PID Process Control/PLC

kkidd09

Member
Join Date
Jul 2017
Location
Wisconsin
Posts
2
I am currently trying to implement a process control of the temperature of water going down a drain after it has gone through 2 radiators. I have 5 RTDs and an electric actuator. I am trying to use a setpoint, the last RTD in the process as the Input(Process Variable), and the electric actuator as the output(Control Variable) which will inject cool water as needed to maintain setpoint. The actuactor works with an AC modulating 4-20mA input and 4-20mA output. Any tips on the best method of supplying the actuator with the 4-20mA input? Any tips on the best way to set up my PID in RSLogix500? Any help at all would be great!(y)
 
post the model of the actuator but in general they get the 4-20mA DC signal from the PLC's analog output card the AC is normally only to power the unit and motorized parts the mA signals are isolated from the AC and almost always DC.

I say almost always as I have never seen a 4-20mA AC signal but they might exist for whatever reason.

What PLC are you using and do you have an Analog Output module?

You PLC analog output would tie to your actuators Analog input for "position setpoint" to tell the actuator where to go. the actuator analog output would normally be used for "position feedback" to tell you the current position, and tied into a PLC analog input module.

As for the PID setup, tuning wise, it will depend on a lot of things in your process.
 
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http://sharpevalves.com/images/Electric Actuators/SEA Spec Sheet 2-17-10.pdf

Above is the link to the model actuator. It is a SEA-3 model. According to the wiring I could do an AC or DC modulating setup. The PLC I am currently using is a SLC 5/05 CPU. I do have an analog output card but do not have any more free slots in the chassey, so I was hoping to be able to do this through the Point I/0 I have set-up through this PLC if possible? I understand your explanation of what would need to occur. Would this be possible through Point I/O?
 
According to the wiring I could do an AC or DC modulating setup.

in terms of power supply it actually depends on the option you ordered it with, from what it looks like
SEA3-1 = 220VAC
SEA3-2 = 24VAC

SEA3-3 = 12VDC
SEA3-4 = 24VDC

I would double/triple check the nameplate for voltage ratings. Blue smoke is not cool.

The 4-20mA control signaling however will always be DC for these units. Are there any free channels on your SLC AOUT card.

I didn't think point I/O worked with the SLC line, but I don't real deal much in either of those products.
 
These kinds of actuators are quite common in the HVAC world. That market typically uses a 115Vac/24Vac step-down transformer to step down 115Vac line voltage for the motor supply voltage.

As Labeledas says, the control signal is always DC.
 
Red Flag - Soon to be obsolete

Kkid09,

You have been given a task to limit the discharge temperature, with economical effectiveness, by only using cool water when necessary.

I am not here to offer ideas to the simple task at hand, using the near obsolete control system.

My concern is the well being of your employers long term goals, as well as your awareness of long term goals.

The SLC 5/05 was developed when I was kidd88. I endorse that you first migrate your controller, and then take on the "tasks" of process and profit improvements.

http://www.rockwellautomation.com/g...gration-solutions/product-search/results.page

Regards
 
Though some may consider my methods a bit unorthodox, I'm a get it done with what you have kind of guy when I feel it is feasible. Even if they are old, those 5/05's are some nice processors. There is no need to upgrade because you want to control a valve.

Take your control cabinet, put a 3" KO in the side and set yourself a small panel with a 2.5-3" nipple. In that panel add an old 4 slot rack you have lying around and pull you some power from your panel off one of the buses into that nipple. Run you a ribbon cable over, insert your analog out card (and the analog in card if you need one). Quick question here: Are you SURE there isn't a single spare analog output you can use instead of this? Is the RTD already in the program? Anywho, if you need some extra rack space this solves your problem providing you have enough room to mount a small panel. No need for point IO. You are trying to do this cost-effectively, not re-engineer the whole system that works just fine.

Now, I would need specifics on that transducer you have, but it likely takes a 4-20mA and outputs it to something like a 0-1A valve or such. Now you will need to do some fiddling to get it right, but you can just use an SCP instruction. Couple key questions to help you with this. What is the hottest the water from the radiator ever gets? What is the lowest temperature the water from the radiator ever gets? You can then scale that valve to open and close as needed to cool the water going into the drain.

You would be using a PID if you were trying to keep the water temperature constant, which you are not. You are just trying to keep it below a set point. This system would be a bit reactive, but I feel it would accomplish what you are trying to do, be much simpler than you think you need this to be and would be a LOT cheaper, especially if you have a small PSU and 4-slot rack laying around.

If you think this system would be expanded in the future, go ahead and wire the analog IO up to terminal blocks, put yourself a hot bus/24VDC bus depending on what kind of descrete IO you use, and a neutral/0VDC bus, once again depending on what kind of IO you use. Then as you add a card you can add/label the terminal blocks for future expansion.

Anywho, that is my two cents with the information you have provided. Good luck!

EDIT: Just checked your valve specs. You don't even need feedback from this. It is just something else to go wrong. Your RTD IS your feedback.
 
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