Motion Main Topics Vibration Acceleration Illusions during Motion Motion Sickness.
Post on 24-Dec-2015
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Motion
Two General Classes of Motions
Volitional, Low-SpeedIssues Concerned:
Non-Volitional, High-SpeedIssues Concerned:
– Tolerance
– Safety and protection
– Impact and acute effects
– Illusion
* Vibration a special entity
–Permanence (S-A trade-off)
–Strategy to minimize stress
–Accumulative effects (low force)
–Acute effect (forceful exertion)
Motion
Senses Related to Motion
Sensory ReceptorsExteroceptors
Proprioceptors
Semicircular Canals
Vestibular Sacs
—postural/balance sensors
(Figure 19-1)
Conflict between visual perceptions and actual gravity
—deal with stimuli external to the body
—stimulated by body’s own actions
—acceleration/deceleration sensors
Motion
Ways to Describe Vibration
Type (Wave Form)– Sinusoidal vs. Random
Direction– Forward-Backward
– Left-Right
– Up-Down
Frequency– Cycles per second
Intensity
–Amplitude (Displacement)
–Velocity
–Acceleration
–Rate of acceleration change
Motion
Effects of Vibration on the Body
1. Transmission
Attenuation
Amplification
Resonance
3-4 Hz Resonance in cervical (neck vertebrae)4 Hz Peak resonance in lumbar
(upper torso) vertebrae5 Hz Resonance in shoulder girdle20-30 Hz Resonance between head and shoulders60-90 Hz Resonance in eyeballs
Motion
Effects of Vibration
2. Physiological Effects
Short-term exposure
– increased HR
– increased muscle tension
– urge to urinate
– chest pain
Long-term exposure
– increased risk of disc herniation
– increase risk of low-back pain
– increased risk of Reynaud’s Syndrome or Traumatic Vasopastic Disease (TVD)
Motion
Effects of Vibration
3. PerformanceVisual Performance
– impaired by vibration of 10-25 Hz
– minor effect in low frequency range due to head/eye compensatory motion
Motor Performance
– vertical sinusoidal vibration of 4-20 Hz most detrimental
– dependent on display and control
Neural Process– central neural processes (e.g., RT,
pattern recognition) highly resistant to vibration effect
– tension in muscle increases vigilance
Motion
Subjective Responses Whole-Body Vibration
Comfort scale– mildly uncomfortable
– annoying
– very uncomfortable
– alarming
Attempt to link frequency & acceleration to comfort scales
Equivalent-comfort contours
Large inter-person variability makes design considerations challenging
Motion
Limits of Exposure to Whole-Body Vibration
Criterion-basedcomfort, task performance, or
physiological response
ISO 2631most applicable for transportation
and industrial type vibration exposures
Fatigue-Decreased Proficiency (FDP)
Figure 19-7, page 634
Motion
Criticisms of FDP:1. Comfort and FDP limits for short
exposures maybe too high
2. Appear to be based on mean results
3. Imply the effects of multiple single-axis vibrations are additive
4. Similar shaped contours are an oversimplification
5. Comfort contours may be inaccurate at extreme frequencies
6. Assumes time/intensity trade-off with little support
Limits of Exposure to Whole-Body Vibration
Motion
Control of VibrationSource Control
– Reduce intensity
– Avoid resonance
– Provide tool balancing
– Use non- or less vibratory tools
Path Control– Provide rest period
– Reduce transmission (attenuate)
– Use isolator
Receiver Control– Use isolating or damping apparatus– Adopt more “resistant” postures– Reduce grip force– Reduce contact area
Motion
Acceleration
Terminology
1. Acceleration: Rate of change of motion
2. Linear acceleration: Rate of change of velocity
3. Rotational acceleration: Rate of change of direction
– Radial (centrifugal) acceleration
– Angular (tangential) acceleration
– Nystagmus:
involuntary oscillatory movement of the eyeball
Motion
Acceleration
Direction (Figure 19-8)
1. X-Axis: Forward/Backward
2. Y-Axis: Left/Right
3. Z-Axis: Headward/Footward
Follows right-hand rule (RHL)
Look at motion of the eyeballs to determine the direction of acceleration
Eyeballs go opposite of acceleration,and same direction as deceleration
Motion
4. Tumbling
5. Spinning
Acceleration
Head over heels
Around main body axis- spiral nose dive- forces alternate +/-
Motion
Acceleration Duration
Sustained
Abrupt
Begins at 2/10 second and continues
Effects are primarily physiological
Shorter acceleration, less than2/10 second
Mainly effects of impact or rapiddeceleration
Effects are primarily physical
Motion
Acceleration Duration
Three Categories
Short
Intermediate
Long
- less than 1 second- impact or acute effect
- 1/2 to 2 second duration- very abrupt
- greater than 2 seconds through several minutes
Motion
Methods of Study
Tracks
Centrifuges
Suicides/Accidents
•Usually acceleration/deceleration studies performed on tracks
•Slide/ejection tests in impact laboratories
•Help to study the effect of non-linear acceleration
•Rotary chairs or vehicles
• Haven - Golden Gate Bridge and Brooklyn Bridge
• “Real field studies” if caught on tape
• Reconstruction or simulation
Motion
-Gz “Eyeballs Up”
-Gy “Eyeballs Right”
+Gx“Eyeballs In”
-Gx“Eyeballs Out”
+Gz“Eyeballs Down”
+Gy“Eyeballs Left”
Resulting Forces on the Body
Motion
Effects of +Gz (Figure 19-9)
Acceleration headward
– Increase in weight; drooping of face and soft tissues
– Difficult or impossible to raise oneself
– Blackout; loss of consciousness
– Cardiac output and stroke volume decrease while HR, aortic pressure, and vascular resistance increase
Maximum Tolerance = ~16 G
Effects of Directional Forces
Motion
Effects of -Gz
Acceleration footward
– facial congestion
– headache
– blurring, graying, reddening of vision
Limit at -5 G is about 5 s
Maximum Tolerance = ~10 G
Effects of Directional Forces
Motion
Effects of +Gx (Fig 19-10, -11)
Acceleration sternumward
– Speech difficult
– Progressive tightness and pain in chest
– Difficulty in lifting body parts
– Blurring of vision
– Dyspnea
Maximum Tolerance = ~30 G
Effects of Directional Forces
Motion
Effects of -Gx
Acceleration spineward
Effects the opposite of +Gx
Tolerance = ~30G
Effects of Directional Forces
Motion
Effects of +/- Gy
– little information on these effects
– mainly encountered in an aircraft
– magnitude is relatively small compared to other directions
– less common in occupational settings
Effects of Directional Forces
Motion
Exposures less than 2/10 second
Extremely abrupt
Reverse acceleration
Mainly in forward/backward direction
Deceleration (Impact)
Motion
Factors affecting the impact of an impact
Rate of Onset
Peak G
Stopping Distance
Angle of Impact
Deceleration (Impact)
Motion
Tolerance
Survivablelimit around 30-40 G’s
can only endure for 0.25 seconds
Injury
Death
60 G with rate of onset 5000 G/sec
200 G with rate of onset 5000 G/sec
Motion
Protection
1. Restraining Devices seat belt
2. Absorbing Devicesair bag
3. Special Contoured Seatssecondary collision minimized
4. Body Posturedirection-dependent stiffness or
resistance
5. Water Immersiondamping
6. Anti-G Suitscan take up to 9 G
Motion
WeightlessnessTwo Aspects
– absence of weight itself
– tractionless condition
Both remain to be fully investigated
Physiological Effects
Performance Effects
– space sickness (space adaptation syndrome)
– anthropometric change: height growth 3%
– relaxed posture assumed
(Figure 19-13)
– exhaustion due to the added third dimension in locomotion
Motion
Illusions During Motion
Human senses are not designed for extremely dynamic motions and unusual, prolonged forces encountered in special settings
Disorientation from False Sensations (due to inaccurate sensory information)
– disrupted vestibular-visual coordination: illusion of spinning in opposite direction
– Coriolis illusion: illusion of roll during turning or circling motion
– oculogravic illusion: impression of tilt during a sudden increase of forward speed
Motion
Disorientation from Misperception
(due to brain’s misinterpretation or misclassification of accurate sensory information
– Autokinesis:
fixed light appears to be moving against a dark background
Illusions During Motion
Motion
Motion Sickness
Kinesthetic(body position)
Vestibular(inner eartubes)
Eyes
Cause: incongruities among senses
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