The 7 Essential Parts of a PLC System

 

     The 7 Essential Parts of a PLC System

In this post I am going to share about seven essential parts of a PL 

Seven essential parts of a PLC

 A PLC consist of seven main Parts they are.

1.      CPU

2.     Input & Output Module

3.     Analog Input & Output Module

4.     Then we have specialty inputs and outputs

5.     Then have programming tools

6.      we have HMI which represents human machine interface and then finally the last one or the seventh essential part of the PLC is

7.     Networking we're going to top quickly talk

On each subject and as you have a better understanding of the overall system of the PLC so let's start with CPU.

CPU

            C P U stands for central processing unit and it is actually the brains of the PLC.

    Now we have different manufacturers who manufacture these PLC's and they want to solve them by themselves so they will have their own chip set that they use within the CPU and the brains of the PLC and because of that there has been a concerted effort in order to standardize on the languages that we use to program the PLC's so we have IEC 61131-3 standard   and it defines five different languages that we can use to program the PLC .

1.         The first one is structured text(ST)

2.       The second one is function block diagram (FBD)

3.       The third one is sequential function chart (SFC)

4.        Then we have instruction list(IL)

5.         and then finally have ladder diagram(LD),  now 95% of the installations are ladder diagram.

 However the other methods of programming are still valid and they work well now remember even though a manufacturer says that he will comply to this standard it doesn't mean that the it's exactly the same and you can't just.

 

           Take one program from one man factures controller and put it into another without some to rewrite the software and look at everything within that controller so this gives you the format or the look and feel of what that's look it looks like but it could be different for different manufacturers and the other thing that they were looking at is usually the manufacturer.

 

      Will specify the PLC  scan and the scan literally is the reading of the inputs executing the program not doing the diagnostics of communication updating. Check your outputs and scan times can be synchronous or asynchronous meaning that asynchronous usually has multiple chips involved in the CPU and it will read the i/o at any time during that scan and update without you actually knowing it so you have to be specific as to when you read your output.

        So the next one is inputs and outputs and plc inputs they

Come from a variety of different things it'll be things like digital i/o like photo electrics proximity sensors. The sensors also acting as switches & contacts.                 

                    That here's my input coming in my indicator card. After writing the plc program in software you need to dump the program in plc module. When you are on the input the indicator will display in plc module.

              Diagram what it looks like on the input side and what you'll know this is that the difference between the input and your outputs and your CPU is that everything is isolated by light waves so light actually is triggering the actual input the fire that's why we can what we call isolation. if we look at the outputs again digital outputs here's my ladder diagram I turn the output on it fires the output and then it can be either you  know controlling a coil a light a motor turning on and off.

 

       The next one is analog input and analog Output here what we're doing is taking the analog output we are then going to drive this motor and usually it's going to be a motor speed, and when we talk analog we talk bit resolution so instead being just on/off it's a range.

           So there's my analog output and then we have an analog input.  In this particular case I'm looking at the height of the  level of the tank of the stuff that's in the bottom here so then we have specialty i/o and a lot of the times manufacturers will actually create a lot of specialty items within the controller itself such as high speed counters position control units etc or they come as a separate card that you can purchase.

        In the PLC next is programming tools and in programming tools what you have is you here's a typical PLC here and then what we'll do is have a computer to program this and the computer will have PLC module with software and there's usually multiple ways of connecting, it could be Ethernet it could be  serial port.

                 There's a method at which to talk to that PLC to program it in one of those five different languages to do the work .

 

The next thing we want to take a look at is HMI and HMI the human machine interface so prime example that is a corner PLC which is the HMI is actually built right into the PLC unit itself so here we have the full screen with the function keys available I would say that we could also use programming software.

           Such as my previous screen here this actually showed the advanced HMI software running on that computer talking to the PLC in order to take a look at that also with programming packages,

            a lot of them will actually come with a simulator and what you can do with simulators is that with the advanced HMI screen. actually PLC simulator Will simulate the plc programming and it will display on the HMI screen, Before that everything should be correct and hmi simulator should be in on condition.

           So we can duplicate everything that we see before we actually go on-site and try to troubleshoot this the system so here you see my stack light I have a series of valves here and what I'm doing is just clicking on it and triggering it the program to fire so here I'm checking my sequence of operation.

            And the last thing we talked about is networking is a vast range to discuss and in that what we can do is we can have peer-to-peer communication meaning that one PLC will talk to another PLC.

       When that happens most manufacturers will provide some method of doing that and typically it's my group of registers that are shared amongst all the network that you have connected networking can also include talking or send an email out as you see it on the screen.

            Here now I have plc it sends the email through Gmail into the phone or computer indicating something's up or something's wrong we can also grab information through PLC's using protocol and protocol is just the way in which the PLC can understand and so that we can extract information from it and up here we can use and typically some of the open protocols.

            These can be  like Modbus TCP Ethernet IP that kind of thing Modbus, RTU just to name a couple of examples are on host link would be another one and we can do several things. when we network things together we can bring things right into Excel and I'll just forward this a little bit so we can actually see our Excel display information as we in real time as it as the program is being executed.

          so that's it for now again that's a the seven essential parts of the PLC system again CPU inputs and outputs analog i/o ,special specialty i/o,  programming tools HMI networking and that's about it see it's very confusing sometimes,  so if you really want to manufacturers are more than willing to help you out you can give them a quick email with your specifications that’s what you need in a PLC system .