Fr. Tom McGahee Says:
The electrode interface begins with an isolated instrumentation amplifier with two "active" differential inputs and a third "guard" electrode that establishes the ground reference.
The "guard" electrode is physically located between the two differential inputs. This helps to cancel out much of the common mode signal.
Robert Rolf says:Typically the 'Guard' electrode surrounds the two active electrodes to effectively short out surface leakage currents and improve signal/noise ratio. Typical separation is 2 cm. If placed in between the electrodes it is to ensure the best possible balance in the noise pickup to maximize the CMRR.
... The three wires are twisted together to reduce common mode noise. The "ground" electrode then goes in the center to maintain electrical symmetry. You can also surround the two signal wires with a ground, but this is a bit messier to implement in the real world. You can obviously use coaxial wiring instead of the twisted 3 pair wire, but for the target application you do not really gain much for the extra expense. Remember that the target application is simply to get a *useable* signal from a muscle movement. If we were interested in the exact waveform (as in electrocardiographs), then the extra expense is warranted.
In regard to the spacing between the active electrodes, it depends on the signal source. For the heart you would want them separated by at least a few cm since the heart is located rather deep. For muscles located nearer the surface you can reduce the spacing. Note that if separation of the electrodes is too far, then you get a mixture of several muscle signals, and that is fairly useless.
Robert Rolf says:
You can get readymade electrodes (as used for myoelectrically controlled prosthesis) from Otto-Boc, or other electrically powered prosthetic makers (Schmidel?). Check with your local prosthestist or hospital rehab department for local sources and options.
The Otto Boc electrode has a single switched output (with adjustable thresholds) for controlling open/close of an electric hand so you may be able to use it directly as input to mouse switches or the 'mousekeys' program (Microsoft accessibility option pgm) with a suitable scan code stuffer.
A now defunct local company (Leaf electronics) made an interface that allowed a single EMG electrode to control both opening and closing with a single muscle site by use of a graded muscle contraction. A light contraction would open the hand, a strong one would close it. I presume that this is now commercially available elsewhere, but I haven't checked.
Another local company specializes in building devices to assist handicapped individuals with computer control.
There are many other companies out there, and most of their engineers are quite happy to answer questions about their technology.
David Cary says: EEGs (which are intended to detect electrical currents from neurons in the brain) use practically the same electrodes and electronics as EKGs (which detect electrical currents from muscles in the heart) and EMGs (which detect electrical currents from the muscle to which it is attached).
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