Control of Living Processes 2
Consider the differences in the ways that organisms respond to fixing dilution
rate and fixing nutrient concentration. There must be a very roundabout
scheme for adjusting metabolic rates so that the consumption of limiting
nutrient matches the feed rate to reach the exact concentration of S that
determines the specific growth rate coefficient equal to the dilution rate.
Actual experiments show undershoot or overshoot when a chemostat is disturbed,
and the system tends to be quite unstable at high dilution rates.
Control of an auxostat is complicated by requiring sensors
and pumps but is simple in terms of response. The concentration of limiting
nutrient will adjust quickly to the new set point and usually without overshoot
or undershoot if the controller is tuned properly.
Go "with the flow"