ELECTRIC POWER GENERATION USING PIEZOELECTRIC DEVICES

Transcription

1 ELECTRIC POWER GENERATION USING PIEZOELECTRIC DEVICES Husain Maruwala & Rushali Thakkar husainmaru52@gmail.com,rushalithakkar@ymail.com Abstract - Piezoelectric materials (PZT) can be used as mechanisms to transfer ambient vibrations into electrical energy that can be stored and used to power other devices. With the recent surge of micro scale devices, PZT power generation can provide a conventional alternative to traditional power sources used to operate certain types of sensors/actuators, telemetry, and MEMS devices. In this paper, the dynamics of piezoelectric materials for the use of power generation devices has been experimentally investigated. The objectives of this work are to estimate the amount of power that PZT can generate, and to identify the feasibility of the devices for real-world applications. The energy produced by the PZT was stored in two different ways. The first was in a capacitor that allows for immediate access to the stored energy, and the second method charged a nickel metal hydride battery. Index Terms Piezoelectric, feasibility, telemetry, capacitor, battery. I. INTRODUCTION The trends in technology allow the decrease in size, weigh, and power consumption of complex digital systems. This decrease in size, weigh, and power gives rise to the concept of wearable devices in which digital systems are integrated in everyday personal belongings, like clothes, watch, glasses, etc... Batteries are the power source used for wearable devices. However, the disadvantage of batteries is the need to either replace or recharge them periodically, and their size and weigh that sometimes reach a high percentage of the system. An alternative to batteries is harvesting energy from the environment (energy scavenging). Walking is one of the usual human activities that have associated more energy. The electric energy provided by the piezoelectric film must be converted to a stable voltage to supply wearable devices. Therefore, it is necessary to design a power converter. The electrical model of the piezoelectric is well known. It has a charge source in parallel with a capacitor and a leakage resistor. The waveform of the charge source depends on the stress applied. This paper presents a way to obtain the parameters that characterize the electrical model of the source waveform using voltage measurements between the piezoelectric electrodes. The structure of the work is as follows: Section II presents a review of the principles governing the conversion of mechanical to electrical energy of piezoelectric materials. Section III explains the electrical model, its analysis and design in base of the physical principles. Section IV shows the results. Section V gives some conclusions. II. MATHEMATICAL MODEL The piezoelectric effect causes the generation of an electrical charge between the two faces of the material when it is mechanically stressed. If the electrodes are not short-circuited, a voltage associated with the charge appears. The materials which have this property also have the inverse piezoelectric effect that consists of the deformation of the material when an electrical field is applied. Therefore, piezoelectrics are materials that allow conversion between electrical and mechanical energy and vice versa. This property may be used for harvesting energy from the environment. The materials are composed of polar molecules aligned along a certain axis (poling axis), and therefore present a net dipolar moment. Figure 2.1: Piezoelectric film polarity. If the stress compresses the piezoelectric, the voltage polarity is the same that the poling axis. If the 71

2 stress stretches the piezoelectric, the voltage polarity is opposite to the poling axis. When a stress is applied to the piezoelectric film, the distance between the dipoles is modified and electric field is created, so a charge appears at the surface in order to compensate this field. Figure 2.4: Spring compression Figure 2.2: The piezo electric effect in a quartz crystal. Figure 2.3: First hand detailed block diagram When a stress is applied to peizo material it gets deformed i.e strain is acted upon it, which can be shown as follows : Y= Stress / Strain, where,y= young s modulus (which can be obtained from specification sheet of springs) In the above formula, stress can be found as follows where, Force = m * a Stress = Force / Area Now, peizo can be correlated with spring by spring equation which is Force = k * x where, x = D standing -D deflecting D standing = Height of spring when standing i.e force is not applied. D deflecting = Height of spring when force is applied. k = Spring rate of compression spring(constant). Force = force exerted on spring. By comparing above force equations,we get m * a = k * x (a) In the above equation acceleration can be found as k is constant, x can be found by above given equation and mass can be considered as ideal mass i.e 68kg. The electrical model that describes the piezoelectric material under a mechanical low frequency stress is given by: V = S* P The change in pressure is directly proportional to the change in voltage Here, S= k * h where S = sensitivity constant. k= peizo electric constant. Sr.no Crystal K=piezoelectric constant(v/g per unit applied pressure per unit thickness,v-m/n) Coupling coefficient (effeciency) 01. Quartz % 02. Rochelle salt % 03. Ammonium % dihydrogen phosphate 04. Ethylene % diamine tartirate 05. Lithium % sulphate 06. tourmaline % Table 2.1:Piezoelectric crystal properties 72

3 Further V = S*(F/A) As P=F/A Also F = m*a. Thus, V= S*((m*a)/A) (B) i.e. V is directly proportional to acceleration. Hence from equations A and B we can conclude that displacement is directly proportional to Voltage. III. ANALYSIS AND DESIGN A. Piezo-Electric Material The piezo used in the project plays the vital role in efficiency of the overall project. There are different piezo materials exhibiting different behaviour when exposed under various condition. The efficiency of each power harvesting device was tested under different excitation conditions to show the relative performance and allow the work to be compared to other studies. The results of the study found that both the Quick Pack and the monolithic piezoceramic material PZT were capable of recharging the batteries. However, the PZT was shown to be more effective in the random vibration environment that is usually encountered when dealing with ambient vibrations. Signal PZT efficiency (%) MFC efficiency (%) Resonant Chirp Hz Random Hz Table 3.1:Piezo electric efficiency It can be clearly seen that efficiency of PZT is greater than MFC. Hence, PZT is preffered over MFC. B. SPRING Rate which is the change in load per unit deflection, may be determined by the following procedure: 1. Deflect spring to approximately 20 percent of available deflection and measure load (P1) and spring length (L1). 2. Deflect spring to approximately 80 percent of available deflection and measure load (P2) and spring length (L2). Be certain that no coils (other than closed ends) are touching L2. 3. Calculate rate (R) lb./in. (N/mm) R = (P2 - P1) / (L1 - L2) The solid height of a compression spring is defined as the length of the spring when under sufficient load to bring all coils into contact with the adjacent coils and additional load causes not further deflection. Solid height should be specified by the user as a maximum, with the actual number of coils in the spring to be determined by the spring manufacturer.as square or rectangular wire is coiled, the wire cross-section deforms slightly into a keystone or trapezoidal shape, which increased the solid height considerably. The dimensional change is a function of the spring index and the thickness of the material. C. Darlington Pair Figure 3.1: Basic circuit diagram of darlington pair amplifier Considering entire darlington pair(with two transistors), we have IB1=200uA and we need IC2=60mA to drive the dc-dc converter. Therefore, the Total gain needed is IC2= β*ib1 i.e β= 300 Consider β1=30 and β2=10 DC ANALYSIS (Open all capacitors) Figure 3.2: Dc Model 73

4 IE1=IB2= (1+ β1)*ib1 =(1+30)200u IE1=IB2 =6.2mA IC1= β1*ib1=30*200u IC1=6mA IC2= β2*ib2=10*6.2m IC2=62mA IE2=(1+β2)*IB2=(1+10)*6.2m IE2=68mA TO FIND R E (EMITTER RESISTANCE) We know that, VBE=VB-VE VBE1+VBE2=VB-IE2*R E (From the data sheet for transistors BC-548 and SL-100 VBE1=VBE2=6V) 6+6=VB-68m*R E..(1) VBC1=VB-VC (From the data sheet for transistors BC-548 and SL-100 VB=30+10=40V Sub in eqn(1). R E =411.76Ω=430 Ω VCB1=30V) (Assume VO=approx10V) TO FIND R C (COLLECTOR RESISTANCE) I0=IB1*β1* β2 VO=IO*R C 10=200u*30*10*R C R C =166.7Ω=160 Ω TO FIND R1 AND R2 VBC1=(R B *IB1)+(IC1*R C ) 30= R B *200u+6m*166.7 R B =144.9K Ω=150k Ω Assume R1=600k Ω As Rb=(R1*R2)/(R1+R2) R2=200K Ω Figure 3.3:Final ckt diag after design completion A. Result IV. RESULT AND DISCUSSION 1. The model prepared delivers voltage of around 5-7 volts and current of around 300uA. 2. L.E.D used as a load, glows with full intensity, and continuously. 3. The Lithium-ion battery (rechargeable) of 2 volts/100mah is successfully charged. B. Discussion Initially, we decided to charge the mobile battery with the specifications as 3.7 volts/1800ua. But as we proceeded,this task was not achievable due to following reasons: 1) The piezo of required size were not available. 2) The DC-DC converter IC (Maxim 710 ) is not available in single piece. 3) Large number of such model is required. V. CONCLUSION The model prepared generates low voltage and current approximately 5 volts/200 ua in single stepping. Hence applicable only to low-power applications. If more such models are developed then it can be used for medium-power applications also. The model developed is robust since it can sustain a Human-weight. VI. REFERENCES [1] Ariponnammal, S. and Natarajan, S. (1994) Transport Phonomena of SmSel X Asx, Pramana Journal of Physics Vol.42, No.1, pp

5 [2] Barnard, R.W. and Kellogg, C. (1980) Applications of Convolution Operators to Problems in Univalent Function Theory, Michigan Mach, J.,Vol.27, pp [3] National Institute of Standards and Technology (NIST), Information Technology Laboratory (ITL), Advanced Encryption Standard (AES), Federal Information Processing Standards (FIPS) Publication 197, November [4] Stallings, W. Cryptography and Network Security: Principles and Practice. Prentice Hall Publishing, Third Edition, [5] Global spec. URL: [6] NVisionIP, Network Situational Awareness Tool, [Online]: 75