When I saw what eNABLE does and started reading the problems eNABLE community members face when designing, printing and assembling the prosthetic hands in any variant, I was able to identify a couple of problems that were common to almost all cases, and I decided to start working on a way to try and solve those the best i could.
So I first identified the scale/size problem; when a printed hand involves screws or bolts and other standard-sized hardware, it becomes very problematic to properly scale to a recipients custom size.
Then it comes the mechanic part of it; all systems I have seen, use the force the recipient has in his/her forearm/wrist to open/close it. (V/O, V/C) This has a lot of disadvantages since every recipient is different and a design that may seem to work for some, may not for work for others, (different flexion and extension) and the strength exerted by it varies accordingly, but always inside certain values (never too strong) dictated by the forearm/wrist itself.
We encounter limit in the 3dprinters, too, that cannot accurately print smaller features and also, the plastic itself reaches a limit in its mechanical properties as features get smaller.
Also regarding scale/size, there´s the thing about trans radial amputation, where the above (wrist actuated) does not apply.
Besides, the thing about the individual fingers controlled all at once seems very disadvantageous, since, in the most cases all fingers share a common pivot point, which in turn makes grasping non-cylindrical objects, very difficult, if not impossible.
So I started working on these topics and designed this pieces I present you here.
I designed an enclosure where the mechanical side of things happen, that doesn’t need scaling, and that can interact with any type of 3dprinted hand (PTC), eliminating the problems with varying hardware sizes.
Also, I designed a pulley system that multiplies the work that the actuating servo can produce, along with a second pulley system that allows the fingers to move somewhat independently, giving them more freedom to grasp conical objects or other non-cylindrical ones.
This enclosure can be axially (collinear) mounted on the recipient´s residual limb, if there´s enough room for it between the elbow and the wrist, or it can be mounted on the underside of the residual limb, via a couple adapters. (This part is still in progress, but the current version works pretty much ok, I think).
Everything was designed to be 3dprinted without any support, making it easier for anyone to assemble it without much knowledge/skill required.
Also I tried to remove as much material as possible so it could be as light but as strong as possible.
All 3dprinted parts weigh at 178 gr.
The heaviest parts are the battery and the linear servo that controls the movement.
The movement is actuated via an Arduino Micro microcontroller that reads the values from a Myoware sensor placed on the biceps muscle group. The Arduino actuates the servo, that moves the fingers via a multiplying pulley and a tensor system that allows independent motion between the fingers.
There´s an Arduino sketch (code/program) I wrote, that calibrates the sensor each time the system is turned on, so if the readings from the Myoware sensor vary depending on its placement on the recipient´s arm, the system adapts to this.
Hardest part of this system while assembling it, is that it requires soldering, and that´s it, pretty simple.
I tried to make it as simple, light, efficient, cheap and strong as possible.
I am using red PLA, printed at 195c, with .3mm layers, and varying percentages of infill, all printed on a Makerbot Replicator 2, using a Flexy-Hand (by Gyrobot) to demonstrate the working principles of my design.
Clearly, this design can and will work with any PTD design that actuates (V/O, V/C) via pulling strings (tendons).
You will find the complete list of materials with its recommended print settings and characteristics in the files attached, as well as the Arduino code involved and instructions for assembly.
Even when the challenge is for a PTD with several technical requirements, and my design doesn’t fully comply with all of them, I think it´s a great time and opportunity to share this work that I believe can be very helpful (mainly in concept but also in execution).
I really hope it’s something that contributes to the eNABLE community.
Thanks for reading, and again, please feel free to comment/ask me on this design.
José Santos Lizcano.