Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 36
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The actuator can be replaced with a passive link, making this a one-DOF leg whose second segment doesn’tswing out as much as the leg shown in Figure 7-2.Walkers203204Chapter 7WalkersFigure 7-1 One-DOF leg forframe walkersFigure 7-2 Two-DOF leg usinglinear actuatorsChapter 7Walkers205Figure 7-3 Two-DOF leg usinglinear actuators withchassis-mounted knee actuatorRotary actuators (Figure 7-4) are the most elegant, but make the jointslarge. The cable driven layout (Figure 7-5) takes up the least volume andhas no exposed actuators.
Both of these methods are common, mostlybecause they use motors in a simple configuration, rather than linearactuators. Their biggest drawback is that they need to be big to getenough power to be useful. iRobot’s Genghis robot used two hobby servos bolted together, acting as rotary actuators, to get a very effective twoaxis hip joint. This robot, and several others like it, use simple straightlegs. These simple walker layouts are useful preliminary tools for thoseinterested in studying six-legged walking robots.To turn the two-DOF linear actuator layout into a three-DOF, a universal joint can be added at the hip joint. This is controlled with an actuator attached horizontally to the chassis.
Figure 7-6 shows a simpledesign for this universal hip joint. The order of the joints (swing first,then raise; or raise first, then swing) makes a big difference in how thefoot location is controlled and should be carefully thought out and prototyped before building the real parts.The three-DOF rotary actuator leg (Figure 7-7) adds a knee joint tothe Genghis layout for improved dexterity and mobility.
There are manyvarieties of this layout that change the various lengths of the segments206Chapter 7WalkersFigure 7-4 Two-DOF leg usingrotary actuatorsFigure 7-5 Two-DOF leg usingcable driven actuatorsChapter 7Walkers207Figure 7-6 Three-DOF leg usinglinear actuatorsFigure 7-7 Three-DOF leg usingrotary actuators208Chapter 7Walkersand the relative location of each actuator. It is quite difficult to drive atwo-DOF hip joint with cables, but it can be done. The general layoutwould look much like what is shown in Figure 7-7.WALKING TECHNIQUESStatically-stable walkers are easier to implement than dynamically-stable walkers.
A method used to group statically-stable walkers is the technique used to move the legs. There are three useful sub groups: wavewalking, independent leg walking, and frame walking. Wave walking iswhat animals with many legs use, like millipedes. Independent leg walking is used by just about every four, six, and eight-leg walker, althoughsome simplify things by moving their legs in groups for certain speeds ormotions.
Frame walking exists in nature in the form of an inchworm, andis the simplest of the three, but to have high mobility still requires manyactuators. As we shall see, frame walking can be a very effective mobility method for a mobile robot.Wave WalkingCentipedes and millipedes use a walking technique that must be mentioned, although it is simple in concept, for walking robots, it is less efficient than other methods. The robot lifts its rear-most set of legs andswings them forward and sets them down, then the next set of legs ismoved similarly. When the front-most set of legs is moved, the wholerobot chassis is moved forward relative to the legs.
The process can besmoothed out some by averaging the position of the body as each set oflegs moves forward. This technique can be used with six- or more-leggedrobots, but is not very common in robots because of the large numbers ofjoints and actuators.Independent Leg WalkingVirtually all other legged animals in nature that don’t use wave walkingcan control each leg independently. Some animals are better than others,but the ability is there. Figures 7-8 and 7-9 show four- and six-leggedwalkers with three rotary-actuated joints in each leg. An eight-leg layoutwould have no less than 24 actuators. The four- and six-legged versionsChapter 7Walkers209Figure 7-8 Independent legwalker, four legs, twelve DOFFigure 7-9 Independent legwalker, six legs, eighteen DOF210Chapter 7WalkersFigure 7-10 Extra wide feetprovide two-legged stabilitytheoretically have very high mobility.
Many research robots have beenbuilt that use four or six legs and are impressively agile, if very slow.Although it would seem impossible to build a two-legged staticallystable robot, there is a trick that toys and some research robots use thatgives the robot the appearance of being dynamically stable when they areactually statically stable. The trick is to have feet that are large enough tohold the robot upright on one foot without requiring the foot to be inexactly the right place.
In effect, foot size reduces the required accuracyof foot placement so that the foot can be placed anywhere it can reachand the robot will not fall over.The wide feet must also prevent tipping over sideways and are so widethat they overlap each other and must be carefully shaped and controlledso they don’t step on each other. Two-legged walking, with oversizedand overlapping feet, is simply picking up the back foot, bringing it forward, and putting it down.
The hip joints require a second DOF in addition to swinging fore and aft, to allow rotation for turning. Each leg musthave at least three DOF, and usually requires four. The layout shown inFigure 7-10 can only walk in a straight line because it lacks the hip rotation joint. Notice that even with only two legs and no ability to turn, thislayout requires six actuators to control its six degrees of freedom.This layout provides a good educational tool to learn about walking.Although in the final implementation it may have eight DOF and itsChapter 7Walkers211knees bend backwards, it is familiar to the designer.
One leg at a time canbe built, tested, and debugged and then both attached to a simple platefor a chassis.Frame WalkingThe third general technique for walking with a legged robot is framewalking. Frame walking relies on the robot having two major sections,each with their own set of legs, both sections statically stable. Walking isaccomplished by raising the legs of one frame, traversing that frame forward relative to the frame whose legs are still on the ground, and thensetting the legs down.
The other frame’s legs are then raised and traversed forward.The coupling between the two frames usually has a second rotatingDOF to facilitate turning, rather than by adding a rotation in each leg.Figure 7-11 shows a mechanism for traversing and turning the two partsof the body. In nature, an inchworm uses a form of frame walking.
Thetwo frames are the front and back sections of the worm. The coupling isthe leg-less section in between. In the case of the inchworm, the couplinghas many degrees of freedom, but two is all that is required if the legseach have their own ability to move up and down. Unfortunately forrobot designers, the inchworm also has the ability to grasp with its claw-Figure 7-11 Mechanism forframe traversing and rotating212Chapter 7WalkersFigure 7-12 Traversing/rotatingframe eight-leg frame walker withsingle-DOF legsFigure 7-13 Eight-leg framewalker with two-DOF legsChapter 7Walkerslike feet, making it quasi-statically stable.
Figure 7-12 shows an implementation of the traversing/rotating frame with simple one-DOF legs.This layout has 10 DOF. Figure 7-13 removes the rotating joint, whichforces placing a second joint in each leg to be able to turn. The actuatorcount goes up to 17 with this layout. The advantage of having the secondjoint in each leg is the ability to place each leg in the most optimum pointto maintain traction and stability.
Still, 17 actuators is a lot to control andmaintain.A six-leg tripod-gait frame walker could, however, have just threedegrees of freedom, all in one joint between the two frames. This jointwould have a linear motion for traversing, a rotary motion for steering,and a vertical motion to lift one frame and then the other. Mobility wouldsuffer with such a simple platform because the robot would lack the ability to stand level on uneven terrain. Perhaps the best is a six-leg tripodgait frame walker with one linear DOF in each leg and two in the coupling, bringing the total DOF to eight.
Figure 7-14 shows just such alayout, perhaps the best walking layout to start with if designing a walking robot.Figure 7-14 Six-leggedtripod-frame walker withsingle-DOF legs213214Chapter 7WalkersROLLER-WALKERSA special category of walkers is actually a hybrid system that uses bothlegs and wheels.
Some of these types have the wheels mounted on fixedlegs; others have the wheels mounted on legs that have one or twodegrees of freedom. There doesn’t seem to be any widely accepted termfor these hybrids, but perhaps roller walkers will suffice.A commercially available roller walker has one leg with a wheel on itsend, and two jointed legs with no wheels, each with three DOF.
Themachine is a logging machine that can stand level even on very steepslopes. Although this machine looks ungainly with its long legs with awheel on one of them, it is quite capable. Because of its slow traversespeed, it is transported to a job sight on the back of a special truck.Wheels on legs can be combined to form many varieties of rollerwalkers. Certain terrain types may be more easily traversed with thisunusual mobility system.
The concept is gaining wider appeal as itbecomes apparent a hybrid system can combine the better qualities ofwheeled and legged robots. If contemplating designing a roller walker, itmay be more effective to think of the mobility system as a wheeled vehicle with the wheels mounted on jointed appendages rather than a walking vehicle with wheels. The biggest limitation of walkers is still topspeed. This limitation is easily overcome by wheels. A big limitation of awheeled vehicle is getting over obstacles that are higher than the wheels.The ability to raise a wheel, or reconfigure the vehicle’s geometry toallow a wheel to easily drive up a high object, reduces this limitation.There are several researchers working on roller walkers.
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