research summary xiaodan xi

Research Summary of GEAR in NCSU Xiaodan Xi, ZJU

Self-Powered Environmental Vapor Concentration Monitoring

Wristwatch Based on Micro-fabricated Tuning Forks

Xiaodan Xi, EE College of Zhejiang University, ECE College of NCSU

Abstract The product is a self-powered chemical monitoring wristwatch based on micro-fabricated

tuning forks, which is designed for patients and doctors to detect the concentration of chemical

vapor (such as ethanol) in the environment and display the real-time results in case of exceeding

chemical concentration. The principal of the detection is the output voltage frequency shifting

because of change of chemical vapor concentration around the tuning forks. We use AD5930 and

the crystal oscillator as the frequency generator and generate various frequency input voltages for

tuning forks. To detect the resonant frequency of the tuning forks and control the sampling velocity,

we use the microcontroller MSP430 and the ADC10 inside it. For the power supply of the

microcontroller and frequency generator, we use solar cells as the energy harvester, and use the

DC/DC converter IC LTC3105 to get 2.5V output voltage, storing the power in a super capacitor,

and finally get a steady output voltage of 2.5V using the LDO linear regulator IC TLV71325. We

design the circuits and program and simulate to verify it. Finally, we build the real system platform

to test the performance of the product.

Key words Energy harvesting; DC/DC converter; Tuning forks; MSP430

Objectives

The objective of my research is to design the power supply circuits and control system of a self-

powered vapor concentration monitoring device. Because the device uses small ICs,

microcontrollers and micro-fabricated tuning forks, so the product can be made into a wristwatch

so that patients with diseases such as asthma can wear it and detect the concentration of some

harmful vapor such as ethanol in the air wherever there is sunlight.

The specific objectives are as follows :

Choosing the method of power supply based on solar energy harvesting

Simulating the performances of the power supply circuits

Building testing platform of the power supply and test the performances in real condition

Designing the system of vapor concentration detecting system based on MSP430

Writing program of MSP430 to control the AD5930 to generate voltage with specific

frequency

Research Methods

Solar Energy Harvesting Power Supply Design

Energy Harvesting Method

One of the solar energy harvesting methods is shown as Fig.1, which consists of three main

parts, thus IS, MPPT and OS :


• Input Stage (IS) : Storing the energy collected from the solar panel into the energy

reservoir;

• Maximum Power Point Tracker (MPPT) : Sensing the light intensity and controlling

input stage forcing the photovoltaic panel to work in most efficient conditions;

• Output Stage (OS) : Generating a stable voltage supply for low-power applications.

Fig.1 A strategy of solar energy harvesting[1]

Solar Cell and Power IC Selection

• Solar Cell : IXYS KXOB22-04X3

For the solar cell of the energy harvesting circuit, we choose the IXYS KXOB22-04X3 solar

cell, and use three parallel connected solar cells as the energy collector. And here are some of the

characteristics as follows.

Fig.2 Three parallel connected solar cells Fig.3 Some characteristics of IXYS KXOB22-04X3

• Power IC : LTC3105

The LTC3105 is a high efficiency step-up DC/DC converter that can operate from input voltage

as low as 225mV. A 250mV start-up capability and integrated maximum power point controller

(MPPC) enable operation directly from low voltage, high impedance alternative power sources.

From Fig.5 we can know that this IC integrates the IS and MPPT parts in a single chip, which

is indicated by the red frames. For the input stage, it uses the Boost converter circuit to get a high

enough input voltage, and for the MPPT part, with an MPPC resistor, he MPPC circuit dynamically

regulates the average inductor current to prevent the input voltage from dropping below the MPPC

threshold, and Fig.4 shows the principle of MPPT.

Fig.4 The I-U and P-U curves of the solar cell[2]

Current @ Pmpp 13.38mA

Voltage @ Pmpp 1.5V

Short Circuit Current 15mA

Open Circuit Voltage 1.89V


Fig.5 Block diagram of LTC3105

Power Supply Circuit

Fig.6 shows the circuit of solar energy harvesting using IXYS KXOB22-04X3 solar cells and

LTC3105 as the DC/DC converter IC. And as shown in Fig.6, a resistor divider connected between

the VOUT and FB pins programs the step-up converter output voltage, the Vout equation is as follows :

1.004 ∙ 1

According to the datasheet of the MSP430 and AD5930, we find that the power supply is

proper to be around 3V, and so we finally choose 2.5V as the output voltage of the circuit to meet

the requirement of the controlling devices. Therefore, we choose resistors as 1.19 and

800 .

Fig.6 Circuit diagram of the solar energy harvesting power supply

Tuning Forks Working Principle

We all know that if the frequency of the input voltage is about the same with the intrinsic

frequency of a tuning fork, then the output voltage of the tuning fork will be higher than usual. This

frequency is called a resonant frequency.

Input Stage

MPPT part


By measuring the output voltage, we can determine the resonant frequency, and it has been

proved that injection of vapor to the tuning fork can result in a shift of resonant frequency. And the

shift value can be different due to different vapor concentration. Fig.7 shows the frequency shifts of

tuning forks with different types of polymers on them. And Fig.8 shows the frequency shift curves

with different ethanol vapor concentration. According to the curves, we can determine whether the

concentration exceeds limit.

Fig.7 frequency shifts with different polymers[3] Fig.8 Frequency shift curves to ethanol concentration[3]

MSP430 Detecting Control System Design

Fig.9 shows the connection of frequency detecting system, consisting of AD5930, MSP430,

tuning forks and PC. The MSP430 receives voltage signal from tuning forks, convert it to digital

data using its internal ADC10, and gives a control command to AD5930 to control the frequency of

the voltage it generates. In our project, the AD5930 generates sine wave with frequency changing

from to . We set the 31368 and 34208 . And the increment

frequency is 9Hz. Fig.10 shows the flow shapes of the control strategy of MSP430.

Fig.9 Connection of frequency detecting system

Fig.10 Control strategy of MSP430

START

Set the initial dataof frequency

Data of frequency increment

End of data of frequency ?

Data output to AD5930

N

Y Data sentto PC

Polymer wires

Data Frequency


Fig.11 shows the hardware connection between MSP430 and AD5930. And Fig.12 shows the

serial communication and control timing sequence chart of AD5930. To control the frequency

generator, MSP430 should give a CTRL signal to start the AD5930, and give an FSNC signal as the

synchronizing signal. Besides, because the MSP430 uses UART to send data, and SCLK is chosen

as the clock, so this clock signal should also be send to AD5930. Finally, the por SDATA transmits

control register data and frequency information data to AD5930, shown as Fig.13.

Fig.11 Hardware connection of MSP430 and AD5930

Fig.12 Serial communication and control timing of AD5930

CTRl register 0x0F7F

∆ 0x2000

∆ 0x3000

∆ 0x3800

0xC000 + (Freq & 0x000111)

0xD000 + (Freq & 0x111000)

Fig.13 Data sent to AD5930

Results and Analysis

Solar Energy Harvesting Power Supply

Fig.14 shows the testing circuit platform of solar energy harvesting power supply and the

testing result in the sunlight. From the result, we can see that the output voltage of the power supply

is a steady 2.76V, which satisfies the requirement of the devices.

Fig.15 shows the results of the power supply with a voltage source as the input. From the results,

we can see that the output voltage is 2.348V, and the ripple voltage of it is 157.8mV, which is 6.72%


of the output voltage. The results satisfies the requirements of the system.

Fig.14 Testing circuit and the output voltage in the sunlight

Fig.15 Output voltage and ripple voltage with voltage source as the input

Frequency Detector and Generator

Fig.16 shows the Testing circuits of MSP430 and AD5930. Fig.17 shows the result of SDATA

sequence chart and FSNC signal from the P1.2 of MSP430. And Fig.18 shows the sine wave

generated by AD5930. The results indicate that the The MSP430 functions well and the AD5930

board can give an output voltage with a continuously changing frequency.

Fig.16 Testing circuits of MSP430 and AD5930


Fig.17 SDATA sequence chart and FSNC signal sent from MSP430

Fig.18 Sine wave generated by AD5930

Future Work

There are some extra work to do apart from the work we have done, to improve the

performances of the system and make a better interface with the user.

To add a filter in the power supply, so that the output voltage can have less noise and high

frequency ripple;

To use coaxial cables instead of wires in the connection of MSP430 and AD5930, so that the

signals transmitting between the two boards can be more smooth;

To connect the MSP430 with PC, so that the real time results can be displayed on the screen

to let people analyze and modify more easily.

References

[1] D.Dondi et al. A Solar Energy Harvesting Circuit for Low Power Applications : ICEST 2008

[2] Nathan Bourgoine. Harvest Energy from a Single Photovoltaic Cell : LT Journal of Analog

Innovation 2011

[3] Minghan Ren et al. Chemical Sensor Based on Microfabricated Wristwatch Tuning Forks : Anal.

Chem. 2005, 77, 2700-2707

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