Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

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166 Chapter 5 Tracked Vehicle Suspensions and Drivetrains On a more practical scale for mobile robots, urethane belts with molded-in steel bars for the drive sprocket and molded-in steel teeth for traction are increasingly replacing all-metal tracks. The smaller sizes can use solid urethane belts with no steel at all. Urethane belts are lighter and surprisingly more durable if sized correctly. They also cause far less damage to hard surface roads in larger sizes. If properly designed and sized, they can be quite efficient, though not like the mechanical efficiency of a wheeled vehicle. They do not stretch, rust, or require any maintenance like a metal-link track. The much larger surface area in contact with the ground allows a heavier vehicle of the same size without increasing ground pressure, which facilitates a heavier payload or more batteries. Even the very heavy M1A2 has a ground pressure of about eighty-two kilo pascals (roughly the same pressure as a large person standing on one foot). At the opposite end of the scale the Bv206 four-tracked vehicle has a ground pressure of only ten kilo pascals. This low ground pressure allows the Bv206 to drive over and through swamps, bogs, or soft snow that even humans would have trouble getting through. Nevertheless, the Bv206 does not have the lowest pressure. That is reserved for vehicles designed specifically for use on powdery snow. These vehicles have pressures 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 can appear 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 would have suspension components that take up nearly the same volume. In fact, the volume of a six wheeled rocker bogie suspension is about the same as that of a track unit when the negotiable obstacle height is the baseline parameter. The last advantage of tracks over wheels is negotiable crevasse width. In this situation, tracks are clearly better. The long contact surface allows the vehicle to extend out over the edge of a crevasse until the front of the track touches the opposite side. A wheeled vehicle, even with eightwheels, would simply fall into the crevasse as the gap between the wheels 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, but requires 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-one drive units require only power and control systems to be added. They are
Chapter 5 Tracked Vehicle Suspensions and Drivetrains 167 extremely robust and come in a large variety of styles and are made for both steel and rubber tracks. Nearly all are hydraulic powered, but a few have inputs for a rotating shaft that could be powered by an electric motor. They are not manufactured in sizes smaller than about 1m long, but for larger robots, they should be given consideration in a design because they are designed by companies that understand tracks and undercarriages, they are robust, and they constitute a bolt-on solution to one of the more complex systems of a tracked mobile robot. STEERING TRACKED VEHICLES Steering of tracked vehicles is basically a simple concept, drive one track faster than the other and the vehicle turns. This is exactly the same as a skid-steer wheeled vehicle. It is also called differential steering. The skidding power requirements on a tracked vehicle are about the same, or perhaps a little higher, as on a four-wheel skid steer layout. Since brakes were required on early versions of tracked vehicles, the simplest way to steer 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 major drive source, but gives fine steering and speed control. A second improvement to drive-and-brake steering uses a fantastically complicated second differential powered by its own motor. One output of this differential 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 of the two tracks. This also gives fine steering control, but is quite complex. Another method for steering tracked vehicles is to use some external steerable device. The most familiar vehicle that uses this type of system is the common snow mobile. This is a one-tracked separately steered layout. For use on surfaces other than snow, the skis can be replaced with wheels. A steering method that can improve mobility is one called articulated steering. This layout has two major sections, both with tracks, which are connected through a joint that allows controlled motion in at least one direction. 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. These systems can aid mobility further if a second degree of freedom is added which allows controlled or passive motion about a transverse pivot joint at nearly the same location as the steering joint.
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vi Contents Commutation 34 Installa
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viii Contents Chapter 4 Wheeled Veh
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x Contents Industrial Robots 258 In
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xii Introduction a unique robot. Wh
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xiv Introduction outdoor and indoor
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xvi Introduction Rapid prototyping
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xviii Introduction Because the phot
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xx Introduction Meanwhile, the opaq
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xxii Introduction Laminated-Object
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xxiv Introduction Figure 5 Fused De
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xxvi Introduction ton. This process
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xxviii Introduction Figure 8 Ballis
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xxx Introduction laser fusing of ce
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xxxii Introduction material to form
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4 Chapter 1 Motor and Motion Contro
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10 Chapter 1 Motor and Motion Contr
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72 Chapter 2 Indirect Power Transfe
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100 Chapter 2 Indirect Power Transf
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110 Chapter 3 Direct Power Transfer
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Page 240 and 241: 202 Chapter 7 Walkers when crossing
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Page 252 and 253: 214 Chapter 7 Walkers ROLLER-WALKER
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216 Chapter 7 Walkers Walkers have
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220 Chapter 8 Pipe Crawlers and Oth
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230 Chapter 9 Comparing Locomotion
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242 Chapter 10 Manipulator Geometri
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266 Chapter 11 Proprioceptive and E
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292 Index CAM-LEM, Inc., xxiii camm
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294 Index (ground pressure cont.) a
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296 Index (mobility systems cont.)
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298 Index shear pin torque limiters
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About the Author Paul E. Sandin is
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