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View Full Version : Summary of Responses to "Medicotest Blue Sensor NF-50-K/USAElectrodes"



Gregory S. Sawicki Um
02-10-2003, 11:43 AM
Thanks to all that responded.

Not many options out there for truly low profile electrodes that have shielded
leads. Pre-amps at the skin surface are clearly the best solution, but make the
electrode profile too large for our application.

FYI:
Some responses suggested noise from the powered orthosis. Our exoskeletons use
pneumatic muscles......no interference from electric motors etc.

Best,
Greg Sawicki

Original Posting:

We are using low profile disposable electrodes from Medicotest (Blue Sensor NF-
50-K/USA) to collect from 8 muscles of the lower leg during walking with a
powered orthosis. The low profile of these electrodes allows application under
the carbon fiber shell of our exoskeltons.

We are experiencing problems with noise (we think maybe electromagnetic
interference) and are searching for other companies who sell low profile
electrodes that have shielded leads.

Any suggestions, recommendations on where to find low profile, electrodes with
shielded leads would be appreciated.

Also any comments on your experience with Blue Sensor electrodes would help.

Responses will be collected and posted.

Thanks.


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Hi Gregory,

I use Dantec pre-gelled disposable inside lower limb prosthetic sockets (a
similar application to your own.) I have encountered some problems, but I
don't think these are caused by the electrodes per se, rather the way I
pick up he signal by wiring into preamps from another system.

Anyway, they certainly meet your requisites of being sheilded and low
profile. My brochure just has a phone number 1800 303 3748. I've never
actually purchased any, as we had some surplus stock in our department.

Wesley Pryor
PhD Candidate
National Centre for Prosthetics and Orthotics
La Trobe University
VIC Australia 3086

phone: (03) 9479 5729
fax: (03) 9479 3742
mob: 0417 525 153
email: w.pryor@latrobe.edu.au

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Dear Gregory Sawicki,

You can usually figure out the source of the noise if you look at the
data - in my experience there are basically 4 kinds of noise found in
EMG traces:

1. AC line noise - typically 50/60Hz although occasionally
higher harmonics show up (i.e. 100/120Hz). An FFT spectral
analysis of the data will show up strong signals at specific
AC line frequencies.
2. Movement artifact - covers a wide range of signals but generally
it's low frequency and ceases when the subject stops moving.
Small amounts of LF artifact can be removed by LF filtering.
Large LF artifact at the input may prevent the EMG system from
recording anything sensible.
3. RF or "out-of-band" signals - any signal that is outside the
expected EMG signal band. This can be difficult to track but
generally you have a high frequency signal that either upsets
the input amplifiers or causes aliasing when the signal is
sampled. This is usually intermittent.
4. External noise within the EMG bandwidth - this is not common but
would be a possibility if you have a powered orthosis. Does the
noise disappear if the orthosis is not powered/operational?

Once you've identified the source of the noise you can start dealing
with it. Assuming that it's #4, I think your options are:

a. Shield the EMG pickup cables (I don't think that the type of
electrode will have much to do with the problem, assuming that
it's a good quality, new gel electrode.
b. Add an additional ground reference electrode connected to the
subject (anywhere) and the EMG system common reference or
ground (make sure that you don't break any electrical isolation
when you do this).
c. Ask your EMG system manufacturer to help.
d. Shield the device that's generating the interference - it's
often easier to prevent the interference getting out that trying
to deal with it after it's escaped.
e. Amplify the EMG at the skin surface so that the EMG signal is
shielded and immune from electrical interference from that point
onwards.

Our EMG systems use the last approach and we have very few problems
with electrical interference even under extreme conditions (users in
aircraft, operating linear motors, using cell phones etc). Information
on our preamplifiers is available at
http://www.motion-labs.com/electrod.htm if you are interested.

Regards,
Edmund Cramp
--
Motion Lab Systems, Inc. 15045 Old Hammond Hwy, Baton Rouge, LA 70816
+1 225 272-7364 (voice), +1 225 272-7336 (fax)
email: eac@motion-labs.com
http://www.motion-labs.com

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Is the noise low frequency or high frequency? What do you mean by a
"powered orthosis"? Might the noise come from a motor within the orthosis?

Low frequency noise is likely due to movement artifact and should be
attenuated using a high pass filter at about 30 Hz. Higher frequency noise
could be due to lack of shielding for 60 Hz hum (in which case shielded
leads should help) or pick up of radio frequencies if very high frequency
(low pass filters should help). If your reference electrode is connected
appropriately and you are using amplifiers with high CMRR you should not
have 60 Hz noise. I see you are at Michigan so you might want to talk to
Susan Brown as she is quite experienced with EMG, although primarily with
the upper limb during more constrained movements. You should try some
standard EMG electrodes to see if the noise remains, in which case it is
not resulting from use of your electrodes.

Good luck and say hi to Susan for me.

Sincerely,

Warren G. Darling, Ph.D.
Associate Professor,
Department of Exercise Science,
The University of Iowa
Iowa City, IA 52242

phone: 319-335-9514
fax: 319-335-6966

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Hello Gregory,
We use the flat electrodes from Biosemi along with their Active Two system
(http://www.biosemi.com/flat_electrode.htm). The noise resistance of these
electrodes should be quite good since the input amplifier is inside the
electrode itself. They are almost immune to environmental noise. Also, since
the input impedance is so high, they are much less susceptible to movement
artifact as well. They can be purchased in the US from Cortech Solutions
(http://www.cortechsolutions.com/). I hope this information is useful to
you. Please note that these electrodes are not truly flat, just flat
relative to the profile of other low-noise electrodes.
Best,
Christopher

/************************************************** **
Christopher J. Poletto, Ph.D.
Staff Research Biomedical Engineer
Laryngeal and Speech Section
Medical Neurology Branch, NINDS
National Institutes of Health
Building 10, Room 5D38
10 Center Drive, MSC 1416
Bethesda, MD 20892-1416

Phone: (301) 402-1496
FAX: (301) 480-0803

################################################## ######################

--
Gregory S. Sawicki, M.S. MechE
Human Neuromechanics Lab
Dept. of Movement Science
Division of Kinesiology
401 Washtenaw Avenue
Ann Arbor, MI 48109-2214

ph: 734-615-1711
e-mail: gsawicki@umich.edu

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