Rockwell Automation 20D PowerFlex 700S AC Drives with Phase II Control Reference Manual - Manual (Page 82)

82 Rockwell Automation Publication PFLEX-RM003E-EN-E - January 2011
Chapter 1 Detailed Drive Operation
Feed Forward Low Pass Filter
Parameter 730 [PLL LPFilter BW] specifies the bandwidth of the speed feed
forward low pass filter in units of radians/second. Setting this parameter to zero
(0) bypasses filtering.
One use of the filter is to better time the feed-forward and reference encoder
signals when parameter 728 [PLL Ext Spd Ref ] is linked to the master drive’s
speed reference (a master drive is one that controls the PLL encoder). In this case
set parameter 730 [PLL LPFilter BW] to the bandwidth of the master drive. This
will compensate for the drive’s delayed response from input reference to output
signal. (Master bandwidth can be approximated as the reciprocal of the small
signal step response to 63% output. Values ranging from 2…20 radians/second
would not be unreasonable for a master drive.)
ACOMP
When parameter 720 [PLL Control], bit 1 “Ext Vel In” = 0, feed forward is
selected from the “X to V” converter. Set parameter 720 [PLL Control], bit 2
“Trckng AComp” = 1 to provide acceleration compensation. Bit 2 of parameter
720 [PLL Control] should be set to 0 when the feed forward comes from an
external source, unless the arrival of the external input is perfectly timed.
Noise Filtering
Parameter 730 [PLL LPFilter BW] affects the level of filtering. Input tracking is
generally maintained regardless of this setting. However, in extreme cases an
abrupt input change can occur and the drive should quickly respond. There will
always be a trade off between noise and PLL high frequency response that must
be resolved by PLL bandwidth.
Encoder Placement Affects Noise
The reference encoder can be mounted anywhere, however it is best to choose a
shaft that is minimally disturbed. A stub shaft off of a relatively high kinetic
energy roll should be considered. The mechanical ratio that exists between where
the encoder is mounted and where the preferred mounting location is can be
factored into the Electronic Gear Ratio (EGR).
Virtual Encoder
The PLL’s output is generated by a virtual encoder calibrated in Counts Per
Revolution and a base RPM.