Problem: control PQW to a input analog

Hi,
I have a pump connected to a VLT and I want to control is PQW to a certain flow.

A-----PUMP1----Flowmeter1--------------> OUT

B-----PUMP2---------Flowmeter2--------|

I will explain. Pipe A I have a variable flow and I want to control PUMP2 to 25% of the flow in point. How can put the correct value in PQW for VLT to pump2?

When Flowmeter1 is 100m3/h, pump2 need to send 25m3/h. Allways in that porpotion. If flow1 200m3/h flow 2 is 50m3/h. How can I adapt my PQW to VLT to control to that porpotion?

How I do that? Thank's for all.

PS

I did a similar thing for blending sauces.

If you have 2 flowmeters its easy, feed each flowmeter back into the PLC and you then know the speeds you are running for adjustment.

Your starting point for reference should be calculated from your motor reference frequency, the full speed rpm rated at that frequency, the gear ratio of the gearbox between the motor and pump and the litres per revolution of the pump.

With the VSD you can set up the output range to input, i.e. if 4-20mA the settings should allow you to set 4mA = 0 and 20mA = 100Hz for example.

With all this info the rest is maths and scaling.

Without looking atv books I believe the PQW range is 0 to 27648.

I would try to scale the inputs and ratio's into realistic numbers, and convert to output format prior to writing to the PQW's.

This looks like a closed loop control system to me. The feedforward term will be as Peter describes above. A PID controller will trim the feedforward term using FlowMeter1/4 as the setpoint and FlowMeter2 as the feedback.

Yes I agree with PeterW, but if the pressure is much high in A than B? Is not easy.

Like Simon said with a PID control I can do that. But PID control from siemens are not simple to use. There is any PID or PI control simple to work to siemens?

Thanks for all,

PS

A use my own simple PI controller

ProportionalTerm = ProcessError * ProportionalGain
IntegralTerm = IntegralTerm + ProcessError * IntegralGain
OuputTrim=ProportionalTerm+IntegralTerm

You'll need a reset to zero the integral when the process is not running and you'll need to apply some limits, but something this simple should be adequate.

Maybe I am missing the question but..

how about this example


L piwxxx // analog input- flow meter
itd // convert to a double
dtr // double to a real
L 1.250 // load 1.25 or 25%
*r // multiply
rnd // round
t pqwxxx // Transfer to VLT drive - analog out

I used it when blending red sauce to meat sauce for bolognese blending.

The ratio was something like 3:1 red:meat, similar to your ratio, the pipes if I remember correct (this was 10+ years ago) were something 3" for red and 1 1/4" for the meat going into a combined 3" pipe to the fillers.

The flowmeters were micromotion and installed between the positive displacement pumps and an in-line mixer.

I used no PID, it wasn't necessary, any back pressure was detected by the flowrate and compensated with the pumps, which were on very inaccurate VSD's (eddy current TASC drives), therefore the whole accuracy was controlled by the flowmeters.

I have no idea what your application is, nor the accuracy you require but for mine it worked fine, as a matter of fact if overcame a major problem with the product.

Jay Wilson said:

Maybe I am missing the question but..

how about this example


L piwxxx // analog input- flow meter
itd // convert to a double
dtr // double to a real
L 1.250 // load 1.25 or 25%
*r // multiply
rnd // round
t pqwxxx // Transfer to VLT drive - analog out

Click to expand...

The only problem with that is that you have no positive control feedback to correct error.

When I controlled similar, I had a ratio required and a full speed target. I then calculated what throughput was required from each and controlled each independantly with an error on each, which if exceeded would stop both pumps.

Jay - the way I see it is that if the flow was exactly as set by the drive speed, there would be no need for a flowmeter and you would just output two speeds in the ratio required. I'm guessing that this is not the case so to get the flows in proportion you must measure both flows and adjust the drive speed for pipe B to maintain the desired proportion.

SimonGoldsworthy said:

Jay - the way I see it is that if the flow was exactly as set by the drive speed, there would be no need for a flowmeter and you would just output two speeds in the ratio required. I'm guessing that this is not the case so to get the flows in proportion you must measure both flows and adjust the drive speed for pipe B to maintain the desired proportion.

Click to expand...

No NO NO NO....

This was exactly what we had, which is where the problems came from, we received record complaints from the product.

It would work OK'ish for awhile but without any feedback to check the control it could wander.

The TASC drives didn't help, but the back pressure from the blending process would play havoc. Flowmeters were the solution and positive feedback control.


EDIT: just read the whole bit andrealised you were arguing the a same

pmsilva said:

Yes I agree with PeterW, but if the pressure is much high in A than B? Is not easy.

Click to expand...

It looks like line B is discharging into A. If that is the case, then if the pressure in A is higher than the pressure in B you will never have flow from B into A. (Water runs down hill, flow is from higher to lower pressure ALWAYS.)

If the lines are not connected then the difference in pressure doesn't matter.

It would help to know what kind of pumps you have in each case. This is as much a mechanical engineering problem as a control problem. A positive displacement pump, like a gear or peristaltic pump, will behave differently than a centrifugal pump.

simon your PID is in step7? Could you explain better how it works, or give the code? thanks.

I need to know the flow in A and I need to control flow in B... with your given solutions my conclusion is that I need really to use a PID control.

If someone have a simple code PID control to present thanks.

PS

If you are not comfortable coding a PI block yourself I suggest you use the Siemens PID block.