µRack
Cod. CAREL +03P220431 rel. 2.2 dated 10/02/12
20
Proportional and integral control (PI)
To minimise any deviations in stable operating conditions between the controlled value and the set point, typical of proportional control, a proportional plus integral
strategy (P+I) can be sued.
This strategy helps overcome situations of stalemate in which the working point remains steadily at a value other than the set point.
PI control adds the integral action to proportional control. This action, when a control error persists, has an increasing effect over time on the overall control action.
The parameter that defines the integral action is the integral time (r22).
The default value is 600 s (10 min). The integral time corresponds to the time taken by the integral action, with a constant error, to balance the proportional action.
The lower the integral time, the faster the response of the control.
For further information, refer to classic control theory.
N.B.: Make sure the integral time is not set too low, otherwise control may become unstable.
The following figure highlights the difference between the proportional control and proportional plus integral control (with inverter):
Proportional control
Propor Integral control
T
[s]
SP
RP
T
[s]
10 Volt
Min In
T
[s]
RP
SP
T
[s]
10 Volt
Min In
Fig. 5.e
Key:
RP
Pressure read
SP
Set point
T
Time
Min In
Minimum inverter output value
Dead band control
This control requires the setting of inverter set point, the inverter pressure differential for “dead band” control (parameter r21) and the “inverter ramp up time”
(parameter r28).
Three zones are defined: activation zone DOnZ, dead band NZ and deactivation zone DOffZ, in which the program behaves differently (see the figure).
In the activation zone DonZ, the fans are started as follows:
-
The inverter is activated as soon as there is demand, with a value no less than the minimum inverter opening MinIn;
-
The inverter output is increased according to the times set by parameter r23.
-
If the inverter output reaches 100%, the situation persists
In the dead band NZ, the inverter output does not undergo any variation.
In the deactivation zone DoffZ, the fans are stopped as follows:
-
The inverter output is progressively brought to the minimum value, according to the times set by parameter r24.
When reaching the minimum value, the fans are stopped.
Key:
InPress
Discharge pressure
B
Set point + differential
StpM
HP set point
DOnZ Activation
zone
DOffZ Deactivation
zone
NZ Dead
band
T [s]
Time
Inverter Inverter
status
NFan
Number of fans on
Fig. 5.f
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