In order to simplify their installation and operation, modern home security systems must be able to monitor their environments wirelessly. This includes monitoring perimeter security, intrusion detection, as well as the security of sensitive areas within the home, such as a medication containers, safes, or other places where valuables are hidden.
While video monitoring is possible, it requires sophisticated image processing and analysis to detect unauthorized activities, and also raises concerns about privacy. Conventional motion sensors eliminate the privacy issue but they require a power supply and support electronics, which add to the product’s BOM cost. Motion detectors also require some sort of wired or wireless interface to relay the sensor’s output to the cloud. In many cases, cameras and motion detectors require professional installation, making them uneconomical for many cost-sensitive applications.
This article presents an alternative solution more suitable to the needs of many consumers and small businesses: the use of RFID tag technology to enable low-cost, battery-free, and easy-to-install home security systems.
RFID tags can be used as the foundation for a series of robust security sensors. A motion detector can be made by integrating the RFID tag with a reed switch. Likewise, a shock event detector can be fabricated by substituting the reed switch for a shock switch. In both instances, the sensor switch is connected between the two terminals of the antenna, and the two terminals of the RFID chip as well. In this configuration, the RFID tag’s operation is enabled or disabled according to the ON or OFF state of the switch.
The sensors provide a low-cost, compact solution for the security monitoring system shown in Figure 1. RFID tags equipped with motion sensors can be attached to the window, the door and the safe, while a shock sensing tag can be attached to the medication box. A commercial RFID reader serves two functions: a) it detects any sensor that was brought to its active state by its motion or shock sensing switch b) the reader also floods the area with low-power RF radiation that powers the tags when they are activated. Since each RFID tag has its own unique ID, the RFID reader knows where the detected activity occurs. Once captured, the activity data can be transmitted to the cloud for analysis and, when appropriate, notification of the user via smartphone or an email alert.
Figure 1. Security monitoring using RFID tag.
Sensor working principles
The functional diagrams in Figure 2 illustrate the working mechanisms of the shock sensor and motion sensor.
Figure 2. A shock sensor is comprised of an RFID chip, an antenna, and a shock sensor (a). In its normal (undisturbed) state, the metal ball shorts the two contacts of the shock sensor (b). When the sensor is disturbed, its metal ball no longer shorts the two contacts, allowing the antenna to receive power for the RFID chip, and to transmit its signal (c). A motion sensor is composed of an RFID chip, an antenna, and a reed switch (d). In its normal state, the reed switch is held in its ON state by a magnet affixed nearby. This shorts the antenna, preventing the RFID chip from receiving power (e). When the magnet is removed, the reed switch transitions to its OFF state, thereby enabling the RFID device (f).
As stated earlier, the shock sensor consists of an RFID chip and an antenna, as shown in Figure 2a. The shock switch composes of a cavity and a tiny metal ball that rolls around inside. When the shock switch is placed upright, the ball rolls onto the two conductive contacts sticking out of the cavity, creating a conductive path (Figure 2b). If the sensor tilts towards another direction, the contacts are disconnected, making a simple means for detecting motion or orientation. When used as a security sensor, the shock switch is connected between the two terminals of the antenna and placed upright, so that shorts them in its passive position. The resulting short breaks the impedance matching between the RFID chip and the antenna, preventing the RFID tag from radiating a signal. If the shock sensor is tilted, the metal ball moves off the contacts, enabling the RFID tag to receive power and to radiate a signal, which is then read by RFID reader.
The motion sensor system shown in Figure 2d consists of an RFID chip, an antenna, a reed switch, and a magnet. A reed switch is comprised of two ferromagnetic flexible metal reed contacts in a sealed glass cavity. The switch’s two contacts are normally open, until the presence of a magnetic field causes them to close (Figure 2e). When the senor is moved out of proximity of the magnet, its contacts return to the original open condition as in Figure 2f, thereby enabling the RFID chip and alerting the reader that the tag has been moved. This is illustrated in Figure 1 where, for example, the RFID tag with the reed switch is affixed on the window, and the magnet is attached on the wall near the RFID tag. When the window is closed, the magnet keeps the reed switch in a closed position thereby shunting the antenna and keeping the chip inactive. When the window is moved up or down, the magnet is no longer close enough to keep the reed switch closed. At this point, the reed switch reverts to its open position and the RFID tag becomes active.
Dumb sensors, smart system
Since each RFID tag has its own unique ID, it can be correlated to the specific object or location being monitored. This information can be used by the RFID reader or the cloud-based security application to understand more about the nature of the event, including identifying the number of possible intruders, where they are located, and making some inferences about their activities. Such analysis may also reduce the number of false alarms caused by pets or other benevolent disturbances.
RFID technology is a promising tool for creating robust, reliable, and low-cost security systems. The tags are easy to install and do not require batteries, ensuring their long-term reliability. In real-world applications, however, some care must be paid to ensuring that the RFID readers are positioned to provide complete coverage of the area under surveillance. Commercialization will also require development of “smart” RFID readers that can perform at least the first layer of processing of the activity produced by the RFID tags.
>> This article was originally published on our sister site, EDN.
|Wei Wang, Cihan Asci, and Sameer Sonkusale are with the Department of Electrical and Computer Engineering at Tufts University in Medford, MA. More information about their research activities can be found here.|