CONTROL AND SUPERVISION
FOR AN INDUSTRIAL GRAIN DRYER
Clemente Cárdenas, Eduardo J. Moya, David García and Oscar Calvo
Fundación CARTIF, Parque Tecnológico de Boecillo. Parcela 205. 47151 Boecillo, Valladolid, Spain
Keywords: Supervision, Control, Industrial Process, PLC, SCADA, Profibus, PID.
Abstract: Automation and control of processes in a food industry is a very important aim. The main reasons are:
guaranteeing a better quality of the final product, reducing cost time and improving the use of the raw
materials. Specifically, drying and storage grain industries have plants which, in many cases, are out of
phase. Besides, they are controlled by machine operators. Our work has consisted in developing a total and
supervision automated system to control most of the processes. A first step has been to automate four cereal
dryers in order to collect data. Subsequently, a control has been designed to get a constant value of moisture
of the grain. At the same time, these data have been used to obtain a total traceability of the process.
1 INTRODUCTION
In most cereal drying industries it is very important
to store the final product in optimal conditions along
time in order to achieve a good preservation.
Combination of several measures is necessary:
• Grain cleaning and sorting, avoiding any
undesired product or seed.
• Drying until a moisture level is reached, to
guarantee the correct preservation.
• Storing temperature Control during all the time
that the product remains in the facilities.
In general, once recollected, grains don’t have a
suitable degree of humidity and temperature to be
stored in silos for a long period of time. That is why
it is necessary to increase the temperature in order to
reduce humidity, making the drying a process of
great relevance. Therefore, supervision and
automation offer the operator the necessary tools to
control the drying process accurately, using
historical and real-time process performance
information.
Improving control enhances consistency and
saves energy by ensuring key process variables are
more stable. Processes may also be operated closer
to optimum values or constraints.
Process automation is not innovative, but if
supervision and control solutions are customized, as
in this case, we can deduce, then, that we are
innovating.
In the following sections we describe an example of
control of such processes.
2 DRYING AND STORAGE
PROCESSES
Basically, the cereal drying process consists in
passing a hot air current through the product, in
order to reduce the moisture inside the grain.
There are several factors to take into account from
the point of view of the process and also from that of
the product:
• The product can have different humidity
percentages.
• Moisture reduction depends on each type of
grain.
• Each product has a temperature upper limit
and a humidity lower limit to consider .
• The goal is to achieve a maximum
performance in Tons/hour, as well as a
minimum energetic consumption.
A horizontal grain dryer consists of a perforated
metal sheet connected to a source of heated forced
air supplied by a diesel or gas burner. The grain
conduit has upper and lower ends to receive and
discharge, respectively, a quantity of grain to be
dried by heat conveyed to the grain through the
perforated sheet. Rollers with an agitator keep grain
moving downward into the dryer. It is also necessary
405
Cárdenas C., Moya E., García D. and Calvo O. (2009).
CONTROL AND SUPERVISION FOR AN INDUSTRIAL GRAIN DRYER.
In Proceedings of the 6th International Conference on Informatics in Control, Automation and Robotics - Robotics and Automation, pages 405-408
DOI: 10.5220/0002192904050408
Copyright
c
 SciTePress

that several extractors ease the ejection of humid air
out of dryer.
The inside of a horizontal dryer and a detail of
the rollers are showed in figure 1.
Figure 1: A typical horizontal dryer.
Before the final storage into silos, the grain is
cooled.
Periodically, once the product is stored, it is
advisable to control the grain temperature to avoid a
lost of quality or possible explosions due to high
temperatures. Implantation of a temperature and
humidity control and supervision system in silos,
guarantee a quality for the final customer. When the
temperature rises above a reference signal, fans are
switched on to introduce cool air. At the same time,
the warm air is put out of the silos by means of
extractors. More details of these processes are
showed in (de Dios, 1996).
3 SUPERVISION, AUTOMATION
AND CONTROL PROCESSES
It is necessary to integrate inside processes with
many interacting elements, automation and control
systems. Next, each of these systems will be
explained.
3.1 Automation of the Process
Automation operation is a first step to control and
supervise any process. It is used to carry out
sequential motors start and stop, processes stop due
to failures, motor speed control or temporisation
actions. Automation architecture consists of:
• A first PLC´s to control four dryers and a
second PLC to control silos fans.
• Two PC to install Silos and Dryers SCADA
systems.
• Two Grain moisture measuring instruments.
• A PC-PLC PROFIBUS Communication
Card.
The PLC is connected with decentralized
periphery devices using PROFIBUS DP. This has
been possible by letting one unique PLC control
four dryers. The moisture equipment is connected to
the Dryer SCADA PC through a serial RS-232C
port. In figure 2 an automation scheme is depicted.
Figure 2: Automation scheme for silos and dryers control.
Each silo contains twenty four temperature
sensors located in six levels and one outside the silo.
There are also two humidity sensors (one inside and
one outside). These sensors are connected to a ADC
through four channels, which are connected to the
Silos SCADA PC . (see figure 4 for a detailed view).
3.2 Supervision
One of the most important tasks has been the design
of the supervision system. Instead of acquiring a
commercial SCADA, a supervision programs have
been developed with the Visual C++ tools.
According to the type of product, the supervision
system makes it possible to change some parameters
in order to control the drying conditions. It is
possible to activate the number of extractors, to time
the grain feeder, to change the discharge time, even
to switch on a second flame in the burner.
In the stored grain process a complex supervision
and control system has been developed. Not only
can we supervise the temperature of each silo to six
levels, but we can control the temperature based in
different choices. For example, depending on the
external air humidity and temperature, a time period
or a temperatures difference can activate extractors
and fans.
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Figure 3: A window of dryers SCADA application.
In every reading, the control compares the actual
values with the ones defined by user. If for instance
the actual temperature is higher that the limit set ,
the system turns on the aerations system. Once the
temperature below that set point the PLC shuts off
the aeration system. Examples of such controls are
showed in (Silva, 2003) and (Srzednicki, 2005).
A larger and more complex system totally
controllable and observable through the internet, by
user our SCADA Web (Janeiro, 2006) is being
developed.
The parameters of both supervision systems can
be changed from a window as depicted in Figure 3
and figure 4 .
Figure 4: Temperature Silos SCADA application.
The prime emphasis in these processes has been
on improving the security of existing machines. Due
to different controls implemented it is possible to
generate a important number of alarms. The most
important one is the temperature control, used to
avoid possible fires inside of the dryers.
The communication between all equipments;
computers, humidity measuring instruments and
PLCs, is continuously being checked.
All these alarms and other parameters are registered
in a database, to be analysed in order to verify the
stored and drying conditions, and improve the
processes.
Figure 5: Alarms registration.
3.3 Moisture Control System
Most development work has centered on the
optimization of machine design and capacity, and
the application of these machines to existing
processing strategies. The result has been the
development of more compact dryers in recent
years. The manner in which processing itself is
carried out, must be considered. There are works
related to to the engineering aspects of the process,
but researchs carried out on dryers control are not
quite extensive. Recent studies are based on the use
control techniques principally PID (Guofang,
2006),predictived (Qiang, 2001) and fuzzy control,
(Zhang and Litchfiled, 1993),(Bremmer, 1997) and
(Chunyu et al., 2007).
Control techniques principally consist of a
computer program and a number of sensors
measuring process properties. It is also necessary to
have some forms of SCADA/PLC systems.
The goal is to design a feedback controller for
the plant shown by the block diagram in figure 6,
which includes the feedback interconnection of the
plant and controller, and elements associated with
the performance objectives.
The moisture error of the discharged grains was
used as input parameter of the controller. The output
parameter of the controller was the speed of the
rolls.
Continuous-flow grain drying is a non-linear
process with a long delay; it is often subjected to
large disturbances and therefore is difficult to
control. The ON/OFF and PID designed controllers
have been an adequate control method in this type
of machines.
The grain humidity has to be controlled by
changing the temperature inside the dryers.
CONTROL AND SUPERVISION FOR AN INDUSTRIAL GRAIN DRYER
407

Figure 6: A dryer control diagram.
It has been verified that it is more effective to
switch the two flames of the burner than modifying
the speed of the rolls. The combination of an
ON/OFF control of burner with a PID control
actuating in roll speed makes it possible to achieve
an optimal control of humidity grain in these dryers.
In spite of this good behaviour of this type of
controls, we are developing robust control due to
long delays and disturbances in some cases. A work
of an implemented robust control in a similar
process can be viewed in (Cárdenas, 2003).
Simulations test show that the robust controller
performed well over a wide range of drying
conditions.
4 CONCLUSIONS
Process automation and supervision seem to promise
significant potential for development in the future.
The efficency of dryers has been increased
significantly. This has been achieved by making
them larger, more space efficient and by increasing
control and supervision systems. The incorporation
of these controls has also made itpossible to reduce
the grain humidity before it is stored into silos. In
addition, data collection and analysis, as well as
product traceability, ensures optimum quality for
customers and tools to enhance profitability.
The control method provide a new solution for
grain drying process.
Advanced controllers are being simulated with
good results and we expect to implement them in the
factory in the future for a better optimal energy
consumption.
ACKNOWLEDGEMENTS
This work was supported in part by “Programa
Nacional de Recursos y Tecnologías
Agroalimentarias”, (PROFIT) from the Spanish
Technology and Science Ministry.
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Building and industry publications.
CONTROL
PLANT
Humidity Reference
Feeding Time
Second flame ON/OFF
Roll Speed
Error
+
-
Measured Humidity
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