Hello Experts!

Perhaps this inquiry is somewhat out of place, but because of its urgency, I hope that, through its small relevance, it may be allowed, and, hopefully, be assisted, in this very helpful and informative place.

I just want to know your professional thoughts on my first implementation of a (slightly) complex PID control as a possible solution to a requirement of controlling a tank level, at the same time, the ratio of it's two inlet flow.

Please see the attached file for my (i know I can do better, but please forgive me for now!) sketch of the simple problem, as well as my proposed solution, through Cascade and Ratio Control Strategy.

I think this can be simpler (without cascade, just divide the output of the Level controller into the SP ratio, then send to the two valves), but I also think this proposed solution has it's merit while being more complex (the advantage of cascade control, the possibility of independent flow control, etc.), but I can probably be wrong, or missing something, for, as I've said, this is my first implementation, so please help me.

I would greatly appreciate any response, especially a quick one, for I need this to implement very soon.

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Here's the attachment:

http://wikisend.com/download/947514/20130221_222954.jpg

Thanks much!

 

This cannot work: A ratio controller simply reads an input variable (typically called the “wild flow”) and multiplies it by the value of the SP. This – normalized w.r.t. the range of the secondary controller - gives the output of the ratio (in %).

In your scheme, the level controller changes the SP of the ratio. Thus all it would do is trying to change the distribution between the two streams (which it will not be able to in your scheme, since you have foreseen 2 outputs – which does not make sense for a ratio).

Better: Have the level controller act directly on one stream (valve or flow controller SP – if the dynamics of the flow loop vs. the level loop allow a cascade). Read the flow rate of this stream as input into the ratio which then steers the second flow controller.

H.H. Eder
www.act-control.com

 

Hello H. H. Eder!=')

Sorry for the delayed response!

> This cannot work: A ratio controller simply reads an input variable (typically called the “wild flow”) and
> multiplies it by the value of the SP. This – normalized w.r.t. the range of the secondary controller - gives the
> output of the ratio (in %).

Yap, I see, this is the typical application of Ratio Control. And yap, here, I don't have a "wild flow" from which the second flow should refer it's SP.

Though, the only difference I see is that instead of referring to the wild flow's PV, I referred to it's SP in deriving the controlled flow's flow SP. Would this really make a huge effect on my control outcome?

If so, then I think I can solve it by just deriving my controlled flow SP to the wild flow's PV, not to it's SP. While the wild flow's SP would be derived directly from the Level Controller's CV(Cascade). What do you think sir?

> In your scheme, the level controller changes the SP of the ratio. Thus all it would do is trying to change the
> distribution between the two streams (which it will not be able to in your scheme, since you have foreseen 2
> outputs – which does not make sense for a ratio).

It seems that this has some inaccuracy. The Level Controller, which is the primary controller of the cascade Level-Flow control, do not change the *ratio* per se, but the ratioed *SP*, which would be fed to the two flows (thus, the two would have a ratioed flow SP). The ratio itself will be constant (though can be changed by the operator).

> Better: Have the level controller act directly on one stream (valve or flow controller SP – if the dynamics of the
> flow loop vs. the level loop allow a cascade). Read the flow rate of this stream as input into the ratio which
> then steers the second flow controller.

This is close to similar to my proposed solution above (on my first paragraph), it's just you seem to suggest that I should not have a cascade anymore, but the Level controller should directly have the wild flow valve as it's CV/MV (in contrast to the Level controller's CV to be fed to the wild flow controller's flow SP).

I think this is a good solution, and a simpler one. Though, there also seems a disadvantage/s. First, the wild flow would not have flow controller anymore, which the plant seem to prefer to have (they want to have an option to have an independent automatic control to the wild flow, if desired). Though, I think I can still implement both, by having a selector in which it will determine in which controller the valve would be a slave (to the Level or to the Flow, but no cascade anymore, both would be single loops).

Anyways, can I ask you to give your thoughts about the difference (advantage/disadvantage) of our seemingly two viable options available (cascade or direct control, each would have the controlled flow deriving its SP to the wild flow's PV, not SP anymore)?

Anyways, thank you very much for the very helpful response sir!=')

I hope you can further respond.

Thanks much!