Weigh in motion

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:

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 passes by. Force sensors with quartz crystals are the most rigid and measure only in one direction along the vertical axis. When a force is applied to the top surface of the sensor, quartz crystals produce an electric charge proportional to the applied force. The signal is a very high impedance electric charge, which is not susceptible to electrical interference.

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. 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

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Wikipedia