Pid For Process Regulation; Feedback; Reference (Set Point); Inverse Regulation - Danfoss VLT 6002-6011 Installation, Operation And Maintenance Manual

PID for Process Regulation

The built-in proportional, integral, derivative (PID) controller in the
drive maintains a constant condition (pressure, temperature, flow, etc.)
in the system. It adjusts motor speed based upon a reference and/or
setpoint and the feedback signal. The PID controller is activated by
setting parameter 100, Configuration, to Closed loop.
A transmitter supplies the PID controller with a feedback signal from the
process to indicate its current state. The type of feedback signal in use
depends upon the type of process.
This means that deviations that occur between the reference/setpoint
and the actual process state are sensed. Such deviations are
compensated for by the PID regulator by adjusting the output frequency
as required by the difference (or error) between the reference/setpoint
and the feedback signal.
The PID controller in the drive has been designed for use in HVAC
applications and to perform a number of specialized functions.
Previously, it was necessary for a building management system to
handle these special functions by installing extra modules to program
the system.
With the VLT 6000, there is no need for extra modules to be installed.
Only the reference and/or setpoint and a selected feedback signal need
to be programmed. An advanced feature of the VLT 6000 is the drive's
ability to accept two feedback signals, making two-zone regulation
possible.
Voltage drop in long control cables can be compensated for by using
the scaling parameters for the analog inputs.

Feedback

The feedback signal must be connected to the drive according to the
table below, depending on the type of feedback signal and the settings
of the associated parameters listed.
Feedback type Terminal
Pulse 33
Voltage 53 or 54
Current 60
Bus feedback 1* 68 and 69
Bus feedback 2* 68 and 69
* The bus feedbacks can only be set by serial communication.
If two analog feedback signals are to be used, they both must be
voltage signals.
The Minimum Feedback and Maximum Feedback, parameters 413
and 414, must be set for the feedback signals. The type of process unit
for the signals is selected in parameter 415, Process Units.
64

Reference (Set Point)

In parameter 205, Maximum Reference, the maximum value of the sum
of all reference signals is set.
The Minimum Reference, set in parameter 204, indicates the smallest
value that the resulting reference can assume.
The reference range cannot exceed the feedback range.
If multiple Preset References are required, use multiple setups or set
them in parameters 211 to 214 Preset Reference. See Reference
Handling.
If a current signal is used as a feedback signal, voltage can be used as
an analog reference. Use the table below to decide which terminal to
use and which parameters to program.
Reference type
Pulse
Voltage
Current
Preset reference
Setpoints
Bus reference *
* The bus reference can only be set by serial communication.
Terminals that are not used should be set to No function.

Inverse Regulation

Normal regulation means that the motor speed increases when the
reference/setpoint is higher than the feedback signal. Inverse regula-
tion means that the motor speed decreases when the reference/setpoint
is higher than the feedback signal. If there is a need for inverse
regulation, inverse must be programmed in parameter 420, PID
Normal/Inverse Control.

Anti-Windup

The process controller is factory set with an active anti-windup function.
Parameters
This function ensures that when either a frequency limit or a current limit
307
is reached, the integrator will be reset to zero. If the frequency limit or
308, 309, 310 or
current limit is cleared, the integrator will turn on again. This function
311, 312, 313
can be disabled in parameter 421, PID Anti-windup.
314, 315, 316
537

Start-up Conditions

538
In some applications, the optimum setting of the process regulator takes
a long time to reach. In such cases, a start-up frequency can be
entered in parameter 422, PID Start-up Frequency. The drive will
ramp directly to this frequency before PID control begins. This avoids
control by deviation between the reference/setpoint and the actual state
of the process during acceleration. It also avoids overshoot of the
process requirement during acceleration due to lag in the feedback
signal.
Terminal Parameters
17 or 29 301 or 305
53 or 54 308, 309, 310 or
311, 312, 313
60 314, 315, 316
211, 212, 213, 214
418, 419
68 and 69