Design of man-machine interfaces in process control

Digi tal Computer Applications to Process Control, Van © IFAC and orth-Holland Publishing Company (1977)

DESIG

auta Lemke, ed.

OF MA -MACHI E I

l.E.

Rijnsdorp

\ .B.

Rouse

(Twente

TERFACES

I

5-7

PROCESS CO TROL

niversity of Technology, Enschede,

(University of Illinois,

Urbana,

L

etherlands)

Illinois, U.S.A.)

Computerized CRT-displays are rapidly being introduced into man-machine interfaces for process control and supervision. The flexibility of these devices can promote the incorporation of human factors in interface design. The paper gives a survey of design criteria and guidelines for: Allocating system functions between "man" and "machine"; coping with operator skills; evaluating CRT-displays and choosing controls and dialogue structures for these type of displays. Also, attention is paid to experimental comparison of design alternatives, and to the influences of the human organization, high mental stress and job satisfaction on interface design. I

TRODUCTIO There is still another side to the problem: Human beings cannot merely be reduced to system components; they have needs, likes and dislikes, and aspirations. Of course, the conscientious designer is aw~re of"this.ba~ic dif~erence between "man and machine and, if he forgets it, the human beings in the system will sooner or later remind him about it, either directly or indirectly. In some countries, new legislation even requires guarantees for an optimal human" working environment (in the wide sense of the word 191)·

The digital computer shows a remarkably persistent progress towards still higher speed, greater versatility, and lower costs. This does not only apply to computer hardware and software, but also to peripherals in man-computer interfaces. Of particular interest is the introduction of a variety of computerized visual displays, which combine flexibility and economy with suitability for industrial environments. These developments are leading manufacturers to promote new principles of information presentation, which appeal to the progressive user, but might also complicate his (her) choice. This is even more true when the user tries to design the interface himself.

In the following sections, these problems will be analysed in more detail. The text will be focused on the question: "How should the design process be arranged to cope with all relevant human factor s ?" (P u r due \.J 0 r k s hop I 10 I ) . Special attention will be paid to a critical evaluation of some present trends, in order to provide arguments for a more balanced point of view.

It seems appropriate to utilize the present freedom in design for improving or even optimizing system performance. As man-machine interfaces are only there because of man, their performance depends strongly on human factors. The designer would therefore be ver happy to possess a set of consistent data about the human being as a "system component" just like those data available for other system components.

THE DESIG

PROCESS

There is a growing literature about the methodology of design. Some methods (Bosman Ill, 121)stress the "sociotechnical" approach, where human factors are treated in a wa similar to technical and economic factors (de long 1131). Fig.l shows a possible way to look at the sociotechnical design of automation systems (it does not indicate the various jumps and iteration cycles, which unavoidably occur in practice). After the problem definition phase (van den Kroonenberg 11 I), which results in preliminary specifications, follows the functional design phase. Here one determines the functions the system has to perform.

Indeed, the designer will find much infotmation in handbooks about human factors (Ergonomics 11 1 in particular about suitable di ensions of equipment, readability of displays, criteria for the physical environment (such as temperature', humidity, noise), etc. Further, human performance is described for simple well-defined tasks. However, can the designer integrate these elementar data into a total picture, corresponding to the actual human jobs in system operation and maintenance?

-181)

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PROBLEM DEFI ITIO PHASE FU. CTIO AL

" man "

design selection and training methods final

Fig.

STEPS I

specifications;design result

THE SOCIOTECH,ICAL DESIG.

In the shaping phase, these functions are all 0 cat e d bet wee n " man" and " ma chi ne". For the case of automation systems, such as found in process control, functions are allocated a ong operating and maintenance personnel on the one hand, and an- achine interfaces, instruments and computers on the other hand. The arrows in Fig. 1 indicate an order of preference: The allocation should not result in offering re aining bits and pieces to the hu an side of the system. This would interfere ith the integration of hu an tasks into satisfactory jobs and job structures (Grandjean 1151). The term" an-machine dialogue" is used for the t pe of interaction between " an" and" achine". One has to choose bet een alternatives, such as man- initiated and machine-initiated, displa -

OF AUTO.1ATIO.

SYSTEMS

on-demand or display-by-exception,

etc.

The application software serves as the ultimate "sink" in the design process. Of course, the accompanying develop ent costs cannot be left unconstrained, but they can be reduced by using a suitable problem-oriented language. S STEX F . CIIO S Syste functions can be categorised in three groups (Bibby 1161, Rijnsdorp 1171) I. Supervision and control of proces operation; 11. Coping \ ith alfunctioning; Ill. S stem e aluation and i prove ent. The first group refers to supervision and control of the nor al operation of the process(es) going on in the auto ated

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OF MA. -MACHI E I

system. Deviations of process variables from desired values have to be detected and corrected. The second group refers to abnormal operation due to faults in process equipment, instruments and computers. It consists of fault detection, identification, ~ompensation and correction. This group is gaining in importance with the increased awareness for system reliabilit and safety, and the resulting new safety legislation. When the risk standards which are common in the nuclear power industry will also be enforced in other fields, there will be quite a change in the design of automation systems~ The third group deals with system evaluation and improvement. Process performance is reported to operators, supervisors, the accounting department, management, R&D experts, etc. The results can lead to improvements in methods and equipment, and to suggestions for new designs. This emphasizes "improvability" as a desirable characteristic of automation systems, which can be enhanced by utilizing the inherent flexibility of modern visual display systems and application of software. In Fig.2 the first group of functions (supervision and control of normal operation) has been arranged in a multi-layer scheme (Mesarovic!181).A distinction has been made between steady operation of continuous processes on the one hand, and dynamic operation of continuous and batch processes on the other hand. The first layer refers to simple switching actions of a binary nature. The second layer adds more intelligence b utilizing measurements for adjusting control variables in feedback or in feedforward fashion. The third layer, which has been set aside fro the second layer because of its great practical importance, refers to adaptation of the settings in the second layer to compensate for process changes, product quality deviations, etc. The fourth layer optimizes the econo of process operation b utilizing the degrees of freedo for control hich have not alread been fixed in the third 1 aye r . fhe fifth and sixth la ers onl pertain to dyna ic operation. The ai is the allocation of raw materials and products to various processes, and the scheduling of this allocation as a function of time. In some applications, e.g. steel ills,

TERFACES

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traffic control, this is a very important part of process control and supervision; in other cases (e.g. the manufacturing of standard chemicals) it hardly pia s a role. ALLOCATIO OF SYSTEM FU CTIO S TO " A " A D "MACH lE" In the allocation of functions between "men" and "machines" (the ulural is more to the point than the sino~lar) there are three possibilities: ~ , a) manual operation b) automatic operation c) computer-assisted manual operation. Strictly spoken, the last possibility is very common because the extremes of purely manual and purely automatic operation are often not very feasible. Even the ikings on Mars do not function completely automatically~ Assisted manual operation is also inherently sound, as it utilize_ the complementary nature of man and machine (15\ ,Schmidtke 119\ by combining th ir strong sides while avoiding their weak sides. For the lowest layer of Fig.2, frequent switching actions can be allocated to the machine and infrequent ones to man. The choice should not be merely a matter of economics: Manual switching can augment the updating of the mental picture the operator has about the state of the process. In modern processes, the second layer is highly automated. The operators only have to run the controls manually in case of malfunctioning or maintenance of the automation system. In contrast, the third and the fourth layer usually rely on computer-assisted manual operation. In fact, together with the detection, identification and compensation of faults (grc'!p 11 in Fig. 2), this often constitutes the most critical part of the operator/co puter interface design. The scheduling laver offers good opportunities for enriching the job of the operating personnel (Ketteringham 120 for instance b "playing" with a odel in off-line software (e.g. background mode in on-line. co puters).

I)

The planning la periods of ti e action between is not suitable

er reckons ith long and ith the interany processes, hence it for the operator.

In practice, the second group (Fig.2) is usuall realized b so e for of co puter-assisted anual operation. Here the operating personnel takes care of

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1.AYER III SYSTE E ALUATIO A D I

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improvement by modifications and small extensions

-

PRO EME T

4

correction faults

3 compensation

process equipment,

C

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in

2 identification

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1 detection

computers

and

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control layer

steady operation of continuous processes

P R

o C

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o T R

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,batch p~ocesses & dynamic operaltion of continulous processes lallocating feed operators toff-line to proplanning I computer Icesses & procesdepartment I (e.g. LP) I sin g mo_d_e__s_. ~-_--,----------------Ischeduling switch job enrichment loff-line ---overs based on for operating I software (e.g. I expected supply & personnel? I simulation).

6.planning of operation

I & prod.

5.scheduling of operation t--

operators; diagnostic maintenance I software personnel monitoring o~alarms, process state I warning signals by operators I by parallel equipm and/or Ion-line comp.

---t~_ _---------L.I_d_e_m_a-n-d-~p-a-t--t-e-r-n-s---II+

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done, or I advisory or 4.optimi- optimisation loptimum start-up of process monitored by supervi'sory sing . b Iswitch-over control operation y control room control by adjust~ent of shut-down operator computer set pOints or I switching conI trol schemes I 3. trimcompensation 'adaptation of control room I advice to operaming of for errors, Isequential control operator; quali-I tor by on-line stabiliprocess chan-to compensate for t anal sis by comp.; data sing con- ges, product lerrors, etc. laborator ana- I transmission trol qualit devi1 st I from laborator ations, etc I 2.stabifeedback and sequential conoccasionally: I "conventional" lising feedforward Itrol (program manual control instruments control control with Icontrol ith feed b operators I or DDC given desi- ,back) I red values I Ifixed program infrequent frequent I.switching actIcontrol (withactions: (rov- I actions: (micro) ions lout feedbac_k_) ~_i_n_g_)_o_p_e_r_a_t_o_r_s __I_c_o_m_p_u_t_e_r_s _

I

F CTIO S AI.D F SYSTE

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ALLOCATIO

OF THE PROCESS COl TROL ADS PER IS101

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OF .1A. - 1ACHI . E I . TERFACES

first-line identification and compensation, and the maintenance personnel for second-line identification and correction. The operator can have a share in system evaluation and improvement (e.g. by modifying display lay-out and format). This would require a simple problemoriented language for communication with the computer system. It could enhance job satisfaction, which tends to suffer from a high level of automation, and so yield a contribution to humanisation of work. An alternative approach to function allocation has recentl been suggested (Rou e 121 I Recognizing that there are many tasks that could be performed by either man or machine, it seems unreasonable to force a strict or static allocation of responsibility between man and machine. Instead, responsibility should be dynamically allocated to the decision maker (man or machine) that has the most spare capacity at the moment. Theoretical and experimental investigations of this idea are currently under way. In this allocation section, different groups of personnel have been given various tasks. As the operators ha e the most urgent and frequent contacts with the automation system, the following sections are mainly devoted to their job, functioning, and interfaces. (See also Edwards and Lees 122, 231; Johannsen and Sheridan 124 1 ) . OPERATI. G SKILLS In the past, when the degree of automation was lov;, many operators had welldefined tasks, often of a repetitive nature. 1uch research has been done to model human beha ior in acti ities such a anual feedback control of fast processes (McRuer & Krendel 1 25 1 , Kleinman c.s I 6 1 ) slow processes (Crossman and Cooke I 71, e t and Clark 128 , Brigham and Laios 1291, \eldhuijzen 1301 Pa ernotte 1 31 I ) anual progra control (Bainbridge c. s. 139') etc. Ho~e

er, in odern highly auto ated processes it i much more difficult to describe what the operator has to do. For econo ic reasons he is often kept relati el' busy b gi ing him the supervision of a larger section of the total process or of se eral processes. Consequentl the operator performs a multitude of tasks in a time-var ing pattern, ith periods of relatively calm and other ones with frenzied acti ity. Moreover, as a result of automation, the majorit of

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the tasks emphasize typically human abilities, such as pattern recognition and prediction, deduction and induction, improvisation, etc. Such abilities and skills are by nature more difficult to define and to model. Still, progress is being made in studying mental skills in the experimental psychologist's laboratory. Here the human subjects are usuall' .)ffered one task-at-a-time, thus all m2ntal functions ( uch as perception, short-term memory, long-term memory, etc) can be completely dedicated to the task-at-hand. Unfortunately, this is widely different from the actual ituation of the process operator, who ha to share his mental function amongst many parallel tasks. The superposition princi~le cannot be 32 1) because of applied here (Singleton the limited access rate and capacity of the human short term memory. Good performance for many parallel tasks requires external memory aids, such as recorders, log sheets, alarm arrays, etc. 1

It seems desirable to load human subjects in laboratory experiments with secondary and/or parallel primary tasks in order to obtain results which are more amenable for practical application. Apart from research in the laboratory, investigations have also been performed in the field. The operator's job has been analyzed by observing his activities in the control room (Dirken c.s1 33 1 Daniel c.s 1 3 ,de Jong 1351, Dallimonti 361, van Droffelaar 137\, Drury 138 ). For example, researchers have obtained the distribution of total working time over different types of activities (Fig. 3). In all cases it was noted that general scanning of displa s occupies a more or les constant fraction of the total working time, irrespective of the time of day or night, the amount of upsets occurring in the process, etc. It seems that the operator regularly follows changes in the tate of the process, or even anticipates future de elop ents. The i portance of state obser ation ha also beco e apparent fro the anal 'sis of verbal protocols (Bainbridge c.s. '39 Kragt 01). These were ade after the operator was encouraged to sa' ~hat he was doing or thinking. Ras ussen 1, has used erba pro 0cols to construct an integrated ode of ental activities bet een initiation of the response and the manual action. This model resembles a ladder (Fig./), \·:ith one leg up ards for anal sis of a situation, and another downv;ard for planning

DESIG' OF

710

A - ACHI E I TERFACES I' PROCESS CO'TROL

morning shift

activity

evening shift

scanning of displays

37%

39%

manipul. of controls

6

5

external communication

night shift

begin shift

end shift

34%

34%

32%

4

5

4

internal communication

29

16

6

23

16

reporting

9

10

11

10

12

maintenance

0

0 12

9

12

32

19

21

0

personal care unrelated activ.

Fig.

17

11

DISTRIBUTIO

3

5-7

OF OPERATOR ACTIVITIES

(van Droffelaar,

1974) .

"data processing" activities state of "knowledge"

•

jumps

-- --,/--'/

--/'"

Fig.

",

~----

",

----~-

SI PLIFIED SCHEME OF MODEL OF

E~TAL

ACTI ITIES

(Rasmussen,

1976)

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DES I G~' 0 F MA~ -:1 ACHI El. T E RF ACE S I T

the proper actions. The skilled operator often jumps intuitively from somewhere on the first leg to somewhere on the second leg and thereby bypasses higher, more cognitive steps. He can do this by holistic perception instead of detailed observation, and by associations based on experience. The advantage of field work is its focus on the actual situation of the process operator; disadvantages are the qualitative and superficial nature of the results, which are not very suitable for developing general models of human behavior. FU CTIOl S OF THE MAN-MACHI E INTERFACES The various functions in process control and supervision require a variety of "men" and "machines", which logically leads to a number of interfaces. The following ones can be distinguished, at least functionally (See also Fig.S). a. On-line operator interface b. Off-line operator interface c. Reporting interface e. Programmer's interface f. System improvement interface. The on-line interface encompasses both operator/process and operator/computer interaction. An important requirement is that the operator should not notice any difference between direct interaction with the process and indirect interaction via the computer system. This point has also been stressed b Dallimonti 143\ who has found that operato~s are not willing to go to another display, even if this is more suitable (See also Crossman c.s. 1441). He also indicates the need for easily surveyable displa s of the state of the process (see preceding section, RasmussenI4S!, Smithl461. The type of information to be displayed will depend on the operator's interest: At some moments of time he wishes to obtain a global impression; at other times he has to go into more detail for a specific part of the process. Ihis suggests the desirability of rapidly adaptable displays and controls. However, the operator should not be confused by a varying la -out. It seems desirable to allocate a given "field" (e.g. in a picture shown on a CRI) consistently to a gi en process variable. Within this field, different t pes of infor ation can be displa ed "on de and" e.g. present value, set point, predicted value, historic values, control action, control parameters.

~

711

PRO CE S S CO.' I R0 L

The off-line operator interface has been separated from the on-line interface, so that an individual operator can concentrate on scheduling or system improvement, when relieved of his on-line duties by one of his colleagues. The laboratory interface communicates with the on-line operator interface for the transmission of product quality data. The reporting interface is only rarely of interest to the operato~ while on duty, hence it can be locdted at some distance or even in a separate room. This also reduces noise interference from line printers and the like. The programmer's interface is usually built into the computer system. Finally, the system improvement interface (to be used by control engineers, process technologists, software experts, etc.) can be combined with the off-line operator interface, provided the total work load is not too high. Important interface components are displays and controls. It will not be tried here to survey the variety of possibilities (see e . . ierwille 1 47 /, Control Engineering 481, Singleton 49, 501 Williams 151 , JOn g kind ls BernotatlS31 Union 154 , AndreievlSS ,56 Instead some attention will be paid to CRI-displays and the associated controls.

2!,

DISPLAYS 'hen faced with the need to choose a CRT-like display, the designer has man options. He can choose from almost one hundred commercially available alphanumeric displays (Granholm 157 I) or from a lesser number of graphics displays. Alternatively, he can use a general purpose CRT and design his own character and line formats. In either case, human tactors considerations should influence his decisions. In this section we will briefly sum arize a few hu an factors guidelines for choosing or designing displays. ~1uch of this material is excerpted from Rouse 1581. Hov.:e er, important additions are also included, not the least of ~hich is the use of international units. Considering the i par ant CRI para,eters (definition and units co e fro Gould 1 1 59 ', Sherr'60 , .1cGor ic' 151, Grether 61 I , and JEDEC 62'), lu.ina ce is the intensity of a surface easured in candela per eter squared. For a surface without internal sources of light, lu inance equals reflectance ti es illu ination where illu ination is easured in lumens per meter squared. Lu inance de-

DESIG

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OF MA. - ACHI.E I.TERFACES I.

functions

name of interface

o

-LI E

OPERATOR

11 TER-

PROCESS CO TROL

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layer (see Fig.2)

display of process state (actual, predicted, historic values)

11.2 identification of malfunctioning 1.3 trimming

alarms

11.1

display and control of process line-up

1.1

detection switching actions

-----------------------------------------------------FACE

/ switching and control tuning

1.2 stabilising control

display and control of set points

1.3 trimming

display of product qualities

1.3 trimming

~-----------------------------------------------------

OFF-LI E

display of optimum set values

1.4 optimizing control

display and control for e.g. simulation

1.5 scheduling

~------------------------------------------------------

OPERATOR

display and control system improvement

for

Ill. system improvement

I1TERFACE LABORATORY I TERFACE REPORTI G I TERFACE

PROGRA~1ER'S

I.TERFACE S

S T E~1

I.1PRO E~E1 T I:TERFACE

Fig.s

reading-in of product quality data

1.3 trimming

"hard copy" reports for operation supervisors, accounting dept., management, etc.

Ill.

access to working memories and computer modes

11.

displa sand controls odification for progra

system evaluation

system malfunctioning III. system improvement I I I.

men t.

F . CTIO. S OF .1A. /.1ACHI E 11 TERFACES

s

stem improve-

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creases as the cosine of the viewing angle measured from a perpendicular to the displa . Brightness is perceived luminance. Adequate luminance is 80 candela per meter squared while 160 candela per meter squared is preferred. At the time of Gould's revie\ (Gould 1591) most available CRTs were adequate and there is no reason to assume that the situation has changed. The ratio of symbol luminance plus background luminance to background luminance is termed contrast ratio. A contrast ratio of 15 is acceptable while 30 is preferred. To obtain a contrast ratio of 15, character parts should be separated by 7'. Further, the ambient lighting should be controlled if possible. Gould's review (Gould 1591) indicated that the contrast ratio of many CRTs was inadequate and this situation appears to continue to some extent, especially in areas with high levels of ambient lighting. This is important as contrast ratio affects human performance more significantly than luminance (Gould 159\, Sherr 1601) . Flicker occurs when the regeneration rate of the display is lower than the critical fusion frequency which in turn depends on the luminance, CRT scanning patterns, and persistence of the phosphor. Safe regeneration rates are 50 Hz if the illumination is greater than 54 lumens per meter squared (a well-lit area) or 20 Hz if the illumination is less than 5.4 lumens per meter squared (Sherr 160 I). Gould indicated that many CRTs, available at the time of his review, flickered. One minute of arc is the accepted standard of visual acuity but three minutes is a more conservative estimate. Characters should subtend 12' to 15' which is exceeded by most displays (Gould 591 McLaughlin 1631, Granholm 1571). Character sizes above 3.05 mm do not significantly improve performance (at a iewing distance of 750 m). Lower case letters appear to result in inferior performance when compared to upper case letters. The choice of character generation method (i.e., matrix versus stroke) is not important (fro a u an factors point of vie) as long as the atrix has a sufficient number of ele ents (\artebedian 16

I)·

The t pical 5x7 character atrix is arginal and 10 or 11 dots in height is preferred (Gould 1591; Sherr 160 Of the man displa s a ailable in 1973, ost used the 5x7 atrix (.1cLaughlin 1631). However, an displa s now use larger matrices (Granholm 1571). (It is

I).

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interesting that dot matrix size was essentially the onl displa parameter of potential human factors importance noted in these surveys). Height to width ratio of characters should be from 7:5 to 3:2 (Sherr 1601; McCormick 151 ). B 1 ink in g cur so r s are pr e fer red; wit h a 3 Hz blink frequency being appropriate ( artebedian 1661; Smith 1651). Boxshaped cursors are best for use with alphanumeric information ( artebedian 1661 while more pointer-like cursors appear to be appropriate for manipulating graphical information. The use of straight-line interpolation for the graphical display of discrete points is convenient from a hardware/ software perspective and a typical feature of most graphic displays. Unfortunately, the properties of the graph may be distorted by such straight-line interpolation and humans transmit this distortion. Spline interpolation has been used to avoid this difficulty (Rouse 1671). To provide the illusion of continuity, lines should be composed of at least 20 points per cm which appears to be satisfied by most displays (Gould 159 I ) . In the design of CRT-generated instruments, counters are preferred to dials for static reading ( ason 1681) while hybrid displays are preferred to either pure analog or di ital for dynamic reading (Schubert 169 ). CO . TROLS The designer of the human-computer interface may select from a variety of input devices. While we will mention several alternatives, many of them are inappropriate for use in process control. Thus, the set of alternatives quite naturally limits itself and actually akes the choice easier. . 1ost displa s are a ailable with standard (Q ERTY) ke boards. From hu an factors point of view, these keyboards are awkward because of their geometr (Cha bers 170; Kroe er 1711). Further, in an industrial environ ent, the ke s are too nu erous and too s all for the operator who does not necessarily sit at the ke board like a t pist. A keyboard like that on a touchtone telephone has been suggested as a possible alternati e. \hile such x3 keyboards li it the size of a co mand language, this a not be a proble in process control situations. More robust languages for these ke boards have been investigated (S ith I 72 I) .

714

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further alternati e is the use of touch panels which allow the user to simply touch the display to indicate his command input (Ebeling 1731). Of course, both of these alternatives would not be attractive in situations" here robust dialogues are used. Other input devices such as light pens, mice, and tablets (Keats 741) are useful in offices and laboratories but are inappropriate in situations where the pen, mouse, or stylus might be dropped or misplaced. Joysticks or trackballs are useful for positioning objects but are cumbersome for specifying input commands. The choice of an input device depends on the tasks of the user. If the user spends most of his time sitting at a console, monitoring many processes, and making relatively long-range decisions then a standard keyboard and light pen may be quite suitable. On the other hand, if the potential user of the input device spends his time moving throughout the plant, then simple touchtone keyboards or touch panels, perhaps with enlarged keys, are appropriate. Finally, if the task of the user is remote positioning of workpieces, then joysticks or manipulator-like devices are appropriate. To conclude this discussion of input devices, it is important to note a few particularly poor choices as input devices. The use of two independent potentiometers to position the cross-hairs of a cursor provides a task that is difficult for the human to master. Unfortunately, a well-known (and inexpensive) graphics display has this type of device as standard equipment. Special purpose buttons, switches and levers are inexpensi e and ay be appropriate when the same operator continually uses the s ste However, when a variety of operators utilize the syste the designer should choose de ices that are ore standard and can thus exploit the operator's training with ot er equip ent. A -.1ACHI.'E DIALOGUES ~ithin

this section,"e ill briefl su arize a fe~ pri ciples upon hich the de ign of dialogues can be based . 1artin •1ost of this material co es from 175 PIa e 76 Rouse 581 The reader shou d refer to Martin's text for a fairthorough treat ent of mo t of the ideas discussed here.

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keyboards, light pens, etc. Another alternative is voice which, for somehat limited applications, has been fairI y s u c ce s f u I (R ab in er I 7 7 I ). Th re e fa i rly common ways of soliciting operator input is via computer-directed questioning, presentation of menus, and operator directed command languages. Keyboards work well with all three alternatives while light pens are best suited for menu-picking. oice input systems are feasible for all three alternatives if the vocabulary is sufficiently controlled. Considering the choice of input language, the feasibility of natural language is increaSinjlY being explored (Martin 1751, Plame 176 ). However, the successful operational systems in this area are highly dependent on contextual knowledge and thus, the user must know what the computer will understand. Therefore, one still has the problem of designing response sets. In pursuing such designs, a most important gui~eline is to choose mnemonics or command phrases that have some inherent meaning for the eventual user. This may be facilitated by the designer talking with the operators who will utilize the system and letting them pick the phrases. Another useful idea is that communication, especially with menu picking, may be enhanced by using symbols instead of words. Thus for example, symbols that represent the processes invoked by the input command may improve performance by exploiting the operator's ability to visualize what he wants faster than he can verbalize it. Regardless of how well the dialogue has been designed, users are bound to make mistakes. A mistake frequently made by beginning users is not knowing what to respond. This difficulty can be lessened by providing a "help" command that results in the set of acceptable responses being explained. Another frequent problem is that of becoming entrapped in a sequence of commands that are executing but which one wishes to revoke. A solution to this problem is to provide an "escape" co mand that allows the user to stop execution of those co mands, but without stopping the whole s ste

T

The first issue of i portance concerns how the operator will ake his inputs. In the pre ious section, . e discussed

Considering input errors, tht system should be designed to accept anything but to check it for credibility before the co and is executed. One idea is to read e er co and in alphanu eric and then decode the input into floating point or integer if necessar Of course the need to do this t pe of decoding depends on the program ing language used. If the input co and is not acceptable, then the operator should be informed in

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DESIC\ OF :lA\-:l.-\CHI\E

I\TERFACES

a r.:anner appropriate to his level of under.standing of the systen. l)n the other hand, if the con~and is acceptable but upon execution ~ay have disastrous cons e que n c e s (i. e ., s hut tin g d 0 \,' n 0 f the f act 0 r y), the nth e s y s t e ~ S:l 0 u 1 d ask the operator if he really ~ants to execute such a comnand. As a final comment, if the operator's commands are such that they take a considerable time to execute, then he should b e pro v i d e d \,' i t h S 0 me i n t e r me d i ate fee d b a c k a b 0 ut,,,, hat ish a p pen i n g. 0 the r \,' i se, hem a y fee 1 t hat s 0 met h i n g has g 0 n e \,' r 0 n g and therefore initiate diagnostic actions or perhaps just re-enter his original commands. HO~

CA\ THE

DESIG~ER

OBTAI\ DATA ABOlT

A L T E R ~ AT I \' E S ?

Csually the design process is constrained by tight deadlines, ~hich severely restrict the possibility of thoroughly evaluating alternatives, e.g. for the operator/computer interface. Still, several possibilities exist for obtaining valuable information: a)

Hock-ups

A powerful, yet simple aid is a mock-up of alternative interfaces, preferably on actual scale. Such mock-ups can be made of inexpensive and easily used mate r i a 1 s ( \,' 0 0 d, f 1 ann el, e t c .) and b e pro v i d e d \,' i t 1 d r a h' i n g s 0 r p hot 0 g rap h s 0 f displays and controls.

1\

PROCESS

CO\TROL

715

~LS imul a~~

A sinulator can be used to tryout design alternatives.Complete simulation is usually too expensive and time consuming, but partial simulation, directed to certain alternatives, can be of great help to reach a decision. Although these approaches cannot guarantee an optimal result, the designer can use all available information in order to try to avoid bad designs. For instance it is impractical to sea;.·ch for the best mnemonic code for designating process v a r i a b 1 e s, but i t i S \,T 0 r t h \,' h i 1 e t 0 ski p bad ones, such as straight numbers. THE

I\FLlE\CE OF THE ORGA\IZATIO\

The r e h a v e bee n cas e s \,' her e car e f u 1 1 y designed man/machine interfaces have not performed as expected. The cause ~as an unsatisfactory allocation of tasks among operators, responsible for different sections of the process. For instan~e (Bibby ' 79 :) in a sequence of processing steps each operator tends to balance his section by transferring incoming fluctuations in a do~nstream direction. This can cause an impossible situation for the last operator in the sequence. The solution is to allocate the responsibility for each section to a pair of operators (one in the section andt"he other in the upstream section) and to provide appropriate additional displays and co~rnunication channels.

A mock-up is also very suitable for collecting the ideas and suggestions from all people involved in the design, and fro~ the future users of the design results (Cornelissen-Engels '7S'). It may perhaps come as a surprise that fe~ people are able to form a threedimensional mental picture fron engineeri n g d r a \,' i n g s .

It is advisable to analyse the hu~an o r g ani z a t ion, e s p e cia 1 1 y \,' it: 1 res p e c t to the allocation of responsibilities and t ;1 e c 0 r:1 ::: u n i cat ion s t r u c t u re, b e for e starting ~ith specifications for nan :::achine dialogues and interfaces (~oo~­

:1 0 c k - ups

Provided o~vious design pitfalls have a \' C 1 d e cl, t:1 ere suI tin gin t e r f ace ~ill usual_y result in satis:actory perfor:::a:1ce u:1der average conditions. H u::: a :1 j e :1 a ': ~ 0 u r i s ~ y n a t u re:: i g :: 1 y a d a ;::' t i \. e and t:, us:":'. i nor s 11 0 r t c () :":'. i n g s ',,- ill j e c 0 :-:~ p e :1 S a ~ C: d ~ 0 r a s 1 0 :1 gas t:1 e :, u ~ a n cap a ~ i ~ i tie s are :1 0 t f u 1 1 y e x plaited.

can fur the r b e use d for c :1 0 0 sin g o \' e r - all dim ens ion s i n \' i e h' 0 fan t i1 r 0 p 0 metric data (see Ergono~ic Handjooks) and for studying environmental factors suc~ as illu~ination and glare. b)

Tachvstoscopic Slide

Pr£i~~rs

Jisplay characteristics, such as structures and formats for pictures to be s~o~n on visual displays can be co~­ pared by sho~ing them to human subjects on a tachystoscopic slide projector. T~is device allo~s adjusting very short exposure tines, ~hich can be used to study narginal observability. It is a successor to an older device, the tachystoscope, ~hich is perhaps more fa~iliar.

::.aa~er

SO!).

JESIG\ FOR SIIlAIIO\S OF HIGH STRESS

h e e:1

Eo -\,' e \- er, so::: e t i :":'. e s t :: ere are pea ~ per i 0 cl S \,- her e :-:-: any t h i n g s g 0 \,' r 0 :1 g s i ~ u 1 tan e 0 u s ly. In centralized process supervisio:1 e s p e cia 1 1 y h' :1 ere 0 n e 0 per a tor i s responsible for ~any process variables lfor instance 100 controlled variables and 500 non-controlled variables) very stressful situations can occur.

DESIC: OF .1A . '- lACHI . E I . 'TERF CES

716

There are strong indications that human beings, when under stre s, fall back upon more primitive response patterns For instance, incompatible displa sand controls (where direction of movement is against the natural feeling of the majority of people of a given culture), can be managed when the operator is at ease, but are of cause of many errors when the operator is under stress. Another problem is the human tendency, once started, to continue on a chosen track of thought and the great difficulty of starting afresh. Also, once human beings focus their attention on a given problem, they tend to become insensitive to other phenomena. These human characteristics point towards the desirability of parallel presentation of information within an easily surveyable angle of vision, instead of sequential presentation common on many man/computer interfaces. Human errors can lead to unsafe situations, and, in some cases to actual catastrophies. Fortunately, these events are rare and shortcomings of operator/ computer interfaces go unnoticed. Or, the operator receives all the blame. The probabilit of human errors can also be diminished by avoiding unnecessary mental gymnastics. For instance, direct pointing(e.g. on a process scheme displayed on a visual display, or on an array of dedicated push-buttons) is preferable to t ping in mnemonic codes for i portant process variables.

I.' PROCESS CO . TROL

5-7

information from, for example, the operator/computer interface (Daniel 183 , Stri~enec 1821). Communication and co-operation can be enhanced by designing the interface for use by two or more persons (this i also very desirable in view of on-the-job training, maintenance, and coping with large process upsets). Display of process performance can improve meaningfulness of work as this allows the operator to evaluate process operation and can encourage him to contribute towards improving the system. Personal development has already been touched upon in the section about the allocation of tasks between man and machine. Job satisfaction is negatively influenced by mental stress during emergencies. This is particularly bad in very highly automated systems (van Droffelaar 1371) where the operator has almost nothing to do except wait for emergencies. Human beings need activity and they can cope with danger better when their skills have been exercised. CO CLUSIO.S The introduction of computerized CRTlike displays can promote the primary goal of sociotechnical design: Adaptation of the man/machine interface to human skills, needs, limitations and aspirations. The allocations of functions between "man" and "machines" should not be seen as a static separation, but rather as a dynamic co-operation.

JOB DESIC" A.. D JOB SATISFACTIO. Thus far we have looked at the operator's job from the point of view of perforance: ~hat can his skill do for the s stem, and how can the s stem assist hi in his tasks? . 'O\J \Je shall consider the personal side: ,hat does he experience in his job? Job satisfaction has been in estigated bv factor analysis of the an wers gi en to questionnaire 0 e such in estigation, pertaini g to 1 10 ~orkers in the steel industr (lartensson 181 resulted in the following four .ai factors: a) Independence and responsibility b) Co unication and co-operation c) . 1eaningfulness of ork d) Perso al de elop ent. 7

The operator can only be independent and responsible if he receives adequate

There is a great need for human factors research on combinations and/or" integrations of various operator skills. It is ad isable to separate the overall an/machine interface into a number of interfaces, dedicated to on-line of off-line functions, and matched to the interests of different groups of personnel. The on-line operator interface should pro ide for easil sur e able displa's of the process state. Contrast ratio and flicker still are weak points in an CRT-displa s. The typical 5:7 character atrix is onl argi all satisfactor Dedicated function keys, s a I l ke boards, and touch panels are preferable for online operator interfaces.

5-7

DESIG: OF .IA. -.IACHI

E L. TERFACES

In the design of interfaces it makes sense to compare alternatives by dedicated experiments.

10

I.

PROCESS

-

~

717

CO. TROL

_ _....JL_

..:.<___

yZ-

___=_ _= _

It is advisable to analyse the human organization before specifying the interface. 11 The on-line operator interface should be designed for situations of high mental stress. nder these circumstances the operator should be able to exercise his skills and to co-operate with colleagues.

Bosman,

D.,

"-OY' .·Jan-'1a

ine ::;y

Bosman, D., der, H.F.,

van Heusden, A.R., Xulyttenboogaart, J.D.,

t;:" . etherlands, 12

":;e~ool-

.. ea. 7A.Y'e m env and

vOY'.:~

ol

"(in Dutch), Xens en Onder;;ming 30 (1976) no.2 (Xarch/April) 109-129-:..00

ACK Ot.JLE DGE.1E

T

D. Bosman and P.A.A. van Boxtel are to be thanked for suggestions and critical remarks.

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he 13

-

I." PROCESS

CO TROL

S-7