Page 87 - Handbook of Biomechatronics

82 Jeff Christenson

Fig. 25 Preamplifier electrode.

A preamp electrode is short for preamplifier electrode (Fig. 25). These
devices contain two electrodes and a ground. The preamp electrode collects
the signal and also amplifies it, although further amplification may occur
downstream. Many preamp electrodes have an input for adjusting the ampli-
fication of the signal. The metal electrodes of the preamp electrode also must
contact skin. The advantage of preamp electrodes is the ease of setup and
portability.
Once sensed and amplified, the EMG signal is still very noisy, and must
be filtered and rectified. EMG is often filtered through a band-pass filter
within the power spectrum of the signal, with a low passband of around
10Hz and a high band pass of 500Hz (Raez and Hussain, 2006; Soares
et al., 2003). The EMG is then rectified (the absolute value is taken) and
filtered through a third-order low pass filer of around 5Hz to envelope
the signal (Fig. 26). These processing techniques return a proportional signal
which can be used in biomechatronic devices.
Surface EMG has been used successfully in many biomechatronic
devices. However, even after processing, surface EMG is a noisy signal
and does not necessarily correlate to one muscle. The surface EMG sensor
will pick up all muscle activity in surrounding area, meaning a combination
of different muscle neurons and their fibers.

Fig. 26 EMG processing.