Agriculture Reference
In-Depth Information
12
Climate Management - Mechanization
12.1
Regulation and Control
Systems
between the measured and the set point val-
ues of the parameter. In this example, the
motors opening the vents would start oper-
ating, opening the vents to ventilate and
approximate the air temperature of the
greenhouse to the set point (23°C).
12.1.1
Introduction
The regulation of a process, in a physical
system, calls upon a set of dedicated tech-
niques and materials to ensure that, under
all circumstances, the physical quantity to
be regulated equals a desired value, which
is called the set point (Urban, 1997a).
The general principle of regulation is as
follows:
12.1.2
Input-output systems
In classical control, the processes to be reg-
ulated are considered as input-output sys-
tems. It may occur that there is more than
one input or output (Fig. 12.1). The inputs
can be of two types: (i) control inputs; and
(ii) exogenous inputs (or disturbances).
Figure 12.1 schematizes an input-
output system (Fig. 12.1a), an input-output
system with a disturbance (Fig. 12.1b), and
the inputs and outputs for the climate con-
trol of a greenhouse (Fig. 12.1c). In this last
case, CO
2
enrichment, heat supply and vent
opening for ventilation are considered as
inputs of the system. The disturbances are
the outside temperature, the outside wind
direction and velocity, and the external glo-
bal radiation, humidity and CO
2
. The out-
puts are the inside temperature, humidity
and CO
2
. Considering radiation as a distur-
bance, even though it is essential for photo-
synthesis, is due to the fact that it is not a
value that can be controlled by the user.
1.
The value of the parameter to be regu-
lated is quantified, by direct measurement
or by calculation. For instance, to regulate
the opening of the vents as a function of the
greenhouse air temperature, the air temper-
ature is measured, which happens to be
25°C.
2.
This measured value is compared with
the parameter's set point, which can be pre-
defined by the user or be the result of a
calculation or the application of a pre-
established rule. The air temperature set
point value, in this example, can be fixed in
23°C, which is lower than the measured
value (25°C).
3.
Finally, by means of one or several actu-
ators, one or more pieces of equipment
begin to operate, to decrease the difference
