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Automatic Compensation of Skin Conductance in Electromyographic Sen sing

IP.com Disclosure Number: IPCOM000121654D
Original Publication Date: 1991-Sep-01
Included in the Prior Art Database: 2005-Apr-03
Document File: 3 page(s) / 117K

Publishing Venue

IBM

Related People

Koperda, FR: AUTHOR [+2]

Abstract

In sensing the minute electrical signals that trigger muscular movement, the signal to the amplifier input may have an impedance that varies over several orders of magnitude. For a fixed amplifier input impedance, the skin-sensor resistance and skin impedance act as a voltage divider which causes a voltage drop in signal amplitude. This invention provides a consistent amplitude signal over a wide range of skin impedance and contact resistance of the skin-sensor interface.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Automatic Compensation of Skin Conductance in Electromyographic Sen
sing

      In sensing the minute electrical signals that trigger
muscular movement, the signal to the amplifier input may have an
impedance that varies over several orders of magnitude.  For a fixed
amplifier input impedance, the skin-sensor resistance and skin
impedance act as a voltage divider which causes a voltage drop in
signal amplitude. This invention provides a consistent amplitude
signal over a wide range of skin impedance and contact resistance of
the skin-sensor interface.

      In applications that require sensing of muscle movement, it is
convenient to use the small electrical voltages associated with
muscle contractions.  This voltage is known as an electromyographic
(EMG) signal because it is associated with muscle movement.  To sense
the EMG signal, usually requires electrical contact with the skin
with some conducting material.  The boundary between the skin and the
contacting sensor is poor and its characteristics will change over
time.  There are both short and long term effects that will cause
interference with the desired EMG signal.

      In trying to process EMG signals, contamination of the signal
occurs because of noise induced in the skin-sensor interface and
amplitude variations caused by the conductance of the skin.  These
conditions are very dynamic and require methods to minimize their
effects.

      The resistance of this signal path may vary from 50K ohms to
200M ohms.  The effect of the amplitude of the signal can be
calculated using the following formula for a voltage divider:
      V = Z1 / (Z1 + Z2) where:
      V = voltage drop from the source to amplifier
      Z1 = the skin resistance + skin-sensor contact resistance
      Z2 = the input impedance of the amplifier.

      For example, operating into an amplifier which has an input
impedance of 10M ohm can give a drop of .005 (50K/(50K + 10M)) to 20
(200M/(200M + 10M)).  This range of variations is large and can cause
problems in analyzing the signal.

      Continuous adjustment of the desired EMG is possible by using a
reference signal and adjusting the gain of amplification derived from
this received signal.  A test signal of low amplitude (10 millivolts)
in inserted into the skin and picked up by another sensor.  This
reference signal experiences the same changes in skin resistance and
skin- sensor resistance as that of the desired EMG signal.  It thus
becomes a reliable gage of signal variations.

      The figure provides a block diagram of the steps involved in
processing the signal.  After the EMG and reference signals have been
received, they are sp...