Weigh-In Motion (WIM) Weigh-In Motion (WIM) DetectorsDetectors

Shreenath Rao

CEE 398 KUC

March 6, 2002

Presentation OutlinePresentation Outline

Applications of WIM WIM classification WIM detector technologies

Bending Plate WIM Piezo-Electric WIM

Sources of error in WIM data Summary

WIM DefinitionWIM 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

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

WIM ApplicationsWIM Applications

Data CollectionData 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.

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

Broad Classification of WIMBroad Classification of WIM

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

Classification of WIMClassification of WIM(According to Installation Type)(According to Installation Type)

WIM ComponentsWIM 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 TechnologiesWIM Detector Technologies

TechnologyPermanent Portable

On-BoardLow-

SpeedHigh-Speed

Low-Speed

High-Speed

Bending Plate ► ● ● ●

Piezo-Electric ► ● ●

Load Cell ● ● ●

Capacitive Mats ● ●

Pressure Sensor ●

Bending Plate WIMBending 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 WIMBending 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 WIMBending Plate WIMInstallationInstallation

Bending Plate WIMBending Plate WIMHow Does It Work?How Does It Work?

Bending Plate WIMBending Plate WIMHow Does It Work?How Does It Work?

Bending Plate WIMBending Plate WIMComputer Screen ViewComputer Screen View

Bending Plate WIMBending Plate WIMOther InformationOther 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 WIMPiezo-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

Piezo-Electric WIMPiezo-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 WIMPiezo-Electric WIMHow Does It Work?How Does It Work?

Piezo-Electric WIMPiezo-Electric WIMOther InformationOther 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 DataSources 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

SummarySummary

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