PID loop weighted sum

hello,
I am a beginner and also new here, I was reading something about PID loop, there is two tanks , each with a sensor of pH, the control narrative says that a weighted sum (to assign priority) of these two measurements is taken and considered as control variable. then a dosing pump is used to reach the setpoint.
I don't understand how is this going to work .
please help

PID Controllers are actually weighted sum of the 3 control parameters i.e. P & I & D the sum of these parameters are Weighted sum approach is used to optimize both settling time and maximum overshoot.
Simultaneous tuning of PID parameters for both control areas are performed.
This really applies to closed loop control which is the norm.
Perhaps others who may be more up to date with PID control can explain it better or correct me if I'm wrong.

(to assign priority)

is there only a single source of dosing?

The size of the tank will have a big impact on the dosing as will the flow of the ph correction chemicals and if you agitate/mix

I would say you need to provide more info to get a proper answer or good suggestions

cjd1965 said:

(to assign priority)

is there only a single source of dosing?

The size of the tank will have a big impact on the dosing as will the flow of the ph correction chemicals and if you agitate/mix

I would say you need to provide more info to get a proper answer or good suggestions

Click to expand...

yes there is only one source of dosing.
I am trying to understand why there was a weighted sum of two measurements

yes, but the weird thing is the summing of two different measurements before adjusting the loop

Are the tanks connected together ?, there only may be one source of dosing but does it dose both tanks via separate valves ? Are the tanks mixed together on discharge ? it all makes a difference.

I'm imagining two tanks that are different sizes or otherwise have differing response to dosing. so your PH feedback for the dosing loop might be 75% PH sensor 1 + 25% PH sensor 2.

Nick

Without a P&ID of how these elements are connected together as well as sizes, it's hard to say why the weighed sum is used. I got a feeling that the tanks are connected or the dosing doesn't have selection valves.

pH control is not a good place for beginners to start. pH control is non-linear, except for small sections of the "S" curve. There are many, many, many variables to consider, when adding a reagent, it may be several minutes or more before you see a response. Some times there are cascading vessels, and you dose the first one, but control off the last one.
For a weighted sum, you may have double tank-A, plus tank-B and divide by 3 for your process variable. Just a guess.

Ken Moore said:

pH control is non-linear, ...

Click to expand...

... to say the least; even at linear sections of the S curve it is still a log plot, right?



The math of a "weighted sum" (weighted average?) usually assumes linearity, so a proper weighted sum would require transformation of variables before doing the weighting. What is to stop some external process biasing one tank's pH value and the control system driving the "weighted sum" to the setpoint, e.g. one tank at 6, one at 3, setpoint at 5?


I'm with @cardosocea here: we need to know more about this process; can the weighted sum formula be posted here? Schematic of tanks, piping of dosing, piping of any inter-tank continuous mixing, piping of inlet and outlet, piping of measurements, whether this is a batch or continuous process?