Can someone elaborate 3-point step control on Auto and Manual mode in general? I have a KS90 controller controlling a servo motor from KROM SCHRODER GT50 to actuate a combustion air valve as part of gas mixing station.
Generally 3 point control is to provide basic step control to a device such as open/close a valve as in your example. 3 Term control (PID) is too complex and is impractical as you have only open and close commands without any idea of valve position.
Assuming your valve servo motor can inch your air valve open, depending on process requirements you can open it fully in one go or inch it very slowly, or anything inbetween to suit. Writing software is not difficult - I have written algorithms for penstocks controlling water flow ... inch close to reduce, inch open to increase flow. A similar algorithm can be used for increasing/decreasing the speed of a variable speed motor driving a water pump.
This is Auto - for manual control you would merely inch the valve by hand.