Danfoss VLT AutomationDrive FC 301 Series Design Manual page 30

Basic Operating Principles
3 3
Figure 3.9 Process PID Control Diagram
Table 3.5 sums up the characteristics that can be set up for the process control.
Parameter
7-20 Process CL Feedback 1 Resource
7-22 Process CL Feedback 2 Resource
7-30 Process PID Normal/Inverse Control
7-31 Process PID Anti Windup
7-32 Process PID Controller Start Value
7-33 Process PID Proportional Gain
7-34 Process PID Integral Time
7-35 Process PID Differentiation Time
7-36 Process PID Differentiation Gain Limit
7-38 Process PID Feed Forward Factor
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Design Guide
Description of function
Select from which source (i.e., analog or pulse input) the Process PID should receive its
feedback
Optional: Determine if (and from where) the process PID should get an additional
feedback signal. If an additional feedback source is selected, the two feedback signals are
added together before being used in the Process PID control.
Under [0] Normal operation, the Process Control responds with an increase of the motor
speed if the feedback falls lower than the reference. In the same situation, but under [1]
Inverse operation, the Process Control responds with a decreasing motor speed instead.
The anti-windup function ensures that when either a frequency limit or a torque limit is
reached, the integrator is set to a gain that corresponds to the actual frequency. This
avoids integrating on an error that cannot in any case be compensated for with a speed
change. This function can be disabled by selecting [0] Off.
In some applications, reaching the required speed/set point can take a very long time. In
such applications, it might be an advantage to set a fixed motor speed from the
adjustable frequency drive before the process control is activated. This is done by setting
a Process PID Start Value (speed) in 7-32 Process PID Controller Start Value.
The higher the value, the quicker the control. However, a value that is too large may lead
to oscillations.
Eliminates steady state speed error. Lower value means quick reaction. However, a value
that is too small may lead to oscillations.
Provides a gain proportional to the rate of change of the feedback. A setting of zero
disables the differentiator.
If there are quick changes in reference or feedback in a given application - which means
that the error changes swiftly - the differentiator may soon become too dominant. This is
because it reacts to changes in the error. The quicker the error changes, the stronger the
differentiator gain is. The differentiator gain can thus be limited to allow setting of the
reasonable differentiation time for slow changes.
In applications where there is a good (and approximately linear) correlation between the
process reference and the motor speed necessary for obtaining that reference, the feed
forward factor can be used to achieve better dynamic performance of the process PID
control.
Rev. 2014-04-04 All rights reserved. MG33BF22