PLC and its selection
In the industry, the old command relay circuits have been replaced by industrial processors called PLCs or programmable logic controllers, which are a kind of brain and decision center of most industrial processes. The correct choice of PLC is one of the things that must be considered according to a series of parameters and technical specifications.
A PLC or a controller consists of two basic parts: the processor or central processing unit (CPU) and the input and output interface. Or its main processor as well as the amount of main and external memory.
In general, the following can be mentioned for the correct choice of PLC:
PLC selection criteria include the following:
* Functions and objectives of the control system
* Program requirements
* Input / output capacity required
* Type of input / output required
* Memory size required
* CPU speed required
* Electrical requirements
* Operating speed
* Communication requirements
* Software.
* Operator interface
* Physical environments
Tasks and objectives of the control system
* The starting point in determining any solution is to understand what needs to be achieved.
* In program design, tasks should begin with a number of simple comprehensible decomposition elements, each of which can be easily explained.
Program requirements
* Input and output needs of the device After determining the performance of the system, the next step is to determine what input and output in the controller required by the system
* Identify the list of required functions and a specific type of device.
* Requires special operations in addition to discrete logic (on / off).
* List of advanced functions required alongside simple discrete logic.
Electrical requirements
Electrical requirements for system inputs, outputs and power; When determining the electrical requirements of a system, consider three things:
Input power (control system power);
Device input voltage and output voltage and current.
PLC operation speed
What is the speed of the control system (speed of operation).
When determining the speed of operation, consider the following points:
- What is the speed of the process or the machine?
Do you have operations or events that are critical? They should be identified if any.
- How long should the fastest operation occur (detect the device input speed and device activated output)?
Does the control system need to count the pulse of an encoder or flowmeter that needs to respond quickly?
Communication requirements
The need to share information outside the process means communicating. Communication involves sharing program or status information with another electronic device, such as a computer or monitor, at the operator station. Communication can be done locally via a twisted pair cable or remotely via a telephone or radio modem.
Operator interface
If the system needs control of the operator or interaction with him. Many applications require an operator interface to convey machine or process status information or allow an operator to enter data. The traditional operator interface includes pushbuttons, lights and a numeric LED display. Electronic operator interface devices display device status messages in descriptive text, display of component sections, and route alerts. They can also be used to enter data manually.
Physical environments
Consider the physical environment in which the control system is located. In harsh environments, place the control system in a computed area with the appropriate IP. Of course, keep in mind the availability for maintenance, troubleshooting, or rescheduling