Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 35
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The four-wheeledlayout is not well suited for anything but flat terrain unless at least onewheel module is made vertically compliant. This is possible, but wouldproduce the complicated mechanism shown in Figure 6-8.Chapter 6Steering History197Figure 6-8 Drive/steer modulewith vertical complianceAll-terrain cycles (ATCs), when they were legal, ran power through adifferential to the two rear wheels, and steered with the front wheel in astandard tricycle layout. ATCs clearly pointed out the big weakness ofthis layout, the tendency to fall diagonally to one side of the front wheelin a tight turn. Mobility was moderately good with a human driver, butwas not inherently so.Quads are the answer to the stability problems of ATCs.
Four wheelsmake them much more stable, and many are produced with four wheeldrive, enhancing their mobility greatly although they cannot turn inplace. They are, of course, designed to be controlled by humans, whocan foresee obstacles and figure out how to maneuver around them. If amobility system in their size range is needed, they may be a good placeto start. They are mass-produced, their price is low, and they are amature product.
Quads are manufactured by a number of companiesand are available in many size ranges offering many different mobilitycapabilities.As the number of wheels goes up, so does the variety of steeringmethods. Most are based on variations of the types already mentioned,but one is quite different.
In Figure 4-30 (Chapter Four), the vehicle isdivided into 2 sections connected by a vertical axis joint. This layout iscommon on large industrial front-end loaders and provides very goodsteering ability even though it cannot turn in place. The layout also198Chapter 6Steering Historyforces the sections to be rather unusually shaped to allow for tighter turning. Power is transferred to the wheels from a single motor and differentials in the industrial version, but mobility would be increased if eachwheel had its own motor.Chapter 7WalkersCopyright © 2003 by The McGraw-Hill Companies, Inc.
Click here for Terms of Use.This page intentionally left blank.There are no multi-cell animals that use any form of continuouslyrolling mechanism for propulsion. Every single land animal usesjointed limbs or squirms for locomotion. Walking must be the best wayto move then, right? Why aren’t there more walking robots? It turns outthat making a walking robot is far more difficult than making a wheeledor tracked one. Even the most basic walker requires more actuators,more degrees of freedom, and more moving parts.Stability is a major concern in walking robots, because they tend to betall and top heavy.
Some types of leg geometries and walking gaits prevent the robot from falling over no matter where in the gait the robotstops. They are statically stable. Other geometries are called “dynamically stable.” They fall over if they stop at the wrong point in a step.People are dynamically stable.An example of a dynamically-stable walker in nature is, in fact, anytwo-legged animal. They must get their feet in the right place when theywant to stop walking to prevent tipping over. Two-legged dinosaurs,humans, and birds are remarkably capable two legged walkers, but anychild that has played Red-light/Green-light or Freeze Tag has figured outthat it is quite difficult to stop mid-stride without falling over. For thisreason, two legged walking robots, whether anthropomorphic (humanlike) or birdlike (the knee bends the other way), are rather complicateddevices requiring sensors that can detect if the robot is tipping over, andthen calculate where to put a foot to stop it from falling.Some animals with more than two legs are also dynamically stableduring certain gait types.
Horses are a good example. The only timethey are statically stable is when they are standing absolutely still. Allgaits they use for locomotion are dynamically stable. When they want tostop, they must plan where to put each foot to prevent falling over.When a horse’s shoe needs to be lifted off the ground, it is a great effortfor the horse to reposition itself to remain stable on three hooves, eventhough it is already standing still. Cats, on the other hand, can walk witha gait that allows them to stop at any point without tipping over. They donot need to plan in advance of stopping.
This is called statically-stableindependent leg walking. Elephants are known to use this technique201202Chapter 7Walkerswhen crossing streams or difficult terrain. They stand on three legswhile the forth leg is moved around until it finds, by feel, a suitableplace to set down.These examples demonstrate that four-legged walkers can havegeometries that are either dynamically or statically stable or both.Animals have highly developed sensors, a highly evolved brain, and fantastically high power-density muscles, that allow this variety of motioncontrol. Practical walking robots, because of the limitations of sensors,processors, and fast acting powerful actuators, usually end up being statically stable with two to eight legs.The design of dynamically-stable, walking mobile robots requires anextensive knowledge of fairly complicated sensors, balance, high-levelmath, fast-acting actuators, kinematics, and dynamics.
This is all beyondthe scope of this book. The rest of this chapter will focus on the secondmajor category, statically-stable walkers.LEG ACTUATORSFirst, let’s look and leg geometries and actuation methods. There arethree major techniques for moving legs on a mobile robot.• Linear actuators (hydraulic, pneumatic, or electric)• Direct-drive rotary• Cable drivenHydraulics is not covered in this book, but linear motion can be doneeffectively using two other methods, pneumatic and electric. Pneumaticcylinders come in practically any imaginable size and have been used inmany walking robot research projects. They have higher power densitythan electric linear actuators, but the problem with pneumatics is that thecompressed air tank takes up a large volume.Linear actuators have the advantage that they can be used directly asthe leg itself.
The body of the actuator is mounted to the robot’s chassisor another actuator, and the end of the extending segment has a footattached to it. This concept has been used to make robots that useCartesian and cylindrical coordinate walkers. These layouts are not covered in this book, but the reader is urged to investigate them since theycan simplify the development of the control code of the robot.Chapter 7Direct-drive rotary actuators usually have to be custom designed toget torque outputs high enough to rotate the walker’s joints.
They havelow power density and usually make the walker’s joints look unnaturallylarge. They are very easy to control accurately and facilitate a modulardesign since the actuator can be thought of conceptually and physicallyas the complete joint. This is not true of either linear actuated or cabledriven joints.Cable-driven joints have the advantage that the actuators can belocated in the body of the robot. This makes the limbs lighter andsmaller.
In applications where the leg is very long or thin, this is critical.They are somewhat easy to implement, but can be tricky to properly tension to get good results. Cable management is a big job and can consumemany hours of debug time.LEG GEOMETRIESWalking robots use legs with from one to four degrees of freedom(DOF).
There are so many varieties of layouts only the basic designs arediscussed. It is hoped the designer will use these as a starting point fromwhich to design the geometry and actuation method that best suits theapplication.The simplest leg has a single joint at the hip that allows it to swing upand down (Figure 7-1). This leg is used on frame walkers and can beactuated easily by either a linear or rotary actuator. Since the joint isalready near the body, using a cable drive is unnecessary. Notice that allthe legs shown in the following figures have ball shaped feet.
This is necessary because the orientation of the foot is not controlled and the ballgives the same contact surface no matter what orientation it is in. A second method to surmount adding orientation controlled feet is to mountthe foot on the end of the leg with a passive ball joint.The following four figures show two-DOF legs with the differentactuation methods. These figures demonstrate the different attributes ofthe actuation method. Figure 7-2 shows that linear actuators make thelegs much wider in one dimension but are the strongest of the three.Figure 7-3 shows a mechanism that keeps the second leg segment vertical as it is raised and lowered.
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