Process Control Relevant Parameters - Danfoss VLT Midi Drive FC 280 Design Manual

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2.5.3 Process Control Relevant Parameters

Parameter
Parameter 7-20 Process CL Feedback 1 Resource Select from which source (analog or pulse input) the process PID gets its feedback.
Parameter 7-22 Process CL Feedback 2 Resource Optional: Determine if (and from where) the process PID gets an additional feedback
Parameter 7-30 Process PID Normal/ Inverse
Control
Parameter 7-31 Process PID Anti Windup
Parameter 7-32 Process PID Start Speed
Parameter 7-33 Process PID Proportional Gain
Parameter 7-34 Process PID Integral Time
Parameter 7-35 Process PID Differentiation Time Provides a gain proportional to the rate of feedback change. A setting of 0 disables the
Parameter 7-36 Process PID Diff. Gain Limit
Parameter 7-38 Process PID Feed Forward
Factor
•
Parameter 5-54 Pulse Filter Time Constant
#29 (Pulse term. 29)
•
Parameter 5-59 Pulse Filter Time Constant
#33 (Pulse term. 33)
•
Parameter 6-16 Terminal 53 Filter Time
Constant (Analog term 53)
•
Parameter 6-26 Terminal 54 Filter Time
Constant (Analog term. 54)
Table 2.9 Process Control Parameters
MG07B102
Design Guide
Description of function
signal. If an additional feedback source is selected, the 2 feedback signals are added
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 is lower than the reference. 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 be compensated for by a speed change. Press
[0] Off to disable this function.
In some applications, reaching the required speed/setpoint can take a long time. In such
applications, it may be an advantage to set a fixed motor speed from the frequency
converter before the process control is activated. Set a fixed motor speed by setting a
process PID start value (speed) in parameter 7-32 Process PID Start Speed.
The higher the value, the quicker the control. However, too large a value may lead to
oscillations.
Eliminates steady state speed error. A lower value means a quicker reaction. However, too
small a value may lead to oscillations.
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, use the
feed forward factor to achieve better dynamic performance of the process PID control.
If there are oscillations of the current/voltage feedback signal, use a low-pass filter to
dampen these oscillations. The pulse filter time constant represents the speed limit of the
ripples occurring on the feedback signal.
Example: If the low-pass filter has been set to 0.1 s, the limit speed is 10 RAD/s (the
reciprocal of 0.1 s), corresponding to (10/(2 x π))=1.6 Hz. This means that the filter
dampens all currents/voltages that vary by more than 1.6 oscillations per second. The
control is only carried out on a feedback signal that varies by a frequency (speed) of less
than 1.6 Hz.
The low-pass filter improves steady state performance, but selecting a too long filter time
deteriorates the dynamic performance of the process PID control.
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