On board weighing (OBW) systems are fitted to vehicles, rather than to infrastructure, and enable the weight data to be communicated at any time from a moving vehicle.
Components of an on-board weighing solution
An OBW solution consists of several components that work together to form a complete operational system: the sensors, the interconnecting internal harness (e.g. cables) and the “box”:
1. The three main types of sensors are load cell (esp. in steel-sprung suspensions), air pressure transducer (APT) (esp. in air bag suspensions) and strain gauge. Additionally accelerometers and displacement sensors may be used.
2. The installed sensors are connected via cables to transfer the raw data to the vehicle unit.
3. The “box” is the vehicle unit, including data processing units and communication interfaces, such as the HMI for the driver, an interface to a printer, etc.
4. Localization and communication modules might also be part of the box, such as GNSS, GPRS and DSRC. However, interfaces to these modules elsewhere in the vehicle might also be possible.
Following product development activity in the interconnecting internal harness, some current solutions are no longer hard wired but transmit data via Bluetooth or other wireless technology.
Static and dynamic systems and accuracy
Static OBW systems have been used in the trucking industry for many years. They weigh the vehicle when it is stationary, e.g. at parking lots, rest area or red traffic lights. The main objective is to optimise truck fleet management and routing with respect to their capacity and load limits.
The technology has improved and currently also dynamic OBW systems that weigh the vehicle when it is in motion are available on the market. In a dynamic system the weight values are monitored continuously with a certain sampling frequency and the “real” weight value is based on complex algorithms. In principle, a dynamic systems could be build out of any sensor type, and may include, for example, components that can control and balance the loads per drive axle while driving. This may result in longer tires life, smooth riding and roll stability, and better braking capabilities.
Static OBW is currently the most reliable system. Static data readings provided by OBW systems are required to provide an accuracy of 2% within 95% of readings, or an error of ±500 kg. To ensure accuracy of these readings, OBW systems are periodically (at least twice a year) calibrated at weigh stations, where a calibration certificate is completed.
Use and cost of OBW systems
OBW could contribute to improved weight compliance in road transport in various ways. Four basically different approaches are conceivable:
1. In-vehicle weight display for drivers
2. Voluntary active demonstration of weight compliance
3. Mandatory on-board weighing for pre-screening
4. Mandatory on-board weighing for direct enforcement
The road transport sector in Australian has taken a proactive approach by adopting OBW systems for compliance management purposes, leading to improved productivity, efficiency and safety outcomes. The use of OBW systems are being linked to Australia’s Intelligent Access Program (IAP), which provides a 21st century approach to manage heavy vehicle access and compliance to the road network.
OBW solutions appear to have similarities to the Digital Tachograph, not only regarding technical aspects, e.g. when equipped with a DSRC interface for external readouts by enforcement officers, but also regarding the total process including organisational and political aspects. Actual weight information should be visible to the driver allowing to take appropriate actions.
OBW systems for compliance monitoring should be installed and maintained only in trusted workshops. Present WIM stations could also be used to verify the weight detected by the OBW system. This redundancy would certainly increase the significance of the calibration methods. If the values are diverging, this is a good indication that either the calibration is outdated or the OBW system has been manipulated.
Currently OBW systems operating in static cost below EUR 1,000 per truck and they are available for all vehicle types. In cased such systems would become mandatory and would be fitted in-line during production for all models, hardware costs may even come down to around EUR 500. OBW dynamic systems cost from EUR 5,000 to 13,000.