How to design a good HMI Display
This article provides engineers with an introduction to designing screen graphics.To learn more, read on…
All modern control room systems use visual displays to convey information to plant operators; often it is the
primary source of operational data. The clarity of these displays can determine
how well the plant is run; lack of information can result in poor operational
efficiency, excessive wear and tear and, in extreme cases, failure to see a problem may be dangerous.
Display clarity becomes doubly important when multiple VDU’s compete for an operator’s attention especially when
different displays have different purposes and are designed by different vendors
to differing standards. Often by engineers who have no training in design and little experience of working in control rooms.
Remember also that whilst the designer will sit close up in front of the screen, in use control-room VDUs will be
viewed at distance along a desk or even across the room. Whilst detail will be
lost, the presence of a plant problem should be evident at a distance – even if the operator has left his spectacles at home!
Our objective here is to provide some guidelines for screen design so that common pitfalls can be avoided and the
customer gets user-friendly displays, which are legible, useful and fit for purpose.
Display pages need to be clear, easy to use and legible across a control desk. Here's a quick summary:
- Backgrounds - use muted tones – light grey is good.
- Shadowing is a recommended method for subdividing a display.
- Background graphics should use muted tones, alarm colours should be avoided.
- Use black outlines to highlight objects.
- Text – use a standard font, which will be available on all PC’s.
- Use San serif fonts for on-screen clarity – e.g. Arial.
- Text size needs to be large enough to read at a distance e.g. Arial 16.
- More detailed text should be added as pop-up windows or ‘tool-tips’.
- System Alarm status should be visible on all displays.
- Alarm status should be displayed across the top of the display.
- Alarm colours will follow the convention: Red = alarm, Yellow = warning, green = status OK.
- For Alarms - use additional non-colour dependent indications; position, text, etc.
- Flashing of unaccepted alarms and automatic screen switching on alarm should be avoided.
- Sounds are a good method for highlighting and prioritising alarms.
- Dynamic data should be positioned on scanned areas of the screen; i.e. across the top, centrally and lower right.
- Data should be grouped logically.
- Data values should be clear & units should be obvious (but not repeated with every value).
- Data Resolution should be appropriate to use, avoid too many decimal values; 546.45 C is pointless and unclear.
- If red & green are used for ‘Running’ & ‘Stopped’ indications, add text to make it clear.
- Plant status indication must not rely solely upon colour changes.
- Colour combinations should be chosen with care and use appropriate colours and contrasts.
- Navigation buttons should be obvious and large enough to select quickly.
- ‘Next Screen’ button should be ideally placed at the lower right side of the screen.
- A ‘Home / Overview’ button should appear on each screen (ideally lower left corner).
- Other navigation buttons are best placed at the bottom of screen.
- Control buttons must invoke a ‘confirm action’ dialog box.
- All pages must have a consistent look & feel. Navigation buttons should be in the same place.
- Everything should line up.
Screen Layout
Before designing any screen it is useful to understand how a user
will use it. Generally, users will scan a screen in the same way as they
would scan a page in
a magazine, which in the west means from the top left corner to the
right and reading down the screen. Unlike a book, we have no lines to
guide us, so we
usually only do 2 or 3 incomplete scans of the screen as suggested in
example below.
In taking this into account, the display designer should ensure that
important items should be on the ‘scan’ line. Alarms should therefore be
across the top of the
page, key data in centre right and maybe buttons and controls on the
lower right. Whilst supporting graphics and the company logo are better
placed on the lower left of the screen.
Colour Issues
Before deciding what to put on a display it is necessary to have some understanding of how colour can be used. Colour is
a powerful tool for enhancing visibility of key data. However excessive use of colour can make a page confusing or overwhelming.
On-screen colours are created from 3 primary colours; Red, Green
& Blue and the secondary colours; Cyan, Magenta and Yellow are
created from pairs of primary colours.
Colour Wheel
A colour wheel is often used to demonstrate the relationship between colours. Example:
Complementary contrast is created by positioning a secondary colour
with the primary colour from the opposite side of the wheel. Example:
Whilst useful to create impact, using the colours together can make it difficult for the eye to focus.
Contrast of hue created from pairs of adjacent colours can be more
subtle, with pairs of secondary colours (cyan, yellow, magenta)
producing higher contrasts than pairs of primary colours (red, green,
blue).
Example:
Selecting Suitable Display Colours
The eye senses primary colours via the cones, which are sensitive to
red, green or blue light, as there are fewer blue cones then our
perception of blue is less.
This is why thin blue lines are more difficult to see. Blue should be
avoided for small display objects however blue does make a good
background colour.
In selecting colours for a screen it is essential to agree colours for plant status and alarms.
The preferred colour convention, following the standard for safety signs (BS 5378) defines:
- Red = stop, prohibition, danger
- Yellow = caution, risk of danger
- Green = safe condition
- Blue = mandatory action.
Any screen design should ensure that these key safety colours are
clearly visible on the VDU and that these colours
are not overused for other purposes. Large blocks of primary colours,
e.g. red, should be avoided because they will cause complimentary colour
image retention
on the retina. Try it for yourself; if you look at an area of bright red
then look at a white wall you will see the same image in green.
Designers also need to be aware that 1 in 12 men have some degree of
colour blindness. This normally means that there
is some confusion in the perception of reds and greens (also yellows,
oranges, & browns) which mean that screen designs should not solely
rely on colour to indicate plant condition.
Background Colours
Whilst black and white provide good colour contrast for text they
produce too much glare and do not provide a good
background for a range of colours. Muted tones are best for backgrounds,
greys, browns and blues; these provide a good contrast for the brighter
colours used
for important dynamic data, such as red, yellow, green, magenta or
white.
Different background colours can sometimes be used to good effect to immediately identify groups of screens. For
example, light grey may be used for the main plant, very light brown for the oil tank farm, light blue for the water plant.
The use of subtle shading to create the illusion on raising or
lowering a section of the display can be used with great effect to
differentiate areas of the display.
Static Pictures
Screen displays often include a representation of the process plant.
When well done, these can often help the
operator to immediately visualise the plant and the location of the
measurements. However too much detail can clutter the display and make
it difficult to see the important dynamic data, so keep it simple.
Muted tones are best used for plant representations. Large areas of
strong colours will detract from important
dynamic data. A good method to make items stand out is to outline the
object in black because the eye can easily see a black shape.
Photographs of the plant are sometimes used as backgrounds; often these photographs are poorly lit and invariably
contain too much detail and produce very cluttered backgrounds with little opportunity to optimise colour contrasts.
Photographs should only be used in exceptional circumstances and should be of good quality.
Sometimes background pictures can be enhanced by the use of graphics
from a wide range of graphical object libraries. To add a series of 3D
cylindrical tanks in shades of grey, can make
a display instantly identifiable, but adding 3D images of pipes, pumps,
valves, etc. adds little and will make the display too cluttered.
The temptation to create realism in a background should be avoided,
unless you are especially gifted. It is virtually
impossible to create good perspective, scale and shading. It will take a
long time to produce and the results will be disappointing. Realism is
best avoided.
Text & Data Values
Perhaps the most common complaint about displays is that the text is
difficult to read, which is shame because text is the most versatile way
of conveying information to an operator.
Selecting a Font
There are hundreds of fonts available but a few simple ground rules
will avoid problems. Select a common font, one that will exist on all
computers, such as Arial, Helvetica, System, etc. If you
are designing for a Windows operating system and you use an unusual font
you may find that when all your careful work is moved onto a different
PC that all fonts
will be re-mapped. Unexpected gothic script in operator displays is not
often welcomed!
Fonts are normally classified as ‘Serif’ and ‘San-Serif’. A serif
refers to the curly bits at the end of each letter, which lead the eye
onto the next letter. Books and newspapers will almost
certainly be set in a Serif font in the body text. However, due to the
resolution of most VDU screens, it is better to use a San-Serif font
such as
Arial;
because the resolution is not good enough to clearly render the detail of a Serif font.
Having selected a font the next task is to decide on a size.
Basically, it should be possible to read key information from several
feet away without the need for reading glasses (an affliction
suffered by many of us over the age of 45!). Arial at 16 point should be
a good starting point. Up to 2 larger sizes should be selected for
labels and
headings. Once you have selected a basic style – stay with it and avoid
the temptation to use different fonts or more than 3 sizes. If you find
you need to
provide more detail in your text, use a pop-up window or ‘tool-tip’ so
that the user can select to see the additional text.
TOO MUCH UPPER-CASE TEXT CAN BE DIFFICULT TO READ AND CAN CAUSE EYE STRAIN, ESPECIALLY IF AN UNDERLINE IS USED. Upper case should be reserved for headings. Text should be in lower case with the
first letter of the leading word a capital. It can sometimes be a problem to know where to put capitals – I favour
‘Exhaust Valve Temperatures’ rather
than
‘Exhaust valve temperatures’ but which ever you choose, you should be consistent.
Text Data Values
Text, and especially data values, should be grouped in areas of the
screen. Randomly distributing data values around a picture makes it
difficult to scan for data values. If you want the user to
compare data values, then put them next to each other in a table. If you
have several tables with the same data types, e.g. temperature,
pressure, speed; ensure that the data is presented in the same order in
each table.
Care should be taken with units. It is important to know that
pressure is displayed in millibars but writing ‘mBar’ next to every
measurement is unnecessary and clutters the display.
Avoid unnecessary decimal places. The displayed Data Resolution
should be appropriate to use; 546.45 C is pointless and difficult to
read, (the accuracy of measurement is probably limited to +/- 2C).
Subtle changes in background relief can often be used to enhance the
presentation of text and data. If you choose to include this in your
design, it is important to ensure that you are consistent.
Alarms and Plant Status Animation
The key aspect of an HMI display is the dynamic plant data. There are
2 basic types of dynamic data: Alarms and normal Plant Status data.
Alarms
Alarm status for the overall plant, preferably organised into groups,
should be visible on every screen and there
should be a simple navigation route to access the screen containing
additional details about the alarm. The alarm colours should follow the
safety convention:
- Red = stop, prohibition, danger
- Yellow = caution, risk of danger
- Green = safe condition
- Blue = mandatory action.
As 1 in 12 men have some form of colour-blindness, which can affect
the perception of red and green, colour
cannot be used as the sole indicator of alarms. Any colour change must
be supplemented with a pictorial change and if the alarm is critical, an
audible alert.
Pictorial changes could include shape changing, a change in the
position of an indicator, having additional text or objects appear on an
alarm. Flashing of unaccepted alarms is very irritating
and stressful and should be avoided. Similarly, automatic changing of
displays should be avoided – to have a page disappear when you are
working on it is
irritating and in extreme cases a cascade of alarms can produce so many
screen refreshes, an operator can be locked out of the system.
Audible alerts can be very useful, especially if the system is able
to create multiple tones and pitches. These tones can be used to
transmit the importance of an alarm, research has shown
that a high pitched, fast pulsing sound automatically conveys urgency,
whereas a lower pitch, slow pulsing sound is less urgent.
Whatever convention is used, alarms should be placed where they can
be easily seen, our preference is along the top of the screen.
Plant Status Animation
Whilst conventions are generally well defined for alarms, some
designers have problems when using colours for plant status. Some are
tempted to use green for go to indicate running plant and red
for stopped, whereas the alarm logic would suggest using red for
dangerous, running plant and green for safe, stopped plant. The
convention in the power
industry is RED for RUNNING, but to be sure (and to account for colour
blindness) plant status should always be indicated by another means
preferably
text –
‘Running’. If there is risk of confusing a red status indicator for an alarm then other colour combinations should be used.
Navigation and Control
Navigation
To use a VDU system, the operator must be able to change between
pages quickly and easily. With the wide acceptance of Microsoft Windows,
the operator control device of choice are ‘mice’ or
similar pointing devices, which have largely superseded keyboards as a
primary navigation tool. Perhaps the most intuitive pointing device is
the touch screen.
Whatever the choice of pointing device, the screen ‘hot spot’ must be
obvious and large enough to be hit easily. There are 2 main preferred
methods of denoting a ‘hot spot’; the most obvious is to
add a button to the display. These should be clearly labelled to
indicate the effect of the button. The other approach is to embed the
hot-spot in the
picture. This can be used to great effect in an overview display showing
a schematic layout of the site; ‘clicking’ over any area will switch
the screen to a more detailed display.
Navigation between displays should be simple, obvious and quick, and
will normally be organised in a logical tree structure. The next step in
a navigation sequence should always be the most
obvious; the lower right hand part of the page is a good place to put a
‘next step’ button. It should also be possible to back-track and return
to the
previous display. All screens should also have a button or hot-spot to
reset the screen to the top-level display, often called the ‘Overview’.
This is the
equivalent of a ‘Home’ page on a well-designed web-site. This ‘Overview’
button need not be too obvious and should be located in the same
position on every page; the bottom left is often preferred.
If possible, navigation buttons should be grouped together to make it
easy for the operator to use them without having to move a mouse or
trackball too far. If the user is expected to routinely
switch between two or more screens, it is a nice idea to locate the
button for the next screen in the sequence at same point on each screen,
so that the user can click quickly between displays without moving the
pointer.
Control
Where buttons and hot-spots are used for control, then a pop-up
window should be used to confirm the action selected:
e.g. ‘Start Feed Pump – OK or Cancel’. This avoids accidentally
activating plant and allows the new user to explore the system without
fear of causing damage or injury.
Summary of Recommendations
There are no absolutes when designing HMI displays, and everyone will have their own preferences.
We hope that in preparing this paper, many of the common failures can be avoided.
Whatever style you adopt when designing an HMI display, it most
important that the style should be followed throughout the suite of
displays. Each display should have the same look and
feel; navigation buttons should be in the same place and everything
should line up.
Display pages need to be clear, easy to use and legible across a control desk:
