The override control solution consists of two PID controllers, each controlling one Process Variable (PV). A switch determines which control output to select. The switch condition is often stated as: select the smallest (or largest) output. As you can see in the block-diagram, there is a feedback path from the selected output m to both PID controllers. This is done to allow for output tracking: the control output of the controller that is not active should track the selected output, to avoid bumps. The PIDs need to be tuned individually, when they are active. Of course you can use the DotX PID Tuner
In practice, the switching conditions are often chosen 'suboptimal' resulting in sawtooth like behavior, instead of the desired smooth transition from one controller to the other.
Getting the switching conditions right is essential (for a good result). In (Schuurmans 2019) we showed that the standard switching conditions may require some (minor) adjustments. We found that in the presence of stochastic disturbances (which are usually present!), the switching may take place too often, resulting in lousy results. You can avoid that by filtering the signals before making the selection (so use a filtered m1
to decide), or by introducing some simple additional dynamics in the decision for switching (see paper for details).
A.H. Glattfelder and W. Schaufelberger, Stability of discrete override and cascade limiter single loop control systems. IEEE Transactions on Automatic Control 33, 1988
M. Cannon, Q. Cheng, B. Kouvaritakis, and S. V. Rakovi´c. 'Stochastic tube MPC with state estimation.' Automatica, 48(3):536 – 541, 2012.
A.A. Lopez, J.A. Joseph, 'On the stability of override control systems', LAAS-CNRS, Toulouse France, Rapport N° 98304, 1996
J. Schuurmans, 'Review of override control methods', Proceedings of the 29th European Symposium on Computer Aided Process Engineering, 2019
Please let me know what you think, if you would like to respond to this post (or any other). If you would like to read more on override control, press the donwload button.