Multivariable Cross Direction control for optimal paper machine performance

Learn about Multivariable Cross-machine Direction (MCD) control, including: 

  • What is MCD control?
  • How does it differ from CD control? 
  • Why is MCD control needed?

Watch introduction to MCD control video

Challenges of single-variable CD control

Single-variable, Cross-machine Direction control, or simply CD control, became a key area for advanced process control applications in the early 1970s. CD control takes one profile, or the combination of two profiles, as its input and generates a control action to one set of CD actuators. These controls are handled independently. For example, moisture profiles with water-spray actuators, and caliper profiles with induction actuators.

However, many paper machines have multiple sets of CD actuators working independently on the same machine and this approach can cause problems. That is because each set of CD actuators could influence multiple sheet properties. For example, the slice actuator in the headbox can influence weight, moisture and fiber orientation.  There is no easy way to coordinate multiple CD actuators through independent, single-input, single-output CD control applications. 

There is no easy way to coordinate multiple CD actuators through single-input, single-output CD control.

MCD control: Benefits and definition

Paper machines with multiple CD actuator sets and multiple sheet property profiles require a different level of process control complexity.

Multivariable CD control uses control algorithms to systematically make the best utilization of control ranges of all actuators and controls sheet property profiles toward their targets. It is designed to take in multiple sheet profiles and will control multiple sets of the CD actuators simultaneously, taking into account interactions between multiple input and multiple outputs, and their constraints. It can also optimize profile variability based on the priority you set to each profile. 

The end goal is to produce a sheet where all profiles (weight, moisture, caliper, etc.) are uniform. 

MCD control goal: achieve uniform CD profiles of sheet properties simultaneously according to their priorities.
An example of the pulp and paper process, where profiles may come from several machine locations and control actions are applied to multiple sets of CD actuators.

Examples of MCD control

MCD control dashboard for paper machine application

Paper machine application

For a linerboard paper machine, we want to control weight and moisture profiles, usually across two or more headboxes, using steambox and waterspray CD actuators. This means you need to accommodate the following:

  • Different actuator resolution
  • High resolution profiles
  • Response Interaction
  • Conflicting actions among actuators (actuator going in opposite direction)

MCD control: 

  • Simultaneously controls multiple sets of CD actuators while prioritizes participating profile
  • Prevents conflicting control actions
  • Provides better overall profiles and less rejects

Supercalender application

For the supercalender, the goal is to get uniform high gloss on both sides of the sheet while also maintaining specified caliper profile. To achieve that, you have four profiles to monitor - caliper, top gloss, bottom gloss, and the difference of gloss - as well as two induction CD actuators. This results in more profiles than actuators sets to be controlled, calling for a trade-off among profiles. 

MCD control:

  • Manages MD average of gloss profile
  • Reduces expensive coating additives (used to improve gloss) 
  • Improves reel building by optimizing MD and CD gloss, moisture and caliper, simultaneously
MCD Control for supercalender

Explore potential benefits of paper machine applications and advanced controls

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