MisterEzell
Member
Hi guys,
I'm currently a PLC student in my 40's. I just started going to a local tech school, and really am kinda slipping in the mud trying to get my old gray matter wrapped around all this. My instructor seems to enjoy dumping projects on us that we are ill prepared for, and seeing where the pieces fall. He takes a sink or swim approach. He handed out a project the other day that I know just enough about to know that I don't know much about it. Especially where to start. He does, however, highly recommend that we use every resource we can get our hands on to learn, and accomplish the project. The internet is a resource. Right? Probably the single greatest resource ever! With all this in mind, I'm not asking anyone to do my homework. I'm simply asking for guidance, and education. Point me in the right direction, and I'll learn as I see the different components come together and function. Below will be word for word what the project asks us to accomplish. Thanks in advance for any and all input. Right now I'm using TLP Logix Pro Simulator at home, but I use MicroLogix 1200 at school.
I'm currently a PLC student in my 40's. I just started going to a local tech school, and really am kinda slipping in the mud trying to get my old gray matter wrapped around all this. My instructor seems to enjoy dumping projects on us that we are ill prepared for, and seeing where the pieces fall. He takes a sink or swim approach. He handed out a project the other day that I know just enough about to know that I don't know much about it. Especially where to start. He does, however, highly recommend that we use every resource we can get our hands on to learn, and accomplish the project. The internet is a resource. Right? Probably the single greatest resource ever! With all this in mind, I'm not asking anyone to do my homework. I'm simply asking for guidance, and education. Point me in the right direction, and I'll learn as I see the different components come together and function. Below will be word for word what the project asks us to accomplish. Thanks in advance for any and all input. Right now I'm using TLP Logix Pro Simulator at home, but I use MicroLogix 1200 at school.
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Pump Control with a PLC
PLC LAB
Engineers and technicians are now designing their own pump station controls, using a PLC and assorted sensors with other inputs, instead of buying one preconfigured and preprogrammed. This "do-it-yourself" method of pump station control gives you exactly the type of control you need and want, and you aren't forced to buy something that almost fits. There are a number of things you have to remember to include in the PLC programming.
So, here are some design considerations for you to keep in mind when writing the pump sequencer for your PLC. Let's say you have five pumps. Four will be in the sequence; one will be on standby. A good way to equalize wear on these pumps is to have the sequence alternate. First you should set it up so that the first pump to come on is the first pump to go off (FIFO) as demand drops. So, the first sequence is 1-2-3-4 on and then 1-2-3-4 off. But adding in alternation, the next sequence should be 2-3-4-5 on and then 2-3-4-5 off. The next would be 3-4-5-1 on and then 3-4-5-1 off, and so on. Sometimes, you may want to base your alternation scheme on pump run time, instead of FIFO, or in addition to FIFO. This complicates the programming because now you also have to take into account which pumps have been running the longest. You should also establish a hysteresis, so that your pumps are not bumping on and off. Pumps go on at low level and go off at high level. Your logic should keep them off until low level is reached again. If you have an analog input (flow or level) to the controller, you can begin to deselect pumps as you approach high level, and begin to call pumps on as you approach low level. You need to be able to sense the "pump running" condition. There are lots of ways to do this, and they all take time. Flow, pressure, current, shaft rotation, all have been used to sense pump run. Remember to program in the necessary time lag to have the pump come on line and up to speed before calling "pump fail".
I suggest that it is also very useful to add a contact to the H-O-A switch "auto" position so that the PLC can tell when the pump is not ready to be called. A pump can be out of service for maintenance, or it can have previously failed. Pulling in this contact moves the pump out of the alternation scheme until the contact breaks again. This is all part of working out the pump fail logic, which, of course, an old-fashioned pump controller did automatically for you out of the box.
Finally, I warn, don't forget to take into account pump/control operation in the event of a power fail, program restart and/or level sensor fail. You probably don't want all your pumps to start at the same time on power up, or when the level signal goes to zero. When you design a PLC-based pump controller, you have complete control over what the controller will do. This means that you have to be sure you've thought out the entire functionality you want the controller to have. It is too late when you start the pump station up and pumps go on and off when you don't want them to.
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How's that for a relative beginner?