Wireless SAW Sensing
SAW technology has very successfully been used in the telecommunications industry to build frequency controlled oscillators and band pass filters. This same technology also has the capability of providing wireless and sensing capabilities. SAW sensing elements can also be powered without an external power supply. When connected to an antenna, they can be powered by wirelessly harnessing energy from electromagnetic bursts from a near by transmitter, very much like RFID. The transduced signals from the sensing element can then be transmitted to a receiver through the same antenna thereby providing an effective wireless and energy-passive sensing mechanism.

SAW Pressure Sensing
The figure below shows a SAW wireless pressure sensor. The sensing element is powered by an electrical pulse using the Input IDTs (inter-digital transducers; the comb-like pattern of metal on the device that converts the electric field energy to mechanical wave energy and then back to an electric field). This pulse, as mentioned earlier, can be obtained from an electromagnetic burst funneled into the IDTs via an antenna. The electrical pulse at the Input IDTs induces a surface wave in the sensing element as shown in the figure below. The pressure that the sensing element is exposed to influences the frequency of this wave and the resulting electromagnetic burst that is created by the Output IDTs that is funneled through the same antenna. A simple transceiver, which emits an electromagnetic pulse to power the sensor and then waits to receive the transduced signal from the sensor provides for an effective wireless pressure sensing mechanism.

Figure-1: Wireless SAW Pressure Sensor

Wireless sensors are typically targeted to work within the Industrial, Scientific and Medical (ISM) radio bands. These bands include the 433.05-434.79 MHz range, the 902-928 MHz range and the 2.40 - 2.50 GHz range. Radio-frequency transceivers that operate within these bands interrogate wireless SAW temperature sensors by emitting an electromagnetic burst and then "listening" for a response from the sensors. The frequency of the returned radio-signal is used to determine the temperature measured by the sensors. The sensors, depending on the application, can be calibrated to provide a wide temperature span and very high resolutions. Interrogation distances, i.e. the range within which the sensors can operate, typically range between 1.5 to 3.0 meters. The interrogation distance can significantly be increased by optimizing the designs of the antennae used both at the transceiver end and the sensor end.

Typical and Potential Applications
A key application of the wireless SAW pressure sensor is in tire pressure monitoring systems (TPMS). The United States government recently passed legislation that requires from September 2007 all new passenger vehicles and light trucks under 10,000 pounds of gross vehicle weight to be equipped with tire pressure monitoring systems. The ability of vehicles, especially critical transport vehicles, to carry the specified load is a direct function of maintaining correct tire pressure. The correct pressure results in the avoidance of catastrophic failure and allows for increased fuel economy and tire life.

Further, there is a significant market opportunity in the commercial, agricultural and construction vehicle markets, where continuous monitoring of tire pressure offers opportunities for operating cost reductions and elimination of expensive manual maintenance programs. Industrial applications for wireless pressure sensors include HVAC industries, medical applications, semiconductor fabrication plants and process control requirements.

What SenGenuity Offers Today
SenGenuity is actively pursuing a TPMS solution in conjunction with Transense Technologies, a leading technology transfer company. In addition, SenGenuity is also developing wireless pressure sensors for industrial applications. Should you have any questions about these efforts or would like to obtain more information, please contact Kerem Durdag, Director of Sales and Marketing at 1-603-578-4066 or e-mail at sensors@sengenuity.com.