PID flow control help in RSLogix 5000

Hi all, this is my first post here.

I am an industrial electrician but have done some PLC courses and basic programming.

A client of ours is building a new additive system for their process line, and wants us to do the program.

Basically, I need to control the flow rate (l/min) of an additive depending on the product type selected, and the speed of the product being extruded (cuts/min).

I have a SEW variable speed drive controlling the pump, an ABB flow meter will be giving me my flow rate, and the average cuts/min is already being calculated and used in the existing program.

I have done the logic already, but I have never used the PID instruction before.

I just want to know if I am on the right track, or if there are still things I am missing or doing incorrectly.

FLOW_METER_L_MIN will be the analog flow meter converted to L/min

DRIVE_SUGAR_PUMP_SPEED_SET will be the analog speed signal being sent to the drive (I am not sure what unit or format this has to be in yet, I still need to research the drive)

There is logic set up to move the appropriate flow rate required to PID_SUGAR_SYSTEM.SP depending on the average cuts/min.

Is there anything else I am missing?

How will the PLC know how quickly to ramp up the speed of the drive depending on the setpoint compared to the current flow rate? Will it need to run through some sort of tuning?

What format will DRIVE_SUGAR_PUMP_SPEED_SET be in from the PID instruction?

Thanks in advance for any help.


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Make sure the units of your flow signal and setpoint are the same. The output will likely be scaled 0-100. I'm not familiar with the SEW drive. If it accepts a 4-20 mA drive signal then you will have to scale the output of the PID to whatever is the full range of counts that your PLCs output card takes.
The ramping and speed will be determined by the P, I and D settings. Do a search on the forum for PID and you will get many hits.

I hope that the pump supplier knew that you were planning on controlling flow with the pump RPM. This can be tricky and requires someone to evaluate the pump curves and the system operating range (based on the system flow characteristics). This can be frustrating to do if you are not familiar with pumps, piping systems and other mechanical characteristics of the system.

Few things to note:

The fluid will not move until you meet a minimum head pressure, then depending on the system pluses and minuses the flow may drastically overshoot (if there is any siphon effect) when it does start flowing. So once the flow requirement drops below a point that you will have to determine at startup there is no reason to keep the pump running.

Ask the mechanical engineer for the pump curve. Try to understand what is shown on that graph and relate that to what you are trying to accomplish with the PID. Maybe that will help with adjusting the system.

Good luck,

Darren

Also, at least look at the online help for the PID. You need to schedule it's execution with the time period (loop update time) that you setup in the PID configuration tab. You can do this with a timer for most pump applications (remember the mechanical system can only react so fast). I would think that depending on what you are doing 500 - 1000ms should be fast enough (maybe even greater could be acceptable).

Thanks for the advice so far, I will read the online help regarding PID.

I don't think the system has to be that accurate. They are running just fine as it is now with the operators checking the cut rate and adjusting the additive flow rate manually at the moment.

I am thinking of having some sort of 'prime' stage, where when the system is first started, the pump is set at say 50% for 30 seconds or so until the flow rate and cut rate are being measured accurately and have levelled out, then letting the PID control will take over.

I will look into the specifications of the pump also.

If your PID is not in a scheduled task you will need to put a timer bit in to execute it at a fixed rate. The Preset of the timer must be converted to seconds and put as the update time for the PID - otherwise your tuning values will be meaningless.

My PIDE is located inside a periodic task.
I´m using PIDE instead of PID, I use a control valve and a pump instead of a variable speed pump, just because I already had both.
Flow1.jpg shows PIDE instruction, PV is taken from Flow2.jpg.
I read 4-20 mA data from analog input ( coriolis flowmeter) and convert it to 0-30000 kg/h this is the same value i have on the PIDE parameter, though i have hart interface on the flowmeter i do not use it because scanner is too slow and may cause problems to mi PIDE, so i read 4-20 ma directly.

A mass flow controller would be more appropriate. PIDE is not even required .