Capacitive Sensor Front-end Using Carrier Demodulation

Nowadays, indoor human localization plays an important role in security system, smart house and health-care monitoring. There are many technologies for indoor human localization have been developed in past decades. The application is usually based on WI-FI, GPRS, infrared RFID, bluetooth, ultrasonic sensors. Every technology has its own advantages and limitations. Capacitive sensors are considered in this project due to their low cost, low power consumption and effective sensing. The purpose of this research project is to design a long range, low cost indoor human localization with a front-end interface of a capacitive sensor. The circuit for this project was designed by the previous researcher. It is based on carrier demodulation, the phase and amplitude are affected by presence of human. The basic idea is to generate a carrier with sine wave of 10kHz by the PWM block of microcontroller ATmega328P. The sine wave carrier passes a RC filter which is formed by plate sensor and an adjustable resistor. The capacitance is affected by the distance between a person and the plate sensor. Since the change of the capacitance would affect the phase and amplitude of the signal, converted both the affected sine wave signal and the original one into square waves by comparators. And compare them with an xor gate. The demodulated output is measured by the built-in ADC of the microcontroller. My contribution was to test the circuit, characterize a new front-end and design PCB board. For testing part, first was to test the sensitivity in which a person stand in front of the plate sensor at different range and record the value from microcontroller on server using an XBEE radio module. Then to test the stability by running the circuit for hours in two ways. One was testing with a fixed capacitance capacitor instead of the plate sensor and the other was testing with the plate sensor while no person stands in front of it. By performing these tests, the fourth order butterworth BP filter part was found that it contributed more noise than without it. So that it was considered to be removed. Other problems were the large drift over time and the noise. Thus I did resoldering some parts of the circuit in a nicer way, changing some resistance or capacitance to several filters, and also replacing some components. The power supply part need to be redesigned. For analog part, the virtual ground need to be precious at the half of the analog voltage, thus a dual-output with Vref and Vref/2 outputs component is considered. For digital part, a switching voltage regulator is needed. And to reduce the board size, a dual-out component is considered. Because of the components limitation, the needed power supply is about 5.5V, so that an extra external boost regulator was to be designed as a voltage adaptor. In the end, design the new PCB board. The basic rule was to design different parts of the circuit depending on whether they would affect each other, and to make the board size as small as possible. It was to reduce the noise affection from the components to each other. First was to design the power part at the left-bottom corner to make sure the power traces connect the components in short and in a tree shape. Then to design the XBEE and microcontroller part at the right-bottom corner, the power filtering parts are close to the microcontroller. For analog part, the different functional parts were designed at the top of the board one after one as close as possible.