Tank evel conrol (powerflex 525 pump feeding tank)

Hi Everyone,
I need help in setting up the tank level control using power flex 525 to control a pump which feeds the tank. I am planning to use the PIDE to control the level but according to my customer request he wants me to ramp the pump to fill around 60HZ (Max) speed initially until I fill the tank half (20 Litres).

My plan is to use a manual speed command to Output frequency until I reach the required setpoint keeping the PIDE disabled. once I reach the desired level I will activate the PIDE in Auto mode. This way I can have my PIDE manual mode as a fail safe entry for the operator to enter the speed directly into the PIDE CV Value.

Please let me know if you need any further info and if my scenario is correct.

Thanks in advance.

Might want to consider using the HandMode feature of the PIDE instead of disabling the loop for bumpless transfer when you go into auto. This document describes the different modes:

https://literature.rockwellautomation.com/idc/groups/literature/documents/wp/logix-wp008_-en-p.pdf

Thanks for your reply Robert.
I looked at the manual.
Could you please elaborate on what's the difference between Manual Mode & Hand Mode?

[email protected] said:

Thanks for your reply Robert.
I looked at the manual.
Could you please elaborate on what's the difference between Manual Mode & Hand Mode?

Click to expand...

They're mostly the same, but give you two hooks into setting the CV directly and returning to Auto via bumpless transfer. So Manual mode would be used for your ramp speed and Hand Mode for your Operator override control. The point is, don't disable the loop during the ramp.

Got it.
What would be the best practice for the mode switching...

When the operator starts the shift I want to provide him a Start Button on the HMI Screen which puts my PIDE into manual mode (ramp to 50hz) from CV INIT Value until the Tank is filled half & when it reaches the desired level I will switch to auto mode using the PLC Logic. (Reached the level)

For any reason if the operator wants to override the process. I will provide an Override button to enter into Hand Mode and also provide an entry to enter the required % of speed.
At this point both my manual & auto mode triggers becomes zero.
Please correct me If I am wrong.


Thanks.

Do you really need PID control?
Depending on the application, you can set High Level and Low Level set points and then turn the pump ON and OFF accordingly.

Pump is currently being controlled by a Power Flex 525 which needs to be modulated by a PID loop to maintain the level accordingly.

[email protected] said:

Got it.
What would be the best practice for the mode switching...

When the operator starts the shift I want to provide him a Start Button on the HMI Screen which puts my PIDE into manual mode (ramp to 50hz) from CV INIT Value until the Tank is filled half & when it reaches the desired level I will switch to auto mode using the PLC Logic. (Reached the level)

For any reason if the operator wants to override the process. I will provide an Override button to enter into Hand Mode and also provide an entry to enter the required % of speed.
At this point both my manual & auto mode triggers becomes zero.
Please correct me If I am wrong.


Thanks.

Click to expand...

Yes in general....Not sure what you meant by "At this point both my manual & auto mode triggers becomes zero". By using the Hand and Manual mode of the PIDE, the resultant CV is always the value that controls the pump. So the logic is simply scale the CV appropriately for the Pump speed. Nothing ever goes to zero. In Manual mode, you write the value you want directly to the CV. In Hand mode, you write the value to HandFB and the PIDE copies it to CV for you. You could do it all in Manual mode and selectively write the CV based on Operator override or Ramp, but it just is a little clearer using two modes for two purposes.

Not sure what you meant by "At this point both my manual & auto mode triggers becomes zero"
Prog Auto Req for Auto Mode
Prog Manual Req for Manual Mode
Prog Hand Req for Hand Mode.
When Hand Req is set to 1 I want to send 0 to Prog Auto Req & Prog Manual Req....

What is the expected set point for the tank level? 50%?

Its going to be 75%.

[email protected] said:

Not sure what you meant by "At this point both my manual & auto mode triggers becomes zero"
Prog Auto Req for Auto Mode
Prog Manual Req for Manual Mode
Prog Hand Req for Hand Mode.
When Hand Req is set to 1 I want to send 0 to Prog Auto Req & Prog Manual Req....

Click to expand...

You don't need to do that unless it's a visualization thing on your HMI. The PIDE will route the correct value to the CV based on mode. For example, the Manual SP is 60Hz for the fill to 50%. You don't need to zero out that SP. When in Manual mode, just MOV Manual_SP to CV. When in Auto, the CV is updated by the PIDE and the MOV instruction is disabled. When in Hand mode, the PIDE copies whatever value is in the HandFB parameter to CV. You can link to this via your HMI. Then all you do is scale the CV always to your pump SP.

Great Robert
One last question .
I want to set High Level & Low Level Limits to Deccel the Pump to 0 hz without filling the tank anymore. Sending a 0 to CV should work right?


Thanks

Why set it to manual when filling the nature of PID is that at empty level or low level the PID should ramp up to full output until the algorithms of the PID determine that it is close enough to set point to ramp it down. That is one of the defining features of PID. The chances are that if the level goes above set point then it will ramp down until it's output would reach zero. You should only need high level limits to stop the pump (possibly alarm to hold the process if this state continues for a period of time). Operator set point can be introduced just by a button on the HMI to set the output but then you need to consider if the operator puts in a higher set point or it goes above the normal high level. What do you want to happen in this case.

If the PID is set up correctly then it should try to reach set point as quickly as the system will allow without overshoot.

This should be simple!!!!

parky said:

If the PID is set up correctly then it should try to reach set point as quickly as the system will allow without overshoot.

Click to expand...

Yes!!!! This requires only a PI controller.
WHY?



However, of the set point can be 75% with an acceptable variation of say 2% then this can be done with a SCP block or similar. The pump output is 100 when the water level is below 75% - 2%. The pump is off when the level is above 75% + 2%. The pump output ramps up from 100% flow at 73% level down to 0% flow at 77% level.


There is no need for an integrator unless maintaining exactly 75% level is required.


There are formulas for computing the controller gain and integrator time constant as a function of pump flow and surface area of the tank. It is possible to place the closed loop poles on the negative real axis is the s place so there will be no overshoot.



It would be nice if Phil had a sticky area or a place more obvious than the download section. No one goes there.


As engineering manager I would ask.
Is the pump capable of supplying flow greater or equal to the maximum demand?
How much greater?
If not then for how long will the demand be greater than the pump flow.
Is the tank surface area constant as a function of level? This affects the gain.


I don't see why this drags on and on. It should be simple.