differentials,but higher traction would be obtained if the differentials were limitedslip, or lockable.
Even better would be to have each wheel driven with itsFigure 4-17 Chassis link-basedpitch averaging mechanism148Chapter 4Wheeled Vehicle Suspensions and Drivetrainsown motor. This design cannot turn in place, but careful layout can produce a vehicle that can turn in little more than twice its width.Greater mobility is achieved if the center joint also allows a rollingmotion between the two sections. This degree of freedom keeps all fourwheels on the ground while traversing uneven terrain or obstacles. It alsoimproves traction while turning on bumps.
Highest mobility for this layout would come from powering both the pivot and roll joints with theirown motors and each wheel individually powered for a total of sixmotors. Alternatively, the wheels could be powered through limited slipdifferentials and the roll axis left passive for less mobility, but only threemotors. Figures 4-18 and 4-19 show these two closely related layouts.An unusual and unintuitive layout is the five-wheeled drivetrain. Thisis basically the tricycle layout, but with an extra pair of wheels in theback to increase traction and ground contact area. The front wheel is notnormally powered and is only for steering.
Figure 4-20 shows this is afairly simple layout relative to its mobility, especially if the side wheelpairs are driven together through a simple chain or belt drive. Althoughthe front wheels must be pushed over obstacles, there is ample tractionfrom all that rubber on the four rear wheels.Figure 4-18 Two-sections connected through vertical axis jointChapter 4Wheeled Vehicle Suspensions and Drivetrains149Figure 4-19 Two sections connected through both a verticalaxis and a longitudinal axis jointFigure 4-20 Five wheels150Chapter 4Wheeled Vehicle Suspensions and DrivetrainsSIX-WHEELED LAYOUTSBeyond four- and five-wheeled vehicles is the large class of six-wheeledlayouts.
There are many layouts, suspensions, and drivetrains based onsix wheels. Six wheels are generally the best compromise for highmobility wheeled vehicles. Six wheels put enough ground pressure, traction, steering mobility, and obstacle-negotiating ability on a vehiclewithout, in most cases, very much complexity. Let’s take a look at themore practical variations of six-wheeled layouts.The most basic six-wheeled vehicle, shown in Figure 4-21, is the skidsteered non-suspended design. This is very much like the four-wheeleddesign with improved mobility simply because there is more traction andless ground pressure because of the third wheel on each side. The wheelscan be driven with chains, belts, or bevel gearboxes in a simple way,making for a robust system.An advantage of the third wheel in the skid-steer layout is that themiddle wheel on each side can be mounted slightly lower than the othertwo, reducing the weight the front and rear wheel pairs carry.
The lowerweight reduces the forces needed to skid them around when turning,reducing turning power. The offset center axle can make the vehiclewobble a bit. Careful planning of the location of the center of gravity isrequired to minimize this problem. Figure 4-22 shows the basic concept.Figure 4-21 Six wheels, allfixed, skid steerChapter 4Wheeled Vehicle Suspensions and Drivetrains151Figure 4-22 Six wheels, allfixed, skid steer, offset center axleAn even trickier layout adds two pairs of four-bar mechanisms supporting the front and rear wheel pairs (Figure 4-23).
These mechanismsare moved by linear actuators, which raise and lower the wheels at eachcorner independently. This semi-walking mechanism allows the vehicleto negotiate obstacles that are taller than the wheels, and can aid in traversing other difficult terrain by actively controlling the weight on eachwheel. This added mobility comes at the expense of many more movingparts and four more actuators.Skid steering can be improved by adding a steering mechanism to thefront pair of wheels, and grouping the rear pair more closely together.Figure 4-23 Six wheels, all corner wheels have adjustableheight, skid steer152Chapter 4Wheeled Vehicle Suspensions and DrivetrainsFigure 4-24 Six wheels, frontpair steerThis has better steering efficiency, but, surprisingly, not much bettermobility.
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