What are the necessary components for a DIY biomed pong game interface?

[teckjunkie]

Hey guys how's it going this is my first post so take it easy on me . I'm in my last year of electrical engineering technology, and we need to design and construct a final project. On the list of possible topics was the idea of a bio-med game interface. It would be biomed probes detecting muscle activity to control a video game. Rather than getting an open source game, rewriting the code, building a cable to connect it to the computer, and etc I found an old think geek DIY pong game.

http://www.thinkgeek.com/geektoys/science/8546/
(avaible via partsexpress.com)

So I want to replace the buttons with connector probes attached to circuitry. I figured I would have the biomed sensors on the arms going through a high gain op-amp, and that through a ADC. Whichever output became high when then arm was flexed I would tie to a relay or something to close the switch. Simulating someone pushing the buttons.

I'm great with the amplificiation of volages, transitors, and such. However if anyone can any point me to where I would purchase biomed probes and any documentation you might know of it would be much appreciated. I know nothing about biomedical stuff. How an arm would react when flexing versus not flexing and reading signals from it. Any input would be much appreciated. Googling has gotten me nowheres and I'm really lloking forward to getting started on the project.

 

[Danger]

My next Hallowe'en costume (if I can get it done in time) incorporates myoelectric controls, so I've checked into it a bit. My first stop was a local prosthetics distributor. Unfortunately, the surface electrodes in use these days are made of gold and cost over $700 Canadian per.
I have an idea that I am looking into, but haven't had time to get very far. I'm thinking of using metal acupuncture needles to infiltrate the flesh and rest up beside the muscles. Nothing approaching the sensitivity of a transcutaneous pick-up should be necessary because of the proximity to the source. On the other hand, I can foresee it being somewhat uncomfortable, and I'll possibly have to wrap my arms in anti-biotic bandages.

 

[berkeman]

I googled medical supply electrode +emg, and got a bzillion good hits.

I'll tell you from personal experience, though, electrodes (which you can get cheap disposable versions of, Danger) are pretty messy and uncomfortable to put on and wear. They would certainly put me off of playing your game. Plus, there are some pretty strict medical safety standards that you would have to design to (I think UL510? here in the US) to reduce your liablity risk. Whenever you hook electrodes to the body, you are somewhat bypassing the normal insulation level that your skin provides, so shock risks go up significantly. Danger would probably get a kick out of the experiment I did with a 9V batter and a couple EEG electrodes may years ago -- just about shot myself across the room...

So I'd suggest that you re-think your sensor approach to use some other angle. How about some optical pickup or exercise device (play and work out at the same time)?

 

[dlgoff]

Berkeman,

I have to disagree a little with you on not hooking up electrodes to your body. Sure you need to make sure that there will be no leakage currents involved with your circuit; but using a good instrument op-amp with a medical grade power supply should be okay. The OP should check the UL510? requirements however to understand how to achieve and test for low leakage currents. e.g. hospital grade power cables and plugs.

BTW How did you get jolted with a 9 volt battery? You must have been switching it through some big coils or something?

Regards

 

[Danger]

Berkeman, I'd certainly appreciate anything that you can steer me toward in the line of disposable myoelectric surface pick-ups.
Now, as much as I appreciate that you're one of the most 'electric savvy' guys on PF, I have to agree with digoff about the feedback thing. I have absolutely never heard of anyone getting jolted by a prosthetic limb. You must have screwed up your connections somehow. At the very least, you must have been running your 9 volts through a frequency generator of some sort, which might have resulted in a Taser-like effect if that frequency interfered with neurological impulses. The main thing to remember in this context is that the electrodes are a passive sensor system. No power is applied to the electrodes. They provide the power to fire a Schmitt trigger.

 

[NoTime]

This might work
http://www.alibaba.com/catalog/11691565/Disposable_ECG_Electrode.html

Or you might ask your doctor to sell give some to you.

 

[Danger]

The thing here is that those electrodes are EKG units. They wouldn't be discriminatory enough for prosthetic control, but maybe for this application.

 

[berkeman]

BTW How did you get jolted with a 9 volt battery? You must have been switching it through some big coils or something?

Nope, just a straight 9V. Okay, nobody try this at home, okay? I was designing and prototyping a biofeedback EEG device back in my consulting days (it turned out pretty cool in the end), and I obviously used myself for a lot of the EEG testing. So I had EEG electrodes on my head pretty often. One day I was working on some part of the circuit (I don't remember what), and I ended up putting a small voltage across a couple of the electrodes on my forehead. It caused a little tingle and twitch which surprised me.

So I uttered a line from the movie Real Genius, "Up the voltage!" I pulled out a 9V battery and touched the electrode lead ends to the buttons, and WHAM! That shot me backwards out of my chair (and luckily made me let go of the battery) and gave me a hell of a shock to my head.

So I learned my lesson about how electrodes drop the skin resistance a lot. I mean, it's common to use your tounge to battery test 9V batteries, right? And all that does is give a little tingle for fully charged batteries. And think how wet and low-Z your tongue is compared to your dry skin.

 

[Danger]

That doesn't sound right, Berkeman.
I'm not denying that it happened, but I have an alternate explanation that seems to make more sense. I'm thinking that the placement of the electrodes was such that you accidentally triggered neuronal firing in your motor cortex. We'd need somebody like Moonbear to confirm or deny the possibility of that, though; I don't know much biology.

 

[berkeman]

Oh it triggerd some firing all right! Yeah, I wasn't saying the forehead electrodes directly coupled to any major muscles. That was my brain complaining!

 

[NoTime]

The thing here is that those electrodes are EKG units. They wouldn't be discriminatory enough for prosthetic control, but maybe for this application.

Don't know about that.
As I understand it, muscles generate small electric currents when they move.
Most muscles are fairly large compared to the size of the electrode.
So I might think there should be fair discrimination.
The operating principal of these electrodes seems to only be to reduce skin resistance.
Proper placement would be important.

The only other posibility I can think of is nerve implant electrodes.
That would require surgery.

 

[NoTime]

Oh it triggerd some firing all right! Yeah, I wasn't saying the forehead electrodes directly coupled to any major muscles. That was my brain complaining!

Electoshock therapy might be a relavent topic.

 

[Danger]

Don't know about that.
As I understand it, muscles generate small electric currents when they move.
Most muscles are fairly large compared to the size of the electrode.
So I might think there should be fair discrimination.
The operating principal of these electrodes seems to only be to reduce skin resistance.
Proper placement would be important.

The only other posibility I can think of is nerve implant electrodes.
That would require surgery.

You're both right and wrong there, NoTime. Transcutaneous electrodes can't do anything about reducing skin resistance; nothing can. The point is to isolate the impulses produced by a particular muscle. You're right that placement is important, but so is 'focus' (which is what I meant by 'descrimination'). Do you realize how close together the muscles are that control your index finger and your bird finger or your thumb, never mind the individual joints of each? A disposable EKG electrode wouldn't be able to tell the difference.
And neurological implants today go beyond simple surgery; it's almost a mating process. The original nerve tissue is prompted to grow around the coiled platinum (or similar) electrode in order to create a full fusion of the two. It's still no easy task to translate an electrochemical impulse into a strictly electronic one.

 

[NoTime]

You're both right and wrong there, NoTime. Transcutaneous electrodes can't do anything about reducing skin resistance; nothing can. The point is to isolate the impulses produced by a particular muscle. You're right that placement is important, but so is 'focus' (which is what I meant by 'descrimination'). Do you realize how close together the muscles are that control your index finger and your bird finger or your thumb, never mind the individual joints of each? A disposable EKG electrode wouldn't be able to tell the difference.
And neurological implants today go beyond simple surgery; it's almost a mating process. The original nerve tissue is prompted to grow around the coiled platinum (or similar) electrode in order to create a full fusion of the two. It's still no easy task to translate an electrochemical impulse into a strictly electronic one.

The electrodes do decrease skin resistance, by a few orders of magnitude.
I never measured what a good electrode can obtain, but just wetting your fingers and holding the ohm meter probes will show that.
The medical ones have salts that increase the effect.

The fine control muscles are close together, stacked even.
Don't recall anyone trying to read those, just the major muscles.
But, I've never researched ths, just miscellaneous sutuff I'be read.

 

[Danger]

I see where you're going about the resistance, but that's the paste at work, not the electrodes themselves, and it's a surface effect. It's more about ensuring continuity between the skin and the sensor, rather than making the skin more conductive.
The range of motion of new designs in hands requires very sophisticated control systems, and my costume needs flex and extend pickups on all ten fingers. (That's muscular intent, by the way; my fingers won't actually be able to move, so switches won't work.)

 

[NoTime]

Not entirely, the skin surface is composed of dead dry cells.
The skin is permeable, quite a problem with some chemicals.
Plain water and body salts or the special paste bypass the dry layer insulation properties and provide electrical connection to the body interior.

Since your fingers and arms still can exert force even when constrained, have you considered pressure transducers for your costume control.
Sounds simpler to me.

Or perhaps smearing your arm with goop and wrapping it with a contact sensor array, using a computer with pattern recognition to determine muscle activity.
Unfortunately, pattern recognition is still a very immature technology.
Gold would only be important for long term corrosion in the contact array.

 

[Danger]

Okay, I'll concede the point about the conductance. I've never heard of that as an issue, but it makes sense. As for the use of gold, I never bothered asking why they're made out of it; I quit the conversation when I heard the price... and that's the only kind they sell.
As for transducers, I don't think that they'd work. There's a possibility, so I'll do some figuring. Thanks for the idea. The reason that switches (or overly sensitive transducers) won't work is because both hands will have a firm grip on something at all times. If I can find transducers with the proper trigger pressure, though, I might get by with them. They'd have to be double-acting, with up to about 0-10 lbs. of pressure unreactive, 10-20 lbs. giving a positive output, and 20+ lbs. giving a negative one (to make up for having only flex rather than flex/extend). Learing to operate it that way will be tough, but it's going to be anyhow.

 

[NoTime]

How complex is your control requirement.
Could you for instance, use one or both little fingers to switch command sequences (along the line of morse code)

How about your toes. Apparently some people have enough toe control to tie knots.

Just throwing out ideas here.

 

[Danger]

My feet are going to be somewhat tied up as well, so no go there. (And I have barely enough toe control to walk. )
As for complexity, the outfit has 10 major bi-directional servo control channels, plus 2 momentary contact solenoids. (Hmmm... just got a thought there; I can put the solenoids on a timer. There's 2 switches gone.)

 

[NoTime]

Are you building an exoskeleton?

 

[Danger]

No, although I've been toying with one of those for years. This is a Pierson's Puppeteer as invented by Larry Niven for his Tales of Known Space and Ringworld series.
"[URL=http://img224.imageshack.us/my.php?image=nessusfistgodkk0.jpg[/URL]

It's hard to see in that picture, but each of those python-like necks has one eye and one mouth with articulated lips.

 

[teckjunkie]

I know its been a while since I posted this but if your interested I did get it up and running if your interested check out http://teckjunkie.com/wiki/

 

[Danger]

Very cool, TJ! Congratulations, and thanks for posting that.

 

[xez]

Hmm sounds like there are some nifty projects that are
afoot!

Here's my $0.02 on the subject.

Material dermal biocompatibility --
it's a semi-non-issue for most people for most common
materials, especially when the contact duration and
frequency is relatively minor (a few hours once in a blue
moon or several minutes intermittently).
The problem is just to avoid things that are highly
toxic, as well as things that are allergens to some minority
of the people. The situation is basically like jewelry,
but to a much lesser extent since you might wear a
watch, ring, necklace almost all the time, and things like
ear-rings are in such intimate contact (potentially through
a pierced hole in a very sensitive area) that they're almost
in the "implant" category.

Because the skin and tissue resistance is so high, and
because you'd be using high impedance (multi-megohm)
sensing (most likely), the contact resistance differential
is basically insignificant whether you were to use
lead, silver, aluminium, gold, platinum, or anything else.
The thing that would degrade contact reliability would be
the formation of thick layers of oxide / corrosion or grime,
or provoking a callous / inflammation that'd block the
electrode from having good clean skin contact.

So if you think about the materials and scenarios
commonly experienced with jewelry, various forms
of stainless steel, silver, gold, various plated forms of
those, et. al. you'll find both information
on corrosion / allergen / durability issues, as well
as possibly finding much cheaper and more common kinds
of material *sources* than things that are intended to
be medical electrodes! I'd probably just get some
good purity stainless steel 'washers' and start with that
as an experiment. It's (relatively) cheap / easy enough
to get various kinds of 'gold' (plate / alloy / whatever)
jewelry wires / bars / whatever if you decide you want
to work with those. Careful of the soldering, though,
obviously leaving semi-toxic metal or flux residues even
on a very inert platform wouldn't be good for the wearer!
You might try to do some electric spot / arc welds if
working with steel or platinum or something not so easily
soldered. For silver / gold alloys just use the kind of
either silver-solder or brazing material or MAPP / acetylene
type of welds and fillers that a jeweler would use on a
ring or necklace or something that would have skin
contact in the treated area (unless that'd be on the
reverse face).

Silverware, or electronic component leads may be other
sources of some pretty bio-inert electrode type supplies.

As noted previously, the saline gel can help with skin
resistance and intermittent contacts.

Electrode placement / geometry -- Well you could
do a bit of research on what people have done wrt.
biofeedback, TENS devices, polygraphy, et. al., and
consulting some basic anatomy books to see where the
involved muscles and their nerves run would be
informative.

However the old experimenters maxim applies well --
hook her up and see what you get! I expect if you
used something like an instrumentation amp feeding into
something like a PC based sound card or custom ADC
reading out into the game port / serial port / USB or
whatever you could easily do oscillographic plots of
the signal responses, geometric differential responses,
correlations or anti-correlations of responses between
differential point pairs (A-B) vs (C-D), etc. quite easily
when tinkering around with electrodes and their placement
long before you have to finalize the design of the
production system. I imagine that in a couple of hours
of tinkering you could easily identify the signal
shapes, amplitudes, and physiological actions to
selectively trigger several distinctly recognizable
sensor events. It shouldn't be NEARLY as complicated
as some of the more advanced EEG or even EKG type
monitors that are done routinely. And since you're
a part of the control loop you can always invent patterns
of movements that would be easily recognized by the
system as being artificial and control oriented
vs accidental twitch responses -- e.g. do the same action
left then right then left again in rapid succession or
'double-grip' (is this the new 'double-click?') quickly
or whatever works for your system.

Signal processing -- yeah I think just some garden
variety low voltage battery powered instrumentation
amps followed by some variable gain stage of x5 to x2000
should suffice for sensing pretty much anything the body
can easily emit wrt. neuro-muscular galvanic activity close
to the surface. Just feed it into an o-scope or PC
digitizer while you're playing around then work on whatever
more compact digitization and signal processing system
you may eventually want. I'd probably just stick an
mini-ITX or smaller type of low power PC into the thing
if I was building something of the size of an immersive
costume since space and a few watts of power aren't
too hard to come by and it's easier than doing 8051
embedded code for a quick hack.

I'd think it'd be easier to hook up about (at least)
eight sensors per forearm, giving several resultant
combinatorial differential pairs to look for cross correlations
against.. that should give you plenty of data to generate
several different signalling modes reliably.

If you want to get more complicated with the sensing
wrt. muscular activity you could always actively
look at things like sub-surface impedance to a low
power induced RF field like the body fat monitors do --
generate a signal that travels through the body,
different frequencies for different depths of penetration,
and look at the AC impedance vs frequency to see what
the tissue type / density is... And clearly when you
contract a muscle that'll change the position and density
of the tissue above / around it in a detectable way,
though really I expect that's not needed.

Other ideas -- I understand that a common technique in
TV type special effects is to have manual actuation of
one part of a given 'creature' and then programmatically
control the other motions of the creature relative to
believable patterns that follow suit the action of the
manually controlled parts, so you get composite behaviors
like 'looking around', 'smelling', 'hiding', 'dancing' or
whatever based on pre-programmed motion
control / behavior algorithms that extrapolate from the
manual inputs that are given to produce more intricate
and 'naturally flowing' results.

You might look at using things like face expressions /
muscles too if the hands / toes are too busy for much
control, those are easily picked off with electrical
galvanic sensors or strain gagues etc.

Good luck!

 

[dlgoff]

Thanks for the follow-up teckjunkie. Most of the time we never get to see the results from project questions.

Regards