Ex. PGS 1037
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`EX. PGS 1037
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`A Brief History of Automatic Control
`
`Stuart Bennett
`
`hence the rate of combustion and heat output. Improved tempera­
`
`
`Automatic feedback control systems have been known and
`ystem s were devised by Bonnemain (circa 1743-
`ture control s
`used for more than 2000 years; some of the earliest
`examples
`1828), who based his sensor and actuator on the differential
`
`
`
`are water clocks described by Vitruvius and attributed to Ktesi­
`expansion of different metals. During the 19th century an exten­
`bios (circa 270 B.C.). Some three hundred years later, Heron of
`
`
`sive range of thermostatic devices were invented, manufactured,
`
`
`Alexandria described a range of automata which employed a
`and sold. These devices were, predominantly,
`con­
`
`
`variety offeedback mechanisms. The word "feedback" is a 20th
`direct-acting
`
`trollers; that is, the power required to operate the control actuator
`
`century neologism introduced in the 1920s by radio engineers to
`
`describe par asitic, positive feeding back of the signal from the
`was drawn from the measuring system.
`The most significant control development during the 18th
`
`
`
`
`output of an amplifier to the input circuit. It has entered into
`
`century was the steam engine governor. The origins of this device
`
`common usage in the English-speaking world during the latter
`
`lie in the lift-tenter mechanism which was used to control the gap
`half of the century.
`Automatic feedback is found in a wide range of systems;
`
`between the grinding-stones in both wind and water mills. Mat­
`
`
`thew Boulton (1728-1809) desclibed the lift-tenter in a letter
`
`
`Rufus Oldenburger, in 1978, when recalling the foundation of
`IFAC, commented on both the name and the breadth of the
`
`(dated May 28,1788) to his partner, James Watt (1736-1819),
`
`
`who realized it could be adapted to govcrn thc speed of the rotary
`
`
`
`subject: "I felt that the expression 'automatic control'
`covered
`all systems, because all systems involve variables,
`steam engine. The first design was produced in November 1788,
`and one is
`
`concerned with keeping these variables at constant or given
`
`and a governor was first used early in 1789. The original Watt
`
`governor had several disadvantages: it provided only propor­
`varying values. This amounts to conccrn about control of these
`tional control and hence exact control of speed at only one
`
`
`variables even though no actual automatic control devices may
`
`operating condition (this led to comments that it was "a modera­
`
`
`
`
`be intentionally or otherwise incorporated in these systems. I was
`tor, not a controller"); it could operate only over a small speed
`
`
`
`of biological, economic, political as we\1 as engineering
`thinking
`range; and it required careful maintenance.
`
`systems so that I pictured the scope ofIFAC as a very broad one."
`The first 70 years of the 19th century saw extensive efforts to
`
`
`
`
`
`
`This divcrsity poses difficultics for historians of the subject
`improve on the W att governor,
`and this article
`(and for editors of control journals),
`does not
`
`aud thousands of governor patents
`
`attempt to cover all application areas.
`were granted throughout the world. Many were for mechanisms
`
`
`Thc history of automatic control divides conveniently into
`designed to avoid the offset inherent in the Watt governor.
`four main periods as follows:
`Typical of such mechanisms were the governors patented by
`
`William Siemens (1823-1883) in 1846 and 1853, which substi­
`
`• Early Control: To 1900
`
`
`tuted integral action for proportional action and hence produced
`
`• The Pre-Classical Period: 1900-1940
`
`
`
`"floating" controllers with no fixed set point. Practical improve­
`Period: 1935-1960
`• The Classical
`ments came with the loaded governor of Charles T. Porter (1858):
`• Modern Control: Post-1955
`his governor could be run at much higher speeds, and hence
`
`
`This article is concerned with the first three of the above; other
`
`greater forces could be developed to operate an actuator. A little
`
`articles in this issue deal with the more recent pcriod.
`later Thomas Pickering (1862) and William Hartnell (1872)
`invented spring-loaded governors, which also operated at higher
`Early Control: To 1900
`
`speeds than the Watt governor and which had the added advan­
`
`Knowledge of the control systems of the Hellenic period was
`
`tage of smaller physical size than the Watt and Porter governors.
`
`preserved within the Islamic culture that was rediscovered in the
`From the early years of the 19th century there were reports of
`
`
`West toward the end of the Renaissance. New inventions and
`
`problems caused by governors "hunting," and attempts to ana­
`
`applications of old principles began to appear during the 18th
`
`
`lyze thc governor mechanism to determine the conditions for
`de Reamur (1683-
`
`example, Rene-Antoine Ferchault
`century-for
`stable (non-hunting) operation were made. IV Poncelet (1788-
`
`
`1757) proposed several automatic devices for controlling the tem­
`
`1867) in 1826 and 1836, and G.B. Airy (1801-1892) in 1840 and
`
`perature of incubators. These were based on an invention of
`1851 produced papers that showed how dynamic motion of the
`
`
`Cornelius Drebbel (1572-1663). The temperature was measured by
`
`governor could be described using differential equations, but
`
`the expansion of a liquid held in a vessel connected to aU-tube
`
`both met difficulties when they attempted to determine the
`containing mercury. A float in the mercury operated
`an ann which,
`
`conditions for stable behavior. Airy, in 1851, stated the condi­
`
`controlled the draft to a fumace and
`
`through a mechanical linkage,
`
`tions for stable operation, but his report is so terse that it is not
`
`
`possible to determine how hc arrived at thcse conditions. In 1868,
`
`The author is with the Department of Automatic Control & Systems
`
`
`James Clerk Maxwell (1831-1879) published his now-famous
`
`
`paper entitled "On Governors." In it he described how to derive
`
`
`
`Engineering. The University of Sheffield, Mappin Street, Sheffield
`
`the linear differential equations for various governor mecha­
`
`S1 3JD, U.K., telephone: +44 (0)114 282 5230, email: s.bell­
`
`
`nisms. At this time mathematicians and physiCists knew that the
`
`nett@sheffield.ac. uk.
`
`June 1996
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`0272-1708/96/$05.00©19961EEE
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`17
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`Ex. PGS 1037
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`The Pre-Classical Period (1900-1935)
`
`stability of a dynamic system was determined by the location of
`
`
`
`
`
`the roots of the characteristic equation, and that a system became
`The early years of the 20th century saw the rapid and widespread
`unstable when the real part of a complex root became positive;
`
`
`
`application of feedback controllers for voltage, current, and
`
`the problem was how to determine the location of the real parts
`
`frequency regnlation; boiler control for steam generation;
`of the complex roots without finding the roots of the equation.
`
`
`
`electric motor speed control; ship and aircraft steering and auto
`
`
`Maxwell showed, for second-, third-, and fourth-order systems,
`
`
`
`stabilization: and temperature, pressure, and flow control in the
`
`
`that by examining the coefficients of the differential equations
`
`
`process industries. In the twenty years between 1909 and 1929,
`
`
`the stability of the system could be determined. He was ahle to
`
`sales of instruments grew rapidly as Fig. 1 shows. The majority
`
`
`
`
`give the necessary and sufficient conditions only for equations
`
`
`up to fourth order; for fifth-order equations he gave two neces­
`
`
`
`of the instruments sold were measuring, indicating, and
`
`
`
`recording devices, but toward the end of the period the sales of
`
`
`SillY conditions. Maxwell's paper, now seen as significant, was
`
`
`controllers began to increase. The range of devices designed,
`little noticed at the time, and it was not until the early years of
`
`built, and manufactured was large; however, most were designed
`
`thi s century that the work hegan to he assimilated as engineering
`
`without any clear understanding of the dynamics buth of the
`knowledge.
`
`system to be controlled and of the measuring and actuating
`The problem formulated by Maxwell was taken up by
`
`
`
`devices used for control. The majority of the applications were
`
`Edward J. Routh (1831-1907), whose first results were pub­
`
`concerned with simple regulation, and in such cases this lack of
`
`lished in 1874. In 1877 he produced an extended treatise on
`
`understanding was not a serious problem. However, there were
`
`
`the "Stability of Motion" in Which, drawing on the work of
`
`some complex mechanisms involving complicated control laws
`Augustin-Louis Cauchy (1789-1857) and Charles Sturm
`
`being developed-for example, the automatic ship-steering
`(1803-1855), he expounded what we now know as the Routh­
`
`
`Hurwitz stability criteria. In 1895, the Swiss mathematician
`mechanism devised by Elmer Sperry (1911) that incorporated
`
`Adolf Hurwitz ( 1859-1919) derived the criteria inde­
`
`
`
`PID control and automatic gain adjustment to cOIIlpensate for the
`
`
`disturbances caused when the sea conditions changed. Another
`pendently (basing his work on some results of C. Hermite).
`He had been asked for help with the mathematical problem
`
`exampJe is the electricity supply companies concerned about
`
`achieving economic operation of steam-generating boilers.
`
`
`by his colleague Aurel Boleslaw Stodola (1859-1942), who
`
`Boiler control is of course a multivariable problem in that both
`was working on a turbine control problem.
`
`
`water level and steam pressure have to be controlled, and for
`
`
`Most of the inventions and applications of this period were
`
`efficient combustion the draught to the boiler has also to be
`
`
`
`concerned with the basic activities of controlling temperatures,
`
`
`
`controlled. During the 1920s several instrument companies
`
`
`
`
`pressures, liquid levels, and the speed of rotating machinery: the
`develop complete hoiler coutrol systems.
`
`
`desire was for regulation and for stability. However, growth in
`
`the size of ships and naval guns, and introduction of new weap­
`
`
`
`ons such as torpedoes, resulted in the application of steam,
`As control devices and systems hegan to be used in IIlany
`
`hydraulic, and pneumatic power systems to operate position
`
`different areas of engineering, two major problems became
`
`
`control mechanisms. In the United States, Britain, and France,
`
`
`apparent: (I) there was a lack of theoretical understanding with
`
`
`engineers began to work on devising powered steering engines
`160
`
`to assist the helmsman; on large ships the hydrodynamic forces
`140
`on the rudder were such that large gear ratios between the helm
`120
`and the rudder were required and hence moving the rudder took
`100
`
`a long time. The first of powered steering engine, designed by
`88
`60
`
`
`Frederick Sickels in the U.S. (patented 1853) was an open-loop
`40
`
`
`
`system. The first closed-loop steering engine (patented 1866)
`20
`was designed by J. McFarlane Gray for BruneI's steamship the
`0
`0:; (J)
`Great Eastern. In France, around the same time, Jean Joseph
`0: 0;
`
`Fareot designed a range of steering engines and other closed-loop
`position control systems. He suggested naming his devices
`
`
`"servo-moteur" or "motcur asservi," hence our terms "servo­
`100
`mechanisms" and "servomotors."
`90
`80
`
`Further applications for control systems became apparent
`70
`
`with the growth in knowledge of electricity and its applications.
`60
`50
`For example, illC lamps required the gap between the electrodes
`40
`30
`
`
`to be kept constant, and generally it was helpful to all users if
`20
`
`either the voltage or the current of the electricity supply was kept
`10
`o
`
`
`
`constant. Electricity also provided additional tools-for meas­
`
`
`
`urement, for transmission and manipulation of signals, and for
`(b)
`
`
`actuation-which engineers began to use. The electric relay,
`
`which provided high gain power amplification, and the spring
`Fig. 1. (a) Ratio of instrument
`to machinery sales in the United
`
`biased solenoid, which provided (crude) proportional control
`Stales,
`instrument sales in
`1918 to 1936 (1921 = 100). (b) Index of
`the United States,
`
`action, were significant devices.
`1909 to 1936 (1921 = 100).
`
`(a)
`
`18
`
`IEEE Control Systems
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`Ex. PGS 1037
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`

`
`no common language in which to discuss problems, and (2) thcrc
`
`to be uscd within AT&T in 1931.lnformation about the amplifier
`
`
`was not published in the open literature until 1934. In developing
`
`were no simple. easily applied analysis and design methods. The
`
`
`
`the practical amplifier and in understanding its behavior, Black
`
`
`equation and the only available analysis tool was the differential
`
`
`was assisted by Harry Nyquist (1889-1976), whose papcr "Rc­
`
`
`
`application of the still not widely known Routh-Hurwitz stability
`of the so-called
`generation
`process, dependent on being able to
`Theory" laid down the foundations
`tesL This is a laborious
`
`Nyquist analysis and was published in 1932.
`
`obtain values for the parameters, and one that gives no guidance
`to the designer on the degree of stability, or what to do to make
`device-the negative feed­
`This work provided a practical
`the system stable,
`back amplifier- and led to a deeper understanding
`of the benefits
`of negative feedback in systems. It also, eventually, led to a
`
`As applications multiplied, engineers became puzzled and
`method of analyzing and designing control systems which did
`
`confused: controllers that worked satisfactorily for one applica­
`
`
`
`not require the derivation and manipulation of differential equa­
`
`
`tion, or for one set of conditions, were unsatisfactory when
`
`tions, and for which experimental data-the measured frequency
`applied to different systems or different
`
`conditions: problems
`response- could be combined with calculated data; from the
`arose when a change in one part of the system (process,
`control­
`
`combined response the degree of stability of the system could be
`
`
`ler, measuring system, or actuator) resulted in a change in the
`
`
`estimated and a picture of changes necessary to improve the
`
`major time constant of that part. This frequently caused instabil­
`
`performance could be deduced.
`ity in what had previously been, or seemed to have been, a stable
`
`
`system. Some acute ohservers, for example Elmer Sperry and
`with Black's work, Clesson E. M ason of
`Contemporaneously
`
`Morris E. Leeds, noted that the best human operators did not use
`
`the Foxboro Company developed a pneumatic negative feedback
`an on-off approach to control but used both anticipation,
`backing
`
`
`amplifier. Edgar H. Bristol, one of the founders of the Foxboro
`variahle approached the set-point,
`off the power as the controlled
`
`Company, had invented the flapper-nozzle amplifier in 1914.
`
`and small, slow adjustments when the error persisted. Sperry
`
`
`The early versions of the flapper-nozzle amplifier were highly
`tried to incorporate these ideas into his devices, and for many
`
`
`
`non-linear (effectively on-off behavior), and during the 1920s
`years Leeds resisted attaching simple on-off control outputs to
`
`had only succeeded in increasing its
`
`extensive modifications
`
`
`his recorders because he realized that this would not provide
`linear range to about 7% of full range. In 1928, Mason began
`good control.
`
`experimenting with feeding back part of the output movement
`of the amplifier, and in 1930 produced a feedback circuit that
`In 1922. Nicholas � inorsky (1885-1970) presented a clear
`
`
`
`
`
`
`linearized the valve operation. This circuit enabled integral (or
`analysis of the control involved in position control systems and
`
`reset) action to be easily introduced into the behavior of the
`
`
`formulated a control law that we now refer to as three-term or
`system. In 1931, the Foxboro Company began selling the Sta­
`
`
`the way in which PID control. He arrived at his law by observing
`
`bilog pneumatic controller which incorporated both linear am­
`a helmsman steered a ship. This work did not become widely
`
`
`and plification (based on the negative feedback principle)
`
`known until the late 1930s, after Minorsky had contributed a
`
`integral (reset) action (Fig. 2). There was some initial market
`
`series of articles to The Engineer. But even if designers had been
`
`resistance to this device, on the grounds of cost and because its
`aware of M inorsky's work they would still have lacked suitable
`
`behavior was not understood. Foxboro responded by producing,
`
`
`linear, stable, amplification devices to convert the low power
`explaining the principles of the system in clear
`in 1932, a bulletin
`
`signals obtained from measuring instruments to a power level
`
`and simple terms and stressing how the behavior was different
`
`
`suitable to operate a control actuator. Slide and spool valves
`
`
`that is, those from what it termed "narrow-band" controllers,
`developed during the early part of the 20th century were begin­
`with limited linear range.
`ning to provide the solution for hydro-mechanical systems,
`The electronic negative feedback am plifier and the pneumatic
`
`although valve overlap that resulted in dead space and stiction
`
`
`controller were the outcomes of work on industrial problems.
`were problcms that had to be overcome. Howevcr, there was an
`
`During the same period, extensive work was being carried out
`impasse with respect to amplifiers for electronic and pneumatic
`
`on analog calculating machines under the direction of Vanevar
`
`
`systems. As early as 1920 the amplification problem was proving
`
`
`Bush at the Massachusetts Institute of Technology. This work
`
`a serious obstacle to the further development of long-distance
`
`
`
`resnlted in the differential analyzer, which provided a means of
`
`telephony. Improvements in cable design and the use of imped­
`simulating the behavior of dynamic systems and of obtaining
`ance loading had extended the distance over which telephone
`equations. It also led to the
`
`numerical solutions to differential
`
`transmissions could take place without amplification. yet the
`
`study and design of a high-performance servomechanism
`by
`transcontinental service in the U.S. was dependent on amplifica­
`
`
`Harold Locke Hazen (1901-1980) and his students. In addition
`
`
`tion. Telephone repeaters based on electronic amplification of
`
`to designing a servo system, Hazen also undertook the first major
`
`the signal were used around 1920, but the distortion they intro­
`
`
`theoretical study of servomechanisms. His papers, published
`in
`
`duced limited the number that could be used in series. Expansion
`1934, provided the starting point for
`the next generation of
`
`of traffic on the network was also causing problems since it
`
`control system specialists.
`
`necessitated an increase in bandwidth of the lines with the
`consequent
`
`increase in transmission loss. Harold Stephen Black
`The Classical Period: 1935-1950
`
`
`began work on this problem in the early 1920s. He
`(1898-1983)
`if some of the amplification of a high-gain amplifier
`
`
`of During the period 1935-1940, advances in understanding
`realized that
`control system analysis and design were made independently by
`
`were sacrificed by feeding back part of the output signal, the
`
`several groups in several countries. The best known and most
`
`distortion due to noise and component drift could be reduced. On
`
`influential work came from three groups working in the U.S. The
`
`August 2, 1927, he sketchcd a circuit for a negative feedback
`development in Europe and in Russia during this period followed a
`
`amplifier. Following extensive
`work. full-scale
`development
`
`
`
`somewhat different path deriving from Vyschnegradsky's work in
`
`
`practical trials were: carried out in 1930, and the amplifier began
`
`June 1996
`
`19
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`Ex. PGS 1037
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`

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`
`
`Fig. 2. Internal view of the Foxboro Stabilog circa 1936.
`
`
`
`Russia and then Barkhausen's work in Germany, followed by
`
`
`developments due to Cremer, Leonhard, and Mikhailov.
`
`AT &'1' continued with its attempts to find ways of extending
`
`the bandwidth of its communication systems, and upon obtaining
`
`good frequency response characteristics. The ideal which they
`were sceking was a constant gain over a wide bandwidth with a
`sharp cut-off and with a small phase lag. Engineers in the Bell
`
`
`Telephone Laboratories worked extensively on this problem, but
`
`found that if they achieved the desired gain characteristic then
`the phase lag was too large. In 1940, Hendrik Bode, who had
`
`
`been studying extensions to the frequency-domain design
`
`
`
`method, showed that no definite and universal attenuation and
`
`
`
`phase shift relationship for a physical structure exists, but that
`
`
`there is a relationship between a given attenuation characteristic
`
`
`and the minimum phase shift that can be associated with it. In
`
`the same paper he adopted the point (-1,0) as the critical point
`rather than the point (+1,0) used by Nyquist, and he introduced
`
`the concept of gain and phase margins, and the gain-bandwidth
`
`limitation. Full details of Bode's work appeared in 1945 in his
`book Network ,1nalysis
`and Feedback Amplifier Design.
`became clear during 1941 that the cumbersomc systcm of relay­
`
`
`The second important group, mechanical engineers and
`
`ing manually the information obtained from radar devices to the
`
`
`physicists working in the process industries in the U.S., encour­
`
`gun controllers was not adequate to combat the threat of fast
`aged by Ed S. Smith of the Builders Iron Foundry Company,
`
`aircraft and that there was a need to develop a system in which
`
`
`
`
`began systematically developing a theoretical understanding of
`
`
`an automatic tracking radar system was directly linked to the gun
`
`the control systems they used. They sought to establish a com­
`
`
`director, which was in tum linked to the gun position controller.
`
`mon terminology and tried to develop design methods. They
`
`
`Work on this "systems" problem brought together mechani­
`persuaded the American Society of Mechanical Engineers to
`
`
`
`cal, electrical, and electronic engineers, and an outcome of this
`
`
`form an Industrial Instruments and Regulators Committee in
`
`
`cross-fertilization of ideas was a recognition that neither the
`
`
`
`1936, thus becoming the first major professional body to form a
`
`frequency response approach used by the communication engi­
`
`
`
`section specifically to deal with automatic control. Several mem­
`neers nor the time domain approach favored by the mechanical
`
`bers of this loose grouping were aware of developments in
`
`
`
`
`engineers were, separately, effective design approaches for ser­
`
`
`Germany and in England. During this period the manufacturers
`
`vomechanisms. What was required was an approach that used
`
`
`of pneumatic controllers continued to improve and develop their
`
`the best features of each.
`
`
`
`instruments, and by 1940 field-adjustable instruments with PID
`
`control were available-for example, an improved version of the
`Work by Gordon S. Brown and his students at -'1IT showed
`
`
`Stabilog and the Taylor Fulscope. In 1942, J.G. Ziegler and N.B.
`
`
`how many mechanical and electrical systems could be repre­
`
`
`Nichols of the Taylor Instrument Companies published papers
`
`sented and manipulated using block diagrams. Albert C. Hall
`
`
`describing how to find the optimum settings for PI and PID
`
`
`showed, in 1943, that by treating the blocks as transfer functions
`
`
`control-the so called Ziegler-Nichols tuning rules. These were
`
`(he used the Laplace transform approach) the system transfer
`
`extended in the mid-1950s by Geraldine Coon (Taylor Instru­
`locus could be drawn, and hence the Nyquist test for stability
`ment).
`
`could be used. More importantly the gain and phase margin could
`
`be determined, and he introduced the use of M and N circles
`
`
`The third group was located in the Electrical Engineeling
`
`which enable estimates of the dosed loop time domain behavior
`Department of MIT and was led by Harold L. Hazen and Gordon
`to be made. Another group working the so called Radiation
`
`
`
`S. Brown. They used time-domain methods based on operator
`
`
`Laboratory at MIT (this laboratory was concerned with develop­
`
`
`techniques, began to develop the use of block diagrams, and used
`ing radar systems for the detection and tracking of aircraft)
`
`the differential analyzer to simulate control systems. Scholarly
`
`designed the SCR-584 radar system, which, linked with the M9
`
`
`
`interchanges between MIT and the University of Manchester led
`
`
`director, was deployed in southeast England and had a high
`
`to a ditferential analyzer being built at Manchester University
`
`
`
`success rate against VI rockets. The M9 director was designed
`
`and, in 1936, Douglas Hartree and ArtllLlr Porter assisted A.
`
`by a group led by Bode and including Blackman, C.A. Lovell,
`Callender of ICI to use the machine to simulate an industrial
`and Claude Shannon, working in the Bell Telephone Laboratory.
`control system and to derive design charts for the system.
`
`Out of the work on the SCR-584 came the Nichols chart design
`
`The advent of the second world war concentrated control
`
`
`method, work by R.S. Phillips on noise in servomechanisms, and
`
`
`
`system work on a few specific problems. The most important of
`
`W. Hurewicz's work on sampled data systems. After the war,
`
`these was the aiming of anti-aircraft guns. This is a complex
`
`details of the work were published in the seminal book Theory
`
`
`
`problem that involves the detection of the position of the air­
`oj Servomechanisms.
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`plane, calculation of its future position, and the precise control
`
`of the movement of a heavy gun. The operation required up to
`
`
`The Radiation Laboratory group used phase advance circuits
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`14 people to carry out complicated observation and tracking
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`in the forward loop to modify the performance of their control
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`tasks in a coordinated way. The design of an adequate servo­
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`system. Several other workers, particularly in the U.K., used
`
`
`
`mechanism to control the gun position was a ditficult task. It also
`minor loop feedback to modify system response and hence found
`
`20
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`IEEE Control Systems
`
`Ex. PGS 1037
`
`

`
`Fundamental Theory of Servomechanisms,
`
`began to
`MacColl's
`the Nyquist approach difficult. In 1942, A,L, "John" Whiteley of
`
`the British Thomson Houston Company proposed an approach
`
`
`by the British govcrn­set out the new approaches. Encouraged
`
`
`based on plotting the inverse functions on a Nyquist diagram; in
`
`
`
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`ment, the Institution of Electronic Engineers held a conference
`the same year HoT, Marcy (Kellog Company) independently
`
`in London in 1946 on radar, and the interest shown in the papcrs
`
`proposed a similar method.
`
`
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`relating to servomechanisms resulted in a further conference
`
`The problems raised by anti-aircraft control were system
`devoted to control held in 1947. In the United States the govern­
`
`design problems in that several different units, often designed
`ment agreed to continue paying key people for a period of six
`
`
`
`and manufactured hy different groups, had to be integrated; the
`months after the end of the war to enable them to write up their
`
`
`overall performance was dependent not so much on the perform­
`
`
`work. One outcome was the Radiation Laboratory Series of
`
`ance of the individual units but on how well they worked
`Theory of Servomechanism.
`books, including
`
`
`together. Difficulties experienced in getting units to work to­
`
`
`The conference on "Automatic Control" held in July 1951 at
`
`gether led to a deeper understanding of bandwidth, noise, and
`
`
`Cranfield, England, and the "Frequency Response Symposium"
`
`non-linearities in systems .. By the end of the war people such as
`
`held in December 1953 in New York marked the beginnings of
`W.
`
`Arnold Tustin (1899-1994) in England and R.S. Phillips,
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`the transition period leading to modern control theory. The first
`
`
`
`Hurewicz, L. McColl, N. Minorsky, and George Stibbitz in the
`
`
`
`of these, organized by the Department of Scientific and Industrial
`
`
`U.S. were concentrating on nonlinear and sampled data systems.
`
`Research, with the assistance of the IEE and the [\i{echE, was
`
`The other major development to emerge from the fire control
`
`
`
`the first major international conference on automatic control.
`
`
`work during thc war was the study of stochastic systems: Norbert
`
`
`Arnold Tustin chaired the organizing committee, and 33 papers
`
`
`Wiener (1894-1964) wished to contribute to the war effort and
`
`were presented, 16 of which dealt with problems of noise,
`
`
`proposed tackling the problem of predicting the future position
`
`
`
`non-linearity or sampling systems. There were also sessions on
`of an aircraft.
`was based on the work he had done
`His proposal
`
`
`analog computing and the analysis of the behavior of economic
`
`
`in the 1920s on generalized harmonic analysis (Wiener, 1931).
`
`
`systems (this latter reflecting both the particular interest of
`
`He worked with John Bigelow on implemcnting his prediction
`
`
`Arnold Tustin and the growing interest in applications of feed­
`
`
`
`system, and they succeeded in developing an electronic system
`back theory).
`
`for prediction. Wiener was disappointed that in the end his
`The wartime experience demonstrated the power of the fre­
`
`
`
`system was only able to achieve a marginal improvemcnt (less
`
`quency response approach to the design of feedback systems; it
`
`
`than 10%) over the system developed empirically by the Bell
`also revealed the weakness of any design method based on the
`
`Telephone Laboratory. The work did lead to Wiener producing
`
`
`
`
`assumption of linear, deterministic behavior. Real systems are
`
`
`the report "The Extrapolation, Interpolation and Smoothing of
`
`
`non-linear; real measurements contain errors and are contami­
`
`
`
`(OSRD Applications" Stationary Time Series with Engineering
`nated by noise; and in real systems both the process and the
`Report 370, February 1, 1942), known as "the yellow peril"
`
`
`environment are uncertain. But what design techniques can be
`because of its yellow covers and the formidable difficulty of its
`
`
`
`
`used that allow the designer to consider non-linear and non-de­
`
`
`mathematics. It was eventually published in the open literature
`
`
`terministic behavior and to allow [or measurement errors and
`in 1949.
`noise? Also, the design problem changed from that of simply
`By the end of the war the classical wntroltechniques-with
`
`
`
`the "best" achieving a stable controller to that of achieving
`
`
`the exception of the root locus design method of Walter Evans
`
`controller. But what is the "best" controller?
`
`(1948, 1 950)-had been established. The design methodologies
`
`Ziegler and Nichols had shown how to choose the parameters
`
`
`were for linear single-input systems-that is, systems that can
`
`of a given type of controller to obtain an "optimum" performance
`
`
`
`be described by linear differential equations with constant coef­
`
`
`of a given control structure (PI, PID). Similarly, Whiteley's
`
`ficient� and that have a single control input. The frequency
`
`
`
`standard forms enabled designers to choose a particular perform­
`
`
`
`response techniques, based on the use of Nyquist, Bode, Nichols,
`
`
`ance for a range of systems. Work was done on evaluating a
`
`
`and Inverse Nyquist charts, assessed performance in terms of
`
`
`
`whole range of performance indicators including I AE, ISE,
`
`
`bandwidth, resonance, and gain and phase margins and provided
`
`
`ITAE, and ITSE (Graham and Lathrop, 1953). Sterile arguments
`
`
`
`a graphical, pictorial view of the system behavior. The alternative
`
`
`developed about which the performance
`was the "best"
`indicator
`
`
`
`approach based on the solution of the ditlcrential equations using
`
`
`until it was accepted that what was important was the choice of
`
`
`
`
`Laplace transform techniques expressed performance in terms of
`
`
`
`an appropriate performance indicator for a particular application.
`
`
`rise time, percentage overshoot, steady-state error, and damping.
`
`
`
`In addition to performance criteria based on minimizing some
`
`
`Many cngineers preferred the latter approach because the pcr­
`
`
`
`error function there was, for certain classes of system, interest in
`
`
`formance was expressed in "real" terms, that is, the time behavior
`
`
`minimizing the time to reach a set-point (obvious applications
`
`of the system. The disadvantage, of course, is that until the
`
`
`are military target seeking servomechanisms and certain classes
`
`development of the root !locus method there was no simple and
`
`
`
`of machine tools). Donald McDondald's "Non-Linear Tech­
`
`
`easy way in which the designer could relale parameter changes
`
`niques for Improving Servo Performance" (1950) was followed
`to time beh