Weigh in motion
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Weigh-in-motion or weighing-in-motion (WIM) devices are designed to capture and record the axle weights and gross vehicle weights as vehicles drive over a measurement site. Unlike static scales, WIM systems are capable of measuring vehicles traveling at a reduced or normal traffic speed and do not require the vehicle to come to a stop. This makes the weighing process more efficient, and, in the case of commercial vehicles, allows for trucks under the weight limit to bypass static scales or inspection.
- 1 Accuracy of WIM and Quality of Data
- 2 Road applications
- 3 Types of Weigh in Motion System
- 4 System Basics of Most Systems
- 5 Rail applications
- 6 Air applications
- 7 References
- 8 External links
Accuracy of WIM and Quality of Data
The accuracy of weigh-in-motion data is generally much less than for static weigh scales where the environment is better controlled. The European COST 323 group developed an accuracy classification framework in the 1990's. They also coordinated three independently controlled road tests of commercially available and prototype WIM systems, one in Switzerland, one in France (Continental Motorway Test) and one in Northern Sweden (Cold Environment Test). Better accuracy can be achieved with multiple-sensor WIM systems and careful compensation for the effects of temperature. The Federal Highway Administration in the United States have published Quality Assurance criteria for WIM systems whose data is included in the Long Term Pavement Performance project.
Especially for trucks, gross vehicle and axle weight monitoring is useful in an array of applications including:
- Pavement design, monitoring, and research
- Bridge design, monitoring, and research
- To inform weight overload enforcement policies and to directly facilitate enforcement
- Planning and freight movement studies
- Toll by weight
- Data to facilitate legislation and regulation
The most common road application of WIM data is probably pavement design and assessment. In the United States, a histogram of WIM data is used for this purpose. In the absence of WIM data, default histograms are available. Pavements are damaged through a mechanistic-empirical fatigue process that is commonly simplified as the fourth power law. In its original form, the fourth power law states that the rate of pavement damage is proportional to axle weight raised to the fourth power. WIM data provides information on the numbers of axles in each significant weight category which allows these kinds of calculations to be carried out.
Weigh in motion scales are often used to facilitate weight overload enforcement, such as the Federal Motor Carrier Safety Administration's Commercial Vehicle Information Systems and Networks program. Weigh-in-motion systems can be used as part of traditional roadside inspection stations, or as part of virtual inspection stations. In most countries, WIM systems are not considered sufficiently accurate for direct enforcement of overloaded vehicles but this may change in the future.
The most common bridge application of WIM is the assessment of traffic loading. The intensity of traffic on a bridge varies greatly as some roads are much busier than others. For bridges that have deteriorated, this is important as a less heavily trafficked bridge is safer and more heavily trafficked bridges should be prioritized for maintenance and repair. A great deal of research has been carried out on the subject of traffic loading on bridges, both short-span, including an allowance for dynamics, and long-span.
Recent years have seen the rise of several "specialty" Weigh-in-Motion systems. One popular example is the front fork garbage truck scale. In this application, a container is weighed—while it is full—as the driver lifts, and again—while it is empty—as the container is returned to the ground. The difference between the full and empty weights is equal to the weight of the contents.
Types of Weigh in Motion System
One of the earliest WIM systems, still used in a minority of installations, uses an instrumented existing bridge as the weighing platform. Bending plates span a void cut into the pavement and use the flexure as the wheel passes over as a measure of weight. Load cells use strain sensors in the corner supports of a large platform embedded in the road. The majority of systems today are strip sensors - pressure sensitive materials installed in a 2 to 3 cm groove cut into the road pavement. In strip sensors, various sensing materials are used, including piezo-polymer, piezo-ceramic, capacitive and piezo-quartz. Many of these sensing systems are temperature-dependent and algorithms are used to correct for this.
Bridge Weigh in Motion
The concept of Bridge WIM was first proposed by Moses in the United States. It fell into disuse but re-emerged in Europe in the 1990's. The disadvantage of Bridge WIM is that it requires a bridge to be present at the location of interest. An advantage is that it is portable - the same system can be moved between bridges in a matter of hours. The concept of Bridge WIM is that the bridge flexes under the weight of the passing truck. Truck axle weights are found by minimizing the sum of squared differences between the theoretical and measured responses. Strain transducers are generally used to measure the bridge response but other responses are possible including deflection. Users of Bridge WIM claim similar levels of accuracy to the best of the other WIM technologies though there are few tests that provide direct comparisons. Since it was first developed, many innovations have been proposed to improve accuracy. One of the more complex is a dynamic version known as Moving Force Identification though this poses practical challenges for calibration. One of the more significant other innovations is the development of Nothing-On-Road (NOR) or Free-of-Axle-detector (FAD) systems which allow installation to take place without access to the road surface.
System Basics of Most Systems
WIM systems can employ various types of sensors for measurement. Strain transducers are used in Bridge WIM systems. Strain gauges are used to measure the flexure in bending plates and the deformation in load cells. The strip sensor systems use piezo-electric materials in the groove. Finally, capacitive systems measure the capacitance between two closely placed charged plates.
High impedance charge signals are amplified with MOSFET based charge amplifiers and converted to a voltage output, which is connected to analysis system.
Inductive loops define the vehicle entry and exit from the WIM station. These signals are used as triggering inputs to start and stop the measurement to initiate totaling gross vehicle weight of each vehicle. They also measure total vehicle length and help with vehicle classification. For toll gate or low speed applications, inductive loops may be replaced by other types of vehicle sensors such as light curtains, axle sensors or piezocables.
The high speed measurement system is programmed to perform calculations of the following parameters:
Axle distances, Individual axle weights, Gross Vehicle Weight, Vehicle Speed, Distance between vehicles, and the GPS synchronized time stamp for each vehicle measurement.
The measurement system should be environmentally protected, should have a wide operating temperature range and withstand condensation.
Variety of communication methods need to be installed on the measurement system. A modem or cellular modem can be provided. In older installations or where no communication infrastructure exists, WIM systems can be self-operating while saving the data, to later physically retrieve it.
A WIM system connected with any available communication means can be connected to a central monitoring server. Automatic data archiving software is required to retrieve the data from many remote WIM stations to be available for any further processing. A central database can be built to link many WIMs to a server for a variety of monitoring and enforcement purposes.
Weighing in motion is also a common application in rail transport. Known applications are
- Asset protection (imbalances, overloading)
- Asset management
- Maintenance planning
- Legislation and regulation
- Administration and planning
There are two main parts to the measurement system: the track-side component, which contains hardware for communication, power, computation, and data acquisition, and the rail-mounted component, which consists of sensors and cabling. Known sensor principles include:
- strain gauges: measuring the strain usually in the hub of the rail
- fiber optical sensors: measuring a change of light intensity caused by the bending of the rail
- load cells: Measuring the strain change in the load cell rather than directly on the rail itself.
- laser based systems: measuring the displacement of the rail
Yards and main line
Trains are weighed, either on the main line or at yards. Weighing in Motion systems installed on the main lines measure the complete weight (distribution) of the trains as they pass by at the designated line speed. Weighing in motion on the mainline is therefore also referred to as "coupled-in-motion weighing": all of the railcars are coupled. Weighing in motion at yards often measure individual wagons. It requires that the railcar are uncoupled on both ends in order to weigh. Weighing in motion at yards is therefore also referred to as "uncoupled-in-motion weighing". Systems installed at yards usually works at lower speeds and are capable of higher accuracies.
Some airports use airplane weighing, whereby the plane taxis across the scale bed, and its weight is measured. The weight may then be used to correlate with the pilot's log entry, to ensure there is just enough fuel, with a little margin for safety. This has been used for some time to conserve jet fuel.
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