Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 41
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It should be remembered thatthis process will include several iterations, trial and error, and perseverance to guarantee that the best system is being incorporated. The moreinformation that can be obtained about the operating environment, themore likely the robot will be successful. In the end, one of the morecapable and versatile mobility systems, like the six-wheeled rocker bogieor the four-tracked front-flipper layouts will probably work well enougheven without complete knowledge of the environment.A generic rule of thumb for mobility system design can be extractedfrom the investigations done in this chapter.
Relative to the size andweight of the vehicle the mobility system is carrying, make the mobilitysystem big, light, slow, low (or movable) cg, and be sure it has sufficienttreads. If all these are maximized, they will make your robot a highmobility robot.237This page intentionally left blank.Chapter 10Manipulator GeometriesCopyright © 2003 by The McGraw-Hill Companies, Inc. Click here for Terms of Use.This page intentionally left blank.Manipulator is a fancy name for a mechanical arm. A manipulator isan assembly of segments and joints that can be convenientlydivided into three sections: the arm, consisting of one or more segmentsand joints; the wrist, usually consisting of one to three segments andjoints; and a gripper or other means of attaching or grasping.
Some textson the subject divide manipulators into only two sections, arm and gripper, but for clarity the wrist is separated out as its own section because itperforms a unique function.Industrial robots are stationary manipulators whose base is permanently attached to the floor, a table, or a stand. In most cases, however,industrial manipulators are too big, use a geometry that is not effectiveon a mobile robot, or lack enough sensors -(indeed many have no environmental sensors at all) to be considered for use on a mobile robot.There is a section covering them as a group because they demonstrate awide variety of sometimes complex manipulator geometries. The chapter’s main focus, however, will be on the three general layouts of the armsection of a generic manipulator, and wrist and gripper designs.
A fewunusual manipulator designs are also included.It should be pointed out that there are few truly autonomous manipulators in use except in research labs. The task of positioning, orienting,and doing something useful based solely on input from frequently inadequate sensors is extremely difficult.
In most cases, the manipulator isteleoperated. Nevertheless, it is theoretically possible to make a trulyautonomous manipulator and their numbers are expected to increase dramatically over the next several years.POSITIONING, ORIENTING, HOW MANYDEGREES OF FREEDOM?In a general sense, the arm and wrist of a basic manipulator perform twoseparate functions, positioning and orienting. There are layouts wherethe wrist or arm are not distinguishable, but for simplicity, they aretreated as separate entities in this discussion. In the human arm, the241242Chapter 10Manipulator Geometriesshoulder and elbow do the gross positioning and the wrist does the orienting.
Each joint allows one degree of freedom of motion. The theoretical minimum number of degrees of freedom to reach to any location inthe work envelope and orient the gripper in any orientation is six; threefor location, and three for orientation. In other words, there must be atleast three bending or extending motions to get position, and three twisting or rotating motions to get orientation.Actually, the six or more joints of the manipulator can be in any order,and the arm and wrist segments can be any length, but there are only afew combinations of joint order and segment length that work effectively. They almost always end up being divided into arm and wrist.
Thethree twisting motions that give orientation are commonly labeled pitch,roll, and yaw, for tilting up/down, twisting, and bending left/right respectively. Unfortunately, there is no easy labeling system for the arm itselfsince there are many ways to achieve gross positioning using extendedsegments and pivoted or twisted joints. A generally excepted genericdescription method follows.A good example of a manipulator is the human arm, consisting of ashoulder, upper arm, elbow, and wrist. The shoulder allows the upperarm to move up and down which is considered one DOF.
It allows forward and backward motion, which is the second DOF, but it also allowsrotation, which is the third DOF. The elbow joint gives the forth DOF.The wrist pitches up and down, yaws left and right, and rolls, givingthree DOFs in one joint. The wrist joint is actually not a very welldesigned joint.
Theoretically the best wrist joint geometry is a ball joint,but even in the biological world, there is only one example of a true fullmotion ball joint (one that allows motion in two planes, and twists 360°)because they are so difficult to power and control. The human hip joint isa limited motion ball joint.On a mobile robot, the chassis can often substitute for one or two ofthe degrees of freedom, usually fore/aft and sometimes to yaw the armleft/right, reducing the complexity of the manipulator significantly.Some special purpose manipulators do not need the ability to orient thegripper in all three axes, further reducing the DOF. At the other extreme,there are arms in the conceptual stage that have more than fifteen DOF.To be thorough, this chapter will include the geometries of all thebasic three DOF manipulator arms, in addition to the simpler two DOFarms specifically for use on robots.
Wrists are shown separately. It is leftto you to pick and match an effective combination of wrist and armgeometries to solve your specific manipulation problem. First, let’s lookat an unusual manipulator and a simple mechanism—perhaps the simplest mechanism for creating linear motion from rotary motion.Chapter 10Manipulator GeometriesFigure 10-1 E-chainE-ChainAn unusual chain-like device can be used as a manipulator. It is based ona flexible cable bundle carrier called E-Chain and has unique properties.The chain can be bent in only one plane, and to only one side. Thisallows it to cantilever out flat creating a long arm, but stored rolled uplike a tape measure. It can be used as a one-DOF extension arm to reachinto small confined spaces like pipes and tubes.
Figure 10-1 shows asimplified line drawing of E-chain’s allowable motion.Slider CrankThe slider-crank (Figure 10-2) is usually used to get rotary motion fromlinear motion, as in an internal combustion engine, but it is also an efficient way to get linear motion from the rotary motion of a motor/gearbox. A connecting rod length to / crank radius ratio of four to one produces nearly linear motion of the slider over most of its stroke and is,therefore, the most useful ratio. Several other methods exist for creating243244Chapter 10Manipulator GeometriesFigure 10-2 Slider Cranklinear motion from rotary, but the slider crank is particularly effective foruse in walking robots.The motion of the slider is not linear in velocity over its full range ofmotion.
Near the ends of its stroke the slider slows down, but the forceproduced by the crank goes up. This effect can be put to good use as aclamp. It can also be used to move the legs of walkers. The slider crankshould be considered if linear motion is needed in a design.Chapter 10Manipulator GeometriesIn order to put the slider crank to good use, a method of calculatingthe position of the slider relative to the crank is helpful.
The equation forcalculating how far the slider travels as the crank arm rotates about themotor/gearbox shaft is: x = L cos Ø+ r cos Ø.ARM GEOMETRIESThe three general layouts for three-DOF arms are called Cartesian, cylindrical, and polar (or spherical). They are named for the shape of the volume that the manipulator can reach and orient the gripper into any position—the work envelope.
They all have their uses, but as will becomeapparent, some are better for use on robots than others. Some use all sliding motions, some use only pivoting joints, some use both. Pivotingjoints are usually more robust than sliding joints but, with careful design,sliding or extending can be used effectively for some types of tasks.Pivoting joints have the drawback of preventing the manipulator fromreaching every cubic centimeter in the work envelope because the elbowcannot fold back completely on itself. This creates dead spaces—placeswhere the arm cannot reach that are inside the gross work volume.
On arobot, it is frequently required for the manipulator to fold very compactly. Several manipulator manufacturers use a clever offset jointdesign depicted in Figure 10-3 that allows the arm to fold back on itselfFigure 10-3 Offset jointincreases working range ofpivoting joints245246Chapter 10Manipulator GeometriesFigure 10-4 Gantry, simplysupported using tracks or slides,working from outside the workenvelope.180°.
This not only reduces the stowed volume,but also reduces any dead spaces. Many industrial robots and teleoperated vehicles use this or asimilar design for their manipulators.CARTESIAN OR RECTANGULARFigure 10-5 Cantilevered manipulator geometryOn a mobile robot, the manipulator almostalways works beyond the edge of the chassis andmust be able to reach from ground level to abovethe height of the robot’s body. This means themanipulator arm works from inside or from oneside of the work envelope. Some industrial gantrymanipulators work from outside their work envelope, and it would be difficult indeed to use theirlayouts on a mobile robot.
As shown in Figure10-4, gantry manipulators are Cartesian or rectangular manipulators. This geometry looks likea three dimensional XYZ coordinate system. Infact, that is how it is controlled and how theworking end moves around in the work envelope.There are two basic layouts based on how theChapter 10Manipulator Geometriesarm segments are supported, gantry and cantilevered.Mounted on the front of a robot, the first twoDOF of a cantilevered Cartesian manipulator canmove left/right and up/down; the Y-axis is notnecessarily needed on a mobile robot because therobot can move back/forward. Figure 10-5 showsa cantilevered layout with three DOF. Though notthe best solution to the problem of working offthe front of a robot, it will work. It has the benefitof requiring a very simple control algorithm.CYLINDRICALThe second type of manipulator work envelope iscylindrical.
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