Research Of Wireless Passive Multi-Parameter Microsensor System

In the last decades, Radio Frequency Identification (RFID) has been widely used in many fields, such as gaining entrance to buildings, collecting tolls without stopping, tracking a wealth of assets in supply chain management, homeland security and so on. However in many occasions, a series of environmental parameters like vibration temperature, air pressure, humidity need to be detected. For example, in hermetic space like industrial painting room, to achieve the optimum spraying, the temperature, humidity and air pressure should be monitored during the producing; put the temperature monitoring system into the revolving bearings can timely discover the hidden danger. These demands facilitate the combination between micro-sensor and RFID technology, forming a wireless sensing monitoring system, also known as "RFID Sensor". The wireless sensing system has unique advantages in monitoring the implantable medical instrument, mechanical rotating structure, sealed workshop and harsh environment. It has been reforming the traditional data collection system and the lifestyle of human.The ways to implement the wireless sensing systems could be either active or passive telemetry. Active telemetry systems provide relatively long distances of bidirectional data transmission, but with large size and limited lifetime when internal batteries are used. Passive telemetry systems have reduced transmission distances but infinite life as battery-free. Currently, there are two ways to achieve passive telemetry:one is that put the capacitive sensor in series with an inductor to form a micro LC sensor node, by detecting changes in the resonant frequency of the LC node to complete the monitoring of environmental parameters. Another is integrating the micro-sensors with the sensor detection circuits, by back reflection modulation to transmit the measurement data to the outside. LC passive wireless node has simple structure. But it relies on an external device to detect the resonant frequency, and has a short measurement distance. In this paper, a passive wireless multi-sensor monitoring system based on back reflection modulation has been proposed. The main contents and innovations include:First, the theoretical basis of inductive coupling system is introduced and several passive wireless sensor telemetry systems are compared. The research status of passive wireless multi-sensor monitoring systems based on back reflection modulation is analyzed, which provides direction for the following design. Then the theoretical analysis for each module of passive wireless sensing system based on back reflection modulation is described. Circuit simulation is adopted to find feasible implementation for the system.Then the inductively coupled passive wireless multi-parameter sensors system for monitoring hermetic space environment is proposed. The system consists of two parts:the main system and subordinate system. The main system transmits power and receives measured data through a coupled transformer composed of main and sub inductors printed in their sub-systems, respectively. There are three micro capacitive temperature, pressure and relative humidity sensors implanted in the subordinate system. A relaxation oscillator is used to convert capacitance to frequency. The time division multiplexing unit is used to simultaneously measure multiple sensors. The converted frequency signal is used to vary the amplitude of the waveforms on the coupling inductors. The envelope detection unit is used to recover the frequency signal from the modulated carrier. A clock signal used in the asynchronous communication is extracted from the recovered signal. The clock signal is used to distinguish three sensors’ converered frequencies. Electromagnetic resonance is utilied to relize the maximum coupling distance. Finally, test the system in the measurement chambers, the measurement curves have good linearlity and consistency. The measurement results indicate that the sensors’ conversion accuracies of the temperature, relative humidity and air pressure of the passive wireless multi-parameter sensor system are 0.52 kHz/℃,-0.132kHz%RH, and -0.156kHz/kPa, respectively.At the end of the thesis, several improvement plans are presented. Such as, through the inductor design and impedance matching to improve the coupling efficiency of the system; by adding a fixed reference capacitor to realize the differential measurement of the capacitive sensor to reduce the effect of the parasitic parameters and temperature effects. In the further study, the integrated circuit(IC) is preferred than the PCB circuit. Using the IC can greatly reduce the size and power consumption of the system, along with the coupling distance of the system greatly increased. Thus the passive wireless multi-parameter sensors system will has a promising prospective in medical testing, industrial control, food safety and other areas.