By nature most people are reactive when it comes to dealing with life’s big expenditures. Think of happy occasions such as graduations and weddings. Then consider more somber milestones like retirement. By most accounts the average worker is wholly unprepared to absorb the costs linked to these and other of life’s events. Even though the timing and amount of those costs can be forecasted, the typical approach to saving for the future isn’t up to the task. Face it: As a species we’re wired to be reactive!
The PID controller is similarly reactive. While the PID is programmed to address variability within a process from slight to severe it lacks the ability to enact the necessary adjustments proactively. Indeed, a feedback controller has no sense of upstream activity until a given upset hits and is computed by the PID in the form of Error. What’s more, the controller’s response is limited to the amount of Error that’s computed – it’s not based on additional Error that’s known to be on its way.
Feed Forward Control is an advanced control scheme that is well-suited for processes which are routinely affected by a known upstream source of disturbances. With an accurate model of the disturbance the Feed Forward architecture enables proactive adjustments. Those adjustments deliver an appropriate response at the appropriate time, and they are thereby able to minimize the negative effects of the disturbance. They may even blunt the disturbance altogether and eliminate any negative effects.
Following are a few specific ways that Feed Forward Control can benefit your process:
Feed Forward is an ideal control scheme when there is a known or defined source of routine disturbances. Feed Forward is able to key in on the source, and by modeling the range of disturbances emanating from that source Feed Forward Control can prepare an appropriate response.
Whereas PID Control is reactive, Feed Forward Control is proactive. Once an upstream disturbance is identified Feed Forward immediately seizes the initiative and prescribes an appropriate change. It is Feed Forward’s proactive nature that either minimizes or eliminates the negative effects of a known disturbance.
Since the PID responds to Error it is always catching up as the effects of a disturbance steadily build over time. In contrast Feed Forward’s use of a disturbance model allows it to assess the full magnitude of the disturbance and to implement an appropriate countermeasure.
The value of Feed Forward’s timeliness shouldn’t be underestimated. If Feed Forward Control simply delivered a countermeasure as soon as the disturbance was identified, then it might do more harm to the process. Fortunately it is designed to deliver the countermeasure as the disturbance hits the process, essentially nullifying the negative effects of the disturbance.
If these benefits address problems that you experience in your facility, then Feed Forward may be the right regulatory control solution for you. While Feed Forward does involve additional instrumentation and engineering costs, it typically pays for itself quickly when deployed under the right circumstances.