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biohand - Low cost 3D printed hand prosthesis

Development of a robust low cost 3D printed hand prosthesis using off-the-shelf components.

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Though technology gets cheaper by the day, the prices of health-related tech haven't seen drastic changes in the last years. While I do understand that there is a lot involved in developing live-supporting devices, I feel that there's lots of room to increase affordability in the field.
Current prosthetics don't fall short, either. Current bionic replacements for upper- and lower-extremity amputees can cost several hundreds of dollars. Living in the third world, it's clear to me that such devices simply can't reach all of those who need them.
As many other Makers already, I wish to change this situation, even if by the tiniest bit.

This project (biohand) aims to develop a cheap (around the U$300 mark) prosthetic hand, that is 3D printable, and uses affordable and easy-to-find components.

Still actively on the making!

CONSIDERATIONS

I live in Brazil. Here - unfortunately - lots of components are expensive or hard to get. This constrained a lot of design choices, a few of which are presented below.

Also, I tried my best to use solely open-source development tools (or, in worst cases, free-of-charge ones).

Documentation

I just finished my thesis on the biohand. The project is thoroughly detailed there, with relevant steps in mechanism synthesis, electronics design, software structure and prototype assembly. Check it out! :)

MECHANICS

Each finger is actuated individually, and relies on a 4-bar linkage mechanism to synchronize the movement of the proximal and distal phalanges (see picture below). The actuator is fully enclosed in the palm, to guarantee that even patients with wrist disarticulation or transradial amputation can use it.

Lots of thought went in the choice of the actuator itself. 'Proper' DC motors (eg. Maxon or Faulhaber) are generally too expensive for this application; same goes for linear actuators. Chinese DC motors with enough torque and decent reductions are hard to find and may easily be discontinued - a danger for any design.

I've noticed that standard RC servos, on the other hand, are available *pretty much* anywhere in the world, and its internals (specially regarding mechanics) are roughly the same across a wide range of manufacturers. With that in mind, I used the DC motor and two gears of a standard metal RC servo to design a 3D printable linear actuator. A standard 25mm M3 screw acts as a leadscrew.

While it looks flimsy, it was able to lift 1kg. On top of that, the servo's potentiometer is used to supply feedback about the finger's position - something hardly available in commercial-grade hand prosthetics. The long screws in the servo's enclosure can also be used as dowels for the joints.

The thumb, uses a common arrangement with 2 degrees of freedom, with a linear actuator moving the first metacarpal joint, and a micro-servo driving a rotary motion for the carpometacarpal joint.

Lastly, I designed all the finger's parts in such a way that they're printable laying flat on the printbed. The part's orientation during print is of great relevance in its final strength. With this setup, the assembled finger mechanism was able to individually hold 2kg in a non-destructive test.

The prototype still doesn't feature aesthetic covers (yet to come!), and it's being assembled.

The mechanical designs were made using Autodesk Inventor. While the program isn't open-source, 3-year Pro licenses are supplied for students. I'm considering moving to Onshape in a (somewhat not so) distant future.

ELECTRONICS

I'll be driving the whole system off of a 2S LiPo battery. The thumb's micro servo is getting a proper regulator (an LM317 or an LM1117). The DC motors are being driven with TI's DRV8801: 2A RMS current output (for real! Heard that, L298?), internal logic and an amplifier for sense current made it an easy pick.

For the processor/controller, I went with an STM32F103 ARM Cortex M3. I was already familiar with the processor, and ST's recent lineup of Discovery and Nucleo boards are a significant step for lowering the entry price for ARM development. I'm currently using an Nucleo-F103RB both as dev-board and SWD probe.

Electronics are still in its early stages. For now, I hooked up an STM32 (on a breakout board) to the SWD probe [left of the pic], and designed/milled a carrier board for the DRV8801 [right].

The complete board is still being designed under KiCad.

SOFTWARE

Toolchains for embedded ARM development used to cost a kidney (or be insanely hard to use). In the last years, however, many great open-source tools have become available. I'm currently using a software setup that consists of:

Since the hardware is still being actively developed, I've had...

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  • 5 × Standard RC Servo
  • 1 × Micro RC Servo
  • 1 × 2S LiPo Battery
  • 5 × 3-pin 90 degree header (100mil spacing)
  • 2 × 4-pin 90 degree header (100mil spacing)
  • 1 × 5-pin 90 degree header (100mil spacing)
  • 1 × LM1117, 3.3V, 800mA, LDO voltage regulator
  • 1 × SMD switch
  • 1 × Spool of ABS plastic (minimum of 200g required)
  • 1 × STM32F103R6Tx Main processor for the control board.

View all 14 components

  • Simple EMG control and planned upgrades

    Martin Vincent Bloedorn09/20/2015 at 22:32 0 comments

    As I progress in the making of the biohand, I'm starting to note all things I believe have to be changed or improved. And the list is starting to get quite long:

    • I'm not exactly satisfied with the potentiometers in the fingers; I'll redo the cabling and their attachment to the phalanges.
    • The palm is (very clearly) incomplete, but I'll redesign it completely. The control board, currently located in the inner palm, will be moved out to the dorsal region of the palm.
    • The thumb currently flexes around the Metacarpophalangeal joint; I'll move the active joint to the Carpometacarpal (in the human hand, the thumb's joint that's inside the palm).
    • Software must be reestructured.

    Yea. Before this, I won't consider any tests with subjects, so I'm looking at a minimum of 3-4 months range of work.

    In the meantime, (just for fun) I ran some tests on the current biohand prototype. I've interfaced it with Myo Armband, a great off-the-shelf intelligent EMG sensor, using Niklas Rosenstein's python bindings. The setup requires a PC or smartphone to be connected to the Myo, as it cannot be directly connected to the biohand's ARM Cortex-M3 processor (not in any way I'm aware of).

    Check the video on Instagram, or click the pic below to see it :)

  • Thesis presentation video

    Martin Vincent Bloedorn09/13/2015 at 14:56 0 comments

    The last couple weeks have been a bit chaotic, and I ended up having little time to work on the Biohand.

    In the meantime, I've presented my thesis. I presented an overview of the Biohand's development, some assembly stages and tests. There you go:

    (english subtitles available)


  • First tests with the control board

    Martin Vincent Bloedorn08/31/2015 at 13:02 0 comments

    I've received the finished control boards for the biohand about 3 weeks ago. Due to some time constraints, I finished the board and minimal test program for the STM32F103 only during the last week. Despite two minor pinout mistakes on the board (that I've managed to work around), it worked nicely.

    Check the board demo video on Instagram.

    The board was attached to the palm, as shown below:

    At some point, this seemed like a good idea. However, the attachments I designed for the board didn't worked out as well as planned (ergo the hot glue all around). Also, the board makes the assembly thicker than I've planned, so adding another layer of a 3D-printed protective cover wiill make the hand too large.

    So, I'm already working on an obvious fix; swapping the board to the "back" of the hand, instead of the "inner" palm. I'll post a sketch as soon as I come up with a consistent idea.

    'til next time!

  • Short-term To-Dos

    Martin Vincent Bloedorn08/17/2015 at 18:45 0 comments

    As mentioned before, I've spent the last month mostly working on my thesis (about biohand). Now that I've finished it, I finally have some spare time to invest in the project again. I tought I'd briefly list some of the 'To-Do's I have planned for the upcoming two/three weeks:

    • In the previous logs I've mentioned that the custom control PCBs had arrived from the board house. They are now fully soldered and functional, but the processor (STM32F103, an ARM Cortex M3) still has to be programmed to offer minimal functionality (drive the motors with PWM, drive the micro RC servo on the thumb's base, etc).
    • Once the ARM is programmed, a small frame will be printed to hold the control board on the biohand's palm. All wiring (motors, servo and potentiometers) will be hooked up to the control board.
    • As clearly visible in the pictures, there's still no cosmetic covering. The motors and internals are visible and unprotected. A proper cover will then be designed/printed to enclose the biohand's palm.

    Other modifications are planned in the long-term future too; the thumb mechanism will be fully replaced and the control board will undergo a large revision. However, these more significant changes will only take place after the aformentioned short-term goals are properly consolidated.

  • First video!

    Martin Vincent Bloedorn08/17/2015 at 17:57 0 comments

    I recorded a small video with biohand's basic functionalities at the moment. The main control board is still being programmed, so moving all 5 motors + RC servo in the hand isn't currently possible.

    In the video, a standard Arduino controls the thumb's servo and two DC motors, using an Rugged Motor Driver shield (that has the same DRV8801 drivers employed in the custom board).

    As soon as the complete board gets minimally programmed and hooked up to the prototype, I'll post another video.

  • Some tests and results

    Martin Vincent Bloedorn08/17/2015 at 13:21 0 comments

    As a part of my thesis on the biohand, I ran some relevant tests (mostly still with no control board, which came in a few days later). Some of these tests are presented briefly here. For more detail, check my thesis! :D

    Prehensions

    (Weir et al., 2003) presents a minimal list of required hand grips (prehensions) to accomplish the majority of the Activities of Daily Living (ADLs). This list encompasses a total of 7 different poses, shown below:

    They are:

    • a1) Tridigital palmar prehension (tripod grip)
    • a2) Bidigital palmar prehension (pinch)
    • b) Tip prehension
    • c) Lateral grip
    • d) Hook grip
    • e) Spherical grip
    • f) Cylindrical grip (power grip)

    The image below shows the prototype's CAD model implementing this minimal set of prehensions (numbering is equivalent to the figure above):


    With the 6 active Degrees of Freedom of the biohand, the list could be achieved with no problems. Following the CAD proof of concept, the poses were tested on the prototype itself. The image below shows some of the tested grips:

    Those are:

    • A) lateral grip
    • B) bidigital palmar prehension
    • C) cylindrical grip
    • D) adduction grip: using the space/angle between adjacent fingers to hold thin objects.
    • E) bidigital palmar prehension

    Load test

    It's a know fact that parts printed in an FDM 3D-printer have stronger bonds inside layers than between them (an interesting study shown here). All parts in the biohand take advantage of this fact: they are designed to be printed aligned to the printing bed in such a way, that the main stress and load gets distributed along the layers.

    For example, the image below shows the proximal phalanx of the index finger. The red lines show were the phalanx connects to the MCP (metacarpophalangeal) and PIP (proximal interphalangeal) joints, as well as the linear actuator that moves the finger. The part gets printed with the orientation shown in the figure:

    This grants an increased robustness. No destructive tests were conducted, but the test below shows the biohand succesfully whitstanding a load of 40N with no signs of demage:

    The test was carried out with no power supply, since the fingers employ a non-backdrivable mechanism.

    Speed, strenghts and weight

    Upper-limb prosthetics still have a hard time achieving speeds and strenghts comparable to those in human arms and hands. To contextualize the biohand's streght, speed and weight, it shall be compared to two of the market-leading devices: the i-limb and the bebionic v2 (the previous version of the current v3 model). The data from these devices is based in the wok of (Belter et al., 2013):

    The biohand weights in at approx. 335g (planned with the control board and cosmetic cover). This represents 54% and 62% of the the commercial prosthetics.

    The index' angular speed, when measured at the MCP joint (at the base of the finger), reaches a peak of 56°/s, which is 58% and 93% of the other two devices.

    Lastly, an individual finger of the biohand is capable of exerting up to 6.3N*, which represents 43% and 83% of the commercial devices.

    Considering that the biohand uses modified RC servos as linear actuators for each finger, I can't say I'm not pleased with the results. Four fingers working simultaneously can exert more than 36N in a prehension. This is sufficient for the majority of lighter ADLs, as stated in (Smaby et al., 2004).

    The reduction in weight is also very relevant. The work by (Pylatiuk et al., 2007) shows that excessive weight is a complaint of most amputees carrying some type of prosthetic device.

    * The force test used only 60% of the biohand's nominal tension, to avoid motor demage during stall (for the maximum-force tests).

    Cost

    Lastly, one of the great thriumphs of the biohand is, in my opinion, its very low cost. With the prototype almost completed, the cost table can be calculated as follows:

    ItemCost per unit (USD)QuantitySubtotal (USD)
    Standard RC servo12,95564,75
    Micro RC servo9,9519,95
    Black ABS plastic (filament)19,19 (kg)155g2,97
    Other hardware costs (e.g., screws,...
    Read more »

  • Boards

    Martin Vincent Bloedorn08/15/2015 at 22:01 0 comments

    So. Finished the bulk of my thesis, which frees time to work in this project again. Almost synchronized, the first iterations of the biohand's control boards arrived from the board house!

    I rushed to test them. But yea. With all those small piched components, I didn't got around milling a version in my CNC before the design was sent off for manufacturing. So, it was kind of all-or-nothing. And as it's (almost) always the case, I soon discovered some mistakes. Namely, I swapped two VDD/VSS pairs from the STM32 processor. [...] It took me around 24h to find and solder a workaround and get a minimal setup (processor + regulator + caps) to work. LED blinking in the next pic:

    Time to keep soldering (and eventually spotting other dumb mistakes, *sigh*).

    BTW, more info on the project will soon be available on my personal page.

    'til next time!

  • Looking a bit sharper

    Martin Vincent Bloedorn07/18/2015 at 20:30 0 comments

    I'm currently writing my thesis (about the biohand, BTW), a process that will take me at least a couple more weeks. Thus, I haven't had much time to further develop the hand itself. For now;

    • Electronics are in the oven. Sent out my boards for manufacture with a local supplier.
    • Replaced the somewhat odd red-test-thumb with a more definitive (and cool looking) dark one.

    That's it, for now (sadly). I'll leave some pics on the hand's current state. I'll also try to upload a video of a simple function demo in the near future. 'til next time!

    Clenching a fist:

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Discussions

maciejwozniak94 wrote 11/15/2016 at 09:22 point

ola 

Tenho uma pregunta sobre os motores que voce usou no seu projecto. Agora estou a constuar o Mao Robotic e agora tenho escolhar os motores. Podes diz me que power e empresa escolhaste ?

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Frédéric wrote 10/27/2016 at 09:47 point

Do you still plan to put files on github? I would like to try your design!

  Are you sure? yes | no

Martin Vincent Bloedorn wrote 10/31/2016 at 02:23 point

Hi Frédéric! Man, sorry about that... I never quite got around uploading them. I'll take the time to do it ASAP (as more people have already asked me too :( ) and I'll let you know! Thanks for the interest :) 

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John Boyd wrote 06/29/2015 at 15:01 point

I am curious if there is a way to make this back-driveable while keeping the linear drive screw. Perhaps add some sort of spring or 'suspension system' to the linear drive screw? this would give the power saving benefit of the drive screw (since the motors do not need to constantly run), but also allow some flexibility if the fingers bump something hard.

Just a suggestion. Love your project!

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John Boyd wrote 06/29/2015 at 15:05 point

Here's another unsolicited idea :)

Instead of 3D printing each finger, it would be interesting if you 3D printed just the bone structure and put a rubbery/fleshy material over the bones, more like an actual hand.

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Martin Vincent Bloedorn wrote 07/02/2015 at 21:51 point

Hey! First off, thanks for the interest in the project, it means a lot to me :) Sorry for the long reply time! 


Regarding the linear actuation, I put some hard thought in trying to keep it back-driveable, but I simply couldn't find any alternative that would be cheap, easy to build, strong and reliable all at the same time. For now, the 'bump-safe' is actually the fact that the structure itself isn't perfectly stiff. Specially when an impact hits the outer side of the fingers, the structure kind of bends inwards, absorbing some of the shock. 

In the end, the goal is making it simple and affordable enough, so that even if something does break, you just print it out again and pop it in. But suggestions are always welcome! Thanks again =)

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middelbeek wrote 06/26/2015 at 09:37 point

I would like to have the 3D files to try this myself! Great project!

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Martin Vincent Bloedorn wrote 06/26/2015 at 19:05 point

I'm still working out some quirks. As soon as I have a more definitive version, I'll upload all the files and link them here. Thanks a lot for the interest! :D

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thiago.ebel07 wrote 06/25/2015 at 01:23 point

Awesome!

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