KVFF
KVFF
is the velocity feed forward gain. In the absence of
velocity feed forward (
KVFF
= 0), the commanded velocity is
proportional to the position (following) error. This means that the
actual position will lag the commanded position by a value
proportional to the speed. The error will be smaller for larger
values of
KPP
.
The following table gives a feel for the following error
magnitude.
Speed (rpm)
KPP (Hz)
Following Error
(revolutions)
1000
10
0.27
2000
10
0.53
5000
10
1.33
1000
20
0.13
2000
20
0.27
5000
20
0.66
Note:
The following error can easily exceed one complete motor
revolution. In many electronic gearing applications, such
following errors are not acceptable (real gears don’t have
following errors!) Also, stepper systems don’t have such errors.
Feed forward takes advantage of the fact that the SC900 DSP
knows the frequency of the encoder or step inputs and hence
knows how fast the motor should be going at a given instant. All
or part of this velocity can be added to the velocity command to
reduce following error. If
KVFF
is set to 100 (%), then the
steady state following error reduces to zero.
MA950
4 - 9
