Rockwell Automation DeviceLogix System User Manual - Manual (Page 32)

32 Publication RA-UM003B-EN-P - February 2010
Chapter 2 Navigate the Function Block Editor Interface
Filter Category
The Filter category includes a single instruction type: Low Pass Filter.
Low Pass Filter
The DeviceLogix Low Pass Filter (LPF) function block provides a filter
to attenuate input frequency above the cutoff frequency.
Let W
lag
stand for the value of the attribute WLag and T
sample
is the
value of the sampling period. Therefore, the sampling frequency f
s
is:
According to W
lag ,
the expected analog cutoff frequency f
p
is:
With the basic equation between the analog input frequency f and the
digital frequency
we can obtain the digital frequency
p
corresponding to f
p
The bilinear transformation method is applied to convert the analog
filter into the digital filter. The bilinear transformation is defined as
follows.
To adjust the frequency shift due to the bilinear transformation, you
must use the prewarping equation to calculate the prewarping analog
frequency. The prewarping equation is as follows.
Then the value of w
p
is taken as the real analog cutoff frequency and
substitute for w
lag
..
Ω
Ω