McGraw-Hill - Robot Mechanisms and Mechanical Devices Illustrated - 2003 Part 12 doc

In other words, there must be atleast three bending or extending motions to get position, and three twist-ing or rotating motions to get orientation.ori-Actually, the six or more joints

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divided into three sections: the arm, consisting of one or more segments

and joints; the wrist, usually consisting of one to three segments and

joints; and a gripper or other means of attaching or grasping Some texts

on the subject divide manipulators into only two sections, arm and

grip-per, but for clarity the wrist is separated out as its own section because it

performs a unique function

Industrial robots are stationary manipulators whose base is

perma-nently attached to the floor, a table, or a stand In most cases, however,

industrial manipulators are too big, use a geometry that is not effective

on a mobile robot, or lack enough sensors -(indeed many have no

envi-ronmental sensors at all) to be considered for use on a mobile robot

There is a section covering them as a group because they demonstrate a

wide variety of sometimes complex manipulator geometries The

chap-ter’s main focus, however, will be on the three general layouts of the arm

section of a generic manipulator, and wrist and gripper designs A few

unusual manipulator designs are also included

It should be pointed out that there are few truly autonomous

manipu-lators in use except in research labs The task of positioning, orienting,

and doing something useful based solely on input from frequently

inade-quate sensors is extremely difficult In most cases, the manipulator is

teleoperated Nevertheless, it is theoretically possible to make a truly

autonomous manipulator and their numbers are expected to increase

dra-matically over the next several years

POSITIONING, ORIENTING, HOW MANY

DEGREES OF FREEDOM?

In a general sense, the arm and wrist of a basic manipulator perform two

separate functions, positioning and orienting There are layouts where

the wrist or arm are not distinguishable, but for simplicity, they are

treated as separate entities in this discussion In the human arm, the

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shoulder and elbow do the gross positioning and the wrist does the enting Each joint allows one degree of freedom of motion The theoreti-cal 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 twist-ing or rotating motions to get orientation.

ori-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 effec-tively 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 respec-tively 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 for-ward 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 is

a 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 left

to you to pick and match an effective combination of wrist and armgeometries to solve your specific manipulation problem First, let’s look

at an unusual manipulator and a simple mechanism—perhaps the plest mechanism for creating linear motion from rotary motion

An unusual chain-like device can be used as a manipulator It is based on

a 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 This

allows it to cantilever out flat creating a long arm, but stored rolled up

like a tape measure It can be used as a one-DOF extension arm to reach

into small confined spaces like pipes and tubes Figure 10-1 shows a

simplified line drawing of E-chain’s allowable motion

Slider Crank

The slider-crank (Figure 10-2) is usually used to get rotary motion from

linear motion, as in an internal combustion engine, but it is also an

effi-cient way to get linear motion from the rotary motion of a

motor/gear-box A connecting rod length to / crank radius ratio of four to one

pro-duces nearly linear motion of the slider over most of its stroke and is,

therefore, the most useful ratio Several other methods exist for creating

Figure 10-1 E-chain

linear 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

Figure 10-2 Slider Crank

In order to put the slider crank to good use, a method of calculating

the position of the slider relative to the crank is helpful The equation for

calculating how far the slider travels as the crank arm rotates about the

motor/gearbox shaft is: x = L cos Ø+ r cos Ø.

ARM GEOMETRIES

The three general layouts for three-DOF arms are called Cartesian,

cylin-drical, and polar (or spherical) They are named for the shape of the

vol-ume that the manipulator can reach and orient the gripper into any

posi-tion—the work envelope They all have their uses, but as will become

apparent, some are better for use on robots than others Some use all

slid-ing motions, some use only pivotslid-ing joints, some use both Pivotslid-ing

joints 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 from

reaching every cubic centimeter in the work envelope because the elbow

cannot fold back completely on itself This creates dead spaces—places

where the arm cannot reach that are inside the gross work volume On a

robot, it is frequently required for the manipulator to fold very

com-pactly Several manipulator manufacturers use a clever offset joint

design depicted in Figure 10-3 that allows the arm to fold back on itself

Figure 10-3 Offset joint increases working range of pivoting joints

180° This not only reduces the stowed volume,but also reduces any dead spaces Many indus-trial robots and teleoperated vehicles use this or asimilar design for their manipulators.

CARTESIAN OR RECTANGULAR

On 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 enve-lope, and it would be difficult indeed to use theirlayouts on a mobile robot As shown in Figure10-4, gantry manipulators are Cartesian or rec-tangular manipulators This geometry looks like

a 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 the

Figure 10-4 Gantry, simply

supported using tracks or slides,

working from outside the work

envelope.

Figure 10-5 Cantilevered manipulator geometry

arm segments are supported, gantry and

can-tilevered

Mounted on the front of a robot, the first two

DOF of a cantilevered Cartesian manipulator can

move left/right and up/down; the Y-axis is not

necessarily needed on a mobile robot because the

robot can move back/forward Figure 10-5 shows

a cantilevered layout with three DOF Though not

the best solution to the problem of working off

the front of a robot, it will work It has the benefit

of requiring a very simple control algorithm

CYLINDRICAL

The second type of manipulator work envelope is

cylindrical Cylindrical types usually incorporate

a rotating base with the first segment able to

tele-scope or slide up and down, carrying a

horizon-tally telescoping segment While they are very

simple to picture and the work envelope is fairly

intuitive, they are hard to implement effectively

because they require two linear motion segments,

both of which have moment loads in them caused

by the load at the end of the upper arm

In the basic layout, the control code is fairly

simple, i.e., the angle of the base, height of the

first segment, and extension of the second

seg-ment On a robot, the angle of the base can

sim-ply be the angle of the chassis of the robot itself,

leaving the height and extension of the second

segment Figure 10-6 shows the basic layout of a

cylindrical three-DOF manipulator arm

A second geometry that still has a cylindrical

work envelope is the SCARA design SCARA

means Selective Compliant Assembly Robot

Arm This design has good stiffness in the

verti-cal direction, but some compliance in the

hori-zontal This makes it easier to get close to the

right location and let the small compliance take

up any misalignment A SCARA manipulator

replaces the second telescoping joint with two

vertical axis-pivoting joints Figure 10-7 shows a

SCARA manipulator

Figure 10-6 Three-DOF cylindrical manipulator

Figure 10-7 A SCARA manipulator

POLAR OR SPHERICAL

The third, and most versatile, geometry is thespherical type In this layout, the work envelopecan be thought of as being all around In real-ity, though, it is difficult to reach everywhere.There are several ways to layout an arm withthis work envelope The most basic has a rotat-ing 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 spher-ical geometry manipulator

Figure 10-8 Basic polar coordinate manipulator

Figure 10-9 High shoulder

polar coordinate manipulator

with offset joint at elbow

Figure 10-10 High shoulder polar coordinate manipulator

with overlapping joints

Figure 10-11 Articulated polar coordinate manipulator

Figure 10-12 Gun turret polar coordinate manipulator

THE WRIST

The 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 ball-in-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)

Figure 10-14 Two-DOF wrist (yaw and roll)

Figure 10-15 Three-DOF wrist (yaw, roll, and pitch)

The 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 door-knobs, 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 one

Figure 10-16 Simple direct

drive swinging jaw

turns the other This pulls the two jaws together The jaws can be moved

through a linear actuator or can be directly mounted on a motor

gear-box’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 the

gripper This and similar designs have the drawback that the jaws are

always at an angle to each other which tends to push the thing being

grabbed out of the jaws

Figure 10-17 Simple direct drive through right angle worm drive gearmotor

Figure 10-18 Rack and pinion drive gripper Figure 10-19 Reciprocating lever gripper

A 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 until

it 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 10-20 Linear actuator

direct drive gripper

Figure 10-21 Parallel jaw on

linear slides