International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 2
Number 3
September 2008
297
Integrated Software for Implementation of
Control Systems Based on PLC Controllers
S. Surma & J. Mikulski
Silesian University of Technology, Faculty of Transport, Katowice, Poland
ABSTRACT: Industrial processes’ control systems based on PLC controllers play today a very important role
in all fields of transport, including also sea transport. Construction of control systems is the field of
engineering, which has been continuously evolving towards maximum simplification of system design path.
Up to date the time needed for system construction from the design to commissioning had to be divided into a
few stages. A mistake made in an earlier stage caused that in most cases the next stages had to be started
anew. Modern software allows for quicker implementation of system design. Available debugging systems
allow for defect detection at an early stage of project implementation. Application of additional protective
devices will allow for remote diagnosing of the equipment state and introducing changes (depending on
authorities granted). The paper presents general characteristic of integrated software for implementation of
complex control systems. The issues related to the software use for programming of the visualisation
environment, control computer, selection of transmission medium and transmission protocol as well as PLC
controllers’ configuration, software and control have been analysed.
1 INTRODUCTION
The control systems based on PLC controllers find
numerous applications both in the industry and in
mobile transport means of inland and sea transport.
A large field of opportunities entails a broad
spectrum of applications of such systems. Possibility
of cooperation with the detectors and actuating
devices provided with practically any
communication interface allows implementation of
such systems in the existing solutions without a
necessity to modify the system itself, and in
many cases also simultaneous use of these elements
of the system for the purpose of preliminary safety
tests.
Navigation, as a component of the system,
provides information about position in the space.
This information has to be compared on a current
(quasi-continuous) basis with position of units
located nearby. It is unacceptable that the data are
lost or they are incorrect because of hanging up of
the controller or freezing of part or entire picture.
Both these situation have to result in transition of the
system into a safe condition and ensure an
appropriate safety level by warning the personnel
and giving them a possibility to control the ship. It is
also important that the condition just before
the malfunction is restored for the diagnostics
purposes and the statuses necessary to restore
the equipment behavior in case of malfunction
are archived.
2 ARCHITECTURE OF CONTROL SYSTEMS
BASED ON PLC CONTROLLERS AND
INDUSTRIAL COMPUTERS
Architecture of control system is various and
depends of the solution applied. It is based upon
computer network topology because of a need to
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cooperate and exchange the information between
controllers.
Main features of the system’s structure is use of a
central computer that coordinates operation of all
controllers or distribution of the logics between the
controllers so they become either autonomic or semi-
autonomic. An advantage of using a centralized
system is a possibility of full archiving with a
practically unlimited memory capacity (for the
purpose of archiving, we may use disk array, whose
safety is greater that the one of a single disk (using
RAID array)). A centrally controlled system reduces
system readiness because of a need to ensure a safe
condition in the case of communication system
malfunction and depending of the assumed safety
strategy to shut down all the controllers or only those
with broken communication (a controller, when
facing loss of communication, should enter into a
safe state which may not mean shutting down all the
output signals).
Giving autonomy to the controllers in a
decentralized system enables performance of certain
tasks in spite of loss of communication. It has to be
related with a possibility to archive all changes in the
controller’s memory up to the moment of restoration
of communication, when all the archved data in the
controller are transferred to the central archive. Data
concerning the activities performed have to be time
stamped i.e. bear the information about exact time of
occurrence of an event.
Decentralizing of the system realized during
completely independent activities brings advantages
in the form of lack of communication problems,
although renders difficult or impossible to perform a
central control of a distributed system because of
problems that may occur with the safe transmission.
Full decentralizing enforces also a guaranteed full
archiving of the controller’s operation in its memory.
Thus, the capacity of archive memory has to be
adapted to the average number of activities
performed by the controller in the time period
required for keeping of entries.
The communication between particular system
components may be effected in many ways.
Transmission media being used may be split up into
two basic categories: open standards and private
standards. As per today, because of a dynamic
development of computer network and also
availability of Ethernet standard based equipment
(10Base-TX, 100Base-TX, 1000BASE-T, etc.) based
on TCP/IP protocol, more and more actuating
devices has such interface implemented. This
enables, of course with a proper data protection
technique (such as CRC) to use this medium for
control of equipment, while maintaining guaranteed
transmission safety (using a non-safe transmission
medium). Methods of digital cryptography based
on long keys that are available today could enable
to use an open Internet network for data
transmission over long distances and access to the
equipment from any place on Earth. Anyhow, small
possibility to influence the availability of Internet
providers’ net results in a fact that the public
networks are rarely used for control of the devices
(Lewiński & Perzyński 2003, Młyńczak & Gorczyca
2004).
Protection of the system against access of
unauthorized persons requires implementation of not
only access system for the computers, but also such
organization of internal network as to separate it
completely from the public network. Various
solutions may be used for this purpose. One of them
is creation of a gate server allowing the access of
authorized (verified) persons to the separated zone of
system diagnostics and control.
The above solutions allow to establish a system
organization diagram taking into account the
communication layer, which is shown on Fig 1.
Control
computer
Switch
Firewall / Gateway
Control
computer
Redundant system
Firewall / Gateway
Firewall / Gateway
Firewall / Gateway
SwitchSwitch
WAN
Execution
device
PLC
PLC
PLC
PLC
Diagnostician
Execution
device
Execution
device
Execution
device
Execution
device
Execution
device
Execution
device
Execution
device
Execution
device
ETHERNET CAN
Fig. 1. Construction of a system with devices protecting against
access of unauthorized persons
3 TYPICAL SOLUTIONS OF SOFTWARE
STRUCTURE FOR THE CONTROL SYSTEMS
BASED UPON THE PLC CONTROLLERS
3.1 Structure of software
Software for the control systems consists of several
basic layers:
− visualization,
− communication,
− control,
− decision.
Each level features its individual safety class that
has to be ensured to classify this system as safe one.
The lowest safety requirements apply to the
visualization level because it is not responsible for
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decision-making that might influence the safety, and
each command issued through an interface is
verified several times, which enables elimination of
interface and transmission errors in the control
commands bypassing the decision logic (the error
would have to appear several times in different
telegrams while not causing change in redundancy
code of each of these telegrams) (Lewiński 2001).
The communication layer safety depends of the
assumed solution. If we use a safe medium, it is
required that such medium has very strict parameters
in terms of quality and resistance to the interferences
so the transmission over this medium is not
distorted. The simplest solution that fulfills such
requirements is optic cable. This medium is not
vulnerable to the electromagnetic interferences.
The Ethernet standard used in the communication
layer contains 4-byte redundancy code (CRC) for
the purpose of transmission correctness control.
A package with CRC code that was calculated in
the transmitter and stored in the frame being sent and
that differs from the code calculated in the receiver
will be ignored and a request will be sent to the
transmitter to repeat the transmission. Application of
such a transmission protection method results in the
fact that we may use medium with less strict
parameters, because the transmission is protected
against errors by verification of redundancy code
(CRC).
The control layer covers actuating systems such
as PLC controllers and end devices (pumps. Motors)
if their control takes place with bypassing of PLC
controllers (this is possible when they are provided
with an appropriate interface – such as Ethernet).
These devices may be current or voltage controlled
by the controllers but if they are provided with the
communication interfaces and diagnostic electronics,
it is possible to perform a current analysis of
equipment condition i.e. continuous diagnostics that
may detect not only two states – operating – non-
operating but also discover worn-out or damaged
bearing based upon the vibrations or certain
components of vibration that are characteristic to the
specific malfunctions.
The decision layers contains most frequently the
interlocking computers that supervise the total
control process and issue commands to the
controllers and devices. They are provided with one
or several communication standards and enable
cooperation with many system elements, such as
redundant computers, PLC controllers or actuating
equipment. These computers fulfill also function of
archive servers for the entire system.
The control and decision layer have to supervise
the communication layer and one another. If one of
the layers is damaged entirely or in part the system
has to work out a safe situation that would guarantee
elimination of possible hazard to human health or
life and further also material losses.
3.2 Creation of programs in a modern software for
control systems construction
Changes that took place during the last several years
in the creation of programs for each system layer
dramatically changed the system construction
method. This had an advantageous impact upon the
costs of the system and time of its construction, Fig.
1 shows the evolution of software with time.
Today, due to the modern software integrating all
layers and reducing the construction of a program to
combining the entire system of blocks representing
specific devices that mostly are repeatable and at the
same time enabling design of entirely new elements
(devices) with their own interlocking systems, the
complete construction of the system may take place
quicker and at the same time with minimum
requirements concerning the knowledge of
programming languages by the designer. The process
itself consists in connection of blocks with inputs or
outputs of the controllers or communication bus. The
blocks are programmed by persons with broader
knowledge and extensive information enables their
implementation by persons with minimum
programming knowledge. Configuration of specific
system elements based on the blocks may be
performed by assigning input constants and
description of units such that they have unique
identification in the system.
in the past
Today
New
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Tool #2
Tool #3
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Fig. 2. Evolution of programming methods in the control
systems based on PLC controllers
Exchange of information between the system
components takes place based upon global variables
predefined in the system, similarly as in the older
software. However, here we do not have to deal with
necessity to know how to define – the interface is
intuitive, enabling to decide which variable has to be
put for reading outside the controller – without going
deeper into the structure of the program.
300
Communication between the controllers may be tine-
depending i.e. when the attempt to read does not take
place or readout of variable fails the controller has to
react in appropriate way. Besides this there is also a
possibility to couple the controllers into a redundant
system increasing the readiness of the system (2003)
or/and its safety (2002) (Lewiński 2001).
Exchange of information between the actuating
devices and controllers is as important as
visualization of the process for the service. Hitherto
it was an element requiring large programming
knowledge especially when the program itself was
written in the lower level language (assembler) now
is based on a graphic editor. Embedded animated
elements representing the statuses of devices or
informing the service about their operation (also
operation of the entire system) are represented as
hyper-macros that issue, as output function, a
function that subsequently is used for control of the
visualization of the given element used anywhere in
the system (regardless whether the logic of
visualized component is located in the controller or
in the controlling computer).
The time of system construction based upon
predefined blocks is considerably shorter. The only
element that requires working out in the project is
logic of its operation. The construction is the longest
part of designing. It has to contain all acceptable
variants and protect the system against human errors
as much as possible, as well as allow for automatic
operation of the system in certain assumed situations
based upon the date written in memory and
containing information about the given procedures
(such as control of distance between two ships,
supervision of entry to the port with location device
data taken into account and compared with the
stored land parameters with selection of an optimum
route which may be displayed to the navigator or
take the decision immediately by controlling the
operation of onboard devices).
4 CONCLUSIONS
Time and cost of the system construction largely
depend of the expenditures necessary for creation of
program part of the system. Shortening of
programming time by its simplification enables
saving in terms both of time and money due to the
possibility of system operation simulation already at
the stage of programming.
Use of an intuitive interface of programming
environment renders possible to work on the system
not only for IT engineers – programmers but also
suitably trained engineers having more knowledge
about the system being designed and constructed.
Modern software allows not only for a complete
construction of programming for all control levels
but also to carry out the full diagnostics of the
system being started up and already existing, due to
the implemented libraries.
Use of integrated programming environment
cooperating with a database where all information is
collected about the changes of system status, allows
quick determination of the occurring problem, its
diagnostics and working out of procedures to prevent
the existing situation in the future based upon
analysis of the data, current diagnostics and entries
concerning similar system conditions written in the
database.
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Lewiński A. & Perzyński T. 2003. The safety of multi-
computer systems for railway transport management and
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th
TST International Conference, Katowice-Ustroń
13-15 November 2003.
Młyńczak J. & Gorczyca P. 2004. Model of remote diagnostics
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th
TST International Conference,
Katowice-Ustroń 04-06 November 2004.
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