Agriculture Reference
In-Depth Information
the vents in the previous ventilation exam-
ple) is proportional to the difference between
the average value and the set point value
(temperature in the ventilation example).
In a PI the amplitude of the action is
proportional to the integral of the differ-
ences between the average value and the set
point. It eliminates the deviations step by
step (Fig. 12.2).
In a similar way, the PI control is used
in thermosyphon heating systems. If the
interior temperature control is not linked to
the external climate conditions (tempera-
ture and wind velocity), the control oper-
ates in feedback mode, so it only acts when
the interior temperature changes and
induces, after checking the set point, activa-
tion of the equipment if applicable. If, on
the contrary, the alteration of the external
climate parameters influences
a priori
the
control (before it affects the internal tem-
perature) the control operates in 'feedfor-
ward' mode. For example, a heating system
by feedforward control can start or increase
the energy supply induced by an increase
in the wind velocity (which contributes to
the cooling of the greenhouse), although the
Proportional regulation
In the ventilation process of high temperature
or high humidity, when the temperature or
the humidity reaches the set point the vent
opening is operated. If this opening is oper-
ated proportionally to the measured tempera-
ture or humidity excesses, in relation to the
threshold value, a proportional regulation is
applied. Thus, if the difference is small, the
vents will open a little and if the difference is
very large they will open 100%.
The band of proportionality opening
must be defined. If the band is 6°C and the set
point temperature is 20°C, the vents will open
50% when the greenhouse temperature is
23°C, and will open 100% when it is 26°C.
These band values may be increased or
decreased, depending on the external tem-
perature and wind velocity. Proportional reg-
ulation is also used in thermosiphon heating
systems although in a more complex way.
Proportional integrated control (PI)
In a proportional controller (P) the ampli-
tude of the action (the percentage opening of
(a)
Real
temperature
Proportional
control (P)
Set point
Time
(b)
Real
temperature
Proportional
integrated
control (PI)
Set point
Time
Fig. 12.2.
Graphical representation of the performance of a good ventilation control system depending
on the temperature. (a) Proportional control (P); (b) proportional integrated control (PI).
