Introduction
— why automatize.
The main reasons
why to waste money on automatization seem simple and
plain:
·
to increase productivity;
·
to cut prime cost;
·
to gain continuous quality;
·
to furnish more
information about the subject, i.e., the "fowl" that brings golden
eggs.
What problems might
occur, which can intrude the implementation of automatization,
even if the situation is ready for that:
·
There is an absence of clear ideas what and how should
be automatized, i.e., there is no algorithm.
·
Obscurity how to keep a balance between requirements,
i.e., how to act economical, but not following the principle that avaricious
pays twice.
Automatization is usually carried
out by a certain company relying on a request of a client, which is called the
Control System Integrator, because it bounds automatical
elements, programming and technological process in one system.
What should be
considered before staring the automatization:
1.
Preferably, that your object is placed in service area
of the provider.
2.
If the chosen provider is foreign company, the related
company or expert is required in service area.
3.
You should avoid of buying cheep out-of-date
equipment, because that could cause serious exploitation problems.
4.
The advance of control system integrator compared with
PLC manufacturing company representative is that it has wider rage of
alternatives in choosing automatization solutions.
General automatization tendencies in different counties should be
similar, though, there can be differences in approach,
small aspects and motivation.
Sensors
Sensors,
that are inserted in the production process environment record environment
condition and dynamics of its alterations. Production environment could be a
container, in which lies the product or intermediate product (for example,
paste compound, milk at different stages, beer yeast or leaven, chemical
product mass etc.) surrounding atmosphere, or the product itself (for example,
for production of meat products temperature sensors are inserted in the product
itself). Range of the usable sensors might be very wide, however for the
production it is common to use only sensors approbated in practice, as possible
sensor error may provide incorrect information about procedure of the process.
In industry the
most popular sensors for measuring of theses parameters are: temperature,
pressure, pH, flow in capacity and aggregation
indices, humidity. Of course there could be sensors and devices for measurement
of other values too, selection and adequacy of which, in any case, is
determined by the exact demands for its application. For example in food
industry outer encasement of the sensor should be made from the stainless steel
or other inert material. Demands lessen (but not always) if the sensor should
be inserted in the environment, which has no direct contact with the produced
product (for example, thermostat water in the encasement of technological
tank).
By choosing sensor
without the above mentioned, it is important to take into consideration its
precision (distinction ability)', inertial characteristics, stability and
stability in the given production conditions. For example, quite often
temperature regulation problems do not occur due to the distinction ability but
because of the inertial characteristics. It occurs in a way that sensor reacts
to the environment temperature alterations with delay. In such case you should
choose smaller mass temperature sensor in construction of which other
parameters from which depends inertia are optimized too. Accordingly, physical
characteristics of other sensor should be analyzed in the context of concrete
application. Only in such way it is possible to find out its adequacy and if
necessary to take additional actions for improvement of measurement
correctness.
Executing
mechanisms
Work on executing
mechanisms is performed with the help of power elements (magnetic starters,
relays, softstarters, inverters etc.) These elements
are usually mounted in power unit control boxes. However, inverters and softstarters (according to the leakage level and environment
conditions) are mounted separately on the walls or other fixture. Usually
Control System Integrator provides acquisition of inverters and softstarters as well as their putting into operation, int.al. installation of optimum
operation parameters. Inverters are produced by almost all large companies
whose basic business is PLC's. Producers of inverters also use to be producers
of power systems as well as companies specializing in production of different
electric drives.
Programmable logic
controllers (PLC)
Programmable logic
controllers (PLC) with programmed process automation program are central
executers of the automation process. There are several stable PLC developing
and manufacturing companies in the world. Therefore, question which PLC to choose
for automation of certain processes appears quite often.
In this regard, at
first it is necessary to acknowledge number and types (digital, analogue) of
inputs/outputs, as well as available periphery (operator panels, indication
devices etc). If this task is relatively larger then it is important to assess
number of PLC's, type of their communications as well as application
opportunities of distributed modules.
In fact, PLC of
popular companies are with equal options and if any new additional option has
been invented for PLC of one company, then after quite a short period of time
these options become available in products of other companies, therefore PLC
selection quite often is determined by the following collateral factors:
1.
Customer usually chooses PLC of a certain company if
products of this company are dominating in equipment of the given object. In
such case it is easier for user to handle questions connected with service and
maintenance;
2.
Control System Integrator prefers PLC of a certain
company, if analogical project, based on this PLC, has been realized before;
3.
4.
Flexible price policy. It means, how capable, PLC
provider-company is, to offer discounts depending from the order size as well
as to offer opportunity of deferred payments by adjusting to financing
specifications of the certain project.
Companies that has
emerged in the PLC market quite lately try to attract attention with different
innovative solutions, however in the classical understanding of PLC they ensure
quite narrow spectrum of goods.
Process
visualization systems — SCADA
More and more often
PC's and their networks are integrated in the systems.
Usually PC task in
the automation systems is to ensure automated control of processes,
registration and visualization. Internationally such systems are usually called
SCADA (Supervisory Control And Data Acquisition). The
most common principle of process tasks distribution among PLC and PC is the
following: PLC controls and regulates the process, but PC registers and performs
functions of extended process operator panel. PLC's of several companies (for
example, Allen — Bradley) are created with PC like ideology. In such case
function distribution principles could be different.
Software packages
for SCADA system are offered almost by all common PLC producers (Siemens
— WinCC, General Electric Fanuc — Cimplicity, etc.). Some companies specialize only in SCADA
software (for example, Citect, Wonderware).
Any SCADA program tries to find an opportunity of ensuring communication with
the help of different exchange protocols. Interface between SCADA program, PLC
of other producers and hardware of other companies could be ensured with
several methods, the most progressive of which is considered to be exploitation
of OPC server.
General basic
functions ensured by SCADA programs of almost all companies are:
1. Data
registration
SCADA should ensure
continuous information reading from PLC and other devices that should be
analyzed in compliance with programmed formulas and logic interconnections, as
well as to ensure its demonstrative presentation in a graphic and tabular form.
Data is collected, ie. archived,
so that procedure of the process could be observed for any period of time
during system operation.
2. Operators'
assistant-interface
SCADA collects all
information about the process and ensures its demonstrative visualization, so
that the operator could follow the process procedure. Therefore, it is
important to workout — program process visualization diagram, visual
version of which in the consciousness of a man would nicely associate with the
real process. For this occasion visual figures of the devices are fashioned,
where colors of the devices or other features change depending from the their
status (devices could be turned on/off, could be in the manual or automatic
mode, at the state of readiness to complete operation, in alarm or warning
situation etc.) At the process visualization diagram it is often important to
see the upcoming process progress way.
3. Alarm and events
monitoring
SCADA system must
detect, depicture and save alarms and other important process events. Usually,
such systems consider operator registration option, as well as option to record
time, when alarm message has appeared, when operator has found it and when
averted it. Convenient option to inspect alarms and other messages in the
archive allows engineers and programmers to analyze causes of failures, in
result of which it is possible to improve system operation safety.
4. Databases
An important
component of SCADA is an opportunity to save databases (in SQL and other
formats) and export them to other structures of the enterprise. For example,
quite often it is necessary to provide information about certain product
consumption to the department of financial accounting.
Communication
protocols
There are quite a
lot of common communications protocols, which ensure link between PLC, PC,
executing mechanisms, the Internet etc.
Among most well
known and most utilized communication protocols we can name — Profibus, DeviceNet, Ethernet, Modbus, AS-I, Lionwork, CAN,
HART. In a lot of cases, it is not at all unambiguously clear, on basis of what
protocol (or protocol combination) to realize certain automation project.
Optimum protocol choices are determined by the equipment location topology of
the given process control task, total number of devices, high-speed as well as
field of application. Popularity of several protocols using in
Conclusion —
process control tendencies with a sight to the future
1. Lower prices,
smaller dimensions
You may feel that
there is a tendency in the PLC offering, that prices decrease and sizes become
more compact. It is mainly connected with the development of quasi-conductor technology, that allows ensuring of the same solution with
less number of components and with more compact sizes.
2. Operator panels
— PCs'
More and more
frequent, computerized process visualization programs SCADA are used for the
automation of production processes Visualization program also performs operator
panel functions and in that case, there decrease the need to use specialized
PLC operator panels.
3. PLC + PC —
new approach in the process control
Continually
increasing complex application, changes approach to the process control and
management. By basing on PC created options and provided information as well as
friendly Windows interface, operator is actively involved in the process
control improvement (quite frequently without even knowing that).