Agriculture Reference
In-Depth Information
The main goal of ventilation regulation
systems is to avoid the interior air tempera-
ture exceeding the fixed threshold.
Secondary objectives are to dehumidify and
favour the input of CO
2
. Temperature,
humidity and CO
2
sensors are needed for
their management. They may be limited by
the rain or the wind.
The only function of the shading con-
trol system is to decrease radiation, nor-
mally to reduce the temperature at times of
high radiation load.
The supply of CO
2
is only practised
during the daytime, with intervention of the
CO
2
, radiation and vent-opening sensors.
Humidification tends to maintain the
hygrometry, using humidity and air temper-
ature sensors.
For the regulation of all these systems
(thermal screens, dehumidification systems)
several sensors can be used. The simplest
control systems use clocks.
circulation. Its functions are to measure
different parameters, perform calculations
with resident programs, and give activation
orders to existing equipment, to maintain
the regulated parameters within the desired
values (set points).
When sensors are monitored using
analogue technology, the signals must be
converted into digital information before
they can be interpreted by the computer.
For this, an analogue-digital converter
(ADC) is used. When a sensor generates a
measurement signal that is not interpret-
able by the ADC, an interface that adapts
the signal (to make it readable by the ADC)
is used. For instance, a solar radiation sen-
sor generates a potential difference, which
is proportional to the incoming radiation,
in the form of an analogue signal that is
converted into a digital signal by the ADC
converter in order to be interpreted by
the computer.
The activation orders of the computer
or output signals, at low tension (24 volts),
activate relays that operate the different cor-
rection equipment.
Until now, different computers per-
formed the fertigation and climate manage-
ment. Nowadays, the trend is to integrate
them, which allows for a better joint
management.
12.2.2
Digital control systems
Systems developed during the Second
World War enabled analogue technology,
which used electrical circuits to obtain
inputs (measurement of environmental
parameters) and calculate, automatically,
outputs, to control mechanisms and equip-
ment. The arrival of digital control systems,
which could manage more complex sys-
tems at lower cost, has helped digital con-
trol systems to supersede analogue control
systems.
A digital control system is basically
composed of: (i) the controller, that is, the
climate computer; (ii) the correction
equipment (heating, ventilation, etc.); and
(iii) sensors, to measure the different para-
meters to be regulated.
control computers
It is impossible to provide a full list of all
the possible functions of climate control
computers, because each user has specific
requirements. The ones detailed below are
the minimum required for a well-equipped
greenhouse.
The set points are generally different
during the day and the night, and can vary
even during the same period (day or night).
A clock can perform the day/night changes,
or changes can be triggered by measure-
ments of the radiation or by calculations
of sunrise and sunset (depending on the
latitude and date; i.e. the astronomical
clock).
The climate control computer controls
different processes to regulate, mainly,
temperature, humidity, light, CO
2
and air
