McGraw-Hill - The Robot Builder''''s Bonanza Episode 2 Part 3 doc

Attach the aircraft cable FIGURE 27.3 Construction detail of the basic two-pincher gripper, made with Erector set parts.. The spring built into the toy arm opens the gripper when power i

You can add pads to the fingers by using the corner braces included in most Erector Setkits and then attaching weather stripping or rubber feet to the brace The finished grippershould look like the one depicted in Fig 27.5.

ADVANCED MODEL NUMBER 1

You can use a readily available plastic toy and convert it into a useful two-pincher gripperfor your robot arm The toy is a plastic “extension arm” with the pincher claw on one endand a hand gripper on the other (see Fig 27.6) To close the pincher, you pull on the handgripper The contraption is inexpensive —usually under $10 —and it is available at manytoy stores

406 EXPERIMENTING WITH GRIPPER DESIGNS

FIGURE 27.2 An assortment of girders from an Erector Set toy construction kit.

TABLE 27.2 PARTS LIST FOR TWO-FINGER ERECTOR SET GRIPPER.

2 4 1/2-inch Erector Set girder

1 3 1/2-inch-length Erector Set girder

4 1/2-inch-by-6/32 stove bolts, fender washer, tooth lock washer, nutsMisc 14- to 16-gauge insulated wire ring lugs, aircraft cable, rubber tabs, 1/2 by

1/2-inch corner angle brackets (galvanized or from Erector Set)

Chop off the gripper three inches below the wrist You’ll cut through an aluminumcable Now cut off another 1 1/2 inches of tubing—just the arm, but not the cable File offthe arm tube until it’s straight, then fashion a 1 1/2-inch length of 3/4-inch-diameter dowel

to fit into the rectangular arm Drill a hole for the cable to go through The cable is centered because it attaches to the pull mechanism in the gripper, so allow for this in thehole Place the cable through the hole, push the dowel at least 1/2 inch into the arm, andthen drill two small mounting holes to keep the dowel in place (see Fig 27.7) Use 6/32 by3/4-inch bolts and nuts to secure the pieces

off-You can now use the dowel to mount the gripper on an arm assembly off-You can use asmall 3/4-inch U-bolt or flatten one end of the dowel and attach it directly to the arm Thegripper opens and closes with only a 7/16-inch pull Attach the end of the cable to a heavy-duty solenoid that has a stroke of at least 7/16 inch You can also attach the gripper cable

to a 1/8-inch round aircraft cable Use a crimp-on connector designed for 14- to 16-gaugeelectrical wire to connect them end to end, as shown in Fig 27.8 Attach the aircraft cable

FIGURE 27.3 Construction detail of the basic two-pincher

gripper, made with Erector set parts.

1/2" x 6/32 bolt

Fender washer Pivot bar Finger Tooth lock washer Nut

Gap between finger and pivot bar

A

B

FIGURE 27.4 Hardware assembly detail of the pivot bar and fingers of

the two-pincher gripper a Assembled sliding joint;

b Exploded view.

to a motor or rotary solenoid shaft and activate the motor or solenoid to pull the gripperclosed The spring built into the toy arm opens the gripper when power is removed fromthe solenoid or motor.

ADVANCED MODEL NUMBER 2

This gripper design uses a novel worm gear approach, without requiring a hard-to-find(and expensive) worm gear The worm is a length of 1/4-inch 20 bolt; the gears are

408 EXPERIMENTING WITH GRIPPER DESIGNS

3"

FIGURE 27.5 The finished two-pincher gripper, with

fin-gertip pads and actuating cables.

FIGURE 27.6 A commercially available plastic two-pincher robot arm and claw

toy The gripper can be salvaged for use in your own designs.

standard 1-inch-diameter 64-pitch aluminum spur gears (hobby stores have these for about

$1 apiece) Turning the bolt opens and closes the two fingers of the gripper Refer to theparts list in Table 27.3

Construct the gripper by cutting two 3-inch lengths of inch aluminum channel stock Using a 3-inch flat mending “T” plate as a base, attach thefingers and gears to the “T” as shown in Fig 27.9 The distance of the holes is critical anddepends entirely on the diameter of the gears you have You may have to experiment withdifferent spacing if you use another gear diameter Be sure the fingers rotate freely on thebase but that the play is not excessive Too much play will cause the gear mechanism tobind or skip

41/64-inch-by-1/2-inch-by-1/16-Secure the shaft using a 1 1/2-inch-by-1/2-inch corner angle bracket Mount it to thestem of the “T” using an 8/32 by 1-inch bolt and nut Add a #10 flat washer between the

“T” and the bracket to increase the height of the bolt shaft Mount a 3 1/2-inch-long inch 20 machine bolt through the bracket Use double nuts or locking nuts to form a free-spinning shaft Reduce the play as much as possible without locking the bolt to the bracket Align the finger gears to the bolt so they open and close at the same angle

1/4-TWO-PINCHER GRIPPER 409

Arm tube

DowelSet screw

Hole for cable

End view

FIGURE 27.7 Assembly detail for the claw gripper and

wooden dowel Drill a hole for the ing cable to pass through.

actuat-Coupling Cable to claw

(spring loaded inside claw) Steel aircraftcable

Motor spindle

FIGURE 27.8 One method for actuating the gripper: Attach the solid

aluminum cable from the claw to a length of flexible steel aircraft cable Anchor the cable to a motor or rotary solenoid Actuate the motor or solenoid and the gripper closes The spring in the gripper opens the claw when power to the motor or solenoid is removed.

To actuate the fingers, attach a motor to the base of the bolt shaft The prototype gripperused a 1/2-inch-diameter 48-pitch spur gear and a matching 1-inch 48-pitch spur gear on thedrive motor Operate the motor in one direction and the fingers close Operate the motor inthe other direction and the fingers open Apply small rubber feet pads to the inside ends ofthe grippers to facilitate grasping objects The finished gripper is shown in Fig 27.10.Figs 27.11 through 27.14 show another approach to constructing two-pincher grippers.

By adding a second rail to the fingers and allowing a pivot for both, the fingertips remain

410 EXPERIMENTING WITH GRIPPER DESIGNS

TABLE 27.3 PARTS LIST FOR WORM DRIVE GRIPPER.

2 3-inch lengths 41/64-inch-by-1/2-inch-by-1/16-inch aluminum channel

2 1-inch-diameter 64-pitch plastic or aluminum spur gear

1 2-inch flat mending “T”

1 1 1/2-inch-by-1/2-inch corner angle iron

1 3 1/2-inch-by-1/4-inch 20 stove bolt

2 1/4-inch 20 locking nuts, nuts, washers, tooth lock washers

2 1/2-inch-by-8/32 stove bolts, nuts, washers

1 1-inch-diameter 48-pitch spur gear (to mate with gear on driving motor shaft)

1 1 / 2 " x 1 / 2 "

corner angle iron

Locking nut

Nut Nut

Tooth lock washer

3 1 / 2 " x 1 / 4 "-20 bolt

48 pitch spur gear

3" "T"

Gears 3"

A

B

FIGURE 27.9 A two-pincher gripper based on a homemade work drive system a.

Assembled gripper; b Worm shaft assembly detail.

TWO-PINCHER GRIPPER 411

FIGURE 27.10 The finished two-pincher worm drive gripper.

FIGURE 27.11 Adding a second rail to the

fingers and allowing the points to freely pivot caus-

es the fingertips to remain parallel to one another.

412 EXPERIMENTING WITH GRIPPER DESIGNS

Pivot points

FIGURE 27.12 Close-up detail of the dual-rail

fin-ger system Note the pivot points.

Pull cables to close

FIGURE 27.13 A way to actuate the

gripper Attach cables to the fingers and pull the cables with a motor or solenoid Fit a torsion spring along the fingers and palm to open the fin- gers when power is removed from the motor

or solenoid.

parallel to one another as the fingers open and close You can employ several actuationtechniques with such a gripper Fig 27.15 shows the gripping mechanism of the still-popular Radio Shack/Tomy Armatron Note that it uses double rails to effect parallel clo-sure of the fingers You can model your own gripper using the design of the Armatron oramputate an Armatron and use its gripper for your own robot.

Flexible Finger Grippers

Clapper and two-pincher grippers are not like human fingers One thing they lack is a

com-pliant grip: the capacity to contour the grasp to match the object The digits in our fingers

can wrap around just about any oddly shaped object, which is one of the reasons we areable to use tools successfully

You can approximate the compliant grip by making articulated fingers for your robot

At least one toy is available that uses this technique; you can use it as a design base Theplastic toy arm described earlier is available with a handlike gripper instead of a claw grip-per Pulling on the handgrip causes the four fingers to close around an object, as shown inFig 27.16 The opposing thumb is not articulated, but you can make a thumb that moves

in a compliant gripper of your own design

414 EXPERIMENTING WITH GRIPPER DESIGNS

FIGURE 27.15 A close-up view of the Armatron toy gripper Note the use of the

dual-rail finger system to keep the fingertips parallel The gripper

is moderately adaptable to your own designs.

Make the fingers from hollow tube stock cut at the knuckles The mitered cuts allow thefingers to fold inward The fingers are hinged by the remaining plastic on the topside ofthe tube Inside the tube fingers is semiflexible plastic, which is attached to the fingertips.Pulling on the handgrip exerts inward force on the fingertips The result? the fingers col-lapse at the cut joints.

You can use the ready-made plastic hand for your projects Mount it as detailed in theprevious section on the two-pincher claw arm You can make your own fingers from a vari-ety of materials One approach is to use the plastic pieces from some of the toy construc-tion kits Cut notches into the plastic to make the joints Attach a length of 20- or 22-gaugestove wire to the fingertip and keep it pressed against the finger using nylon wire ties Donot make the ties too tight, or the wire won’t be able to move An experimental plastic fin-ger is shown in Fig 27.17

You can mount three of four such fingers on a plastic or metal “palm” and connect allthe cables from the fingers to a central pull rod The pull rod is activated by a solenoid ormotor Note that it takes a considerable pull to close the fingers, so the actuating solenoid

or motor should be fairly powerful

The finger opens again when the wire is pushed back out as well as by the natural springaction of the plastic This springiness may not last forever, and it may vary if you use othermaterials One way to guarantee that the fingers open is to attach an expansion spring, or

a strip of flexible spring metal, to the tip and base of the finger, on the back side Thespring should give under the inward force of the solenoid or motor, but adequately returnthe finger to the open position when power is cut

Wrist RotationThe human wrist has three degrees of freedom: it can twist on the forearm, it can rock up anddown, and it can rock from side to side You can add some or all of these degrees of freedom

to a robotic hand A basic schematic of a three-degree-of-freedom wrist is shown in Fig 27.18

WRIST ROTATION 415

FIGURE 27.16 Commercially available plastic robotic arm and hand toy The

gripper can be salvaged for use in your own designs The opposing thumb is not articulated, but the fingers have a semi- compliant grip.

With most arm designs, you’ll just want to rotate the gripper at the wrist Wrist rotation

is usually performed by a motor attached at the end of the arm or at the base When themotor is connected at the base (for weight considerations), a cable or chain joins the motorshaft to the wrist The gripper and motor shaft are outfitted with mating spur gears Youcan also use chains (miniature or #25) or timing belts to link the gripper to the drive motor.Fig 27.19 shows the wrist rotation scheme used to add a gripper to the revolute coordinatearm described in Chapter 25

You can also use a worm gear on the motor shaft Remember that worm gears introduce

a great deal of gear reduction, so take this into account when planning your robot Thewrist should not turn too quickly or too slowly

416 EXPERIMENTING WITH GRIPPER DESIGNS

Digits

Pull cable

Set screw

Cable eyelet

Wire tie (1 of 5) Grommetfingertip

FIGURE 27.17 A design for an experimental compliant finger Make the finger

spring-loaded by attaching a spring to the back of the finger (a strip of lightweight spring metal also works).

FIGURE 27.18 The three basic degrees of

free-dom in a human or robotic wrist (wrist rotation in the human arm

is actually accomplished by ing the bones in the forearm).

rotat-Another approach is to use a rotary solenoid These special-purpose solenoids have aplate that turns 30° to 50° in one direction when power is applied The plate is spring-loaded, so it returns to its normal position when the power is removed Mount the solenoid

on the arm and attach the plate to the wrist of the gripper

From Here

Using DC motors and shaft encoders Chapter 18, “Working with DC Motors”Using stepper motors to drive robot parts Chapter 19, “Working with Stepper Motors”Different robotic arm systems and assemblies Chapter 24, “An Overview of Arm Systems”Building a robotic revolute coordinate arm Chapter 25, “Build a Revolute Coordinate Arm”Building a robotic stationary polar Chapter 26, “Build a Polar Coordinate Arm”coordinate arm

Interfacing feedback sensors to computers Chapter 29, “Interfacing with Computers andand microcontrollers Microcontrollers”

FROM HERE 417

FIGURE 27.19 A two-pincher gripper (from the plastic toy robotic arm detailed

earlier in the chapter), attached to the revolute arm described in Chapter 25 A small stepper motor and gear system provide wrist rotation.

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“Brain, brain, what is brain?” If you’re a Trekker, you know this is a line from one of theoriginal Star Trek episodes of the 1960s, entitled “Spock’s Brain.” The quality of the storynotwithstanding, the episode was about how Spock’s brain was surgically removed by a race

of women who needed it to run their air conditioning system Dr McCoy rigged up

a gizmo to operate Spock’s brainless body by remote control Clearly, without his brainSpock wasn’t much good to anyone, least of all to Dr McCoy, who never got the hang ofthe buttons he needed to push to start Spock walking

“Brains” are what differentiate robots from simple automated machines—brainlessSpocks who might as easily crash into walls as move in a straight line The brains of a robotprocess outside influences, like sonar sensors or bumper switches; then based on the pro-gramming or wiring, they determine the proper course of action Without a brain of sometype, a robot is really nothing more than just a motorized toy, repeating the same actionsover and over again, oblivious to anything around it

A computer of one type or another is the most common brain found on a robot A robotcontrol computer is seldom like the PC on your desk, though robots can certainly be operat-

ed by most any personal computer And of course not all robot brains are computerized Asimple assortment of electronic components—a few transistors, resistors, and capacitors—are all that’s really needed to make a rather intelligent robot Hey, it worked for Mr Spock!

In this chapter we’ll review the different kinds of “brains” found on the typical hobby

or amateur robot, including the latest microcontrollers—computers that are specially made

to interact with (control) hardware Endowing your robot with smarts is a big topic, so

additional material is provided in Chapters 29 through 33, including individual discussions

on using several popular microcontrollers, such as the Basic Stamp II

Brains from Discrete ComponentsYou can use the wiring from discrete components (transistors, resistors, etc.) to control arobot This book contains numerous examples of this type of brain, such as the line-trac-ing robot circuits in Chapter 38, “Navigating through Space.” The line-tracing functional-ity is provided by just a few common integrated logic circuits and a small assortment oftransistors and resistors Light falling on either or both of two photodetectors causes motorrelays to turn on or off The light is reflected from a piece of tape placed on the ground.Fig 28.1 shows another common form of robot brain made from discrete componentparts This brain makes the robot reverse direction when it sees a bright light The circuit

is simple, as is the functionality of the robot: light shining on the photodetector turns on arelay Variations of this circuit could make the robot stop when it sees a bright light Byusing two sensors, each connected to separate motors (much like the line-tracers ofChapter 38), you could make the robot follow a bright light source as it moves By simplyreversing the sensor connections to the motors, you can make the robot behave in the oppo-

422 AN OVERVIEW OF ROBOT “BRAINS”

Ground

Q1 2N2222 e b c

RL1

M1

D1 1N4003 R1

FIGURE 28.1 Only a few electronic components are needed

to control a robot using the stimulus of a sensor.