Need Help With PID Control

Dear Members,
I am doing a SCADA project on S7400H system. I am new to PID control and am facing a problem with programming PID control for three pumps to maintain a constant pressure at the outlet water header. The control phylosophy is that based on the pressure Set Point, I need to start one pump (lead) and increase its frequency from 0 - 50 Hz until the frequency is achieved. If the pump reaches 50Hz freq. and the SP is not reached then start the second (lag) pump and again increase its frequency in the range 0 - 50Hz to achieve the pressure Set-point. If during operation, any of the duty pumps fail then start the standby pump. I really need urgent help from you guys as i have a very tight schedule to complete the job. I know i am posting this a bit later at the project stage but i need your expert opinions and help on this one. If any of you have implemented similar control phylosopy in past on siemens controller, i would very much appreciate if i can get a copy of the logic used.
Will be awaiting your kind relpies.

Inayat
[email protected]

If the pumps are equal and have a VSD each, I would run all pumps in parallell. If a pump stops or starts, the controller will adapt the speed, and you will not have to make a haywayer program with much testing to get a robust solution.

Kalle

I usually split the PID output into two.
0-50% = x-50Hz on pump 1.
50-100% is x-50Hz on pump 2.
If one doesn't start (eg. monitor the frequency) stat pump 3 in it's place.

What kind of pumps are you using? The reason I'm asking is that not all pumps move any water at low speeds. A centrifugal pump usually start moving water at 15-20Hz. Hence the x-50Hz.

Thanks alot Jasper and Kalle for your prompt reply. Splitting the output into two seems a good way ahead. I request you if possible kindly send me a snapshot of the logic so i can use it for reference and understand it better. Your kind consideration will be very much appreciated.

Jesper how do i split the output into two?..and how should i scale the %age output to produce an analog signal for a Analog Output module (0-10V) for teh VFD speed control. A snapshot of the logic will b very much appreciated as mentioned earlier. Thankyou for your time.

Here is a sample, This has a flow chart included. It is for an ammonia plant with multiple compressors.

You may be able to adapt it for your needs as it is specifically for Lead, Lag, Lag Lag control.

Split the CV like this:

-----------------[FC106 (in=cv, out=pump1)]-

--[if CV>=50%]---[FC106 (in=cv, out=pump2)]-

--[if CV<50%]------------[MOV 0 -> pump2]-

FC106 is a build in unscale function if i remember correctly.

Dear All
Thanx for your support. I think i am able to implement the logic at last after some hurdles. Many thanks to all of you and especially esper. I have simulated it and works fine but I will be sure about it when the plant is in operation and the logic is tested.
By the way i have a new task now .... I have to fetch some data from around 12 Siemens S7300 PLCs via ethernet network. I would like to use the GET/PUT function. I only need to GET some data from them for monitoring purposes. The data from each PLC should not be more tehn 10 bytes at the most. I need your recomendations for what SFB to use. Should i use the GET function. How i can estalblish a connection b/w the S7300 PLCs and my S7400H system. Both sides have respective ethernet CPs. Also i would like any advice about the connection resources i have. I am alrady having redundant ethernet connection with another H stationa and around 6 WinCC stations.
Any recomendations will b very much appreciated.
Thanx and have a good day.
Inayat