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| Subject - PLC Overview | |
| JimmyDee |
Programmable Logic Controllers (PLC) When we discuss what a Programmable Logic Controller is, we must first realize what it is. I will try to define a PLC to the best of my ability by describing what its components are and how it works. A PLC is a computer that has the ability to interface real world inputs, (tangible, usable, physical, voltage or current signal producing devices) with real world outputs, (tangible, usable, physical, voltage or current utilizing or controlling devices). The computer part of the PLC is a relatively slow but highly reliable computer. In today’s standards of the high speed desk top computer, these are dinosaurs but are becoming much faster than their predecessors. The central processing unit in the PLC is very stable and works with many checks for eliminating errors. Without going into the mechanics of how it works, it is necessary to say that if the processor or anything else, adds or subtracts one extra bit of information, the processor will go into a fatal error and crash. If it gets caught in a continuous loop and takes too long to go through an entire scan, it will also crash. Every precaution for reliability and safety has been manufactured into the PLC. The input terminals are the part of the PLC that communicate with the PLC by accepting a voltage or current from an input device and allowing that signal to become a meaningful piece of data the PLC computer can utilize. This would be similar to a keyboard or mouse is an input device that the computer recognizes as meaningful data and utilizes in its working programs. This data is turned into a digital signal, either on or off, or a multiple bit digital signal that would communicate a range of 0% through 100%. This information is not directly inserted into the computer. It is usually transferred through a device called an optical isolator. This is nothing more than a LED that when on turns on a photo transistor which acts like a switch. The light produced hits a photo transistor that turns on when light hits it and that allows a 5 volt signal to communicate directly with the CPU sending it data. The purpose of this optical isolation is to keep any electrical spikes or other harmful or erroneous electrical problems from direct contact with the CPU. The output terminals also communicate with the CPU by means of the optical isolator. No electrical current from the CPU is allowed to intermingle with either the inputs or outputs. All links to and from the CPU are done by light connections by means of this device called an optical isolator. Outputs are typically nothing more than a switch that is turned on and off by the CPU. These switches are either a set of contacts or a solid state switch device called a triact. There are other types of outputs but we will not discuss these at this time. I will say, PLC manufacturers make about any type of signal producer anyone can imagine, that act as an output. A few of these PLC outputs would include: 4ma-20ma, 0-10 volt, BCD, hard relay contacts, Pneumatic valves blocks and a raft of others. OK, that is a brief overview of the hardware that we call a PLC. So how can we get this thing we call a PLC to do some work for us? We will briefly discuss the programming part of its operation. Here is a simple one line program: |-------] [-----------------------( )----| |--------01----------------------- 02--| This is a sample of a program that allows an input that we addressed as input 01, when it is turned on or goes true, will turn on an output that we addressed output 02. How do we get out PLC to do some real work for us? First we need to hard wire to the PLC to get it to function the way we would like it to. Using an external power supply of some type that matches the input requirements of the PLC input section we will power one side of the input terminal strip with the neutral or one of the wires we designate as the common. Most PLCs have a common terminal on their input terminal strip. The other side of the power supply output feeds any type of control device like a switch. The load side of that switch or control device will then feed an input. In our example, it will feed input terminal #1. Now when the control device or switch is turned on, the PLC will see a true on input 1 and every place that input #1 is used in the program, (in some processors it can be used 9999 times) it will go true when turned on and false when it is turned off. Input # 1 in our example will turn on output #2. So what is output #2? We can have several types of outputs but the typical is nothing more that a single pole switch. Lets say we have an output rack with 8 outputs numbered 1-8. We are using output (switch) #2 for out example. When we have all conditions on our line of programming logic true, switch #2 has turned on and we can control anything we want to with this switch just like it was an individual, discrete circuit. Once the PLC is in run mode and has our simple program in it, lets turn on the switch controlling input #1. It goes true, causes output #2 to go true, which turns on the switch associated with output #2, which will turn on the device we want to control with output #2. The controlled item could be a starter, solenoid, pilot light or what have you. This is a brief overview on how a PLC works and is not meant to be an exhaustive study but a very simple introduction into PLCs. I have seen programs with over 3000 lines of logic in them being controlled by thousands of inputs and outputs. The PLC is the workhorse for our modern automation in industry. Jim |
| zol_man | jimmy, there is a link to a plc forum somewhere on my computer. i will post it as soon as i can find it. might be on the pc at my office. carl |
| JimmyDee | http://www.barn.org/FILES/historyofplc.html Here is a great link on the History of the PLC. Jim |
| lctrc789 | Jimmy, I work on PLC s almost everday and they are to say the least scary to any one who is not familiar to them, What we have uses device nets and ethernet cables and they operate many of the agitators for mixers, to PH balancing, to temp controls. They use them in the entire plant and the entire pharmecuetical buildings all the way down to controlling the tepmeratures of the building to the lights... I would venture to say that we have well over 2000 I/O panles and more everyday coming in to use. Ladder logic programs and instrumentation courses are very helpful to learn and undertsand for this type of work, to those who do it they can give you headaches everyday as well...LOL |
| Allphase | http://www.thelearningpit.com/ |
| JimmyDee | quote: Wow the LogixPro simulator looks like a great buy for training. At $30 it is a bargain. Jim |
| stedder | This is a great topic for me since I've just been introduced to these types of controlers, yes I was scared and I'm sure the panels I was working in was simple compared to most, but man when every thing started to click (fig and lit) I got pretty happy (since it was a push job and my part worked . I'm going to check out the links but do you guys have any suggestions for me on the best way to continue the learning of such systems? The stuff I worked / am working on is pretty much just simple line drawings connect the dots stuff but the possibilities seem limitless! I'm way too excited, I'll shut up now.
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| Afterhrs | Steddar, I would contact the manufacturer of the PLC that you just installed, they may have a trial or demo version of the program for that PLC. The trial versions do not include all the capabilities, but they offer some insight on that particular model. |
| drew2 | I would like to know if there is a book or a site specifically for Allen-Bradley PLC's that someone could point me to? Also I would prefer if the book or site had something about programming also |
| Afterhrs | Jimmy, I would also like to say this one best introductions I have witnessed. You condensed half a text book of information into a few short pages. OUTSTANDING!!!!!!!!!!! Jerry |
| JimmyDee | Thanks. I wrote it to see if we could get some traffic started in the automation area. Looks like it worked. Sort of. Jim |
| Allphase | http://plcguide.mrplc.com/index.html This site is AB specific (SLC-500 and RSLogix-500) There are downloadable sample programs as well |
| JimmyDee | quote: This is a good site to learn from. Jim |
| gftodd | PLc's are great process controllers. We use SLc'505s & PLc 5's mostly to controll our maufacturing. Hundreds of them, what I don't like about them are saftey issues. I am a floor electrician and have a area of responsibility. PLc's are our tool for trouble shooting manufacturing processes in our area's. Could be robotics, conveyors,welding, sealing ect. The fact someone in a remote location can & has manipulated logic without having knowledge of what caused a breakdown can put people in dangerous situations. WE try to have safe guards in place. Does anyone use any type of MPS safety system? |
| Afterhrs | I have placed all my SLC-5/04's in run mode and pulled the key out of all them. This is supposed to stop all remote programming except for data tables.  I have not found a fix for a preventing access to the panelview HMI as of yet.  I am not sure what you mean by "MPS safety system." Please explain. Regards, Jerry |
| gftodd | Monitored power systems. reduntant safety relays. We have lockout boxes at entrance to all of our work cells. These gate boxes have major & minor selector switches that will diable either type motion. Major motion (robots) minor motion a conveyor or turn table. Depending on the job you will do while your in the cell. Here's the problem we are only allowed to use this system during production, after production hours we have to go to full lock out. So how is this system safe for me during production but not after? This system is used in part because of osha regulations against software (PLC)safe guards. |
| Afterhrs | gftodd, The engineer or maintenance manager would be explaining all aspects to me about the legality of this LOCK OUT / TAG OUT procedure or the company can can explain it to O.S.H.A. I dont consider myself a trouble maker, but I will always stand up on SAFETY and CODE ISSUES ! Best Regards, Jerry |
| gftodd | The problem is MPS system are OSHA regulations! We can use it during production but must lock out after production. It's not that it is unsafe, I just don't understand double standard. By the way are your panelviews AB if so what style. |
| Afterhrs | I would have to see the OSHA regulation and the schemetic for myself. LOCK OUT / TAGOUT is supposed to prevent any and all forms of energy release. If does not follow OSHA 1910.147, then it is not compliant. The double standard does not seem right, it is my guess that the regulations may have misinterpeted or somewhat bent. It is my understanding that OSHA does not officially approve any devices, but a company can claim that a product does meet OSHA specification.......Lawyers?!?!?! We run various panelviews, 1400e, 1000, and 550. Best regards, Jerry |