If remote sensors are to be autonomous and report their data in real time, they must have a reliable power source and a stable Ethernet connection. The United States Geological Survey (USGS), for example, monitors water conditions with a nationwide array of automated sensors, and uses their data to update the USGS Web site with the latest information. Many of these sensors must function in locations where there is no access to the power grid and no Internet infrastructure, and where regular human interaction with the sensor installation would be impractical.
This project integrated a sensor with a solar panel and an NEMA 3R weatherproof enclosure that contained an IP67- rated outdoor radio transmitter with a high-gain antenna and a maintenancefree battery (see figure). The goal was to create a water-monitoring station that could reliably transmit its data 24 hours a day, seven days a week, and up to 40 miles.
The first step was to determine the daily power requirements for each sensor and its attached radio transmitter. A typical sensor using an analog output might use 0.60 Amp hours per day (0.4 W x 24 hours at 12 V). A digital sensor might use 0.72 Amp hours per day. If connected to an IP67-rated outdoor radio transmitter that requires 2.4 Amp hours per day, the total power requirement would be 3.12 Amp hours per day.
There is no need for an onsite generator when the power requirements are that low. A solar panel and attached battery were sufficient. The size and cost of the required solar panel will be a function of the location and its solar insolation, the measure of solar energy received on a given surface area over a specified period of time. In a location like Chicago, for example, with an average yearly solar insolation level of 3.72, a 50W solar panel would provide enough power to operate a transmitter and its associated sensor, while simultaneously charging the attached 12V battery so transmission can continue during the hours of darkness. After calculating the power requirements for each USGS installation, an appropriate solar panel was selected.
Currently, the most commonly used unlicensed frequencies are 2.4 GHz and 900 MHz, although the move to digital television has freed up new bandwidth at lower frequencies. At 2.4 GHz, radio has greater bandwidth, but less range and more interference from other transmitters. Additionally, the higher the radio frequency, the more it will be affected by multipath propagation, a phenomenon that occurs when broadcast radio signals bounce off intervening objects and arrive at the receiver at different times, and out of sequence.
At 900 MHz, radio has greater range, less competition from other devices, and is less susceptible to multipath propagation, but it provides less bandwidth. The successful remote installation involved experimenting with both options to see which one provided the best signal strength in the local environment.
This work was done by B&B Electronics. For more information, Click Here .