control pH with step7 SFB41

I have to get a pH set point in a tank working with 2 4/20mA controlled pumps (acid and basic),i know that pH is not a linear process, so is it right to use SFB41?

PID is not suitable for non-linear process like pH control. The way I usually do this is to use a ramp to drive the output up until the pH is within a certain range from the desired set-point say 0.1 or 0.2 pH, then stop the ramp and start it again only to stop when you get even nearer or reach the desired set-point.

Whether you then switch over to some sort of control at the point when you arrive at the set point depends on the process. For slow processes like a water treatment plant it may suffice to stop the ramp and then restart it when the error exceeds certain value. For faster processes or if you require more precise regulation then you can start a normal PI controller like SFB41 when you get to within, say 0.1 pH of the desired set-point.

RMA said:

PID is not suitable for non-linear process like pH control. The way I usually do this is to use a ramp to drive the output up until the pH is within a certain range from the desired set-point say 0.1 or 0.2 pH, then stop the ramp and start it again only to stop when you get even nearer or reach the desired set-point.

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Hi RMA, with this approach do you have good result? because the slope pH/reagent (titration curve) change a lot (almost vertical around pH=7)

I've used it mostly for water treatment plants where we're using a screw drive to deliver chalk in powder form. Running at maximum speed the screws deliver 50 - 100 kg / min. running the ramp over 30 - 60 seconds usually gave good enough control that we didn't need any further control.
The key point is that by stopping the ramp abruptly you go from delivering up to 100 kg / min to delivering < 5 kg / min in a few seconds. With carefully chosen switching points this enables pretty good and yet still fast control.