Control; Control Actions; Proportional, Derivative And Integral Action Influence On The Controlled Process Feedback; Self-Tuning - gefran 3850T Use And Installation Manual

7.5.

Control

7.5.1.

Control actions

Control actions are divided into 3 categories:
• Proportional Action: action where the output contribu-
tion is proportional to the input deviation.
• Derivative Action: action where the output contribu-
tion is proportional to the input deviation variation rate.
• Integral Action: action where the output contribution is
proportional to the input deviation time integral.
Deviation is the difference between the value read by the
adjusted variable and the desired value.
Control actions serve to achieve the optimised regulation of
the controlled process at each stage.
7.5.1.1. Proportional, Derivative and Integral action
influence on the controlled process feedback
The controlled process feedback depends on the type of
control action set. Specifically:
• Increasing the Proportional Band reduces oscillations
but increases deviation.
• Decreasing the Proportional Band reduces deviation
but causes oscillations of the regulated variable (too
low values of the Proportional Band make the system
unstable).
• Increasing the Derivative Action, corresponding to an
increase in Derivative Time, reduces deviation and
avoids fluctuations up to a critical value of Derivative
Time beyond which the deviation increases and pro-
longed oscillations occur.
• Increasing the Integral Action, corresponding to a
decrease in Integral Time, tends to cancel the devia-
tion between the controlled variable and the setpoint
(setpoint).
• If the value of the Integral Time is too long (weak inte-
gral action) deviation may persist between the con-
trolled variable and the setpoint.
For more information about control actions, contact Gefran
customer service.
7.5.2.

Self-Tuning

Self-tuning is a simplified and automatic tuning mode, de-
pending on the state of the process.
The purpose of self-tuning activation is to calculate the op-
timal adjustment parameters at process startup. The vari-
able (i.e.: temperature) must be that which can be detected
at zero power (room temperature).
You can automatically activate tuning using the appropriate
key on page PID CFG > Base > 2.
The procedure is automatically done by optimizing the ap-
proach in relation to the actual temperature value.
Power is initially forced to 100%, up to half of the value
between PV and SP, and then to 0% to assess over-elonga-
tion of the system.
At the end of this procedure, the new parameters will be
memorised in all 10 PID groups:
• proportional band,
• integral and derivative times calculated for active
action (heat or cool). In the case of double action (heat
+ cool)
The active tuning condition is signalled by a LED.
80703B_MHW_2850T/3850T_02-2019_ENG
7. EXAMPLES AND APPLICATION NOTES
Warning! Self-Tuning is not applicable with an ON/
OFF control.
Single action example, PV less than SP / 4
7.5.3.

Cascade control

Two controllers are arranged in cascade when the output
signal from the first becomes the input signal in the second,
setpoint
PRIMARY
CONTROLLORE
CONTROLLER
PRIMARIO
IN1
PID1
PRIMARY
SENSORE
interferences disturbi
SENSOR
PRIMARIO
PROCESS
PROCESSO
which in turn sends a signal to the controller. The controller
that compares the controlled variable with the setpoint is
defined as primary, while the one that compares the value
of the variable using the signal from the primary controller is
defined as secondary.
The advantage of cascade control consists of faster control
of the value of the primary variable in response to distur-
bance of the secondary variable. Moreover, the primary
variable is less subject to deviations. The secondary con-
troller keeps the flow constant by changing it only on the
directions of the primary controller.
The cascade controller is used especially in very slow
processes. In such processes, in fact, the error is recovered
over a long period of time: when a problem enters the pro-
cess, the error occurs after a long time, so corrective action
does not start immediately.
After corrective action has started, you will still have to wait
a long time to see the result of the action itself.
Performing a cascade control consists in finding interme-
diate controlled variables that can act with quick corrective
action due to possible disturbances.
The two controllers (primary and secondary) are arranged
in cascade: each has its own process variable and only the
SECONDARY
CONTROLLORE
CONTROLLER
SECONDARIO
IN2
PID2
SECONDARY
SENSORE
ACTUATOR
ATTUATORE
SENSOR
SECONDARIO
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