The traditional approach for wireless sensors involves interrogators that communicate with each other (i.e., the two boxes “talk” to each other). In contrast, surface acoustic wave (SAW) sensors allow for a single box that can communicate with multiple sensors. One of the biggest advantages of SAW sensors over competing wireless sensors is that the SAW sensors do not require a power source.
Researchers at Kennedy Space Center and the University of Central Florida (UCF) have developed a SAW multiplexing technology that allows for an increase in the number of sensors to the 40-60 range (or even higher) and resolves collision problems and echoes overlapping between the sensors.
Many applications require more than one sensor, so a multiplexing scheme is needed to allow wireless access to a number of sensors. Coherence multiplexing, i.e. noise radar, assigns a different pair of path lengths or propagation distances to each SAW sensor, providing each with an ID. Then, a series of noisy pulses (each one different) is sent to the sensors and the returned echoes are correlated against the noise to find the signals associated with each sensor. Averaging these signals allows the noise floor to be reduced.
In the past, orthogonal frequency coding of SAW sensors has been demonstrated to be preferable to most other multiplexing schemes. Orthogonal frequency coding works by placing a number of reflectors on the SAW device, each one reflecting a desired frequency band while transmitting other frequencies; the order of these reflectors determines the code, or ID of the sensor.
NASA’s multiplexing technology is an improvement over existing orthogonal frequency devices by increasing the number of sensors that can be multiplexed and reducing the signal-to-noise ratio.