Snapshot of the front panel of the simulator:
In this simulator two temperature control systems of a liquid tank heated with hot liquid in a jacket are simulated in parallel (i.e. simultaneously), so that the benefits of cascade control are easily observed:
Both control systems are excited by the same temperature setpoint and the same disturbances.
In both control systems the process to be controlled is given by the following model based on energy balances:
where Tp is product liquid temperature, Th is heating liquid temperature, cp and ch are specific heat capacities, Fp and Fh are flows, Tpin and Thin are inlet temperatures, Vp and Vh are volumes of jacket and tank respectively, Te is environmental temperature, dp and dh are liquid densities, Uhp and Ueh are heat transfer coefficients between jacket and tank, and between jacket and enviroment (air) respectively. (All parameter values are available from the front panel of the simulator.)
To account for unmodelled dynamics, a pure time delay is added to the measured temperatures Tp and Th.
The measurements of Tp and Th are fed to measurement lowpass filters with time constant Tf.
The aims of the tasks given below are
Cascade control is a common control structure in both process control systems and in servomechanisms. In process control cascade control is used for temperature control, level control, pressure control, and quality control. These are the primary control loops, while the secondary loop typically performs flow control or pressure control.
In servomechanisms the primary loops perform positional control or speed control, while the secondary loop typically performs speed control (in a primary positional loop), or current control (in electrical servomechanisms).
Cascade control can give a much better compensation for disturbances than single loop control can do.
In the tasks below the starting point is that the process is in it's nominal operating point which is characterized as follows: