Here Pd is the partial disturbance gain, Py is the gain from y2 to y1, and Pu is the partial input gain from the unused inputs u1. If we look more carefully at (2.4) then we see that the matrix _Bgives the effect of disturbances on the primary outputs y1 when the manipulated inputs u2 are adjusted to keep y2 constant, which is consistent of the original definition of the partial disturbance gain given by Skogestad and Wolff (1992). Note that no approximation about perfect control has been made when deriving (2.4). Equation (2.4) applies for any fixed value of s (on a frequency-by-frequency basis).
The above equations are simple yet very useful. Relationships containing parts of these expressions have been derived by many authors, e.g. see the work of Manousiouthakis et al. (1986) on block relative gains and the work of H¨aggblom and Waller (1988) on distillation control configurations.
Note that this kind of analysis can be performed at each layer in the control system. At the top layer we may assume that the cost is a function of the variables y1 , and we can then interpret y2 as the set of controlled outputs c. If c is never adjusted then this is a special case of indirect control, and if