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Category: Grip mechanics

3D-print molded Protosil RTV 245 (durometer shore 40A) silicone covers for Toughware Equilux [proof of concept, demo of "bionic" grip]

Cite this article:
Wolf Schweitzer: Technical Below Elbow Amputee Issues - 3D-print molded Protosil RTV 245 (durometer shore 40A) silicone covers for Toughware Equilux [proof of concept, demo of "bionic" grip]; published February 4, 2018, 11:50; URL: https://www.swisswuff.ch/tech/?p=8248.

BibTeX: @MISC{schweitzer_wolf_1571786866, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - 3D-print molded Protosil RTV 245 (durometer shore 40A) silicone covers for Toughware Equilux [proof of concept, demo of "bionic" grip]}}, month = {February},year = {2018}, url = {https://www.swisswuff.ch/tech/?p=8248}}


It is sometimes more fun to present the technical results before or even instead of explaining exactly why.  So in short, I 3d-designed and then printed molds to make grip covers for really serious grip performance of a Toughware Equilux device.

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Scientific approach taken for implementing a successfully marketable microprocessor-controlled knee - history of Otto Bock C-leg [lessons for prosthetic arms?]

Cite this article:
Wolf Schweitzer: Technical Below Elbow Amputee Issues - Scientific approach taken for implementing a successfully marketable microprocessor-controlled knee - history of Otto Bock C-leg [lessons for prosthetic arms?]; published January 2, 2018, 15:10; URL: https://www.swisswuff.ch/tech/?p=7790.

BibTeX: @MISC{schweitzer_wolf_1571786866, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - Scientific approach taken for implementing a successfully marketable microprocessor-controlled knee - history of Otto Bock C-leg [lessons for prosthetic arms?]}}, month = {January},year = {2018}, url = {https://www.swisswuff.ch/tech/?p=7790}}


This blog post takes a few relevant observations, and assumptions, throws them up in the air and sees if they turn into sunshine.

  • If anything has brought us forward, it is also the ability to find relevant short cuts. We do not always have to invent the wheel when really we just want a variation of it.
  • If there is any acutal success story where academic research was required to leverage consumer market for a prosthetic limb, it is that of Otto Bock's C-leg.
  • If we can understand what the concepts are for getting a C-leg successfuly built, marketed and sold, we should be able to take generalized aspects of it to formulate success elements for prosthetic hands, grippers or arms.

Background

While the idea of a microprocessor controlled knee was created earlier [link], no marketable solution was available in due course. "In the early 1990s, Kelly James, an engineer at the University of Alberta, Edmonton, Canada, developed the C-Leg, the first leg with microprocessor-controlled swing and stance phases. Buying the rights from the university, he traveled around the world to interest prosthetic manufacturers in his invention ("A Leg Up," by Isabelle Gallant, U of A Engineer, Spring 2011). However, he didn't receive any commercial interest until German manufacturer Ottobock bought the patent in 1992 and launched the groundbreaking technology.".

Then, based on work betweeen 1995 and 1998, a doctoral thesis at the ETH Zurich described an intelligently, microprocessor controlled knee for above knee prostheses built from available and affordable materials [1].

That research was performed 1995 to 1998, financially supported by Otto Bock, and Otto Bock presented its first C-Leg in 1997.

The rest is history. If ever there was a leap in performance of prosthetic function, ever, it was the C-Leg. No prosthetic hand ever came close to achieving this level of success.

So this particular doctoral thesis seems to contain some possibly interesting ingredients worthwhile looking at. As any doctoral thesis here is public record, and a copy of it must be made available at the public library, I borrowed a copy for further information.

There are some other prosthetic developments, however, nowhere else is academic research anywhere near that successful as in the instance of the C-leg:

  • Otto Bock Michelangelo hand; the mechanism seems to come from American DARPA or other army research and probably was just built, the first glove was a great design work. So there is no analytical approach comparable to the C-Leg. It is too heavy, it does not work with prosthetic gloves really, it is not sturdy.
  • i-Limb: This cannot possibly have suffered too much analytical thought. The device looks more like it was born out of something else. While it does not always function as maybe intended, it is really lovable. It does not have a reliable precision grip, it is really weak, it tears up its paper thin gloves within minutes.
  • TRS prosthetics: Bob Radocy as end-user himself developed by far the greatest useful solutions. But they are not the result of extensive academic efforts, so they cannot be compared to the C-Leg. They are extremely good though and any analysis must start there.
  • Toughware PRX: These devices are extremely well made, mechanics wise - but we lack an analytical model that precedes the engineering there as well, comparing this to the C-leg approach.
  • Becker Mechanical Hand: Also the Becker hand was clearly built by someone with great practical and pragmatic understanding. No analytical effort of the magnitude of a C-Leg preceded it though.
  • Hosmer hooks: they came out of a practical development, no scholarly work appeared to be prepared for these either.

 

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[1] D. Zlatnik, "Intelligently controlled above knee prosthesis," PhD Thesis, 1998.
[Bibtex]
@phdthesis{zlatnik1998intelligently,
  title={Intelligently controlled above knee prosthesis},
  author={Zlatnik, Daniel},
  year={1998},
 school={ETH Zuerich, Switzerland}
}

Grip performance enhancement through modifying terminal device gripper surface [overview]

Cite this article:
Wolf Schweitzer: Technical Below Elbow Amputee Issues - Grip performance enhancement through modifying terminal device gripper surface [overview]; published July 19, 2017, 16:00; URL: https://www.swisswuff.ch/tech/?p=7431.

BibTeX: @MISC{schweitzer_wolf_1571786866, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - Grip performance enhancement through modifying terminal device gripper surface [overview]}}, month = {July},year = {2017}, url = {https://www.swisswuff.ch/tech/?p=7431}}


When using a prosthetic arm with a terminal device, grip performance is a key issue.

Usually, a bare steel hook such as the Hosmer model 5 works through just about every situation. That is just because that is how it is. A closer look reveals, however, that that view may be overly simplistic. If anything, it requires explanation.

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Prosthetic hand and gripper options: grip analysis, grip construction [summary/post list]

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Wolf Schweitzer: Technical Below Elbow Amputee Issues - Prosthetic hand and gripper options: grip analysis, grip construction [summary/post list]; published October 11, 2016, 12:36; URL: https://www.swisswuff.ch/tech/?p=6663.

BibTeX: @MISC{schweitzer_wolf_1571786866, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - Prosthetic hand and gripper options: grip analysis, grip construction [summary/post list]}}, month = {October},year = {2016}, url = {https://www.swisswuff.ch/tech/?p=6663}}


This website features a range of posts regarding grips and both analytic and constructive aspects. Here is a short overview of the most relevant.

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iLimb Ultra Revolution - global news: a fitting work glove has been found [product tip]

Cite this article:
Wolf Schweitzer: Technical Below Elbow Amputee Issues - iLimb Ultra Revolution - global news: a fitting work glove has been found [product tip]; published May 25, 2014, 12:38; URL: https://www.swisswuff.ch/tech/?p=3072.

BibTeX: @MISC{schweitzer_wolf_1571786866, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - iLimb Ultra Revolution - global news: a fitting work glove has been found [product tip]}}, month = {May},year = {2014}, url = {https://www.swisswuff.ch/tech/?p=3072}}


(Updated 10/2018)

Durability of iLimb gloves - given the overall iLimb's failure to really assist with hard grips [link] - is a bit of a joke anyway. Even though, prices are high for replacement gloves. Now, a work glove has been found that fits it.

This is global news as several user forums failed to elicit cogent answers. Also, a request to the prosthetist who allegedly had forwarded that problem to Touch Bionics also failed to elicit a useful reply. That means that, again, this website digs into uncharted terrain.

Again!?

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Manualization of the body and time and effort spent to implement it [options for people missing an upper extremity part]

Cite this article:
Wolf Schweitzer: Technical Below Elbow Amputee Issues - Manualization of the body and time and effort spent to implement it [options for people missing an upper extremity part]; published September 11, 2011, 20:37; URL: https://www.swisswuff.ch/tech/?p=469.

BibTeX: @MISC{schweitzer_wolf_1571786866, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - Manualization of the body and time and effort spent to implement it [options for people missing an upper extremity part]}}, month = {September},year = {2011}, url = {https://www.swisswuff.ch/tech/?p=469}}


If a part (or more parts) of upper extremities are missing, absent function can be replaced to a small degree.

Replacing at least some basic aspects of a hand's function with the rest of the body and immediate environment is what upper extremity amputees including myself do every day.

There are a few questions along the road, but other than that, "manualization of the rest of the body" as well as manualization of surrounding environment is what is going on. There are simply no other options and interestingly, problems are similarly in nature regardless of the type of solution one chooses.

When evaluating a prosthetic hand, when doing evaluation of a prosthetic arm or hook, when evaluating myoelectric or ""bionic"" prostheses such as iLimb, BeBionic or Michelangelo by Otto Bock, then this should be considered thoroughly.

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What makes a good grip - gadget for non-disabled people [product]

Cite this article:
Wolf Schweitzer: Technical Below Elbow Amputee Issues - What makes a good grip - gadget for non-disabled people [product]; published April 10, 2011, 14:28; URL: https://www.swisswuff.ch/tech/?p=394.

BibTeX: @MISC{schweitzer_wolf_1571786866, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - What makes a good grip - gadget for non-disabled people [product]}}, month = {April},year = {2011}, url = {https://www.swisswuff.ch/tech/?p=394}}


Disability is a relative term, as it appears.

One may turn out to be disabled facing a certain task, and non-disabled facing another.

As it appears, seemingly non-disabled people cannot handle all of today's medical containers or connectors too well manually.

So, grip prosthetics enter the world of non-disabled people.

Matthew Ostroff developed Medegrip.

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Grip geometry and prosthetic solutions analyzed - stump, Krukenberg arm and grip [what we really want / Sci Fi]

Cite this article:
Wolf Schweitzer: Technical Below Elbow Amputee Issues - Grip geometry and prosthetic solutions analyzed - stump, Krukenberg arm and grip [what we really want / Sci Fi]; published March 26, 2011, 20:40; URL: https://www.swisswuff.ch/tech/?p=390.

BibTeX: @MISC{schweitzer_wolf_1571786866, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - Grip geometry and prosthetic solutions analyzed - stump, Krukenberg arm and grip [what we really want / Sci Fi]}}, month = {March},year = {2011}, url = {https://www.swisswuff.ch/tech/?p=390}}


1 Comment

To grip well and with ease (as already investigated earlier here), the gripper parameters usually are constrained to reside along that diagram:

Heavy design

With a solid slippery gripper, a complex, heavy mechanism requires at least 5 fingers to stabilize an object in 3D space (if not seven). See left side of the x-axis of that diagram above: the harder the grip surface (metal, wood, plastic), the more fingers are needed and thus the more complex the mechanism will be. Advantages of slippery smooth hard surfaces are the possibility to clean them thoroughly, and their longevity.

Light design

With increasing surface friction, less fingers are required. A simple prosthetic hook just has two "fingers" and optimally, both grippers are covered with silicone tubing. Grip is just as good as a heavy complex hard surface hand - but construction is a lot less complex and the weight can be kept a lot lower (or at same weight, the gripper is more sturdy). See right side of that diagram just above.

For grippers at that end of the diagram (far right), it is worthwhile to note that despite gripping beauty, soft deformable high friction surfaces wear down and require replacement. I have gone through tapes or gloves as rapidly as one set a week. Then, you need materials you do not have to order through special channels that cost a fortune but are straight forward.

Caveat

Now, we are not free in our choices for everyday usage of a prosthetic. I recently learned of a ~ 7kg research prosthesis for robotic research of a multi-articulated prosthetic hand. The researchers could wear it for 20 minutes before their arms would start shaking.

V2P - grip strength and grip geometry - Gecko Skin

Cite this article:
Wolf Schweitzer: Technical Below Elbow Amputee Issues - V2P - grip strength and grip geometry - Gecko Skin; published February 24, 2011, 19:02; URL: https://www.swisswuff.ch/tech/?p=385.

BibTeX: @MISC{schweitzer_wolf_1571786866, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - V2P - grip strength and grip geometry - Gecko Skin}}, month = {February},year = {2011}, url = {https://www.swisswuff.ch/tech/?p=385}}


Whereas one may try to optimize grip quality by increasing finger control and grip geometry sophistication with over-underactuation, increased amount and thus weight of electronics and batteries, increased risk of failure and breakage due to increasingly finely structured constructions of endless sophistication, grip quality itself is a different subject. Not altogether, but somewhat. Due to that, grip optimization also is a somewhat different subject.

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V2P Prehensor - general reflection on grip strength and test of multi layered customized claw covers

Cite this article:
Wolf Schweitzer: Technical Below Elbow Amputee Issues - V2P Prehensor - general reflection on grip strength and test of multi layered customized claw covers; published July 26, 2010, 01:29; URL: https://www.swisswuff.ch/tech/?p=342.

BibTeX: @MISC{schweitzer_wolf_1571786866, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - V2P Prehensor - general reflection on grip strength and test of multi layered customized claw covers}}, month = {July},year = {2010}, url = {https://www.swisswuff.ch/tech/?p=342}}


After reading a scientific article about grip strength, after reflecting about my current experience, I figured it was time for an update on grip strength.

Based on a very nice article of Markenscoff et al. (1990), it becomes clear that to securely grip an object in three-dimensional space one requires a rather large number of fingers if there is no friction at all.

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