PLC SELECTION CRITERIA

Selecting a PLC system requires planning as various PLCs are available in market so for selecting a PLC one must consider the following guidelines:

* System (task) requirements.
* Application requirements.
* What input/output capacity is required?
* What type of inputs/outputs are required?
* What size of memory is required?
* What speed is required of the CPU?
* Electrical requirements.
* Speed of operation.
* Communication requirements.
* Software.
* Operator interface.
* Physical environments.

System requirements

The point of departure for any solution must be to understand what is to be achieved. The program design begins by dividing the task into a number of simple comprehensible elements, each of which can be described easily.

Application requirements

*The next step is to determine which input and output devices the system requires after determining the operation of the system.

*List the required function and identify the device type.

*In addition to discrete (On / Off) logic, the need for special operations.

*List the required advanced functions in addition to a simple, discrete logic.

Electrical Requirements

The electrical requirements for inputs, outputs, and system power;

Consider three items when determining a system's electrical requirements:

• Input power (control system power 24VDC or 220VAC)

• Input voltage of the device (24 VDC or 220VAC)

• Output voltage and current (mV or mA signal)

Speed of Operation

How fast it is necessary to operate the control system (operation speed).

Consider these points when determining operating speed:

·         How fast does the process or machine work? Are there any operations or events "time critical" that need to be detected?

·         In which time frame the fastest action must take place (detection of the input device for output device activation)?

·         Does the control system need to count and respond quickly to pulses from an encoder or flow meter?

Communication

If the application requires non-process data sharing, i.e. communication. Communication involves sharing data or status of applications with other electronic devices, such as a computer or monitor at the station of an operator. Communication can take place locally via a twisted pair wire or by telephone or radio modem remotely.

Operator Interface

Many applications require operator interfaces to convey machine or process status information or allow an operator to input data. Traditional operator interfaces include pushbuttons, LED numeric display and pilot lights.

Physical Environment

The physical environment where the system of control will be located. Consider the location of the control system environment. Housing the control system in a suitable IP-rated enclosure in harsh environments. Recall considering maintenance, troubleshooting or reprogramming accessibility.