Finally ,controller performances may be compared according to a combination of the following objectives:

• Decrease the rise time ,defined as 10% to 90% of the time needed by a controlled variable to move from the initial constant value to a new constant value(e.g.:after a change in set-points).
• Decrease the overshoots that may occur if the controlled variable has an unstable behaviour .
• Decrease the steady-state error,which is the error between the set-point and he controlled variable when thedynamic behaviour has disappeared.
• Improved the disturbances rejection : a good controller is requested to rapidly attenuate the effects of the disturbances over the controlled variables (e.g.:without control, a change in atmospheric humidity affects the final moisture content of a product).
• Improve the robustness : a good controller is requested to be able to account for a large variety of similar products,even if some uncontrolled drying conditions or feed characteristics change (e.g.:the initial moisture content of a products).