Rockwell Automation Allen-Bradley PowerFlex 700S - Nomenclature; Low Pass Filter; Second Order Low Pass Filter

Rockwell Automation Publication PFLEX-RM003E-EN-E - January 2011 49
Detailed Drive Operation Chapter 1
Nomenclature
Low Pass Filter
A low pass filter is designed to pass low frequencies and attenuate high
frequencies. The break point between high and low is called the cut off
frequency.
Figure 7 - Bode Plot Low Pass Filter (ω
co
= 10 radians/second)
The Process Control Loop has a low pass filter immediately after the error signal.
The break frequency is set by parameter 184 [PI Lpass Filt BW]. The filter is used
to eliminate unwanted noise in the feedback. Typical range is between 10
radians/second to 50 radians/second.
Second Order Low Pass Filter
A second order low pass filter is similar to a low pass filter, however the
magnitude rolls off twice as fast as a first order low pass filter. Also the phase shift
of a second order filter is from 0 to 180° compared to 0 to 90° of a first order filter.
Symbol Description of Symbol Units
s Laplace Operator
ω Frequency radians/sec
ω
co
Cut-off Frequency radians/sec
Bode Diagram
Magnitude (dB)Phase (deg)
Frequency (rad/sec)
Frequency
0
-5
-10
-15
0
-30
-60
-90
10
0
10
1
10
2