1. Weigh-In Motion (WIM) Detectors
Shreenath Rao
CEE 398 KUC
March 6, 2002

2. Presentation Outline Applications of WIM WIM classification
WIM detector technologies
Bending Plate WIM
Piezo-Electric WIM
Sources of error in WIM data
Summary

3. WIM Definition
“The process of measuring the dynamic tire forces of a moving vehicle and estimating the corresponding tire loads of the static vehicle”
American Society for Testing and Materials (ASTM) Standard Specifications E

4. WIM Applications
Provides reliable and accurate data on the weight and types of vehicle using roads
WIM data can be used for:
Weight enforcement
Data collection ►
Industrial / military

5. Data Collection
“Understanding "why" some pavements perform better than others is key to building and maintaining a cost-effective highway system.”
Long Term Pavement Performance (LTPP)  program — a comprehensive 20-year study of in-service pavements.

6. Broad Classification of WIM
Low speed systems (<15 km/h)
Provide the high level of accuracy needed for enforcement or toll collection
High speed systems (>15 km/h)
Minimize or eliminate vehicle disruption while permitting data of sufficient accuracy for planning and screening

7. Classification of WIM (According to Installation Type)
Permanent
Sensors and the data acquisition systems collect data at the same location
Semi-permanent
Sensors are built into pavement while the data collection system is moved from site to site
Portable
Sensors and equipment are moved from site to site

8. WIM Components Accurate weighing device - weighpads or axlepads
Very high speed A/D converter to change the voltage signal into required data
Software package for data collection and analysis

9. WIM Detector Technologies
Technology
Permanent
Portable
On-Board
Low-Speed
High-Speed
Bending Plate ► ●
Piezo-Electric ►
Load Cell
Capacitive Mats
Pressure Sensor

10. Bending Plate WIM Steel rubber plates w/ strain gauges
Generated signal proportional to the deflection of the plate under a vehicle axle
Signal is amplified and processed to produce the vehicle axle mass 

11. Bending Plate WIM
Typically consists of two in-road weigh platforms in a travel lane
Inductive loop vehicle detectors before and after bending plate scales

12. Bending Plate WIM Installation

13. Bending Plate WIM How Does It Work?

14. Bending Plate WIM How Does It Work?

15. Bending Plate WIM Computer Screen View

16. Bending Plate WIM Other Information
Cost
In-ground:  $15K to $85K 
Portable:   $18K to $22K
Expected Lifetime:  8 to 12 years
Accuracy (individual axles and GVW)
In-ground:  0 to12%  ±1SD  
Portable:  0 to 6%  ±1SD
Vendors
In-ground:  International Road Dynamics, International Traffic Corp., Pat America, Inc.  
Portable:  Pat America, Inc.

17. Piezo-Electric WIM
Piezoelectric materials convert mechanical stress or strain into proportionate electrical energy
these materials mechanically expand or contract when voltages of opposite polarities are applied
Piezoelectric polymer films are also piezoelectric, converting heat into electrical charge

18. Piezo-Electric WIM
Permanent or temporary installation into or onto the road surface
Can be installed directly into the road in a flexible format
Can conform to the profile of the road
Flat construction, small cut

19. Piezo-Electric WIM How Does It Work?

20. Piezo-Electric WIM Other Information
Cost
$3K to $60K  
Accuracy
In-ground:  3 to 30%  ±1 SD 
Portable:  3 to 30%  ±1 SD    
Vendors
In-ground: Cardinal Scale Manufacturing Co., ECM, Inc., International Road Dynamics, International Traffic Corp., Kistler Instrument Corp, Pat America, Inc., Peek Traffic, TDC Systems  
Portable: ECM, Inc., International Road Dynamics, International Traffic Corp., Peek Traffic 

21. Sources of Error in WIM Data
Vehicle dependent
Tire characteristics, suspension systems, lift, acceleration
Environment dependent
Temp. gradients, wind, ice, snow, rain
System dependent
Vertical height, non-uniformity, spring-back delay, compression-repeatability
Roadway dependent
Pavement design, obstructions (potholes, etc.), road contour

22. Summary
WIM detectors are becoming increasingly popular for collecting traffic data at highway speeds
Several technologies of WIM detectors exist to collect accurate and quick traffic data
Selection of WIM depends on application
Good installation and calibration of WIM required for accurate and reliable data