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Publication Number:  FHWA-HRT-12-072    Date:  May 2013
Publication Number: FHWA-HRT-12-072
Date: May 2013


Smart Pavement Monitoring System


Researchers studied the feasibility of equipping existing off-the-shelf pressure and humidity sensing systems with RF power-harvesting modules that would alleviate the need for wired power delivery.


A commercial moisture cell was tested in this study. The Dynamax SM200 gauge was selected (see figure 215 and figure 216). The input voltage of the cell was between 5 and 15 V, which allows for powering using a regular 9- or 12-V battery. The output is a voltage signal varying from 0 to 1 V directly proportional to the level of moisture. This format of the output can be integrated with a wireless transmitter. Initial testing was conducted. The main objective was to test the powering scheme of the cell and identify the format of the output. The cell was attached to a 9-V battery, and the output was measured using a commercial multimeter (figure 218). Water was added gradually to a box of sand, and the output voltage was monitored as shown in figure 217. Testing showed that a commercial battery-powered RF module can be used to transmit moisture data.

This photo shows a top view of the Dynamax sensor model (SM)200 moisture gauge. It is a black cylindrical object and is sticking into the sand. There is a cord coming out of the top.
Figure 215. Photo. Overhead view of Dynamax SM200 moisture gauge.

This photo shows a side view of the Dynamax sensor model (SM)200 moisture gauge. It is a cylindrical object with a rounded top. A cord is coming out of it. The bottom is flat and has two very thin sticks coming out of it.
Figure 216 . Photo. Dynamax SM200 moisture gauge.

This graph shows measured output voltage of the moisture cell powered by a 9-V battery. The x-axis shows moisture levels, and the y-axis shows measured voltage. The graph begins at a moisture level of 1 at 0 mV and increases to a moisture level of 12 at 600 mV.
Figure 217. Graph. Measured output voltage of the moisture cell powered by a 9-V battery.

This photo shows a testing setup for a moisture cell. There is a red bucket containing sand, and the moisture gauge is sticking into the sand. A black cord is running for the moisture gauge to an apparatus that is giving a reading of 1,788. The apparatus also has a cord running to a battery that is connected to three other wires.
Figure 218. Photo. Testing setup for the moisture cell.

Testing showed that a commercial battery can be used to power the module and transmit the moisture data, which will eliminate the need for wires. It was also concluded that complete RF powering is not achievable using current commercially available passive modules. The depth at which these gauges are to be embedded causes the RF wave to attenuate to levels below the required minimum power. The non-feasibility of a completely continuous RF powered module will not affect the long-term monitoring performance for the purpose of fatigue prediction. Time snapshots of moisture and subgrade pressure levels can be still acquired and used.