Many methods have been developed for interrogation of wireless passive sensors. Surface acoustic wave (SAW) sensors and RF reflection sensors can receive and reflect electronic magnetic waves that are broadcast and received by the antennas. The interrogation distance can range from several meters to tens of meters. These previously developed technologies have limitations. The signal frequency is very high (usually at GHz level), which increases the difficulties in signal processing and interrogation system development, and the interrogation distance is limited by the power attenuation in the space. Longer interrogation distance requires higher-power-density electromagnetic (EM) waves in signal broadcasting, which increases the EMI hazard to environments.
This innovation addresses wireless passive sensor interrogation and sensor network technology. More specifically, it is a new method to power and interrogate wireless passive sensors using a ubiquitous conductor in the sensing environments. The ubiquitous conductor refers to the conductive media available in the sensing environments, such as power cable and metal frame. The new method uses the environment-available conductor as the antenna to power and interrogate wireless passive sensors. This innovation leads to a breakthrough in sensor network technology.
The monopole antenna method has many advantages over the traditional loop antenna method:
- The monopole antenna length can be much greater than the sensor size. That means a very small sensor can be interrogated by a very long antenna.
- The monopole antenna requires no closed-loop circuit as the loop antenna. It has much more flexibility in antenna and sensor system configuration.
- One monopole antenna can interrogate a number of sensors along the antenna. Each sensor has its own unique resonant frequency. The interrogation system uses the signal having different frequencies to interrogate different sensors.
The method uses near-field coupling to power and interrogate sensors; it does not rely on broadcasting EM waves. Therefore, there is no critical limitation on the antenna geometric parameters, such as length and shape.
Using a ubiquitous conductor to power and interrogate wireless passive sensors is a breakthrough technology for building a sensor system and sensor network. The sensor system can be built upon environment-available conductors, eliminating the wiring process for power supply and signal transmission. The sensor system and sensor network can achieve a larger distance than previous wireless sensor network solutions using EM-wave broadcasting, but with reduced cost, complexity, size, and power consumption. The wireless signal is limited in the small distance of the near-field coupling. The method has the minimum EMI influence to the sensing environments because there is no strong EM-wave broadcasting in the space. The method requires minimum change to the sensing environments. The sensor nodes are wirelessly coupled to the conductor media, so no additional wires, cables, antennas, or connectors are needed. The system can be applied without any mechanical change to the sensing environments.
Using this method, the sensor system and sensor network are extremely easy and simple to install and use. Building a sensor system and sensor network is as easy as attaching the sensor nodes (LC resonant tags) on the interested position near the conductor media, and connecting an interrogator unit to the conductor network.