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I have been analysing the signal from a rotary encoder which provides
displacement time data in an attempt to determine the optimal cut-off
frequency for filtering the data prior to differentiation to provide
velocity and acceleration data. I have been smoothing the data using
a Butterworth 4th Order digital filter with cutoff frequencies ranging from
1 to 60Hz and subsequently calculating the residual as the mean square
difference between the filtered and raw data. Having plotted the residual
against cutoff frequency I have been attempting to determine the optimal
cutoff frequency by projecting the linear part of the resulting curve to
the vertical axis and then back to the curve to determine the cutoff. The
process is outlined on pages 41-43 of David Winter's book
"Biomechanics and Motor Control of Human Movement, 2nd Edition".
My problem is that the plot at the higher cutoff frequencies is not linear
but curvelinear and I an unable to determine over what range of cutoff
frequencies should I project my line from. The calculated optimal cutoff
frequency is affected to a great extent by what range I define as the
linear part of the curve.
Can anyone provide advice on how I might determine my optimal
cutoff frequency? Is there a source of a more detailed explanation of the
method for determining optimal cutoff frequency?
Any assistance is greatly appreciated. As always I will post a summary of
the replies to the List.
Regards
Robert Newton
--
Robert Newton Phone Int+ 1 814
865 7107
Center for Sports Medicine Fax Int+ 1 814 865 7077
The Pennsylvania State University Email RUN1@PSU.EDU
117 Ann Building
University Park, PA 16802
United States of America
displacement time data in an attempt to determine the optimal cut-off
frequency for filtering the data prior to differentiation to provide
velocity and acceleration data. I have been smoothing the data using
a Butterworth 4th Order digital filter with cutoff frequencies ranging from
1 to 60Hz and subsequently calculating the residual as the mean square
difference between the filtered and raw data. Having plotted the residual
against cutoff frequency I have been attempting to determine the optimal
cutoff frequency by projecting the linear part of the resulting curve to
the vertical axis and then back to the curve to determine the cutoff. The
process is outlined on pages 41-43 of David Winter's book
"Biomechanics and Motor Control of Human Movement, 2nd Edition".
My problem is that the plot at the higher cutoff frequencies is not linear
but curvelinear and I an unable to determine over what range of cutoff
frequencies should I project my line from. The calculated optimal cutoff
frequency is affected to a great extent by what range I define as the
linear part of the curve.
Can anyone provide advice on how I might determine my optimal
cutoff frequency? Is there a source of a more detailed explanation of the
method for determining optimal cutoff frequency?
Any assistance is greatly appreciated. As always I will post a summary of
the replies to the List.
Regards
Robert Newton
--
Robert Newton Phone Int+ 1 814
865 7107
Center for Sports Medicine Fax Int+ 1 814 865 7077
The Pennsylvania State University Email RUN1@PSU.EDU
117 Ann Building
University Park, PA 16802
United States of America