How to trigger a prosthetic hand

General project help for Adafruit customers

Moderators: adafruit_support_bill, adafruit

Please be positive and constructive with your questions and comments.
Locked
markjames
 
Posts: 74
Joined: Tue Oct 01, 2013 1:28 pm

How to trigger a prosthetic hand

Post by markjames »

So a friend of mine is missing an arm. She got a new one and she asked me if I'd like the old one. That was a pretty easy question, lemme tell you. So now I have an arm, two batteries, and a charger.

The only thing is I have no idea what triggers this thing. I've got a couple of pictures here - one shows the inside and the other the outside complete with the battery pack.

On the inside there are two round areas that have 4 gold contacts on each. I wanna take the sleeve (skin) off to expose the mechanism but the default position is closed so the fingers and thumb are currently gripping. I guess when she 'thinks' it to open it opens and when she stops thinking it it closes. I don't know how to simulate this 'thinking' part though. Is it a low level voltage? A change in skin conductivity? Would it be a current measured across some of the contacts or from side to side?

Image

Image


Any ideas would be muchly appreciated.

User avatar
adafruit_support_mike
 
Posts: 67446
Joined: Thu Feb 11, 2010 2:51 pm

Re: How to trigger a prosthetic hand

Post by adafruit_support_mike »

My guess is that the two long pads on the sides establish a reference voltage and the two round pads are connected to an instrumentation amplifier (basically a super-op amp) and would be looking for a small change in skin voltage corresponding to a nerve impulse.

That's pure speculation though, and there's no way to tell without either opening the control unit or finding some documentation from the maker.

markjames
 
Posts: 74
Joined: Tue Oct 01, 2013 1:28 pm

Re: How to trigger a prosthetic hand

Post by markjames »

So....

I'll try to find some sort of documentation. A prosthetics site would probably be a good place to ask. In the meantime I may try just putting a few millivolts across the pads to see if that makes it go.

any voltage that it would sense coming from muscle stimulation would be DC - right?

mark

User avatar
adafruit_support_mike
 
Posts: 67446
Joined: Thu Feb 11, 2010 2:51 pm

Re: How to trigger a prosthetic hand

Post by adafruit_support_mike »

I doubt it.. over time, any DC current would create a vast depletion of something in one part of the body and a vast excess of it somewhere else.

I did some digging on the mechanics of nerve impulse transmission, and it looks like it's a burst-and-recover process.

The natural voltage inside a nerve cell is about -70mV due to the action of structures in the wall called 'ion pumps'. The pumps push three sodium ions out of the cell for every two potassium ions they allow in. Left to themselves, the ion pumps would eventually drive all the sodium out of the cell, but there are other structures called 'sodium channels' and 'potassium channels' that allow the ions to move back where they came from. The -70mV 'resting potential' is the equilibrium point between the action of the ion pumps and the sodium/potassium channels.

If the voltage inside the cell rises to about -50mV, the sodium channels open and allow a lot of sodium ions in. That creates a local region of higher voltage, which triggers the sodium channels nearby to do the same thing. The process lasts for about 500 microseconds and creates a voltage spike up to about +50mV. This phase is called 'depolarization'.

The positive voltage inside the cell causes the potassium channels to open, allowing an out-rush of potassium ions, and taking the voltage inside the cell negative again. As with most such systems, there's some overshoot, taking the voltage inside the cell down to about -80mV. The process of releasing potassium ions is called 'repolarization' and the overshoot is called 'hyperpolarization'. Repolarization also lasts for about 500 microseconds, and it takes 2-3 milliseconds for the ion pumps to undo the hyperpolarization.

The polarization and depolarization thresholds are constant, so the signal that travels along the nerve path is a pulse about 1 millisecond wide with an amplitude of about 100mV. The strength of a signal doesn't depend on the amplitude of the pulse, but on the frequency of the pulses. Strong signals have high-frequency pulse trains, weak signals have low-frequency pulse trains.

Odds are you're looking at a control circuit based around a frequency counter.

markjames
 
Posts: 74
Joined: Tue Oct 01, 2013 1:28 pm

Re: How to trigger a prosthetic hand

Post by markjames »

Thanks for going to all that trouble and for your very thoughtful reply.

I think that while this is a pretty nifty toy to experiment with that I should be thinking more about how this thing might be better used.

I did a bit of research too and found that there are a few organizations that take used prosthetics and 'recycle' them. Apparently they can't be reused in North America 'as is' (legal issues) but they can be disassembled and the electronics reused in developing countries to make prostheses there. I've contacted a couple of these organizations and I'm waiting to hear back from them about how to get this to them.

It would have been fun to play with but when I weighed my fun vs someone else having a usable limb it became a pretty easy choice.

Thanks again for your help,

Mark

User avatar
adafruit_support_mike
 
Posts: 67446
Joined: Thu Feb 11, 2010 2:51 pm

Re: How to trigger a prosthetic hand

Post by adafruit_support_mike »

That's a good choice too.

User avatar
bobbooth
 
Posts: 1
Joined: Mon Jun 22, 2015 1:58 am

Re: How to trigger a prosthetic hand

Post by bobbooth »

Let me now what you find out i have a brother anď a ñeice becided my self that are handicapt

Locked
Please be positive and constructive with your questions and comments.

Return to “General Project help”