Often, when new sensor or actuator hardware be-
comes available for use in a control system, it is desirable to
retain the existing control system and apply the new control
capabilities in a gradual fashion rather than decommissioning
the entire existing system and replacing it with an altogether new
control system. However, this requires that the existing controller
remains in action, and the new control law component is added to
the existing system. This paper formally introduces the concept
of Plug-and-Play control and proposes two different methods
of introducing new control components in a smooth manner,
providing stability guarantees during the transition phase as well
as retaining the original control structure. The applicability of the
methods is illustrated on two different practical example systems,
a livestock stable climate control system and a laboratory-scale
model of a district heating system.