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

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

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

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

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.

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

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

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

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

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 

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

Bending Plate WIM Installation

Bending Plate WIM How Does It Work?

Bending Plate WIM How Does It Work?

Bending Plate WIM Computer Screen View

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.

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

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

Piezo-Electric WIM How Does It Work?

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 

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

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