Grip planning and carrying out [TRS Prehensor vs. iLimb Ultra Revolution - theory and practice]

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To reliably plan and successfully carry out a grip using a prosthetic hand or gripper, it helps having a few requirements met.

This is obviously not clear at this moment, as recent experiences at the Cybathlon show. All the same, others and myself have addressed various related grip aspects previously so this is something we can come back to.

So let us start by watching the iLimb hand trying to grasp a clothespin.

iLimb using uncoordinated "finger theater" to mimic "bionic" hand appearance

The Cybathlon revealed once more that the iLimb, despite being typically presented as "Cyborg"-defining "revolutionary" "bionic" prosthesis is currently, or, in its present form, maybe not more than uncoordinated "finger theater".

Sure may make for great TV talks, but not so great item grab sessions. Reconsidering that, the fun does lies in the unpredictable here.

The iLimb makes for great hand impressions, to a degree at least - but actually planning a grip can be difficult as also the Cybathlon prosthetic arms race showed.

It is a reality that the actual configuration of the curved finger tip surfaces due to their very small diameter in combination with an unknown (and variable) relative finger configuration makes for different actual grasps every single time. Not one grip is like the other.



Pflumfs falera:


And .... yippie:


"iLimping" around, that was.

The iLimb does not fail every time. It is far more treacherous than that. But it will get you, eventually.

With that, it can be concluded that the following impressions from our iLimb Touch Bionics "pilots" at the recent Cybathlon prosthetic arms race were not at all to be blamed for the prostheses presenting them with a miracle grasp space, a surprise adventure into random finger salad configurations.

The following sequence contains a full misalignment between thumb and index to the point where no sensible grasp is obtained.

I guess they may want to fix that.


The same problem again leads to a problem carrying out a grip here.


Conversely, a recent TRS Prehensor test yielded these results:


And testing at home confirmed that this is a repeatable result.


In essence this is already putting to life what Ma et al. published regarding a "stable grasp" [bibcite key=ma2016spherical]: and hooks definitely provide a common point N where the grip will, and if planned, is about to happen.

For many prosthetic hands with individual finger motion and configuration options, this will be a relevant research path to the future of stable grasping.


Cite this article:
Wolf Schweitzer: Technical Below Elbow Amputee Issues - Grip planning and carrying out [TRS Prehensor vs. iLimb Ultra Revolution - theory and practice]; published October 17, 2016, 17:49; URL:

BibTeX: @MISC{schweitzer_wolf_1582366344, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - Grip planning and carrying out [TRS Prehensor vs. iLimb Ultra Revolution - theory and practice]}}, month = {October},year = {2016}, url = {}}