Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 39
Текст из файла (страница 39)
There needs tobe some way to even the playing field so it is the effectiveness of themobility system that is being compared regardless of its size. In thischapter, we’ll investigate several ways of comparing mobility systemsstarting with a detailed discussion of ways of describing the mobilitysystem itself.
Then, the many mobility challenges the outdoor environment presents will be investigated. A set of mobility indexes that providean at-a-glance comparison will be generated, and finally a practical specific-case comparison method will be discussed.THE MOBILITY SYSTEMTo level the playing field, the mobility systems being compared have tobe scaled to be effectively the same size. This means that there needs tobe a clear definition of size. Since most robots are battery powered,energy efficiency must also be included in the comparison because thereare advantages of shear power in overcoming some obstacles that batterypowered vehicles simply would not have. This limited available power inmost cases also limits speed.
In some situations, simply going at anobstacle fast can aid in getting over it. For simplicity and because of the229230Chapter 9Comparing Locomotion Methodsrelatively low top speeds of battery powered robots, forward momentumis not included as a comparison of mobility methods in this book.One last interesting criteria that bears mentioning is the vehicle’sshape. This may not seem to have much bearing on mobility, and indeedin most situations it does not. However, for environments that arecrowded with obstacles that cannot be driven over, where getting aroundthings is the only way to proceed, a round or rounded shape is easier tomaneuver.
The round shape allows the vehicle to turn in place even if itis against a tree trunk or a wall. This ability does not exist for vehiclesthat are nonround. The nonround shaped vehicle can get quite inextricably stuck in a blind alley in which it tries to turn around. For most outdoor environments, simply rounding the corners somewhat is enough toaid mobility. In some environments (very dense forests or inside buildings) a fully round shape will be advantageous.SizeOverall length and height of the mobility system directly affect a vehicle’sability to negotiate an obstacle, but width has little affect, so size is, atleast, mostly length and height.
The product of the overall length andheight, the elevation area, seems to give a good estimate of this part of itssize, but there needs to be more information about the system to accuratelycompare it to others. The third dimension, width, seems to be an importantcharacteristic of size because a narrower vehicle can potentially fit throughsmaller openings or turn around in a narrower alley. It is, however, theturning width of the mobility system that is a better parameter to compare.For some obstacles, just being taller is enough to negotiate them. Forother obstacles, being longer works. A simple way to compare these twoparameters together would be helpful.
A length/height ratio or elevationarea would be useful since it reduces the two parameters down to one.The length/height ratio gives an at-a-glance idea of how suited a systemis to negotiating an environment that is mostly bumps and steps or onethat is mostly tunnels and low passageways.Width has little effect on getting over or under obstacles, but it doesaffect turning radius. It is mostly independent of the other size parameters, since the width can be expanded to increase the usable volume ofthe robot without affecting the robot’s ability to get over or under obstacles.
Since turning in place is the more critical mobility trait related towidth, the right dimension to use is the diagonal length of the system.This is set by the expected minimum required turning width as determined by environmental constraints. It may, however, be necessary tomake the robot wider for other reasons, like simply adding volume to theChapter 9Comparing Locomotion Methodsrobot. A rule of thumb to use when figuring out the robot’s width is tomake it about 62 percent of the length of the robot.The components of the system each have their own volume, and moving parts sweep out a sometimes larger volume. These pieces of the robotare independent of the function of the robot, but take up volume.Including the volume of the mobility system’s pieces is useful. As will beseen later, weight is critical, so the total mass of the mobility system’scomponents needs to be included.
Since mass is directly related(roughly, since materials have different densities) to the volume of agiven part, and volume is easier to calculate and visualize, volumenegates any need to include mass.EfficiencyAnother good rule of thumb when designing anything mechanical is thatless weight in the structure and moving parts is always better. This ruleapplies to mobile vehicles. If there were no weight restriction and littleor no size restriction, then larger and therefore heavier wheels, tracks, orlegs would allow a vehicle to get over more obstacles.
However, weightis important for several reasons.• The vehicle can be transported more easily.• It takes less of its own power to move over difficult terrain and, especially, up inclines.• Maintenance that requires lifting the vehicle is easier to perform andless dangerous.• The vehicle is less dangerous to people in its operating area.For all these reasons, smaller and lighter suspension and drive traincomponents are usually the better choice for high mobility vehicles.There are three motions in which the robot moves: fore/aft, turn, andup/down, and each requires a certain amount of power.
The three axes of astandard coordinate system are labeled X, Y, and Z, but for a mobile robot,these are modified since most robot’s turn before moving sideways. Therobot’s motions are commonly defined as traverse, turn, and climb. Arobot can be doing any one, two, or all three at the same time, but thepower requirements of each is so different that they can easily be listedindependently by magnitude. Climbing uses the most power and turning inplace usually requires more power than moving forwards or backwards.This does not apply to all mobility systems but is a good general rule.231232Chapter 9Comparing Locomotion MethodsTHE ENVIRONMENTMoving around in the relatively benign indoor environment is a simplematter, with the notable exception of staircases.
The systems in this bookmostly focus on systems designed for the unpredictable and highly variedoutdoor environment, an environment that includes large variations intemperature, ground cover, topography, and obstacles. This environmentis so varied, that only a small percentage of the problems can be listed, orthe number of comparison parameters would become much too large.Hot and cold may not seem related to mobility, but they are in that themobility system must be efficient so it doesn’t create too much heat anddamage itself or nearby components when operating in a desert. Themobility system must not freeze up or jam from ice when operating inloose snow or freezing rain.
As for ground cover, the mobility systemmight have to deal with loose dry sand, which can get everywhere andrapidly wear out bearings, or operate in muddy water. It might also haveto deal with problematic topography like steep hills, seemingly impassable nearly vertical cliffs, chasms, swamps, streams, or small rivers. Themobility system will almost definitely have to travel over some or all ofthose topographical challenges. In addition, there are the more obviousobstacles like rocks, logs, curbs, pot holes, random bumps, stone or concrete walls, railroad rails, up and down staircases, tall wet grass, anddense forests of standing and fallen trees.This means that the mobility system’s effectiveness should be evaluated using the aforementioned parameters. How does it handle sand orpebbles? Is its design inherently difficult to seal against water? Howsteep an incline can it negotiate? How high an obstacle, step, or bumpcan it get over or onto? How wide a chasm can it cross? Somehow, allthese need to be simplified to reduce the wide variety down to a manageable few.The four categories of temperature, ground cover, topography, andobstacles can be either defined clearly or broken up into smaller moreeasily defined subcategories without ending up with an unmanageablylarge list.
Let’s look at each one in greater detail.ThermalTemperature can be divided simply into the two extremes of hot andcold. Hot relates to efficiency. A more efficient machine will have fewerproblems in hot climates, but better efficiency, more importantly, meansbattery powered robots will run longer. Cold relates to pinch points,Chapter 9Comparing Locomotion Methodswhich can collect snow and ice, causing jamming or stalling. A usefulpair of temperature-related terms to think about in a comparison ofmobility systems would then be efficiency and pinch points.Ground CoverGround cover is more difficult to define, especially in the case of sand,because it can’t be scaled. Sand is just sand no matter what size the vehicle is (except for tiny robots of course), and mud is still mud. Driving onsand or mud would then be a function of ground pressure, the maximumforce the vehicle can exert on a wheel, track, or foot, divided by the areathat a supporting element places on the ground.
Lower ground pressurereduces the amount the driving element sinks, thereby reducing theamount of power required to move that element. Higher ground pressureis helpful in only two cases: towing a heavy load behind the robot, andclimbing steep slopes.Robots are infrequently required to be tow trucks, but this may changeas the variety of tasks they are put to widens. Climbing hills, though, is acommon task. The effect of ground pressure on hill climbing can beovercome with careful tread design (independent of the mobility system), which combines the benefits of low ground pressure with hightraction.
Lower ground pressure should be considered to indicate a morecapable mobility system.The theory that sand and mud are not scalable can’t be applied tograss however, because tall field grass really is significantly larger thanshort lawn grass. Grass seems benign, but it is strong enough whenbunched up to throw tracks, stall wheeled vehicles, and trip walkers.These problems can be roughly related to ground pressure since a lighterpressure system would tend to ride higher on wet grass, reducing its tangling problems.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
