User guide

Chapter 6. Following 215
The bandwidth of the low-pass filter is controlled with the FFILT command:
FFILT Setting Low pass Filter Bandwidth
Ø ∞ (no filtering) – default setting
1 120 Hz
2 80 Hz
3 50 Hz
4 20 Hz
NOTE
Increasing the FFILT command value increases the filtering effect (lowers the bandwidth),
but at the expense of increasing the phase tracking error (phase lag) of the slave axis. For
more information on phase tracking, refer to Factors Affecting Following Accuracy below.
When considering whether or how much master position filtering to use, consider the
application requirement itself. The application requirements related to filtering can be
categorized into these three types:
Type I: If an application requires smooth motion but also high slave tracking accuracy,
then a heavy filtering should not be used. It should not be used because it may
introduce too much phase lag, although the motion may be smooth. In other
words, the master axis in the first place should produce very smooth motion and
low sensor measurement noise such that a higher level of master filtering is not
needed.
Type II: If slave tracking error is not critical but smooth slave axis motion is desired, then
you can use a higher level of master filtering to deal with sensor noise or master
vibration problems.
Type III: If it is determined that under certain dynamic conditions the master position's
oscillatory measurement is purely caused by its vibration motion (noise is
insignificant), and it is necessary for the slave to follow such motion, then the
filter command should not be used or only use the highest bandwidth (FFILT1).
Following Error
As soon as an axis becomes configured as a slave, the slave's position command is
continuously updated and maintained. At each update, the position command is calculated
from the current master position and velocity, and the current ratio or velocity of the slave.
Steppers only: This continuously updated position command is used as the target position
for the Dynamic Position Maintenance feature, described on page 216.
Whenever the commanded position is not equal to the actual slave position, a Following error
exists. This error, if any, may be positive or negative, depending on both the reason for the
error and the direction of slave travel. Following error is defined as the difference between the
commanded position and the actual position.
Following Error = Commanded position - Actual position
If the slave is traveling in the positive direction and the actual position lags the commanded
position, the error will be positive. If the slave is traveling in the negative direction and the
actual position lags the commanded position, the error will be negative. This error is always
monitored, and may be read into a numeric variable (VAR) at any time using the PER
command. The error value in slave steps is scaled by SCLD for the axis. This value may be
used for subsequent decision making, or simply storing the error corresponding to some other
event.