CoCo-80 User Manual
98
acceleration integrated to compute velocity and result in a drift and eventually an
infinite large magnitude of displacement after double integration.
Acceleration
Velocity
Displacement
Figure 107. A small error in acceleration results in a DC offset in velocity
and a huge drift in displacement.
Of course, the computed velocity and displacement signals are unrealistic. They
are artifacts of the integration errors. In order to remove such a problem caused
by inaccurate measurement and digital integration, a high pass filter can be
applied before or after the integration. It should be noted that the high-pass filter
will distort the waveform shape to some extent because it alters the low frequency
content of the signal. However this effect must be tolerated if numerical
integration is used.
Digital High-Pass Filter
The most effective way to remove the DC drift effect as described above is to apply
a high pass digital filter to the continuous time streams. In CoCo, a unique
algorithm is realized so that even the data is sampled at high rate, the high pass
filter can still achieve very low cutoff frequency.
The filter cutoff frequency is specified at -3dB attenuation.
To remove unwanted signals at or near DC, please set up the cutoff frequency of
the digital high-pass filter as high as possible as long as it won’t chop off useful
frequency content of your interest.
To give an example, if you are not interested in any frequency less than 20Hz,
then you can set the cutoff frequency to approximately 10Hz. With this setting,
the amplitude attenuation at 20Hz will be less than 1dB.
The following picture shows that ch1 sets the high-pass filter at 15Hz and ch2 at
50Hz. Others are at 1Hz.
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