http://www.instructables.com/id/Modifications-to-Robot-Arm-for-Opto-Coupler-Feedba/

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Modifications to Robot Arm for Opto Coupler Feedback, OWI 535, Edge etcby jomac_uk on January 14, 2011

Table of Contents

Modifications to Robot Arm for Opto Coupler Feedback, OWI 535, Edge etc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Intro:   Modifications to Robot Arm for Opto Coupler Feedback, OWI 535, Edge etc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Step 1:   Surgery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Step 2:   Mounting the Opto Coupler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Step 3:   Modifying the Encoder Gear. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Step 4:   Wiring up and re-assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Step 5:   Re-assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Step 6:   And.....Finally!! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Related Instructables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

http://www.instructables.com/id/Modifications-to-Robot-Arm-for-Opto-Coupler-Feedba/

Author:jomac_ukA semi retired engineer with too much time on his hands, I just love, building and tinkering with anything electrical or mechanical. I also love chatting topeople and swapping ideas. Just send me a message if you want to know more.

Intro:  Modifications to Robot Arm for Opto Coupler Feedback, OWI 535, Edge etcFor those of you into robotics, and especially robotic arms, many of you have seen the cheap but impressive robotic arm that is currently on the market for less then $30(£30 in the UK) These arms have 5 motors, and some models come equipped with a built in USB interface allowing the PC to control the Arm.

Whilst good value for money, these Robotic Arms do have certain limitations, and the biggest by far, is the lack of feed back, not that there is anything to feed back from!

These robotic arms dont actually have servo's as such, but a simple motor and worm drive gearbox with 4 cogs inside it. Anyone interested in Inverse Kinetics, wouldntbe impressed with the way this arm performs, and the supplied software for the USB version just simply relies on a timer, hoping to get the arm in a very near position towhere it was programmed to go.

After reading a few very interesting topics on here, where people have interfaced these arms with an Arduino, the lack of positional feedback was very obvious,especially if you wanted any degree of accuracy, ie better then 2" resolution.

After playing around with small variable resistors to give feedback, I decided to strip down one of the "servo's" to see what room i had to play with. The following Tutorial,explains how these "servo's" can be simply and easily modified using 1 part, and a little time to give that much needed feedback, with surprising accuracy.

Parts needed:- 4 infra red slotted opto's.3 or 4 core flat cable.

Tools needed:- Soldering iron and solder.Craft knife.Super glue.Patience.Endless supply of coffee!

Please note, the servo shown below is a finished one, hence the extra cable.

Step 1: Surgery.The chances are, if you have one of these arms, you will have built it yourself and know how it goes together, so in which case, i will skip the obvious and get right downto the basics. There are 4 cogs or gear wheels inside the gearbox, driven by a worm drive. Take all of the parts out leaving the bare shell. Just to the right of the motormount, and almost opposite the brown gear wheel, we need to cut some of the plastic away from the divide wall, this isnt critical, but a piece about 3mm X 10mm shouldbe suffiicent, this is where the slotted opto will be glued.

Next modify the slotted opto, the mounting lug on the end opposite the chamfered LED needs to be cut off, next cut off about 1mm from the other lug. Next with care, cutoff about 1mm of the chamfered housing for the LED, this is best done with the craft knife, be careful with your fingers, and make sure you dont crack the housing, as it isfragile, we have to remove enough plastic to expose the face of the LED, you may need to apply a drop of super glue afterwards, to hold the infra red LED in place. Thereis nothing special about these slotted opto's, i just happened to have a few in my junk box, practically any type will do.

Once the Slotted opto is cut and modified to fit, its time to prepare the connections, looking at the rear of the opto, and the LED with the chamfered edge nearest to you,cut the top left and bottom right connection short, to use as a terminal post. Next bend the bottom left diagonally to the top right and solder the two together, this forms thecommon connection or 0 volts.

http://www.instructables.com/id/Modifications-to-Robot-Arm-for-Opto-Coupler-Feedba/

http://www.instructables.com/id/Modifications-to-Robot-Arm-for-Opto-Coupler-Feedba/

Step 2: Mounting the Opto CouplerNow its time to mount the opto coupler in position, note that the pictures show the wire already connected, but i would leave this off until last, its a lot easier! Andappologies for the couple of blurred pictures, but they are clear enough to get the idea. This step has got to be the fiddliest of the whole operation, the opto has got to bemounted in eacactly the right position, if its too high or too low, then the opto will touch the cog, if it too far to the left, the motor wont fit, too far to the right, and the otherhalf of the case wont fit. This is the way i did mine.

The next step would be easier, if the bottom half of the case was held in a mini vise. Temporarily fit the motor, and hold it in place with a rubber band, then fit the two gearwheels,one grey/black the other brown to its shaft, and fit that in place.

At this stage, it would be useful to try a couple of dummy runs, getting the opto into place, BEFORE you apply the glue.

Apply super glue to the lower mounting lug of the opto, and carefully fit that into position (you may have to move the 2 gear wheels to one side to allow access. Once inpostion, press the opto into place, making sure that the faces of the opto dont touch the faces of the gear wheel, and also making sure the opto is snug against the motor.Holding it in place with a small screw driver wedged between the opto and the plastic helps, you will see how when its in position.

Once the opto is in place, and you are happy that it IS in the right place, carefully remove the two gear wheels, and place the gearbox shell to one side to allow the glue toset.

Now comes the next fiddly bit.

http://www.instructables.com/id/Modifications-to-Robot-Arm-for-Opto-Coupler-Feedba/

Step 3: Modifying the Encoder Gear.Now we have to modify one of the gear wheels, the brown one in fact, to turn it into the encoder wheel.

The very nice people who made this gear wheel, left 6 small circular markings on the underside of the gear wheel, these are in a perfect position for the holes that wewant. Basically the more holes we have, the better the final resolution. I certainly wouldnt advise going for more then 6 holes, as it would leave the wheel too weak. Thefirst picture shows the 6 markings, highlighted with a pen.

Great care needs to be taken with the next step, as the wheel can be easily damaged, Please DONT be tempted to drill each hole out in one go, The finished hole size is4mm. I first took a piece of wood, and drilled a hole big enough to take the smaller half of the gear wheel, to allow the larger face to sit on a flat surface. The first hole idrilled was 1.5mm, this was my pilot hole, the next size up was 2.5mm, and for the last drilling, i did this by hand holding the drill bit with a cloth wrapped around it. Thehole being so close to the edge, i didnt want the risk of the drill bit grabbing, and wrecking the gear wheel.

With the gear wheel drilled, we can now move onto the next step.

Step 4: Wiring up and re-assemblyNow we can solder the wires into the opto coupler. I used flat cable, because i didnt have to make any further mods to the casing of the motor gearbox. In my case, all icould find was 3 core from an old computer modem lead. Thats the reason why i had to join the -ve's of the LED's, if you have 4 core, you could connect the two diodes inthe opto coupler seperately, but it doesnt make any difference.

With the wires connected, they can now be moved clear of where the motor sits, and then with a spot of super glue, glue the cable into position, it needs to be glued toone side, to avoid catching the motor.

http://www.instructables.com/id/Modifications-to-Robot-Arm-for-Opto-Coupler-Feedba/

Step 5: Re-assemblyWith the connections made, and the cable secured in place we can now replace all of the gear wheels, making sure that nothing catches or obstructs the gears, oncesatisfied, carefully thread the motor cable through the space next to the opto cable, and press the motor into place. Tidy up any wires which may catch on the lid, andthen replace the top cover, and screw in the 3 case screws. Clip the motor wires into their lugs to hold them in place.

And thats it, the motor/gearbox has now been converted to provide accurate optical feedback.

Step 6: And.....Finally!!The outer gear wheel/drive mount can now be added, and the whole assembly fitted back onto the robot arm.

One down, three to go!! Once I was up to speed, i could convert one of these motors in about 45 mins.

The ratio bewteen the gear wheel and the motor is 32 to 1, calculating the ratio of the other gear wheels, shows that for a 180 degree turn of the output shaft, you will get1024 pulses out of the gearbox, and looking at the output waveform on a scope, the opto was producing nice and clean square waves.

Early playing around has proved that i can get the arm to any position along the horizontal axis to within 3 to 4 mm, this i feel sure can be bettered if the backlash or slackin the gears is allowed for within the software, im pretty new to writing Scripts in the Arduino world, having come from the world of PICS.

The gripper or the jaws were not modified in this way for two reasons, the first reason was, there wasnt really enough room to get an opto inside the gearbox without a lotof fabrication, but the main reason, was, i wanted better feedback from the gripper. I wanted to sense the actual grip pressure, and i did this using a sensitive currentsensing module like this:-

http://www.technobotsonline.com/sparkfun-acs712-low-current-sensor.html

Once fitted, i get an accurate feedback of the pressure applied to the jaws of the robot arm, and i can therefore program the Arduino to switch off the motor when a presetpressure is reached.

A similar sensor could be fitted to possibly the elbow joint, and the arm calibrated to give weight readings as well.

I hope you found this instructable of interest, and i welcome any questions or comments.

Happy Modding..;o)

http://www.instructables.com/id/Modifications-to-Robot-Arm-for-Opto-Coupler-Feedba/

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Control an OWIRobotic Armwith Arduino byzlite

Arduino Robot -Quadruped -body kinematicstests (video) byc0ffeepowder

ROBOTIC ARMUSING 6SERVOS (video)by mshayan27

Sugru + Servos= Robot bymarc.cryan

Wendell theRobot bymarc.cryan

PROTEI_007 bygabriellalevine

Comments

32 comments Add Comment

 e024576 says:  Jan 15, 2012. 11:29 AM  REPLYHi,

I considering a project like this.

Regarding the suggested Sparkfun current sensor board - the spec as I read it say " 66 to 185 mV/A output sensitivity."

On my arm, measured gripper motor current is about 200 mA when the grip is just moving, and about 350 mA when the grip motor is stalled and themechanical clutch starts to slip.

Given these low current values is seems like the sensor board voltage output would be quite low. Has the recommended Sparkfun board actually beentested in this application?

Thanks

 DieCastoms says:  Jul 31, 2011. 8:45 AM  REPLYI am considering purchasing the OWI 535 robot arm. I have found one for as little as $34US. I have a few questions about the robot that I hope you or someof your commentors will answer for me. Some of these are a little off-topic but all relate to the OWI 535 or to the optos.

I have a small pile of 'dead' PC mice. I know that many of the older ball-type use optos to 'count' the movements of two rollers touching the ball. How wouldthose optos compare in size and function to the ones being used in this 'Ible? Would it likely be possible to salvage optos from PC mice for this? could anopto sensitive enough to count the actual gear teeth be obtained?

Is this robot worth purchasing? I've had both the original Armitron robot arm and it's clone from the Discovery Store. Those two 'toys' are the extent of myrobot arm experience.

If it is "new in box" how much assembly is required? Does one have to actually build each gearbox? This would be a benefit to me as I will inevitably end-uptaking the thing apart if I DON'T have to assemble it from scratch ;)

Can someone tell me in layman's terms how much the gripper can lift? I know everywhere says "100 grams" but I'm honestly not sure how much that is >.<

Can people please show pictures of how they've cleaned up all the dangling wires around the arm? These are a real turn-off to me >.< The OP mentionsadding flat cables. I would like to see examples.

I look forward to any and all answers and if this is too off topic, will the OP please ask commentors to PM me with their answers instead?

Michael, at "DieCastoms"

http://www.instructables.com/id/Modifications-to-Robot-Arm-for-Opto-Coupler-Feedba/

 Kozuko says:  Jan 3, 2012. 3:27 AM  REPLYThe author mentions that any opto will work. However I have a salvaged collection and I can tell you this isn't quite true. Some opto have longerdistances in between emitter/receiver for them to fit. So a small one from a mouse would probably work. It took me about 2.5 hours to assemble this non-stop with about two times having to redo gearboxes cause I forgot/messed up a step. It can lift up a single AA battery. The most I have tried to lift withmine was a small knife. Um cable management is key. the original contents include small cable harness with sticky stuff on the bottom. These attach tothe arms and body of the robot down to the core control. No way around it. Keep in mind if you do this, you will have lots of wires to work with

 Dominion-Network says:  Oct 22, 2011. 9:09 AM  REPLYI do have a question, I'm currently designing a series of PCB's (Arduino clones and clone shields) I'm just wondering what input would you connect the RXout to? an analog input? or a digital input for HIGH/LOW ?

 Kozuko says:  Jan 3, 2012. 2:38 AM  REPLYIt's analog. Check out datasheethttp://pdf1.alldatasheet.com/datasheet-pdf/view/105412/ETC/EE-SX1235A-P2.html

at page two on bottom right corner it give you an idea on how it works. Let me know if you need further help

 BarryNL says:  Dec 30, 2011. 10:48 AM  REPLYNice - I just finished melding joint M2 on mine with a hacked Hitec HS300 servo to turn the motor into a proper servo (the potentiometer actually fits into thebit where the gripper rotates quite nicely). But, I think your way may be simpler.

 travster says:  Feb 4, 2011. 7:53 AM  REPLYGreat work! I've been looking for something like this for my Orbduino project.http://www.tnhsmith.net/Other/Orbduino/Thanks again!

 BobsLanOnTheWan says:  Nov 20, 2011. 11:21 AM  REPLYDude. Tha is the coolest thing ive ever seen!Do you have an instructable on this?

 travster says:  Nov 21, 2011. 6:32 AM  REPLYThank you Bob. I've been meaning to write one up, but haven't gotten to it yet. However, wiring diagram and source code are published on my site.

 www.robotscience.co.za says:  Nov 14, 2011. 8:15 AM  REPLYthis is seriously cool. well done. i was thinking of getting one of these yellow robo arms because they are available and cheap - thanks for saving me thetrouble of having to figure out how to retro-fit better controls! as a return favour check out my video of TANK TRACKs robot with LEGO NXT controller andSONY PLAYSTATION PS2 remote at www.robotscience.co.za

 mrrepel says:  Oct 28, 2011. 7:10 AM  REPLYI've ordered the arm and still waiting for it to arrive.

@Anobium, is it possible to make an howto from your setup ? I really like how it works !

 Anobium says:  Aug 23, 2011. 11:41 PM  REPLYProject completed! Thank you for the initial gearbox concept!

See http://www.zen21667.zen.co.uk/

Anobium

 Anobium says:  Jul 17, 2011. 9:35 AM  REPLYThis is the current state of my development program - I have added more mods to the gearbox and added a psuedo CNC controller.

http://www.youtube.com/watch?v=5wiKAZYmUq0

Thank you for the inspiration.

 fuentesg says:  Jul 17, 2011. 5:32 PM  REPLYHi Anobium, what software are you using? and how do you make the interface with picAxe?

Best Regards from Mexico.

 Anobium says:  Jul 18, 2011. 6:42 AM  REPLYThe software is a language I have developed, this language is very simple.

The command strings are interpreted and then sent from the PC to the Picaxes.  The interpretor is written in AWK and the communication interface inPerl, both are very simple. I chose these languages for portability.

The Picaxes control the axis movement, sense the position and find the ‘home position.

http://www.instructables.com/id/Modifications-to-Robot-Arm-for-Opto-Coupler-Feedba/

An example may help.

     G91A25B50C75D100E50     G28ABCDE

The G91 is the instruction to move an axis. Each axis is given a unique identifier A, B, C, D or E. The number is the unit of travel.The G28 is the instruction to send an axis to its home position. S0, with two commands I can find any location within the span of the robot armmovements. These ‘G’ codes, called GCODES, are then interpreted and sent to the Picaxes which then move the axis appropriately. Seehttp://en.wikipedia.org/wiki/G-code (I am currently documenting the codes for this robot arm).

So, I have written few pieces of software and some microcode in the Picaxes.

Interested in trying to adapt a robot arm?

 fuentesg says:  Jul 21, 2011. 7:08 AM  REPLYYes I'm working in that, but i don't using picaxe, i need more flexibility in the software, thinking in opensource and C++ programing language.

 Anobium says:  Jul 30, 2011. 1:19 AM  REPLYI am interested as I gave my mods a lot of thought. In the software I have a huge amount of flexibility. I currently have the robot executing'Towers of Hanio' in the Gcode language - this means no changes to the microcode, the comms or the PsuedoCNC software - very cool!

What approach will you be using? Opensource and C++ - do tell more. I want to learn more.

 Anobium says:  Jul 18, 2011. 11:22 AM  REPLYhave written the first manual, I think the first of many....

See http://www.zen21667.zen.co.uk

 Anobium says:  Mar 22, 2011. 3:19 PM  REPLYSame comment. What opto did you use? Part number? Or, any recommendation?

I have tried to fit two different types this evening with no luck.... So, I am asking the Oracle.

Most grateful of a response.

Anobium

 jomac_uk says:  Mar 23, 2011. 12:46 PM  REPLYSet up your Opto first with a multimeter on the receiving diode, then connect a 3 volt button cell to the transmitting diode, you have 4 combinations ofconnection to get it right (8 if you dont know which is the transmitter or receiver!). In the correct combination the meter will change from high resistance tolow resistance when you connect the button cell to the transmitting side, leave connected, and then break the beam with a piece of card, the meter willthen show high/low resistance.

 phoenix62_80 says:  Mar 23, 2011. 6:29 AM  REPLYI have been looking at Farnell's offerings and have found this one:

Vishay Semiconductor - TCST1202Farnell Number: 1470058£0.97 for 1 off

I think it would do the job required. I haven't bought and tested this yet but I think this would work.

Hope this helps

Phoenix62_80

 jomac_uk says:  Mar 23, 2011. 12:50 PM  REPLYYes you are right Phoenix, however this opto doesnt have the mounting lugs that the GE one did, if you remember, i used one of the lugs to glue theopto in place.

Also this one is a simple transistor output, this i havent tried, just the darlington output.

 jomac_uk says:  Mar 23, 2011. 12:42 PM  REPLYThe Opto i used was a General Electric H21B1 as i had loads in my junk box, they can be simply tested using a multimeter and a 3 volt battery. There isnothing special about them. If you connect the receiver side to your multimeter, and then connect a 3v button cell to the transmitter or diode side, when thetransmitting led transmits, the multimeter will show that the receiving diode is now turned on and conducting.

More information can be found on the Digikey site, the Opto i used had darlington outputs, but literally ANY opto that you can get to physically FIT will work.

http://parts.digikey.com/1/parts/947374-sensor-opto-slot-3mm-darl-thru-h21b1.html

http://www.instructables.com/id/Modifications-to-Robot-Arm-for-Opto-Coupler-Feedba/

 phoenix62_80 says:  Mar 21, 2011. 11:46 AM  REPLYExcellent Instructable!!

I'm looking to do these mods but I was wondering which infra red slotted opto did you use? Do you have a part number and somewhere I can buy them. Ihave loads of general electronic components but I don't think I have any slotted infra-red optos....Obviously I need to get some!!!

Could you post your code at some point too? Coding is not my strong point either!

 jomac_uk says:  Mar 21, 2011. 2:33 PM  REPLYHi Phoenix,There no code, i dodnt get any further, i used a Seeeduino to count the feed back. There isnt even a reference point to reset back to, you have to usebasic software to count the pulses back from each joint.

The sensors were from my junk box, any slotted opto will do, as long as the size fits in, my opto was an "off the shelf" device, the hard and critical bit isthe drilling of the holes in the gear wheel, take care and fit the cheapest opto's they will all work.

Jomac

 jrienks says:  Mar 5, 2011. 10:05 PM  REPLYNice job.I have built the same kind of thing only I put potentiometers on the outside. Not as clean as yours.you can view the video at:http://www.youtube.com/watch?v=ArEjPsYxM4A

 jomac_uk says:  Feb 4, 2011. 8:53 AM  REPLYIm impressed Travster with your site, since i finished this project, i have now modified my arm with a proper wrist, so the jaws now rotate 180 degrees. I mayadd this as another instructable, if i get the time.

 travster says:  Feb 7, 2011. 9:29 PM  REPLYThanks Jomac!Rotating wrist sounds great, looking forward to that. Any pics/vids?

 jomac_uk says:  Jan 20, 2011. 10:46 PM  REPLYSInce i finished this, i didnt like all of the extra cables dangling all over the place, so i removed the flat 3 core and the two single strands of cable, ndreplacedit with 1 sigle cable with 4 cores and a screen. This now looks a lot neater.

 jomac_uk says:  Jan 15, 2011. 10:06 AM  REPLYI was really surprised with the results, i am still playing with the software, which isnt my strong point, but ive even managed to allow for the back lash or slackin the gears along the horizontal axis. i can move the arm back to the same point each time to within 3 or 4 mm, certainly a lot better then the original set up.

 mowdish says:  Jan 15, 2011. 8:06 AM  REPLYNice project. I'm suddenly much more interested in getting one of those arms.

 killerjackalope says:  Jan 15, 2011. 9:41 AM  REPLYI have that arm it's really quite good.