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What
is a control?
A control is a device that allows you to
'communicate' with objects and to manipulate them. Controls can enable
you to direct equipment or machinery that can help you
to generate more power, more reach and to reduce effort and risk. In
some cases, the control also provides the force to make an action
happen, for example, a foot pump pedal.
 The
controls on your mobile phone allow you to make calls, send text
messages and set your favourite ring tone. The keys on your keyboard and
your mouse buttons are all controls that enable you to view this page!
The main function of a control is to
transmit information to an object. When you use a control, information
is flowing from you to the object (brain signals pass to the muscles in
your hands which move to activate the control). You gather information
from the object via feedback, for example, in the form of a change in a
visual display when you press a key on your keyboard, or illumination of
a room when you operate the light switch, etc.
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Types of controls
The type of
information that is transmitted by a control can be discrete
(separate) or continuous. Discrete controls use a limited number
of conditions, for example, a light switch, which is either on or off.
Continuous controls use any value between the outer limits, for
example, the gas control knob on a cooker hob can regulate the
flow of gas anywhere between the minimum and maximum flows.
Controls often have an associated display to provide you with
information about the results of your control actions. These
displays can also show discrete or continuous information. Read
more on displays.
Another major
difference between controls is the amount of force needed to use
them. Some controls may require a light touch with fingertip
operation, for example, keyboard push buttons or lighting toggle
switches. Others need more force using the strength and grip of
the whole hand or foot, for example, door knobs, gear levers,
brake pedals. Can you think why there should be a difference?
See the Effective controls information below to find out.
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DISCRETE
CONTROLS |
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Hand
push button
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Foot
push button
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Toggle
switch
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Rotary
selector switch
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CONTINUOUS
CONTROLS
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| Knob
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Crank
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Wheel
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Lever
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Pedal
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Effective
controls |
To be effective,
controls must be accessible, identifiable and usable. Accessibility
- since almost all controls require physical contact to be
operated, they must be able to be reached comfortably and
efficiently. You must consider the body size of the user - it's
no use putting an emergency button out of reach! Identifiability
- a control needs to be able to be identified in terms of what
it does in order to operate (twist, lift etc.), what it controls
and what state it is in (on, off, partially open, etc.). This is
especially true for groups of controls that may have similar
functions. For example, the controls on the cooker at home
probably look almost identical, but the labelling, and often the
position of the control, enable you to tell them apart. The
controls on a cooker rotate when operated to give you an idea of
their state. Feedback is given by the oven, grill, burners or
plates as they produce heat. Feedback is an important element of
controls, giving you confirmation of your actions. Usability
- a control must be able to be used! You should be able to
operate it with the required force, speed and accuracy. For
example, it would be difficult to operate a foot pump with your
hand because you need a lot of force to push air into a tyre.
Using your foot for this enables you to put your weight behind
it and use the large leg muscles to push. However, you don't
need to be precise, unlike setting a rotary central heating
control. The control knob is designed to be operated with the
fingers and does not require force. |
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?????
ACCURACY
v
PRECISION
Accuracy
indicates how close a measurement is to an acceptable value. For
example, a balance should read 100g if you place a 100g weight
on it. If it does not, then the balance is inaccurate.
Precision
indicates how close together or how repeatable results are. A
precise measuring instrument will give very nearly the same
result each time it is used.
It
is possible for an instrument to be precise, but inaccurate. Thermometers
found in school labs are often more precise than they are
accurate. It is quite easy to read a thermometer to the nearest
0.2°C, but the calibration can often be out by a degree or
more. |
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Guidelines for
design
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Control compatibility
The control should operate in the way that people expect it to
operate. There are certain directions of control movement which are expected by the majority of people. They are
called population stereotypes and control movements which conform to these stereotypes are said to be compatible.
In western Europe, for example, a movement to the right, a
movement forwards and away from the body, or a clockwise
rotation, instinctively suggests a start or increase in
operation.
People learn how to use
controls more quickly if compatible control
movements are used. Compatible controls are safer to use in
emergencies. When people are under stress, they tend to use the
'expected' direction of movement. Control movements that seem 'natural' to the operator are more efficient and less
tiring because there is less need for thought and assessment,
and therefore operation is faster.
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The
use of force
The maximum force (and speed, accuracy, or range of body movement) required to operate a control should not exceed the limits of the least capable
operator. 'Normal' requirements for control operation should be a
lot less than the capabilities of most operators.
Control movements should be as short as possible.
Controls should have enough resistance so that they can't be
accidentally operated, especially where the consequences are
serious. For example, the emergency stop button on an escalator
should resist an accidental light touch. For controls like
emergency stop buttons, requiring a single application of force, or short periods of continuous force exertion, a reasonable maximum resistance is half of the operator's
maximum strength. For controls operated continuously, or for long periods, the resistance should be lower.
Power-assisted, or fully powered controls should be applied in some types of equipment where the operator cannot apply enough force
unaided e.g. electric garage door controls.
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Coding
of controls
Controls that are different (coded) in terms of either shape, size, mode of operation,
labelling, or colour are easier to identify than controls which
are similar in these respects.
Shape coding - the use of a distinctive shape for controls improves their visual and tactile
(touch) identification. Standardised shapes should be used and sharp edges should be avoided on the parts of the control that are grasped.
Size coding - where size is used to distinguish controls
(usually control knobs), the larger control should always be at
least 20%
larger than the smaller one for controls ranging from 15-150mm in diameter, in order to avoid confusion in selection.
Colour coding
- colour should not be used as the sole method of coding of the
control. It is more
effective when combined with other methods. Use red, orange, yellow,
green and blue as they are the only colours that are effectively identified and
recognised. However
about 7% of men and under 1% of women suffer from
red-green colour blindness - they are unable to differentiate reds
and greens and tend to see these colours as shades of greyish-yellow. Therefore, in situations where colour coding and recognition are
vital, you should check that all operators can distinguish the colours
used.
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Labelling
If controls are to be labelled, adequate space and lighting must be
provided to enable the labels to be clearly visible. Labels should be either on the control or immediately adjacent to
it. They should not obscure
the control and should be
difficult to remove. Letters
and numbers used should be standard.
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Hand controls
You should use hand controls in preference to foot controls
if:
- accuracy of control positioning is important;
- speed of control positioning is important;
- continuous or prolonged application of force is not necessary.
You should use:
- Push buttons or toggle switches for tasks involving speed of
operation.
- Fingertip or hand-operated rotary knobs for fine adjustment and small forces, e.g. radio
tuning.
- Control sticks or levers when you
need to apply moderate to large forces intermittently, e.g. gear changing or using
the hand brake in a car.
- Sticks, levers or wheels for continuous adjustment or tracking
tasks.
- Controls that use one handed
operation for precision and speed.
- Controls that use two handed
operation for larger forces.
A hand control should offer some resistance to movement or accidental
operation, for example, the keys on your keyboard need a definite push
to operate them (try pushing a key gently - you should feel a slight but
definite resistance).
A hand control should allow you to get a good grip. You should make sure
that the control material is suitable for the environment where it will
be used. For example, if the control is to be used outdoors, it may need
a more textured surface to allow good grip in the wet, or it may need to
be insulated so that it is not too cold or hot to touch. Materials for
hand controls should be poor conductors of heat and electricity. They should be
non-porous so that they will not soak up liquids. They should be strong enough not to chip or crack and injure
your hand.
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Foot
controls
You should use foot controls in preference to
hand controls if:
- continuous control of the task is required but the precision
of the position of the control is not important.
- large forces need to be applied.
If a large force is needed, you should use a pedal on which pressure is applied by the whole leg, e.g.
car brake pedal.
If a smaller force and continuous control is needed, you should
use a pedal on which pressure is applied mainly from the ankle, e.g. car
accelerator pedal.
For all foot controls, the direction of movement should be down or away from the body and in line with the
centre of the body.
The angle at which the pedal is positioned should allow the foot to be placed on the pedal surface with the ankle at an angle of 90º.
This angle should increase with operation of the pedal, that is,
the pedal should be pushed away from the body to operate.
A pedal should be flat and have a large enough contact area so
that the foot does not slip off (a width of at least 90mm). A
textured surface also helps to keep the foot in position.
Foot controls should only be used if you are seated, or for very
short standing times. It can place strain on the other leg/foot
if you operate a foot pedal while standing. |
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Choosing
a control
You should choose a
control to suit the type of task required of the equipment.
| Control
type |
Suitability
for tasks involving: |
| Speed |
Accuracy |
Force |
Range |
| Push
button |
Good |
Unsuitable |
Unsuitable |
Unsuitable |
| Toggle
switch |
Good |
Unsuitable |
Unsuitable |
Unsuitable |
| Rotary
selector |
Good |
Good |
Unsuitable |
Unsuitable |
| Knob |
Unsuitable |
Fair |
Unsuitable |
Fair |
| Small
crank |
Good |
Poor |
Unsuitable |
Good |
| Large
crank |
Poor |
Unsuitable |
Good |
Good |
| Wheel |
Poor |
Good |
Fair/Poor |
Fair |
| Horizontal
lever |
Good |
Poor |
Poor |
Poor |
Vertical
lever
(to/from body) |
Good |
Fair |
Short:
Poor
Long: Good |
Poor |
Vertical
lever
(across body) |
Fair |
Fair |
Fair |
Unsuitable |
| Joystick
(lever) |
Good |
Good |
Poor |
Unsuitable |
| Pedal |
Good |
Poor |
Good |
Unsuitable |
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Controls
FAQs
Q. I
wish to know what is the distance a child has to sit from a set
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