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.

Road applications

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.^[1]

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.

System basics^[2]

Sensors

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.

Charge amplifiers

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

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.

Measurement system

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.

Communications

Variety of communication methods need to be installed on the measurement system. A modem or cellular modem can be provided. If no communication infrastructure exists, WIM systems can be self-operating while saving the data, to later physically retrieve it.

Data archiving

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 variety of monitoring and enforcement purposes.

Rail applications

Weighing in motion is also a common application in rail transport. Known applications are^[3]

• Asset protection (imbalances, overloading)
• Asset management
• Maintenance planning
• Legislation and regulation
• Administration and planning

System basics

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^[4]
• fiber optical sensors: measuring a change of light intensity caused by the bending of the rail^[5]
• 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.

Air applications

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.

References