On identification and control tuning of cylinder dryers

The steam pressure control loops in the drying section of a paper machine play an important role for the quality of the moisture content control. The steam pressure loops are essential for the effective use of advanced control schemes for the moisture control. From mathematical model building and experiments on industrial paper machines it is found that the dynamics from the steam valves to the steam pressure can be described by a simple process model, the so called IPZ model. This model has an integrator, one pole, and one zero. The process is characterised by three parameters that vary significantly with the operating conditions. A typical step response of the open loop process has a fast and a slow transient. The steam pressure in the cylinders is in general controlled by PI-controllers. Some previous work on this subject has been done by Nelson and Gardner.The paper discusses the specifications for control of the IPZ process. Different controller structures are discussed and general tuning rules for the IPZ process are derived. It is shown that the tuning should be based on a compromise between reference signal tracking and disturbance attenuation. The disturbance attenuation is essential to avoid coupling between the different steam groups.An important property of the tuning rules is that it is possible to determine the controller parameters based on quite simple experiments on the process. The new tuning rules are compared with the common practice of making lambda-tuning based on the fast dynamic of the process.The main results of the paper are a controller structure for steam pressure control and a set of tuning rules for the controller. The tuning rules have been tested on paper machines in the industry. (Less)