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Human–Machine System: Controls The boundary between the human and the

machine is the “user interface.” Information flows from human to machine

through controls.

HUMAN

CONTROLS

MACHINE

DISPLAYS

Physical Environment

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G1: Select the proper type of control

Types of systems: Open-loop Closed-loop

Discrete vs continuous Select part of the body to implement mental

command Choose mechanical interface between

human body and machine interior Consider amount of human power required Consider errors and speed

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G2: Select the proper control characteristics

Force People variables:

Muscles used User population Percentile designed for Control variables

Control options Keys and pushbuttons Knobs Cranks

Control variables: Purpose Design

Hand wheels Foot/leg controls

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Control Options

Keys and Pushbuttons Keys should be non-round, concave on top, and

consistently displaced. Use key interlocks. Feed back activation to operator. Use shape to indicate function. Avoid palm buttons.

Knobs Circular knobs depend on hand friction on

circumference. Knob shape can compensate for slippery grip. Increasing diameter increases torque.

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Control Options (cont.)

Cranks Provide high coefficient of friction. Allow grip to rotate on crank. Consider a sphere.

Hand Wheels For valve control, increase radius to

reduce tangential force. Use recommended heights. Rim diameter should be 20–50 mm. For vehicle control, follow diameter

and orientation recommendations. Consider permitting angle adjustment.

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Control Options (cont.)

Foot–Leg Controls Provide greater force and free the hands. May be switches or pedals. Consider friction problems (shoe sole, dirt, debris). For continuous control, bend the ankle by

depressing the toe. Consider knee switches.

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G3: Prevent unintended activation

More severe consequences require greater precautions.

Consider that operators may bypass the guard if it makes operation difficult.

All equipment should have an emergency stop control.

Consider two reaction times: Human (sensing, deciding, carrying out) Machine

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Methods of reducing accidental activation

Key or special tool activation (locks) Interlocks Barriers or covers Recessing Spacing Resistance Direction

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G4: Prevent incorrect identification

Labeling Color Shape Size Mode of operation Location

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Labels should be: Legible Located properly Understandable

Color 8% of males, 0.4% of females have color perception

problems Use 8 or fewer colors. Consider lighting requirements. Consider color stereotypes. Consider cultural differences in color stereotypes

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Shape Can be a visual signal. Can be identified in the

dark. Use up to 9 different

shapes. Tactual shape coding is

slower than color coding. Tactile signals (e.g.

pimple, ridges) can help.

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Size Only 2 or 3 different sizes can be differentiated. Larger sizes give mechanical advantage. Use larger sizes in cold environments.

Mode of Operation Consider push/pull vs. rotate vs. slide. Mode of operation may be a backup for identification

methods. Location can identify controls and groups of controls.

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G5: Make accomplishments equal intentions

Require verification of critical commands. Make complex sequences of actions user-

friendly. Use more care and testing for actions with

multiple steps. Check input for validity. For continuous controls, consider

control/response ratio. Control positions often act as displays.

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Population Stereotypes

Engineer’s habit patterns may not be the user’s habit patterns.

Stereotypes are often country-specific (light switches, water faucets, car blinkers). See table 15.9, pg. 305 for US conventional control

movements

Consider labeling.

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G6: Properly locate and arrange controls

Arrangement of Controls Group related controls and

displays together. Minimize layout

complexity. Have blank space on the

panel. Hand Controls

Keyboards Manipulative controls

Knobs Switches

Force controls

Foot Controls Avoid foot controls for

standing operators. For continuous control, leg

should fully extend at bottom of stroke.

For discrete control, use one leg.

Have straight line between pedal and back support.

Comfort may be more important than force capability.

Design to avoid fatigue.

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Human–Machine System: Displays Displays include instruments, labels, and

accompanying printed instructions and warnings.

HUMAN

CONTROLS

MACHINE

DISPLAYS

Physical Environment

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Causes of Failure

Legibility or detectability Understanding

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Displaying InformationInform

atio

n

Display

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G1: Select legible characters Font

Use printed characters Avoid Roman numerals Upper/lower case:

For short messages, use all caps For text or longer messages, use mixed-case with large

open spaces in letters For VDT, use more pixels and lower dot pitch

Size Character height = K × Distance from eye

K = 0.004, 0.006, 0.0017, 0.0073 radians Visual angle should be 15–25 min of arc

1 min of arc = 0.00029 rad

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G2: Arrange characters & symbols

Text – clear and legible Print text in columns 10 -11-point type minimum Use space between number and unit Use double-spacing Do not justify right side Use headings for organization

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Guideline 2 (cont.)

Codes – don’t rely on memory Automate the code transfer Make codes checkable Use short codes Make the code meaningful

Abbreviations – use with caution Use rules to form abbreviations Rules best for encoding are not necessarily the best for

decoding Truncating works better than vowel deletion in testing,

but best rule of thumb is to be consistent! Do not include period

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G3: Decide on type of display

General Considerations Task/job – what’s the purpose of the display? User characteristics

language, education, culture, expectations, etc. Standardization (see examples, next slide)

Menus – lists of options Deep vs Shallow Structure according to how information is used Minimize complexity / maximize understanding

Avoid multiple pages Highlight options Use blank space and grouping

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Guideline 3 (cont.)

Tables – make the information easy to find Round data to 2 significant digits Use explicit tables Avoid matrix tables Make the primary comparison down the column Reduce row alignment errors and column selection

errors

Formulas – simplify calculations Use to permit exact calculations Present in units that the user will enter Decide on significant digits necessary

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Guideline 3 (cont.)

Graphs – make relationships visible Use to compare complex relationships Use instead of tables when:

Displayed data have inherent structure Structure is relevant to the task

Provide titles, labels, units Place close to the text it illustrates Guidelines for Good Graphs, for example

Make graphs wider than tall Show scale subdivisions with tick marks Avoid hatching Use only a few curves on a single graph Indicate data points with open symbols

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Guideline 3 (cont.)

Symbolic Messages – be sure they’re understood Include shapes and colors, diagrams, pictographs Use color to identify categories

makes navigation easier but use ONLY as a redundant code

Use icons with care Weigh benefits vs. costs

Maps – locate in space and time Can show data distributions and location relationships May be scaled to distance or time May be not to scale

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G4: Project your message Slides

35 mm slides Computer projection

Transparencies Video

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G5: Select the instrument display

Discrete: finite choice of options Continuous: point on a scale

Analog Digital

Representational: provide diagram or picture Video

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Warning messages

Type of failure: False signals Missing signals Multiple signals

Should be both visual and aural Should be within primary field of view Should provide guidance information Urgency / immediacy of warning should match

severity of danger

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G6: Locate/arrange the displayLocation Locate to be seen easily Provide appropriate lighting Consider eye height and head orientation Keep data within a 20º cone of line of sight Angle should be about 30º below the Frankfurt Plane

Arrangement Determine what the operator is required to do Maintain consistency on panel and within facility Decide on grouping logic Consider computer simulation