After designing the process and equipments we have to choose the right controller. For selecting a PLC controller first step, we have to choose the type of PLC based on the following criteria.
Criteria for selecting PLC:
Number of logical inputs and outputs
Memory - Selected based on the size of the program
Number of special I/O modules: Depends on the application. For exotic applications, a large number of additional cards may be required.
Scan Time: Depending on the size of the program, a larger program needs shorter scan times. And, the shorter the scan time, the higher the cost.
Communications: serial and network connections allow the PLC to be programmed and to communicate with other PLCs. The needs are determined by the application.
Software: the availability of programming software and other tools determines the ease of programming and debugging.
Based on the criteria, PLC can be chosen among these types:
– Types of PLC
Basic types of PLC:
A unitary PLC is the simplest type of controller and contains all the basic components of the system within a single housing or box. These components usually include the processor, in addition to the ports for the input and output connections. Unitary PLCs generally connect directly to the device or application that is being controlled.
A modular PLC contains several different modules that can be attached to build a custom controller. Normally, a base module contains basic functions such as the regulation of electrical power, the processor of the computer and the input connections.
Additional modules, including analog-to-digital signal converters or additional outputs, can be added to this central unit as needed. This modular design allows you to easily customize and change a PLC.
Rack Mounting PLC:
Each module in a modular PLC is connected directly to the base unit, a rack-mount PLC keeps each module separate. All the additional modules are connected through a network, and the modules are kept in organized racks.
This approach allows larger systems to be built without becoming too crowded and complicated. The modules are well organized in the rack and can be removed and reinserted as necessary.
Selecting procedure for right PLC:
The process of selecting a PLC can be broken into the steps listed below:
Understand the process to be controlled, which means:
-Identify the number of inputs and outputs needed.
-Determine how the process will be controlled.
-Determine special needs, such as the distance between the parties to the process.
Selecting the vendor, The factors that could be considered are:
-Manuals and documentation.
-Support while developing program and while troubleshooting
-The range of products available
-Track record of the company and business experiance.
Plan the ladder logic for the controls
Check if there is enough memory in the chosen PLC:
Use the instruction times and memory requirements for each instruction to determine if the PLC has sufficient memory and if the response time will be adequate for the process.
Create a table to calculate the scan time and memory, example: