Pid For Process Control - Danfoss VLT 8000 AQUA series Operating Instructions Manual

PID for process control

The PID controller maintains a constant process
condition (pressure, temperature, flow, etc.) and
adjusts motor speed on the basis of a reference/
setpoint and the feedback signal.
A transmitter supplies the PID controller with a
feedback signal from the process to indicate its actual
state. The feedback signal varies with the process load.
This means that deviations occur between the
reference/setpoint and the actual process state. Such
deviations are evened out by the PID regulator, in
that it regulates the output frequency up or down
in relation to the deviation between the reference/
setpoint and the feedback signal.
The integral PID regulator in VLT 8000 AQUA units
have been optimised for use in water applications.
This means that a number of specialised functions
are available in VLT 8000 AQUA units.
Using the VLT 8000 AQUA, there is no need for
extra modules to be installed. For example, only
one required reference/setpoint and the handling
of feedback need to be programmed.
There is a built in option for connecting two
feed-back signals to the system.
Correction for voltage losses in long signal cables
can be carried out when using a transmitter
with a voltage output. This is done in parameter
group 300 Min./Max.scaling.
Feedback
The feedback signal must be connected to a
terminal on the frequency converter. Use the
list below to decide which terminal to use and
which parameters to program.
Feedback type Terminal
Pulse 33
Voltage 53, 54
Current 60
Bus feedback 1 68+69
Bus feedback 2 68+69
Please note that the feedback value in parameter
535/536 Bus feedback 1 and 2 can only be set via
serial communication (not via the control unit).
Furthermore, the minimum and maximum feedback
(parameters 413 and 414) must be set to a value in
the process unit that corresponds to the minimum
and maximum scaling value for signals connected
to the terminal. The process unit is selected in
parameter 415 Process units.
= factory setting. () = display text [] = value for use in communication via serial communication port
MG.83.A2.02 - VLT is a registered Danfoss trademark
Reference
In parameter 205 Maximum reference, Ref
a maximum reference that scales the sum of all
references, i.e. the resulting reference, can be set. The
minimum reference in parameter 204 indicates the
smallest value that the resulting reference can assume.
The reference range cannot exceed the feedback range.
If Preset references are required, set these in parameters
211 to 214 Preset reference. See Reference type.
See also Reference handling.
If a current signal is used as a feedback signal,
voltage can be used as analog reference. Use
the list below to decide which terminal to use
and which parameters to program.
Reference type
Pulse
Voltage
Current
Preset reference
Setpoints
Bus reference
Please note that the bus reference can only be
set via serial communication.
Inverse regulation
Normal regulation means that the motor speed
increases when the reference/setpoint is higher than
the feedback signal. If there is a need for inverse
regulation, in which the speed is reduced when
Parameters
307 the reference/setpoint is higher than the feedback
308, 309, 310 or
signal, inverse must be programmed in parameter
311, 312, 313
420 PID normal/inverse control.
314, 315, 316
535
Anti Windup
536
The process regulator is factory preset with an active
anti-windup function. This function ensures that
when either a frequency limit, current limit or voltage
limit is reached, the integrator will be initialised for
a frequency that corresponds to the present output
frequency. This avoids integration on a deviation
between the reference/setpoint and the actual state
of the process, the controller of which is not possible
by means of a speed change. This function can be
disabled in parameter 421 PID anti windup.
®
VLT 8000 AQUA
Terminal
17 or 29
53 or 54
60
68+69
NB!:
Terminals that are not in use may preferably
be set to No function [0].
,
MAX
Parameters
301 or 305
308, 309, 310 or
311, 312, 313
314, 315, 316
211, 212, 213, 214
418, 419
137