Abstract:
A vibrational energy harvesting system is disclosed. Included is a first energy harvesting unit and a second energy harvesting unit that convert mechanical vibrations into first and second AC signals, respectively. A first AC-DC converter coupled to the first energy harvesting unit and a second AC-DC converter coupled to the second energy harvesting unit are configured to convert the first AC signal and the second AC signal into a first DC signal and a second DC signal, respectively. A DC-DC converter is coupled between the second AC-DC converter and a controller, and is configured to receive the second DC signal and provide a regulated DC signal by using energy from the second DC signal in response to a periodic signal generated by the controller. Typically, an energy storage unit is coupled to the DC-DC converter and is configured to receive and store energy from the regulated DC signal.

Description:
FIELD OF THE DISCLOSURE 
       [0001]    The present disclosure relates to energy harvesting systems for electronic devices. 
       BACKGROUND 
       [0002]    Energy harvesting is a technique that captures energy from an environment in which an energy harvester is placed. Power sources for energy harvesters include electromagnetic sources (e.g., RF signal), kinetic sources (e.g., motion of walking), thermal sources (e.g., temperature gradients), and biochemical sources (e.g., glucose). Kinetic sources are particular attractive because they are readily available. In particular, vibrational sources are relatively common. As such, vibrational energy harvesters such as piezoelectric cantilevers and resonators are especially desirable. 
         [0003]    Due to the ability of piezoelectric cantilevers and resonators to withstand large amounts of strain, using them to harvest vibrational energy provides sufficient output for small-scale power applications. The strain can come from many different sources such as human motion, low-frequency seismic vibrations, and acoustic noise and radio frequency (RF) propagations. However, the majority of vibration sources with strong amplitude lie in a range between 4 Hz and 300 Hz with an acceleration of around 1G. Output power depends strongly on the frequency of vibration. Typically, more power is generated at resonant frequencies. 
         [0004]    Power harvesting from these electric materials has been investigated for several different potential applications. One such application identified in the prior art pertains to harvesting energy using piezoelectric materials embedded in shoes. Another energy harvesting application involves a piezoelectric polymer backpack strap which generates electrical energy from an oscillating tension in the strap during walking. Even the motion of breathing in and out has been studied for energy harvesting using piezoelectric polymers. Yet another idea has been to use a relatively small windmill to induce vibration in a series of piezoceramic beams. Piezoelectric polymers have also been investigated for generating electrical power from water currents. Another study investigated the storage of electrical energy from energy harvesting devices in batteries and capacitors. Others have characterized various piezoelectric materials, while still others have built wireless self-powered strain sensors that use harvested energy as both power sources and sensing signals. Further still, at least one study has formulated a model of a power harvesting system that comprises a cantilever beam with attached piezoelectric patches. Moreover, others have performed a comparison of piezoelectric, electromagnetic, and electrostatic configurations as a means of harvesting energy from a variety of vibration sources. These studies have generally concluded that the selection of a particular energy harvesting configuration is application dependent, but that piezoelectric materials based harvesters are the simplest to implement overall. What is needed is a vibrational energy harvesting system that can harvest energy from the environment where it is placed. 
       SUMMARY 
       [0005]    A vibrational energy harvesting system is disclosed. Included is a first energy harvesting unit and a second energy harvesting unit that convert mechanical vibrations into first and second AC signals, respectively. A first AC-DC converter coupled to the first energy harvesting unit and a second AC-DC converter coupled to the second energy harvesting unit are configured to convert the first AC signal and the second AC signal into a first DC signal and a second DC signal, respectively. A DC-DC converter is coupled between the second AC-DC converter and a controller, and is configured to receive the second DC signal and provide a regulated DC signal by using energy from the second DC signal in response to a periodic signal generated by the controller. Typically, an energy storage unit is coupled to the DC-DC converter and is configured to receive and store energy from the regulated DC signal. Moreover, the disclosed vibrational energy harvesting system can be completely self-powered. 
         [0006]    Those skilled in the art will appreciate the scope of the disclosure and realize additional aspects thereof after reading the following detailed description in association with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The accompanying drawings incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure. 
           [0008]      FIG. 1  is a schematic diagram of a vibrational energy harvesting system that is in accordance with the present disclosure. 
           [0009]      FIG. 2A  depicts the multilayer piezoelectric cantilever undergoing an upward deflection due to a vibration applied through the supporter. 
           [0010]      FIG. 2B  depicts the multilayer piezoelectric cantilever undergoing a downward deflection due to a vibration applied through the supporter. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the disclosure and illustrate the best mode of practicing the disclosure. Upon reading the following description in light of the accompanying drawings, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims. 
         [0012]    It will be understood that when an element such as a layer, region, or substrate is referred to as being “over,” “on,” “disposed on,” “in,” or extending “onto” another element, it can be directly over, directly on, directly disposed on, directly in, or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly over,” “directly on,” “directly disposed on,” “directly in,” or extending “directly onto” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. 
         [0013]    Relative terms such as “below” or “above” or “upper” or “lower” or “horizontal” or “vertical” may be used herein to describe a relationship of one element, layer, or region to another element, layer, or region as illustrated in the Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. 
         [0014]    The present disclosure provides a vibrational energy harvesting system that will facilitate versatility and viability of high data rate wireless sensing and monitoring systems for remote and difficult to reach locations. The operational performance of the vibrational energy harvesting system of the present disclosure will enable new classes of applications for medical and environmental sensing. For example, the vibrational energy harvesting system of the present disclosure harvests energy from the environment in which it is placed, which makes it suitable for powering environmental sensors embedded in structures such as buildings, vehicles, and medical implants. Moreover, the vibrational energy harvesting system of the present disclosure can be completely self-powered. 
         [0015]    A vibrational energy harvesting system  10  has a multilayer piezoelectric cantilever  12  that includes a first energy harvesting unit  14  and a second energy harvesting unit  16 . The first energy harvesting unit  14  includes a first top electrode  18  for collecting charges, a first piezoelectric multilayer  20  comprising a plurality of piezoelectric materials  20 - 1 ,  20 - 3  and  20 - 5  with an embedded plurality of electrodes  20 - 2  and  20 - 4 , and a first bottom electrode  22  for collecting charges. The plurality of piezoelectric materials  20 - 1 ,  20 - 3  and  20 - 5  are typically arranged in relatively thin layers that are sub-millimeters thick. As a result, the first piezoelectric multilayer  20  generates a lower voltage and higher current relative to a single piezoelectric layer of similar total thickness. The embedded plurality of electrodes  20 - 2  and  20 - 4  collect currents from the plurality of piezoelectric materials  20 - 1 ,  20 - 3  and  20 - 5  that add together to provide an output current that is collected by the first top electrode  18  and the first bottom electrode  22 . 
         [0016]    A first interface layer  24  provides electrical isolation between the first energy harvesting unit  14  and the second energy harvesting unit  16  that has a second top electrode  26  for collecting charges. The second energy harvesting unit  16  also includes a second piezoelectric multilayer  28  comprising plurality of piezoelectric materials  28 - 1 ,  28 - 3  and  28 - 5  with an embedded plurality of electrodes  28 - 2  and  28 - 4 , and a second bottom electrode  30  for collecting charges. The plurality of piezoelectric materials  28 - 1 ,  28 - 3  and  28 - 5  are typically arranged in relatively thin layers that are sub-millimeters thick. As a result, the second piezoelectric multilayer  28  generates a lower voltage and higher current relative to a single piezoelectric layer of similar total thickness. The embedded plurality of electrodes  28 - 2  and  28 - 4  collect currents from the plurality of piezoelectric materials  28 - 1 ,  28 - 3  and  28 - 5  that add together to provide an output current that is collected by the second top electrode  26  and the second bottom electrode  30 . 
         [0017]    A supporter  32  is coupled to the multilayer piezoelectric cantilever  12  through a packaging isolation layer  34  that provides electrical isolation between the supporter  32  and the multilayer piezoelectric cantilever  12 . A second interface layer  36  situated between the supporter  32  and the multilayer piezoelectric cantilever  12  holds the multilayer piezoelectric cantilever  12  and the packaging isolation layer  34  to the supporter  32 . In an exemplary embodiment, the second interface layer  36  provides additional electrical isolation between the supporter  32  and the multilayer piezoelectric cantilever  12 . 
         [0018]    The vibrational energy harvesting system  10  is configured to provide decoupling of a harvested energy output  38  from the first energy harvesting unit  14 , and the second energy harvesting unit  16 . However, coupling of the harvested energy output  38  to the first energy harvesting unit  14  and the second energy harvesting unit  16  is necessary for a sustainable energy harvesting operation. A highly desirable feature is provided in that the vibrational energy harvesting system  10  can be completely self-powered. 
         [0019]    A first AC-DC converter  40  is electrically connected to the first energy harvesting unit  14  through a first electrical bus  42 , which in this exemplary embodiment is made up of a first connector  44  and a second connector  46 . A second AC-DC converter  48  is electrically connected to the second energy harvesting unit  16  through a second electrical bus  50 , which in this exemplary embodiment is made up of connectors  52  and  54 . Harvested power propagates from the first energy harvesting unit  14  and the second energy harvesting unit  16  by way of output signals. Each output signal includes an alternating current component and an alternating voltage component. 
         [0020]    Characteristics of the output signals such as amplitude and frequency depend upon various parameters that include but are not limited to, elasticity and piezoelectric coefficients of the materials making up the multilayer piezoelectric cantilever  12 , as well as dielectric constants of the materials making up the multilayer piezoelectric cantilever  12 . Further still, various structural geometries for the vibrational energy harvesting system  10  and/or linear acceleration impact the characteristics of the output signals. Each of the output signals are converted from alternating current to direct current by way of the first AC-DC converter  40  and the second AC-DC converter  48 , respectively. It is to be understood that the first AC-DC converter  40  and the second AC-DC converter  48  can each include local energy storage to facilitate a constant energy feed into the DC-DC converter  64 . The local energy storage can be capacitors or inductors or combinations thereof. 
         [0021]    A DC voltage output from the first AC-DC converter  40  provides DC bias for a controller  56  through a third electrical bus  58 , which in this exemplary embodiment is made up of connectors  60  and  62 . A DC voltage output from the second AC-DC converter  48  provides DC bias for a DC-DC converter  64  through a fourth electrical bus  66 , which in this exemplary embodiment is made up of connectors  68  and  70 . A periodic signal output from the controller  56  is transmitted to the DC-DC converter  64  over a fifth electrical bus  72  made up of connectors  74  and  76 . The periodic signal output from the controller  56  switches a transistor (not shown), integrated within the DC-DC converter  64 , ON and OFF to regulate energy output from the DC-DC converter  64 . 
         [0022]    Regulation of the energy output from the DC-DC converter  64  can be a boost or a buck of an input voltage applied to the fourth electrical bus  66  by the second AC-DC converter  48 . The DC-DC converter  64  is configured to boost voltage of the second DC signal and thereby generate the regulated DC signal as an average voltage of the second DC signal falls below a first predetermined voltage level and buck voltage of the second DC signal and thereby generate the regulated DC signal as an average voltage of the second DC signal rises above a second predetermined voltage level. 
         [0023]    In at least one embodiment, the first predetermined voltage level and the second predetermined level are the same. The energy output from the DC-DC converter  64  is transferred to an energy storage unit  78  by way of a sixth electrical bus  80  made up of connectors  82  and  84 . The energy storage unit  78  can be, but is not limited to, electrochemical batteries and/or capacitors. Energy stored in the energy storage unit  78  is released on demand to a load (not shown) over a seventh electrical bus  86  made up of connectors  88  and  90 . 
         [0024]      FIGS. 2A and 2B  are diagrams depicting operating principles of the multilayer piezoelectric cantilever  12 . In operation, a vibration puts the multilayer piezoelectric cantilever  12  beam into motion. In particular,  FIG. 2A  depicts the multilayer piezoelectric cantilever  12  undergoing an upward deflection due to a vibration applied through the supporter  32 , which is typically fastened to a vibration source (not shown). During the upward deflection of the multilayer piezoelectric cantilever  12 , the first energy harvesting unit  14  and the second energy harvesting unit  16  generate AC signals that are transmitted through the harvested energy output  38 . The AC signal generated by the first energy harvesting unit  14  is carried on the first electrical bus  42 , whereas the AC signal generated by the second energy harvesting unit  16  is carried by the second electrical bus  50 . The upward deflection of the multilayer piezoelectric cantilever  12  compresses piezoelectric materials  20 - 1  ( FIG. 1 ) along the first top electrode  18  ( FIG. 1 ), while simultaneously placing piezoelectric materials  20 - 5  ( FIG. 1 ) along the first bottom electrode  22  ( FIG. 1 ) under tension. As a result of the upward deflection, a net negative charge collects on the first top electrode  18  and a net positive charge is collects on the first bottom electrode  22 . Similarly, the upward deflection of the multilayer piezoelectric cantilever  12  compresses piezoelectric materials  28 - 1  ( FIG. 1 ) along the second top electrode  26  ( FIG. 1 ), while simultaneously placing piezoelectric materials  28 - 5  ( FIG. 1 ) along the second bottom electrode  30  ( FIG. 1 ) under tension. As a result of the upward deflection, a net negative charge collects on the second top electrode  26  and a net positive charge collects on the second bottom electrode  30 . 
         [0025]    As the multilayer piezoelectric cantilever  12  moves towards the downward deflection depicted in  FIG. 2B , the multilayer piezoelectric cantilever  12  compresses piezoelectric materials  28 - 5  ( FIG. 1 ) along the second bottom electrode  30  ( FIG. 1 ), while simultaneously placing the piezoelectric materials  28 - 1  ( FIG. 1 ) along the second top electrode  26  ( FIG. 1 ) under tension. As a result of the downward deflection, a net positive charge collects on the second top electrode  26  and a net negative charge is collects on the second bottom electrode  30 . 
         [0026]    Returning to  FIG. 1 , the AC signal conveyed on the first electrical bus  42  is converted to a DC signal by the first AC-DC converter  40 , and the AC signal conveyed on the second electrical bus is converted to a second DC signal by the second AC-DC converter  48 . The DC signal provided by the first AC-DC converter  40  powers the controller  56 , which in turn generates a periodic switching signal that controls the switching frequency and/or duty cycle of the DC-DC converter  64 . The second DC signal received by the DC-DC converter  64  on the fourth electrical bus  66  is regulated to a fixed voltage that is appropriate for application to the energy storage unit  78  and is received by the energy storage unit  78  over the sixth electrical bus  80 . Energy is released on demand by a load (not shown) that couples to the seventh electrical bus  86 . 
         [0027]    Those skilled in the art will recognize improvements and modifications to the embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.