It’s not uncommon for process control discussions to focus on use of the PID to improve a loop’s ability to track Set Point. Oftentimes tweaks to controller tuning parameters either seek incremental reduction in Settling Time or strive to limit Overshoot. Indeed, many PID tuning courses position optimization of a control loop’s Set Point Tracking as the ultimate goal. Such a bias overlooks the value of other strategies like Feed Forward Control.
While Feed Forward Control is not an appropriate control strategy for improving Set Point tracking, it’s been widely shown to help improve a process’ capacity for disturbance rejection. If a disturbance to a process can be isolated then it can often be modeled and defused using the Feed Forward architecture. Unlike traditional feedback control Feed Forward utilizes a model to proactively trigger an appropriate corrective action. That action anticipates the end state of a disturbance rather than reacting to Error produced by the disturbance as it accumulates. By getting a jump on the disturbance’s impact Feed Forward Control prepares the process to mute any ill effects.
Not every application is ideal for Feed Forward Control. Even so most processes can benefit to some degree from its use. Following are process attributes for which Feed Forward is well suited and for which the architecture can often improve disturbance rejection:
Manufacturing processes that are subject to frequent load changes are excellent candidates for Feed Forward. Think of Food & Beverage manufacturers along with Utilities – especially power companies. The load changes experienced by these and similar process manufacturers are largely predictable, readily modeled, and they can be neutralized through the application of appropriate counter-measures.
Processes that can be characterized as slow due to a large Dead-Time are often good candidates for Feed Forward Control. Manufacturers in the Oil & Gas and Basic Materials have many such processes. The delayed responses of their processes offer a window of opportunity for the type of preemptive correction achieved through Feed Forward. The counter-measure can be timed for implementation just as the disturbance impacts the process.
For a variety of reasons large process infrastructure tends to be slow, important, and very expensive. Multi-zone furnaces, steam turbines, and distillation columns are good examples and span a variety of industry segments from Chemicals and Oil & Gas to Basic Materials and Pulp & Paper. The proactive control achieved through Feed Forward protects costly equipment that is also big and bulky.
These characteristics are shared by processes which are often good candidates for a disturbance rejection strategy such as Feed Forward. While Feed Forward does not impact Set Point Tracking, it does serve an often equally important purpose – rejecting the negative effects of process disturbances.