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Understanding Smart Sensors - Nomads.usp

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184 <strong>Understanding</strong> <strong>Smart</strong> <strong>Sensors</strong>Interdigitated electrode transducerPiezoelectric substrateReflectionless surface wave terminationStableoscillator0 f1 f1+ ∆fUpconverter0 f0 f0+ ∆f0 f1Frequencycontrolledoscillator0 ∆f ²fFigure 8.5 SAW device.designed to meet cost-sensitive applications such as blind-spot detection inautomobiles. The approach detects the echoes that reflect back over a limitedrange from continually propagating electromagnetic impulses. The sensor isnot affected by most materials, so its location is not a factor in its ability tofunction properly. The unit can see through walls and distinguish betweenwood and metal wall. The radar noise is coded so an unlimited number of sensorscan be colocated without interfering with each other.The proximity sensor’s antenna transmits a subnanosecond pulse at anoise-dithered repetition rate. The pulse can be from 50 ps to 50 ns long. Ditheringrandomizes the time of the transmission in a spread spectrum that appearsas noise to other detectors. The echoes returned to the (patented) receiver aresampled typically at a frequency of 1 MHz and in a time frame (plus delay)determined by the transmitter. The range of the sensor is limited to 20 ft (6m)or less. However, several applications are possible with the sensor, including carsecurity, voice-activated navigation, and detectors for locating one materialembedded in another, such as steel within concrete.Production microwave proximity sensors operate on radar principles. Asimple microwave sensor consists of a transceiver, an antenna, and signal

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