1. Introduction
Multivariable system involves more than one control loop, These loops interact with each other, in such a manner that single input not only affects its own output but also affects other process outputs. [1], [2] QTP has four tanks and two pumps, this benchmark is simply a water level control problem. The aim of the process is to keep the liquid level in the lower tanks at the desired values. Related laboratory process introduced by Johansson in Figure 2.1. [3], the process is used to show multivariable interactions which are also known as coupling and it limits performance in multivariable control systems. Multivariable interactions in a QTP are each output (water levels) of the system has affected by two pumps. Due to these reasons, it can be regarded as a prototype for many MIMO control applications in industry such as paper production processes, chemical processes, metallurgy and biotechnological areas, medical industries.