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Weigh-In Motion (WIM) Detectors

Weigh-In Motion (WIM) Detectors

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Weigh-In Motion (WIM) Detectors

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  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 1318-94

  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

  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 WIMInstallation

  13. Bending Plate WIMHow Does It Work?

  14. Bending Plate WIMHow Does It Work?

  15. Bending Plate WIMComputer Screen View

  16. Bending Plate WIMOther 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 intoproportionate 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 WIMHow Does It Work?

  20. Piezo-Electric WIMOther 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