Agriculture Reference
In-Depth Information
(temperature, humidity, CO 2 ) close to pre-
established set point values, respecting
certain rules (absolute or conditional pro-
hibitions, priorities, time delays) imposed
by the user. The climate computer manages
the climate.
We can distinguish several levels of
climate management:
The 'on/off' mode is usual in dynamic
ventilation, CO 2 injection, humidification,
shading and hot air heating.
A disadvantage of the 'on/off' mode is
the frequent starts and stops around the
set point value. There are three ways to
avoid it:
1. Using a time delay: After the equipment
starts, it cannot stop until a certain mini-
mum time has lapsed. It is used, for
instance, in air heating. Once the equip-
ment starts (for example, because the air
temperature is at 14°C, lower than the
fixed set point of 15°C) it has to operate,
for instance, for 5 min before stopping,
although the set point is reached again
before this period ends. Therefore, the air
temperature will exceed the set point value,
delaying the next start.
2. Using a dead zone: Around the set point
value a dead zone is fixed ( x ), so the regu-
lated equipment for a certain set point ( c )
starts when the value ( c x ) is reached and
stops when ( c + x ) is exceeded. In this way
frequent starts and stops can be avoided. In
the previous example, we could fix a set
point value ( c ) at 15°C, and the dead zone
( x ) at 1°C, so the system would start at 14°C
and stop at 16°C.
3. Using average values: These are used
for those parameters that can change a
great deal, such as wind velocity or the
light. As average values are used as set
points, the variability is highly cush-
ioned. For instance, as the wind velocity
oscillates a lot because the wind is fre-
quently gusty, the average value of the
measurements of a set period is used instead
of the last instantaneous air velocity
measurement.
Level 1
: (base level) The time scale is
very short (about 1 min). It excludes
processing of the information. Most of
the actual climate control computers
employ
this
level
of
management
nowadays.
Level 2
: The time scale is of the order
of 1 h or a whole day. The objective is
the management of the physiological
functions involved in the growth and
development of the plants in the
short term (photosynthesis, transpira-
tion). It involves the use of models.
Level 3
: The time scale is longer than
1 day. This level is the bio-economic
optimization and strategic decision
support. It allows solutions to be
obtained that are close to the economic
optimum, in each case. The realization
of level 3 is ideal and has not yet been
achieved in practice.
12.1.5 Types of controllers
The controllers can be classified depending
on the type of regulation, which is the way
in which the correction calculation can
be made.
There are two main types of regulators:
(i) non-progressive controllers that only
regulate fixed positions of the controlled
device; and (ii) progressive controllers that
control any position.
Progressive controllers
In the progressive type of controllers, the
operation of the equipment is modulated to
maintain the parameter values to be control-
led inside the interval of pre-established set
point values.
The most common progressive control-
lers are: (i) the proportional control (P);
(ii) the proportional integrated control (PI);
and (iii) the proportional integrated deriva-
tive control (PID).
Non-progressive controllers
In the 'on/off' type, the actuator can only
take two positions: on or off. Mechanical
ventilation, for instance, starts if the interior
temperature exceeds 23°C and stops if it
decreases below 20°C, pre-fixed set values.
A set point value can be used for a 'dead
zone', detailed later.
 
 
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