Editing Feed forward (control) (section)

== Overview ==
With feed-forward or feedforward control, the disturbances are measured and accounted for before they have time to affect the system. In the house example, a feed-forward system may measure the fact that the door is opened and automatically turn on the heater before the house can get too cold. The difficulty with feed-forward control is that the effects of the disturbances on the system must be accurately predicted, and there must not be any unmeasured disturbances. For instance, if a window was opened that was not being measured, the feed-forward-controlled thermostat might let the house cool down.

The term has specific meaning within the field of CPU-based [[automatic control]]. The discipline of feedforward control as it relates to modern, CPU based automatic controls is widely discussed, but is seldom practiced due to the difficulty and expense of developing or providing for the [[mathematical model]] required to facilitate this type of control. Open-loop control and feedback control, often based on canned [[PID controller|PID control]] algorithms, are much more widely used.<ref name="ISA1">{{cite web|title=Fundamentals of Motion Control|url=http://www.isa.org/Content/ContentGroups/Motion_Control2/Departments1/Motion_Fundamentals/200227/20020421.pdf|publisher=ISA|access-date=23 February 2013|archive-url=https://web.archive.org/web/20110927022329/http://www.isa.org/Content/ContentGroups/Motion_Control2/Departments1/Motion_Fundamentals/200227/20020421.pdf|archive-date=27 September 2011|url-status=dead}}</ref><ref>{{cite journal|author1=Book, W.J. |author2=Cetinkunt, S. |title=Optimum Control of Flexible Robot Arms OR Fixed Paths|journal=IEEE Conference on Decision and Control|date=December 1985}}</ref><ref>{{cite journal|author1=Oosting, K.W. |author2=Dickerson, S.L. |title=Control of a Lightweight Robot Arm|journal=IEEE International Conference on Industrial Automation|year=1986}}</ref>

There are three types of control systems: open loop, feed-forward, and feedback. An example of a pure open loop control system is manual non-power-assisted steering of a motor car; the steering system does not have access to an auxiliary power source and does not respond to varying resistance to turning of the direction wheels; the driver must make that response without help from the steering system. In comparison, [[power steering]] has access to a controlled auxiliary power source, which depends on the engine speed. When the steering wheel is turned, a valve is opened which allows fluid under pressure to turn the driving wheels. A sensor monitors that pressure so that the valve only opens enough to cause the correct pressure to reach the wheel turning mechanism.  This is feed-forward control where the output of the system, the change in direction of travel of the vehicle, plays no part in the system. See [[Model predictive control]].

If the driver is included in the system, then they do provide a feedback path by observing the direction of travel and compensating for errors by turning the steering wheel. In that case you have a feedback system, and the block labeled ''System'' in Figure(c) is a feed-forward system.

In other words, systems of different types can be nested, and the overall system regarded as a [[black-box]].

Feedforward control is distinctly different from open loop control and [[teleoperator]] systems. Feedforward control requires a mathematical model of the plant (process and/or machine being controlled) and the plant's relationship to any inputs or feedback the system might receive. Neither open loop control nor teleoperator systems require the sophistication of a mathematical model of the [[physical system]] or plant being controlled. Control based on operator input without integral processing and interpretation through a mathematical model of the system is a teleoperator system and is not considered feedforward control.<ref name=onote1/><ref name=onote25/>