Feed forward, sometimes written feedforward, is a term describing an element or pathway within a control system that passes a controlling signal from a source in its external environment to a load elsewhere in its external environment. This is often a command signal from an external operator.
A control system which has only feed-forward behavior responds to its control signal in a pre-defined way without responding to how the load reacts; it is in contrast with a system that also has feedback, which adjusts the output to take account of how it affects the load, and how the load itself may vary unpredictably; the load is considered to belong to the external environment of the system.
In a feed-forward system, the control variable adjustment is not error-based. Instead it is based on knowledge about the process in the form of a mathematical model of the process and knowledge about or measurements of the process disturbances.
Some prerequisites are needed for control scheme to be reliable by pure feed-forward without feedback: the external command or controlling signal must be available, and the effect of the output of the system on the load should be known (that usually means that the load must be predictably unchanging with time). Sometimes pure feed-forward control without feedback is called 'ballistic', because once a control signal has been sent, it cannot be further adjusted; any corrective adjustment must be by way of a new control signal. In contrast, 'cruise control' adjusts the output in response to the load that it encounters, by a feedback mechanism.
These systems could relate to control theory, physiology, or computing.
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 control algorithms, are much more widely used.
The Three types of Control System (a) Open Loop (b) Feed-forward (c) Feedback (Closed Loop) Based on Hopgood (2002)
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 you include the driver 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.
History
Historically, the use of the term “feedforward” is found in works by D. M. MacKay
as early as 1956. While MacKay's work is in the field of biological
control theory, he speaks only of feedforward systems. MacKay does not
mention “Feedforward Control” or allude to the discipline of
“Feedforward Controls.” MacKay and other early writers who use the term
“feedforward” are generally writing about theories of how human or
animal brains work.
The discipline of “feedforward controls” was largely developed by professors and graduate students at Georgia Tech, MIT, Stanford and Carnegie Mellon.
Feedforward is not typically hyphenated in scholarly publications.
Meckl and Seering of MIT and Book and Dickerson of Georgia Tech began
the development of the concepts of Feedforward Control in the mid 1970s.
The discipline of Feedforward Controls was well defined in many
scholarly papers, articles and books by the late 1980s.
Benefits
The
benefits of feedforward control are significant and can often justify
the extra cost, time and effort required to implement the technology.
Control accuracy can often be improved by as much as an order of magnitude if the mathematical model is of sufficient quality and implementation of the feedforward control law is well thought out. Energy consumption
by the feedforward control system and its driver is typically
substantially lower than with other controls. Stability is enhanced such
that the controlled device can be built of lower cost, lighter weight,
springier materials while still being highly accurate and able to
operate at high speeds. Other benefits of feedforward control include
reduced wear and tear on equipment, lower maintenance costs, higher
reliability and a substantial reduction in hysteresis. Feedforward control is often combined with feedback control to optimize performance.
Model
The
mathematical model of the plant (machine, process or organism) used by
the feedforward control system may be created and input by a control engineer or it may be learned by the control system. Control systems capable of learning and/or adapting their mathematical model have become more practical as microprocessor
speeds have increased. The discipline of modern feedforward control was
itself made possible by the invention of microprocessors.
Feedforward control requires integration of the mathematical
model into the control algorithm such that it is used to determine the
control actions based on what is known about the state of the system
being controlled. In the case of control for a lightweight, flexible robotic arm, this could be as simple as compensating between when the robot arm is carrying a payload
and when it is not. The target joint angles are adjusted to place the
payload in the desired position based on knowing the deflections in the
arm from the mathematical model's interpretation of the disturbance
caused by the payload. Systems that plan actions and then pass the plan
to a different system for execution do not satisfy the above definition
of feedforward control. Unless the system includes a means to detect a
disturbance or receive an input and process that input through the
mathematical model to determine the required modification to the control
action, it is not true feedforward control.
Open system
In systems theory, an open system is a feed forward system that does not have any feedback loop to control its output. In contrast, a closed system
uses on a feedback loop to control the operation of the system. In an
open system, the output of the system is not fed back into the input to
the system for control or operation.
Applications
Physiological feed-forward system
In physiology,
feed-forward control is exemplified by the normal anticipatory
regulation of heartbeat in advance of actual physical exertion by the
central autonomic network. Feed-forward control can be likened to learned anticipatory responses to known cues (predictive coding).
Feedback regulation of the heartbeat provides further adaptiveness to
the running eventualities of physical exertion. Feedforward systems are
also found in biological control of other variables by many regions of
animals brains.
Even in the case of biological feedforward systems, such as in the human brain, knowledge or a mental model
of the plant (body) can be considered to be mathematical as the model
is characterized by limits, rhythms, mechanics and patterns.
A pure feed-forward system is different from a homeostatic
control system, which has the function of keeping the body's internal
environment 'steady' or in a 'prolonged steady state of readiness.' A
homeostatic control system relies mainly on feedback (especially
negative), in addition to the feedforward elements of the system.
Gene regulation and feed-forward
The
cross regulation of genes can be represented by a graph, where genes
are the nodes and one node is linked to another if the former is a transcription factor for the latter. A motif which predominantly appears in all known networks (E. coli, Yeast,...)
is A activates B, A and B activate C. This motif has been shown to be a
feed forward system, detecting non-temporary change of environment.
This feed forward control theme is commonly observed in hematopoietic cell lineage development, where irreversible commitments are made.
Feed-forward systems in computing
In computing, feed-forward normally refers to a perceptron network in which the outputs from all neurons go to following but not preceding layers, so there are no feedback loops. The connections are set up during a training phase, which in effect is when the system is a feedback system.
Long distance telephony
In the early 1970s, intercity coaxial transmission systems, including L-carrier, used feed-forward amplifiers to diminish linear distortion. This more complex method allowed wider bandwidth than earlier feedback systems. Optical fiber, however, made such systems obsolete before many were built.
Automation and machine control
Feedforward control is a discipline within the field of automatic controls used in automation.
