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Comparison of a touch-sensitive VDU and computer-aided keypad for plant control
Comparisonof a touch-sensitive VDU and computer-aided
keypad for plant control D.M. U S H E R
The technology of touch screens is discussed with emphasis on the development of appropriate software and human factors.A touch screen was compared with an intelligentkeypad in an experiment in which C E G B personnel controlled Simulated plant. Users preferred the touch screen in normal conditions and particularly for emergency working. Also, a statisticallysignificantimprovement in plant controllabilitywas recorded when the touch screen was used in place of the keypad. Touch screens are seen to possess many advantages over conventional man-machine interfaces.Their use is shown to be appropriate in applications suchas alarm analysis,data and mimic selectionand interactive graphics as well as for controllingplant. Keywords : display devices (computers); ergonomics; electric power stations. The increasing use of microprocessors and minicomputers in the electricity supply industry and their falling cost make it feasible to consider new man-machine interface devices for the control room. The complexity of modern generating plant makes demands on the man-machine interface that the traditional dedicated knobs and switches cannot always meet, and the concept of an on-screen input device shows clear promise. The rapidly develop.ng technology and expertise in computer graphics combined with the touch screen allow a VDU to become a bi-directional instrument for both viewing and controlling plant status. On grounds of reliability and simplicity of construction, infra-red beam obscuration was chosen as the basis of operation of a prototype touch screen which we built in the laboratory several years ago. Since then, similar devices have become available commercially. Clearly, comparing a single highly developed interface device with existing low technology equipment would be invidious and meaningless as a guide to the choice of the best way forward, since any new device would surely be an improvement of some kind. Therefore, a second manmachine interface concept Was developed in parallel: a multiplexed keypad, possessing many advanced intelligence features comparable with those offered by the touch screen, but based upon longer established technology. An experiment was designed to compare the two devices in a realistic environment. Possible refinements and modifications to both hardware and software are indicated throughout the following description of the touch screen and its use. Development of the software is continuing.
PRINCIPLES
OF TOUCH
SCREEN
OPERATION
The many ways of rendering a VDU touch-sensitive include interrogating cross-wires for shorted loops, measuring the The author is at the Central Electricity Generating Board, South West RegionalHeadquarters, Bedminster Down, Bristol, Avon, UK.
DISPLAYS. JULY 1983
resistance between conductive transparent laminas, sensing changes in the capacitance of areas of thin film, and timing the reflection of acoustic (quasi-Rayleigh) waves in a glass overlay. However, we have not encountered a method which performs better than infra-red beam obscuration. The following is a description of a product marketed by Carroll Inc, which, having certain operational advantages over our prototype, was preferred for the experiment. The VDU screen is enclosed on two adjacent sides by infrared light emitting diodes (LEDs) spaced 7mm apart, opposite which are infra-red photodiode receivers. The horizontal (X) LEDs are pulsed in sequence, and the corresponding receivers are gated synchronously. If no light is received by any of the diodes, their numbers are stored and scanning of the vertically mounted (Y) LEDs begins. Pulsing the LEDs in this way prevents interaction between neighbouring receivers and allows them to be positioned with considerable latitude, as the incoming beam intensity is above 80% of its peak over a transverse distance of some 15 mm. Compensation for changes in room lighting level is achieved in a 'dynamic level shifter' which uses the LEDs continuously to measure the background illumination and sets the diode switching threshold. Black Perspex, transparent to infra-red light, may be used to improve the appearance of the device, at the cost of some loss of beam intensity. The spatial resolution of a touch screen is adequate if it is small compared with the width of the user's finger. This criterion is met in Carroll's device by interpolating 'soft' beams, as described below. Parallax A persistentproblem with on-screen devices is that parallax causes the user to touch not his desired point but one slightly nearer the centre of the screen. The use of a planar matrix of beams to determine the position of a finger on a spherically curved screen was found to exacerbate the problem to such an extent that our prototype had to be aban-
0141-9382/83/030157-05 $03.00 © 1983 Butterworth & Co (Publishers) Ltd
157
Scan horizontal
I
Store broken beam
I
Celculate
I I
numbersXn forO= 1 to/
Ic-II XO= 2
ZXn
YO =
j2_. ~Ym
No
Yes
|
I
beams
brokenbeam numbers Ym
i
DEC's VTV30-H character graphics generator was used to generate the mimic for reasons of economy, convenience, performance and precedent. In its 8 x 8 pixel mode, 36 rows of 64 characters are made available in a choice of eight foreground and background colours, with a 1.6 Hz blinking attribute. The alphanumerics are 0.0146 times the height of the VDU frame diagonal l and subtend the angle of maximum legibility (about 24 minutes of arc) at a viewing distance of about 2l.
I
Send 17/XO/YO/18
I
Read
X0, Y0
Wakescreen
I
updating Job
I
Scale and shift into VT30 coords
(X,Y)
1
00=10000
I
protocol flags
protocol flags
Calculate distance D from (K Y) to active point i, where
Clear J#
I
Set J#
Emit 'cancel'
i Emi, i
D = (X-X(/))2+ (y-y(i))2
No
'identify' 'action' or 'confirm'
tone
tone
hit el J # ?
,
" ~
Register at hit at J#, where
L J# = J
ill
J
Fig. 1 Flowchart for registering a 'hit' at an active point J # out of N. The Part above the line is performed by the touch screen controller; the remainder by a Swepspeed job
doned. However, the difficulty is reduced to an acceptable level in the current device by: curving the LED arrays to the same radius as the screen, using a screen of smaller diagonal; and using in situ calibration software to map the beam coordinates onto the character positions in a way which automatically compensates for the angle of approach of the finger. The mapping process is described in more detail below. Displays As the VDU moves nearer the centre of an operator's purview, it becomes increasingly important to present him with an image of as high a quality as possible. Our concern is the detailed performance of the graphics generator, and whether it is compatible with anthropometric criteria: can the
158
Clearly, the viewing distance is limited to below about 500 mm to prevent operator arm strain. This suggests an l of about 250 mm, but such a small screen is prohibitively expensive and compounds the touch registration problems. As a compromise, a Barco CDCT 3/37 (with l = 300 ram) was used and the alphanumerics were 4.5 mm in height, exceeding the optimum of 3.5 ram. It was nevertheless unexceptionable. In its 6 × 6 pixel mode, the VTV30-H produces characters which subtend 24 minutes of arc at a distance of 1.5l. These are best displayed for touch screen use by a 330 mm frame diagonal (provided, for example, by Digivision's MCHW14) but are not as well defined as the 8 x 8 characters.
Logic Controller The logic used by the microprocessor (an 8748) in the touch screen controller is shown in the top part of Fig. 1. The factor of two in the summations over the broken beams effectively doubles the beam density (and hence resolution) by interpolation.
Was previous
[:5oesa hit at J follow the protocol?
operator read the data and distinguish the special symbols when he is near enough to touch the screen? If so, is he comfortable? Off-screen devices are advantageous in this context, because the user may position himself to optimize independently both viewing the screen and using the manmachine interface, whereas a compromise must be made when using the touch screen.
Inhibiting the output when more than (perhaps) three beams are broken simultaneously (I,J < 4) or when beams are broken for fewer than a certain number of scans, would render the device more secure against accidental operation by insects flying into the beams, for example.
Mapping The coordinates (,I"0, Y0) of the centre of the finger are read by the host software and the nearest character position (X, Y) is found through the mappings: X = ROUND(FLOAT(XO)*%X + %C) and Y = ROUND (FLOAT(YO)*%Y + %D) where the floating point scalars %X, %C, %Y and %D are the 'scaling' and 'shifting' parameters in the X and Y directions. Their values may satisfactorily be determined ab initio using an interactive program, but an improvement would be to update them continuously as 'rolling averages' to account dynamically for changes in operating position. Implementation of this idea is pending. It is important to realize that the mapping cannot be l : 1 and that there will be character grid coordinates (typically three X and two Y')
DISPLAYS. JULY 1983
which cannot be addressed by touch. This restriction does not apply in the case of dot-addressable graphics, of course.
/ vou
Operating protocol A protocol involving three basic stages has been found appropriate for plant control: 'identification', 'action', and 'confirmation'. As seen from the flowchart in the lower part of Fig. 1, a 'touch' becomes a 'hit' if the touched position has a single nearest 'active point' within a distance whose square is nine square characters, provided it follows the protocol. The positions of the active points are of course determined by the mimic (Fig. 3, for example) and may be representations of pumps, valves and mills, as well as limit markers, display selectors, data, menus and so on. The control protocol sequence starts when a 'hit' is registered on an item which is not blinking. This is the 'identification' stage, at which the item begins to blink, and appropriate 'action areas' are created at the bottom of the screen for controlling it. The 'action' progresses by touching these areas in ways designed to be self-evident to the user. Two-state inputs (such as on/off and open/close) require two 'action areas', colour-coded to assist the user, which begin to blink when touched. Numerical data are input by touching an analogue scale to bring up a fine scale with a cursor. When the operator is satisfied he may 'confirm' the 'action' by touching the 'identified' item again. At each stage prior to 'confirmation', a hit may be cancelled, and the previous protocol stage regained, by retouching the last point. The stages are accompanied by audible tones of ascending pitch, produced by a laboratory modification to the commercial controller. The level of security may be increased to include other confirmation stages, or reduced so that actions take place without the possibility of cancellation. Barring certain sequences of actions, limiting control areas to those personnel with knowledge of code numbers, ganging valves together, and printing prompts on the screen are all aids to improved operation which may be easily incorporated. Feedback The audible feedback, atlhough excellent for the user, has been found to irritate third parties. Feedback of some kind is a very important part of any man-machine interface, being the equivalent of the 'click' obtained from a switch or keypad. The outside rows and columns of characters may be flashed in various ways if audible tones prove unacceptable. The smaller the delay between the touch and some indication of its receipt, the more the user feels in control of the machine, and the better the interface.
KEYPAD CONCEPTS But for the problem of character perception at close range, only the advantages of touch screens have so far been emphasized. However, there are many questions to which the answers were unknown at the start of the project: Can operators readily assimilate such a new operating method? Is the protocol really self-evident? Can those who wear glasses see the screen easily? Is it tiring on the eyes, neck or arms? Given the computer back-up, might not a simple keypad be improved greatly?
DISPLAYS. JULY 1983
1220mm
.-,1
,o,ooo
AG'hLI / Fig. 2 Subjects' operating.position during the experiment
In order to answer these questions, an experimental comparison was devised. The touch-sensitive VDU was mounted in a mock-up control desk, and a small keypad was fitted to the desktop in a convenient position for the hand, leaving room for resting the arms and for consulting A3 documents. The console was designed to optimize the ergonomics of both man-machine interfaces within the constraints discussed below. A section through the desk is shown in Fig. 2. Each plant item had a code number displayed against it on the mimic, and its control was initiated by keying in that number in a process broadly equivalent to the 'identification' stage in the touch screen protocol. Thus, the keypad was divided into two parts: a number pad and an 'action' pad, with keys labelled: stop, start, open, close, cancel and enter. Dedicated keys for pumps were also included. The protocol was written such that the items began to blink when 'identified' and the number keyed in appeared at the centre of the bottom of the screen. Only 'action' keyings were subsequently accepted by the host software, being accompanied by the audible tones as before. 'Confirmation' was by pressing the enter key, before which a single key (cancel) was available to negate the key-strokes in reverse order. Numerical data were of course entered through the number pad. This protocol has proved no more difficult conceptually than that of the touch screen. Physically, the keypad was a commercial unit, with 9 x 9 mm keys, separated by 3 mm, in two groups of 16. It is smaller than those in general use in Central Control Rooms and does not provide back-illumination, but is representative of future trends. Clearly, the extra keys that would be necessary to a real application would be deleterious to the manmachine interface, causing the operator to spend longer searching for the required key. The main advantages of the keypad perceived at the outset were that numerical data could be entered more quickly and precisely, that it would be more familiar and hence more acceptable to control room personnel and that, like all off-screen devices, it would not obscure the display. In this experiment, the fact that a keypad may be mounted remote from the screen (in any position convenient to the
159
were maintained to the half-full mark (indicated on the mimic by a dotted line) as follows: D=
£
(d(i)-50)'
+
£
i=0
(e(i)-50) 2
i=0
where d(O and c(O are the percentage water levels in the de-aerator and condenser respectively at 3 i seconds from the start of a test of length 3 N seconds. The parameter D may reach over 80 if the system has been badly controlled and large excursions have occurred, or may be reduced to below 10 by close control of the system.
RESULTS
Quantitative Fig. 3 The mimic used for the touch screen. The fight-hand bar (coloured red) in each vessel indicates the current water level and the other indicates the level at the previous times shown. Flow paths axe indicated by white pumps and valve symbols; red ones stop the flow. The two-state 'action areas' for isolator valves are shown. Since the 'identified valve' (GF19) is open, the 'close' area is yellow
operator in the case o f portable keypads) is neglected. But to use a keypad, in whatever position, the operator must transfer his attention to it from the screen. It is this process which has been found to be particularly detrimental to his performance.
EXPERIMENTAL
DETAILS
It was considered important to compare the man-machine interfaces in as realistic a situation as possible, rather than to use the more usual simple item-hunting type of test, because of the importance o f gathering the opinions and reactions o f potential users. Therefore, a typical power station feed-water system was simulated numerically in a rudimentary way, and CEGB operators were used as subjects. The experiment took place at the CEGB's Nuclear Power Training Centre at Oldbury. As seen in Fig. 3, which is a monochrome representation of the mimic used in the experiment, the system consisted of a condenser hot-well from which water could be extracted with either o f two pumps, passed through several valves and pipework branches and into a de-aerator vessel. The rate of flow out o f the de-aerator, which was equal to the flow into the condenser, was proportional to the load on the generating set, which was displayed to the operator but beyond his control. Forty-seven subjects were given ten-minute tasks of different form but similar complexity, using each interface alternately. The device on which the subjects started their sequences was alternated. They were encouraged to use the plant items following their usual operational procedure. Twenty-five o f the subjects performed four pairs o f tasks and the remainder only one pair, due to shortage of time. The tasks were sufficiently distinct to prevent subjects anticipating changes and to confine them to 'load following'. The tasks involved maintaining the water levels in the condenser and the de-aerator against a rapidly changing load, and coping with two fault conditions : an extraction pump failure and a full steam leak. A measure of performance D was defined according to how closely the levels of water
160
The performances achieved with the keypad were not radically different from the touch screen ones. A statistical difference can best be shown, however, by subtracting the value of D obtained during the keypad run in each pair froln the value obtained during the touch screen run. The distribution of these difference data ~99 = D(touch screen) D ( k e y p a d ) is shown in Fig. 4 to be centred at about AD = - 4. Since the mean of the deviation distribution is about 40, we may conclude that a 10% improvement in controllability is effected by using a touch screen instead of a keypad. Non-parametric statistical tests show this to be a significant result at p < 0.05. Breaking down the series o f run-pairs shows that the controllability differences reduce slightly as the users" experience increases. The 'learning curve' of the touch screen is steeper than that o f the k e y p a d ; t o u c h screens make the plant more readily accessible. Of the first runpairs, 39 subjects showed a difference greater than 5%. Nineteen o f these started on the keypad and changed to the touch screen for their second run, and 20 started on the touch screen. As many as 17 (90%) of the first group showed improvement, compared with only ten (50%) of the second. This emphasizes that the touch screen is an
I I
18
I //
16
14 ta
o=
12
o
I0
~z
8
Z Z
6--
4
-
2 0 -60
-50
-40
-30
-20
-lO
0
I0
20
30
40
Deviation differences, A 0
Fig. 4 Distribution of run-pair deviation differences. Its mean is -4,0
DISPLAYS. JULY 1983
easier interface to operate, being more easily assimilable and providing less of a barrier between man and machine.
any feelings of fatigue despite the considerable degree of concentration demanded by the tasks.
Performance differences as small as these are arguably trivial in operational terms. Indeed, no great significance should be attached to a 10% improvement in an arbitrarily defined measure of performance. The potential for closer control has, however, been shown to be greater.
CONCLUSION
Qualitative More definite are the subjects' preferences as expressed through a questionnaire. Nearly five times as many preferred the touch screen to the keypad as vice versa, and for emergency operation this ratio was increased to over 10:1 in the case of the long-term subjects. The main difference between the views expressed by subjects with limited experience of the devices and those with longer term experience was that more of the latter group preferred the touch screen. Very few members of either group expressed
DISPLAYS. JULY 1983
Touch screens are shown to be useful and beneficial input devices for the control of plant, particularly when utilizing infra-red beam obscuration. An experimental comparison has found the touch screen superior to a comparably intelligent keypad. Exploration of the enormous range of uses to which touch screens may be put has hardly begun. Acknowledgements The author expresses his gratitude to the staff of the Nuclear Power Training Centre, Oldbury. This paper is published by permission of the Director-General, South West Region, CEGB. Bibliography Shackel, B. 'The applied ergonomics handbook' (Butterworths, 1974) Siegel, S. 'Non-parametric statistics' (McGraw-Hill, 1956)
161
keypad for plant control D.M. U S H E R
The technology of touch screens is discussed with emphasis on the development of appropriate software and human factors.A touch screen was compared with an intelligentkeypad in an experiment in which C E G B personnel controlled Simulated plant. Users preferred the touch screen in normal conditions and particularly for emergency working. Also, a statisticallysignificantimprovement in plant controllabilitywas recorded when the touch screen was used in place of the keypad. Touch screens are seen to possess many advantages over conventional man-machine interfaces.Their use is shown to be appropriate in applications suchas alarm analysis,data and mimic selectionand interactive graphics as well as for controllingplant. Keywords : display devices (computers); ergonomics; electric power stations. The increasing use of microprocessors and minicomputers in the electricity supply industry and their falling cost make it feasible to consider new man-machine interface devices for the control room. The complexity of modern generating plant makes demands on the man-machine interface that the traditional dedicated knobs and switches cannot always meet, and the concept of an on-screen input device shows clear promise. The rapidly develop.ng technology and expertise in computer graphics combined with the touch screen allow a VDU to become a bi-directional instrument for both viewing and controlling plant status. On grounds of reliability and simplicity of construction, infra-red beam obscuration was chosen as the basis of operation of a prototype touch screen which we built in the laboratory several years ago. Since then, similar devices have become available commercially. Clearly, comparing a single highly developed interface device with existing low technology equipment would be invidious and meaningless as a guide to the choice of the best way forward, since any new device would surely be an improvement of some kind. Therefore, a second manmachine interface concept Was developed in parallel: a multiplexed keypad, possessing many advanced intelligence features comparable with those offered by the touch screen, but based upon longer established technology. An experiment was designed to compare the two devices in a realistic environment. Possible refinements and modifications to both hardware and software are indicated throughout the following description of the touch screen and its use. Development of the software is continuing.
PRINCIPLES
OF TOUCH
SCREEN
OPERATION
The many ways of rendering a VDU touch-sensitive include interrogating cross-wires for shorted loops, measuring the The author is at the Central Electricity Generating Board, South West RegionalHeadquarters, Bedminster Down, Bristol, Avon, UK.
DISPLAYS. JULY 1983
resistance between conductive transparent laminas, sensing changes in the capacitance of areas of thin film, and timing the reflection of acoustic (quasi-Rayleigh) waves in a glass overlay. However, we have not encountered a method which performs better than infra-red beam obscuration. The following is a description of a product marketed by Carroll Inc, which, having certain operational advantages over our prototype, was preferred for the experiment. The VDU screen is enclosed on two adjacent sides by infrared light emitting diodes (LEDs) spaced 7mm apart, opposite which are infra-red photodiode receivers. The horizontal (X) LEDs are pulsed in sequence, and the corresponding receivers are gated synchronously. If no light is received by any of the diodes, their numbers are stored and scanning of the vertically mounted (Y) LEDs begins. Pulsing the LEDs in this way prevents interaction between neighbouring receivers and allows them to be positioned with considerable latitude, as the incoming beam intensity is above 80% of its peak over a transverse distance of some 15 mm. Compensation for changes in room lighting level is achieved in a 'dynamic level shifter' which uses the LEDs continuously to measure the background illumination and sets the diode switching threshold. Black Perspex, transparent to infra-red light, may be used to improve the appearance of the device, at the cost of some loss of beam intensity. The spatial resolution of a touch screen is adequate if it is small compared with the width of the user's finger. This criterion is met in Carroll's device by interpolating 'soft' beams, as described below. Parallax A persistentproblem with on-screen devices is that parallax causes the user to touch not his desired point but one slightly nearer the centre of the screen. The use of a planar matrix of beams to determine the position of a finger on a spherically curved screen was found to exacerbate the problem to such an extent that our prototype had to be aban-
0141-9382/83/030157-05 $03.00 © 1983 Butterworth & Co (Publishers) Ltd
157
Scan horizontal
I
Store broken beam
I
Celculate
I I
numbersXn forO= 1 to/
Ic-II XO= 2
ZXn
YO =
j2_. ~Ym
No
Yes
|
I
beams
brokenbeam numbers Ym
i
DEC's VTV30-H character graphics generator was used to generate the mimic for reasons of economy, convenience, performance and precedent. In its 8 x 8 pixel mode, 36 rows of 64 characters are made available in a choice of eight foreground and background colours, with a 1.6 Hz blinking attribute. The alphanumerics are 0.0146 times the height of the VDU frame diagonal l and subtend the angle of maximum legibility (about 24 minutes of arc) at a viewing distance of about 2l.
I
Send 17/XO/YO/18
I
Read
X0, Y0
Wakescreen
I
updating Job
I
Scale and shift into VT30 coords
(X,Y)
1
00=10000
I
protocol flags
protocol flags
Calculate distance D from (K Y) to active point i, where
Clear J#
I
Set J#
Emit 'cancel'
i Emi, i
D = (X-X(/))2+ (y-y(i))2
No
'identify' 'action' or 'confirm'
tone
tone
hit el J # ?
,
" ~
Register at hit at J#, where
L J# = J
ill
J
Fig. 1 Flowchart for registering a 'hit' at an active point J # out of N. The Part above the line is performed by the touch screen controller; the remainder by a Swepspeed job
doned. However, the difficulty is reduced to an acceptable level in the current device by: curving the LED arrays to the same radius as the screen, using a screen of smaller diagonal; and using in situ calibration software to map the beam coordinates onto the character positions in a way which automatically compensates for the angle of approach of the finger. The mapping process is described in more detail below. Displays As the VDU moves nearer the centre of an operator's purview, it becomes increasingly important to present him with an image of as high a quality as possible. Our concern is the detailed performance of the graphics generator, and whether it is compatible with anthropometric criteria: can the
158
Clearly, the viewing distance is limited to below about 500 mm to prevent operator arm strain. This suggests an l of about 250 mm, but such a small screen is prohibitively expensive and compounds the touch registration problems. As a compromise, a Barco CDCT 3/37 (with l = 300 ram) was used and the alphanumerics were 4.5 mm in height, exceeding the optimum of 3.5 ram. It was nevertheless unexceptionable. In its 6 × 6 pixel mode, the VTV30-H produces characters which subtend 24 minutes of arc at a distance of 1.5l. These are best displayed for touch screen use by a 330 mm frame diagonal (provided, for example, by Digivision's MCHW14) but are not as well defined as the 8 x 8 characters.
Logic Controller The logic used by the microprocessor (an 8748) in the touch screen controller is shown in the top part of Fig. 1. The factor of two in the summations over the broken beams effectively doubles the beam density (and hence resolution) by interpolation.
Was previous
[:5oesa hit at J follow the protocol?
operator read the data and distinguish the special symbols when he is near enough to touch the screen? If so, is he comfortable? Off-screen devices are advantageous in this context, because the user may position himself to optimize independently both viewing the screen and using the manmachine interface, whereas a compromise must be made when using the touch screen.
Inhibiting the output when more than (perhaps) three beams are broken simultaneously (I,J < 4) or when beams are broken for fewer than a certain number of scans, would render the device more secure against accidental operation by insects flying into the beams, for example.
Mapping The coordinates (,I"0, Y0) of the centre of the finger are read by the host software and the nearest character position (X, Y) is found through the mappings: X = ROUND(FLOAT(XO)*%X + %C) and Y = ROUND (FLOAT(YO)*%Y + %D) where the floating point scalars %X, %C, %Y and %D are the 'scaling' and 'shifting' parameters in the X and Y directions. Their values may satisfactorily be determined ab initio using an interactive program, but an improvement would be to update them continuously as 'rolling averages' to account dynamically for changes in operating position. Implementation of this idea is pending. It is important to realize that the mapping cannot be l : 1 and that there will be character grid coordinates (typically three X and two Y')
DISPLAYS. JULY 1983
which cannot be addressed by touch. This restriction does not apply in the case of dot-addressable graphics, of course.
/ vou
Operating protocol A protocol involving three basic stages has been found appropriate for plant control: 'identification', 'action', and 'confirmation'. As seen from the flowchart in the lower part of Fig. 1, a 'touch' becomes a 'hit' if the touched position has a single nearest 'active point' within a distance whose square is nine square characters, provided it follows the protocol. The positions of the active points are of course determined by the mimic (Fig. 3, for example) and may be representations of pumps, valves and mills, as well as limit markers, display selectors, data, menus and so on. The control protocol sequence starts when a 'hit' is registered on an item which is not blinking. This is the 'identification' stage, at which the item begins to blink, and appropriate 'action areas' are created at the bottom of the screen for controlling it. The 'action' progresses by touching these areas in ways designed to be self-evident to the user. Two-state inputs (such as on/off and open/close) require two 'action areas', colour-coded to assist the user, which begin to blink when touched. Numerical data are input by touching an analogue scale to bring up a fine scale with a cursor. When the operator is satisfied he may 'confirm' the 'action' by touching the 'identified' item again. At each stage prior to 'confirmation', a hit may be cancelled, and the previous protocol stage regained, by retouching the last point. The stages are accompanied by audible tones of ascending pitch, produced by a laboratory modification to the commercial controller. The level of security may be increased to include other confirmation stages, or reduced so that actions take place without the possibility of cancellation. Barring certain sequences of actions, limiting control areas to those personnel with knowledge of code numbers, ganging valves together, and printing prompts on the screen are all aids to improved operation which may be easily incorporated. Feedback The audible feedback, atlhough excellent for the user, has been found to irritate third parties. Feedback of some kind is a very important part of any man-machine interface, being the equivalent of the 'click' obtained from a switch or keypad. The outside rows and columns of characters may be flashed in various ways if audible tones prove unacceptable. The smaller the delay between the touch and some indication of its receipt, the more the user feels in control of the machine, and the better the interface.
KEYPAD CONCEPTS But for the problem of character perception at close range, only the advantages of touch screens have so far been emphasized. However, there are many questions to which the answers were unknown at the start of the project: Can operators readily assimilate such a new operating method? Is the protocol really self-evident? Can those who wear glasses see the screen easily? Is it tiring on the eyes, neck or arms? Given the computer back-up, might not a simple keypad be improved greatly?
DISPLAYS. JULY 1983
1220mm
.-,1
,o,ooo
AG'hLI / Fig. 2 Subjects' operating.position during the experiment
In order to answer these questions, an experimental comparison was devised. The touch-sensitive VDU was mounted in a mock-up control desk, and a small keypad was fitted to the desktop in a convenient position for the hand, leaving room for resting the arms and for consulting A3 documents. The console was designed to optimize the ergonomics of both man-machine interfaces within the constraints discussed below. A section through the desk is shown in Fig. 2. Each plant item had a code number displayed against it on the mimic, and its control was initiated by keying in that number in a process broadly equivalent to the 'identification' stage in the touch screen protocol. Thus, the keypad was divided into two parts: a number pad and an 'action' pad, with keys labelled: stop, start, open, close, cancel and enter. Dedicated keys for pumps were also included. The protocol was written such that the items began to blink when 'identified' and the number keyed in appeared at the centre of the bottom of the screen. Only 'action' keyings were subsequently accepted by the host software, being accompanied by the audible tones as before. 'Confirmation' was by pressing the enter key, before which a single key (cancel) was available to negate the key-strokes in reverse order. Numerical data were of course entered through the number pad. This protocol has proved no more difficult conceptually than that of the touch screen. Physically, the keypad was a commercial unit, with 9 x 9 mm keys, separated by 3 mm, in two groups of 16. It is smaller than those in general use in Central Control Rooms and does not provide back-illumination, but is representative of future trends. Clearly, the extra keys that would be necessary to a real application would be deleterious to the manmachine interface, causing the operator to spend longer searching for the required key. The main advantages of the keypad perceived at the outset were that numerical data could be entered more quickly and precisely, that it would be more familiar and hence more acceptable to control room personnel and that, like all off-screen devices, it would not obscure the display. In this experiment, the fact that a keypad may be mounted remote from the screen (in any position convenient to the
159
were maintained to the half-full mark (indicated on the mimic by a dotted line) as follows: D=
£
(d(i)-50)'
+
£
i=0
(e(i)-50) 2
i=0
where d(O and c(O are the percentage water levels in the de-aerator and condenser respectively at 3 i seconds from the start of a test of length 3 N seconds. The parameter D may reach over 80 if the system has been badly controlled and large excursions have occurred, or may be reduced to below 10 by close control of the system.
RESULTS
Quantitative Fig. 3 The mimic used for the touch screen. The fight-hand bar (coloured red) in each vessel indicates the current water level and the other indicates the level at the previous times shown. Flow paths axe indicated by white pumps and valve symbols; red ones stop the flow. The two-state 'action areas' for isolator valves are shown. Since the 'identified valve' (GF19) is open, the 'close' area is yellow
operator in the case o f portable keypads) is neglected. But to use a keypad, in whatever position, the operator must transfer his attention to it from the screen. It is this process which has been found to be particularly detrimental to his performance.
EXPERIMENTAL
DETAILS
It was considered important to compare the man-machine interfaces in as realistic a situation as possible, rather than to use the more usual simple item-hunting type of test, because of the importance o f gathering the opinions and reactions o f potential users. Therefore, a typical power station feed-water system was simulated numerically in a rudimentary way, and CEGB operators were used as subjects. The experiment took place at the CEGB's Nuclear Power Training Centre at Oldbury. As seen in Fig. 3, which is a monochrome representation of the mimic used in the experiment, the system consisted of a condenser hot-well from which water could be extracted with either o f two pumps, passed through several valves and pipework branches and into a de-aerator vessel. The rate of flow out o f the de-aerator, which was equal to the flow into the condenser, was proportional to the load on the generating set, which was displayed to the operator but beyond his control. Forty-seven subjects were given ten-minute tasks of different form but similar complexity, using each interface alternately. The device on which the subjects started their sequences was alternated. They were encouraged to use the plant items following their usual operational procedure. Twenty-five o f the subjects performed four pairs o f tasks and the remainder only one pair, due to shortage of time. The tasks were sufficiently distinct to prevent subjects anticipating changes and to confine them to 'load following'. The tasks involved maintaining the water levels in the condenser and the de-aerator against a rapidly changing load, and coping with two fault conditions : an extraction pump failure and a full steam leak. A measure of performance D was defined according to how closely the levels of water
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The performances achieved with the keypad were not radically different from the touch screen ones. A statistical difference can best be shown, however, by subtracting the value of D obtained during the keypad run in each pair froln the value obtained during the touch screen run. The distribution of these difference data ~99 = D(touch screen) D ( k e y p a d ) is shown in Fig. 4 to be centred at about AD = - 4. Since the mean of the deviation distribution is about 40, we may conclude that a 10% improvement in controllability is effected by using a touch screen instead of a keypad. Non-parametric statistical tests show this to be a significant result at p < 0.05. Breaking down the series o f run-pairs shows that the controllability differences reduce slightly as the users" experience increases. The 'learning curve' of the touch screen is steeper than that o f the k e y p a d ; t o u c h screens make the plant more readily accessible. Of the first runpairs, 39 subjects showed a difference greater than 5%. Nineteen o f these started on the keypad and changed to the touch screen for their second run, and 20 started on the touch screen. As many as 17 (90%) of the first group showed improvement, compared with only ten (50%) of the second. This emphasizes that the touch screen is an
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16
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12
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8
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-
2 0 -60
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Fig. 4 Distribution of run-pair deviation differences. Its mean is -4,0
DISPLAYS. JULY 1983
easier interface to operate, being more easily assimilable and providing less of a barrier between man and machine.
any feelings of fatigue despite the considerable degree of concentration demanded by the tasks.
Performance differences as small as these are arguably trivial in operational terms. Indeed, no great significance should be attached to a 10% improvement in an arbitrarily defined measure of performance. The potential for closer control has, however, been shown to be greater.
CONCLUSION
Qualitative More definite are the subjects' preferences as expressed through a questionnaire. Nearly five times as many preferred the touch screen to the keypad as vice versa, and for emergency operation this ratio was increased to over 10:1 in the case of the long-term subjects. The main difference between the views expressed by subjects with limited experience of the devices and those with longer term experience was that more of the latter group preferred the touch screen. Very few members of either group expressed
DISPLAYS. JULY 1983
Touch screens are shown to be useful and beneficial input devices for the control of plant, particularly when utilizing infra-red beam obscuration. An experimental comparison has found the touch screen superior to a comparably intelligent keypad. Exploration of the enormous range of uses to which touch screens may be put has hardly begun. Acknowledgements The author expresses his gratitude to the staff of the Nuclear Power Training Centre, Oldbury. This paper is published by permission of the Director-General, South West Region, CEGB. Bibliography Shackel, B. 'The applied ergonomics handbook' (Butterworths, 1974) Siegel, S. 'Non-parametric statistics' (McGraw-Hill, 1956)
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