6. Identification of controlled and manipulated variables
One of the main problems to develop the control system
has been to determine a continuously measurable
variable clearly related to dryer production.
Analysing the dryer behaviour, the production grows
with the cylinder load. The intensity of the cylinder electrical
motor has shown as the best, between measurable
variables, to characterise the dryer load, and therefore, it
is well related to dryer production. The difference between
outlet dry and wet gas temperatures also reflects
the dryer load because the heat transfer increases with
it, lowering the temperature difference. Fig. 3 shows that
there is a good correlation within these variables.
Thus, cylinder motor intensity and the difference of
those temperatures have been chosen as controlled variables
in two different control systems tested, the Difference
of Temperatures Control (DTC) and the Intensity
Control (WC).
The inlet and outlet gas temperatures are measured
without difficulty and therefore, they can be used also
as controlled variables in a cascade-type control
configuration.
Both feed and fuel flow rates can be easily manipulated
by means of variable frequency drives changing
the screw motors speed.
Under manual control, the motor intensity shows two
fluctuations types, a high frequency one (coincident to
rotational speed), due to product build up on interior
walls, and a low frequency fluctuation, due to periodic
product accumulation in the cylinder followed by a large
outflow. For controlling purposes the instantaneous
intensity is time-averaged over a 2 min period to filter