Feedforward Control
Encyclopedia
Feedforward control is a term that has specific meaning within the field of CPU
based Automatic Controls
. The discipline of “feedforward controls” 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
algorithm
s) are much more widely used [4, 32, 34]. See: http://www.isa.org/Content/ContentGroups/Motion_Control2/Departments1/Motion_Fundamentals/200227/20020421.pdf
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 drives 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; see: http://www.bgu.ac.il/chem_eng/pages/Courses/oren%20courses/Chapter_9.pdf. [1,3, 30, 31]
While the term “feedforward control” has been adopted by other fields and even popularized in articles and textbooks about management, its meaning in modern controls is made clear in the following citation:
“In feedforward control there is a coupling from the set point and/or from the disturbance directly to the control variable, that is, a coupling from an input signal to the control variable. The control variable adjustment is not error-based. In stead 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.” From: Basic Dynamics and Control, Haugen, F, 2009, ISBN 978-82-91748-13-9
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. [1, 25]
The mathematical model of the plant (machine, process or organism) used by the feedforward control system may be created and input by a controls engineer
or it may be learned by the control system. See: http://www.tagonline.org/articles.php?id=42. 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 Controls was itself made possible by the invention of microprocessors. [1, 25]
Feedforward systems are also found in biological control by human and animal brain
s. One helpful article on this type of feedforward system can be found at: http://psychology.jrank.org/pages/1155/feedback-feedforward.html
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. [37, 38]
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.[37] See article on motor skill
s.
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. [38, 13,14]
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. [1, 2, 31, 33, all]
Feedforward control is also discussed in the field of artificial intelligence
. See Feedforward neural network.
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
based Automatic Controls
Automatic control
Automatic control is the application of concepts derived from the research area of modern control theory. Automatic control is also a technology for application of control strategies. The implementing requires prior of analyzing and modeling of the subject to be controlled...
. The discipline of “feedforward controls” 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
Mathematical model
A mathematical model is a description of a system using mathematical concepts and language. The process of developing a mathematical model is termed mathematical modeling. Mathematical models are used not only in the natural sciences and engineering disciplines A mathematical model is a...
required to facilitate this type of control. Open-loop control
Open-loop controller
An open-loop controller, also called a non-feedback controller, is a type of controller that computes its input into a system using only the current state and its model of the system....
and feedback control (often based on canned PID control
PID controller
A proportional–integral–derivative controller is a generic control loop feedback mechanism widely used in industrial control systems – a PID is the most commonly used feedback controller. A PID controller calculates an "error" value as the difference between a measured process variable and a...
algorithm
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
s) are much more widely used [4, 32, 34]. See: http://www.isa.org/Content/ContentGroups/Motion_Control2/Departments1/Motion_Fundamentals/200227/20020421.pdf
The benefits of feedforward control are significant and can often justify the extra cost, time and effort required to implement the technology
Technology
Technology is the making, usage, and knowledge of tools, machines, techniques, crafts, systems or methods of organization in order to solve a problem or perform a specific function. It can also refer to the collection of such tools, machinery, and procedures. The word technology comes ;...
. Control accuracy can often be improved by as much as an order of magnitude
Order of magnitude
An order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to the class preceding it. In its most common usage, the amount being scaled is 10 and the scale is the exponent being applied to this amount...
if the mathematical model is of sufficient quality and implementation of the feedforward control law is well thought out. Energy consumption
Energy consumption
Energy consumption is the consumption of energy or power. It is covered in the following articles and categories:* World energy consumption* Domestic energy consumption* Fuel efficiency in transportation* Electric energy consumption* Electricity generation...
by the feedforward control system and its drives 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
Hysteresis
Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...
. Feedforward control is often combined with feedback control to optimize performance; see: http://www.bgu.ac.il/chem_eng/pages/Courses/oren%20courses/Chapter_9.pdf. [1,3, 30, 31]
While the term “feedforward control” has been adopted by other fields and even popularized in articles and textbooks about management, its meaning in modern controls is made clear in the following citation:
“In feedforward control there is a coupling from the set point and/or from the disturbance directly to the control variable, that is, a coupling from an input signal to the control variable. The control variable adjustment is not error-based. In stead 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.” From: Basic Dynamics and Control, Haugen, F, 2009, ISBN 978-82-91748-13-9
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
Physical system
In physics, the word system has a technical meaning, namely, it is the portion of the physical universe chosen for analysis. Everything outside the system is known as the environment, which in analysis is ignored except for its effects on the system. The cut between system and the world is a free...
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. [1, 25]
The mathematical model of the plant (machine, process or organism) used by the feedforward control system may be created and input by a controls engineer
Control engineering
Control engineering or Control systems engineering is the engineering discipline that applies control theory to design systems with predictable behaviors...
or it may be learned by the control system. See: http://www.tagonline.org/articles.php?id=42. Control systems capable of learning and/or adapting their mathematical model have become more practical as microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
speeds have increased. The discipline of modern Feedforward Controls was itself made possible by the invention of microprocessors. [1, 25]
Feedforward systems are also found in biological control by human and animal brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
s. One helpful article on this type of feedforward system can be found at: http://psychology.jrank.org/pages/1155/feedback-feedforward.html
Even in the case of biological feedforward systems, such as in the human brain
Human brain
The human brain has the same general structure as the brains of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size. Estimates for the number of neurons in the human brain range from 80 to 120 billion...
, knowledge or a mental model
Mental model
A mental model is an explanation of someone's thought process about how something works in the real world. It is a representation of the surrounding world, the relationships between its various parts and a person's intuitive perception about his or her own acts and their consequences...
of the plant (body) can be considered to be mathematical as the model is characterized by limits, rhythms, mechanics and patterns. [37, 38]
Historically, the use of the term “feedforward
Feed-forward
Feed-forward is a term describing an element or pathway within a control system which passes a controlling signal from a source in the control system's external environment, often a command signal from an external operator, to a load elsewhere in its external environment...
” 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.[37] See article on motor skill
Motor skill
A motor skill is a learned sequence of movements that combine to produce a smooth, efficient action in order to master a particular task. The development of motor skill occurs in the motor cortex, the region of the cerebral cortex that controls voluntary muscle groups.- Development of motor skills...
s.
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
Robotic arm
A mechanical arm is a robotic, usually programmable, with similar functions to a human arm. The links of such a manipulator are connected by joints allowing either rotational motion or translational displacement. The links of the manipulator can be considered to form a kinematic chain...
, 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. [38, 13,14]
The discipline of “feedforward controls” was largely developed by professors and graduate students at Georgia Tech, MIT
Massachusetts Institute of Technology
The Massachusetts Institute of Technology is a private research university located in Cambridge, Massachusetts. MIT has five schools and one college, containing a total of 32 academic departments, with a strong emphasis on scientific and technological education and research.Founded in 1861 in...
, Stanford
Stanford University
The Leland Stanford Junior University, commonly referred to as Stanford University or Stanford, is a private research university on an campus located near Palo Alto, California. It is situated in the northwestern Santa Clara Valley on the San Francisco Peninsula, approximately northwest of San...
and Carnegie Mellon
Carnegie Mellon University
Carnegie Mellon University is a private research university in Pittsburgh, Pennsylvania, United States....
. 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. [1, 2, 31, 33, all]
Feedforward control is also discussed in the field of artificial intelligence
Artificial intelligence
Artificial intelligence is the intelligence of machines and the branch of computer science that aims to create it. AI textbooks define the field as "the study and design of intelligent agents" where an intelligent agent is a system that perceives its environment and takes actions that maximize its...
. See Feedforward neural network.