Split parallel control - a little known control structure

We describe a control structure that is commonly used in the process industry, e.g. chemical and petrochemical industries, for switching between manipulated variables (MVs), but which has received little attention in academia. It has one controller for each MV, typically PID-controllers, that control the same process value ( y ) but with different manipulated variables (u i ) and different setpoints ( r i ). The scheme is sometimes called “separate controllers with different setpoints”, but we suggest that a better name is “split-parallel control” (SPC), since the two controllers are placed in parallel in the block diagram, but the active control action is split between the two controllers, similar to in split-range control (SRC). SPC is an alternative to SRC, but it does have some advantages compared to SRC, including ease of implementation and the possibility to have different PID tunings for each MV. We also state some yet unresolved questions regarding the SPC structure, especially in regard to stability. Split-parallel control (SPC) uses setpoint separation to perform the switching, which is an advantage in some cases, for example, for bidirectional inventory control.