Electronic – Hardware to Detect EMG or Skin Resistance

biopotentialembeddedpicresistanceskin-effect

I have been given assignment to design a device that can detect emg signal of Skin Resistance change while moving
fingers.

actually i need to track finger movements using some hardware containing micro controller

My idea was that i will apply voltage at two points one finger tip and one elbow.

and measure voltage change by amplifying it.

Today i tested this idea by applying 1 volt and measuring by Ossiloscope .I donot know why there was 50 Hz AC wave detected there and there was very very slight change.

but i am not much happy with the results.

any idea?may be something mile a glove that sends computer a signal through some PIC/AVR or what ever
Any Idea or help is required.Consider what would you do in this case keeping in mind safety?

Best Answer

Are you in Europe? The 50Hz seems to be coming from the power lines and if that noise is really bothering you, you should design some sort of notch filter to remove it. Depending on how far you go, you could maybe even use an adaptive filter that will cancel out that noise without losing any data you might need (frequencies around 50Hz) but that seems unnecessary for your assignment.

In order to get reasonable voltage levels, you need to amplify your signals tremendously. I can't remember the exact value off the top of my head, but EMG signals are relatively strong. You may be able to get away with a gain of less than 1000 in order to get peak-to-peak values of about 5V. The explanation of the amplifier will come later.

There are two types of electrodes that I am aware of that you can use to measure EMG signals: unipolar and bipolar. Your electrodes are probably unipolar and so I will assume this for the rest of the post.

You cannot just have two electrodes to measure EMG. You MUST have a reference point (i.e. your ground point). This, I would put at the elbow, or somewhere bony that will pick up very little electrical activity. Once you have a reference point, only then can you select two points on the body and then measure the difference in voltage levels between the two. In short, you must have at least three electrodes (if using unipolar electrodes).

You are going to have to use a differential amplifier (or instrumentation amplifier for better gain) in order to achieve this because you need to find the voltage difference between two points of the body. So you would tie your reference point (elbow) to virtual ground, and then tie the other two electrode outputs into the inputs of an instrumentation amplifier. You can look it up on wikipedia if you want to design your own, but it is probably easiest to buy an instrumentation amplifier. The INA128 (I think, from TI) should work fine.

You should double check the typical frequency range of EMG signals, but they are high compared to other biosignals that are typically measured. This being said, you should pass your amplified signal through some sort of bandpass filter for better results and higher immunity to noise. Another method, which might be better, is to integrate the signal (periodic muscle contractions should look almost like a square wave). This way, when you are interfacing with a PIC/AVR microcontroller, you can check voltage levels (which will correspond to intensity of muscle contraction) through the ADC channels.

Long answer, probably got worse as the answer went on but I hope this can be of use to you.

You should use the finest magnet wire with many multiple strands or LITZ WIRE to improve L/R time constant and peak current for lowest loss cable. Your discharge rise time will then drop to xx picoseconds. But for conductor pairs use twisted pair Litz wire to reduce CM emissions.

Otherwise, you can jam many radios/mobiles with high rep rates of > 1pps affecting AGC. !!

In most conductors R(f) rises fast and L(f) drops slow from skin effect. This R effect increases more with iron content since it is from Eddy Currents. In DSL and cable modem skin effects change Zo, phase shift and group delay.... Tony

ref wiki

Although the geometry is different, a twisted pair used in telephone lines is similarly affected: at higher frequencies the inductance decreases by more than 20% as can be seen in the following table.

Characteristics of telephone cable as a function of frequency
Representative parameter data for 24 gauge PIC telephone cable at 21 °C (70 °F).

   (Hz) R (Ω/km)    L (mH/km)   G (μS/km) C (nF/km)
     1    172.24    0.6129      0.000      51.57
    1k    172.28    0.6125      0.072      51.57
   10k    172.70    0.6099      0.531      51.57
  100k    191.63    0.5807      3.327      51.57
    1M    463.59    0.5062     29.111      51.57
    2M    643.14    0.4862     53.205      51.57
    5M    999.41    0.4675    118.074      51.57

The spectrum of your pulse is not at all like a square wave, since it is not repetitive over a small interval. It is a continuous spectrum rolling off similar to The null of 2nd harmonics of the equivalent pulse of a "square" wave and then rolling off above the 0.35Tr rise time. So resonant frequency and group delay calculations of pulses is very poor and affected by skin effects, even in controlled impedances, making Baseband communication much worse than the discrete equalized channel's of a modem for thruput in bps/Hz .....Tony

However hollow copper tubing with interior flash gold plating works wonders in microwave as does ENIG on stripline and gold-plated aluminum cases for RF circuits and enclosures for microwave. I saw this in '77.

Here is a different skin effect from UV on dielectrics (human skin) and how Sodium Bicarbonate helps prevent cancer. ( also reduce causes/reactions of itching). https://www.cancertutor.com/simoncini/. :):)

Variation of resistance within conductor due to Skin effect

The skin effect is the effective reduction in the cross sectional area of a conductor that carries high frequency AC. The current density is higher on the outside of the conductor than it is an the inside. The actual resistivity of the conductor is still uniform throughout.

Its essentially an example of back emf which is strongest in the middle of the wire produced by the changing magnetic field of the AC.

enter image description here

see http://en.wikipedia.org/wiki/Skin_effect

Best Answer

Related Solutions

Electronic – Skin effect for pulse current

You should use the finest magnet wire with many multiple strands or LITZ WIRE to improve L/R time constant and peak current for lowest loss cable. Your discharge rise time will then drop to xx picoseconds. But for conductor pairs use twisted pair Litz wire to reduce CM emissions.

ref wiki

Variation of resistance within conductor due to Skin effect

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