Understanding Smart Sensors - Nomads.usp

Transceivers, Transponders, and Telemetry 193transmit the data to a mobile laboratory that travels to the races [27]. An indicationof a potential problem can initiate a pit stop before the problem becomesa disqualifying failure. Indianapolis 500 and National Association of Stock CarAuto Racing (NASCAR) vehicles use similar telemetry. In fact, proving-groundvehicles equipped with telemetry systems allow automotive engineers to evaluate,from their offices, development vehicles on high-speed test tracks. Cellularcommunications with drivers provide on-the-spot direction for performingvarious tests with real-time feedback on how systems are affected. In both racingand vehicle development, time is short and cost investment high. Telemetrymakes the outcome more predictable.Telemetry is also used on production vehicles in systems that monitorpressure and temperature for each tire. The RF transmitter at each wheel sendsa signal (355 or 433.92 MHz are common frequencies) that is received by aunit mounted under the dash, and a dashboard-mounted display providesinformation to the driver. One system uses pulse code modulation to transmitdigital data by turning the carrier frequency on or off, producing a burst ofradio frequency energy [28]. Switching rate and time are controlled to createthe code.Wheel-mounted transmitters deliver 10 frames of coded data in 128-msbursts at approximately 30- to 35-sec intervals. Each frame has 8 bits of dataand a blanking period. Two bits indicate tire pressure, two identify the wheelbeing measured, and the other four identify the car model and year. Tire pressureand temperature are important vehicle measurements that not only affectthe performance and economy of the vehicle but also have a significant impacton vehicle safety. These systems may be common on future vehicles due to thecombined capability and continued cost reduction of integrated sensing andRF technology.Another difficult-to-get-at measurement is the temperature of the pistonin an internal combustion engine. Telemetry has been used to transmit thetemperature indicated by the variation of a temperature-sensitive chip capacitor.The system schematic is shown in Figure 8.12 [29]. Increasing temperaturedecreases the capacitance, which in turn increases the transmitting frequency. Amultiloop antenna in the oil pan receives the transmitted signal. Data is convertedto temperature through a calibration curve. The system has seven datachannels to map temperatures at a number of locations inside the piston. Asmall 22-pin hybrid package contains the multiplexer and the blocking oscillator.Power is generated based on the piston’s movement. A minimum enginespeed of 1,200 rpm is necessary, and temperatures from 150 to over 600°F(65–315°C) can be measured. To keep track of the data being sent, a referencecapacitor that is not temperature dependent transmits a 1-MHz signal. Theremaining channels are each turned on for 1 sec and then off for 1 sec.