For 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, wouldnt be 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 to where 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.
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, wouldnt be 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 to where 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.
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Signing UpStep 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 down to 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 motor mount, 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 should be 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, cut off 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 is fragile, 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. There is 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 the common connection or 0 volts.
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, cut off 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 is fragile, 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. There is 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 the common connection or 0 volts.
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To help me gather up the materials BEFORE I disassemble my arm it would be really helpful to know the dimensions of the 10/32 gear wheel so that I can try to buy a bagful of them on eBay.
Did you happen to keep a record, or do you have one lying about that you can measure?
Many thanks
Nick
The mods were done to the original gears, if you look at the 10/31 gear wheel from inside, you will see on the surface 8 little circles from the casting, all i did weas to drill 4 small holes using half of these circles as a guide. Doing this doesnt really stop you from trying something else. Saying that, these gear wheels are pretty standard, look in any hobby model magazine or catalogue and you will see these, they are pretty much a standard tooth pitch as BILLIONS of Chinese toys and cogs all use the same pitch. Any problems just shout!
John
The next level is not as high as you think,actually its quite easy even if you have even basic programming skills.
each of the gearbox's is quite simple,its just a motor driving a gearbox. In the original, the system was basically very stupid, there was NO feedback to tell the micro at what position.
Here's my simple advice, take one of the motors out and connect it to your micro, just focus on the one, and not all of them, this will just confuse you. Now that you have added an opto to it, you now have a form of feedback, so your basic motor/gearbox is now a servo. Just play until you can control the new servo the way you want, when you are happy, just apply the rules to the other 'new' servo's
At one point I was tempted to look into replacing the motors with stepper motors. Another idea was to use servos. Your idea should be much cheaper and more fun as it gets into how the whole thing works.
My version does not have the USB inter face. I didn't even know it came with one! I was going to set up my Arduino with several MOSFETs to replace the switches in the controller box. That approach along with what you did would allow me to program the arm for repeated tasks.
A pressure sensor in the pincher wouldn't hurt either.
My son and I call my robot arm Whiny because of the sound the motors make as we move it around.
The original DC motor can be compensated for within the micro if you calculate the backlash and 'overshoot' your stop point in either direction.
The 'pressure' sensor was nothing more then a simple current monitor, eventually i used a small Sparkfun current sensor module that was cheap. I used a 5 amp module which i modified. The current shunt was just PCB foil on both sides of the PCB and plated through. I first removed the plated through connectors to use one side, this effectively made it a 2.5 amp current shunt. A little more hacking with a modelling knife brought the module down to about 800Ma for full output (0 to 255Mv output for 0 to 800 Ma) It didnt need to be accurate just needed to feedback the amount of stalling current on the grippers.
The only weak point of the whole arm IS the gripper assembly. I have seen more robust ones for sale and that will be the next mod.
-- where did you get the optos? where would you suggest I scavenge/buy some?
-- would you please put up more detailed instructions for putting in the low current sensor and gaining pressure sensitivity?
once again, thanks for a great project -- its exactly what I want to get me started in learning about digital fabrication arms!
This is what they look like... http://teacher.en.rmutt.ac.th/ktw/04-710-409/OPTOCOUPLER%20APPLICATIONS.htm
There is nothing scary about the opto,s I used some that i had in my junk box, i didnt even check what they were, i just used them. For those really unsure, buy 1 and test them using the examples above.
For the pressure sensor, this is just as simple....I used a current sensor from sparkfun like this.... https://www.sparkfun.com/products/8883
when the fingers grip an object, effectively the motor hits a brick wall and stalls. The current then starts to rise and this board will sense this. Whatever micro you use, just simply program it to read the rising current when the fingers grip something.
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 the mechanical 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 been tested in this application?
Thanks
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 would those 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 an opto 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 my robot 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-up taking 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 mentions adding 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://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
http://www.tnhsmith.net/Other/Orbduino/
Thanks again!
Do you have an instructable on this?
@Anobium, is it possible to make an howto from your setup ? I really like how it works !
See http://www.zen21667.zen.co.uk/
Anobium
http://www.youtube.com/watch?v=5wiKAZYmUq0
Thank you for the inspiration.
Best Regards from Mexico.
The command strings are interpreted and then sent from the PC to the Picaxes. The interpretor is written in AWK and the communication interface in Perl, both are very simple. I chose these languages for portability.
The Picaxes control the axis movement, sense the position and find the ‘home position.
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 arm movements. These ‘G’ codes, called GCODES, are then interpreted and sent to the Picaxes which then move the axis appropriately. See http://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?
What approach will you be using? Opensource and C++ - do tell more. I want to learn more.
See http://www.zen21667.zen.co.uk
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
Vishay Semiconductor - TCST1202
Farnell 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
Also this one is a simple transistor output, this i havent tried, just the darlington output.
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
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. I have 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!
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 use basic 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 is the drilling of the holes in the gear wheel, take care and fit the cheapest opto's they will all work.
Jomac
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
Rotating wrist sounds great, looking forward to that. Any pics/vids?