.IndustrialControlSystems.Applications.ProcessControl.CascadeLevelControl

Information

Description

Level control with two PI controllers, connected with a cascade arrangement.
The considered system is a tank filled with water. The water level is the process variable to be controlled.
The system (see the figure below) is composed by a tank and one pipe connected to a linear valve that discharges the water in the atmosphere. The valve actuator is represented by a first order filter.
The control system is composed by the measurement part and the controller. The pressure sensor measures the absolute pressure
on the bottom of the tank.

The measured pressure is subtracted from the atmospheric pressure and then divided by the
gravity acceleration and the water density in order to obtain the water level.

  level = (p - p0)/(rho*g)

The PI controller, given the level measurement and the set point reference compute the control action. Such a control
action is the water mass flow rate flowing through the pipe. Such a mass flow rate becomes the set point reference for the second
PI that with its control action regulates the valve aperture

CS = 0 valve closed,
CS = 1 valve fully open

The tank is 2 m height, and the water level at t=0 is L = 1 m.
In the first phase the controller is asked to maintain the level at the initial value (SP = 1 m), at t = 1200 s the level set point
decrease as a step (SP = 0.5 m). The controller has to act on the valve in order to decrease the water level to the desired value.
A disturb represented by a water mass flow rate entering the tank, becomes different from zero at time t = 3600 s.

Set Point reference, water level

Set Point reference (the Control signal of the Level PI) and valve command

The simulation can be perfomed at an initial stage assuming that the controller is a continuous time one (Ts = 0) , that the math
operations are in double precision (FixedPoint = false). In such a phase it is possible to concentrate on the controller design.

Further stages

Once the controller has been designed and the parameters assigned, one should introduce more details in order to simulate a more realistic system. Please refers to the previous example ( LevelControl ) for more information.

Revisions

Industrial Control Systems (v 1.0.0) : April-May 2012