Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 40
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The problems caused by grass, then, can be assumed tobe effectively covered by the ground pressure category.TopographyTopography can be scaled to any size making it very simple to include. Itcan be defined by angle of slope. The problem with angle of slope,though, is that it can be more a function of the friction of the material andthe tread shape of whatever is in ground contact, than a function of thegeometry of the mobility system. There are some geometries that areeasier to control on steep slopes, and there are some walkers, climbers233234Chapter 9Comparing Locomotion Methodsreally, that can climb slopes that a wheeled or tracked vehicle simplycould not get up.
Negotiable slope angle is therefore important, but itshould be assumed that the material in ground contact is the same nomatter what type of mobility system is used.ObstaclesObstacles can also be scaled, but they create a special case. The effectiveness of the mobility system could be judged almost entirely by howhigh, relative to its elevation area, an obstacle it can negotiate. Obstaclenegotiation is a little more complicated than that but it can be simplifiedby dividing it into three subcategories.• Mobility system overall height to negotiable obstacle height• System length to negotiable obstacle height• System elevation area to negotiable obstacle heightThe comparison obstacle parameters can be defined to be the height ofa square step the system can climb onto and the height of a square toppedwall the system can climb over without high centering, or otherwisebecoming stuck.An inverted obstacle, a chasm, is also significant.
Negotiable chasmwidth is mostly a function of the mobility system’s length, but someclever designs can vary their length somewhat, or shift their center ofgravity, to facilitate crossing wider chasms. For systems that can varytheir length, negotiable chasm width should be compared using the system’s shortest overall length. For those that are fixed, use the overalllength.Another facet of obstacle negotiation is turning width. This is important because a mobility system with a small turning radius is more likelyto be able to get out of or around confining situations. Turning width isnot directly a function of vehicle width, but is defined as the narrowestalley in which the vehicle can turn around.
This is in contrast to ratingsgiven by some manufacturers that give turning radius as the radius of acircle defined as the distance from the turning point to the center of thevehicle width. This can be misleading because a very large vehicle thatturns about its center can be said to have a zero-radius turning width.A turning ability parameter must also show how tightly a vehicle canturn around a post, giving some idea how well it could maneuver in a forest of closely spaced trees. There are, then, two width parameters, alleywidth and turning-around-a-post width.Chapter 9Comparing Locomotion MethodsCOMPLEXITYA more nebulous comparison criteria that must be included in an evaluation of any practical mechanical device is its inherent complexity. Acommon method for judging complexity is to count the number of moving parts or joints.
Ball or roller bearings are usually counted as one partof a joint although there may be 10s of balls or rollers moving inside thebearing. A problem with this method is that some parts, though moving,have very small forces on them or operate in a relatively hazard-freeenvironment and, so, last a very long time, sometimes even longer thannonmoving parts in the same system.A second method is to count the number of actuators since their number relates to the number of moving parts and they are the usually thesource of greatest wear. The drawback of this method is that it ignorespassive moving parts like linkages that may well cause problems or wearout before an actively driven part does.
The first method is probably abetter choice because robots are likely to be moving around in completely unpredictable environments and any moving part is equally susceptible to damage by things in the environment.Speed and CostThere are two other comparison parameters that could be included in acomparison of mobility methods. They are velocity of the moving vehicle and cost of the locomotion system. Moving fast over rough andunpredictable terrain places large and complex loads on a suspensionsystem. These loads are difficult to calculate precisely because the terrain can be so unpredictable. Powerful computer simulation programscan predict a suspension system’s performance with a moderate degreeof accuracy, but the suspension system still must always be tested in thereal world. Usually the simulation program’s predictions are proveninaccurate to a significant degree.
It is too difficult to accurately predictand design for a specific level of performance at speeds not very farabove eight m/s to have any useful meaning. It is assumed that slowing isan acceptable way to increase mobility, and that slowing can be donewith any suspension design. Mobility is not defined as getting over anobstacle at a certain speed; it is simply getting over the obstacle at whatever speed works.Cost can be related to size, weight, and complexity.
Fewer, smaller,and lighter parts are usually cheaper. The design time to get to the simplest, lightest design that meets the criteria may be longer, but the endcost will usually be less. Since cost is closely related to size, weight, and235236Chapter 9Comparing Locomotion Methodscomplexity, it does not need to be included in a comparison of suspension and drive train methods.THE MOBILITY INDEX COMPARISON METHODAnother, perhaps simpler, method is to create an index of the mobilitydesign’s capabilities listed as a set of ratios relating the mobility system’slength, height, width, and possibly complexity, to a small set of terrainparameters.
The most useful set would seem to be obstacle height, crevasse width, and narrowest alley in which the vehicle can turn around.Calculating the vehicle’s ground pressure would cover mobility in sandor mud. The pertinent ratios would be:• Step/Elevation Area: Negotiable step height divided by the elevationarea of mobility system• Step/System Height: Highest negotiable wall or platform, whicheveris shorter, divided by mobility system height• Crevasse/System Length: Negotiable crevasse width divided by vehicle length (in the case of variable geometry vehicles, the shortestlength of the mobility system)• System Width/Turning diameter: Vehicle width divided by outermostswept diameter of turning circle• System Width/Turning-Around-a-Post Width: Vehicle width dividedby width of path it sweeps when turning around a very thin post• Ground PressureThese are all set up so that a higher ratio number means theoreticallyhigher mobility.
No doubt, some mobility system designs will have veryhigh indexes in some categories, and low indexes in others. Having a single Mobility Index for each mobility system design would be convenient,but it would be difficult to produce one that describes the system’s abilities with enough detail to be useful. These six, however, should give afairly complete at-a-glance idea of how well a certain design will perform in many situations.THE PRACTICAL METHODA third way to compare mobility systems that may work well for adesigner working on a specific robot design, is to calculate the total vol-Chapter 9Comparing Locomotion Methodsume of everything on the robot not related to the mobility system(including the power supply), and define this volume with a realisticratio of length, width, and height. A good place to start for the size ratiosis to make the width 62 percent of the length, and the height one quarterof the length.
This box represents the volume of everything the mobilitysystem must carry.The next step is to define the mobility requirements, allowing substantial leeway if the operating environment is not well known. The basicsix parameters discussed above are a good place to start.••••••Step or wall heightMinimum tunnel heightCrevasse widthMaximum terrain slopeMinimum spacing of immovable objectsMaximum soil densityAll of these need to be studied carefully to aid in determining the mosteffective mobility system layout to use. The more time spent doing thisstudy, the better the mobility system choice will match the terrain’srequirements.When this study is completed, selecting and designing the mobilitysystem is then a combination of scaling the system to the robot’s boxsize, and meeting the mobility constraints.
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