Agriculture Reference
In-Depth Information
interior temperature has not yet decreased
(which will happen after a certain time). In
these cases, the rule or model that relates
the wind velocity with the later predictable
greenhouse air temperature changes must
be pre-fixed.
The PID control improves the perform-
ance of the PI control.
In the control process of a certain
parameter, the control system starts the
actuators or equipment when it must cor-
rect the values of the parameter. The values
of the parameter are still periodically meas-
ured and this information allows, if neces-
sary, to correct the actions. In this way, the
system can correct the control actions by
means of 'feedback' control.
When a disturbance occurs, and its
effect on the parameter being controlled is
known, immediate action can take place,
preventatively. This action is the 'feedfor-
ward' control, as already detailed.
Frequently, in the management of the
process both feedback and feedforward
intervene simultaneously. For instance,
when the temperature of a greenhouse
exceeds the set point, due to a radiation
increase, the computer opens the vent (feed-
back). If later the wind velocity or the exter-
nal temperature increases, the control
system can adjust the vent opening in
advance (feedforward).
Another type of control configuration
is the cascade configuration that is used in
complex processes.
12.1.7
Models
Introduction
A model is a simplified representation of a
system or one of its parts. The greenhouse,
the crop and its management constitute a
system. Normally, a model is represented by
a number of mathematical equations.
There are a large number of model
types. A
static model
is a set of equations
that relate several aspects such as, for
instance, heat losses, or ventilation, that
occur at a time when, essentially, the sys-
tem is balanced; thus, a static model can be
considered a
steady-state model
. In these
models the equations are based on physical
laws,
so
they
are
called
mechanistic
models
.
A
dynamic model
incorporates the time
variable. These models are necessary when
a process whose response is slow is repre-
sented, such as the heating of the soil. They
are
stochastic models
.
The term
heuristic
or
stochastic
refers
to the mediums used in the resolution of the
models, thus, heuristic models are solved
by exploration or by means of trial and error,
whereas stochastic models are solved using
statistical methods.
The feedforward control systems use
models, which determine the predictable
effects of a disturbance in the regulated
process and, preventatively, adjust the set
points to this new situation.
In greenhouses, two groups of models
can be distinguished: (i)
physical
models,
which focus on the greenhouse microcli-
mate as a function of the external climate;
and (ii)
physiological
models that focus on
the plants and their relations with the green-
house microclimate.
Simulation
models can be of any kind,
from the simplest to the most complex, and
are of great use, if they are well conceived
and validated, to simulate several real situa-
tions at a low cost. A very simple example,
in Mediterranean greenhouses, is the simu-
lation model of transmissivity to solar radi-
ation (Soriano
et al
., 2004b). This has been
of great use in designing new low-cost
greenhouse structures that are more efficient
12.1.6
Selection of the type of
automatic control
In the simplest systems, non-progressive
controllers (on/off) can be used. To select
the type of progressive controller, it is rec-
ommended to choose the P type (simple
proportional control) whenever possible.
If the set points are not to be exceeded, the
PI controllers must be used. When the
speed of the process requires it, PID must
be used.
Once the type of controller has been
chosen, the management value of the param-
eters to control must be selected.
