SOME CHARACTERISTICS OF A HUMAN OPERATOR*By J. A. V. BATES, M.A., M.B., B.Chir.j

(The paper was received 29 th August, 1947.)

SUMMARYThe paper discusses experimental findings which aid an understanding of the basic

limitations of a human operator as an element in a servo system. It is shown howexperiments on threshold effects can guide the choice of both display magnificationand gear ratio. At least 2 sec appears to be required to develop the maximumperceptual acuity, but a time greater than this is usually needed in accurately settinga control for full confidence in accuracy to develop. The mechanisms of muscularmovements are discussed. Reciprocatingrnovements can be performed at higher ratesthan circular movements, although the latter are less fatiguing. Measurements of themechanical efficiency of man may be worth applying under conditions of heavy work.The conditions of tracking are shown to resemble multiple-choice reaction-timeexperiments, with a lag between stimulus and response that is strictly indeterminable,but which is generally of the order of 0 • 3 sec. Discontinuity in the operator is demon-strated by the inability to halt a reacting movement within 0-2 sec of its start. Inpractical design, the advantages are stressed of incorporating the maximum possibledisplay magnification; of abolishing any striction in the control, possibly by using twohands; of using high rates of turning; and of introducing as much inertia as theoperator can manage. The paper concludes with a note on the acquisition of skill intracking, particularly stressing the need for simple training aids which isolate theindividual stimulus-response elements, and which allow accurate scoring and controlof difficulty in the task facing the operator.

(1) INTRODUCTIONIn the paper, a few of the salient characteristics of a human

operator which are relevant to the designer of servo mechanismsare mentioned. The elementary responses to simplified situa-tions are discussed in preference to overall behaviour withparticular complex control systems.

All but the most primitive creatures can be looked upon as avast organization of individual sensory (input) and motor (output)units with an additional set of mechanisms concerned with themaintenance of these units in a state of activity. The sensory cellsconvert a stimulus from the environment (light, pressure, etc.) intoan electrical impulse, and the motor cells, an electrical impulseinto a force; in man there are perhaps of the order of a hundredmillion separate sensory-motor units. The interconnectingmechanism (central nervous system) consists of special fibreswhich can conduct an electrical impulse at a high rate (up to300 ft/sec) and which can recover from each impulse sufficientlyrapidly to pass frequencies up to 400 c/s. The impulses areessentially similar in nature, whether travelling along a nervefrom a sensory cell, or to a motor one. On the sensory sidethe frequency of impulses increases with increasing strength ofstimulus, and with the stimulus held constant the frequency fallsoff. Thus it appears that the sensory system has been primarilyevolved to register change in the environment; it has the essentialcharacteristics of a differentiating mechanism.

On the motor side, muscles are a collection of a large numberof cells (fibres) which tend to shorten on the arrival of a nerveimpulse. The frequency of the impulses, and the number of fibresreceiving them, are both varied to produce grading of the overalltension produced by the muscle. Elastic elements in series withthe fibre produce smoothing of the output. Thus energy is sup-plied to the body by the muscles only when they shorten; whenkinetic energy must be removed, the work done in stretching theactively resisting muscle is converted into heat and dissipated.On both the sensory and motor sides units operate on an "all-or-none" principle, but their multiplicity allows, for practical pur-poses, a continuous gradation. It is also worth noting that thereare sensory cells situated within the motor system which providedata of a "negative feedback" nature, and which increase stability.

The work of interconnecting the input and output elementsleads to a constant activity within the central nervous system.Consciousness has been described as appearing as a by-productof this activity, and like all other evolutionary products it shows

* Measurements Section paper.t Neurological Research Unit, Medical Research Council.

a refined utilitarianism; the broad consequences of bodilyactivity are available to it, but the details of the mechanism neededto achieve that activity are concealed. The interconnectionsvary greatly in their complexity. At the simplest, stimulus andresponse are separated by only a few milliseconds—tap thetendon at the knee, and the leg will have jerked up before the"mind" is aware that there has been any stimulus. Activity ofthis sort is called "reflex," and in the same class but infinitelymore complex there are, for example, the motor activities of thenew-born. In sharp distinction to activity of this sort thereare "voluntary movements," movements which for the instanttotally absorb all our attention, e.g. the corrective movementsof positional tracking. Between these two types of activity,however, there is a class of movements which started asvoluntary, but which have come by constant repetition to beperformed with less and less conscious attention, e.g. themovements of writing. Movements in this class are preciseand consistent, they are seen universally in skilled performance,but their accuracy at once breaks down if an attempt is made tolift them to a voluntary level by focusing attention on them.

From a servo engineer's standpoint, the essential complexityof nervous activity lies perhaps in the multiplicity of availablealternatives among essentially elementary systems, rather thanin any entirely novel features within an elementary system.From a practical point of view, however, it is possible to discoverthe particular limitations to a human operator, and to a lesssuccessful degree some ways in which they can be overcome.These will now be described.

(2) GENERAL FEATURES OF THE OPERATOR IN ACTION(2.1) Visual Perception

It is assumed, unless otherwise stated, that the operator ispresented with a visual display. The visual apparatus has a highsensitivity and resolving power, and in addition provides threedimensional appreciation. It can detect misalignments as smallas five seconds of arc at the eye under suitable conditions, and ithas great power of identifying form, e.g. a friendly from a hostileaircraft. Magnification of display can, if necessary, overcomeany tendency for vision to be the limiting factor in accuracy.Only in distinguishing a constant frequency from a mixedfrequency background, as in Asdic echoes, is the ear in generalmore useful.

(2.1.1) Weber's Law and Display Magnification.Weber's law states that the least change in any perceived

stimulus is a constant proportion of the magnitude of thestimulus—if / is the intensity, then A/// is a constant, known asWeber's constant. This law is of course merely a convenientsummary of experimental results and it applies only over a statedrange for any particular stimulus, but it is known to be closelyobeyed over a wide range of values when the task is the perceptionof asymmetry or misalignment. If, for example, the display con-sists of a simple cross, the least detectable displacement of thevertical line from the centre of the horizontal line is reached whenthere still remains about a 5 % difference in the lengths of the twoadjacent parts. The particular value remains constant for anyone observer so long as the subtense of the horizontal line exceedsabout •$• deg of arc at the eye. But as the subtense is reducedbelow \ deg Weber's constant increases. Data from three war-time reports on the accuracy of stationary aim have been pooled

[298 ]

BATES: SOME CHARACTERISTICS OF A HUMAN OPERATOR 299

"0 10 20 30 4.0 50 60 70 80Mean subtense of the two lengths to

be compared, minutes apparentFig. 1.—Pooled data on the accuracy of stationary aim, showing fall

in perceptual ability as target subtense is reduced.

in Fig. 1, to show the behaviour of this so-called constant. Thelimiting accuracy in this case is a 7% difference, and Weber'sconstant rises to 30% or so at the smallest detectable subtenses.This increase shows how magnifying telescopes will give a gainin perceptual accuracy. If the exact form of the curve is known,the highest useful magnification can be found directly, once thesize of the smallest target of importance has been specified.In this case, for example, if a target which subtends two minutesat the eye is specified to be the smallest in which there is practicalinterest, a ten-fold magnification will increase its apparentsubtense to 20 minutes. The least detectable misalignment isnow reduced from 30 % of 2 minutes with no magnification to8 % of 2 minutes. With magnification in excess of ten there willbe no gain in accuracy, since the relative acuity remains the same.

(2.1.2) Time Course of Perceptual SensitivityA point of some theoretical interest concerns the rate at which

acuity of perception develops. The visual detection of an"obvious" misalignment requires only a few microseconds if thebrightness is sufficient. Some interest, however, surrounds thequestion of the time taken to perceive a misalignment as itsmagnitude is reduced. This arises as a practical question whendiscussing a drill for laying a telescope graticule on to a stationarytarget.

During the war, some data on this were obtained using as atest object a cross with a variable misalignment in its horizontalarm. It subtended | deg at the eye, and was seen at infinity, andwas visible for a time predetermined by a Compur shutter whichcould be varied from 1/200 sec upwards. The actual presentationwas effected by the subject himself operating a key, so that theelusive factor of his attention would be at a more stable level.The data emerged as shown in Fig. 2, namely the probability ofthe subject correctly seeing the direction of the misalignment inthe horizontal arm, after appropriate deductions for chanceguessing, expressed as a function of the exposure time for variousmagnitudes of misalignment. The points representing the meanprobabilities have been omitted, but the curves were all drawnto pass within the range of one standard error of each probabilityestimate, as obtained from the pooled data from four subjectseach making twenty attempts at five set exposure times. It isseen that one needs at least 2 sec to develop maximum acuity,and with longer times than this there is a negligible improvement.If the experiment is arranged differently, with target size as thedependent variable for a uniform probability of correct per-ception, namely 95% or more, the results follow the rising curveshown in Fig. 3, to which an exponential function can be closelyfitted. This result was also reported by Henmoni2 in a similartype of experiment.

These results help to explain the logarithmic increase in thedelay separating successive corrective movements, which can be

1-0

f 08| 0-7

I 0-5*© 0-4

I 03I 3-2

0-1

0<T 05* 1-0* 1-5 2-0 2-5 *30

Exposure time, secFig. 2.—The relation between the probability of perceiving a given

misalignment and the time of observation.

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% 50

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8 30>

0.0 0-5 10 1-5 20 2-5

Choice reaction-time (x), secFig. 3.—The relation between the magnitude of a misalignment and

the time required to choose an appropriate response.The curve, x = 0-34 -f- 4-1 e~°'18|f is an exponential function asymptotic to the

shortest possible choice reaction-time (0-34 sec).

seen in some records of the correction of a misalignment understationary conditions, and which makes the operator's responseto a disturbance closely resemble a damped oscillation.

(2.1.3) Visual Perception of Velocity and Acceleration.An expression for the least perceptible velocity is of course

meaningless by itself. The only useful approach is to expresswhether or not movement of an object can be detected as afunction of the time allowed to perceive it. This has been doneover a limited range by Dimmick and Karl;6 their resultsare shown in Fig. 4. It is noteworthy that this curve is nota rectangular hyperbola, which might be expected on the simplehypothesis that the product of threshold rate and time allowedwould be a constant. In other words it does not appear true tosay that movement is judged to have occurred as soon as theobject has moved through a fixed angle at the eye.

Nothing has been found in the literature on the perception ofchanges in velocity, and attempts to get reliable data largelyfailed for the significant reason that the faculty seemed to takeso long to develop that it was difficult in the laboratory to keepthe accelerating object in the field of view for long enough. Such

300 BATES: SOME CHARACTERISTICS OF A HUMAN OPERATOR

*g 160o

" 140

jlioo

6 60

gI 40

40!U 20 30Exposure time, sec

Fig. 4.—Threshold for the visual perception of velocity (Dimmick andKarl).

da ta as were obtained suggested tha t a constant change in velocityis not perceived unless the velocity at any instant would be doubledby the end of 5 sec.

(2.1.4) Perceptual Confidence and Forced Guessing.I t appears t o be a psychological fact that as a task becomes

increasingly difficult, the level of the subject's confidence fallsbehind his objective accuracy. W e found, for example, in thisconnection tha t if the task is t o achieve a stat ionary a im of"satisfying" accuracy, one is ap t t o spend five seconds or morewithin one 's most accurate zone before leaving the controls .The t ime is spent wholly in gaining confidence, and so far asaccuracy is concerned it is t ime wasted. Another aspect of thesame peculiarity is the phenomenon of "forced guessing." As atask becomes more difficult, a point is reached at which the subjectfeels tha t every estimate he is making is a complete guess. Thereis naturally at this point a s trong tendency to give up the task asimpossible, but it is found, surprisingly, tha t within a part icularrange, if guessing is forced to continue, one is in fact correct farmore often than would be expected by chance. This phenomenonis quite general th roughout the field of perception, and itsrealization can be a useful aid in lowering one's own thresholds.

(2.2) Moto r Activity

(2.2.1) Reciprocal and Circular Movements.The muscles spanning a hinge jo in t such as the elbow can be

divided into two opposing groups . F o r movement one way themuscles responsible are called agonists, and the group which canoppose their action are called antagonists . Agonist and antag-onist groups can produce opposing torques abou t a jo int , andthe movement of the l imb, or more correctly its angular accelera-tion, is a t any instant directly propor t ional to the algebraic sumof the torque produced by these two opposing groups. I t followsthat the full development of strength can occur only when thereis complete relaxation of all antagonist muscles a t the decisiveinstant. Strength is thus largely a mat ter of correct co-ordination,and this faculty, like most others, can be greatly improved bytraining.

Under conditions of slow reciprocating movement , bo thagonists and antagonists are in tension the whole time, and thistype of movement has been called by S te t son^ a "moving fixa-t ion . " I f the ra te of movement is increased to a point a t whichthe system is in resonance at its natura l frequency (in the caseof the forearm and hand abou t 2 c/s) we may n o w find tha t theperiods of activity of the two opposing groups show n o overlapin time. This type of movement is called by Stetson "ball ist ic,"a n d in his view the key to the acquisition of skill, accuracy, andindustrial efficiency lies in replacing moving fixations by ballisticmovements . I f the ra te of reciprocation is increased, by the t imethe ampli tude is zero a frequency of 8-10 c/s has been reached.It can be demonstra ted tha t this limit is determined by the ra te a twhich the nervous system can dole out appropr ia te excitation, and

it is roughly the same for all moving parts, irrespective of theirmass. Much higher angular velocities than this rate implies, asare seen for example in ball-throwing, are obtainable by a trick ofco-ordination which allows a wave of kinetic energy to traveldown a limb, its velocity increasing with the decreasing mass.

Circular movements are a combination of reciprocal move-ments suitably distributed in phase. Their advantage lies in thefact that the work to be done will be spread over several groupsof muscles rather than over two groups, and therefore in anygiven task the onset of the discomforts of fatigue may be delayed.The maximum rate of wheel-turning with the forearm and handis, however, about half the maximum rate of reciprocal move-ment at the same amplitude. Craik,3 for example, found amaximum rate of 200 r.p.m. with an 8-in diameter wheel, but afree reciprocating movement of the same amplitude can be per-formed at nearly double this rate.

(2.2.2) Man's Mechanical Efficiency.A great amount of study has been devoted to this subject,

particularly by A. V. Hill in the 1920's in Great Britain,and at about the same time, but with a more applied emphasis,by Atzler of Dusseldorf. These studies are based on certainconsiderations of muscle chemistry which enable it to be takenthat a rate of oxygen consumption of one litre per minute isequivalent to 0-48 h.p. Thus by measuring simultaneously theoxygen consumption and the work output, man's efficiency canbe found, and expressed in the usual form; in the case of a trainedsprinter running 100 yards, for example, it was found to be 25%.With this data optimum conditions and rates of working can bededuced. For example, it has been shown that the optimumspeed of pedalling a bicycle is 51 rotations of the crank perminute, ii This index is fairly consistent and reliable, becomingmore so the harder the work done, and the technique is worthbearing in mind in any situation where the man is required tomake anything more than a trivial amount of effort.

(3) THE ELEMENTARY RESPONSE

(3.1) Simple Reaction TimeThe time lag that separates stimulus and response in Man has

been the subject of a great deal of quantitative investigationduring the past 100 years. In the simplest test situation, thesubject is instructed to move his finger on a morse key the instanthe sees a light flash before him. An exceptional and practisedsubject will give a mean value of 0 • 15 sec, and a novice a meanvalue of 0-2 to 0-25 sec. In each case the standard deviationwill be between 0-02 and 0-04 sec. The distribution of valuesaround the mean will be skew, with the longer tail extending in thedirection of the longer times. Taking 3(mean — median)/(stan-dard deviation) as the measure of skewness, the value is about+ 0-45. Various alterations in the experimental technique willlengthen this time interval. The figures just given are foundwhen conditions are arranged to give the shortest and leastvariable reaction times possible, conditions which will include awarning of between 1 and 3 sec before the stimulus appears,an adequate but not excessive stimulus seen binocularly, a com-fortable and warm environment and the absence of distraction.

The most obvious way to increase the time lag is to introducesome complexity into the situation, e.g. to have two lights side byside, and instruct the subject to move in a particular directioncorresponding to the appropriate stimulus—the so-called choicereaction. The simplest complication of this kind will at oncedouble or treble the time lag.

The operator's lag in target tracking is of the multiple-choicetype, since he is required to choose both a direction and anamplitude. But his reaction time is strictly indeterminable in anysituation with a continuously changing input. Indirect evidence

BATES: SOME CHARACTERISTICS OF A HUMAN OPERATOR 301

supports a value between 0 • 3 and 0 • 5 sec as being likely, butthe available techniques to estimate this by auto-regressivecorrelation, for example, have not been fully applied. In theequations used by Tustin's the assumption of 0 • 3 sec led toagreement with the experimental data.

The choice reaction may have a place in the selection of opera-tors for manual tracking. It was found during the 1914-18 War byMiles, and in the 1939-45 War by the Foxboro Co.,« that choice-reaction time correlated highly with initial tracking ability. Choice-reaction time is simple to record and its measure is less equivocalthan that from a record of overall target-tracking performance.

(3.2) The Transit ReactionOne particular variety of reaction-time experiment concerns

the task of reacting at the instant of coincidence of two objectsseen to be approaching each other; the task of the bomb aimerwith the simpler types of bomb sight, and also of the tank gunner.Experiments were performed on this at Lulworth during the war.2

The subject saw a black square cross his telescope field at aconstant speed, and he was told that the instant at which thecentre of the moving square was bisected by the vertical graticulemark should coincide with the instant at which his trigger beganto move. He had no means of ascertaining his success or failure,and thus learning during a sequence of aims was excluded.Laboratory subjects were used, but all the conclusions were con-firmed on eight experienced tank gunners.

It was found that the mean point of a group of aims coincidedwith the desired point of aim; there was no systematic bias, onlyan element of random error. This random error, when expressedas an error in terms of time, had the same value as the randomvariability found in the simple reaction to a light flash, and it wasindependent of the rate at which the target was crossing at all ratesabove about •$• deg/sec. Thus the random errors made by the gunnerwere directly proportional to the crossing speed of the target.

In the transit reaction, the stimulus to the output movement isderived from mental processes involving among other things afuture prediction based on a previous estimate of velocity.When we compare the indefiniteness of this process as a stimuluswith the definiteness of a light flash, it seems surprising that theconsistency in the timing of the two reactions is the same.

(3.3) Operator DiscontinuityThese findings naturally prompted more detailed investigation

of the operator during one reaction time, i.e. about 0 • 2 sec,before coincidence would occur. We first demonstrated that aview of the approaching target for a minimum of 0-2 sec, inde-pendent of the rate, was required in order that there should be nosystematic error in aim. We further demonstrated that it wasonly possible to cancel the reaction by a second stimulus providedthat the second stimulus did not come within 0 • 2 sec beforecoincidence would have occurred. In other words, the subject,having made a "decision" to react, is in an irreversible state, andthe reacting movement must follow 0 • 2 sec later.

The results of these experiments clearly define the extent towhich we must consider the human operator to be discontinuousin the servo sense, namely that whatever is involved in makinga decision, once a decision to react has been made, the reactionmust follow about 0-2 sec later. Nothing that happens on theinput side during this period can affect the reaction which hasbeen decided on in any way. These findings strongly support aconcept of operator behaviour, first put forward by Uttley19 onother grounds, as being necessarily discontinuous, and thisconcept is also strengthened by Telford's1? demonstrationthat a stimulus-response reaction disturbs the subject in such away that a second stimulus, separated by •$• sec from the previousone, is not responded to as quickly as if it had occurred whenseparated by a 1-sec period or longer. This whole subject hasrecently been critically discussed by Hick.14

(3.4) Response to Step-Function InputsThe analysis of the operator's response to unit-function

inputs has provided an interesting type of experimental techniquefirst used in this connection by Craik.3 In a typical experiment,a track containing right-angle steps is traced on a drum rotatingin front of the operator, who sees it through a narrow slit placedat right angles to its direction of movement. He attempts tofollow the track with the tip of a pointer moving in the slit, theposition of the tip being controlled by the position of rotation ofhis wrist. With this system all operators give time-displacementtraces typical of a stable system, operating with a variable timedelay. The degree of "stiffness"—torque (acceleration) per uniterror—and "damping" they show varies with their familiaritywith the apparatus, and their own psychological make-up.Broadly speaking, as learning proceeds, the delay before acorrective movement begins progressively approaches a value of0-3 sec found in the fully practised subject. The rapidity ofmovement increases, and the number of individual correctivemovements to each step input (i.e. the random error in any onemovement) decreases. In examining records from this type ofexperiment one often obtains confirmation of the existence of twophases in a voluntary. correcting movement first distinguishedby Woodworth21 in 1898. The first is characterized by being arapid jerk of the limb, the demand to start movement necessarilyincluding also the demand to stop. As we have stated above,nothing can affect a movement of this type within 0 • 2 sec ofits start, and it is seen in the above type of experiment that atleast one reaction time elapses before a subsequent movementcommences. The second phase of corrective movement is foundwhen the limb rate is sufficiently slow for visual control to becontinuously effective. It appears that movement can be underefficient visual control at rates below 10 deg/sec at the eye, andinefficiently or not at all at rates above 50 deg/sec at the eye.

By systematically identifying movements of the first type in hisrecords, Craik was able to express its amplitude error as apercentage of the actual movement intended. Thus he was infact examining the behaviour of Weber's constant which wediscussed in connection with perception. Fig. 5 shows data

1 2 - 3 4Correction required, deg

Fig. 5.—Error of single corrective movements involving variousamplitudes of wrist-joint rotation.

Two subjects, 10 movements at each amplitude.obtained using his technique on wrist-rotation movements.When dealing with perceptual acuity, it was shown that data inthis form gave information of direct interest to the problem oftelescope magnification, once the least misalignment of sig-nificance in the particular situation had been specified. In thesame way, this data is of direct interest to the gear ratio ofcontrols. If a i-deg wrist movement is, from the characteristics ofthe apparatus, the least amplitude for which correction is deemedsignificant, we must provide at least a four-times magnificationto remove us from a zone in which the mean percentage error isof the order of 80 % to a zone in which the error is of the order of15%. There will be no gain in accuracy with any higher magnifi-cation. This approach may be useful in certain design problems.

302 BATES: SOME CHARACTERISTICS OF A HUMAN OPERATOR

(3.5) Muscular BracketingCertain cases arise in which a finer adjustment than that

obtainable by vision alone must be attempted, e.g. in align-ing a telescope, when the target is smaller than the thicknessof a graticule line, or in tuning a radio transmitter. Herea technique known as muscular bracketing may be em-ployed. In this, movement is made until an error is just per-ceptible. The control is then moved through the dead zoneuntil an error is again just detectable. By memorizing the "feel"of the limb in the two positions, the mid-position is estimated,and the control is moved thither. The gain in accuracy whenusing this technique as opposed to leaving the control with a justdetectable misalignment was again briefly investigated by Craik.5He found, under his particular conditions, a three-fold gain inaccuracy when this technique was used. The gear ratio and sizeof the control will of course determine the distance whichseparates the two positions of error. Craik found an optimumzone when a knob of 25 50 mm diameter was linked so that acircumferential movement of 8-20 mm either side of the centralposition was required to give a perceptible error.

(4) SOME PRACTICAL POINTS IN CONTROL DESIGN(4.1) Display Magnification and Tracking Accuracy

The data shown in Fig. 6 are taken from the results obtainedby the Foxboro Co.10 under what appear to have been par-ticularly careful experimental conditions. In this particular setaided handlebar tracking was used, and in the Figure is shown

0-8

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| 0-5

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0-1

0x l x20x6 xlO

Telescope magnificationFig. 6.—Relation between tracking error and display magnification

(Foxboro Co.).Vertical lines indicate range of error.

the mean error of six operators, together with their range, atthree different magnifications. They tracked for a period of onehour on seven separate occasions at each magnification. It isclear that the introduction of a six-fold magnification has reducedthe mean error to less than half its former value. This resultmay at first be surprising if it is recalled that the threshold ofvisual acuity was not in itself a limiting factor in trackingaccuracy. The decrease in overall error can, however, bereasonably attributed to three factors: first, that increasedmagnification will make errors visible sooner, and hence cor-rective movements will follow more quickly; secondly, it willpermit a more accurate observation of target rates; thirdly,and possibly the most important, it will tend to make the operatorwork harder to keep the error within some special limit whichhe sets independently of the magnification of the system.

With regard to practical design, I think all the evidenceendorses the conclusion of the Foxboro Co. that "optimum

tracking results from the employment of maximum magnificationwhich is practicable under service conditions."

(4.2) Friction and its EffectsIt was again the Foxboro Co.9 which was responsible for

the first large-scale attack on this problem, and their main con-clusions have been well borne out by subsequent work. Theyintroduced a new method of expressing tracking error, namelyto convert it into units of time by dividing the angular error asmeasured at the handwheel by the mean rate of handwheelrotation. This measures the average amount the subject leadsor lags the target on a time basis. It can be applied to variable-speed courses, and permits comparison under conditions ofdiffering display magnification and gear ratio. Typical resultsof their findings of the effects of friction in the control are showninFig. 7. Thisparticular set of data was obtainable from operators

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^ V

Friction•—• 7 lb*.-4 2 lbo—o l lbo— -o N o n e

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i

1 2 5 K) 20 DO 100 200Speed of handwheel turning, r.p.m.

Fig. 7.—The effects of coulomb friction on a positional trackingcontrol—constant-speed course (Foxboro Co.).

Light handwheel, small diameter.

tracking for three minutes at a required constant velocity.Coulomb friction was applied by means of a rope soaked in oilaround the control shaft. The stiction effect was said to be verylow, and the frictional loading to be virtually independent of therate of rotation.

It is clear that at any given rate of rotation the mean error isgreater with 7 lb of friction at the handle than it is without it—something like three to four times at the lower rates of turning,and twice at the higher rates; the error is both relatively andabsolutely greater at the lower speeds than at the high. If thestictional element in their control had been at all large, theerrors at the low rates would have been greater still.

It is quite clear from this that a remedy for friction is anincrease in the gear ratio. If for any reason this is impossible,an alternative way out may be to provide two-handed turninginstead of one. Hick,'5 for example, found that the break-down point for smooth rotation occurred at about 14 lb-inat 40 r.p.m., using one hand, and at about 22 lb-in using two

6000r

0 20 255 10 15Resistance at handle, lb

Fig. 8.—Effects of one- and two-handed turning on power output(Hick).

BATES: SOME CHARACTERISTICS OF A HUMAN OPERATOR 303

hands at the same rate. He also compared the power outputwith both methods; his results are shown in Fig. 8. It isseen that the power output with two hands is significantlygreater at all torques above about 10 lb-in at the handle. Tt isnoteworthy that even under conditions favourable for one-handed tracking, both the Foxboro Co. and Hick found a gainin accuracy if both hands were used.

(4.3) Control InertiaThere is considerable evidence that inertia added to the con-

trolled element is an aid to improved performance, and it wasthe Foxboro Co.9 which first provided reliable quantitativedata on this. They found, for example, that the addition of aflywheel of moment of inertia 25 Ib-in2 to a small light hand-wheel, about halved the mean error on a constant-speed courseat 2 r.p.m. It was of decreasing advantage as the rate increased,and gave no appreciable gain above 20 r.p.m.

There is general agreement on the physiological interpretationof these findings. The advantages of inertia on a control aretwofold. In the first place, since the operator is to some extentaware of the force between his hand and the element he is con-trolling, inertia provides data of a feedback nature which isdirectly proportional to the acceleration of the control. Withpure coulomb friction this data is not available to him, and ifstiction is present, the inconstancies in feel at the low ratesmay add appreciably to his confusion. Hick13 has putforward the suggestion that greater constancy of rate mayresult if the operator concentrates on putting out a constantforce rather than a constant rate. Force can be looked upon asthe body's basic output quantity; velocity is thus the singleintegral of this, and displacement the double integral. Toattempt a desired velocity and a desired displacement are thusin theory more complex operations than to attempt a desiredforce. But against this it might be emphasized that our every-day experience is a demand for accurate displacement outputs,i.e. practice in double integration. Craik,4 however, was un-able to demonstrate any difference in tracking accuracy usinga purely "isometric" control, i.e. one so stiffly sprung thatthe displacement needed was negligible, and a normal type ofpositional control.

Another reason for control inertia was stressed by the Fox-boro Co. They point out the importance of eliminating asfar as possible from the total display irregularity that part duesolely to irregularity in the operator's own output. Inertia ofcourse resists high-frequency components which are liable tooccur within one cycle of rotation owing to overaction of onemuscle in the group of those muscles which are active. Irregu-larity of this kind and its abolition by inertia are well shown inFig. 9, taken from their report.

0 5 10 15 20 25 30 35 40 45

40 0 5 ]0 15 20 25 30 35 40 45Time, sec

Fig. 9.—The smoothing effects of inertia on tracking a slow constant-spaed course (Foxboro Co.).

(«) Light handwheel, 2-8 r.p.m.; average error 32-3 ms.(h) „ „ with added inertia, 2 • 8 r.p.m.; average error 7 • 9 ms.

Inertia need be thought of not only in the rotary type of controlmechanism. Its advantages can of course be obtained in thesame way with the joy-stick type of control, by linking a flywheel

to the control through a ratchet rotation device. With regard tofeedback effects from the lever type of control in general, thesuggestion has been made that a spring centring effect might besufficient to give the operator data as to the actual angular dis-placement of the control he is using. We are unlucky here,however, for two reasons: first, that our accommodation topressure sensation is very rapid, and secondly, that our perceptionof changes in pressure is rather crude, about 7 % difference atleast being required over the useful range. This means that quiteprohibitive restoring forces are present by the time the controlhas reached its full deflection.

In conclusion, it must be emphasized that inertia, friction andrate of rotation are all variables which, besides having their ownsignificance, have interactions with one another which are alsosignificant. This point was well brought out in an analysis ofvariance which the Foxboro Co. made on their data. Butwith regard to practical design, first, the evidence of the labora-tory seems to point to the inclusion of inertia in the outputelement in an amount limited only by the strength of the operatorto meet the highest accelerations which his course is likely todemand; secondly the results emphasize the need for the com-plete elimination of any sort of stiction effect; thirdly, one-handedturning should be sufficient for torques below 10 lb-in at thehandle, but above that two-handed turning should be con-sidered; fourthly, for optimum short-period operation the gearingshould ensure that the maximum rate required to follow theinput should coincide with the maximum rate at which thecontrol can be turned—say about 150-183 r.p.m. in the absenceof further data.

(44) Accommodating to the Operator's Habits

It is important to remember that the operator is working ona mechanistic background determined by his everyday ex-periences. That is to say, he will have certain definite biases.A relevant one which has recently been shown to hokpo is that,when faced with a positional-control system, he expects a linearcontrol characteristic. With practice it was found that rapidstrides were made in overcoming a non-linearity, but experimentappeared always able to show some advantage in a linear system.Departure from linearity over the whole control range maybe essential in some types of mechanism, e.g. a tank-turrettraverse control in which the requirement is both for accuratetracking at 0-25 deg/sec rotation of the turret, and at the sametime for switching the turret at 20 deg/sec. There may be anargument for a series of discontinuous linear phases in this typeof mechanism as opposed to a smoothed transition, but it isconsidered to be relatively unimportant.

Familiarity likewise leads the operator to expect a ratio ofunity between the control and the display in a positional system,but in something so unfamiliar as a velocity control an expectedproportionality cannot be defined. Nevertheless, the requirementthat the operator and mechanism shall form a stable systemimposes limits to the possible characteristics, for the control mustalways allow him to make a movement which will have the effectof reducing any misalignment visible to him. Several mechanismshave been seen in which stability has been lost by requiring thata visible display-error needs for its correction a limb movementsmaller than that which can be made with accuracy. In generalit can be said that the least display-error visible should not needfor its correction an angular movement less than 2 • 0 deg at therelevant joint, but if stiction is present this may of course beconsiderably larger.

With regard to the positioning of controls, provided that theycan be operated without discomfort, no clear evidence has beenproduced that details of positioning are of first-order importance.With regard to the direction of movement, the only thing thatcan be said is that there is a slight advantage in making the sense

304 BATES: SOME CHARACTERISTICS OF A HUMAN OPERATOR

in agreement with the expectation of the average operator, anunknown that can be discovered only by experiment.

(5) ACQUISITION OF SKILL IN TRACKING(5.1) Stages in Learning

The process of learning any manual skill has been divided intothree phases. At first the completely untrained operator is facedby "chaos," and various inappropriate control movements areattempted until a broadly appropriate stimulus-response patternsuitable to the particular mechanism is selected. This will bevirtually instantaneous in a positional mechanism, but it may takehours of trial in, for example, a displacement-accelerationmechanism. There then follows a second phase of quantitativemodification of the relationship between stimulus and responsein which certain basic patterns of behaviour will be becomingfixed at an unconscious level. Methods of training may stand orfall according to whether the force-time habits of muscular activitywhich are securely fixed in this way turn out eventually to be themost advantageous ones. While this proceeds, confidence in-creases, and the third stage is reached when the operator's focus ofattention can operate on a wide fluctuating time-base rather thana small and relatively fixed one, i.e. when he is able to cope withwhole groups of input data rather than single stimuli. The track-ing operator, for example, will be improving his accuracy by pre-dictions of future behaviour of the target, for sufficient memory ofthe past has accumulated for his computing system to be able touse it as additional input information, and a momentum andsmoothing of his output will appear, based on these futurepredictions.

The rate of improvement will depend in general on theefficiency of the display data. To ensure the most rapid progressat each stage the maximum amount of insight should be availableto the trainee by ensuring full knowledge both of the results ofan individual movement, and of overall performance.

(5.2) Importance of PracticePractice in a skill, seen physiologically, seems to be essentially

the progressive reduction in the random variability of an outputquantity (e.g. the force of a muscular contraction) so that in-creasingly smaller increments can be made reliably; with thiscomes the ability to make an output of any given accuracy withless and less conscious effort, and hence more rapidly. When thetask is of such a nature that performance in it can be reliablymeasured, a curve can be drawn to show the improvement withpractice. The typical practice curve from a skilled task shows arapid initial improvement as insight into the requirements of thetask develops, and a slow but gradual improvement later as themotor side of the task becomes progressively more refined.Woodworth,22 discussing this, states: "Psychologists havebecome wary of accepting the terminal level in a practice curveas a true physiological limit. Often it has happened that in-creased incentive or improved methods or conditions havebrought a rise from what seemed to be the limit."

The number of hours of actual manipulation of, say, a guncontrol needed even to approach the desired physiological limitis widely underestimated. The Foxboro Co.? found that im-provements in tracking were still occurring after nine hours'manipulation of the controls. In this experiment the error washalved in the first four hours, and reduced by a further 18% inthe subsequent five hours. It was incidentally noted in thisexperiment that during practice the order of the subjects in termsof performance remained sufficiently constant to enable reliableselection to be made after a relatively short practice time.

(5.3) Transference of SkillTransference in this connection is a term used to describe the

carrying over of an act or way of acting from one performanceto another. Much discussion of training and training appliancescentres round the problem of transference of ability. Lookedat in a physiological way, the essential features are agreed to beas follows22:—

(a) If there are no identical stimulus and response elements inthe two tasks, the influence of one task upon another will beneither positive nor negative.

(b) If the new task requires stimulus and response elementsalready formed, there will be positive transfer of ability.

(c) If the new task requires breaking stimulus and responseelements previously formed, there will be interference or negativeeffect.

The practical importance of this is that it may be very simpleto isolate the essential stimulus and response elements in acomplex task, and to train the operator in them by the simplestand most elementary types of training aid. As the author hasargued elsewhere,1 there seems to be a great need for training aidsof a much more elementary nature than are at present conceived.

(5.4) The Control of Difficulty in TrainingAn important aspect of training centres round the actual

difficulty of the task presented to the trainee at any particularstage. It is clear that a quality which psychologists term the"level of aspiration," induced in the trainee, is a potent factorin the rate of his improvement. Further research is needed intothis highly important aspect of training, which is largely a questionof exactly controlling the difficulty of the task so that the manis keyed to his limit in order to produce a performance whichseems to him reasonable.

There is no a priori reason why "difficulty" in training shouldnot exceed at some stage the difficulty in the final task, and itseems that training equipment should always allow an accuratecontrol of difficulty. There is much scope for experimentationin this field, e.g. in the use of displacement-acceleration servocontrol systems for training to use displacement-velocitymechanisms.

(6) REFERENCES(1) BATES, J. A. V.: "The Human Element in Accurate Gunnery." B.P.C. 46/455,

April, 1946.(2) BATES, J. A. V., and HARKNESS, R. D.: "The Accuracy of Aim at Slow Crossing

Targets," B.P.C. 45/425, May, 1945.(3) CRAIK, K. J. W.: "Physiological and Psychological Aspects of Gun Control

Mechanisms " Pt. II, B.P.C. 43/254, March, 1944.(4) CRAIK, K. J. W., and VINCE, M. A.: "Physiological and Psychological Aspects of

Gun Control Mechanisms," Pt. HI, B.P.C. 45/405, Feb., 1945.(5) CRAIK, K. J. W., and VINCB, M. A.: "The Design and Manipulation of Instru-

ment Knobs," M.R.C. Report 46/272, A.P.U., 14th Jan., 1945.(6) DIMMICK, F. C , and KARL, J. C : "The Effect of Exposure Time on the R.L.

of Visible Motion," Journal of Experimental Psychology, 1930, 13, p. 365.(7) FOXBORO CO. (Foxboro, Mass.): "Accuracy of Tracking by Means of Handwheel

Control," O.S.R.D., 3451, Sept., 1942.(8) FOXBORO CO.: "Supplementary Study of Factors Determining Accuracy of

Tracking," O.S.R.D., 3452, Dec, 1942.(9) FOXBORO CO.: "Inertia, Friction and Diameter in Handwheel Tracking,"

O.S.R.D., 3454, Sept., 1943.(10) FOXBORO CO.: "Telescope Magnification and Tracking Accuracy," O.S.R.D.,

Dept. 5304, April, 1945.(11) GARRY, R. C , and WISHART, G. M.: "On the Existence of a Most Efficient

Speed in Bicycle Pedalling," Journal of Physiology, 1931, 72, p. 425.(12) HENMON, V. A. C : "The Times of Perception as a Measure of Differences in

Sensation," Archives of Philosophy, Psychology and Scientific Method, 1906,Vol. 8.

(13) HICK, W. E.: "The Precision of Incremental Muscular Forces," M.R.C. 46/269,Aug., 1945.

(14) HICK, W. E.: "Discontinuity and the Human Operator of Machine Controls,"M.R.C. 47/348, June, 1947.

(15) HICK, W. E., and CLARKE, P.: "The Effect of Heavy Loads on HandwheelTracking," A.P.U., 47, May, 1946.

(16) STETSON, R. H., and BOUMAN, H. D.: "The Co-ordination of Simple SkilledMovements," Archives Nierlandaises de Physiologie, 1935, 20, p. 179.

(17) TELFORD, C. W.: "The Refractory Phase of Voluntary and Associative Responses,"Journal of Experimental Psychology, 1931, 14, p. 1.

(18) TUSTIN, A.: "The Nature of the Operator Response in Manual Control andits implications for Controller Design," see p. 190.

(19) UTTLEY, A.: "The Human Operator as an Intermittent Servo," Report of 5thMeeting of Manual Tracking Panel, August, 1944, GF/171 (SRIA).

(20) VINCE, M. A.: "The Psychological Effect of a Non-Linear Relation betweenControl and Display," S.R.I. (Servo) Report No. 2, February, 1946.

(21) WOODWORTH, R. S.: "The Accuracy of Voluntary Movement," PsychologicalReview Monograph Supplements, 1899, Vol. Ill, No. 3.

(22) WOODWORTH, R. S.: "Experimental Psychology" (Methuen, 1939).The designations B.P.C., M.R.C., and A.P.U. refer to unpublished reports to the

Medical Research Council.