whole body vibration measurements in king county bus drivers

WHOLE BODY VIBRATION
MEASUREMENTS IN KING COUNTY BUS
DRIVERS
James D. Ploger, Rick Neitzel and Peter W. Johnson
University of Washington
Department of Environmental and Occupational Health Sciences
Motivation and Objectives
Background
• Exposure to vibration is a known risk factor for low back injury/pain
• Relative importance of average (TWA) and impulsive exposures not well
understood
Short Term Goals
• Create a portable system to collect continuous vibration exposure and
determine whether there are differences in:
1) Average vs Impulsive vibration exposures/doses
2) Road types
3) Bus vs cars and seat performance
Longer Term Goals
• Evaluate engineering controls to reduce exposures (seats, suspensions, etc)
• Develop methods for the assessment of temporal patterns of exposure
• Evaluate administrative controls to reduce exposures (route assignments,
change/vary exposures, etc)
GPS
Accelerometer
Amplifiers
Loggers
Accelerometers
(Not shown)
PDA with
Data Acquisition
Card
TWA
Interface box
Signal
Splitter
Data Collection System
Data Collection System
• Hardware
• PDA instrumented with a 16 channel, 16-bit NI,
data acquisition card
• Runs LabVIEW PDA data acquisition software
and samples each of the five channels 640 Hz
(4000 samples/sec theoretical maximum)
• 2GB of memory with SD Flash card (88 hour capacity)
• Serial acquisition of GPS once every second
(vehicle location and speed)
• Software
• Easy to use graphical interface, GPS data a powerful
complement to interpret WBV data
• Complete flexibility to chose time windows for analysis
• Multiple vibration values and doses from ISO 2631- 1
and 2631-5, implemented and calculated in Labview
College
Study Design
Freeway
Speed
Humps
• 15 bus drivers , 6 light, 6 medium, 3 heavy
• Drove 40’ (13.3 m) unloaded King County Metro
coach bus with USSC seat (fixed spring, height adj.)
Surface
Streets
• Drove a 40 mile (65 km), 1 hour test route that
included city streets, freeways and speed humps
• Subset of 5 drivers drove a car over the same route
• Compared
Metro Base
-
Road types
Floor vs Seat WBV (Seat performance)
Bus vs car (exposure relative to driving)
Average TWA (ISO WBV Std 2631- Part 1 ) versus
impulsive exposures (WBV Std 2631- Part 5 )
Background
Max Raw Peak
Accleration Amplitude (m/s2 )
1
Time
Raw Signal
2
Rectified Signal
3
Time
Max Wt Peak (Instantaneous)
RMS Signal (Exponential Averaged)
aw
Aeq rms
Weighted Instantaneous and Averaged Signal
Background – ISO 2631-1
Background – EU Directive
Background – ISO 2631-5
Impulses
Example Continuous z-axis seat and floor accelerations
(Gross - 15 minutes)
20
Street
0
Seat
Floor
Bus Stopped at stoplight
-25
Freeway
Speed
humps
(3 min)
Acceleration (m/s2 )
0
900
450
20
0
Expansion Joints
-25
0
900
450
20
Speed Humps-Slow
Speed Humps-Slow
0
Speed Humps-Fast
-25
0
100
Time (s)
200
Results – Road Types
Interpretation
Average TWA
ISO 2631-1
0.5 = Action limit
1.15=Exposure limit
Above 9 interpret
Aeq with caution
9.1 = Action limit
Impulsive
ISO 2631-5
21 = Exposure limit
< 0.5 low, >0.6 high
Highest values for x, y and z -axis
Impulsive exposures likely – interpret Aeq with caution
Potential moderate to high hazard
Significant differences between road types
WBV Results – Routes
Significant differences between routes
WBV Results – Roads
Significant differences between roads of the same type
WBV Results – Road Types
0.53 m/s2 Freeway
0.45 m/s2 Street
0.44 m/s2 Humps
Freeway exposures slightly above the 8 hr action limit
WBV Results
Bus vs Car – Seat vs Floor
Floor
2.00
Relative to Bus Floor
1.75
1.50
Aw z
(rms m/s2 )
VDV z
(rms m/s1.75)
D zd
(rms m/s2)
‡
Max Wt
Peak z
(rms m/s2)
Max Raw
Peak z
(m/s2)
†‡
*
17.5
†‡
0.57
1.25
Seat
†‡
4.9
4.4
1.00
†‡
*5.7
*
0.48
0.45
13.4
14.9*
5.3
*
3.4
0.75
2.6
0.25
0.50
6.2 6.4
3.5
2.9
9.2
*
7.6
3.8
0.25
0.00
Bus Car
Bus Car
Bus Car
Bus Car
Bus Car
Cars had significantly less exposures, Bus seats amplified exposures
* Sig diff btw floor and seat
† Sig diff bus and car floor
‡ Sig diff btw bus and car seat
WBV Results – Driver Weight
n=6
n=6
Light (140 – 155 lbs)
n=3
Medium (171 – 215 lbs)
Heavy (245 – 323 lbs)
1.4
p = 0.02
p = 0.26
p = 0.64
p = 0.33
p = 0.73
6.2
9.6 9.1
Relative to Light Drivers
1.2
1.0
0.51
0.47
0.45
5.2
4.8
18.7
4.8
16.7
16.9
5.7
8.5
4.9
0.8
0.6
0.4
0.2
0.0
Aw z
(rms m/s2 )
VDV z
(rms m/s1.75)
D zd
(rms m/s2)
Max Wt
Peak z
(rms m/s2)
Light drivers tended to have greater exposures
Max Raw
Peak z
(m/s2)
Resonant Frequencies
of Body Regions
Example Data
FFT Power Spectral Density
0.5
Street
(15 min)
Freeway
(15 min)
Speed
Humps
(3 min)
Power Spectral Density (m2 s-4 Hz-1)
15
Seat
Floor
10
0.25
5
0
0.0
15
0.5
Seat
Floor
10
0.25
5
0
0.0
15
0.5
10
0.25
5
0
0
0.0
5
Frequency (Hz)
10
15
Conclusions
• Hardware and software system provides a compact and flexible
and platform to evaluate both TWA and impulsive exposures
• Test route is very sensitive for detecting small differences
in exposure
• GPS has great value to assess components of the exposure
(where the exposure occurred, bus speed, etc)
• System could lead to the develop and evaluation of
administrative and/or engineering controls
- Identify and trigger the need for street repair
- Differences between bus types
- Differences between seats
- Effects of suspension maintenance/life
Potential Future Work
• Obtain vibration profiles for various routes,
attempt to distribute/limit driver exposures
- Seat pad dosimeter, complete
self contained system/logger
to collect WBV exposures
• Evaluate seating alternatives
- Suspension types (fixed vs variable)
- Manufacturers
- Active dampening
Acknowledgements
• Department of Environmental and
Occupational Health Sciences
• Washington State Medical Aid and
Accident Fund
• Sue Stewart
• Terry Compton
• Tim Drangsholt
• Metro Employees
• Amalgamated Transit Union Local 587