Patent Publication Number: US-2015084443-A1

Title: High energy density vibration energy harvesting device with high-mu material

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of and claims the benefit of and priority under 35 U.S.C. §119(e) to U.S. patent application Ser. No. 13/554,263, filed on Jul. 20, 2012, and entitled“High Energy Density Vibration Energy Harvesting Device with High-mu Material,” and to U.S. Provisional Application No. 61/510,781, filed on Jul. 22, 2011, and entitled “High Energy Density Vibration Energy Harvesting Device with High-mu Material,” the disclosures of which are hereby incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure relates generally to high energy density vibration energy harvesting devices with high-mu materials. 
     BACKGROUND 
     Vibration energy harvesting technologies are developing rapidly, showing great potential in many different applications. For example, miniature vibration energy harvesters are often used for applications such as autonomous sensors and system on chip applications. Most applications use one of four major vibration energy harvesting mechanisms, including electromagnetic, electrostatic, magnetoelastic, and piezoelectric mechanisms. However, such vibration energy harvesters achieve different output powers and energy densities. For example, piezoelectric-based vibration energy harvesters often demonstrate a much higher energy density than other counterpart mechanisms, reaching ˜6mW/cm 3 . Specifically, some piezoelectric bare beam based vibration energy harvesters can generate a power of 6.63 mW/cm 3 . Because of this, piezoelectric-based vibration energy harvesters are often more widely used than other forms of vibration energy harvesters. However, they can suffer from narrow bandwidth (or a limited operating frequency range of 2-5% of the center operating frequency), degraded polarization after prolonged use, and/or the negative side-effects caused by a brittle cantilever. 
     SUMMARY 
     In one aspect, an energy harvesting device, includes a first and second solenoid, each solenoid including (a) a wire coil wrapped around (b) a high permeability core with two or more layers, and the first and second solenoid being disposed along a first path, and a magnetic core: disposed between the first and second solenoid such that the first solenoid is mounted on a first side of the magnetic core, and the second solenoid is mounted on a second side of the magnetic core, and mounted on a support such that the magnetic core can vibrate along a second path that intersects the first path, vibration of the magnetic core inducing a flux change in the first and second solenoids. 
     In one aspect, in an energy harvesting device, including (1) a first and second solenoid, each solenoid including (a) a wire coil wrapped around (b) a high permeability core with two or more layers, and the first and second solenoid being disposed along a first path, and (2) a magnetic core disposed between the first and second solenoid such that the first solenoid is mounted on a first side of the first magnet, and the second solenoid is mounted on a second side of the first magnet, the magnetic core being mounted on a support such that the magnetic core can vibrate along a second path that is orthogonal to the first path, a method includes vibrating the magnetic core along the second path to induce a flux change in the first and second solenoids. 
     In one or more embodiments, the magnetic core includes a first magnet. 
     In one or more embodiments, the magnetic core includes a second magnet disposed above the first magnet such that the first magnet and second magnet have anti-parallel moments. 
     In one or more embodiments, the support includes a spring. 
     In one or more embodiments, the spring includes a circular cross-section. 
     In one or more embodiments, the spring has a resonance frequency of 42 Hz. 
     In one or more embodiments, vibration of the magnetic core achieves a power output density of 20.84 mW/cm 3 . 
     In one or more embodiments, each high permeability core is a 28-layer core, each layer including dimensions 2 cm×2 cm×0.002 inch. 
     In one or more embodiments, the magnetic core includes a second magnet, and the first and second magnets are SmCo magnets with dimensions 2.2 cm×1.3 cm×0.2 cm. 
     In one or more embodiments, a total volume of the energy harvesting device is 6.44 cm×3.25 cm×1.4 cm=29.3 cm 3 . 
     In one or more embodiments, the first solenoid, the second solenoid, and the support are mounted to a base such that the first path is substantially parallel to the base, and the second path is substantially perpendicular to the base. 
     In one or more embodiments, the first and second solenoids include a same size. 
     In one or more embodiments, the first and second solenoids include a same shape. 
     In one or more embodiments, the first and second solenoids are joined in series to double a voltage of the energy harvesting device. 
     In one or more embodiments, the magnetic core is vibrated at 42 Hz, and an output power of 610.62 mW is generated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects and advantages of the present invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
         FIG. 1  is a table that lists different comparison metrics for various vibrating energy harvesting mechanisms in accordance with certain embodiments; 
         FIG. 2A  is a schematic of an energy harvesting device in accordance with certain embodiments; 
         FIG. 2B  is a schematic of the energy harvesting device of  FIG. 2A  with its magnetic core in an upper position in accordance with certain embodiments; 
         FIG. 3A  is a top-view image of an energy harvesting device in accordance with certain embodiments; 
         FIG. 3B  is a side-view image of the energy harvesting device of  FIG. 3A  in accordance with certain embodiments; 
         FIG. 4  is a graph of open circuit voltage (V) of the energy harvesting device of  FIGS. 3A-3B  over time (s) using three different springs in accordance with certain embodiments; 
         FIG. 5  is a graph of the output power (mW) of the energy harvesting device achieved using the three springs graphed in  FIG. 4  in accordance with certain embodiments; and 
         FIG. 6  is a graph of the power density (mW/cm 3 ) of the energy harvesting device based on the frequency (Hz) of the support in accordance with certain embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present disclosure provides for magnetic-based vibration energy harvesters that achieve a high energy density by using high permeability magnetic materials. While a piezoelectric bare beam based vibration energy harvester can generate a power energy density of 6.63 mW/cm 3 , theoretically the magnetostatic energy density (½μH 2 ) in high permeability magnetic materials is 10 5 -10 6  times that of the electrostatic energy density (½εE 2 ) in piezoelectrics. Such magnetic-based vibration energy harvesters can achieve, for example, a energy density greater than 20 mW/cm 3  (e.g., with an acceleration of 5 g), which is over 3 times the energy density of known vibration energy harvesters. 
     Before describing in detail the particular components of magnetic-based vibration energy harvesters, in some embodiments the vibration energy harvesters include two fixed solenoids. Each solenoid has a multi-layer high permeability solenoid core. A vibrating magnetic core is disposed between the two fixed solenoids. The multilayer high permeability solenoid cores lead to significantly increased flux change in the solenoid within one period of vibration of the magnetic core than other such devices, without increasing the total volume of the device. In addition, the two solenoids at both sides of the vibrating magnet(s) make full use of the spatially inhomogeneous bias magnetic fields at both sides of the magnets, leading to doubled power output, and a dramatically enhanced power density than previous energy harvesters. 
       FIG. 1  is a table  100  that shows metrics of comparison among various vibrating energy harvesting mechanisms. The mechanisms/products include electrostatic, magnetoelectric, piezoelectric, magnetoelectric sensor based, magnetostrictive, perpetuum, KCF (from KCF Technologies of State College, Pa.), and high permeability (High-μ (1 st  gen), and High-μ (2 nd  gen)) material-based energy harvesting devices. The metrics of comparison include the central frequency f center , measured in Hz; acceleration “a,” measured in g (9.8 m/s 2 ); P max , the maximum output power of the device, measured in mW; and power density, measured in mW/cm 3 . As shown in table  100 , the 2 nd  generation high-μ vibration energy harvester (e.g., which includes two fixed solenoids with multi-layer high permeability cores, and a vibrating magnetic core, as described herein) has the largest output power density of 20.84 for an f center  of 42 Hz. Such a power density is over three times larger than the widely used piezoelectric device. Further, the performance of the High-μ (2 nd  gen) device is greater than that of the High-μ (1 st  gen) device (e.g., upwards of 10× the flux change of the High-μ (1 st  gen) device). The High-μ (2 nd  gen) device uses two stationary solenoids with thicker multi-layer high-μ magnetic core materials (e.g., 20 layers of material), which allows a greater flux change to be induced in one vibration of the magnetic core. In contrast, the High-μ (1 st  gen) device has a single vibrating solenoid, and therefore the solenoid core can not be thick, resulting in less flux change. 
       FIG. 2A  is a schematic of an energy harvesting device  200  in accordance with certain embodiments. The energy harvesting device  200  includes a first solenoid  202 A and a second solenoid  202 B (indicated by respective dotted rectangles for ease of reference, and collectively referred to herein as solenoids  202 ). Each solenoid  202  can include a wire coil ( 204 A,  204 B) wrapped around a multi-layer high permeability (high-μ) core ( 206 A,  206 B). The energy harvesting device  200  includes a magnetic core  208 , which is disposed between solenoid  202 A and solenoid  202 B. Solenoid  202 A is mounted to the base  220  on the left side of the magnetic core  208  via mount  210 A, and solenoid  202 B is mounted to the base  220  on the right side of the magnetic core  208  via mount  210 B. The magnetic core  208  is mounted to the base  220  via support  212  such that the magnetic core  208  can vibrate between the solenoids  202 . While two solenoids  202  are shown in device  200 , any number of solenoids can be used for a particular energy harvesting device (e.g., 1, 3, etc.). Further, the configuration shown in  FIG. 2A  is intended to be exemplary only, and is not intended to be limiting. One of skill can appreciate that other variations of energy harvesting devices can be engineered according to the principles described herein without departing from the spirit of the description. 
     In some embodiments, the solenoids  202  are manufactured to have the same size (e.g., the same three dimensional size) and/or shape (e.g., the same number of layers in the cores  206 A,  206 B, and the same number of coil layers for each coil  204 A,  204 B, the same number of rotations around the core per coil layer, and/or the like). In some embodiments, the multi-layer high permeability cores ( 206 A,  206 B) are formed of multiple layers of a non-oriented 80% nickel-iron-molybdenum alloy, which offers extremely high permeability. The material can be fabricated using hydrogen annealing to maximize permeability. In some embodiments, the high permeability materials are foils provided by The MuShield® Company of Londonderry, N.H. (e.g., foil with thicknesses of 0.002″, 0.004″, 0.006″, and/or 0.010″). Any high permeability magnetic materials can be used as the magnetic core for the energy harvester to achieve similar harvester performance. (e.g., ferrite or other inductor core materials). 
     The magnetic core  208  includes magnets  214  and  216 . Magnet  214  is disposed above the magnet  216  such that the magnets have anti-parallel moments (e.g., the North (N) pole of magnet  214  is disposed above the South (S) pole of magnet  216 , and the South (S) pole of magnet  214  is disposed above the North (N) pole of magnet  216 ). The magnets  214 ,  216  are joined by joining portion  218  (e.g., which can be made from a magnetic or a non-magnetic material). In some embodiments, the support  212  is a spring (e.g., with a circular cross-section, a square cross-section, and/or the like). While the magnetic core  208  is shown with two magnets  214  and  216 , the magnetic core can include any number of magnets. For example, in some embodiments, the magnetic core  208  includes one magnet, or three or more magnets. Further, the magnets can be arranged in other ways than with anti-parallel moments. For example, the magnets can be oriented such that the same moments are aligned (e.g., N above N, and S above S). As another example, the magnets can be partially crossed, such that they do not completely overlap with eachother (e.g., to form an “X” shape). 
     Vibration of magnetic core  208  creates a voltage V across the solenoids  202 . As the pair of magnets  214 ,  216  vibrate up and down, the magnetic field lines inside each solenoid  202  change direction periodically, inducing a large magnetic flux change (M) in both solenoids  202 . The magnetostatic coupling between the solenoids  202  and the time varying inhomogeneous bias magnetic field results in a nonlinear oscillation and a complete magnetic flux reversal in the solenoids  202 . The presence of the multi-layer highly permeable cores dramatically increase the magnitude of magnetic flux inside the coils of the solenoids  202 . The induced voltage can be doubled to form voltage V by connecting the two solenoids in series. 
       FIG. 2B  is a schematic of the energy harvesting device  200  of  FIG. 2A  with the magnetic core  208  in an upper position. As shown in  FIGS. 2A-2B , the solenoids  202  are disposed along first path  250 . In some embodiments, the first path  250  is substantially horizontal to the surface plane of the base  220 . Magnetic core  208  can vibrate along a second path  252  that intersects (e.g., is orthogonal to) the first path  250 . In some embodiments, the second path  252  is substantially perpendicular to the surface plane of base  220 . As shown by arrows  254 A,  254 B, vibration of the magnetic core along path  252  induces a flux change in the solenoids  202  (e.g., arrows  254 A,  254 B point to the right in  FIG. 2B , compared to the arrows in  FIG. 2A  which point left, due to the movement of the magnetic core  208  to an upward position along the path  252 . The mass of the hard magnetic core  208 , the stiffness of the supporting spring, and/or the magnetostatic coupling between the solenoids  202  and/or hard magnetic core  208  can determine the resonance vibration frequency and the output voltage of the energy harvester. 
     While paths  250 ,  252  are shown as straight paths in  FIG. 2B , in some embodiments the paths are nonlinear paths. For example, an equivalent stand-alone spring-mass system becomes a nonlinear oscillation system once introduced into the energy harvesting device  200  due to the magnetostatic coupling between the solenoids  202  and the hard magnetic core  208 . This nonlinear effect can be explained, for example, from a potential energy point of view. The elastic potential energy of a stand-alone spring-mass system is a well-know linear relationship, with only one minimum value, which happens when the mass passes the equilibrium position in the middle. In contrast, the magnetostatic potential energy has two identical minimum values due to the coupling between the magnet(s) and solenoids, which appear when the magnet(s) move a short distance up or down from the equilibrium position in the middle. As a result, the superposition of two different types of potential energy make a nonlinear relationship, leading to a wider oscillation frequency range. 
     In some embodiments, the total induced voltage of the energy harvesting device equals the integral over the whole solenoid, because the magnetic field magnitude varies along the axis. The open circuit voltage, V, can be expressed by Equation (1): 
     
       
         
           
             
               
                 
                   
                     
                       
                         V 
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     where:
 
dφ(t)=the magnetic flux change over time;
 
dt=time;
 
H[x,y(t)]=the magnetic field H at time t, at the spatial position defined by the point (x (along the length of the solenoid), y (along the direction the vibrating magnetic core travels));
 
M[x,y(x,t)]=magnetization M as a function of time t at coordinates x, y(x,t);
 
A=the total cross section area of the multilayer cores of the solenoids; and
 
dN=the number of loops in the infinitesimal length unit of the solenoid, which can be calculated according to Equation (2):
 
         dN=N   L   ·dx/d   w .   (2)
 
     where:
 
N L =the number of loop layers of the coil;
 
dx=the position x along the length of the solenoid; and
 
d w =the copper wire diameter.
 
     Hence, the maximum output power P max , which happens when the load impedance equals the conjugate of the output impedance of the solenoid coil R coil , is defined by Equation (3): 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     where:
 
R coil =the resistance of the solenoid;
 
S=the number of layers in each core;
 
A′=the cross section area of one layer of the core;
 
L=the length of the solenoid; and
 
d w =the diameter d of the wire w.
 
     Equation (3) shows that the output power P max  increases as the vibration frequency increases (e.g., if all other parameters are kept constant). In some examples, when using the same source power, the amplitude decreases if the frequency increases. Moreover, at a particular frequency, the output power P max  can depend on the total magnetic flux change in the solenoid, in one oscillation period, which is directly related to the permeability μ of the magnetic cores. Therefore, multi-layer soft magnetic beams with a high permeability constitute excellent candidates for the solenoid cores. In some embodiments, a multilayer structure of magnetic material generates a much larger flux change than a single layer, as shown in  FIG. 1  by the High-μ (1 st  gen) device, which among other differences, uses a single layer core. 
     In some embodiments, the magnetic coupling between the fixed solenoids  202  with multi-layer highly permeable cores, and the time varying bias magnetic field generated by the vibrating magnetic core  208  results in a large magnetic flux reversal and maximized flux change in the solenoids  202 , leading to a high maximum power of 610.62 mW, and a maximum power density of 20.84 mW/cm 3  at a frequency of 42 Hz. 
       FIG. 3A  is a top-view image of an exemplary energy harvesting device  300 , and  FIG. 3B  is a side-view image of the exemplary energy harvesting device  300  of  FIG. 3A . The device  300  includes solenoids  302 A,  302 B, each with a copper wire coil  304 A,  304 B formed around the core, and wrapping  306 A,  306 B made of a thin insulator (e.g., teflon, paper, etc.) to hold the respective copper wire coil  304 A,  304 B in place about the core. In this example, each solenoid  302 A,  302 B core is a 28-layer high permeability MuShield® material, with dimensions 2 cm×2 cm×0.002 inch for each layer. In some embodiments, the copper wire coils  304 A,  304 B are made of copper wire (e.g., with a diameter of 1 mm or 40 mils). In some embodiments, the copper wires include 2-5 layers of copper coils, and each layer includes 20-50 turns around the core. The coil resistance of each solenoid  302 A,  302 B is 1.3 Ohm. Each solenoid  302 A,  302 B is held in place by a mount  308 A,  308 B made of a dielectric material (e.g., acrylic, or other insulator material). In some embodiments, the copper wires are coated with a thin insulator. 
     The magnetic core  310  includes two SmCo hard magnets  312 ,  314  with dimensions 2.2 cm×1.3 cm×0.2 cm. The two magnets  312 ,  314  are joined by a non-magnetic spacer to produce a fringing field that is coupled to the solenoids on the two ends of the hard magnets. The magnetic core  310  is mounted on spring  320 , which has a circular cross-section. The magnetic core  310  can vibrate within the area formed by the mount  308 A,  308 B and supports  316 ,  318 . In this example, the device  300  is powered by a vibrating stage that is driven by an audio power amplifier, and its mechanical movement is monitored by an accelerometer. The voltage output of the harvester  300  in the time domain is monitored by a digital oscilloscope. Total volume of the energy harvester is 6.44 cm×3.25 cm×1.4 cm=29.3 cm 3 , which includes the solenoids  302 A,  302 B, the magnetic core  310  and the gap between the solenoids  302 A,  302 B and the magnetic core  310 . 
       FIG. 4  is a graph  400  of measured open circuit voltage (V) of the energy harvesting device  300  of  FIGS. 3A-3B  over time (s) using three different springs in accordance with certain embodiments. Each spring has a different resonance frequency: spring # 1  has a resonance frequency of 27 Hz, spring # 2  has a resonance frequency of 33 Hz, and spring # 3  has a resonance frequency of 42 Hz. For the first spring # 1 , the peak voltage is 1.18 V for an acceleration of 2 g (where g=9.8 m/s 2 ), and the maximum output power on a 2.6 Ohm load is 133.88 mW. For the second spring # 2 , the peak voltage is 1.64 V for acceleration of 3 g, resulting in a maximum output power of 258.62 mW on a 2.6 Ohm load. For the third spring # 3 , the maximum induced voltage is 2.52 V for an acceleration of 5 g, with the corresponding power 610.62 mW on a 2.6 Ohm load. Increasing acceleration values were applied to maintain the same source vibration amplitude. Considering that the total practical volume of the device is 29.3 cm 3 , this device demonstrated excellent performance with the maximum power density of 20.84 mW/cm 3  at 42 Hz. Graph  400  demonstrates that a higher resonance frequency leads to a larger output power. 
     The Q factor of the harvester at 42 Hz was 16, which was obtained from the decay curve of output voltage when turning off the source. Almost the entire device damping is generated from the mechanical collision between the spring supported magnetic core and the solenoid supports. Therefore, other crafting techniques that reduce this mechanical collision could achieve a much higher Q factor while using a much lower input force or acceleration (e.g., by designing the magnetic core such that it floats along a rail, to include bearings to reduce the friction against the surrounding supports, to include a material that reduces the friction between the magnetic core and the surrounding supports, etc.). A simple relation between frequency and power can be derived from Equation (3), as shown below in Equation (4), if all other parameters kept constant: 
         P   max ˜(Δ M/ΔT ) 2   ˜f   2    (4)
 
     where:
 
ΔM is the flux change per period;
 
ΔT the period; and
 
f is the frequency.
 
     Measured test results confirm the parabolic curve fitting, as shown in  FIG. 5 , which is a graph  500  of the output power (mW) of the energy harvesting device achieved for the different frequencies (Hz) of the three springs graphed in  FIG. 4 . In some embodiments, the vibration energy harvester design can accommodate different vibrating frequencies of the environment by changing the spring that is connected to the magnetic core. For example, a larger vibration frequency of the environment (for a particular application) can induce a higher output power if matched with a spring with an appropriate resonance frequency. In some examples, if the vibration amplitude of the testing stage is kept the same, the output power and power density are proportional to the second power of the vibration frequency. If P max ˜f 2  can be extrapolated to higher frequencies, a higher output power density can be achieved while maintaining a constant amplitude. 
       FIG. 6  is a graph  600  of the power density (mW/cm 3 ) of the energy harvesting device based on the frequency (Hz). Graph  600  shows that the output power demonstrates a sagging rise before 42 Hz, which achieves a maximum output of 610.62 mW, and then rapidly declines afterwards. The asymmetrical curve can be caused by the nonlinear oscillation, which can increase the mechanical damping as the frequency ascends. The half-power bandwidth of the device with spring # 3  was measured to be 6 Hz, or ˜15% of the central frequency, which is much higher than the typical 2-5% bandwidth of typical piezoelectric cantilever-based energy harvesters. 
     Upon review of the description and embodiments of the present invention, those skilled in the art will understand that modifications and equivalent substitutions may be performed in carrying out the invention without departing from the essence of the invention. Thus, the invention is not meant to be limiting by the embodiments described explicitly above, and is limited only by the claims which follow.