Thursday, March 8, 2012

Programmable Logic Controllers in Process Automation





Modern machinery comprises of both electronic and mechanical parts. Overall functionality is decided by balance between these parts. First selection of planning and solution plays vital role in finishing result.

In control techniques the selection must be made between the relay based circuits, PLC (programmable logic controllers), special devices and new growth electronics. This article is focused on PLCs and knowledge that has composed more than 5 years practical work with PLCs. The sides of strong and weak, practices of positive and negative are discussed. The ideas whether to apply programmable controllers or not have been figured to assist decision making.

The mainly important assessment in planning PLC based system is choosing the type of processor. The mistakes occurred mean extra costs for revisions or even require for completely new devices. The mainly common fault is overestimating possibilities of programming of small-sized processors. There are numerous techniques for choosing PLC. Unluckily most of them concentrate on electrical side of PLCs and the signals maximum count permitted. They don’t engage analysis to decide possible special requires for user communications or program.

Based on some offered techniques, practical knowledge and future trends a new technique for choosing PLC was built up. Ahti Mikkor has increased his knowledge by taking part in more than 15 projects of big-scale automation. These projects comprise growth of power consumption system monitoring in AS factory of Kunda Nordic Cement, renovating testing rig for flow meters, the turbine safety systems of building Ahtme power plant, the plants of water treatment in towns Põlva and Rakvere, the plant of waste water treatment in town Jõgeva and system monitoring development for central heating network in Tartu.

Positive and Negative arguments in application of programmable logic controllers
Positive arguments
The major benefit that programmable controllers offer is flexibility. The behavior of the system can be with no trouble altered by the use of program without any other modifications. The special devices for instance create any changes in the algorithm control very difficult to put into practice. The flexibility makes PLCs well appropriate for often changed applications, for instance in the robotics.

In PLCs the relatives between inputs and outputs are decided by user program. By using the advanced technologies of programming it is much easier to realize complex control algorithms than in any solutions of hard wired. It makes PLCs very ready for action for multifaceted tasks, for instance in controlling processes of chemical.

Special modules let huge amount of dissimilar signals to be linked to the system of PLC. Use of PLCs should be reflected on in the applications that need some special signals of input or output. Characteristic example would be placing with reference data from the input of high-speed. Typically the packages of PC visualization software are created for PLCs. Some special devices have also the packages of PC software. Large range of communication alternatives between PLCs makes it possible to collect all information from field devices into a central control point.

The lines of communication between PLCs let using information composed from other elements of the system in local process control. The technologies of modern communication allow remote configuration and diagnostics. These two drastically decrease overall costs of the maintenance system.

Negative arguments
Programmable controllers are not prepared with sufficient memory to save huge amounts of data. Even though future trends demonstrate growth in the memory sizes of PLC, special devices) are still enhanced matched for individual data logging applications. For the solutions of networked there is option to use packages of visualization software together with PLCs to records composed data in any format of database required. If data amounts of logged are little or here are also functions of control included, it’s logical to still use PLCs. About packages of visualization software it is good to recognize that in versions standard the majority of them do not support offline recording so that after breakdown the communication it is not possible to get data backwards from the PLC.

The options of modern communication for PLCs comprise standard protocols for instance Ethernet. It is attracting to use offered office networks also as data carrier for system automation communications. Time has exposed that it is better perform to maintain these two divided if there is a need for steady online communication. Barely traceable network temporal overloads can cause problems also in the system communications of automation.

All PLCs require to be programmed. The programming operates comprise hazard for accidental faults in the algorithm control. The particular devices are fine tested and usually free of this type of problems. If accessible, it’s inexpensively considerate to utilize particular devices.

The safety applications that need highest reliability degree should include circuits possible and simplest devices. There is a guideline that every new link in demonstration decreases overall consistency. It’s often cost saving to use circuits of relay based as a substitute of PLCs in small applications.



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