National Aeronautics and Space Administration
Small Business Innovation Research & Technology Transfer 2003 Program Solicitations

TOPIC T6 Kennedy Space Center

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T6.01 Batteryless, Wireless Remote Sensors
T6.02 Development of High Temperature Acoustic Liners

T6.01 Batteryless, Wireless Remote Sensors

Proposals are solicited for development of innovative batteryless, wireless, sensor technologies in support of future space operations. Rapid turnaround, coupled with high safety standards, will necessitate that future launch vehicles be densely instrumented to provide high fidelity health information. This need is well recognized and advances in micromachined and wireless sensors are underway to meet future sensing goals. However, many of these new sensors still require significant on-board power, while batteries, for reasons of weight, longevity, and maintenance, are often not a feasible option in aerospace applications. Also, in many applications, no line of site is available to the sensors, limiting the interrogation to radio frequency (RF) signals.

Therefore, new classes of sensors are needed that can be interrogated remotely using RF signals and respond with a signal that encodes both the sensor's identity as well as an environmental parameter; and does this without batteries. Current literature describes two types of sensors that would meet this requirement, passive and semi-active. Passive sensors have no on-board static energy storage and respond in an echo mode. The incoming signal, for example an RF pulse, excites the sensor, it then reradiates some modified version of the received RF pulse encoding the necessary information. Semi-active sensors take energy from the interrogation signal and store this to enable the sensor to operate and transmit a signal, before shutting down. It is not clear which of these two classes is more advantageous; passive sensors may not require as intense an RF signal to operate, but data encoding to operate large numbers of sensors may require logic that can only be incorporated onto a semi-active device.

One class of passive sensors that has received attention lately is the use of surface acoustic waves. In this case, interdigital electrodes located on a piezoelectric material (e.g., quartz) act as a receiving antenna. When an RF pulse appears, the electrodes cause a surface acoustic wave to propagate on the material. This wave then passes by other sets of electrodes causing small RF pulses to be emitted (or a reflector might be used to send the acoustic wave back to original electrodes) whose relative time delays (and possibly amplitudes) encode both the sensor's identification information as well as the parameter being sensed. Such sensors have been shown to monitor temperature and pressure (stress and strain), both of interest to the aerospace community.

As stated above, an alternative to a passive sensor would be one that acquired its power from the incoming RF pulse, rectifying and filtering it to charge a capacitor. This energy would then be used to turn on circuitry, read a sensor, and then broadcast an encoded signal back to a receiver. This might allow smaller and denser packing of sensors than that achieved by the use of SAW devices, but would likely require higher RF powers. Also, higher power RF transmitters pose safety problems both to individuals as well as to flight equipment, so such an approach would need to be carefully thought out.

Innovations are sought for the following sensor applications:

• Hundreds of temperature sensors might be mounted under the vehicle skin with an internal RF interrogator that scanned through each sensor once per second, constantly monitoring the heat load on the vehicle or hundreds of strain sensors being used to track the pressure distribution exerted on the vehicle. In these cases the distance to the sensors might not be more than 20 to 30 feet from the RF source/receiver, though shorter interrogation distances might be possible. The goal though is to minimize running cables throughout the vehicle, yet still obtain a large amount of vehicle health data.
  
• Such sensors would be incorporated into the thermal protection system (TPS) of a future vehicle, e.g., located under foam, or even under the tiles of the Space Shuttle. In this application a field inspector would carry a transmitter/receiver with a short (3-4 feet) sensing range that could scan the flight hardware during ground processing. Possibly, such sensors could be used to sense water intrusion under the TPS, abnormal stresses, or out of range temperatures during fueling operations.
  
• There is a need for better pressure monitoring within the tires of future vehicles, as well as the Shuttle, and locating a passive sensor within each tire would provide assurance to the pilot that he can land. It is known that SAW devices are in development for this application and it might be that a first aerospace use of such sensors will be for tire pressure monitoring.