CHAPTER 5.0 Functional Overview
5-42
CDS7324 (FORMERLY LSF-0819)
Rev. A
INSTALLATION & USER’S MANUAL
5.11.2.5
Velocity feedback filter
A low-pass first order filter is also included on the motor feedback velocity. The feedback velocity is held in the
parameter
velocity_actual
(1151), with the low-pass filtered velocity held in a parameter called
velocity_filtered
(1165). It is this filtered velocity variable that is used to close the velocity loop.
The filter is not a Butterworth, but instead, is a simple Euler approximation filter, characterised by two filter
coefficients. The filter cut-off is specified with a filter cut-off factor (FCF), held in a parameter called
velocity_filter_cutoff_factor
velocity_filter_cutoff_factor
(1164). The two filter coefficients (a
1
and b
0
) are
computed in the embedded software each time the user writes to the cut-off factor
a
1
= 1 / (1 + 2*
π
*
velocity_filter_cutoff_factor
);
b
0
= 2*
π
*
velocity_filter_cutoff_factor
/ (1 + 2*
π
*
velocity_filter_cutoff_factor
);
Setting
velocity_filter_cutoff_factor
to 0.5 sets the filter cut-off to half the velocity loop sample rate. Hence, the
default speed cut-off factor of 0.25 will set the filter cut-off to 625Hz, when the velocity sample rate is 2.5KHz.
