88
Appendix C: Tuning Your System
Figure C-6. Insufficient Velocity Feed-Forward
C.2.5 Acceleration Feed-Forward (K
a
)
Acceleration feed-forward is used with torque (current) controlled servos. Systems with high
inertial loads need more motor current to accelerate or decelerate than systems with light loads.
The Acceleration Feed-Forward term causes the controller to increase the motor control signals
during periods of acceleration and deceleration.
C.2.6 Integration Limit
The integration of position errors is limited to a fixed DAC output. This prevents the integrator
from “windup” in the case of high static friction.
Note that the integration term can be set to active always or active when at rest (command
velocity = 0) only.
C.2.7 Offset (K
o
)
The offset term compensates for small variations in controller DAC outputs and amplifier
offsets. An internal calibration (transparent to the user) is performed at the factory so that when
the offset is zero the analog or pulse output is also zero.
If necessary, the CONFIG program can be used to re-calibrate the analog and step pulse output.
C.2.8 Shift
The shift parameter is used to calculate the overall scale factor K
R
(K
R
= 2
shift
). The overall scale
factor increases the accuracy of the other tuning parameters. Decreasing the shift by 1 will
divide the equation for O
n
by 2. In order to get the same voltage output from the PID, the gains
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com
