Danfoss VLT Design Manual page 23

Introduction
2.8.5 Feedback Handling
2 2
Illustration 2.15 Block Diagram of Feedback Signal Processing
Feedback handling can be configured to work with
applications requiring advanced control, such as multiple
setpoints and multiple feedbacks. Three types of control
are common.
Single Zone, Single Setpoint
Single Zone Single Setpoint is a basic configuration.
Setpoint 1 is added to any other reference (if any, see
Reference Handling) and the feedback signal is selected
using 20-20 Feedback Function.
Multi Zone, Single Setpoint
Multi Zone Single Setpoint uses two or three feedback
sensors but only one setpoint. The feedbacks can be
added, subtracted (only feedback 1 and 2) or averaged. In
addition, the maximum or minimum value may be used.
Setpoint 1 is used exclusively in this configuration.
If [13] Multi Setpoint Min is selected, the setpoint/feedback
pair with the largest difference controls the speed of the
frequency converter.[14] Multi Setpoint Maximum attempts
to keep all zones at or below their respective setpoints,
while [13] Multi Setpoint Min attempts to keep all zones at
or above their respective setpoints.
Example
A two zone two setpoint application Zone 1 setpoint is 15
bar and the feedback is 5.5 bar. Zone 2 setpoint is 4.4 bar
and the feedback is 4.6 bar. If [14] Multi Setpoint Max is
selected, Zone 1's setpoint and feedback are sent to the
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VLT Refrigeration Drive Design Guide
PID controller, since this has the smaller difference
(feedback is higher than setpoint, resulting in a negative
difference). If [13] Multi Setpoint Min is selected, Zone 2's
setpoint and feedback is sent to the PID controller, since
this has the larger difference (feedback is lower than
setpoint, resulting in a positive difference).
2.8.6 Feedback Conversion
In some applications it may be useful to convert the
feedback signal. One example of this is using a pressure
signal to provide flow feedback. Since the square root of
pressure is proportional to flow, the square root of the
pressure signal yields a value proportional to the flow. This
is shown in Illustration 2.16.
Illustration 2.16 Feedback Conversion
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