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.