Understanding Smart Sensors - Nomads.usp

Transceivers, Transponders, and Telemetry 1838.3 RF SensingRF techniques are also used in wireless sensors. RF sensors are a noncontact andintrinsically safe way to measure velocity and distance, detect motion and pressure,and indicate direction of motion. In addition, RF sensors are used forliquid-level sensing and detecting the presence of foreign objects. RF sensingtechniques include SAW sensors, Doppler radar, sonar, ultrasonics, and microwavesensors. Some of these sensing techniques are used in vehicle antitheft andremote entry systems.8.3.1 Surface Acoustical Wave DevicesSAW technology is being used for both RF communication and sensing. SAWdevices are used to enhance the stability of oscillators in transmitters and asfront-end filters to set the bandwidth and improve intermodulation performanceof receivers. SAW devices respond to temperature, pressure, force,and vibration. The SAW device placed in an oscillator circuit, as shown inFigure 8.5, provides a frequency variation that can be measured accurately [11].Differential techniques can be used to compensate for unwanted stimuli. Thefrequency range can be from direct current to several gigahertz.SAW devices are also being used in chemical sensors. In gas sensors, dualSAW oscillators are frequently used. One SAW device acts as the reference, andthe other has a gas-sensitive film deposited between the input and output interdigitatedtransducers. The relative change in the SAW oscillator frequency isdirectly proportional to gas concentration. In one study, porous oxide coatingswere used as the discriminating element in the sensor [12]. The coating microstructurewas evaluated by monitoring nitrogen adsorption-desorption at 77Kusing SAW devices as sensitive microbalances. The frequency shift of a SAWdevice coated with zeolite in a sol-gel microcomposite film was –6,350 Hz formethonal, –10,200 Hz for isopropanol, and +74 Hz for iso-octane.Automotive navigation systems are also potential users of SAW devices.The SAW device is used in an RD-identification (RD-ID) tag to identify avehicle for billing purposes. In that application, SAW devices operate at 856MHz (discussed in Section 8.3.7).8.3.2 RadarRadar (the acronym for radio detecting and ranging) uses reflected radio wavesto measure range, bearing, and other parameters. A recently developed radarproximity sensor (Figure 8.6) uses spread spectrum techniques and is a goodexample of the critical elements of a radar system [13]. This particular sensor is