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223
Advanced design technique of human-machine interfaces
for PLC control of complex systems
Árpád-István Sütő* and Eugen Ioan Gergely**
* SC TenarisSilcotub SA,
450009 Zalau, Romania, E-Mail:s_arpad@yahoo.com
* *Department of Electrical Drives and Automation,
University of Oradea, Faculty of Electrical Engineering and Information Technology,
1 Universitatii, 410087 Oradea, Romania, E-Mail: egergely@uoradea.ro
Abstract - Touchscreen operator panels proved to be a
convenient succesor for clasical operator panels for
implementing human-machine interfaces (HMIs) in
programmable logic controllers (PLC) systems. The
paper introduces a new technique for HMIs design in
such systems, based on the idea of touchscreens
replication. This redundancy allow actions which are not
possible within the menus and sub-menus of a single
touchscreen. Its strenght is revealed especially in complex
systems, where operators can easily be overwhelmed by
the huge amount of process information. The technique
was applied on a mill tube rolling installation. The results
also proved an increase of system security and zero down-
time for HMI maintenance activities.
Keywords: human-machine interface, programmable
logic controller, touchscreen replication
I. INTRODUCTION
Nowadays solutions for visualizing data and control of
automated processes have to fulfill increasing stringent
requests on what does it mean quality and precision of the
presented information from the perspective of process and
the tools of process control.
Today, industrial process operators are challenged to take
over increasing responsibility regarding the control of the
industrial process. Correct decisions in process control can
be taken only if the information the operators possess is
unitary, relevant and well adapted to the human operators
needs. These information in most cases cannot be obtained
using local indicators or machinery, however global
information from the process can be displayed only through
dedicated interfaces, software implementations through
which the operator have access to a powerful and flexible
set of virtual tools adapted to the needs of controlled
process.
Numerous companies are in a permanent competition to be
able to offer the most advantageous solutions from the
point of view of fiability, reconfigurability, easiness in use,
easiness in implementation, and in developing “state of the
art” solutions for process interfacing.
The main approaches in the design of modern industrial
control system interfacing with the human operator seeks
to achieve are those in which the control interface is
available through a software application, installed on an
IBM compatible PC or the interface is done using a special
device called operator panel [5, 7]. In many cases because
of the high complexity of the projects the two solution are
used together to obtain maximum efficiency in the control
[7].
The solutions based on a software which is run on an IBM
compatible PC, present high advantages in those cases
when there is a huge amount of process data, which have to
be displayed, and in the same time there are many data to
store in a data base, but there are only a few intervention in
the control of the process from the human operator [9]. If
the operator has to intervene with a lot of commands to
control the process this method doesn’t guarantee the
expected speed and easiness in use, because of devices
such as: keyboard, mouse etc.
Solution that most probably will replace totally the classic
command pulpits with buttons, selectors, lamps and
measuring devices are those based on specialized panels.
The must advanced type of specialized panels are the touch
sensible panels. These join together the advantage of the
detailed graphical interface with the easiness in operating.
Both of the described solutions have the advantage that
offer a huge amount of relevant information about the
process in a compact manner and save space and money.
The paper introduces an advanced design technique of
HMIs for PLC control systems. It is based on the
touchscreens replication idea and reveals its strenght
especially in complex systems, where operators can easily
be overwhelmed by the huge amount of process
information. Also, multiple time-constrained control tasks
can be initiated, which is not possible within the menus and
224
sub-menus of a single touchscreen. The technique was
tested on a mill tube rolling installation. In order to ensure
the safety of the real-time control and monitoring of the
process, operator touchscreens have been connected to the
PLC through a dedicated Profibus DP network. Increased
security is obtained by assigning passwords for the
personnel. In addition, the technique proved to assure zero
down-time for touchscreens maintenance activities.
II. TOUCHSCREEN-BASED HMIs
A control system can have several types of HMIs in order
to serve different users types. The design of a HMI influen-
ces the simplicity and the efficiency of operator actions.
Among the available techniques for HMI implementation
the paper will focus on touchscreen-based operator panels
(TOPs).
A great advantage of TOPs is the fact that they are
communicating with the PLC of the process through
industrial networks reducing in this way to minimum the
cost of wiring, which in the classical installations was
much higher, not only because of the high number of used
cable, but also because each classic command or
visualizing device (button, selector, lamps) has to be
connected to one input/output point in the PLC.
Programming is fulfilled with specialized software. Even if
this software has a great flexibility and many features in
order to allow any way of programming and design and
there are no technical restrictions it is recommended to
assume a certain level of standardization, in order to avoid
confusion.
Large companies generally define these kinds of standards,
which present numerous advantages. Usually these
standards refer to the dimensions of the objects on the
display, to the method of navigation etc., and in their
wholeness facilitate the understanding of the interface,
making the training process easier.
Having a general rule for organizing the objects on the
display is easier to set the rotation of operators (however
this is recommended because sustains interest and avoids
broadness) [8].
Companies that produce operator panels offer
programming language as well, for them. Some of them
offer the programming language for the panel free of
charge, and others require a certain fee for them. Generally
it can be said about this programming languages that they
are easy to use having an intuitive graphical interface, and
in most of the cases programming is reduces to defining
certain tags which connect with the memory of specific
zone from the PLC, and setting properties for the inserted
object on the panel (buttons, alarm lists, data fields, static
texts etc.).
In order to create easy to understand and easy to use
programs for the operator panel, some rules should be
considered.
First of all we have to consider that the program will be
used by employees which in most of the cases don’t have
advanced knowledge about process control or IT systems
[2]. In many cases this persons have previous experience
and skills in work with classic pulpits, so the upgrade to the
programmable panels have to be done in such way that
even this persons be able to use them easily.
Navigation between the screens (menus) of the panel must
be done pushing the less possible buttons. It is
recommended to design navigation bars through which
must of the pages used in the normal work are accessible
by pressing a single button. Pages which are not used in the
normal work, such as the pages with auxiliary installations,
pages with service information, pages with the historical
alarms, etc, can be accessed from any page without being
present in the navigation bar. However, where it is
possible, ramifications between the pages should be
avoided.
In complex installations with lots of mechanisms where it
is not possible to have all the commonly used pages in the
navigation bar it is recommended to split the installation
into control zones. Each control zone once is opened must
have in the navigation bar the frequently used pages of the
sub zone accessible through one button touch. The design
method proposed in this paper use one separate panel for
each sub zone.
At the choosing of the background color it is highly
recommended to use different color for each page, and the
button through which the page is accessible should be
identical with the background of the accessed page. This is
important because in time the operator associates the color
to the command zone and so, he doesn’t have to read the
label of the navigation buttons. In this way can be achieved
great reaction times and speed in the navigation between
the pages. In the same time, it is possible in this way to
determine by a single look on the background to which
control zone belong the opened screen, without being
necessary further analyzes on the content of the screen.
Another recommendation is to avoid the high load of the
display with objects. A highly loaded display is
burdensome by time, especially because command
elements change their color depending on the status of the
command. In the same time having a crowded display,
because of the small distance between the objects can
happen that one object is pressed accidentally on the
display and an unwanted command is lunched.
The programmable objects from the panels have proprieties
through which is possible to control the color of the object.
It is very important to choose the right color and this color
must be in concordance with the situation from the field of
225
the controlled mechanism. It is used green when the
command was executed successfully and the mechanism is
on or selected, or to confirm any correctly working
situation. Red color is used to display stopped, unplugged
status, or any abnormal situation.
Usually are used historical tables to show the alarms. In a
special page with alarms are displayed the alarms in the
order they appear, together with other relevant information
[3]. It is not an absolute rule but usually all the alarms have
to be acknowledged, even if they already disappear from
the system. For this, it is recommended to use significant
colors. An example of grouping by color is the following:
present and not acknowledged alarms, present but
acknowledged alarms, disappeared but not acknowledged
alarms. Another important thing is the use of the warnings
when in the system there is a not acknowledged alarm. For
this is recommended to display somewhere on each page a
warning with the signification “appear one alarm”, which
disappear after the operator confirms that he notice it by
acknowledging it in the alarm page. This warning should
not overlap other command elements or indications.
It is very important the way of object grouping on the
screen. It is preferred that elements which refer to the same
mechanism be grouped in a well delimitated area. These
elements (such as the selectors for the operating modes
automat/manual for sub zones) have to be displayed in the
same area in every screen [4].
Also, it is very important to have suggestive labels for all
the objects using the minimum possible number of words,
but with a clear and not confusing signification.
Another aspect to be considered is that these panels have
the possibility of securing the access to the installation.
This means that is possible to deny the access to the
operator for some command areas. The most of the panels
offer the possibility to define more than one security level
and to attach a password to each security level. This means
that for each object on the display you can attach a security
level and a password. This is very useful when there are
critical settings to be managed and operators should not
have access to these settings because they are reserved to
highly qualified personnel. Another way for the
management of the access is by defining a user list. You
can attach rights and restrictions to each user depending o
their attributes.
Companies, in many cases take global dimensions, with
plants in the whole word. In order to reduce the cost of
installation and production, the same equipments (with the
same program) are installed in as many locations as
possible. Similarly, in the case of international experience
programs might happen that equipments have to be
supervised by operators which speak different languages
[1]. In this situations unavoidably appear the problem of
the operating language. Generally speaking operator panels
offer the possibility to change the language for the objects
label. Practically, these suppose the necessity to redefine,
rename, the labels (texts) in the requested language. After
this operation it is possible to insert a button for language
selection. There is no need to redefine the functionality of
the objects.
III. THE REPLICATED TOPs TECHNIQUE
The technique presented in this paper has been designed for
and tested on a mill tube rolling installation, this is
controlled by a PLC and the human-machine interface is
implemented with touchscreen-based operator panels,
which will allow commands and will give confirmations
for the whole process.
The only classical elements which remain outside of the
touchscreen based panel are the emergency mushroom and
the reset of emergency which are not recommended to be
included in the panel, because they are hardware safety
elements [6].
This kind of system is very complicated, having lots of
buttons and indications, which have to figure on the screen.
Even with optimal organization, such a system might
contain 20 different screens. Touch panels do not allow
touching in multiple places in the same moment. In
consequence, with complex installations, a solution based
on a single touch panel is unacceptable.
A resolution of the problem is to use as many identical
touch panels as needed, which are run by the same
program, in order to be able to open one page on each
panel. The panels will be connected through Profibus DP
network to the programmable controller.
Another important aspect to be considered is the separation
of the command pulpit from the other devices that are
connected to the PLC through the Profibus DP network
(inverters, encoders, etc). It is worth to consider creating a
separate network dedicated to the operator panels. This can
be achieved by inserting a suplimentary interface module in
the main rack of the programmable controller. In figure 1 is
presented an example of such a system.
It is sufficient a transfer rate of 1, 5 Mb/s for setting of the
transfer speed, because the panels are not able to read tags
from the PLC in intervals less than 100 ms. As a general
idea, it is recommendable in choosing the speed of
communication to consider the distance to where the panels
are situated, which can be in some situations a few hundred
of meters. With the increase of the mentioned distance it is
recommended to use a lower rate of transfer. The used
accessory for the programming of the panel will be
ProTool.
Because the installation has a high rate of complexity, it is
divided in 3 areas, as follows: mechanisms before the
rolling mill, the tube rolling mill, mechanisms after the
rolling mill. The operator has to intervene only in the areas
226
before and after the rolling mill. The rolling mill is initially
set therefore the operator doesn’t need to work in this area
after the rolling mill is started.
In figure 2 is presented the main screen, from where it is
possible to choose the area in which we need to operate by
touching the synoptic in the desired area.
Fig. 1. Connecting of the panels to the PLC through Profibus DP interface
Fig. 2. Main screen of tube rolling mill panel
Screen from figure 2 is opening at the switch on of the
installation. After the selection of the desired area, the
operator doesn’t have to return to this screen only if needs
to change the work area.
Pressing the “System” button on this screen will open a
dialog box that requires introducing the password for
accessing the page with system settings (see Figure 3).
Figure 4 presents the screen with which a subarea is
controlled and shows how the presented specifications have
been implanted. Having in front of them a similar screen
with that presented in figure 4 the operators will be
able to oversee the state of the controlled installation, being
able to intervene in case of some anomalies. It can be
noticed that on this screen the representation of the
equipments is simplified, without insignificant details,
reducing in this way the load of the screen. As follows, the
operator is not overloaded with insignificant details which
can cause tiredness of eyes (and mind) in time.
A question might be raised: how the operator introduces
the numbers in the parameter input fields knowing that a
keyboard is not available? In order to answer this question
it can be noticed that in the moment the operator touches a
certain data input field an “on-screen” keyboard appears
(see fig. 5) which is active only until the characters are
introduced, and validated by pressing the “Enter” button.
CPU IM
467
SIMATIC MULTI PANEL SIMATIC MULTI PANEL SIMATIC MULTI PANEL SIMATIC MULTI PANEL
227
Fig. 3. General view of the main screen when is requested the enter in “System” screen
Fig. 4. The command screen of infilator
IV. CONCLUSIONS
The paper introduces an advanced design technique of
HMIs for PLC control systems using touchscreen operator
panels. It is based on the touchscreens replication idea and
reveals its strenght especially in complex systems, where
operators can easily be overwhelmed by the huge amount
of process information. Also, multiple time-constrained
control tasks can be initiated, which is not possible within
the menus and sub-menus of a single touchscreen. The
technique was tested on a mill tube rolling installation.
The way of hardware connecting of the operator panels is
very simple, because they are using standard
228
communication networks, reducing in this way the
problems which always appear in old installations due to
the complex wiring. In fact, network communication
enables the replication of the panels. In order to ensure the
safety of the real-time control and monitoring of the
process, operator touch screens have been connected to the
PLC through a dedicated Profibus DP network.
The used operator panels are able to allow the definition of
user passwords that grants a high level of security for the
control and monitoring system. In addition, the technique
proved to assure zero down-time for touch screens
maintenance activities. The replacement of the old panel
can be done “online”, meanwhile the spare panels being
used.
Fig. 5. Area of screen with the “on-screen” keyboard displayed
The new technologies allow a very creative redefinition
of what it means industrial process interfacing. Even if
this technologies are very flexible, permissive,
innovators, however we havet to be careful in what it
means the use of the new, because the migration to the
new systems have to take in consideration that human
operators are not always adequately qualified.
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