The Piezoelectric Electronic Circuit - Literature review Example

?With the continually growing demand for electri it has become a “lifeline for human population,” and the rising disparity between its supply and demand, paved way to exploring alternative sources of energy, as well as its sustainable use, and “methods of electrical power generation that does not negatively impact the environment” as burning fossil fuels. (Shah, n.d.) In view of this, “efforts are afoot internationally to incorporate piezoelectricity into the clean energy mainstream,” and in fact no longer just fiction, but have already debuted in globally leading countries, with state-of-the-art applications, ranging from quartz watches, to motion detectors, to piezoelectric floors and recently, Israel’s piezoelectric road—which is, “perhaps the most novel electricity-generating surface to date”. (Diamond, 2009) Much of its role in alternative energy and applications but first, what is piezoelectricity or, the piezoelectric effect? Wayne Tomasi (2004) defines the piezoelectric effect as generating electrical oscillations as varying mechanical stresses—either as, compression, tension, torsion or shearing, is applied across a crystal lattice structure (i.e. quartz, Rochelle salts, tourmaline, etc.) and vice versa. With this, ambient vibrations in and around systems which typically, are lost energy, can be captured and converted to usable energy, available for consumption—the primary goal of power harvesting; but since, as shown in research, the energy generated by piezoelectricity is insufficient to power most electronics, power harvesting technology has, mostly, focused on accumulation and storage techniques that would enable technology to collect enough energy for a variety of applications. (Sodano et al., 2005) In this premise, the researcher came up with a project, entitled “Integrated Circuits for Energy Harvesting Application”, aiming to design and build a prototype circuit that utilizes piezoelectricity—via the PFCB-W14 piezoelectric device, for energizing small electronic systems, which in this case, is the charging of a Lithium-ion rechargeable battery—which have become very popular today. Figure 1. Equivalent Circuit and Power Generation of PFCB-W14 at 27Hz To better visualize the concept of piezoelectricity, illustrated above is an equivalent circuit of a piezoelectric generator—functioning as a capacitor and a resistor in series with the output terminals, as well as a bar chart of the power generation of Advanced Ceramics Incorporated PFCB-W14 at 27Hz, both obtained from PFCB-W14 Specifications Sheet. By closely looking into the chart, it can be seen that with load resistance in the range of 400k? to 600k?, at typical amounts of force applied, there is maximum power. And along the lines of impedance-matching, when the load and source impedances—in this case, the load and internal resistances, were equal, maximum power transfer occurs, an important point to consider in every circuit design. (Boylestad & Nashelsky, 1998) Also, note that the output of the generator is an ac voltage. Disregarding impedance-matching, rectifying the piezoelectric generator, and directly connecting the output to a capacitor or battery would have been a more straightforward approach for the project. Despite its simplicity and the fact that this circuit works, with the enormous mismatch between the resistances of the generator (in the order of millions) and the battery (merely in ohms, and at times even down to milliohms), basically all the power would be dissipated as heat in the generator itself. For a better implementation of the project, the circuit shown below was considered. Figure 2. Simple Charging Circuit using Inductor Illustrated above is a simple charging circuit that utilizes an inductor, on top of piezoelectric generator, a rectifier bridge, a Zener diode and a Lithium battery that is being charged. Inductor Adding an inductor, as shown above, with sufficiently high reactance so as for the piezoelectric generator to “see” a 400k? of resistance (from the 400k??600k? range presented earlier), allows for maximum power transfer and charges the inductor to the same amount of power available in the generator, which can be utilized during the negative cycle of the piezoelectric generator to continually transfer power to the charging battery. In determining the size of the inductor, it is necessary to be familiar with the basic theory on inductive reactance (XL). According to DSC Ray A. Jackson (1998), XL is the opposition from the inertia effect of the counter-electromotive force (cemf)—typically greater than dc, caused by an ac source, and is primarily dependent on the inductance (L) and the frequency (f) of the current reversals, as given by: XL = 2?fL (in ohms). By manipulating this equation, the value for the inductance can be determined as, L = XL/2?f, where XL= 400k? and f = 27Hz (from the frequency response of PFCB-W14), finally giving L = 400k?/(2? x 27Hz) = 2,358 Henries—an extremely large inductance with extremely small current supplied from the piezoelectric generator. With the generator’s typical output voltage of 50Vpeak applied to a 400k? load, this gives current at 125uApeak, or 88uARMS (from the relationship, RMS = Peak). Using the formulae for approximate inductance—L = uru0N2A/l and flux density—B = uru0Ni/l, as presented in a discussion by Glenn Elert (n.d.) (except, instead of considering only the permeability of free air, u0, the relative permeability of the core, ur, was also taken into account), where: N-number of turns, A-area of core, l-length of loop, and i-current, necessary calculations for checking inductor specifications can be done. Initially, the researcher considered using an E55 core and bobbin Epcos B66335 and Beldin AWG38 enameled wire. Although the calculated inductance was very close to the required value (2,605 Henries), and saturation current was greater than the requirement (1.29mA)—which would probably work quite well, the dc resistance of the inductor was considerably very high (4,863Henries) and would be detrimental to the circuit’s operation. Subsequently, a smaller, much cheaper inductor using a Pot Core and Bobbin from Jacar Electronics, and AWG30 wire (0.3mm diameter), was considered as a more practical option. However, calculating for inductance and saturation current using the data found in LF-1060 core and LF 1062 bobbin specification sheets gave results (L = 0.697 Henries, i = 19mA) that were incomparable with the ideal values, mainly because very fine winding wire (0.06mm) was available at the time. Now for an alternative wind, the very fine wire was wound up to 7,500 turns (instead of 404 turns) to achieve a very acceptable value for saturation current, that is in the order of 1 to 2mA. In turn, the wire resistance went up to around 2.5kW, inductance to 240 Henries; hence, inductive reactance would increase to around 40,750W—a much better result. Although it is not exactly as what was originally outlined, any inductance is better done none. Rectifier Bridge ETC Allen F. Carney (1998) states that basically, a rectifier converts an ac signal to a pulsating dc, that is a portion of the input depending on the type of rectifier used—full-wave or half-wave. For this project, a full-wave rectifier was used specifically a bridge (basic circuit shown in Figure 3 below), since per definition—not only does it allow current to flow in the same direction for both positive and negative cycles of the ac source, it also produces the same peak voltage as half-wave rectifiers (nearly twice that of the conventional full-wave circuit) and a ripple frequency same with conventional full-wave rectifiers, plus, a low peak inverse voltage (PIV) to average output voltage ratio. (Carney, 1998) Figure 3. Basic Bridge Rectifier Circuit In this application, the rectifier bridge (D1, D2, D3 & D4) acts as a commutation diode and allows current to flow from the inductor to the battery when no current flows from the piezoelectric generator. Zener Diode A Zener diode is a PN junction designed to operate in the reverse-bias breakdown region, and once its breakdown voltage is reached, the voltage across the diode practically remains constant regardless of the voltage supply, hence making it ideal and popular for use as voltage regulators. (Howard, 1998) The Zener diode (D5) used in this application prevents overcharging of the battery. Moving on, the best part of this circuit is that, since the inductor is current-based, voltage differences between the piezoelectric generator and the battery are fully converted, acting almost a buck converter. But what is a buck converter? A buck converter is a non-isolating type converter used for voltage step-down/reduction by a relatively small ratio—ratios roughly less than 4:1, and with a basic circuit configuration as shown below. (Jaycar Electronics, 2001) Figure 4. Basic Circuit for a Buck DC-DC Converter The Buck DC-DC Converter illustrated above comprises a switching power MOSFET (Q1), flywheel diode (D1), inductor (L), output filter capacitor (C1), and a control circuit—for the monitoring of the output voltage, as well as switching of Q1 (at a fixed rate, but varying duty cycle) to maintain the desired voltage level—a fraction of the input voltage that is actually equal to the duty cycle; and assuming that the converter is perfectly efficient, a quick rule of thumb: Iout/Iin = Vin/Vout, can be established. (Jaycar Electronics, 2001) While the circuit shown in Figure 2 offers a better implementation than the straightforward approach, it does not fit the scope of the project. For the necessary voltage monitoring, both for the piezoelectric generator and the battery, the researcher made use of additional circuitry as illustrated in the following diagram. Figure 5. Actual Piezo Electronic Circuit Circuit Design and Operation Knowing that the circuit should draw as little power as possible due to power available in the piezoelectric generator (in the order of milliWatts), the researcher found a suitable opamp—a very low quiescent opamp from Microchip, MCP6141, that consumes only 0.6uA, with an input offset voltage of 3mV at 1pA, an output current of 0.1mA, and an output rail voltage within 0.5mV. When the battery is charged, only two parts of the circuit are active: the basic circuit shown in Figure 2—except the battery is replaced with a 10uF 50V capacitor (C1) that charges up to 50V as limited by the 50V Zener diode (Z1), and the battery voltage monitoring circuit—comprising IC1 that in turn provides necessary connections between diodes and resistors. The battery voltage is tested against the reference voltage (~1.4V) of the two diodes (D5 and D6) in series. R1 allows precise adjustment of the cut off voltage; that is, 4.2V for as single cell of a Lithium-ion battery, and well within the MC6141 operating range of 1.4 to 5.5V. C2 stabilizes the monitored voltage so that charging pulses do not unduly change the output of IC1 (trigger point at 4.2V). Current through the 10MW resistor R4 is given by, I = (4.2-1.4)/10MW = 280nA, which is 280,000 times greater than the input bias current of 1pA, so the MC6141 will have no effect on the value, and will also be easily driven by the current through R4, D5 & D6. Because all paths pass at least this much current, all other resistor and diode totems will be assumed to carry enough current. The three resistors/potentiometers, R1, R2 and R3, have a total series resistance of 4.2MW, and at 4.2V, the endpoints of R1 will have voltage values of V = 4.2V (1/4.2), and V = 4.2V (2/4.2) = 2V. This means that R1 can be adjusted around midrange for IC1 to trigger at 4.2V. Current though this branch is given by, IR1= 4.2V/4.2MW = 1uA, and with the quiescent current of MC6141 at 0.6uA, the total current in the battery voltage monitoring circuit would be, I = 0.28uA + 1uA + 0.6uA = 1.88uA. Current through R5 will be the same as R4 when IC2 is active, 280nA.When the battery voltage falls below the preset point, the output of IC1 will go ‘High’ now supplying voltage to IC2. IC2 tests the voltage of the capacitor C1 against the reference voltage (~1.4V) of the diodes (D7 and D8) in series, and R6 allows adjustment of the threshold voltage to around 10V. If the voltage of C1 is over the threshold, the output of IC2 will go ‘High’. The totem pole of resistors R6, R7 and R8 is not driven by the battery, but instead by the piezoelectric generator whose total resistance is given by R = 8.2MW + 1MW + 1MW||10MW = 10.1MW, when the voltage falls under the threshold. When the battery is not charging, voltage on the piezoelectric generator can be up to 50V, but when charging, R6 is adjusted for a threshold voltage of 10V. When the generator is just over threshold, R9—connected to 4.2V, provides a small amount of positive feedback, moving the threshold down a little so that IC2 will not turn immediately back off. Additional current consumption of this section is 1.25uA, from the current through R8 when IC2 is turned on: I = 1.25V/1MW = 1.25uA. Total current for this section is given by, I = 0.28uA + 1.25uA + 0.6uA = 2.13uA. C3 and C4 are for rail voltage stabilization of the related opamps. The output of IC2 powers IC3. IC3 is configured as a negative pulse generator, such that when it turns on, output is initially ‘High’. R12 raises the voltage between R10 and R11 a little. The output of IC3 is also connected to C5 via R13. C5 charges till it reaches the voltage set by R11, at which the output of IC3 will go ‘Low’. R12 then lowers the voltage between R10 and R11, causing C5 to now discharge until it reaches the set voltage on R11, and the cycle goes on. R12 alternates in being in parallel with R10 (IC3 is ‘Low’) or R11 (IC3 is ‘High’), giving the same resistance combination of R12||R = 1/(1/R12 + 1/R) = 1/(1/680K? + 1/1M?) = 405k?, since R10=R11=R. This gives voltages on top R10 when IC3 is ‘Low’ and ‘High’ respectively as: V = 405k?/(405k? + 1M?) x 4.2V = 1.2V, and V = 4.2V - 1.2V = 3V. R1 adjusts the positive input between these voltages and the positive rail of 4.2V. The output of IC3 is also connected to the negative input via R13; the input voltage is restrained by C5. With varying output in IC3, the voltage of C5 and the negative input are driven towards the new voltage of the positive input, in alternating directions. Since R11 sets the set point voltage closer to the positive rail, the charging voltage for C5 will be less than the discharging voltage; hence, the time taken to discharge will be shorter than the charging time, and output will be ‘Low’ for a shorter time than it is ‘High’. R11 adjusts the ‘Low’ to ‘High’ time ratio, while R13 adjusts the current in and out of C5, in turn, adjusting the frequency of oscillation. The ‘Low’ time is adjusted to approximately 0.05msec, and the ‘High’ time to 5 to 10 times longer. Assuming R1 is at 50%, then the positive input changes between 2.7V and 3.6V—these can be used, along with the set ‘Low’ time of 0.05msec, in obtaining a capacitor value via the formula for dc voltage discharge: Vt = V0 x e-t/RC; rearranging equation into C = -t/(R x ln(Vt / V0)), gives C = 174pF (220 pF was chosen to make the oscillations slower than the calculated frequency, as varying R13 increases the oscillations). (Antonine Education, n.d.) Furthermore, with the relationships: Q = CV, and I = Qf, also presented by Antonine Education (n.d.), the Q required to charge C5 from 2.7V to 3.6V, and the total current at frequency 3.3kHz, are found as 198pC and 653nA, respectively. Average current in R13 set at 1M? with time period, 1 to 5, is approximately 1.85uA, while current in R10, R11, and R12 totem pole is given as, I = 4.2V/(1M? + 405k? = 2.99uA. The IC3 output is also connected to the P-Channel MOSFET (Q1), 2SJ380, via the voltage shift capacitor C6. C6 charges to the voltage difference between C1 and the upper rail of IC3 via R14, as well as D9 if the voltage on C6 is larger than the difference. This causes Q1 to turn on each time the output of IC3 goes ‘Low’. The diode D9 causes C6 to discharge quickly when the voltage on C1 falls quickly, maintaining the voltage shift from IC3 that turns on Q1. Total current in this section is: I = 2.99uA + 0.6uA + 0.653uA + 1.85uA = 6.09uA. Each time Q1 turns on, charge from C1 passes to L2, where current builds up for the 0.1mSec that Q1 is turned on. Q1 turns off and the current in L2 continues to flow to the battery via the commutation diode D10. The parts Q1, L2 and D10 comprise a discontinuous Buck Converter circuit, converting the voltage and current in C2 to the lower voltage but increase charging current of the battery. Appropriate inductance for L2 was found using a formula given by National Semiconductor (2002) as L = Et/(rIo), where: L -minimum inductance, E-voltage across the inductor, t-time voltage is applied, r-ripple ratio, and Io-output current, giving an inductance L = (4 x 50 x 10-6)/(2 x 0.5 x 10-3) = 200mH (220mH inductor was chosen with saturation current greater than 0.5mA). When all stages are running, total loss is given by, Loss = 1.88uA + 2.13uA + 6.09uA = 10.1uA. And when the battery is charged, only 1.88uA is drawn from the battery. Review of Related Literature Residential Piezoelectric Energy Sources According to Andrew Katz (2004), since the DELTA Smart House will comprise lots and lots of sensors and microelectronic devices that would carry out a variety of applications throughout the house; hence, he came up with a project that would utilize piezoelectricity mainly as a clean and reliable energy source which does not require regular maintenance to power the said applications, as well as in incorporating additional features in the smart house. The piezoelectric technology can be integrated into a residential setting in a variety of lines, the most conventional perhaps would be as substitute to batteries in providing power to sensors throughout the house, ridding off necessary battery replacements—which would have been a menial job considering the large number of sensors. Another application would be in eliminating and/or enhancing vibrations in certain appliances. For the former, Dr. Rob Clark and Dr. Henri Gavin proposed the use of piezoelectric materials in the development of device that autonomously adapts the amount of dampening with the magnitude of the vibrations present; the latter is basically the opposite. By installing proof mass actuators into the floor, say—of a media room, signals that are too low to be heard (below 20Hz) can be converted to vibrations, giving a more realistic feel, for instance—an explosion in a movie can come with a shake in the room. After that, there is the use of piezoelectric cables—a much cheaper alternative for flat piezoelectric transducers, in creating a grid across the floor for tracking both the location and the orientation of the person within the house, probably also identify the person basing on the manner of walking; on top of that, power can also be generated from people moving around. Figure 6. Energy Harvesting Circuit Illustrated above is the piezoelectric circuit that was designed for the project, which is composed of an AC/DC rectifier—for converting the ac signal from the piezoelectric element into dc, a filter capacitor—which smoothes the current flow, and the DC-DC converter—which enables energy storage into the battery, as well as the increase in the power output from the piezoelectric transducer. The major obstacle in the design was basically on optimizing the power transfer, other than that, the project was a success. Piezoelectric Materials for Powering Wireless Remote Sensors Tayahi et al. (n.d.) presented the modeling, design, and experimental validation of a piezoelectric generator for remote and long-term sensing networks, which include local and higher level processing so as to minimize power consumption requirements, and analyze real-time data concurrently with network traffic reduction, respectively. Basically, the piezoelectric (PE) source was modeled as a capacitor which responds—with a charging-action, to vibrations, and assumed to have a frequency as low as a typical walking pace, hence generating a high voltage and a low current. The group came up with a circuit design that is based on a switch-mode DC-DC converter that allows for PE output current amplification illustrated in the following block diagram. Figure 7. Piezoelectric Generator Block Diagram Based on the above diagram, the PE signal is initially rectified, after which, utilized in the charging of a large “bucket” capacitor to a predetermined level—once reached, the capacitor then discharges into the regulator. The discharge circuit is turned off as soon as the regulator output have fallen below a required level, allowing, yet again, the capacitor to charge up. The PE generator can either be a stand-alone power source, a back-up battery or a battery charger, essentially for powering sensor transceivers based on vibrations around, whose design, offers a foundation for self-energized sensors that are useful in process condition monitoring. Miniaturised Battery Charger using Piezoelectric Transformers Navas et al. (2001) have introduced a miniaturized battery charger for mobile phones by utilizing piezoelectric transformers—which offer high power densities (around 25W/cm3), absence of electromagnetic noise and high voltage isolation capability, as well as a size detail which basically made the development of a plug-type travel charger that is smaller than a matchbox ( Read More