Study On The Passive Wireless Temperature Sensor Based On LC Resonant

Temperature sensors are widely used in industrial manufacturing, aerospace, bio engineering field. However, the traditional cable temperature sensors are difficult in the application of rotating parts, high temperature environments and other occasions. The wireless passive temperature sensors do not need physical connection and power supply, which makes the temperature measurement in these special fields become possible. So it is important to study the passive wireless temperature sensor.In this thesis the relationships between the impedance and the operating frequency are deduced by an equivalent circuit based on the theory of the LC resonant circuit and inductively coupled method. Two type LC resonant sensors with double side spiral inductor and capacitor structure are designed. The thickness of the ceramic substrate with a high dielectric constant is 0.2 mm. The diameter of the substrate of the LC sensor 1 is 25 mm. Both the spiral inductance sensor line width and the spacing width are 0.4 mm. The calculated resonant frequency is about 1.18 MHz. The diameter of the substrate of the LC sensor 2 is 13 mm. The line width of the spiral inductors is 0.2 mm and the spacing is 0.4 mm. The calculated resonant frequency is about 5.49 MHz.Secondly, the designed LC resonant sensors are prepared using the lithography, metal thin film depositing and electroplating technology. The impedance characteristics of the sensors were measured wirelessly with an impedance analyzer and a vector network analyzer. The impedance characteristics of the sensors at different temperatures were also measured. The results show that at room temperature the resonance frequency of the LC sensor 1 and 2 are 1.05 MHz and 5.02 MHz, respectively. With the increasing of the temperature from room temperature up to 130 ℃, the resonant frequency of the two sensors decreases linearly with the increasing of the temperatures. The fitting results show that the sensitivity of the sensor 1 is about 1.7 kHz/℃ while that of the sensor 2 is about 9.44 kHz/℃.By measuring the voltage of the interrogation coil, a measurement circuit for the LC sensor had been proposed based on the MSP430F149 microcomputer. The signal generate module AD9850 is controlled by the microcomputer to generate sweep signal. The highest frequency of the AD 9850 module is up to 20 MHz and the sweep interval can be adjusted. The voltage amplitude of the interrogation coil is sampled through a diode rectifier circuit with resistance and capacitance. The sampled voltage is obtained with the 12 bit A/D module of the microcomputer. The results show that the proposed circuit system can operate in the designed frequency range. The resonant frequency of the LC sensor is obtained by searching the maximum voltages. The resonant frequency measured by the designed circuit are the same as that by the impedance analyzer, which indicates that the designed circuit can realize the wireless test function for the LC sensor.