Weigh in motion
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Weigh-in-motion or weighing in motion (WIM) devices are designed to capture and record 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.
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
- Size and weight enforcement
- Legislation and regulation
- Administration and planning
Weigh in motion scales are often used for size and weight 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.
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.
WIM systems can employ various types of sensors for measurement. The most important quantity to measure is the vertical force (z component) without any influence of forces in other directions or speed of the vehicle that pass by. Force sensors with quartz crystals are most rigid and measures only one direction along the vertical axis. When the force is applied on the top surface of the sensor, quartz crystals produce electric charge proportional to the applied force. The signal is very high impedance electric charge, which is not susceptible to electrical interference.
High impedance charge signals are amplified with MOSFET based charge amplifier and converted to 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. For toll gate or low speed application, inductive loops may be replaced with other types of vehicle sensors such as light curtains, axle sensors or piezocables.
Highly dynamic measurement system is programmed to perform the calculations of the following parameters:
Axle distances Individual axle weights Gross Vehicle Weight Vehicle Speed Distance between vehicles GPS synchronized time stamp of measurement for each vehicle
The measurement system should be environmentally protected and should have wide operating temperature and withstand condensation.
Variety of communication methods need to be installed on the measurement system. Modem or Cellular Modem can be provided. If no communication infrastructure exists, WIM system 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 build to link many WIM to a server for variety of monitoring and enforcement purposes.
Weighing in motion is also a common application in rail transport. Known applications are
- Asset protection (imbalances, over loading)
- 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.
Also, the main difference in these platforms, which are basically a "transmission of weight" application, there are checkweighers, also known as dynamic scales or in-motion scales.
- "Expanded CVISN capabilities". Federal Motor Carrier Safety Administration. Retrieved Feb 8, 2012.
- WIM on road traffic use. "Weigh in Motion Systems"
- Buurman, Gerlof and Zoeteman, Arjen. "A vital instrument in asset management", Europen Railway Review, Issue 3, 23 August 2005.
- Gotcha Monitoring Systems "Longer life for track and rollingstock", EurailMag, Issue 22, September 2010.