Just what is a passive wireless sensor? Passive wireless sensors (PWS) have no battery, no expensive electronics at the sensor site and, being wireless, no need for a wired connection between the sensor and the data acquisition system. One form of passive wireless sensor, Surface Acoustic Wave (SAW)-based sensors, which are similar to RFID (Radio Frequency Identification) tags, respond to a wireless interrogation signal from a reader. Unlike RFID, SAW sensors provide real-time sensor data along with a unique tag ID, stored information and range.
Like any sensor, it needs to have power as well as the ability to communicate with the associated host controller. As can be expected, the position and placement of the passive RFID tag has a significant impact on performance, both in terms of energy harvesting from the RF signal and communication reliability. Fortunately, there are multiple ways to connect sensors and processes, including the use of magnetic based passive sensing, coupled with techniques to cancel out undesired signals due to locally induced eddy currents. This allows for passive sensing through metal barriers of significant thickness (as well as water) making it possible to passively sense pressure, temperature, strain and other parameters through metal barriers. Aiding in the ability to communicate through these traditional barriers is a “self-tuning” technology that allows the tag to automatically and precisely adapt to the correct geographical frequency and compensate for its surrounding interference (i.e. the item to which it is attached). This same characteristic of adjusting to the environment can also potentially be used for sensing as well.
One additional way that passive wireless sensors are beginning to be used for connecting to the rest of the world is near field communication (NFC), a low-power wireless technology that is being incorporated into all major brands of cell phones. NFC presents significant advantages over Bluetooth and other sensors that require a battery. For example, combining NFC with PWS technology makes it possible to develop ultrathin diagnostic skin patches and a variety of printable sensors for food safety, smart labels and even advanced medical diagnostic sensors that communicate either with or through your smart phone.
NFC health-care sensors could allow the hospital to continue monitoring discharged patients even after they leave hospital. Illness onset could be detected much more rapidly, thus minimizing the required effort to treat something more advanced.
Building automation is another high-volume potential market and, as part of the Smart Grid initiative, industry has been asked to respond to the low-cost wireless meter challenge and produce a cost-effective, wireless metering system capable of electrical energy measurement at various locations in a building, and then use wireless communications to a remote data collection point within the building complex to control the facility and integrate into the larger grid.
Temperature sensors are an obvious building automation measurement and with passive surface acoustic wave (SAW) temperature sensors, it is possible to build structures the size of a postage stamp.
More complex measurements are also possible, including selective field detection of diverse gaseous analyte species by combining several multivariable sensors to boost selectivity of individual sensors against chemical interferences (e.g. background vapours) and physical interferences (e.g. temperature), to reduce or eliminate sensor aging effects and to bring these sensors to demanding applications outside laboratory conditions. Current performance characteristics of developed sensors include ppm-ppb-sub-ppb detection sensitivity, rejection of high levels of interferences, and quantitation of individual vapours in their mixtures. A simple moisture-humidity sensor is commercially available from more than one supplier.
In the harsher environment of building structures, roads and highways, and aircraft components, including the turbines, development of electro-mechanical switches on flexible substrates lead to passive wireless stickers that can monitor the shape of deployable structures in aerospace applications. These bi-stable devices hold their state without power, so they can report on the shape of a structure even if polling rates are slow because of energy harvesting system constraints.
Finally, as a result of their ability to communicate in harsh environments, these sensors are suited to applications such as high pressure high temperature (HPHT) oil and gas sensing through pipe walls, sensing temperature through sealed containers, such as nuclear waste storage vessels for long term monitoring or cargo shipping containers for tamper detection.
With its considerable potential read-range (separation distance between reader and device), compatibility with extreme environments, small size, autonomy of sensor installation and “no onboard power” capabilities, passive wireless sensors have a wider application arena than traditional wireless sensors and I am confident we will soon see passive wireless technology in the automotive, aeronautical and very high heat metallurgy and manufacturing markets as well as the traditional less severe environments such as building automation.
Passive Wireless Workshop presentations for the past three years can be found at www.isa.org/commdiv and then navigate to the appropriate PWSW tab.
This column originally appeared in the September 2013 issue of Manufacturing AUTOMATION.