Adaptive Control And Autotuning - Carel EVD evolution User Manual

Key:
CP compressor
C condenser
L liquid receiver
F dewatering fi lter
S liquid indicator
For the wiring, see paragraph 2.7 "General connection diagram".
Note: superheat control in a refrigerant circuit with SIAM scroll
compressor requires two probes for superheat control and two probes
downstream of the compressor for discharge superheat and discharge
temperature control. See par. 5.5.
PID parameters
Superheat control, as for any other mode that can be selected with the
"main control" parameter, is performed using PID control, which in its
simplest form is defi ned by the law:
1
∫
u(t)= K e(t) + e(t)dt + T
T
i
Key:
u(t) Valve position
e(t) Error
K Proportional gain
Note that regulation is calculated as the sum of three separate contributions:
proportional, integral and derivative.
•
the proportional action opens or closes the valve proportionally to
the variation in the superheat temperature. Thus the greater the K
(proportional gain) the higher the response speed of the valve. The
proportional action does not consider the superheat set point, but
rather only reacts to variations. Therefore if the superheat value does
not vary signifi cantly, the valve will essentially remain stationary and
the set point cannot be reached;
•
the integral action is linked to time and moves the valve in proportion
to the deviation of the superheat value from the set point. The greater
the deviations, the more intense the integral action; in addition, the
lower the value of T (integration time), the more intense the action
will be. The integration time, in summary, represents the intensity of
the reaction of the valve, especially when the superheat value is not
near the set point;
•
the derivative action is linked to the speed of variation of the superheat
value, that is, the gradient at which the superheat changes from instant
to instant. It tends to react to any sudden variations, bringing forward
the corrective action, and its intensity depends on the value of the
time Td (derivative time).
Parameter/description
CONTROL
Superheat set point
PID proport. gain
PID integration time
PID derivative time
See the "EEV system guide" +030220810 for further information on
calibrating PID control.
Note: when selecting the type of main control (both superheat
control and special modes), the PID control values suggested by CAREL
will be automatically set for each application.
Protector control parameters
See the chapter on "Protectors". Note that the protection thresholds are
set by the installer/manufacturer, while the times are automatically set
based on the PID control values suggested by CAREL for each application.
EEV electronic expansion valve
V solenoid valve
E evaporator
P pressure probe (transducer)
T temperature probe
de(t)
d
dt
Ti Integration time
Td Derivative time
Def. Min. Max. UOM
11 LowSH: t.hold 180 (320) K (°F)
15 0 800 -
150 0 1000 s
5 0 800 s
Tab. 5.c
Parameter/description
CONTROL
LowSH protection threshold
LowSH protection integration
time
LOP protection threshold
LOP protection integration time
MOP protection threshold
MOP protection integration time 20
ADVANCED
High Tcond threshold
High Tcond integration time

5.3 Adaptive control and autotuning

EVD evolution features two functions used to automatically optimise the
PID parameters for superheat control, useful in applications where there
are frequent variations in thermal load:
1.
automatic adaptive control: the function continuously evaluates the
eff ectiveness of superheat control and activates one or more
optimisation procedures accordingly;
2.
manual autotuning: this is activated by the user and involves just one
optimisation procedure.
Both procedures give new values to the PID superheat control and
protection function parameters:
-
PID: proportional gain;
-
PID: integration time;
-
PID: derivative time;
-
LowSH: low superheat integration time;
-
LOP: low evaporation temperature integration time;
-
MOP: high evaporation temperature integration time;
-
HiTcond: high condensing temperature integration time.
Given the highly variable dynamics of superheat control on diff erent units,
applications and valves, the theories on stability that adaptive control
and autotuning are based on are not always defi nitive. As a consequence,
the following procedure is suggested, in which each successive step is
performed if the previous has not given a positive outcome:
1) use the parameters recommended by CAREL to control the diff erent
units based on the values available for the "Main control" parameter;
2) use any parameters tested and calibrated manually based on laboratory
or fi eld experiences with the unit in question;
3) enable automatic adaptive control;
4) activate one or more manual autotuning procedures with the unit in
stable operating conditions if adaptive control generates the "Adaptive
control ineff ective" alarm.
Adaptive control
After having completed the commissioning procedure, to activate
adaptive control, set the parameter:
"Main control"= air-conditioner/chiller or showcase/cold room with
adaptive control.
Parameter/description
CONFIGURATION
Main control
...
air-conditioner/chiller or cabinet/cold
room with adaptive control
The activation status of the tuning procedure will be shown on the
standard display by the letter "T".
"EVD evolution" +0300005EN - rel. 3.1 - 25.07.2011
19
ENG
Def. Min. Max. UOM
5 -40 (-72) superh. K(°F)
set point.
15 0 800 s
-50 -60 (-76) MOP th- °C(°F)
reshold
0 0 800 s
50 LOP thre- 200 (392) °C(°F)
shold
0 800 s
80 -60 (-76) 200 (392) °C (°F)
20 0 800 s
Tab. 5.d
Def.
multiplexed cabinet/cold room
Tab. 5.e