Paper machine grade changes : closed-loop control

This study handles problems and solutions related to the paper machine grade change. The perspective is in the qualitative study of mass-flow phenomena during manipulation of the variables that change the production rate either locally as a transient or more permanently. In many cases the grade changes are done using a method where the machine speed is left out from the manipulated variables. The weakness of this method is that almost all changes include basis weight change and when done at constant machine speed, it results in a production rate change even when the process would not allow this to occur. The phenomena related to the speed change through the drying section are simulated using ramp-like excitation signals for several variables. Some of the simulations cases are such that they can only be implemented in a simulation environment. Basic principles of the multivariable model predictive grade change control are introduced utilising target value trajectories for the controlled variables. Trajectory target values are based on mass-flow calculations while trajectory timing is based on simultaneous machine speed change timing. The problem related to the usage of mass-flows as measurement during the machine speed change in a delay-system is studied. A simple compensation in addition to trajectory timing changes is developed to solve the problem. Finally, the head-box flow change, a typical disturbance source in all paper machine grade changes, is analysed and a non-linear prediction based disturbance compensation method is proposed.