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The Simulink^{®} model watertank includes the nonlinear WaterTank System plant and a PI controller in a singleloop feedback system.
The WaterTank System is shown in the following figure.
Water enters the tank from the top at a rate proportional to the voltage, V, applied to the pump. The water leaves through an opening in the tank base at a rate that is proportional to the square root of the water height, H, in the tank. The presence of the square root in the water flow rate results in a nonlinear plant.
The following table describes the variables, parameters, differential equations, states, inputs, and outputs of the WaterTank System.
Variables  H is the height of water in the tank. Vol is the volume of water in the tank. V is the voltage applied to the pump. 
Parameters  A is the crosssectional area of the tank. b is a constant related to the flow rate into the tank. a is a constant related to the flow rate out of the tank. 
Differential equation 

States  H 
Inputs  V 
Outputs  H 