By Doug Cooper and Allen Houtz¹

The most popular architectures for improved disturbance rejection performance are cascade control and the "feed forward with feedback trim" architecture introduced below.

Like cascade, feed forward requires that additional instrumentation be purchased, installed and maintained. Both architectures also require additional engineering time for strategy design, implementation and tuning.

Cascade control will have a small impact on set point tracking performance when compared to a traditional single-loop feed back design and this may or may not be considered beneficial depending on the process application. The feed forward element of a "feed forward with feedback trim" architecture does not impact set point tracking performance in any way.

Feed Forward Involves a Measurement, Prediction and Action
Consider that a process change can occur in another part of our plant and an identifiable series of events then leads that “distant” change to disturb or disrupt our measured process variable, PV.

   where:
      CO  = controller output signal
      D    = measured disturbance variable
      e(t) = controller error, SP – PV
      FCE = final control element (e.g., valve, variable speed pump or compressor)
      PV  = measured process variable
      SP  = set point

To appreciate the additional components associated with a feed forward controller, we can compare the above to the previously discussed traditional feed back control loop block diagram.

When to Consider Cascade Control
The cascade architecture requires that an "early warning" secondary measured process variable, PV2, be identified that is inside (responds before) the primary measured process variable, PV1. Essential elements for success include that:
 ▪ PV2 is measurable with a sensor.
 ▪ The same final control element (FCE) used to manipulate PV1 also manipulates PV2.
 ▪ The same disturbances that are of concern for PV1 also disrupt PV2.
 ▪ PV2 responds before PV1 to disturbances of concern and to FCE manipulations.

One benefit of a cascade architecture is that it uses two traditional controllers from the PID family, so implementation is a familiar task that builds upon our existing skills. Also,  cascade control will help improve the rejection of any disturbance that first disrupts the early warning variable, PV2, prior to impacting the primary process variable, PV1.

When to Consider Feed Forward with Feedback Trim
Feed forward anticipates the impact of a measured disturbance on the PV and deploys control actions to counteract the impending disruption in a timely fashion. This can significantly improve disturbance rejection performance, but only for the particular disturbance variable being measured.

Feed forward with feedback trim offers a solution for improved disturbance rejection if no practical secondary process variable, PV2, can be established (i.e., a process variable cannot be located that is measureable, provides an early warning of impending disruption, and responds first to FCE manipulations).

Feed forward also has value if our concern is focused on one specific disturbance that is responsible for repeated, costly disruptions to stable operation. To provide benefit, the additional measurement must reveal process disturbances before they arrive at our PV so we have time to compute and deploy preemptive control actions.

The Feed-Forward-Only Controller
Pure feed-forward-only controllers are rarely found in industrial applications where the process flow streams are composed of gases, liquids, powders, slurries or melts. Nevertheless, we explore this idea using a thought experiment on the shell-and-tube heat exchanger simulation detailed in a previous article and available for exploration and study in commercial software.

The architecture of a feed-forward-only controller for the heat exchanger is illustrated below (click for a larger view):

As detailed in the referenced article, the PV to be controlled is the exit temperature on the tube side of the exchanger. To regulate this exit temperature, the CO signal adjusts a valve to manipulate the flow rate of cooling liquid on the shell side. A side stream of warm liquid combines with the hot liquid entering the exchanger and acts as a measured disturbance, D, to our process.

Because there is no feedback of a PV measurement in our controller architecture, feed-forward-only presents the interesting notion of open loop control. As such, it does not have a tendency to induce oscillations in the PV as can a poorly tuned feedback controller.

But this would only be true if:
 ▪ we have perfect understanding of the D ® PV and CO ® PV dynamic relationships,
 ▪ we can describe these perfect dynamic relationships mathematically,
 ▪ these relationship never changes,
 ▪ there are no other unmeasured disturbances impacting PV, and
 ▪ set point, SP, is always held constant.

The reality, however, is that with only a single measured D, a predictive model cannot account for many phenomena that impact the D ® PV and CO ® PV behavior. These may include changes in: