Myoelectric sensor

[PlastBox]

Ok, so here's a real challenge for you gurus!

Could anyone of you come up with a myoelectric sensor? One of those things that pick up the increased voltage across the skin when an underlying muscle contracts.

From http://searchmobilecomputing.techtarget.com/sDefinition/0,,sid40_gci936219,00.html :

Myoelectric signals are detected by placing three electrodes on the skin. Two electrodes are positioned so there is a voltage between them when a myoelectric signal occurs. The third electrode is placed in a neutral area, and its output is used to cancel the noise that can otherwise interfere with the signals from the other two electrodes. The output voltage is processed using a device called a differential amplifier. The output of this amplifier has much higher voltage than the myoelectric signals themselves. This higher voltage, which produces significant current, can be used to control electromechanical or electronic devices.

This type of sensors are used in many prostetic devices, but as far as I know (not beeing an amputee myself), a lower-arm myoelectric prostetic device with only gripping-motion (and no feedback from the arm, so you can't, say, pick up an egg) will run you about $5.000! I find this to be rather horrible, seeing as how the guys in the first YouTube video listed below did it with hobbyist gear and a low-end microcontroller.

Now I don't claim to be an expert (nor anywhere close to one), but by using a voltmeter on your skin you'll get a reading within the range of (I think) >0 to 50-60 mV. With amplification and perhaps some noise-smoothing one could at least make this a really neat experiment! 8-) Has any of you expertly people got any experience using "differential amplifiers"? I have never even used an opamp myself so...


YouTube:http://youtube.com/watch?v=vM5Cr42V_so !
http://youtube.com/watch?v=dhRJqZ33_3I
http://youtube.com/watch?v=bCSVkkhSy3M

[cramar]

I remember reading a paper on this so I googled it and with some luck here it is http://2005.iccas.org/submission/paper/upload/1revision-sensor.pdf  I am not sure if it helps

Cheers

[cramar]

You may also want to look here http://www.openprosthetics.org/myoelectric

[PlastBox]

Ok, thanks for your replies!

After doing some more research, it all seems to boil down to this:


(From: http://www.nexgenergo.com/medical/biodataemg.html)

Two electrodes connected  to the skin above the muscle and the inputs of a differential amplifier, and a third electrode used as refrence/ground. The amplifier boosts the diffrence between ground (voltage read of skin) and the voltage read of the skin right above the muscle. If one wants to be really fancy, one could filter out 50-60Hz (noise from mains wireing) and use high-pass and a low-pass filters.

So, I think I've got the general principal of this down. Now I only need someone to tell me how to do this. I have as I mentioned before, never used an amp-IC and thus I do not know how they work, part-numbers, etc.

Why is there as good as no information on this online? Does the prostetics-manufacturers want to keep this a "trade secret"? "Uuuh, EMG-sensors is sooo advanced, you building one (on the cheap) is about as likely as you going to Mars! So, we'll just keep on charging $5000 for something with 1 or 2 of these sensors, a motor and some gears. Costs <$100 to produce, but you don't know that!" *evil grin*

Also, why is there apparenty no interest in this kind of sensors in the DIY/tinkerer crowd? Who would not want to make a hookup that allowed one to control ones computer or some robotics of some sort by muscle movement??

Controlling Super Mario with homemade-looking EMG sensors on breadboards!
http://youtube.com/watch?v=Z1npwCTQ7-w

[sil]

PlastBox: did you ever get any further with this research? I've been looking at the idea of myoelectric sensors myself, and like you I found very little about them in detail. How goes the research? I'm really intrigued by the idea of using finger movement (picked up by myoelectric sensors) as a way of providing input to an Arduino.

[PlastBox]

I haven't really been able to fint much beyond what is in the links in this thread, ergo very general information. 3 electrodes placed on the skin, the 2 outmost of the 3 connected to the inputs of an OPAMP, with the center electrode used as a refrence-point for the amp. Bandpass-filters for removing very high and very low frequencys, as well as 50-60Hz (from mains).

Please go ahead and build one! I don't have the skills to take general information like this and make something out of it, but perhaps you do?

Another, perhaps even more interesting concept is EEG. http://openeeg.sourceforge.net/ How about using your brain to control stuff, instead of your fingers? Heh.. They even provide all the information you need to build your own functioning hardware, but it would seem that the tricky part is analyzing the signals beeing read of the skin on the head and getting useful information from it.