I'm very new to building any kind of automation, so please forgive my ignorance of best practices, PLC programming, etc. I'm coming to people who are a lot smarter than I am to try and find a solution.
I'm working on a project for a friend. It's a semi-automated bottle filling station. You line the bottles up with the nozzles, select the bottle size and press the cycle start button.
I'm using a PLC to control a 6 valve manifold to fill 6 bottles at a shot. The manifold is fed by an air operated double diaphragm pump at 20 PSI. I'm using time and pressure as my "variables" in controlling the amount of fluid that is dispensed to the bottles. I had thought about using flow meters to help, however my knowledge level is limited and budget is the main constraint on the project.
The problem I'm having is that I'll get everything dialed in to where it's filling the bottles consistently to the volume needed. Then I'll stop processing or shut the system down (even for 30 minutes), then when I start back up (with the pressure being the same, the timers being the same, etc.) it will no longer throw the correct amount of fluid. It usually throws really heavy for about 15-20 cycles. Then it gradually returns to the original amount or close to it.
My suspicion is that it has something to do with the system staying pressurized over a long period of time without cycling, but I'm not sure. Does anyone have an idea of what may be happening? How to test that hypothesis or for what's really happening? Do you need more information?
Any help would be greatly appreciated.
Possibly the system is filling up by gravity when it is off then it needs those cycles to clear. You need to route your tubing to minimize holdup or gravity feeding.
Your pump is probably not a precision volume to stroke pump like a chemical injection or peristaltic pump. A peristaltic pump might be a better pump for your application. Do you really need 20 psig to operate your pump and transfer the fluid?
I suggest your first move is to get in touch with the person who designed the machine and have him take a look at what was actually built to make sure that what was actually built was what the guy that designed it intended.
I strongly suspect that the PLC programming technique is not at issue here, short of just having some kind of mistake that you have not been able to find. I don't see how you can rely on a timer to get any kind of reliable fill.
The fluid temperature is the same as the ambient or machine temperature? The nozzles or some parts might shrink or grow over temperature.