37
ENG
ir33 universale +030220801 - rel. 2.1 - 21.06.2011
6. CONTROL
ON/OFF and PID control
The controller can operate with two types of control:
• ON/OFF (proportional), in which the actuator either operates at full
power or is o . This is a simple control mode that in certain cases can
achieve satisfying results;
• PID, useful for systems in which the response of the controlled value
compared to the changeable value does allow to eliminate the error
in steady operation and improve the regulation. The changeable value
becomes an analogue value that continuously varies between 0 and 100%.
In PID control, the proportional band coincides with the di erential
(parameters P1/P2).
6.1 Type of control (parameter c32)
Par. Description Def Min Max UM
c5 Type control
0=ON/OFF(proportional)
1=Proportional+Integral+Derivative (PID)
001-
Tab. 6.a
This parameter is used to set the most suitable type of control for the
process in question.
With PID, e ective control means the controlled value coincides with
the set point or falls within the dead zone; in these conditions, a
series of outputs may be active even if not envisaged in the original
control diagram. This is the most evident e ect of the integral factor.
PID control, before being applied, requires proportional control only
without swings and with good stability in the di erentials: only
when there is stable P control can PID guarantee maximum e ectiveness;
6.2 ti_PID, td_PID (parameters c62,c63,
d62,d63)
These are the PID parameters to be set for the application
Par. Description Def Min Max UoM
c62 ti_PID1 600 0 999 s
c63 td_PID1 0 0 999 s
d62 ti_PID2 600 0 999 s
d63 td_PID2 0 0 999 s
Tab. 6.b
The table below shows the probe used by PID1 and PID2 based on the
setting of c19.
c19 PID1
(dependence=1)
PID2
(dependence = 2)
1 B1-B2 B1
7 B1 (circuit 1) B2 (circuit 2)
8 max(B1, B2) B1
9 min(B1, B2) B1
0, 2, 3, 4, 5, 6, 10, 11 B1 B1
Tab. 6.c
For the explanation of operation of control based on the setting of
c19, see par. 6.5.
To eliminate the e ect of the integral and derivative factors, set the
respective parameters ti and td=0
Setting td=0 and ti ≠ 0 achieves P+I operation, widely used for
controlling environments in which the temperature does not have
considerable variations.
To eliminate the error in steady operation, PI control can be
implemented, as the integral factor reduces the average value of the
error. Nonetheless, a high impact of this factor (remember that it
contributes in an inversely proportional way to the time ‘ti’) may increase
temperature swings, overshoots and the time taken for the controlled
variable to increase and decrease, bringing instability.
To resolve such overshoots due to the use of the integral time, the
derivative factor can be introduced, which acts as a damper to the
swings. Nonetheless, needlessly increasing the derivative factor
(increasing the time ‘td’) increases the time taken for the controlled
variable to increase and decrease and can also cause system instability.
The derivative factor however has no a ect whatsoever on the error in
steady operation.
6.3 Auto-Tuning (parameter c64)
The Auto-Tuning function is incompatible with independent
operation (c19=7).
The controller leaves the factory with default settings of the PID
parameters; these allow standard PID control, but are not optimised for
the system that IR33 controls. Consequently, the Auto-Tuning procedure
can be used to ne-tune the 3 parameters involved, so as to ensure
control that is optimised for the system where it is installed: di erent
systems, with di erent dynamics, will generate parameters that di er
greatly.
Auto-Tuning includes two operating procedures:
• Tuning the controller when commissioning the system.
• Fine-tuning the controller with parameters that have already
been tuned, during normal operation.
In both modes, the control rst needs to be programmed setting the
following parameters:
c0 =1 or 2, that is, “direct” or “reverse” control;
c5 =1, that is, PID control enabled;
c64 =1, that is, Auto-Tuning enabled;
St1= working set point.
Tuning the controller when commissioning the system.
This procedure is performed when commissioning the system, and
involves an initial tuning of the PID control parameters to analyse
the dynamics of the overall installation; the information acquired is
indispensable for both this procedure and any further tuning operations
performed.
During commissioning, the system is in a stationary state, that is, it is
not powered and is in thermal balance at room temperature; this state
must be maintained when programming the controller before starting
the Auto-Tuning procedure. The controller must be programmed by
setting the parameters speci ed previously, making sure to avoid starting
to control the loads and thus altering the state of the system (that is,
increasing or decreasing the temperature). This can be achieved by not
connecting the control outputs to the loads or keeping the loads o (not
powered). Once programmed, the controller must be switched o , if
necessary the connections of the outputs to the loads must be restored
and nally power connected to the entire system: controller and unit.
The controller will then start the Auto-Tuning procedure, identi ed by the
TUNING icon ashing on the display, performing a preliminary check on
the starting conditions, and assessing their suitability, that is, for a system
in “direct” mode the starting temperature measured by the control probe
must be:
-higher than the set point;
-more than 5°C from the set point;
for a system in “reverse” mode, the starting temperature measured by the
control probe must be:
-lower than the set point;
-more than 5°C from the set point.
If the starting conditions are not suitable, the procedure will be not
be started and the controller will show the corresponding alarm “E14”;
the controller will remain in this status without perform any operation,
awaiting a reset or until switched o and on again. The procedure can
be repeated to check whether the starting conditions have changed and
Auto-Tuning can start. If on the other hand the starting conditions are
suitable, the controller will start a series of operations that modify the
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