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HUMAN FACTORS IN ENGINEERING & DESIGN

Chapter III:HUMAN OUTPUT & CONTROL

Topics:

Basics of Human motion activitiesBio Mechanics of Motion

Functions of Control

Factors influencing controlsDesign considerations of control devices

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HUMAN OUTPUT & CONTROL

Basics of Human Motion Activities:

Human beings engaged in work-related, leisure,sports activities

Body undergoes changes Physiological, Psychological &cognitive

Outside Effecter Stress

Body change Strain

Work intensity within limits no long term wear & tear

Stress: Task factor that impinges on individual

Physical: Heavy work, Heat-Cold etc

Emotional: Danger, insecurity, loneliness, info overload
http://stress.ppt/http://stress.ppt/

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Strain: Effect of stress on individual

Strain measuredby data collected from body changeslike Oxygen consumption, Blood chemistry, heart rate ,attitude, work rate, errors etc

Source of stress for one may not be for another

individual characteristics differ physical, emotional, attitudinal.

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Human Activity Continuum: Indicates classification of

activities & the effects on body due to stresses

Physical CognitivePsychomotor

DynamicActivity

Bicycle

riding

StaticLoading

Holding

work &assembling

SensoryFunctions

Light

assemblywork

Thinking ,Calculating

Arithmetic

work,planning

Physical MentalFunction

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Stress & Performance

Stress

Maximum Performance atOptimum Stress

Increased stress dropsefficiency Forgetting to attendmeetings , lower quality of work

Increasedstressenhancesperformance

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Primary sources of Job Stress

Work Place arrangement: Location, dimensions

Work Content: Speed & Accuracy requirement

Work Organization: Work-Rest cycle, Start-Stop time

Work Environment: Physical & Social

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Muscle Physiology

Types of Muscles:

Striated - SkeletalCardiac - HeartSmooth Walls of blood vessels & internal organs

Focus on Striated & Cardiac

Number of Muscles: 600400 are Skeletal (appearing in pairs) :

80 of which carry out the most vigorous activitiesAre attached to bones through tendons.

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Muscle Physiology

Energy required for muscle to contract is provided bythe digested food Carbohydrates, fats & to a smallextent Proteins.

Muscle fibres are activated by motor nervesemanating from spinal cord either at the conscious

level or reflex level.

A single motor nerve fibre activating a group ofmuscle fibres is called a Motor Unit

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Measure of Physiological StrainKilocalorie is the measure of EnergyDefined as the Heat required to raise the

temperature of 1 KG of water from 15oC to 16oC.

Energy content food is measured as Calorie isactually 1 Kilocalorie.

Physiological Strain can be assessed with energyexpenditure & Consumption

RateofWork

Air Intake

Max Aerobic Power : Oxygen Uptake

Heart Rate : Tread Mill

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Work efficiency

All Energy expended by a person does not end up inuseful work.

70 % ends up in heat & unproductive static efforts.

Work OutputWork Efficiency = x 100 %

Energy Consumption

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Consumption of Energy Energy CostsEnergy consumption on day-to-day activities:

Activity Kcal / Min

Sleeping 1.3

Sitting 1.6Standing 2.25

Walking (Level) 2 Km/hr 2.1

Cleaning / Ironing 2

3Cycling (16 Km/hr) 5.2

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Factors influencing Energy Expenditure

Use the method that utilizes the least energy to keep

the Strain within permissible limitsMethod of Work:

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Factors influencing Energy Expenditure

Tool Design (Type of Implements):

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Normal Recommended Energy Limits

Number of Hours

(Ayoub & Mital)

Allowable energy expenditure

(Average)8 hour day work Men : 5 Kcal/Min

Women: 3.35 Kcal/Min

4 Hour day work Men : 6.25 Kcal/Min

Women: 4.2 Kcal/Min

Permissible HeartRate as per

Allowable energy expenditure

(Upper limit)

Suggs & Splinter 115 Beats/MinSnook & Irvine Leg work: 112 Beats/Min

Arm work: 99 Beats/Min
http://suggs%20and%20splinter.doc/http://historical%20development%20of%20manual%20lifting%20guidelines%20and%20standards.doc/http://historical%20development%20of%20manual%20lifting%20guidelines%20and%20standards.doc/http://suggs%20and%20splinter.doc/

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Work Rest Cycles

If the overall level of workload cannot be maintainedwithin recommended limits, then there must be Restto compensate the excess requirement demanded.Murrells formula on amount of rest required for agiven Work Activity:

T(W S)R =K 1.5

Where R Rest required, Min

T Total Work Time, MinW Ave Energy Consumption, kcal/minS Reco Ave Energy expdt., kcal/min (taken as 5)K Murrells constant, taken as 5.1.5 Approx of Energy expenditure at Rest, kcal/min

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Strength & Endurance:

Exercise & TrainingIf Improves fitness and be able to perform betterphysical work without fatigue.

Strength:

Maximum force the muscle can exert isometrically in asingle voluntary effort Static : fixed objectDynamic : object position changing continuously

Factors effecting strength are age, gender, physique,motivation, work content. Peak strength 25 to 30 yrs.Women perform better at lower extreme activities.

Female mean Strength 2/3rd Men mean strength

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Strength & Endurance:

Endurance:It is the maintenance of a given static muscular forcefor a period of time called the Endurance Time.

Endurance Time, Min

Perc

entofStrength

25

0

50

75

100

0 5 10

@ 25 % Strength,

Endurance time Approx. 10 min

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Biomechanics of Motion

Types / Range of Body Movement:Occur around movable joints.

Types of joints:Synovial Joints:

Hinge Joints Fingers, KneePivot Joints ElbowBall & Socket Joint Shoulder, Hip

Cartilaginous Joints Vertebrae of Spine

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Range of Body Movement:

Types of Movements depends on:Bone structure of the joint.

Amount of muscle or tissue (Bulk) around the joint.Elasticity of muscles, tendons & tissues around the joint.

Eg.: As the biceps muscle of the upper arm increases, the

Flexion - reduction of angle, at the elbow reduces.

Ability to touch toe with fingers without bending knees dependson the flexibility of lower back, upper legs & bulk at abdomen.It is called Muscle-Bound concept.

Can be improved by training regularly.

Flexibility improves ranges of movements, however, increased

flexibility reduces stabilityof Joint. Eg. Football & Wrestling.

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Range of Body Movement - Definitions:

Flexion: Movement of segment of body causing adecrease in the angle of the joint.

Knee, Hip, Ankle, Wrist

Extension: Movement in the opposite direction of

Flexion which causes an increase in the angle at thejoint.Straightening of the elbow.

Hyperextension is the extension of body segmentbeyond its normal extended position, such as archingthe back.

R f B d M D fi i i

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Abduction: Movement of segment of body in a lateralplane away from the midline of the body.

Raising the arm sideways

Adduction: Movement of segment of body towards the

midline of the body.Moving arm from outwards horizontal position todownward vertical position.Rotation: Movement of segment around its own

longitudinal axis.Hand & Forearm rotation called as

Supination: Results in palm down positionPronation: Results in palm up position

R f B d M D fi i i

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Circumduction: A circular cone-like movement of

segment of body swinging of arm in circularmovement about the shoulder.

An experimentation in each case with 5th percentile,Mean & 95th percentile range of movement on 100College students was made.

Fl i & E t i

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Flexion & ExtensionElbow Flexion Hip Flexion

Wrist Flexion (A)

& Extension (B)

Shoulder Flexion (A)

& Extension (B)

Fl i & E t i

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Flexion & Extension

Knee Flexion, StandingAnkle Flexion (B)

& Extension (A)

Add ti & Abd ti

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Adduction & Abduction

ShoulderAdduction (A)&

Abduction (B)

HipAdduction (A) &Abduction (B)

WristAbduction (A) &Adduction (B)

R t ti & Ci d ti

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Rotation & Circumduction

Shoulder Rotation,

Lateral (A) & Medial (B)Arm Circumduction

F R t ti S i ti & P ti

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Forearm Rotation: Supination & Pronation

Forearm Supination (A) & Pronation (B)

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Ch t i ti f M t R

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Characteristics of Motor Responses:

Purposeful physical activity involves the concept ofSkill

Ability to use the correct muscles with the exact force

necessary to perform the desired response withsequence & timing and be able to adapt to changingenvironmental conditions with consistency.

Types of Motor Responses:

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Types of Motor Responses:

Discrete Movements: Reaching a control stationary button.

Repetitive Movements: Hammering a nail.

Sequential Movements:Typewriting.

Continuous Movements: Steering a car, Band saw.

Static Positioning: Hold part while soldering process.

Features of Motor Responses:

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1. Error:

Difference between the response & set goal is error.

Motor skill invariably has a feedback system for betterresponse closed loop control system.

Error of selection:Selecting a wooden club to iron club in golf.Error of execution:Using more reflex than necessary person

requires 30 to 80 ms to initiate response which may just befast enough for preventing a spill from a plastic mug lookinglike a heavy mug.


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2. Speed of Movements:

Quick response are initiated on basis of stimuli applying brakes in emergency.Here, the Response Time is critical.

Reaction Time: Onset of the signal for Response till thebeginning of response.

Movement Time: From the beginning of response to theend of response.

Movement Time

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Movement Time

Start to the end of response depends upon the natureof movement & accuracy required.

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2. Accuracy of Movements:

More often accuracy of movement of greater importancethan speed, within limits.

Occurs due to inconsistency in trajectory generated by

relevant muscles.

Unintentional foot movement to accelerator instead ofbrake pedal inaccuracy results in accidents.

Functions of Controls

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Functions of Controls

Machines have been devised to perform effectively &

efficiently but require instructions / control fromhumans through Control Devices.

Machines cannot be controlled thought, but by act

and transmit information to a mechanism information maybe Discrete or Continuous.

Control Types vary from conventional Push-Button

controls, Slider mechanisms to Touch-Screentechnology.

Types of Controls

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Types of Controls

For transmitting discrete information:

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Types of Controls

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Types of Controls

For transmitting cursor positioning information:

Factors influencing Design of Controls

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Overall utility of a control is influenced by certain factors:

1.Identification: Correct &rapid identification required toprevent accidents, it is essentially a coding problem.

Methods of Identification Coding are:

Shape Coding:

When control operated without visual attention and withtactual sensitivity.

Differentiates different controls so that one can identifyblindfolded & operate effectively.


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Texture Coding:

This is in addition to Shape Coding

Generally done on knobs by machining them differently to prevent confusion while operation.

-Operator could confuse between same design (flute size)but not between different designs (Knurled & fluted)

FlutedSmooth Knurled

9 & 18 Flutes Straight Knurl Diamond Knurl


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Size Coding:

Not as useful as that of shape coding. For best results they are superimposed & knurled

differently for better identification.

Method 1 Method 2

3 Dia

2 Dia

1 Dia

Knurled


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Location Coding:

When visual attention is absent, identifying by locationpossible with a few switches coded with shape.

Shifting from brake to accelerator - Movement through

kinesthetic sense.

Switching on the headlight of car without looking at thedash board if several similar switches in the same

location leads to error.


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Operational Method:

Each control has unique method of operation and canbe operated only using that method.

Push-Pull variety, Rotary variety.

Color Coding:Color used to identify controlsModerate colors with contrasting shades usedColor to attach meaning Red indicates emergency

Disadvantage :Operator to observe & identifyAt lower illumination & dusty environment may

have to use a second coding method alongside,size coding. cannot be used


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Legend or Label Coding::

Most commonly used method for identification.

Large number of controls can be coded and if properlychosen it is easy to comprehend (Nuclear plants have

walls of Control codes in Labels alone difficult toread & comprehend before acting).

Should not be the only means of coding.

Labels should be placed above the controls so that theoperators hand do not cover them.

C l R R i (C R R i )

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Control Response Ratio: (C-R Ratio)

A specific amount of movement will result in a

particular amount of system response.Steering Wheel rotation leads to movement of wheel to the desired level.

System response may or may not be represented onthe display. Control movement display: Linear (lever, vertical)

or Angular (Knobs, Circular displays)

Ratio of movement of the Control devise to themovement of system response is called Control-

Response Ratio.

Sensitive Control: Slight control movement results in largeresponse movement. CR ratio may be low or High Gain.

Gain is the reciprocal of CR Ratio.


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Resistance in Controls:

Represents the Feel or feedback associated to theresistance of the control.

Operator manipulates while he operating control knob

by knowing the amount of displacement required onthe control and the amount of force to be applied tobring about this displacement.

Resistance of control acts as a feedback to operator. Some controls require only force or only displacement.


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Deadspace and Backlash:

Deadspace:

Amount of control movement around the null positionthat results in no movement in the devise being

controlled.

Inevitable that Deadspace exists in Mechanical controlsystems.

Reduces sensitivity & effects performance.


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Backlash:

Movement of control device if regulated by a secondpart.

Small initial movement by the second part may notinitiate a response called the backlash.

Eg. : Gear Meshing

Design of certain Control Devices:

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Design of certain Control Devices:1.Cranks & Hand wheels:

Frequently used for applying force to control an

equipment.

Ketchmer (1957) study on Cranks & Handwheels:

Optimum combinations of different diameter wouldhave different torque conveying capacity increasein diameter increases capacity. Optimum work leveloccurred for moderate torque level of 30 50 lb-in using 5 &7 wheels, while no effect on 4 wheel.

Time of operation: Larger the radius longer thepersons were able to rotate & continue excepthigher torque levels optimum torque for max time was

10 lb-in


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1.Cranks & Hand wheels:

Cranks were adjusted to waist height & rotate at a rapid rate tillthey no longer could continue or 10 min whichever was earlier.


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2. Pedal or Foot Control: (Ayoub, 1967)

Constant angle: Foot pedal to be moved through 120no matter where the fulcrum was located.

It was found that the closer the fulcrum to the heel,

farther the back of the foot must travel to achieve a120arc.

Constant distance: Foot pedal to be operated by

moving through a distance of 1.9 cm.It was found that the closer the fulcrum to the heel,smaller the arc to achieve the activation distance.


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2. Pedal or Foot Control:

Optimum location of fulcrum:

Constant Angle 1/3 rd from toe to back of foot.

Constant Distance At the heel.


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3. Key Board Design:

Since Keyboards are sequential, the key positions areaptly designed for better speed of operation.

Conventional arrangement: QWERTY keyboard

Redesigned : DVORAK simplified keyboardQWERTY known to be slower (for mechanically

operated typewriter) - to reduce pace - thus reducingof error in typing / keys jamming against one another.

DVORAK superior in speed (10-20 % increase) andreduction in hand & finger fatigue.

Question: is this speed required / justified? Maybe, not.

Key Board Design contd.

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ey oa d es g co td

Computer keyboards are designed to switch intoDVORAK set up from QWERTY. The keycaps arenormally are in QWERTY arrangement.

Dual arrangement may be preferable to many.Conventionally, QWERTY remains a standardarrangement because of its familiarity with people.

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