Piezoelectric Generator for Powering Remote Sensing Networks Moncef Benjamin. Tayahi and Bruce Johnson moncef@ee.unr.edu Contact Details of Author: Moncef Benjamin. Tayahi Phone: 775-784-6103 Fax: 775-784-6627 Email: moncef@ee.unr.edu Abstract This paper deals with the design, modeling and experimental validation of a piezoelectric harvest ambient energy generator based on ambient vibrations to be used in remote and longterm sensing networks. Simulations and experimental studies validate the functionality of individual components in an integrated piezoelectric generator unit.
Piezoelectric Generator for Powering Remote Sensing Networks Moncef B. Tayahi moncef@ee.unr.edu Bruce Johnson johnson@ee.unr.edu Abstract This paper deals with the design, modeling and experimental validation of a piezoelectric harvest ambient energy generator based on ambient vibrations for powering remote and long- term sensing networks. Simulations and experimental studies validate the functionality of individual components in an integrated piezoelectric generator unit. 1 INTRODUCTION Piezoelectric (PE) materials when mechanically loaded induce an electric charge on opposite faces of the PE material. Thus, vibrating, compressing or flexing a PE material generates electricity. Over the past, PE materials have been used in actuators, transducers, and resonators. Recent research involves the use of PE materials for applications such as: i) a transformer for a mobile phone battery charger [1], ii) a micro-generator for embedded microsystems such as medical implants or building sensors [2-3] and iii) a shoe mounted device for powering devices such as a Radio Frequency RF tags [4]. The magnitude and frequency of vibration required to develop sufficient power, to drive the sensors are the main concerns for the development of PE devices. Modern day sensors have evolved from first-generation, unamplified charge mode sensors to a second-generation with internally amplified design and now to the current generation featuring mixed mode Bucket capacitor sensors. The current generation sensors are capable of simple unidirectional communication and have the ability to transmit user-defined information that is preprogrammed into memory during initial installation. Once the data transmission from the memory is complete, the sensor returns to a second-generation mode of operation, while outputting an analog signal proportional to the vibration input. 2 DESIGN MODEL AND CHARACT- ERISTICS OF A PIEZOELECTRIC GENE- RATOR The piezoelectric source is fundamentally modeled as a capacitor that charges in response to vibrations. For this application the frequency of operation is assumed to be low (few order of magnitude in Hz) and the PE source has a high voltage and low current. A DC-DC converter approach is deployed to amplify the current. Practically sensors require voltages ranging from 2.7 to 7V and currents on the order of a few milliamps (ma). A dual approach exploring the availability of lower power sensors and amplifying the PE current output with a dcdc-converter is investigated. This PE power source can be used for three different applications: i. stand-alone power source, ii. battery back-up when used in conjunction with super-capacitors, or iii. a battery charger. Bucket capacitor Low Battery Rectifier Discharge Regulator PE V in circuit V out PFET NFET To Regulator Figure 1: Block diagram of power management circuit and schematic of discharge circuit
Figure 2: The LTC1474 setup and output efficiency. The block diagram of the proposed PE power management source circuit is shown in Figure 1. The PE signal is first rectified then used to charge up a large bucket capacitor. When the capacitor charges to a predetermined value, it will discharge into the regulator. When the regulator output falls below a required level, the low battery output signal will go low, turning off the discharge circuit, allowing the capacitor to charge up again. When the capacitor charges to V BD of the diode plus V GS of the P- Field effect Transistor (PFET), the PFET and NFET will both turn on, creating a return path for the capacitor to discharge into the regulator. A Linear Technology LTC1474 regulator, shown in Figure is used for the purpose of stabilizing the output voltage at a preset value. This chip is a high efficiency converter, capable of handling inputs in the range of 3 to 18 V. The LTC1474 requires low supply currents on the order of 10µA and gives a regulated output of 3.3 or 5V with load current as high as 250mA.The operation involves the use of short burst cycles to ramp the current through internal power switches to charge the output capacitor. 3 MODELING The PE material used for the simulation and later for design implementation is the commercially available Macro Fiber Composite (MFC). MFC is an innovative actuator that offers high performance and flexibility with a competitive cost. The MFC consists of rectangular piezo-fingers sandwiched between layers of adhesive and electrode polyimide film. The assembly enables in-plane poling, actuation and sensing in a sealed, durable, ready-to-use package when embedded in a surface or attached to a flexible structure. The power management circuit is integrated into a sensor network and used to power a sensor with its RF transceiver. The applied pressure to the PE material produces vibrations with various amplitude and frequency. These vibrations compress the element and generate an electric charge proportional to the applied pressure magnitude and its frequency. This developed voltage is stored in a capacitor. This stored voltage is linearly dependent on pressure and can be used as a pressure sensor with very high sensitivity. For acc Measurement of the voltage has to be preceded by signal sampling, quantization and encoding. 4 ENERGY CONVERSION The process of energy conversion in a PE material is based on the principle of the piezoelectric effect. When a piezoelectric element is mechanically stressed it generates a charge. The piezoelectric element stores the energy in two forms, as an electric field (electrical energy) and as a strain (mechanical energy) The interaction between the electrical and mechanical energy forms the source for energy extraction. The relation between the mechanical strain (ST) and the electric field (E) is given by [5]: 1 SC = ( SR ( d E)) (1) ST
where SC is the compliance of the piezoelectric element in a constant electric field, SR is the mechanical stress and d is the charge constant. The model is one- dimensional so the direction of applied stress and the orientation of the material need to be consistent. The charge produced in the material when a pressure is applied is given as: Q = D A (2) where Q is the induced charge D is the electric polarization and is given by d P, A is the area on which the pressure is applied. P is the pressure applied. Using the above equation, the induced charge can be given as: Q = d P A (3) By employing multiple piezoelectric stacks on top of each other and electrically connecting them in parallel, the amount of electric charge generated for an applied pressure can be increased. So the total electric charge developed for n number of stacks is given as: Q = n d P A (4) Approximating the stacked piezoelectric rings as a parallel plate capacitor the output voltage generated can be expressed as: n P d A V = (5) Cstack where C stack is the capacitance of the piezoelectric stack. Considering that a typical pressure P is on the order of 100MPa, the resulting voltage would be over 100 V for a centimeter sized stack. In order to reduce the output voltage to a proper working range of several volts a parallel capacitance is employed. So the output voltage can then be expressed as: n d P A V = (6) Cstack + CParallel The energy stored in a capacitor is given as: 1 2 E = C V. Therefore, the total energy 2 converted by the piezoelectric stack is expressed as, 2 1 n d P A E = C 2 C (7) The selection of the number of stacks depends on: 1) the minimum amount of energy needed to generate a detectable output 2) maximum space available for the piezoelectric generator within the subassembly. 5 WIRELESS SENSOR BASED ON PIEZO ELECTRIC MATERIALS A wireless sensor network transmitting temperature and vibration data was developed utilizing the piezoelectric generator in a sensor mode of operation. The need for high bandwidth associated with the transmission of raw vibration signals can be eliminated by measuring and storing the average vibration levels at several frequency bands. This network operates at a few Hz with a data rate of 200 baud/s. Each sensor contains a microcontroller, a clock, and storage memory. The principal purpose of this application is to create an experimental testbed for investigating different transmission protocols and exploring techniques for prolonging battery life. Time-division multiplexing, a signal mixing protocol was used for bandwidth efficient modulation to conserve more power. The simulation to process data and plot the chart is done using Microsoft Excel. The graph generated for two different MFC s is shown in Figure 3. The graph compares the energy produced by the two different MFC materials, properties of which are as given in Table 1. The graph demonstrates that both the MFC s provide a high voltage and low output current, which is a major drawback for the piezoelectric energy sources. The MFC 2 is dimensionally smaller than MFC 1, and thus provides lower energy as seen from the graph. Therefore, depending on the application for which the piezoelectric source is used either of the piezoelectric materials can be employed. Preliminary experiment has shown that, in order to generate a detectable signal that travels through a typical steel plate of 5 cm on thickness, the minimum energy required is in range of 2mJ. Therefore, based on results in
Energy Vs Parallel Capcitance 600.0 550.0 MFC 1 500.0 450.0 400.0 Energy (mj) 350.0 300.0 250.0 200.0 150.0 MFC 2 100.0 50.0 0.0 1.00E-05 1.00E-04 1.00E-03 1.00E-02 1.00E-01 1.00E+00 Parallel Capacitance (uf) Figure 3: Converted energy as a function of parallel capacitance for two10 ring MFC Stacks Figure 3, a 10 stack MFC with a height of 0.3 mm was sufficient to provide enough energy to power a sensor with a size that is acceptable to most sensors who s currents of operation is of order of few milliamps. The 10-stack piezoelectric source has a voltage conversion ratio of 1.5x10-5 V/Pa and a charge conversion ratio of 4.99x10-13 C/Pa. Property MFC 1 MFC 2 Area, A (cm 2 ) 8.5 * 2.5 2.8 * 1.4 Thickness (mm) 0.3 0.3 Stack Capacitance, 188 24.8 C Stack (µf) Charge 2.1E2 2.1E2 Constant,d (pc/n) Number of 10 10 stacks Pressure, 100 100 P(MPa) Parallel Capacitance, C Parallel (µf) 10-5 10-5 Energy, E(mJ) 540 100 Table 1: Sample calculation for energy produced by different MFC materials To design signal transmitters with specific characteristic frequencies for sensor identification, an equivalent circuit model was developed, and the transfer function of the transmitter was calculated and optimized based on the number of stacks, amplitude and vibration frequency of the piezoelectric source. Good agreement between the model and the experimental results has been achieved. The individual components were integrated to form a powerful PE wireless sensor network. The present design of the wireless sensor provides a new generation of self-energized sensors that can be deployed for process condition monitoring and for mobile networks requiring self contained power. 6 CONCLUSION Several fundamental aspects related to the design of a new type of piezoelectric power generator are discussed. The underlying concept of piezoelectric sensor is to extract energy from the vibration that the piezoelectric undergoes. The piezoelectric sensor consists of an energy converter, a DC-DC converter, and a wireless transceiver. The energy converter is designed using a 10-ring piezoelectric stack to provide power to various remote sensors. The high voltage output
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