Control engineering has evolved over time。 In the past humans were the mainmethod for controlling a system。 More recently electricity has been used for control andearly electrical control was based on relays。 These relays allow power to be switched on and off without a mechanical switch。 It is common to use relays to make a simple logicalcontrol a decisions。 The development of a low cost computer has brought the most recent revolution,the Programmable Logic Controller (PLC)。 The advent of the PLC began in the1970s, and has become the most common choice for manufacturing controls。PLCs have been gaining popularity on the factory floor and will probably remainpredominant for some time to come。 Most of this is because of the advantages they offer。

Key words: programmable controllers, PLC, automation, automation systems, process automation。82696

1。 INTRODUCTION 

Modern machinery consists of both mechanical and electronic parts。 Overall functionality is determined by “balance” between these components。 Initial planning and solution selection plays critical role in final result。 

In control methods the selection has to be made between relay-based circuits, special devices, programmable logic controllers (PLC) and new development electronics。 This article is concentrated on PLC-s and experience that has collected over 5-year practical work with PLC-s。 Strong and weak sides, positive and negative practices are discussed。 Suggestions weather to use programmable controllers or not have been formed to help decision making。 

The most important decision in planning PLC-based system is selecting processor type。 Mistakes mean extra costs for modifications or even need for completely new devices。 The most common error is overestimating programming possibilities of small-sized processors。 There are several methods for selecting PLC。 Unfortunately most of them focus on electrical side of PLC-s and maximum count of signals allowed。 They don’t involve analysis to determine possible special needs for user program or communications。 

Based on several existing methods, practical experience and future trends a new method for selecting PLC was developed。 Ahti Mikkor has gained his experience by taking part in more than 15 big-scale automation projects。 These projects include development of power consumption monitoring system in AS Kunda Nordic Cement factory, renewing testing rig for flowmeters, building Ahtme powerplant turbine safety systems, water treatment plants in towns Rakvere and Põlva, waste water treatment plant in town Jõgeva and development of monitoring system for central heating network in Tartu。 

1。1 Ladder logic

 Ladder logic is the main programming method used for PLCs。 As mentioned before, ladder logic has been developed to mimic relay logic。 logic diagrams was a strategic one。 By selecting ladder logic as the main programming method, the amount of retraining needed for engineers and tradespeople was greatly reduced。

The main advantage that programmable controllers provide is flexibility (Jack, 2003)。 Behaviour of the system can be easily changed via program without any other alterations。 Special devices for example make any changes in control algorithm very hard to implement。 Flexibility makes PLC-s well suitable for frequently changed applications, for example in robotics。 

In PLC-s the relations between inputs and outputs are determined by user program。 By using advanced programming technologies it is much easier to implement complex control algorithms than in any hard-wired solutions。 It makes PLC-s very competitive for complex tasks, for example in controlling chemical processes。 

Modern control systems still include a relays, but these are rarely used for logic。 A relay is a simple device that uses a magnetic field to control a switch, as pictured in Figure 1。1。 When a voltage is applied to the input coil, the resulting current creates a magnetic field。 The magnetic field pulls a metal switch (or reed) towards it and the contacts touch, closing the switch。