I have a duty-assist pump set where the assist cut-in point is to be a preset % of the lead pump loop's output (say,80-90%). My first pass would be to use the .MAXO variable in the pid function in conjuction with the .OLH bit and enable the second pump when the .OLH bit is set. The problem that I see with this is that if the SP is only just above what the lead pump has achieved when it requires assistance, as soon as the assist pump makes an impression on the PV, the pair will start oscillating as the lead pump comes out of its enforced saturation, calls on the assist pump and they go round again.
Next thought: add some hysteresis to keep the lead pump CV locked until the PV drops to a lower 'release point'? The only way I can see to do this is to track the CV and when the assist point is reached, use this value (suitably scaled) as the .SO input and hold it there using external comparators until the PV drops to a lower 'release' value.
All sounds very messy. How does the rest of the world do it? Thanks for any suggestions.
Next thought: add some hysteresis to keep the lead pump CV locked until the PV drops to a lower 'release point'? The only way I can see to do this is to track the CV and when the assist point is reached, use this value (suitably scaled) as the .SO input and hold it there using external comparators until the PV drops to a lower 'release' value.
All sounds very messy. How does the rest of the world do it? Thanks for any suggestions.
