Abstract - Piezoelectric materials can be used to convert mechanical energy, such as vibration into electrical energy which can be used to power up devices in a smart dust sensing network. This piezoelectric mechanism exhibits great potentials to resolve many energy supply problems in sensor network to supply energy to radio frequency (RF) circuits, microcontroller, micro electromechanical systems (MEMS) and sensors placed in remote locations. This paper presents the detailed development work of a self powered wireless sensor node which harvests the power energy from surrounding machinery vibration by the piezoelectric generator. The detailed design on a temperature sensing system is described in this paper. This approach is very useful in application in real-time remote monitoring of the machine temperature by sensor network in industries.

Index Terms – Smart dust sensor network, Piezoelectric energy harvesting, Wireless sensor network, Condition monitoring.

I. INTRODUCTIONThe advanced development of wireless sensor modules,

network communication modules, microelectronic devices and Microelectromechanical systems (MEMS) becomes mature in technology over the past years. These devices not only consume low power in the range of milli- or micro- Watts but also decrease in size for integration. Thus, the usage of ambient energy resources brings in great potential for research communities to resolve the need for powering up these devices. It now becomes feasible to replace the conventional way of using electrochemical batteries and to eliminate the limitation on battery life-span. One way to harvesting the ambient energy is to use piezoelectric material which converts the ambient mechanical vibration into electrical energy.

In an industrial plant, the need of continuously monitoring machine conditions such as vibration, noise, and temperature is very important. As the number of machines is increasing and inaccessibility of machine’s location (hard-to reach area), to monitor the status of many machines on 24/7 real-time will take great amount of manpower if it is to be done by human

Y.W. Shwe and Yung C. Liang are with the Department of Electrical and

Computer Engineering, National University of Singapore, Kent Ridge, Singapore 119 260.

© ICITA 2009 ISBN: 978-981-08-3029-8

workers. Thus, the trend in using the smart dust sensor network to track the performance of machines is considered to be a better solution. However, each sensor node with the network requires power energy to operate. It is not possible to provide each sensor node with an individual power supply unit, thus the entire sensor network must be based on self-powered and wireless configuration. With the help of machine’s vibration, a piezoelectric material can generate the required electric power for the monitoring system. In addition, the use of RF communication facilitates remote connectivity. In this paper, such an approach of self-powered wireless sensor network is elaborated in detail.

II.CIRCUIT DESIGNThe system with piezoelectric material for self-powered

wireless temperature monitoring system can be divided into four sections, which are Piezoelectric Generator, Energy Harvesting Unit (EHU), RF communication and Temperature Sensing Unit. The piezoelectric generator is mounted at the machine. The energy conversion occurs from the mechanical energy, i.e. the machine’s vibration, into electrical energy. This energy can be harvested with the EHU effectively and then used by the RF transmitter unit and temperature sensing unit. The data information such as temperature will be transmitted at 433MHz frequency and received by the control station.

The piezoelectric material that was chosen for this project is the bimorph V20W from Mide Technology Corporation under Energy Harvesting Category. The bimorph defined as the two piezoelectric layers sandwiched together to form a dielectric layer [1]. This type of generator is mostly suitable to mount in the cantilever position which is fixed at one-end onto the machine, while the other end of the generator is free to bend with vibration. It has a low frequency range of 50-150 Hz. The specification of V20W is shown as followings.

Specification of V20WApplication Energy HarvestingFrequency Range (Hz) 75 – 175Harvesting Bandwidth (Hz) 3Device Size (in) 2.00 X 1.00 X 0.03Device Weight (oz) 0.28Active Elements 1 stack of 2 piezosPiezo Wafer Size (in) 1.81 X 1.31 X 0.01Device Capacitance (uF) 0.2

The V20W has the measured resonance frequency of 115.4

Smart Dust Sensor Network with Piezoelectric Energy Harvesting

Yee Win Shwe and Yung C. Liang, Senior Member, IEEE

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The 6th International Conference on Information Technology and Applications (ICITA 2009)

Hz with the open circuit voltage of 30.83 V. The measurement was conducted with different resistance value to determine the maximum power output at the resonance frequency with the same g value. From Fig. 1, it observes that resistance value of 17.86 kOhm has the highest harvested output power of 2.81 mW obtained. In addition, the relationship between different g values with the harvested power output has been studied and plotted in Fig.2. It is expected that the higher the g value, the more electrical power can be harvested.

Fig.1 Output power versus load resistance at the piezo-resonant frequency

Fig.2 Output power versus G-level at the piezo-resonant frequency

A tip mass has the effect of driving down the natural frequency of the V20W but with a higher output voltage. Tungsten tip mass is selected for mounting. The product has a weight of 7.8g. The mounted structure is shown in Fig.3.

The similar procedure was carried out to get the resonant frequencies. First, it was observed that the resonance frequency is shifted to 50.2 Hz because of the additional mass. Different attachment style and location in mounting structure would also vary the resonance frequency. Secondly, the Vpp of 92.5V can be obtained which is much higher than the 30.8 V without the mass. This is expected because the gravitational force enhances the magnitude of vibration. Thus, the value of output voltage becomes higher. From Fig. 4, it observes that resistance value of 27.06 kOhm has the highest output power of 12.81 mW.

Fig. 3 Piezoelectric harvesting unit under laboratory test

Fig.4 Output power versus load resistance at the resonant frequency when the piezo tip was mounted with mass

III. ENERGY HARVESTING CIRCUITThe circuit as shown in Fig. 5 was originated from the MIT

research group [4], where the self-powered RF-ID circuit was developed. The circuit was adopted and modified based on the specification and requirement of this project.

Fig.5 Energy harvesting circuit

The circuit works as following. Charges generated by the piezoelectric generator are first transferred to the capacitor C2, while the regulator and transmitter (as load RL) is isolated by the MOSFET,Q2. The zener diode D5 connecting at the base of bipolar transistor Q1 breaks down when the voltage across the capacitor C1 exceeds a preset value. This turns Q1 on. Once Q1 is turned on, the voltage across R2, adjustable by the potential divider formed by R1 and R2, exceeds the threshold voltage of MOSFET Q2 and it turns on Q2. Thus, the source ground and the load ground is

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connected and C1 starts discharging to the load (regulator and the RF transmitter). R3 acts as the latch to ensure that Q1, and in turn Q2, remains on even the voltage across C1 drops below the zener diode’s breakdown voltage. When the capacitor drops below 4.5 V, the low-battery line on the regulator (not shown) is pulled down, transmitting a negative pulse through an external capacitor and turning Q1 off, in-turn deactivating Q2 and halting the discharge of C1.

IV. RF COMMUNICATION UNITRF communication unit contributes to the main energy

consumption. For this wireless application, only power consumption of RF transmitter needs to be determined as it is powered up by the piezoelectric harvester.

In this project, the RF transmitter used is TXE-433-KH2 and it matching the receiver unit which is RXD-433-KH2. It is ideally suited for wireless application with compact SMD package. It combines an optimised RF transmitter/receiver with an on-board encoder/decoder (HT640/HT658). It is ultra low power consumption with simple implementation and no external RF component is required. On the other hand, it is able to encode 10 address lines and 8 data lines and capable of transferring control or command data over line-of-sight distances of up to 3,000 feet. It has low current consumption, which is 1.5 mA and 1µA for operating current and standby current at 2.7 V.

The Power consumption as well as the amount of energy required for the transmitter depends on input voltage and transmission duration. It is observed that the power consumption of 15.4 mW is needed for transmitting 1 packet of data with 5V supply. On the other hand, the amount of energy required with the same supply voltage is “power times duration of transmission time” which is about 700μJ. Furthermore, two pulse-widths represent one data bit. A total of (8x2 data bits + 10x2 address bits + 5 synchronize bits) 41 bits will be included in one packet. The data transmitted are shown in Fig.6, where “one” is represented two thin pulse and “zero” is represented two fat pulses.

Fig. 6 Transmission of data by RF pulses

V. INTEGRATION AND MEASUREMENTThe whole system is integrated and it includes the

piezoelectric generator supply to energy Harvesting circuit, the temperature sensor and RF transmitter Unit. The harvesting system is tested and the output result is shown in Fig.7. Ch1 represents the output of storage capacitor, which is 22 μF. The charging time is 130 ms and discharging time is 50 ms. Ch2 shows the output waveform of Max666 voltage regulator which is 5 V when the capacitor is discharging. Ch4 (zoom in on Ch3) shows the data received at the receiver end and it indicates that the whole packet of data was transmitted successfully.

Fig.7 Output waveform of the sensor node showing the energy harvesting waveform and the data transmission

The energy usage and efficiency figures can be summarized in the following Table.

Energy from the capacitor (22 µF between 10.6V and 5 V)

0.961 mJ

Energy required for transmitting of 1 packet of data

0.709 mJ

Losses in circuit 0.252 mJEnergy efficiency 73.8%Charging and discharging times (ms) 130 and 50

For temperature sensor, a thermistor is used in this project because of its low cost and good sensitivity over a limited temperature range. It is connected to the microcontroller PIC16f688, which gets the input from the temperature sensor and produces the 8 bit digital output to the transmitter.

The final prototypes are shown in Fig.8 where the sensor node and receiving node are made on the printed circuit boards. 6 data packets were transmitted and received as shown in Fig.9 (CH2). The voltage regulator is regulated at 5 V (CH1). CH2 shows the capacitor charged up to about 9.5V. The charging time is 1.16s and discharging time of 550 ms (sufficient for 6 data packets) with the storage capacitor (C=330 μF). CH4 shows the VT pin whereby VT pin goes high when the first two data package is valid. The temperature reading is displayed on the LCD screen at the receiver node.

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(a)

(b)

Fig. 8 The final prototypes of (a) sensor node with piezo-electric energy harvesting and (b) receiving node.

Fig. 9 The test results showing the energy harvesting and data transmission

VI. CONCLUSIONPower energy harvesting with piezoelectric effect is

presented in the paper. One practical application on self-powered wireless smart dust temperature sensor network has been designed and implemented in the project. The prototype system has an energy efficiency of 73.8% and is capable of transmitting data packets successfully without any external power supply.

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