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Revision 3 . . August 11, 2013 8:40 pm by Cmorley [tidbit about PID ff from Peter]
Revision 2 . . July 12, 2011 5:49 am by Jthornton
  

Difference (from prior major revision) (no other diffs)

Added: 19a20,58

From Peter on forum
http://www.linuxcnc.org/index.php/english/forum/39-pncconf/26860-tuning-values


The FF are feed forward values:

FF0 adds a portion of the command to the output

FF1 adds a portion the first derivative of the command to the output

FF2 adds a portion of the second derivative of the command to the output



I tend to think of feed forward terms being used to center the operating

point of the PID loop.


For example, if linuxcnc is controlling a velocity mode servo, almost

all tuning (on linuxCNC) will be done with FF1 and P (FF1 is velocity feed

forward if the PID command is position)


With a velocity mode servo the PID output is a

velocity command to the drive. This means the PID

output will be near full-scale when doing rapids.

If only P was used, there would have to be enough

position error to generate a near full scale signal when

moving at full speed. Practically what this means is that

the actual position will lag the commanded position

by an amount proportional to the full scale PID output / Pgain,

which is quite significant. FF1 is used here to "forward"

the commanded velocity to the PID output so that the

PID output will be set to the required value for the current

commanded velocity _before_ any P correction is applied,

centering the PIDs operating point around the current velocity



Another example is controlling a torque mode drive. In this case, FF2

(acceleration feed forward if the command is position) is used to

compensate for the system inertia.



No controlled system is a perfect velocity or torque mode device so often

a bit of FF2 can be used to tune velocity mode drives error during acceleration,

and a bit of FF1 can compensate for velocity dependent drag on a a torque mode system


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