The paper aims to compare and prove a pair of disturbance/uncertainty rejection
control laws for the well-known four tank control problems. Control requirements are
expressed in terms of a set point sequence as it usual in the literature. Uncertainty class
is defined as the union of four sub-classes: unknown disturbance, parametric
uncertainty, measurement errors and neglected dynamics. Modelling and design allow
insight of the dynamic properties of the problem. They are formulated by a pair of
theorems which fix the range of application. Theorem are confirmed by the results
simulated runs, and indicate the correct way to further broaden control design
applicability. Disturbance rejection (better uncertainty) design is deployed using the
Embedded Model Control methodology: only unknown disturbance and parametric
uncertainty can be rejected, whereas neglected dynamics effects must be filtered. As a
result, simple performance and stability inequality can be formulated in the frequency
domain and lead to closed-loop pole placement. Inequalities are such to reveal whether
pole placement is feasible and how feasibility can be recovered, an issue which at
authors knowledge is rarely encountered in the literature. Simulated runs prove the
design procedure
