In addition to keeping the controlled variables at their setpoints, the control system must “stabilize” the plant. We have here put stabilize in quotes because we use the word in an extended meaning, and include both modes which are mathematically unstable as well as
slow modes (“drift”) that need to be “stabilized” from an operator point of view. Usually,
stabilization is done within a separate (lower) layer of the control system, often called
the regulatory control layer. The controlled outputs for stabilization are measured output
variables, and their setpoints may be used as degrees of freedom for the layers above.
For each layer in a control system we use the terms controlled output (y with setpoint
ys) and manipulated input (u). Correspondingly, the term “plant” refers to the system to
be controlled (with manipulated inputs u $and controlled outputs y). The layers are often
structured hierarchically, such that the manipulated input for a higher layer (u1) is the setpoint
for a lower layer (y2s), i.e. y2s = u1 (These controlled outputs need in general not be measured variables, and they may include some of the manipulated inputs (u).)