University of Minnesota Driven to Discover
U of MNUniversity of Minnesota
Center for Transportation Studies

Battery-less, Wireless Traffic Sensors

Rajesh Rajamani, Professor of Mechanical Engineering, University of Minnesota

Weigh-in-motion (WIM) systems are used to gather data for everything from vehicle load restrictions and tolls to pavement performance. However, current systems have some drawbacks. The sensors can be expensive and the vibration of the truck body over rough roads can interfere with accurate weight measurements.

On Dec. 3, University of Minnesota Professor Rajesh Rajamani discussed a new battery-less, wireless sensor he and his students in the mechanical engineering department have developed to provide real-time traffic flow and weigh-in-motion measurements. The seminar was part of the ITS Institute’s Advanced Transportation Technologies Seminar series.

Weigh-in-motion (WIM) systems are used to measure a vehicle’s weight as it passes over a sensor, which is typically embedded in the pavement. Often, inductive loops are used in addition to a WIM sensor, because they help provide speed information and are less expensive than using a series of WIM sensors in the pavement, Rajamani said.

WIM systems have several advantages: trucks don’t need to exit the road to obtain the data, all vehicles can be weighed, and traffic backups are reduced at highway exits near weigh stations. However, because the systems measure dynamic rather than static load, they often provide less accurate weight data. Current WIM systems are also expensive, costing anywhere from $9,000 to $100,000 per lane to install.

Rajamani said his team’s initial goal was to develop traffic sensors that were battery-less and wireless, harvesting vibration energy to power the devices and the wireless transmission of data.

The team developed a sensor using piezoelectric elements to convert vibration energy into electrical energy. This energy was harvested to send data from multiple sensors to a wireless transceiver located up to 500 feet from the sensors.

The sensors are easy to install and inexpensive, costing about $200 each, compared to $30,000 per sensor in current WIM systems on average. The multiple sensors make it possible to collect additional data such as vehicle count, number of axles, speed, vehicle length, and weigh-in-motion data, and they remove errors due to vehicle suspension vibrations, which is the significant source of error in existing WIM systems, Rajamani said.

A final advantage is that they consume no energy. “With an inductive loop, you need to keep the loop powered all the time, even if it’s the middle of the night because…a car that might come and pass over that loop,” he said.

Rajamani said he and his team ended up with a three-layer design for their sensor. They began with a thin metal beam that allowed them to harvest energy from vehicles as light as a motorcycle but still work reliably with vehicles as heavy as a semi-trailer. For the weigh-in-motion readings, however, they needed less flexible beams that measured strain proportional to weight, so they incorporated thicker beams with less deflection to take heavier loads.

Preliminary data showed that the voltages of the three-beam sensor increased with vehicle weight, but the measurements are not rising in a linear curve with weight.  “The weigh-in-motion part—I would call it a work in progress. We’re still working on getting it to work well,” Rajamani said.

A final part of the research will use a series of three or more sensors in the road to capture the wave from the vibrations as vehicles pass over sensors to extract dynamic and static components of the load, Rajamani said.

The research project, funded by the ITS Institute and the Minnesota Department of Transportation, is expected to wrap up in August 2010.