Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 42
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Cylindrical types usually incorporatea rotating base with the first segment able to telescope or slide up and down, carrying a horizontally telescoping segment. While they are verysimple to picture and the work envelope is fairlyintuitive, they are hard to implement effectivelybecause they require two linear motion segments,both of which have moment loads in them causedby the load at the end of the upper arm.In the basic layout, the control code is fairlysimple, i.e., the angle of the base, height of thefirst segment, and extension of the second segment.
On a robot, the angle of the base can simply be the angle of the chassis of the robot itself,leaving the height and extension of the secondsegment. Figure 10-6 shows the basic layout of acylindrical three-DOF manipulator arm.A second geometry that still has a cylindricalwork envelope is the SCARA design. SCARAmeans Selective Compliant Assembly RobotArm.
This design has good stiffness in the vertical direction, but some compliance in the horizontal. This makes it easier to get close to theright location and let the small compliance takeup any misalignment. A SCARA manipulatorreplaces the second telescoping joint with twovertical axis-pivoting joints. Figure 10-7 shows aSCARA manipulator.Figure 10-6 Three-DOF cylindrical manipulatorFigure 10-7 A SCARA manipulator247248Chapter 10Manipulator GeometriesPOLAR OR SPHERICALThe third, and most versatile, geometry is thespherical type. In this layout, the work envelopecan be thought of as being all around.
In reality, though, it is difficult to reach everywhere.There are several ways to layout an arm withthis work envelope. The most basic has a rotating base that carries an arm segment that canpitch up and down, and extend in and out(Figure 10-8). Raising the shoulder up (Figure10-9) changes the envelope somewhat and isworth considering in some cases. Figures 10-10,10-11, and 10-12 show variations of the spherical geometry manipulator.Figure 10-8 Basic polarcoordinate manipulatorFigure 10-9 High shoulderpolar coordinate manipulatorwith offset joint at elbowChapter 10Figure 10-10 High shoulder polar coordinate manipulatorwith overlapping jointsManipulator Geometries249Figure 10-11 Articulated polar coordinate manipulatorFigure 10-12 Gun turret polarcoordinate manipulator250Chapter 10Manipulator GeometriesTHE WRISTThe arm of the manipulator only gets the end point in the right place.
Inorder to orient the gripper to the correct angle, in all three axes, a secondset of joints is usually required—the wrist. The joints in a wrist musttwist up/down, clockwise/counter-clockwise, and left/right. They mustpitch, roll, and yaw respectively. This can be done all-in-one using a ballin-socket joint like a human hip, but controlling and powering this type isdifficult.Most wrists consist of three separate joints. Figures 10-13, 10-14, and10-15 depict one, two, and three-DOF basic wrists each building on theprevious design.
The order of the degrees of freedom in a wrist has alarge effect on the wrist’s functionality and should be chosen carefully,especially for wrists with only one or two DOF.Figure 10-13 Single-DOF wrist(yaw)Chapter 10Manipulator Geometries251Figure 10-14 Two-DOF wrist(yaw and roll)Figure 10-15 Three-DOF wrist(yaw, roll, and pitch)252Chapter 10Manipulator GeometriesGRIPPERSThe end of the manipulator is the part the user or robot uses to affectsomething in the environment. For this reason it is commonly called anend-effector, but it is also called a gripper since that is a very commontask for it to perform when mounted on a robot.
It is often used to pick updangerous or suspicious items for the robot to carry, some can turn doorknobs, and others are designed to carry only very specific things likebeer cans. Closing too tightly on an object and crushing it is a majorproblem with autonomous grippers. There must be some way to tell howhard is enough to hold the object without dropping it or crushing it. Evenfor semi-autonomous robots where a human controls the manipulator,using the gripper effectively is often difficult. For these reasons, gripperdesign requires as much knowledge as possible of the range of items thegripper will be expected to handle.
Their mass, size, shape, and strength,etc. all must be taken into account. Some objects require grippers thathave many jaws, but in most cases, grippers have only two jaws andthose will be shown here.There are several basic types of gripper geometries. The most basictype has two simple jaws geared together so that turning the base of oneFigure 10-16 Simple directdrive swinging jawChapter 10Manipulator Geometries253Figure 10-17 Simple directdrive through right angle wormdrive gearmotorturns the other. This pulls the two jaws together. The jaws can be movedthrough a linear actuator or can be directly mounted on a motor gearbox’s output shaft (Figure 10-16), or driven through a right angle drive(Figure 10-17) which places the drive motor further out of the way of thegripper.
This and similar designs have the drawback that the jaws arealways at an angle to each other which tends to push the thing beinggrabbed out of the jaws.Figure 10-18Rack and pinion drive gripperFigure 10-19Reciprocating lever gripper254Chapter 10Manipulator GeometriesFigure 10-20 Linear actuatordirect drive gripperA more effective jaw layout is the parallel jaw gripper. One possiblelayout adds a few more links to the basic two fingers to form a four-barlinkage which holds the jaws parallel to each other easing the sometimesvery difficult task of keeping the thing being grabbed in the gripper untilit closes. Another way to get parallel motion is to use a linear actuator tomove one or both jaws directly towards and away from each other.
Theselayouts are shown in Figures 10-21, 10-22, and 10-23.Figure 10-21 Parallel jaw onlinear slidesChapter 10Manipulator Geometries255Figure 10-22 Parallel jaw usingfour-bar linkageFigure 10-23 Parallel jaw usingfour-bar linkage and linearactuatorPASSIVE PARALLEL JAW USING CROSS TIETwin four-bar linkages are the key components in this long mechanismthat can grip with a constant weight-to-grip force ratio any object that fits256Chapter 10Manipulator GeometriesFigure 10-24 Passive paralleljaw using cross tiewithin its grip range. The long mechanism relies on a cross-tie betweenthe two sets of linkages to produce equal and opposite linkage movement.
The vertical links have extensions with grip pads mounted at theirends, while the horizontal links are so proportioned that their pads movein an inclined straight-line path. The weight of the load being lifted,therefore, wedges the pads against the load with a force that is proportional to the object’s weight and independent of its size.Some robots are designed to do one specific task, to carry one specificobject, or even to latch onto some specific thing.
Installing a dedicatedknob or ball end on the object simplifies the gripping task using this mating one-way connector. In many cases, a joint like this can be used independently of any manipulator.PASSIVE CAPTURE JOINT WITH THREEDEGREES OF FREEDOMNew joint allows quick connection between any twostructural elements where rotation in all three axes isdesired.Marshall Space Flight Center, AlabamaA new joint, proposed for use on an attachable debris shield for theInternational Space Station Service Module, has potential for commer-Chapter 10Manipulator Geometries257Figure 10-25 The threedegrees-of-freedom capability ofthe passive capture joint providesfor quick connect and disconnectoperations.cial use in situations where hardware must be assembled and disassembled on a regular basis.This joint can be useful in a variety of applications, including replacing the joints commonly used on trailer-hitch tongues and temporarystructures, such as crane booms and rigging.
Other uses for this jointinclude assembly of structures where simple rapid deployment is essential, such as in space, undersea, and in military structures.This new joint allows for quick connection between any two structuralelements where it is desirable to have rotation in all three axes. The jointcan be fastened by moving the two halves into position. The joint is thenconnected by inserting the ball into the bore of the base. When the jointball is fully inserted, the joint will lock with full strength. Release of thisjoint involves only a simple movement and rotation of one part.
The jointcan then be easily separated.Most passive capture devices allow only axial rotation when fastened—if any movement is allowed at all. Manually- or power-actuatedactive joints require an additional action, or power and control signal, aswell as a more complex mechanism.The design for this new joint is relatively simple. It consists of twohalves, a ball mounted on a stem (such as those on a common trailerhitch ball) and a socket. The socket contains all the moving parts and isthe important part of this invention.
The socket also has a base, whichcontains a large central cylindrical bore ending in a spherical cup.This work was done by Bruce Weddendorf and Richard A. Cloyd of theMarshall Space Flight Center.258Chapter 10Manipulator GeometriesINDUSTRIAL ROBOTSThe programmability of the industrial industrial robot using computersoftware makes it both flexible in the way it works and versatile in therange of tasks it can accomplish. The most generally accepted definitionof an industrial robot is a reprogrammable, multi-function manipulatordesigned to move material, parts, tools, or specialized devices throughvariable programmed motions to perform a variety of tasks.
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