Raimi's Arm - Bionic Arm for Kids

Open source, high functioning, myoelectric, bionic arm for kids.

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I saw a facebook request from the father of Raimi, a ten year old, who was born with a right arm that finishes just below her elbow. She has failed so far to get a bionic arm. Part of the problem is size – kids hands are small, and have a habit of growing. Commercial bionic hands are phenonemally expensive, and mostly only for self funded adults. There are some open hardware options available, notably on Most of them rely on the user having a functional wrist however. There are two, truly open hardware, below the elbow prostheses for kids, that i've managed to find – one mechanical, and one powered model. Both these arms, while brilliant, are quite basic. So, after further research turned up lots of robotic hand projects, but no actual printable arms (a robot hand is no use to an amputee, without some way of fixing it to their residual forearm!), I decided to design one myself

Update: Jan 2017. I have reluctantly decided to stop work on this project, because i've come to the conclusion that the basic premise is flawed. However, i've started a new bionic hand project on new principals. Raimi will still get a bionic hand - i'm determined. Project page coming soon.


To create an open source, low cost, high functioning, bionic arm and hand for children with foreshortened forearms. (Or grown ups. The hard bit is making it small. Making bigger ones will be simple)

The arm must;

  • be entirely open. Open hardware - open source software.
  • be capable of at least four different grip styles.
  • have a rotating wrist joint.
  • have adaptive grip in the fingers.
  • be controlled by myoelectric sensors. (sensors that are placed on a muscle, elsewhere on the body)
  • be capable of doing things human arms can't.
  • be made from readily available, off the shelf parts.
  • cost less than £300 to build at home.
  • be developed for, and with, someone who actually needs it.
  • be practical, durable and useful.
  • be lightweight. (no one wants a brick on the end of their arm, regardless of how amazing the brick is)
  • be reproducible and complete (not just a hand on its own).

My Design

The key elements of my design are as follows;

  1. The hand – a 3d printed, articulated hand. With tendons running through the wrist.
  2. The forearm front section – a 3d printed forearm containing the following;
    a) Arduino – main controller for the arm
    b) Myoelectric control module – I intend to control the hand with myoelectric sensors, placed elsewhere on Raimi's body
    c) Oled display – for showing system status to the user
    d) Internal mounting points for actuators.
  3. The forearm rear section – a 3d printed socket for Raimi's residual forearm, and a rotating 'wrist' section.
  4. The cable – a power supply cable, to connect the arm to the power pack.
  5. The power pack – a belt mounted, rechargeable, 12v battery. Off the shelf.

October 2016

The new style finger and thumb articulation is working well, and the forearm design is mostly done. But the not quite resolved actuator situation means I'm not going to get the whole arm prototype working in time for the finals. Oh well, disappointing indeed, but the important thing is making Raimi a bionic arm, not making the hackaday prize podium. So, the work continues apace.

The new force feedback actuator design is moving forwards at a terrific speed (more on that In the latest update), and is almost ready for integration into the forearm. I'm developing the firmware in parallel with the actuator, to simplify and compartmentalise it. Other than that, there's lots still to do. Oodles of programming work to get it working properly. Myoelectric muscle sensor - this is how Raimi will control the hand, open and close the grip, and cycle through the grip styles. I am yet to experiment with the sensors, despite having them for months. Way overdue. The problem is that I have advanced ALS and don't have any muscles left myself, to test it on! I'll have to experiment on my children instead. I also need to get to grips with the 3d scanner I bought to scan Raimi's residual forearm. This should enable me to make a closely fitting cup, to join Raimi to the arm.

Open Source

The entire Raimi's arm project is licenced under the Creative Commons licence,

Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)

Message me if you would like any of the in progress project files, software, CAD files, etc.

The Story so Far

I made contact with Raimi's dad in December 2015, and he kindly provided me with some measurements of her non foreshortened arm and hand, and her foreshortened forearm. Now I knew the parameters within which I would have to work.

I decided that the finger design would probably be the hardest part. So I'd better start there!

By mid January, the finger had reached the fourteenth prototype, and I was happy enough with it, that I was ready to start designing the hand itself. I looked at various small actuator options, Firgelli linear actuators (beautiful ...

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The current design file for Raimi's arm (october 2nd 2016). Some elements are as yet incomplete.

123dx - 3.45 MB - 10/02/2016 at 16:55



The latest wiring diagram.

Portable Network Graphics (PNG) - 1.21 MB - 08/02/2016 at 16:59


  • The force feedback is strong in this one...

    Patrick Joyce10/02/2016 at 16:39 0 comments

    Since the last update I've been busy refining the design for the force feedback actuator, to increase its sensitivity range as much as possible (to enable a range of grip strengths). I also realised that it would be extremely helpful if it could sense when the tendon wire went slack. In normal operation this only happens when the fingers are opened out flat. Sensing when the fingers were flat would allow them to stop there, without any of the programming gymnastics the previous system would have required. The tendon wire going slack, could also occur in a situation when the closed fingers were physically prevented from opening for some reason. If this happened, then the danger is that the motor would unwind the tendon drum, despite there being no tension in the tendon wire. This would result in a tangled snarl of tendon wire inside the drum housing – potentially stopping it working entirely.

    My early designs had the load arm able to move freely when the tendon wire lacked tension. This worked fine on the stationary test rig, but would have produced unpredictable results when the arm was used in practice (gravity coming from different directions as the arm was used). So I've added a load arm return spring in the latest design.

  • Last minute, total, change of direction!

    Patrick Joyce09/07/2016 at 19:01 0 comments

    Exciting news! I've struggling for months with the design of the fingers on the bionic hand. I haven't been able to get the force needed to operate the finger low enough. I always intended to use tendons to actuate the fingers, and springs or elastic to return the fingers to a straight position. So the actuator has to pull against the return spring as well as the friction within the tendon mechanism. Try as I might, I couldn't get the no load pull weight down below four hundred grams. Very frustrating. It worked, but 400grams just seemed excessive. Then I found the TACT hand online, which uses a combination of tendons and rigid bars to actuate its fingers. It was an epiphany! So I set about designing my own combination actuated bionic finger. After six prototypes, we (I couldn't have done it without Toni, my carer) have got the no load pull weight down to 100 grams. This opened up the possibility of using nano servos as actuators, making each finger individually controllable. So i'm setting about redesigning the hand from scratch – with only four weeks to go before the hackaday prize deadline. Its possible. Just....

  • Soldering (again), fingers, and hubris....

    Patrick Joyce08/26/2016 at 09:42 2 comments

    Toni (my carer) assembling the redesigned hand.

    So, dad had soldered up the prototype forearm lid (containing all the control electronics for the hand). It worked a treat, once we'd re soldered one wire, that was in the wrong hole. Finally, I could dismantle the test rig, with all its unsightly Dupont cables.

    On testing this beautiful new, self contained, bionic arm, one thing soon became apparent. There was still too much friction in the finger tendon design, and the new elastic finger return mechanism was in need of improvement. So, I dismantled the test rig, safe in the knowledge that I wouldn't need it again (oh, the hubris!), and cracked on with a ground up finger redesign. We finally finished printing and assembling the redesigned hand this morning. We bolted it to the forearm, and got ready to test. Plugged in the power pack, turned it on.... and.... nothing happened. Then, after a few seconds.... fizzle BANG!, followed by a little puff of blue smoke.

    Oh dear. And just as i'd thought i'd finally seen a glimpse of the light at the end of the tunnel.

    And unfortunately, my dad is in France until next week. So we're busy putting the test rig back together at the moment. Finding out what went wrong will have to wait for dad's return from holiday. I did a visual inspection, and can't see any burnt components, or obvious shorts.
    My only theory is that the 5v pro micro I am using couldn't cope with the 12.5v that the power pack was delivering? - as its only supposed to run on a maximum of 12v.

    Do any of you lot have any opinions on whether this is likely or not?

  • Soldering, and the limitations of quadriplegia.

    Patrick Joyce08/11/2016 at 19:25 0 comments

    Last Friday my Dad came over, to wire up the prototype arm. I'm a quadriplegic myself, and can't talk or use my hands, so I rely on carers, friends and family to build my projects. Unfortunately, my dad is the only one with good soldering skills, and he has limited time. He managed to get it all soldered up last week, but then he ran out of time, and we didn't get to test it before he had to leave. I tested it myself after he left. The screen worked on usb power, but nothing else did. My troubleshooting stopped there, as its a mess of tangled wires.

    It has been very frustrating sitting here for a week, looking at it, knowing I could fix it in minutes, if only my hands still worked.

    Dads back tomorrow, and we'll sort it then. But meanwhile, i've come up with a new wiring design for the second prototype, with headers on the pro micro. The new design should allow me to test, and troubleshoot each component individually.

    In other news, I've had problems with my homemade torsion springs, getting the spring power right. They also increase the build complexity considerably, which isn't a good thing. So while I remake the springs, I am also testing a different design - with elastic cord as the finger return mechanism. More news on that in the next update.

  • Actuators and the Forearm!

    Patrick Joyce08/02/2016 at 17:21 0 comments

    Things have been busy here at bionic arm HQ. I've been slack about doing updates, so there's lots to tell.

    Firstly, I decided to sort out the actuators. My requirements are fourfold. They need to be strong, know when they have reached their limits, and be capable of sensing when force requires them to stop – some kind of force feedback. They also need to be really small, and easily available off the shelf.

    The force feedback thing was the tricky one. This is necessary when you grip something large, and tell the hand to close around it. It physically can't close all the way - and without some kind of feedback, the actuator will keep trying to close, and potentially damage itself.

    The micro servos I was using were not strong enough, and full size servos are too large to fit in a child's forearm. I explored miniature gearbox motors for a long time, eventually designing a prototype that 'might' have worked, but was fiendishly complex. In the end I reluctantly declared that a blind alley, and bought some Firgelli micro linear actuators. I was hoping to find a cheaper solution, as I want the arm to be cheap to build - three Firgelli actuators costing £180 in the UK. Now that I have them though - I love them! Small, powerful, and with an internal potentiometer, which I can use for limit switching, and a force feedback equivalent.

    You'll see on the picture that I now have the forearm built as well. It was quite a challenge to squeeze all the electronics and three actuators into a tube twelve centimetres long. But I think I've succeeded. I'll know on Friday, when my dad comes to solder everything together. I had to ditch the rtc module, which was a shame, just not enough room. Wiring diagram below. The arm is powered by a rechargeable 12v power pack, which will live in a pocket, or on a belt.

    You'll also notice that the fingertips are rubber coated for grip. After lots of experimentation I found that modelling balloons are the perfect size, and really grippy.

    I've also built the rotating wrist mechanism, and made some nifty changes to the tendon system, but more about that in the next update....

  • Health and computer issues....

    Patrick Joyce05/25/2016 at 11:21 0 comments

    My health has stabilised - luckily. But as soon asI was able to get back to work, my computer reached the limit of its capabilities. As the CAD models of the hand and forearm got more complex, the computer slowed and slowed, til it became unusable. And then, even worse, it died completely. So i needed a new, wheelchair mounted computer - fast. Unfortunately, i already had the most powerful commercially available wheelchair mounted computer, and that was not powerful enough. So i hacked my own!

    I bought a high spec convertible laptop, and made a mounting box out of the bottom half of a waterproof suitcase. I 3d printed the mounting brackets for the front. Inside the case is the mounting bracket for the eyegaze unit, and amplification gear. On the back is a quick release plate, which locates on my existing mount arm. Eventually the backbox will contain all kinds of gadgets, but for the moment i'm concentrating on Raimi's arm again. I've had three very frustrating weeks - but now i'm back in the game!

  • Spring success, and impending dooom...

    Patrick Joyce04/01/2016 at 11:19 1 comment

    So. Progress is being made. Painfully slowly, but progress nonetheless. The internally sprung finger prototype is up and running, as you can see in the video. I'll make a whole hand next...
    The reason for the slow progress is quite serious unfortunately. Recently the progression of my motor neuron disease has taken a turn for the worse. I can no longer swallow with my head upright. This means i'm having regular choking episodes, to avoid which, I need my head lowering down on to my chest. This process sucks the energy out of me. I'm only managing an hour or two's work each day, before becoming utterly exhausted. This raises the possibility that I won't finish this project before I die. I'm fighting hard, but MND/ALS is a relentless and unstoppable disease. Things are not looking rosy at the moment.

  • Spring!

    Patrick Joyce03/21/2016 at 16:06 0 comments

    The three main issues i'm working on at the moment are: the wrist joint, the forearm layout, and the thumb. I realised last week, that I couldn't work on the hand in isolation anymore, I would have to design the forearm and hand together. I hadn't done this from the start, because my wheelchair mounted computer, while powerful for its type, is puny and inadequate for CAD work. Its all I have however, so i've made do. I'm designing the arm in small sections, that my computer can cope with.

    Raimi has a partial forearm, so the space between the tip of her forearm, and the base of the hand, is the space i'll have to fit three actuators and the wrist joint into. It is 12cm long. Which isn't very big. So to maximise the space in the forearm, i've decided that the finger return springs would be better off in either the palm, or the fingers themselves. After a lot of experimentation (with the help of my carers), I have arrived at an excellent solution. I'm going to make my own springs that fit inside each finger joint. I knocked together a rough, proof of concept, prototype, with the help of my dad. Since then i've redesigned the finger joint to suit. Made a jig for the springs, and ordered more spring steel wire.

    The picture is a CAD model i've made to help explain to my carers, what I want them to make. I'll post a video when I get the first finger working – hopefully tomorrow....

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Alishba Imran wrote 07/23/2020 at 20:44 point

Hey! Is it possible to get CAD files for this arm? 

  Are you sure? yes | no

jacob thankGod wrote 05/14/2017 at 11:03 point

this project is very touching because it relates to me directly because there are so many amputees most which are youngsters in my country who dont have functioning prosthetic devices. i am looking on making use of your design files in creating one that can be use in my country.......thumbs up to a game-changing project.......

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yawar wrote 11/01/2016 at 15:07 point

good work and good luck, if you need any help in designing the hardware or electronics layout, I am always here to help



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Patrick Joyce wrote 11/02/2016 at 10:20 point

Thank you Yawar, i'm good at the moment, but i'll certainly ask for help if i run into trouble. 
Best regards

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yawar wrote 11/02/2016 at 10:27 point

You are always welcome :)

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Alvaro Villoslada wrote 10/04/2016 at 19:27 point

Man, this work is amazing! Congratulations! We have reached a similar conclusion on how to implement a compact tendon-based actuator. But your solution for having force feedback is simply outstanding, so simple and functional, great work!

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Patrick Joyce wrote 10/05/2016 at 08:18 point

Thanks Alvaro! Its not amazing just yet, because the prototype isn't ready! But soon - hopefully ....

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DainBramage wrote 05/09/2016 at 12:52 point

You are an inspiration, Sir! Kudos to you for taking much or all of the good time you have in a given day to help that little girl in such a huge way. Simply amazing! I wish I could give your project more than one skull.

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Patrick Joyce wrote 05/09/2016 at 16:20 point

Hi Dain, thanks! But i haven't actually helped her at all yet. Its early days still. Wait til i get her an actual working prototype. Congratulate me then. Up to that point, its just another project...

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DainBramage wrote 05/09/2016 at 16:27 point

I stand by what I said, from one handicapped soul to another. :)

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M. Bindhammer wrote 03/21/2016 at 18:51 point

Hi Patrick, I have seen these kind of arm/hand prostheses projects many times before. How the arm will be controlled? EMG? Rock on!

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Patrick Joyce wrote 03/21/2016 at 20:00 point

myoelectric sensors on muscles elsewhere on the body (shoulder maybe). 

There are similar projects true - but none that i would be able to build for Raimi. Thats why i started this, and i hope to finish it and release it, so that other kids can benefit.

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M. Bindhammer wrote 03/21/2016 at 20:20 point

Ok, thanks. From my experience the design of any prostheses should start from the question how to control the arm/hand, because this is the most difficult part. 

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Audrey Robinel wrote 03/20/2016 at 15:34 point

A comment on more technical stuff :
-Why the arduino uno instead of a nano, more compact?
-For flexible plastics, you should consider PCTPE. For my tests i've been printing little stuff with it, and it works well, without beeing difficult to print. A heated bed is useful to reduce warping, but other than that, it prints easily, at not so high temperatures, and is rigid enough to be easily fed to an extruder, unlike ninjaflex with some extruders. It is much stiffer than ninjaflex, and i'd say at least twice as stiff as semiflex, but still bends. It is hardly elastic, or requires a lot of force to elongate, but it bends pretty well.
Beeing nylon, it's nearly indestructible, has good layer adhesion, good finish, and has that self lubricating property found in other nylons. That would probably be of great help for your joints, reducing greatly friction.

For your project it seems to be a very good material :

-easy enough to print;

-semi rigid, being not as stiff as PLA, but stiff enough to not bend under low force;

-self lubricating;

-extremely resistant.

If you want me to conduct some tests, don't hesitate, i have a roll of taulman3D PCTPE, another of Taulman nylon bridge, and i'll be ordering a few others from taulman (aloy 910 and tech-g among others).

Here are the specifications for the PCTPE :

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Patrick Joyce wrote 03/21/2016 at 08:54 point

Hi audrey!
for all the tips. I am planning to use a nano, or a micro. I will
have to minimise the size of the electronics at some point, but am
concentrating on the more urgent design issues at the moment.

Regarding flexible plastics. My problem with that is my printer.
Its a cube 3. It takes a propriatary filament cartridge, so i'm
limited to pla or abs for the moment. However, i've come up with a
spring solution that works very well indeed – i'll do an update
this week sometime....

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Audrey Robinel wrote 03/21/2016 at 16:01 point

Ah, too bad for the proprietary format!
however, some times constraints are a fuel to creativity, hence your spring solution :)

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Neon22 wrote 03/20/2016 at 10:07 point

Fantastic work. FWIW I concur the whiffletree seems the best system for dealing with grasping randomly shaped objects. Is it working well for grasping? It's possible the tree needs a little more room but its hard to see from the video. Also consider a wheel for the servo arm. Like the ones here: The wheel means the wire will always be the same tension because the path length does not change as the servo arm rotates.

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Patrick Joyce wrote 03/21/2016 at 11:10 point

thanks for your kind words. I did consider pulley wheels, but decided against them in the end, because of the issue of what happens when the fingers grip something large, and the servo can't go as far as it wants. With separate wires for fingers in, and fingers out, there is more scope for springs that allow the servo arm to travel, even when the fingers can't 

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Neon22 wrote 03/21/2016 at 12:53 point

I was not clear enough. I meant use a single pully servo horn replacing the existing single arm servo horn. The whiffletree should deal wit hlarge and smal objects. If its having difficulty grasping large objects then its possible your whiffletree "box" is not long enough (The along-the-arm dimension). Cheers...

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Audrey Robinel wrote 03/20/2016 at 02:30 point

Hey buddy, i don't see how i can not give you a skull. I really wouldn't mind loosing to you this year, considering this great project. We all feel like we're building things that matters, but some obviously matters more. Great project, i wish you best luck!

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Patrick Joyce wrote 03/21/2016 at 10:57 point


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