• Introduction — why automatize?
• Sensors
• Executing mechanisms
• Programmable logic controllers (PLC)
• Process visualization systems — SCADA
• Communication protocols
• Conclusion — process control tendencies with a sight to the future

Introduction — why automatize?

The main reasons why to waste money on automatization seem simple and plain:

What problems might occur, which can intrude the implementation of automatization, even if the situation is ready for that:

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. Automatization of processes is carried out due to the following scheme:

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What should be considered before staring the automatization:

General automatization tendencies in different counties should be similar, though, there can be differences in approach, small aspects and motivation.

Could you. Please, briefly describe specification of automatization in your country?

If you do not mind we shall dispose it here.

Specific characters of automatization in Latvia

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. Opportunity to receive information about programming of certain PLCs' and other application aspects. It depends from the initiative and competency of the local representatives, as well as from the level of consultation services in the central office of producer/developer;
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 USA and Europe is very different. For example AS-I, Profibus and CAN are particularly more popular in Europe, however, DeviceNet is used in USA relatively much more wider than in Europe.

For technical and application characteristics description of the different protocols you may use the following table:

Further we will show an example from our practice, how the system in which were used AS-I, Profibus and Ethernet communication protocols was selected:

Examined project is related to flour transportation, storage, homogenization and other operations in the flour silo. Object elements dimensionally are located quite steady.

In this system number of digital inputs/outputs were around 1500. Performance of the project realization was selected to be based on Siemens Simatic S7-400 (with processor CPU 414).

We analyze three process control system versions:

1. Direct connection of the executing mechanisms' control signals, sensors and end-switches to the centralized inputs/outputs of the centralized controller.
2. Use of distributed input/output stations that are connected with the central PLC with mediation of PROFIBUS communication line.
3. Application of AS interface lines, in order to use distributed input/output modules for operation of electro magnetic valves and installation of sensor information. AS interface modules connect with main PLC by using PROFIBUS line.

All three versions were analyzed within' context of the given task, by taking into consideration technical solution and complex expenses.

First version is improper, because, due to the large number of inputs/outputs, object controller circuit will have pipelines with capacious installation. It will inconvenience and make the installation performance, system diagnostic and maintenance more expensive, as well as will reduce fire safety due to the large number of cables.

Second version is based on the distributed input/output devices that connect with the main controller via PROFIBUS line. Application of this version is profitable if the object is separated into the groups. In addition, these groups should be relatively consistent and distance between them should be quite long. In particular case distances were quite near and object elements were evenly dissipated in the room. Therefore this version, in terms of outer installation will have very few preferences in comparison with the first version. Besides, in the configuration of the task this version will be the most expensive.

The most advantageous, from all aspects, in this case is the third version.

Given version allows to significantly decrease capacity of the installation pipes, ensures convenient system extension, well observable condition for diagnostics and maintenance. Besides this version is also quite economical. Costs of it are for 20% lower than for the second version and almost the same as for the first version.

Of course it was also necessary to use other communication protocols.

Wherewith, in this case main controller connects with system elements in four ways:

1. Main controller inputs/outputs are directly connected with MCC drive for the provision of control commutation signal for its startup/shutdown and overload control. Main controller and MCC direct circuit is based on ensuring operational safety demands.
2. Frequency converters are controlled, by using PROFIBUS communication line. This PROFIBUS line is also used for link with DP/AS interface converters.
3. All electromagnetic valves, level sensors, pressure sensors, conveyer movement sensors and valve end-switches are connected to input/output modules, that ensure information exchange with the main controller via AS interface. Given input/output modules are connected to the appropriate production line of AS interface. For provision of such interface links in the configuration of this task there were necessary 6 DP/AS interface converters that are connected to the central processor via PROFIBUS communication line. It was necessary to use 6 of these converters, because 31 input/output modules could be connected to one DP/AS interface converter. Total number of DP/AS interface converters is 7, as one AS interface line was supposed to be a reserve line. One of such modules has 4 digital inputs and outputs. Modules of analogue values have two inputs or outputs. DP/AS — it was possible to interconnect number of interface converters with the production lines.
4. Link with the computer was ensured with the help of Ethernet line.

If for the follow-up of certain parameter it would be necessary to provide faster operation, then an opportunity that given sensor should be connected straight to the centralized inputs and outputs should be considered. Such version could apply to the recording of the conveyer movement sensor impulses.

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).

4. Improvement of communications > simplification

Producers of PLC and other industrial components spend more resources to ensure connectivity of the devices, ie. develop communication protocols. It would be easier if several unified standards could be introduced, however that will not happen. In the same time, each producer will look forward to ensure, that his produced industrial devices allow more communication. In that way, forms a tendency that protocols of different companies become similar, more available, and as a result, in the future, a tendency towards decrease of the used protocols number can develop.

Among the most common communication protocols there should be named — Device Net, Profibus, Modbus and Ethernet. Methods how to connect products of different companies are also developing. For example, OPC servers are used in order to connect PLC of one company with visualization program of another company. In that way producers of the industrial components are forced to think of the user, as in the opposite case, due to the limited communication opportunities, given industrial product could remain unmarketable.

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