Cross-machine direction (CD) profile control has been a key area of advanced control applications in the paper industry since the early 1970s. In the past 30 years, the number of profile control applications on paper machines has increased dramatically. During this time, the applications of modern multivariable control techniques were also widely adopted in the paper industry. The use of personal computers and the OPC (OLE for Process Control) communication standard in the industry opens opportunities for new ways to control paper machines. Multivariable CD control becomes the latest advanced control technique for paper machines. Multivariable CD control flexibly coordinates multiple sets of CD actuators. The underlying control algorithm systematically makes the best utilization of the control ranges of all actuators and controls sheet property profiles toward their targets. The application and the results of multivariable CD control have been well accepted by papermakers.
process control By SHIH-CHIN CHEN, JONAS BERGGREN, and ANDREAS ZEHNPFUND A multivariable CD control application approach may be beneficial for certain applications; coordination is the key USE A MACHINE’S FULL CAPABILITY C ross-machine direction (CD) profile control has CD control was designed to minimize the variation of the been a key area of advanced control applications weighted profile [4,12]. The implementation was simple in the paper industry since the early 1970s [1-11]. and results were effective for specific applications. In the past 30 years, the number of profile control ap- In other cases where one profile is changed by plications on paper machines has increased dramatically. multiple sets of actuators, one approach  separates The number of CD actuators installed between 1990 and the input profile in multiple profiles based on the spatial 2007 exceeds 6,000 sets. As a result, many paper machines frequency contents in the profile and then feeds one today are equipped with multiple sets of CD actuators. It separated profile to each actuator set. Each actuator set is quite common that different CD actuators are added controls the separated profile independently. sequentially to existing machines for controlling particu- Another approach [14,15] predicts profile changes lar sheet property profiles. based on the control actions derived for the first set of Therefore, many paper machines have multiple sets actuators. The predicted profile changes are removed of CD actuators working independently on the same from the source profile. The difference profile is fed to machine. Even though papermakers recognized that one the second set of actuators for control. Similar steps are set of actuators may influence multiple sheet properties applied to multiple sets of CD actuators in cascade. The and one sheet property may be affected by multiple sets execution of control actions is synchronized. All these of actuators, there is no easy way to coordinate multiple approaches have had limited success and can only be CD actuators through independent single-input, single- applied to special cases. output CD control applications. While alternative solutions have been applied in the Early attempts to coordinate multiple CD actuators past, the computing power and network communication were severely restricted by the capabilities of quality of industrial control systems have significantly evolved in control systems (QCS) and
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