ROBOTICSPERSONAL

IT’S INEVITABLE.When working on a project at your workbench, at some pointyou’ll wish that you had a third hand. When you’re holding together two partsthat need to be soldered or you need a screwdriver but you don’t want to takeyour eyes off of some small parts to reach for it, an extra hand would be awfullyuhm ... “handy.” In response to this need, the industry has developed all sortsof “third hand” devices such as the alligator clip “helping hands” sold byRadioShack (Figure 1) or specialized vises such as the ones sold by PanaVise(Figure 2).Though these tools each do their jobs very well, it’s up to you to do the(re)positioning any time you want to have your item at a different orientation.

Just a short while ago, some robotenthusiasts and I went to see the

movie Iron Man. The hero of thestory had a pair of autonomous, intel-ligent, mobile robot arms that wouldassist him by holding lights and hand-ing him tools as he worked. In onescene, the arm is intelligent enoughto know where to hold a magnifyingglass to help the hero build part ofhis suit (Figure 3) by simply observingthe human and discerning his needs.Though we obviously have a long

way to go before we get to the levelof robotic helpers depicted in sciencefiction movies such as this, it mademe wonder how far off a usefulrobotic bench accessory was. Forexample, would it be possible to re-purpose a hobby-level robotic armto “earn its keep” by helping out onour workbenches?

CHILD’S PLAYThe first robot arm I owned was

the venerable RadioShack “Armatron”(Figure 4), purchased in the early1980s. When I got it home and (ofcourse) disassembled it, I was bothimpressed and a bit disappointed.Turns out the Armatron used a compact (and complex!) plastic gearbox with a single motor to drive allthe motions of the arm. The two joy-sticks mechanically engaged different

ROBOBENCH: Putting the CrustCrawlerAX-12+ “Smart Arm” to Work

UNDERSTANDING, DESIGNING & CONSTRUCTING ROBOTS & ROBOTIC SYSTEMS

■ BY VERN GRANER

■ FIGURE 1.RadioShack“Helping Hands.”

■ FIGURE 2. PanaViseworkbench vise unit.

■ FIGURE 3. Robert Downey Jr. as“Tony Stark” in the Paramount

feature film Iron Man. PhotoCopyright © Paramount Pictures.

PART 1

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gear trains with the constantly spinning motor, causing the variousjoints of the arm to move. Thoughingenious in design, it wasn’t very“hackable” since there were no individually addressable motors tocontrol. Today, most robot arms yousee for sale in electronics/roboticshobby magazines are composed ofsome plastic or metal parts and ahandful of model aircraft-type servomotors. From a hacking/programmingperspective, this is a big step up fromthe Armatron single motor approach.

Most of these robot arms arequite useful for teaching robotic andautomation concepts. They do a goodjob of introducing the subject andshowing someone just how complicateda simple human task such as “hand methe TV remote” can be for a robot.I’ve seen these “educational” roboticarms used to teaching the computational complexity involved in stacking small wooden blocks ormoving disks to solve the “Tower ofHanoi” puzzle. Yet, I had not seenone employed doing real work in areal workbench environment. Couldit be that a sufficiently powerful

and accurate arm just wasn’t available yet?

ENTER THECRUSTCRAWLER!

When CrustCrawler announcedthe AX-12+ intelligent arm, I remem-ber reading reports of its strengthand accuracy. More importantly, thearm used the Dynamixel servo motorsthat had real feedback capabilities.This would make it possible to attainnew levels of accuracy when usingthe arm since, unlike a standardhobby servo, it can determine if thearm had actually reached a position.With these advanced features, I wondered if the AX-12+ SmartArmwould be able to do real work likethe commercial robotic arms used inmanufacturing. Could it hand metools? Could it hold a printed circuitboard (PCB) while I was soldering?The more I thought about it, themore I began to wonder why not?

I began to catalog the varioustasks that I might need the arm toperform. Using it while constructing

devices was the first thing thatcame to mind. Holding parts orfetching the right tool seemeduseful, as well. Need a #2Phillips? The arm can hand it toyou. Need someone to holdthese parts while you solderthem? The arm can do thatAND it won’t burn its fingers!The possibilities seemed endless. All I would need was a CrustCrawler AX-12+ to experiment with.

HEY BUDDY, CAN YOUSPARE AN ... ARM?

I spoke with my top-secret

contacts over at Nuts & Volts andgave them the broad outlines of theidea I had and asked if they thoughtCrustCrawler might be interested in this experiment. I told them Ithought the AX-12+ would be theperfect candidate for testing out theidea of a useful bench-top robotarm. They put me in touch with AlexDirks over at CrustCrawler, Inc., andhe was happy to send an arm outfor torture ... er ... experimentation.I dug through the CrustCrawlerwebsite and examined all the variousconfigurations at length. I finally sentoff a wish-list to Alex of the parts Ithought I would need and, beforeI knew it, I had a bouncing babycardboard box on my porch. Timeto build!

“SOME ASSEMBLYREQUIRED”

When I opened the AX-12+box, I was delighted to see littlebags of clearly labeled parts and anice thick detailed manual. Otherthan some typical hand tools, theonly thing I had to supply was theLoctite liquid to lock down thescrews so they don’t wiggle looseduring use of the arm (good practicaladvice that was stated right at thefront of the manual). The arm wenttogether in a couple of fun eveningsin no small part due to that very nicethick guide!

Now that I had the arm alltogether, I was ready to get it tomove. Besides the AX-12+ itself, thepackage from Alex included a coollittle interface box called the CM-5(Figure 5) and the USB2Dynamixelunit (Figure 6). The CM-5 is arecord/playback device that can beused to play sequences of moves. It also houses a rechargeable battery pack to allow portable use of the arm and is recharged with the included hefty 12V, five amppower supply.

To program the arm, youconnect your PC to theUSB2Dynmixel, connect a three-pincable from the USB2Dynamixel tothe CM-5 unit, then connect another

■ FIGURE 4. The author’soriginal 1980’s Armatronrobot arm from RadioShack.

■ FIGURE 6. The USB2DynamixelUSB to TTL/RS-485 adapter.

■ FIGURE 5.The ROBOTIS CM-5control/power unit for theCrustCrawler AX-12+ SmartArm.

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three-pin cable from the CM-5 to theDynamixel servo in the arm’s base.Once the connections above areestablished, you connect the 12Vsupply to the CM-5 and you’re readyto go. I started out using the Robotis“motion editor” software that comeswith the CM-5. Though the softwareis fairly intuitive, I found it a bit cumbersome to use. It appears tohave been designed with the“Bioloid” plastic models in mind.Sadly (at the time of this writing),there was no “template” for my nice new all-metal AX-12+, makingpositioning the joints a bit tough for me.

TIME FOR SOME GNUSOFTWARE

While digging around for demosequences on the CrustCrawlerwebsite, I discovered a link to the“AX-12+ Arm Sample” project byScott Ferguson. Scott’s code allowsyou to map joystick controls to thevarious AX-12+ joints and then“puppet” the arm. Once you movethe arm into a position, you canrecord the joint values as a “pose”and then step forward/backwardthrough these poses. This is verysimilar to the “teaching pendants”that many commercial robot armsuse to create their processes. To topit off, Scott released the softwareunder the GNU public license so allthe source code is available, aswell (this would come in handydown the road!).

This software looked verypromising, however it was designedto communicate directly with theDynamixel servos and would not talkto them through the CM-5. I wouldhave to remove the CM-5 from thecommunication chain in order to getScott’s software to work. Thoughthis would be fairly simple to do, itwould leave me without a powersource for the arm as the CM-5 washandling converting the 12V fromthe supply down to the acceptable9.6V voltage levels required by theservos. As a quick and dirty solution,I took one of the extra three-pin

cables and spliced a 2.1 mm powerconnector into it and included fourhigh-current diodes in series to dropthe 12V from the supply down to avoltage that would not exceed the10V maximum rating of the servos(Figure 7).

GET A GRIP!I booted up Scott’s software and

mapped an old joystick to the variousjoints of the AX-12+ and, in a matterof minutes, was moving the arm allaround the desk. A word of warningfor those of you that may end upwith this arm: it is strong! I’veknocked things off my workbench,dumped over a perfectly good can of Dr. Pepper, put a nice nick in thecorner of my LCD monitor, and onetime I even managed to smack myselfright in the funny bone! If you areusing the joystick to control the arm,use caution and make sure breakablethings are not in its path. Don’t learnthis the hard way like I did!

Now that I had the arm in place(and a bandage on my elbow!), Iwas finally ready to try some workexperiments. I started with a taskthat was both repetitive and fairlycommon, namely holding a PCB forsoldering. When I build through-holePCBs, I have to flip the board backand forth betweensolder-side andcomponent-side as Istuff components andthen solder them intoplace. I wanted to seeif the arm coulddo this for me toeliminate the timespent manually

repositioning the mechanical “helpinghands” I normally used for this task.

FLIP IT! FLIP IT GOOD!I started by using the joystick to

maneuver the arm into a braced posi-tion with the elbow joint resting atthe end of its travel, then I moved theshoulder joint so that it was holdingthe gripper at a comfortable angle forsoldering. I then fully opened thegripper and named this pose “ReleaseBoard.” Next, I brought the grippercompletely shut and named this pose“Grip Board.” I executed the ReleaseBoard pose, placed a small projectboard in the gripper’s range, and exe-cuted the Grip Board pose. The grippertook a nice strong hold of the board.I now used the joystick to rotate thewrist joint so the board was solderside up and named this pose “SolderSide.” Lastly, I rotated the wrist so thecomponent side was up again andnamed this pose “Component Side.”

So now, with a click of a mouse Icould have the arm grab a PCB andhold it while I used two hands toinstall a component on it (Figure 9),then another couple of clicks flippedthe board over so I could use both

■ FIGURE 7. Four high-current diodes wired in series to drop the

12V power source.

■ FIGURE 8.The AX12 Arm

Controller softwareby Scott Ferguson.

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hands to solder on it (Figure 10). This process was so deceptively easyand the result so useful that I justswaggered on to the next task!

WHEN UNSTOPPABLEOPTIMISM MEETSUNMOVABLE REALITY

The rumble you felt and that

distant screaming sound wasn’t anearthquake, it was just my ambitiousideas coming in direct contact withreality. With the board flipping functionunder my belt, I decided to take thingsup a notch and see about having thearm hand me one of the other morepopular and useful devices on myworkbench: a solder vac.

I started by placing a spool ofhookup wire on the bench and then

placing the solder vac inthe center of the spoolto hold it upright whereit could be easily grippedby the arm. I then usedthe joystick to move thearm through the series ofposes as I had done

above, naming them “Open Gripper”(Figure 11), “Fetch Solder Vac”(Figure 12), and “Offer Solder Vac”(Figure 13). I then placed the soldervac back in the spool and executedthe Open Gripper and the FetchSolder Vac routines. Unfortunately,the gripper missed the vac andended up dumping the spool over!

A bit (okay, more like hours) of experimentation later and I discovered that to reliably retrieve the solder vac, I would have to makea much more accurate resting spotfor it. Since the arm was reliablyrepeating the positions I had recorded, the solder vac had to be at the same angle and height everytime or there was a good chance thearm would miss.

Logically, this also meant that any tool I wanted to have the armhandle would have to be equally well

■ FIGURE 9. Placing components into a PCB held by theAX-12+ SmartArm.

■ FIGURE 10. Soldering components into place while PCBis held steady by AX-12+.

■ FIGURE 11. AX-12+ gripper open,ready to reach for the solder vac.

■ FIGURE 12. The AX-12+ gripper clampsdown on the solder vac.

■ FIGURE 13. A slight tug removesthe solder vac from the AX-12+gripper.

18 November 2008

■ FIGURE 14. Foot pedalcontrol expected to addhands-free control to theAX-12+ motions.

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situated. Could be this “hand me a tool” business is a bittrickier than I had imagined! I sat back and started to thinkabout how I could make sure the robot’s environmentwould be as reliable as the arm itself.

SAME BAT TIME, SAMEBAT CHANNEL!

Luckily, I’ve got tough calluses onmy ambition and I was able to pullmyself from the wreckage of firstcontact and continue on my questfor a robotic helping hand on mybench. I’ve managed to sweet-talkone of my regular roboteers intohelping write some software to allowfoot-pedal control of the arm (Figure14) and another one to help me builda “tool gallery” to hold the toolsexactly where the arm can find them.

Look for all the details of mycontinuing quest for a helping handat the workbench in ROBOBENCH —Part 2! As always, if you have anyquestions or comments, I can bereached at vern@txis.com. NV

■ FIGURE 15.Old schoolmeets cool tool!

RESOURCES■ CrustCrawler (AX-12 Smart ArmSource) — wwwwww..ccrruussttccrraawwlleerr..ccoomm

■ Forest Moon Productions (GNUSmart Arm software) — wwwwww..ffoorreessttmmoooonn..ccoomm//SSooffttwwaarree//AAXX1122AArrmmSSaammppllee//

THANK YOUS■ I would like to take a moment to thank the good folksover at CrustCrawler for providing this most excellentrobotic arm for use in the project, Nuts & Volts Magazinefor helping set it up, and Paul Atkinson and JamesDelaney for their invaluable assistance with this project.

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