How I Designed & Built a Prosthetic Arm

hi guys a couple years ago for my final year engineering thesis I designed 3d printed prosthetic arm and I'm posted a YouTube video of the model I created and over the years I've got quite a few questions relating to how I designed that what kind of servo motors I use details about the electronics so I thought I'd do a quick video on just running through some of the details if you are interested I've posted my thesis online you can find in a home a Google Drive document but I'll just go through some of the details and try keep it fairly quick so I'll just show the video this was the final model so the arm had seven degrees of freedom and basically what that means is there were seven unique ways in which it could move so each finger could move independently and then the wrist could rotate and the elbow could rotate so the human arm has 23 degrees of freedom so basically we have a lot more dexterity so each one of our fingers can move in three unique ways arm and our wrist can not only rotate but it can kind of flex back and forth and side to side so in total arm this model doesn't have the range of motion of the human arm part of the reason for that is because on it's difficult to fit so many motors in such a confined space so the human arm has arm muscles throughout our arm and in the half and some of those muscles are quite small really and they're controlling the different ways in which our hand and forearm wrist and elbow can move so i desided arm for my project I wanted each finger to at least be able to move independently and then the wrist and the elbow to be able to move so this was for my thesis project so basically I started off by doing a literature review on so looking up papers um here's a list of some of the papers I found relating to prosthetic arm designs so there's actually been quite a lot of arm work in this space on only at the hobbyist level but also academic research and there are even a few armed commercial products out there so some of these papers for example would on give details of a specific mechanical design such as here and different ways in which they would arm actuate or move the fingers and joints other papers kind of did a comparison between existing models so they would compare things like you know the wave the size are degrees of freedom number of joints are actuation methods are also finger speeds and the strength of the arm so basically at this stage in general there's not really any prosthetic arms that can compare to the human arm in terms of arm power density and flexibility but then also there are other issues like how do you interface it to the human body for control or because obviously the sense of touch and on localization in space helps us control the movement of our fingers and hands so it's not as simple as on finding an amputee and just working on a mechanical bionic arm and expecting them to have our reasonable use how its integrated into the nervous system and the rest of the body is a big big factor and also obviously the human body has the ability to self repair whereas I'm kind of like a mechanical arm would be on maintained for prolonged use so for my thesis here the first part was arm as I said a literature review and I'll put a link to my thesis I'm online if you want to have a read so basically prosthetics have been around for thousands of years on ancient Egyptians like had prosthetic toes as shown here on even you know Pirates had peg legs which are a basic prosthetic leg and over time we've kind of arm advanced a lot in the field and we've got on purely mechanical prostheses as on a hook which is operated by moving the elbow or the shoulder I should say and then some of the more modern-day advanced prosthetics are coming from places like DARPA and the US military where the arms have like a lot of degrees of freedom and they're integrated into the nervous system and even directly arm into the brain um I believe on there's some work being done where chips are implanted onto the brain and simply thinking can control the movement of the device but one of the things I was looking at in my thesis was trying to arm produce a low-cost prosthetic arm because especially for people in developing countries they don't really have access to this kind of technology and even a simple prosthesis can improve the quality of life so I was looking at on designing a prosthetic that yeah is relatively cheap so I did quite a bit of reading about human anatomy it's obviously good to have some level of understanding of how our body moves and the types of bones we have and as I said before looking at the degrees of freedom and the bingo so for the finger for example I'll essentially arm each joint can move independently so actually I think up fingers have four degrees of freedom so each joint can move independently and the finger can also wiggle from left a lot right which is on adoption and adoption so looking at some fingers from common prosthetic hands they really only have one degree of freedom on the whole thing just kind of bends forward in one way you can't for example hold one part of the finger and then just let say the top half curl around and here are some photos of me I'm demonstrating how we have different degrees of freedom so in all these three photos I'm bending my fingers but they're all bending in slightly different ways whereas in general a prosthetic hand can't do this just bend in in one particular way maybe say this middle picture will be the way it will Bend so yeah I looked at on some commercial products out there I think what was this one called that be bionic 3 so I think there are a few customers worldwide of this device one big thing one feature it has is that the thumb can be manually clicked into two different positions and it allows the device to have different different kind of like holding patterns so you could put it into a position where you can use a mouse or hold a cup for example yeah I think here this demonstrates the opposed and non opposed long position but that farm position had to be adjusted by your other good hand so obviously there's still lots of room for improvement these were pretty interesting the island digit so these could kind of be tailored to the specific missing limbs so if you're missing just a thumb they could you know tailor the design to just replace the time wall if you're missing three fingers they could tailor it to suit that this was I'm an example of a academic um bionic arm and yeah you can you can go through this in your own time if you're interested so for me um I didn't have that much design and manufacturing experience when I was at university but I was interested in 3d printers and did know how to use 3d printers a little bit so I found there's some really cool project called record the in move project arm which is basically designed by a guy called Gail who is on 3d printed basically a whole humanoid robot so this has come a long way since a few years ago when I was looking at it but basically um I looked at some of his work and he gave details of how he made on Bionic fingers so essentially he mimicked artificial mimicked tendons in the human body by using fishing line which is a pretty cool idea so he look at this photo he pass some fishing line through three different pieces which represent the finger here and then use the motor to UM flex that finger forwards and backwards and he also had some yeah other call our pictures and guides on how he designed the the palm and threaded through multiple kind of artificial tendons to control fire five fingers on hand so I didn't um copy his design that I've just kind of used it for inspiration and motivation so I had a little bit of experience with SolidWorks which is a common engineering ed program and basically I started by just modeling the tip of an index finger so yeah and I really kind of like learned SolidWorks as I went along with this like if I if I would have designed this now would be arm better in you know multitude of ways but um sometimes it's good to develop your skills while you're working on you know a fun and interesting project so yeah I spend a bit of time yet designing like starting with the small components and I left yet two holes here which I would pass the artificial tendons through so yeah I built that up into an actual hole index finger which had three models so if I do a section view you can see that yeah each component arm would sort of slot together nicely and I would pass a rod through the middle with two connected components so at the time I actually just used 3d printer filament um for this kind of connecting rod on using like a steel rod with ball bearings would be a better option but yeah I just want to keep it low cost and simple at the time and each piece had these kind of channels for the tendons or fishing line I used at the time to pass through so from there I worked up to creating the the palm so basically just like had these cutouts where the fingers would slide into and again I used on Cynthia a printer filament to slide in in these channels um to kind of hold it all together and yeah that arm was going reasonably well so I then moved on to designing the wrist I wanted the wrist to be able to rotate so I might actually show an earlier design where I used some gears to try rotate the motor so I just hide that so I had a servo motor with a small gear connected to it which was driving a larger gear which was just part of the palm piece and this worked on reasonably well I did get you know pretty decent rotation out of the wrists using this kind of mechanism but I kind of thought it could really be simplified I could just kind of directly mount the palm directly to the servo motor so I came up with this little weird kind of joint here the reason it has this open channel is because I needed space for the tendons to still be able to pass through and yeah like if I went back in redesigned this Oh be a whole lot better this was just kind of the best I could come up with at the time but one issue was that this was fairly weak so you know if this if this palm bed any kind of significant load say on it was holding a brick like a one-kilogram brick for example then it would be pretty easy for this like FDM printed plastic piece to just snap off and fracture right here but um yeah I wasn't really going for a rugged durable model just kind of a proof of concept so this this turned out to be um reasonable for me so yeah and also at the time I didn't really have that much knowledge of how to really do things in SolidWorks properly but yet I kind of learned as I went along and it was a it was a really good learning experience so designing the forearm I basically just the main purpose of it was to house the servo motors so each servo motor would hold the fishing line tendons which would connect to the fingers so I think in my thesis I'll just go through it a bit more I do demonstrate that a bit so yeah I had some early sketches of what I wanted to achieve overall which is always good but yeah this here we can say on how the yellow lines represent the fishing line and how it would pass through the finger so essentially if it was pulled in this direction downwards the finger would curl up but then he feels pulled by the other tendon and the other way the finger would reopen so yeah here's I actually did yeah 3d print that our mechanism with the gear linked palm unattested that out and yet here's where I moved on to the simplified wrist rotation and yet here's a his picture of the fishing line passing through the hole in the wrist which is again why I needed that opening that little cutout there and yeah here I had these costumes servo horns so servo horn is basically something that connects to the shaft of the motor so I designed and printed some custom ones which are basically I used to arm tied in the fishing line to and yeah that's an example of the fishing line connected to the servo horn passing through the opening in the wrist and then going up to the finger so when the servo would rotate in one direction it would pull on the fishing line and tighten the finger like curl at downwards and then if it rotated in the other direction it would open the finger so another option for this was to arm just have the servo horn connected to one tendon on the tendon that would close the finger downwards and then on the reverse side of that finger have some kind of rubber springs or some kind of spring mechanism which would naturally open up the finger when you release the tension on that fishing line so that was yeah another concept I'd go on with um you know I can't remember exactly why I think I didn't really have access to any kind of our small Springs or I couldn't 3d print any kind of rubber Springs at the time so yeah from there I got to this stage fairly quickly within a couple months on where I had designed and built the hand with on independent finger motion and wrist rotation so I did produce on an early video of that I'll just show you quickly and I'll explain these arm I just started again I'll explain these on sensors I used in just a second so here you can see when I'm flexing my arm the bionic arm is mimicking and here I'm just trying to pick up a small container with it so those electrodes on my forearm were AMJ sensors electro my graphic sensors so when we flex the muscles in our body there's a detectable voltage across that muscle so that voltage arm can be picked up just by using basically basic surface sensors so I went to a website called spark fun and bought these arm muscle sensor kids I think they're retired now but you can get an equivalent version of them but these kids were pretty good on basically I think I was just looking at the user manual there was a small board and you provided power to it and then you are plugged in some centers some electrodes which you would put on a bony bit of your body which would represent the kind of ground reference level and then you would put the other electrodes across the center of the muscle and the the base of the muscle where it connected to the bone and you would measure a potential difference of voltage as you flex that muscle and this arm circuit board had onboard rectification and smoothing so basically when you flexed you would get a DC voltage out of this board and the harder you flex the high that DC voltage would be so I used on I used those sensors integrated with a pic microcontroller to control the arm so I'll go into that in just one second but I just finish off the mechanical design so the final step was developing and elbow and an elbow joint so what I decided to do was just on integrate a large gear into the forearm section and then have a servo driving a small gear in the bicep section initially a so servo motors have roughly 80 degrees of rotation something like that and at the time I wanted to have more angular rotation in the elbow I think I wanted yes I'm 260 degrees or something I can't particularly remember so what I did is I opened up the servo motor and took out the potentiometer so the potentiometer is an internal mechanism which is used to control the angular position of that motor so what I did is arm I mounted that potentiometer directly into the large gear so essentially I was reducing the amount that the potentiometer would turn relative to the rotation of the small gear so on yeah if you want to look into that just type Google I'm hacking a servo motor but um in the end I didn't really need it like I found that it was kind of just more effort than what it was worth and 180 degrees on that I was getting out of the server was pretty adequate so here's an exploded view and I can show you in SolidWorks so yeah I had one servo motor mounted up near the wrists to control the the palm rotation then I had five actually you know in the first model I created um I just used one servo motor to control both the ring finger and the pinky things off but I did later on change that so that they were independent the reason I did that was just so on I had a bit more free space in there because I did use a lithium polymer battery to power the whole thing as well but essentially in the forearm I had get five servo motors in the middle to back region controlling the fingers and I'll see if I've got a video where kind of demonstrate that a bit bit clearer so yeah here if you look at the arm the servomotor horns you'll see them rotating as the fingers rotate so what they're doing is they're tightening and loosening the tendons allowing the fingers to control our contract and retry I also did use I think some like silicon grease at the time arm on the palm to help it rotate a bit smoother so yeah a better design would use some ball bearings and whatnot but yeah because these things 3d printed it was pretty weak anyway so on yeah if you if you wanted a really durable thing you'd really look at other manufacturing techniques so yeah and then here you can just see I'm the kind of gears that I incorporated to get the arm elbow rotation and this final servo motor was mounted in the bicep section just in there so I did on some basic calculations calculating the force on I wanted to roughly know if the elbow was armed stationary how much could the hand lift up I can't remember exactly what it was but it was it was a significant amount like it could like lift up a bottle of water but compared to a human arm you know kind of curling like 10 kilograms on this thing definitely wouldn't be able to won't support that not only because the servo motors were too weak they didn't have enough talk but also the thing just wasn't strong enough that probably would have fractured at some point if it was lifting too much load so the 3d printer I used to make all this stuff was the up 2 or the up plus 2 I think it was I did have a time lapse video printing a component but I couldn't find it yeah just did a youtube search and yeah this was basically the machine I used arm to print all the things so in tow like yeah 3d printing is pretty slow and there were quite a few components so it did take like a long time to UM just physically print and assemble components there are removing support material and sanding added a bit more effort so just a caution to anyone I have made my STL files publicly available and I'll link them into the description but on just kind of keep in mind that yeah my design wasn't perfect like things didn't line up perfectly sometimes I had to get a drill and like read real stuff out and then sand some components so if you really do want a 3d print a prosthetic arm I would recommend just going to the in move project and downloading the files available there because that has had a lot more work put into it and there are there is like a community of people to build these things but if you really a lot of people have bought to me for my ACL file so I finally just put them online for people to play around with then yeah so talking about the the control now the server's I use for tower pro servers which just the only reason I used them was because they were cheap and I didn't have much money at the time but they were definitely they were relatively who I talked but there are definitely better quality server motors like Futaba servo motors arm for example that would be better to use and the micro processor I use which is basically a very simple kind of computer chip was a pic 8-bit pic which is like one of the simplest ones you can use so I just used that because I had some experience programming that at university but I would really recommend um using an Arduino for this kind of stuff because I'm yeah something like an Arduino Uno would be on just easier to program because there's more open-source libraries you can use and more kind of YouTube videos and help you can get for this stuff and it's like less fiddly then using a pic so yeah what I had to do is this was the program for it and I actually had to like you know have a little circuit board where I could connect the programming pins to the correct pins on the pig micro processors so another way no would just be much much easier and yeah I had to kind of integrate my own voltage regulators which is all just standard stuff you would get on an Arduino board anyway there's a picture of the EMG muscle sensor kit are used and I used a lithium polymer battery to empower these thinkers yeah I wanted it to be portable obviously on a prosthetic arm would need to be portable in terms of power for it to be of use to an amputee so here's your rough circuit diagram with the stuff by used I did play around with the open-source program called our free program called express PCB and I just used that to help plan the circuit board that I later made on bare board and just manually soldered which did work but yeah it was just not the most efficient way of doing things using using an Arduino would be a lot a lot faster and easier and yeah also to control servo motors it's arm is fairly straightforward but um I kind of shot myself in the foot and um like manually like can wrote the code like using interrupts to create like the correct pulse widths to control the motors whereas with another way you know you could just easily use a servo control ivory to do that stuff so yeah I had um I had some plans to integrate some like proportional control so basically the amount the finger would close would be proportional to how hard you were flexing but yeah I did it I didn't really have enough time to integrate let's create that see some pictures of final model I made and yeah it could it could do a few like grip a few different objects it could on group a water bottle and like rotator and then also like a pen it would be very very difficult though to actually ride like if an amputee have this connected through them they probably won't be able to write much with the pen because yet it's just not the fine motor control in the fingers and also they wouldn't have any sense of touch so they wouldn't have any kind of neural feedback to UM let them know like how hard they should be pushing down and how fast they should be moving the pen but um this arm did have fairly fast movements yeah I think I demonstrated in some of the videos like so a lot of other prosthetic arms out there um the fingers move slower but with more torque but yeah these these fingers did move fairly quickly which was a pretty cool cool aspect of the design and yeah I did some oscilloscope readings of the EMG sensor kid output so it was a bit rough I said before it gave her DC voltage but it wasn't you know a perfect tom flat line there was some noise in there so uh yeah I tried to kind of like do some averaging out I think I tried to use that you're taking difference at a different sample points um can't remember exactly what all this was for and yeah I can really be bothered beating it right now but yeah there's definitely are lots of room for improvement so I'm in comparison to other prosthetic arms that are out there both like commercial ones and academic research ones um I did a bit of analysis so mine was kind of in the middle in terms of weight on Yale was about one kilogram arm in total but yeah it didn't have as much force compared to these other arms so yeah this is my one here they're yellow diamond and the force yeah that like the fingers would produce in terms of Newton's was was pretty low and this proposed a lot of these other hands kind of using gear driven systems and like more power dense motors but in terms of speed well actually no I didn't do it analysis of speed but yeah in my case my actuation was a lot faster so looking at this chart um yeah I had quite a few actuators in in my design compared to other models and yeah I had quite a few joints so uh in terms of complexity of my design you know it was um significant there was like a bit involved but yeah as I said it was just a more of a prototype than like real design that could be attached to an amputee I did I did like intend to um try molds like a piece of plastic around an amputee and then connect the arm to them but at the time like the university couldn't really support me with the health and safety arm issues and I just didn't really have enough time or support too long to do that stuff but it would have been cool if I did get around to doing that and yeah here's a comparison chart so this proposed design was my unit so yeah the blue we can see the actuation speed degrees per seconds which is higher than a lot of the other models but then the grip force in the red is a bit lower in the degrees of freedom is you know similar theaters but then you know if we look at the human hand here on the far right the human hand arm has phenomenal speeds and also phenomenal strength in grip force and a lot more degrees of freedom so yeah the technology has a long way to go before you know we have real kind of Terminator arms that are stronger and faster than human arms so I think yeah that's the basic gist of most the stuff I did yeah I you know if you have any kind of specific questions on feel free to send me a message his photo of me at university kind of presenting my final thesis work and I was fortunate to get a by distinction in these projects which was basically just came down to the amount of time and effort I put into it and I did learn a lot doing this project so yeah if you're looking to design your own prosthetic arm I would say you know do some research look at the in move project look at the kind of design guides on there I'll start playing around with the CAD program you know SolidWorks or fusion 360 inventor Autodesk um there's quite a lot out there and just start small start like with building a Pingo and then trying to get that to move and then a hand and then the electronics yard recommend on using an Arduino and there's plenty of tutorials are out there about how to use on Arduino so if you like this stuff feel free to subscribe to my channel I'm working on a few different projects and kind of tutorials and yeah I hope you enjoyed it thank you

Channel: Mech Designs & SolidWorks Tutorials

Views: 82,701

Rating: 4.971818 out of 5

Keywords: bionic arm tutorial, how to make a prosthetic, arm, prostheses, prosthesis, 3d, printing, how to make a prosthetic arm diy, university, guide, files, emg, sensors, control, how to, design, solidworks, muscle, tendon, model, cad, servo, motor, printed, create, learn, learning, education, lesson, tutorial, help, gear, gears, tendons, artificial, mechanic, mechanical, mechatronics, robotics, educational, engineering, student, build, artificial hand, how do prosthetics work, arduino robot, low cost, cheap, affordable, budget

Id: CIqzeBxkRws

Length: 33min 19sec (1999 seconds)

Published: Sun Jul 02 2017