WIM

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.

System basics:

Sensors

WIM systems can employ various types of sensors for measurement. The most important quantity to measure is the vertical force 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.

Charge amplifiers

High impedance charge signals are amplified with MOSFET based charge amplifier and converted to 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 application, inductive loops may be replaced with other types of vehicle sensors such as light curtains, axle sensors or piezocables.

Measurement system

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, and 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.

Communications

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.

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 WIM to a server for variety of monitoring and enforcement purposes.