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
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 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.
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
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 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
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