7. DTC control system
Fig. 5 shows DTC control system, with a PID based
structure. The gas outlet temperature can be as close as
desired to wet temperature, not existing any restriction
due to solid temperature because it remains at wet temperature.
Decreasing the set point of the DTC controller
increases the dryer load, as well as the cylinder motor
intensity. Thus, there is a low limit because the intensity
should not go beyond 160 A. The gas inlet temperature
is controlled in a loop that manipulates the fuel feeding.
In both loops, the dead times are not great, related to
the characteristic time of the process. The main problem
for the PIDs tuning have been the highly non-linearity
of the process and the saturation of the actuators.
Fig. 6 shows the DTC controller behaviour. It can be
seen that during the whole period (more than 3 hours)
the controlled variable is maintained close to the set