LOW LOAD, LOW FREQUENCY PIEZO-ELECTRIC POWER GENERATOR
FIELD OF THE INVENTION The present invention relates to system for harvesting, generating, storing and delivering energy and implementation thereof, more particularly, the present invention relates to piezoelectric power generators.
BACKGROUND OF THE INVENTION
Generally, electronic devices incorporate onboard power sources such as a miniature electrical cell to power the components present within the electronic devices. Such power sources have a limited operating lifetime, especially if they are required to power their associated tags continuously. Further, there arises a requirement of having power sources consuming less space for implementation. To address these issues, power generators such as piezoelectric generators were used in many applications for better harvesting of the device at a given source of vibration. These generators are portable devices which are capable of being attached to items or personnel wearable. They can be used, for example, for remotely powering some electronic devices, and recharging batteries passively.
Conventional piezoelectric generators are huge in size and require very huge inductors to actually address the need for impedance mismatch and efficient harvesting. Existing piezoelectric generators are bulky and difficult to incorporate within personnel wearable clothing / accessories due to the heavy mass. The problem is further complicated since the device is not being driven at resonance, and also there is a restriction on the size of the piezoelectric element that can be used. Impedance matching is also a problem to be tackled, while operating at very low frequencies.
Therefore, there arises a need for piezoelectric power generators that are compact in size, and portable for different applications, operable at low load and achieve impedance matching thereby, overcoming the problems existing in the art. SUMMARY OF THE INVENTION
The shortcomings of the prior art are overcome and additional advantages are provided through the present disclosure. Additional features and advantages are realized through the techniques of the present disclosure.
Other embodiments and aspects of the disclosure are described in detail herein and are considered a part of the claimed disclosure.
Accordingly, the present disclosure relates to a piezoelectric power generator system comprising: a piezoelectric module configured to receive a low frequency input energy and convert the received input energy into a corresponding low input ac voltage; and a power conditioning circuit coupled with the piezoelectric module and configured to generate a high output voltage based on the low input ac voltage received from the piezoelectric module, wherein the power conditioning circuit comprising a rectification unit for converting the low input ac voltage into a corresponding dc voltage; a first storage means having a predetermined storage capacity and configured to temporarily store the rectified input dc voltage within the predetermined capacity; and a voltage converter coupled with the first storage means and comprising a MOSFET switch operating in ON and OFF phases, a second storage means and an inductor operatively coupled with the MOSFET switch, wherein said inductor is configured to continuously receive the input dc voltage from the first storage means when the MOSFET switch is at ON phase and store therein until the stored voltage exceeds a predetermined threshold voltage, for generating high output voltage and to discharge the generated high output voltage to the second storage means during when the MOSFET switch is at OFF phase.
The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS
The features of the present invention are set forth with particularity in the appended claims. The invention itself, together with further features and attended advantages, will become apparent from consideration of the following detailed description, taken in conjunction with the accompanying drawings. One or more embodiments of the present invention are now described, by way of example only, with reference to the accompanied drawings wherein like reference numerals represent like elements and in which:
FIG. 1 illustrates a schematic diagram of a power generator system in accordance with an embodiment of the present invention.
FIG. 2 illustrates a perspective view of the power generator system in accordance with an embodiment of the present invention.
FIG. 3 illustrates a simplified circuit diagram of the power conditioning circuit in accordance with one embodiment of the present invention.
FIG. 4 illustrates a simplified circuit diagram of the improved power generator system in accordance with another embodiment of the present invention.
FIG. 5 illustrates a graphical diagram illustrating the range of input voltage level used for generating power by the power generator system in accordance with an embodiment of the present invention.
The figures depict embodiments of the disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the disclosure described herein. DETAILED DESCRIPTION
While the disclosure is susceptible to various modifications and alternative forms, specific embodiment thereof has been shown by way of example in the drawings and will be described in detail below. It should be understood, however that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternative falling within the spirit and the scope of the disclosure. The terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a setup, device or method that comprises a list of components or steps does not include only those components or steps but may include other components or steps not expressly listed or inherent to such setup or device or method. In other words, one or more elements in a system or apparatus proceeded by "comprises... a" does not, without more constraints, preclude the existence of other elements or additional elements in the system or apparatus.
Accordingly, the present disclosure relates to a piezoelectric power generator system comprising: a piezoelectric module configured to receive a low frequency input energy and convert the received input energy into a corresponding low input ac voltage; and a power conditioning circuit coupled with the piezoelectric module and configured to generate a high output voltage based on the low input ac voltage received from the piezoelectric module, wherein the power conditioning circuit comprising a rectification unit for converting the low input ac voltage into a corresponding dc voltage; a first storage means having a predetermined storage capacity and configured to temporarily store the rectified input dc voltage within the predetermined capacity; and a voltage converter coupled with the first storage means and comprising a MOSFET switch operating in ON and OFF phases, a second storage means and an inductor operatively coupled with the MOSFET switch, wherein said inductor is configured to continuously receive the input dc voltage from the first storage means when the MOSFET switch is at ON phase and store therein until the stored voltage exceeds a predetermined threshold voltage, for generating high output voltage and to discharge the generated high output voltage to the second storage means during which the MOSFET switch is at OFF phase.
In one embodiment of the present invention, the rectification unit comprises zero-bias Schottky diodes for rectifying the input dc voltage.
In one embodiment of the present invention, the first storage means is having a low ESR value and configured with a predetermined storage capacity based on the ESR value. In one embodiment of the present invention, the system comprising a reverse bias diode coupled with the voltage converter and configured to minimize the reverse flow of output voltage back to the voltage converter.
In one embodiment of the present invention, wherein the power conditioning circuit comprising the voltage converter integrated with the rectification unit, wherein the voltage converter is configured to generate a regulated output voltage higher than a predetermined threshold level, wherein the regulated output voltage is generated by regulating the dc voltage to a first predetermined threshold voltage level and further to a second predetermined threshold voltage level therein.
In one embodiment of the present invention, the piezoelectric module comprising an array of piezoelectric elements connected in parallel or in series; and plurality of copper electrodes housing the said array of piezoelectric elements for transferring the input voltage to the power conditioning circuit.
In one embodiment of the present invention, the piezoelectric element is configured with a higher d33 piezoelectric charge coefficient. In one embodiment of the present invention, further comprising a first storage means having a predetermined storage capacity and configured to temporarily store the energy excess of the predetermined threshold level there within. In one embodiment of the present invention, wherein the first storage means is having a low ESR value and configured with a predetermined storage capacity based on the ESR value.
In one embodiment of the present invention, wherein the voltage converter further comprising a voltage comparator circuit configured to compare the energy thus stored in the inductor with the predetermined threshold voltage and generate a signal indicating the result of comparison.
In one embodiment of the present invention, the voltage converter further comprises a feedback system to regulate the output voltage. In one embodiment of the present invention, the power conditioning circuit is embedded on an integrated chip (IC).
In one embodiment of the present invention, the power generator system is connected to an external storage device for transferring the high output voltage thus generated. In one embodiment of the present invention, the impedance matching is resistive, and is provided by the Discontinuous mode (DCM) operation of the voltage step up converter.
In one embodiment of the present invention, the piezoelectric module is rectangular in shape. In one embodiment of the present invention, wherein the external storage device includes but not limited to a Li-ion or Ni-Mh,or Ni-Cd button cell.
FIG. 1 illustrates a schematic diagram of the power generator system in accordance with an embodiment of the invention.
As shown in fig.l , the piezoelectric power generator is constructed for harvesting of energy at low load and low frequencies. In one embodiment, the power generator converts mechanical energy into electrical energy required for harvesting and storing purposes. In operation, the power generator is activated by an actuator (101) supplying low load which may be for example similar to the load applied by a human being while walking. The power generator includes one or more components such as a piezoelectric module for converting the mechanical load into electrical energy.
The actuator (101) is generally a mechanical stimulus applied onto a piezoelectric module that converts the mechanical stimulus into equivalent electrical energy. The mechanical stimulus for example, the load of an average human being can be provided by means of a pneumatic cylinder that supplies the required pressure to operate the device at a constant load and frequency. The cylinder is provided with a piston that transfers the mechanical load onto the piezoelectric module. The frequency of operation of the piston is controlled by a circuit such as for example a simple 555 timer circuit. The circuitry controls a solenoid valve which regulates the pressure inside the cylinder. The frequency of switching of the solenoid valve can regulate the frequency, the air valves can regulate the impact and the pressure input determines the value of the actual pressure applied.
The pneumatic cylinder is set to any loading equivalent of for example, 60- 100kg, and a frequency of around 0.6 Hz, under low impact force. The impact of the cylinder piston is used as the mechanical stimulus which is converted to an electrical power by the piezoelectric module.
The piezoelectric module includes a pair of copper electrodes (102a and 102b, collectively referred to as 102) and at least two piezoelectric ceramic elements (103) connected through wires (104) to a power conditioning circuit embedded on a printed circuit board (PCB) (105).
In one embodiment, the piezoelectric module is rectangular shaped or a compact box shaped structure comprising the piezoelectric elements (103) arranged either in parallel or series. One or more copper electrodes (102) are disposed above and below for housing the said piezoelectric elements (103) so as to protect the piezoelectric elements (103) from excess external shock, and also to derive electrical charge from the piezoelectric elements when a mechanical force is applied. In one embodiment, a pair of piezoelectric elements (103) is placed in parallel to reduce their total output impedance. The piezoelectric elements are having a higher d33 piezoelectric charge coefficient (PCC) expressed in pC/N. In one embodiment, the piezoelectric coefficient of the elements used is in the range of 550-700 pC/N.
A silver electrode is applied on the surface of the piezoelectric elements (103) to enable the transfer of the energy generated by the piezoelectric elements (103) to the copper electrodes (102). The copper electrodes (102) housing is configured to induce equal force upon the piezoelectric module, and hence generate approximately equal voltage. The charge thus generated is transmitted via the wires (104) for example copper wires that are connected to the power conditioning circuit embedded within the PCB (105).
The power conditioning circuit embedded within the PCB (105) is located out of the pressure axis, to prevent any damage by the mechanical stimulus to the PCB components so that the energy generated via the excitation of the piezoelectric module is transferred to the PCB (105) located at a safe distance.
The PCB (105) comprises the power conditioning circuitry having one or more electrical components to enable the transfer of energy generated by the piezoelectric module for storing onto a storage battery. In one embodiment, the power conditioning circuit comprising a rectification unit and a voltage converter through which the energy generated by the piezoelectric module is rectified, stepped up for storing in a battery. The voltage converter operates in a discontinuous mode (DCM) for increasing the voltage amplitude of the generated energy to a relatively higher voltage magnitude that is required to store the energy in the battery.
Further, the power conditioning circuit comprises a reverse flow stoppage diode from the battery to minimize the reverse flow of the voltage back to the voltage converter. FIG. 2 illustrates a perspective view of a piezoelectric module in accordance with an embodiment of the present invention.
The piezoelectric module is configured to convert the mechanical energy into electrical energy using a piezo-electric generator or module. In one embodiment, the peizo-electric module, as shown in Fig. 2, comprises disk shaped piezoelectric elements (201) (which are 2- 8 mm in thickness and 0.5-2 inches in diameter), housed in a mechanical compartment for protection and also for equal load distribution.
In one embodiment, the piezoelectric module is rectangular in shape or a compact box shaped structure comprising the piezoelectric elements (201) arranged either in parallel or series. One or more copper electrodes (202a, 202b) is housing the said piezoelectric elements (201) to protect the piezoelectric elements from excess external shock, and also efficiently to derive electrical charge from the piezoelectric elements when a mechanical force is applied. In one embodiment, a pair of piezoelectric elements is placed in parallel to reduce their total output impedance. The piezoelectric elements are having a higher d33 piezoelectric charge coefficient (PCC) expressed in pC/N. In one embodiment, the piezoelectric coefficient of the elements used is in the range of 550-700 pC/N.
A silver electrode is applied on the surface of the piezoelectric elements (201) to enable the transfer of the energy generated by the piezoelectric elements (201) to the copper electrodes (202a, 202b). The copper electrodes (202a, 202b) housing is configured to induce equal force upon the piezoelectric module, and hence generate approximately equal voltage. The charge thus generated is transmitted via the wires for example copper wires that are connected to the power conditioning circuit embedded within the PCB. The voltage generated is proportional to the load on PZT. On variation of load from 40-80kg, there is an increase in voltage generation from 8- 12V.
FIG. 3 illustrates a simplified circuit diagram of the power conditioning circuit in accordance with one embodiment of the invention. In one embodiment, the power conditioning circuit as shown in fig.3 is embedded on a PCB or an integrated chip (IC) and located at a safe distance from the pressure axis applied onto the piezoelectric module indicated as 301 herein. The circuit is embedded on the PCB having the form of a rigid or flexible card. The power conditioning circuit comprises a rectifier or a rectification unit (302) for converting the low input ac voltage generated by the piezoelectric module into a corresponding dc voltage. The rectifier (302) may be for example zero-bias Schottky diodes to have minimal drop and voltage loss during the rectification process and also to eliminate high reverse leakage of voltage. Further, the power conditioning circuit comprises a first temporary storage means (303) coupled with the rectification unit (302) and configured to store energy before transferring to the external battery. The first storage means (303) may be an external capacitor having a predetermined storage capacity and configured to temporarily store the rectified input dc voltage within the predetermined capacity. In one embodiment, the first storage means (303) may be a capacitor having a low Equivalent series resistance (ESR) value to prevent any energy losses and configured with a predetermined storage capacity based on the ESR value. In one embodiment, the first storage means may be selected having ESR value of 0.6 ohm. In another embodiment, the first storage means may be selected having ESR values ranging from 0.01 - 1 ohm. Some of the factors to be considered while selecting the first storage means (303) with low ESR value are such as capacitor leakage, size and voltage levels to be obtained for final transfer to the external battery.
The capacitor leakage is considered while selecting the first storage means (303) to have a minimum leakage, otherwise, the charging process would take a long time if the leakage of energy is greater than the supply and most of the energy would be wasted. Further, the size of the first storage means (303) determines the voltage obtainable at a constant load. Furthermore, the size affects the impedance matching as the output energy level heavily depends on the impedance matching of the storage means (303). If the capacitive impedance mismatch is so high, even a slight variation in the capacity of the storage means (303) can cause a major change in the output energy thus derived. Therefore, the storage means (23) is selected considering the above criteria since the storage means (303) accumulates the energy before transferring to the battery. Further, based on the selection of the storage means (303), wastage of energy due to unnecessary continuous transfer by the power conditioning circuit is avoided.
The size of the storage means (303) is determined so that the same will not affect the voltage levels, the power derivable, since ESR drop becomes a vital component. Also, the size is determined so as to avoid loss of energy due to continuous transfer. In one embodiment, the storage means (303) may be low ESR ceramic capacitors. In another embodiment, the storage means (303) may be an ultra-capacitor, or a super-capacitor, as these capacitors have much lesser ESR value.
The power conditioning circuit further comprises a voltage converter for example a DC-DC converter coupled with an inductor (304) and a second storage means (305) and further coupled with the first storage means (303) to receive the energy there from. The voltage converter (306) is configured to operate in a discontinuous mode (DCM) and triggered to operate based on the amount of energy stored in the first storage means (303). In one embodiment, the voltage converter (306) operates in either saturation or cut-off mode within a specific band of voltage. The operation of the voltage converter (306) in DCM mode enables the first storage means (303) to store the energy over a particular period of time and then to transfer the energy as a whole. The operation of the voltage converter so described is achieved through MOSFET switches which have low turn ON and low turn OFF voltages and the first storage means (303). The MOSFET switches are configured to operate in two phases such as ON and OFF phase. When the MOSFET switch is operating at ON phase, the inductor (304) continuously receives the dc voltage thus stored in the first storage means (303) and store therein. The stored energy is then discharged as high voltage to the second storage means (305) during which the MOSFET is operating at OFF phase. The transfer of energy is triggered or activated by a voltage comparator circuit when the circuit determines that the stored energy exceeds a predetermined threshold voltage. The voltage comparator circuit is configured to compare the energy thus stored in the inductor (304) with the predetermined threshold voltage and generate a triggering signal based on the result of comparison i.e., when the stored energy exceeds the predetermined threshold voltage.
The operation of the MOSFET switches in the manner as described above enables the voltage converter to operate in a discontinuous mode and result in huge impedance. The resulting impedance is resistive and therefore achieves resistive impedance matching of the piezoelectric module.
Further, the piezoelectric elements having high piezoelectric coefficient enables generation of high output voltage at low load. In one example, a load having a pressure of 80kg resulted in high output voltage such as, 12 volts. In another example, a load having a pressure of 60 kg resulted in the high output voltage of 10 volts. In yet another example, a load having a pressure of 40 kg resulted in high output voltage in the range of 8-10 V.
The high voltage energy thus transferred by the voltage converter (306) is then extracted or stored in an external battery (307) such as for example Li-ion battery, or Ni-Mh,or Ni-Cd button cell. The output potential of the converter is dependent on the optimum charging potential of the external battery that is used. In one embodiment, the Li-ion cell is used that has the highest energy to space ratio among the current battery modules, also the fact that the Li-ion cells are button cells therefore, again reduces the total size of the device.
Further, the power conditioning circuit comprises a reverse bias diode (308) coupled with the voltage converter (306) and configured for minimizing the reverse flow of output voltage back to the converter (306). Furthermore, the power conditioning circuit comprises a feedback system to regulate the output voltage.
FIG. 4 illustrates a simplified circuit diagram of the improved power generator system in accordance with an alternate embodiment of the present invention. The power conditioning circuit as shown in fig. 4 is embedded on a PCB or an integrated chip (IC) and located at a safe distance from the pressure axis applied onto the piezoelectric module (402). The circuit is embedded on the PCB having the form of a rigid or flexible card. In alternate embodiment, the PCB comprises a voltage regulator or otherwise a voltage converter (404) to regulate the input voltage to obtain the desired output voltage. The voltage converter comprises a rectifier unit internal to the converter that has a very low voltage drop across the diodes. The voltage drop can be used as the base for AC-DC conversion, and simultaneously reducing reverse leakage to a minimum level.
The PCB further contains a temporary storage mechanism coupled with the rectifier unit in the converter and configured to store energy before transferring to the external load. The temporary storage means may be an external capacitor having a predetermined storage capacity and configured to temporarily store the rectified input dc voltage within the predetermined capacity. In one embodiment, the temporary storage means may be a capacitor having a low Equivalent series resistance (ESR) value to prevent any energy losses and configured with a predetermined storage capacity based on the ESR value. Some of the factors to be considered while selecting the temporary storage means with low ESR value are such as capacitor leakage, size and voltage levels to be obtained for final transfer to the external battery.
The capacitor leakage is considered while selecting the temporary storage means to have a minimum leakage, otherwise, the charging process would take a long time if the leakage of energy is greater than the supply and most of the energy would be wasted. Further, the size of the temporary storage means determines the voltage obtainable at a constant load. Furthermore, the size affects the impedance matching as the output energy level heavily depends on the impedance matching of the storage means. If the capacitive impedance mismatch is so high, even a slight variation in the capacity of the storage means can cause a major change in the output energy thus derived. Therefore, the storage means is selected considering the above criteria since the storage means accumulates the energy before transferring to the battery. Further, based on the selection of the storage means, wastage of energy due to unnecessary continuous transfer by the power conditioning circuit is avoided.
The size of the storage means is determined so that the same will not affect the voltage levels, the power derivable, since ESR drop becomes a vital component. Also, the size is determined to avoid loss of energy due to continuous transfer. In one embodiment, the storage means may be low ESR ceramic capacitors. In another embodiment, the storage means may be an ultra- capacitor, or a super-capacitor, for the reason these capacitors have much lesser ESR value. The voltage converter (404) is a DC-DC device that integrates a low-loss full wave bridge rectifier with a high efficiency buck converter to form a complete energy harvesting solution optimized for high output impedance energy sources such as piezo-electric transducers. An ultralow quiescent current under voltage lockout (UVLO) mode with a wide hysteresis window allows charge to accumulate on an input capacitor until the buck converter can efficiently transfer a portion of the stored charge to the output. In terms of regulation, the converter enters a sleep state in which both input and output quiescent currents are minimal. The buck converter turns ON and OFF as needed to maintain regulation. An input protective shunt enables greater energy storage for a given amount of input capacitance. The buck regulator uses a hysteretic voltage algorithm to control the output through internal feedback from the Vout sense pin. The buck converter charges an output capacitor through an inductor to a value slightly higher than a regulation point. The charging is done by ramping the inductor current up to certain value through an internal PMOS switch and then ramping the same down to 0mA through an internal NMOS switch. This efficiently delivers the energy to the output capacitor. In one embodiment, the ramping rate is determined by Vin, Vout, and inductor value.
Harvested energy can be stored on the input capacitor or on the output capacitor. The wide input range takes advantage of the fact that energy storage on a capacitor is proportional to the square of the capacitor voltage. After the output voltage is brought into regulation any excess energy is stored on the input capacitor and the voltage increases. When a load exists at the output the regulator can efficiently transfer energy stored at a high voltage to the regulated output. While energy stored at the input utilizes the high voltage at the input, the load current is limited to what the converter can supply. If larger loads need to be serviced the output capacitor can be sized to support a larger current for some duration.
In one embodiment, the present invention can be implemented in a shoe, especially the PZT module can be placed under the heel of the shoe and rest of the PCB and other elements placed outside the pressure axis. In order to achieve maximum power efficiency, piezoelectric ceramics position should be as near to the heel as possible. On account of this, the piezo- ceramics were placed around 3-8 mm below the heel part of the shoe. The electronic circuitry required for energy regulation is placed at outer portion of the shoe beside rechargeable battery for storing energy. This can be modified such that the electronic circuitry is placed inside the shoe, out of the pressure axis, and the rechargeable battery is placed outside the boot.
Further, the piezoelectric elements having high piezoelectric coefficient enables generation of high output voltage at low load. The voltage generated is proportional to the load on PZT. In one example, on variation of load from 40-80kg, there is an increase in voltage generation from 8- 12V.
The high voltage energy thus transferred by the DC-DC converter (404) is then extracted or stored in an external battery (406) such as for example Li-ion battery, or Ni-Mh,or Ni-Cd button cell. The output potential of the converter is dependent on the optimum charging potential of the Li-ion cell used. The Li-ion cell has the highest energy to space ratio among the current battery modules, also the fact that the ones used are button cells again reduces the total size of the device.
Further, the power conditioning circuit comprises a reverse bias diode coupled with the voltage converter (404) and configured for minimizing the reverse flow of output voltage back to the converter (404). Furthermore, the power conditioning circuit comprises a feedback system to regulate the output voltage.
As illustrated in fig. 5, the graphical diagram shows a plot of input voltage against the time (sec) clearly indicating the higher levels of input voltage transferred into the power conditioning circuit of the power generator system. As shown, a maximum voltage of 4-5 volts is transferred to the power conditioning circuit from the PZT module which is higher as compared to that achieved in existing art.
The present disclosure is not to be limited in terms of the particular embodiments described in this application, which are intended as illustrations of various aspects. Many modifications and variations can be made without departing from its spirit and scope, as will be apparent to those skilled in the art. Functionally equivalent methods and devices within the scope of the disclosure, in addition to those enumerated herein, will be apparent to those skilled in the art from the foregoing descriptions. Such modifications and variations are intended to fall within the scope of the appended claims. The present disclosure is to be limited only by the terms of the appended claims, along with the full scope of equivalents to which such claims are entitled. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting.
While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.