Abstract:
The invention is a system incorporating a self-tuning resonator and method of self-tuning a resonator within a system. In one embodiment, a method of powering a system with energy harvested from a vibrating surface includes receiving a first mechanical energy at a first driving frequency from the vibrating surface, transferring the received first mechanical energy to a suspended structure within the system, vibrating the suspended structure with the transferred first mechanical energy, passively adjusting the resonant frequency of the suspended structure to a first resonant frequency associated with the first driving frequency by moving a movable mass in response to the movement of the suspended structure, vibrating the adjusted suspended structure with the transferred first mechanical energy, generating electrical energy using the vibrations of the adjusted suspended structure, and powering the system with the generated electrical energy.

Description:
FIELD OF THE INVENTION 
   This invention relates to the field of systems incorporating energy scavengers and more particularly to systems incorporating mechanical energy harvesters. 
   BACKGROUND OF THE INVENTION 
   Energy harvesters may be used to convert mechanical energy to electrical energy. The harvesting system typically includes a component that vibrates in response to mechanical energy passed to the harvesting system. For example, the harvesting system may be attached to a motor or other piece of equipment which vibrates. The conversion of mechanical energy to electrical energy may be accomplished using piezoelectric devices, capacitor devices or magnetic devices. In these systems, the most efficient conversion of energy from mechanical to electrical occurs when the resonant frequency corresponds to the frequency of the received vibration. In general, the conversion of mechanical energy to electrical energy may be quantified using the following equation: 
                P        =       m   ⁢           ⁢     ξ   e     ⁢         ω   3     ⁡     (     ω     ω   n       )       2     ⁢     Y   2             (     2   ⁢     ζ   τ     ⁢     ω     ω   n         )     2     +       (     1   -       (     ω     ω   n       )     2       )     2               
wherein
 
   P is the power that may be harvested, 
   m is the mass of the vibrating component, 
   ξ e  is the electrical damping ratio, 
   ω is the excitation frequency, 
   ω n  is the natural frequency of the harvesting system, 
   Y is the amplitude of external vibration, and 
   ζ τ  is the total damping ratio. 
   Thus, the harvested power is maximized when the ratio of the excitation frequency to the natural frequency of the system approaches 1. 
   Harvesting systems that are to be used to harvest energy from a source that vibrates at a single dominant frequency may include a resonator tuned to the particular dominant frequency. This may be accomplished, for example, by adding a mass to a spring or lever arm so as to modify the resonant frequency of the spring or lever arm. For systems that vibrate at various discrete frequencies, the resonator used in the harvesting system may include different springs or levers tuned to the various frequencies or a number of different resonators, each tuned to a different frequency. 
   One can also employ devices which incorporate active components that are used to tune the resonant frequency of the resonator to the frequency then experienced by the device. Active components, however, require some amount of the harvested energy to be consumed, thereby reducing the effective output of the harvesting system. 
   Accordingly, it would be advantageous to provide a mechanical energy harvesting system with a tunable resonator. It would be further advantageous if harvested energy was not required to tune the resonator. 
   SUMMARY OF THE INVENTION 
   Some limitations of previously known systems incorporating mechanical energy harvesters may be overcome by a system incorporating a passive self-tuning resonator or a method of passively self-tuning a resonator within a system. In one embodiment, a method of powering a system with energy harvested from a vibrating surface includes receiving a first mechanical energy at a first driving frequency from the vibrating surface, transferring the received first mechanical energy to a suspended structure within the system, vibrating the suspended structure with the transferred first mechanical energy, passively adjusting the resonant frequency of the suspended structure to a first resonant frequency associated with the first driving frequency by moving a movable mass in response to the movement of the suspended structure, vibrating the adjusted suspended structure with the transferred first mechanical energy, generating electrical energy using the vibrations of the adjusted suspended structure, and powering the system with the generated electrical energy. 
   In a further embodiment, a wireless device includes a support structure configured to receive mechanical energy generated by a vibrating source external to the device, a vibratory member operatively connected to the support structure for receiving mechanical vibrations from the support structure, and a power harvesting subsystem including a resonator with at least one resonant frequency adjustment mass movably responsive to the vibratory member, such that vibration of the vibratory member causes the at least one resonant frequency adjustment mass to move thereby changing the resonant frequency of the vibratory member from a first resonant frequency to a second resonant frequency. 
   In yet another embodiment, a sensing device includes a sensor configured to provide output indicative of a sensed condition, a memory, a microprocessor configured to obtain the output from the sensor and to store data associated with the output within the memory, and a power harvesting subsystem including a resonator with a suspended structure configured to move a resonant frequency adjustment mass such that movement of the resonant frequency adjustment mass causes the resonant frequency of the suspended structure to change. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may take form in various system components and arrangement of system components. The drawings are only for purposes of illustrating exemplary embodiments and are not to be construed as limiting the invention. 
       FIG. 1  shows a schematic diagram of a wireless sensor system powered by a passive self-tuning energy harvesting resonator incorporating features of the present invention; 
       FIG. 2  shows a partially exploded perspective view of a resonator incorporating a suspended member in the form of a spring that may be used with the system of  FIG. 1 ; 
       FIG. 3  shows a perspective view of the resonator of  FIG. 2  with the spring slidingly supported by transfer arms which are slidingly supported by support beams; 
       FIG. 4  shows a side cross-sectional view of the resonator of  FIG. 2  with one transfer arm positioned closer to the center of the spring than the other transfer arm; 
       FIG. 5  shows an exaggerated simplified side view of the spring of the resonator of  FIG. 2  exerting torque of different magnitudes on the transfer arms in response to a first half of a cycle of a driving frequency whereby the spring biases the transfer arms thereby passively modifying the resonant frequency of the system; 
       FIG. 6  shows an exaggerated simplified side view of the spring of the resonator of  FIG. 2  exerting torque of different magnitudes on the transfer arms in response to a second half of a cycle of a driving frequency whereby the spring biases the transfer arms thereby passively modifying the resonant frequency of the system in accordance with principles of the invention; 
       FIG. 7  shows a side cross-sectional view of the resonator of  FIG. 4  after the spring bias has caused the transfer arms to be moved to positions equidistant from the center of the spring; 
       FIG. 8  shows an exaggerated simplified side view of the spring of the resonator of  FIG. 2  exerting torque of equal magnitudes on the transfer arms in response to a first half of a cycle of a driving frequency while the spring is vibrating at the resonant frequency of the system for the driving frequency; 
       FIG. 9  shows an exaggerated simplified side view of the spring of the resonator of  FIG. 2  exerting torque of equal magnitudes on the transfer arms in response to a second half of a cycle of a driving frequency while the spring is vibrating at the resonant frequency of the system for the driving frequency; 
       FIG. 10  shows a side cross-sectional view of the resonator of  FIG. 4  after the spring bias has caused the transfer arms to be moved to positions whereat the transfer arms define nodes for the harmonic frequency of the system for the received drive frequency; so as to maximize the translation of mechanical energy to electrical energy by the piezoelectric component; 
       FIG. 11  shows a perspective view of the resonator of  FIG. 4  in the condition of  FIG. 10 ; 
       FIG. 12  shows a perspective view of an alternative resonator with a suspended member that may be used with the system of  FIG. 1  incorporating features of the present invention; 
       FIG. 13  shows a side cross-sectional view of one of the suspended members of  FIG. 12  with a plurality of negatively buoyant nanobeads within a fluid filled chamber defined by a resilient outer shell; 
       FIG. 14  shows a side cross-sectional view of one of the suspended members of  FIG. 12  with the plurality of negatively buoyant nanobeads forming various antinodes in response to a driving frequency causing vibration of the suspended member; and 
       FIG. 15  shows a side cross-sectional view of an alternative suspended member with a plurality of positively buoyant nanobeads within a fluid filled chamber defined by a resilient outer shell forming various nodes in response to a driving frequency causing vibration of the suspended member. 
   

   DESCRIPTION 
   Referring to  FIG. 1 , the system  100  includes a microprocessor  102 , a receiver  104 , and a transmitter  106 . The receiver  104  and the transmitter  106  allow communication between the system  100  and an external device. In the embodiment of  FIG. 1 , the receiver  104  is used exclusively to receive data over a link which may incorporate radio frequency, inductive coupling, or any other acceptable means for communication. In alternative embodiments, the receiver may additionally function as an energy harvesting device. 
   The system  100  in this embodiment is a wireless sensor. Accordingly, the system  100  includes a sensor  108 . The sensor  108  provides an output indicative of a sensed condition to the microprocessor  102 . The microprocessor  102  is configured to execute commands stored in a memory  110  which cause data associated with the output of the sensor to be stored in the memory  110 . 
   Power for the system  100  is provided from a power harvesting subsystem  112  which includes a resonator  114  and a storage device  116 . The storage device  116  may be a battery which is charged with the resonator  114 . In alternative embodiments, different types of storage devices may be used or storage devices may be omitted. Additionally, while the resonator  114  in this embodiment includes a single device, two or more resonators may be provided depending on the particular power needs and design characteristics of the system. 
   The resonator  114 , along with other components of the system  100 , may be fabricated using microelectrical mechanical system (MEMS) processes, nanoelectrical mechanical system (NEMS) processes, semiconductor processes, or even traditional molding and machining processes. One example of a resonator that may be used with the system  100  is the resonator  118  shown in  FIG. 2 . 
   The resonator  118  includes a support structure  120 , transfer arms  122  and  124  and a spring  126 . A piezoelectric component  128  and a mass member  130  are attached to the spring  126 . The mass member  130  includes a body  132  and two spacer flanges  134  and  136 . The support structure  120  includes a base  138  and side frames  140  and  142 . Two support beams  144  and  146  extend between the side frames  140  and  142  and above the base  138 . 
   The transfer arm  122  includes two support openings  148  and  150  and a spring opening  152 . The support openings  148  and  150  are sized to slidingly receive the support beams  146  and  144 , respectively. To this end, the height of the openings  148  and  150  are configured to be slightly higher than the height of the support beams  144  and  146  such that when assembled, relative movement is possible between the transfer arm  122  and the support beams  144  and  146  but there is not a significant gap between the upper and lower surfaces of the openings  142  and  144  and the upper and lower surfaces of the support beams  144  and  146 , respectively. The transfer arm  124  is identical to the transfer arm  122 , including two support openings  154  and  156  and a spring opening  158 . 
   When assembled, as shown in  FIG. 3 , the transfer arms  122  and  124  support the spring  126 . The height of the spacer flanges  134  and  136  is selected such that the body  132  of the mass member  130  does not inhibit movement of the transfer arms  122  and  124  as best seen in  FIG. 4 . The spacer flanges  134  and  136  extend upwardly from the body  132  between the transfer arms  122  and  124  thereby maintaining a minimum separation between the transfer arms  122  and  124 . 
   When a system such as the system  100  incorporates a resonator  118 , the system is preferentially configured such that the plane in which the spring  126  flexes is parallel to the predominant axis of the targeted vibrations. For example, the spring  126  is configured to flex primarily back and forth in the direction of the arrow  160  in  FIG. 4 . Accordingly, positioning the resonator  118  on a device such that the predominant axis of the targeted vibrations is parallel to the arrow  160  maximizes the energy from the vibration that is available for conversion. 
   When the resonator  118  is positioned on a device which is not presently vibrating, the resonator  118  may initially be in the condition shown in  FIGS. 3 and 4 . In this condition, the position of the transfer arms  122  and  124  along the support beams  144  and  146  is constrained primarily by the relative dimensions of the support openings  148 ,  150 ,  152  and  156  and the support beams  144  and  146 . Likewise, the orientation of the spring  126  is constrained primarily by the relative dimensions of the spring  126  and the spring openings  152  and  158 . Accordingly, the location of the spring  126  with respect to the transfer arms  122  and  124  may not be symmetrical. For example,  FIG. 4  shows the transfer arm  122  located slightly closer to the piezoelectric component  128  than the transfer arm  124 . Additionally, the spring  126  may not be centered between the frames  140  and  142 . Likewise, the spring  126  may not be parallel to the support beams  144  and  146 . 
   Once the device upon which the resonator  118  is placed begins to vibrate at a first drive frequency, the mechanical energy of the vibration is passed from the device to the base  138 , either directly or through other components such as the housing of the system in which the resonator  118  is located. The mechanical energy is passed from the base  138  to the side frames  140  and  142 , all of which vibrate at the first drive frequency. Likewise, the support beams  144  and  146 , which are made of an acceptably stiff material, vibrate at the first drive frequency. Moreover, because the base  138 , the side frames  140  and  142 , and the support beams  144  and  146  are moving as a unit, the movements of the support beams  144  and  146  are synchronized. 
   For the purposes of the following example, the driving frequency is assumed to initially cause movement of the resonator  118  in the direction of the arrow  160 . As the support beams  144  and  146  move in the direction of the arrow  160 , the transfer arms  122  and  124  are moved in the direction of the arrow  160 , causing the portions of the spring  126  located between the upper and lower surfaces of the spring openings  152  and  158  to move in the direction of the arrow  160  at the first frequency. These areas of the spring  126  are the initial nodes of the spring  126 . The initial nodes of the spring  126  which are defined by the spring openings  152  and  158  are identified in  FIG. 4  as N 1  and N 2 , respectively. 
   As the nodes N 1  and N 2  initially move in the direction of the arrow  160 . The inertia of the mass member  130  resists any movement. Accordingly, the flexible nature of the spring  126  allows the spring  126  to initially flex as shown in  FIG. 5  as movement of the nodes N 1  and N 2  precedes movement of the central portion of the spring  126 . As the spring  126  flexes, the piezoelectric component  128  which is located at the central portion of the spring  126  is flexed, thereby translating the mechanical movement of the vibration applied to the resonator  118  into electrical energy. 
   As the driving frequency causes the resonator  118  to reverse direction, the nodes N 1  and N 2  reverse direction. The mass member  130  initially continues to move in the direction of the arrow  160  allowing the spring  126  to resume its original shape. The inertia of the mass member  130  at this point is still in the direction of the arrow  160  while the nodes N 1  and N 2  are moving in the opposite direction. Accordingly, the spring  126  is flexed in the direction opposite to the initial flexure as shown in  FIG. 6  resulting in the generation of more electrical energy in the manner discussed above. 
   Accordingly, the movement of the nodes N 1  and N 2  define two axes which together define a flexing plane. Within the flexing plane, the point at which the mass member  130  is attached to the spring  126  defines an antinode for the resonator  118 . 
   As the spring  126  is flexed, a torque is applied to the transfer arms  124  and  126 . This is explained with reference to  FIGS. 5 and 6  which are simplified and exaggerated depictions of portions of the resonator  118 . As the transfer arms  122  and  124  move in the direction of the arrow  162 , the spring  126  flexes as discussed above. As the spring  126  flexes, the lower surface  164  of the spring  126  contacts the inner wall  166  of the transfer arm  124  at the lower portion of the spring opening  158  resulting in a force in the direction of the arrow  172 . At the same time, the upper surface  168  of the spring  126  contacts the outer wall  170  of the transfer arm  124  at the upper portion of the spring opening  158  resulting in a force in the direction of the arrow  174 . Accordingly, a counter-clockwise torque is applied to the transfer arm  124 . 
   Similarly, as the spring  126  flexes in the manner shown in  FIG. 5 , the lower surface  164  of the spring  126  contacts the inner wall  176  of the transfer arm  122  at the lower portion of the spring opening  152  resulting in a force in the direction of the arrow  178  while the upper surface  168  of the spring  126  contacts the outer wall  180  of the transfer arm  122  at the upper portion of the spring opening  152 . Accordingly, a clockwise torque is applied to the transfer arm  122 . 
   When the flexure of the spring  126  is reversed, as shown in  FIG. 6 , the upper surface  168  contacts the inner walls  166  and  176  at the upper portions of the spring openings  158  and  152 , respectively, resulting in forces in the direction of the arrows  184  and  186 , respectively. Additionally, the lower surface  164  contacts the outer walls  170  and  180  at the lower portions of the spring openings  158  and  152 , respectively, resulting in forces in the direction of the arrows  188  and  190 , respectively. Thus, the transfer arm  122  is torques in a counter-clockwise direction while the transfer arm  124  is torqued in a clockwise direction. 
   Thus, each transfer arm is torqued in both a counter-clockwise direction and a clockwise direction during a complete cycle. In the example of  FIG. 4 , the mass member  130  is closer to the transfer arm  124  and the spacing of the transfer arms  122  and  124  is less than ½ of the wavelength (λ) of the resonant frequency of the system  100  for the driving frequency. Accordingly, the forces in the direction of the arrows  172  and  184  are larger than the forces in the directions of the arrows  174  and  188 . Accordingly, the transfer arm  124  is biased by the spring  126  in a direction away from the mass member  130  which defines the antinode of the spring  126 . 
   Therefore, because the transfer arms  122  and  124  are free to slide along the support beams  144  and  146 , the bias generated by the spring  126  and the mass member  130  causes movement of the transfer arms  122  and  124  so as to center the spring  126  and the mass member  130  between the transfer arms  122  and  124 . A similar process forces the spring  126  to an orientation perpendicular to the transfer arms  122  and  124  and parallel with the support beams  144  and  146 . Thus, when the resonator is in the configuration of  FIG. 4 , the spring  126  forces the transfer arm  122  to the position shown in  FIG. 7  which is to the left with respect to the position of the transfer arm  122  in  FIG. 4 . The transfer arm  122  thus defines a new node N 1 ′. The nodes N 1 ′ and N 2  are symmetrically spaced apart from the antinode of the spring  126 . 
   The nodes N 1 ′ and N 2  are thus spaced apart at a distance which is a multiple of ½ of the wavelength (λ) of the spring  126  while the piezoelectric component  128  spans an antinode of the spring  126 . In the event the frequency of the spring  126  defined by the nodes N 1 ′ and N 2  is the resonant frequency of the spring  126  for the first drive frequency, a standing wave will be generated within the spring  126  which entraps the transfer arms  122  and  124  thereby maintaining the nodes N 1 ′ and N 2  at locations which define the resonant frequency of the spring  126  for the first drive frequency. By way of explanation, the forces exerted on the transfer arms  122  and  124  for a full cycle of flexure by the spring  126  when the spring  126  is vibrating at the harmonic frequency for a particular driving frequency are shown in  FIGS. 8 and 9 . Each of the forces identified by the arrows  192 ,  194 ,  196 ,  198 ,  200 ,  202 ,  204  and  206  are of equal magnitude. Accordingly, the spring  126  does not bias the transfer members  122  and  124  predominantly outwardly or inwardly. Moreover, because the piezoelectric component  128  spans an antinode of the spring  126 , bending of the piezoelectric component  128 , and thus generation of electrical energy, is maximized. 
   In the event, however, that the frequency of the spring  126  defined by the nodes N 1 ′ and N 2  is not the resonant frequency of the spring  126  for the first drive frequency a standing wave will not form in the spring  126 . Similarly, if the frequency of the spring  126  defined by the nodes N 1 ′ and N 2  is the resonant frequency of the spring  126  for the first drive frequency but the drive frequency is changed, the standing wave will be destroyed. 
   In either event, the transfer arms  122  and  124  are not “trapped” by a standing wave and the movement of the spring  126  and the mass member  130  biases the transfer arms  122  and  124  toward positions whereat the frequency of the spring  126  defined by the nodes at the spring openings  152  and  158  is the resonant frequency of the spring  126 . 
   Depending upon the initial starting position as well as the resonant frequency for the new drive frequency, the transfer arms  122  and  124  may be biased away from each other or toward each other. In the example of  FIGS. 4 and 7 , the transfer arms  122  and  124  are biased outwardly, away from each other, to the positions shown  FIGS. 10 and 11  wherein the transfer arms  122  and  124  have shifted away from each other. The transfer arm  122  thus defines a new node N 1 ″ while the transfer arm  124  defines a new node N 2 ′ and the frequency of the spring  126  defined by the nodes N 1 ″ and N 2 ′ is the resonant frequency of the spring  126  for the drive frequency. 
   Referring to  FIG. 12 , an alternative resonator  210  is described. The resonator  210  includes a support  212  having a base  214  and two frames  216  and  218 . A suspended structure  220  extends between the frames  216  and  218 . The base  214  and the frames  216  and  218  may be fabricated using any acceptable material such as silicon or plastic. In one embodiment, the frames  216  and  218  are formed from a conductive material while the base  214  is formed from a non-conductive material. 
   As shown in  FIG. 13 , the suspended structure  220  includes a resilient outer shell  224  which defines a channel  226 . In one embodiment, the outer shell may be formed from a parylene material. The channel  226  is filled with a fluid  228  which may be a liquid or a gas. A number of beads  230  are located within the channel  226 . In this embodiment, the beads  230  are glass nanobeads which are negatively buoyant in the fluid  228 . 
   The resonator  210  operates in a manner similar to the resonator  118 . One difference is that when the ends of the suspended structure  220  are moved by the frames  216  and  218 , the mass of the suspended structure  220  along with the fluid  228  and the beads  230  provide sufficient inertia to cause the suspended structure  220  to flex. Additionally, the beads  230  initially have a greater inertia than comparable volumes of the fluid  228 . Accordingly, the beads  230  each act in a manner similar to the mass member  130 , each bead  230  tending to create an antinode within the suspended structures  220  and  222 . 
   Continued vibration of the suspended structure  220  biases the beads  230  into antinodal groups as shown in  FIG. 14  as the frequency of the suspended structure  220  approaches the resonant frequency associated with the received mechanical energy. The antinodes formed by the groups of beads  230  thus define nodes N within the suspended structure  220  which are spaced apart at a distance which is a multiple of the wavelength (λ) of the resonant frequency of the suspended structure  220 . 
   While a single suspended structure  220  is shown in this embodiment, in alternative embodiments more than one suspended structure is provided. Additionally, the suspended structures may be designed to be adjustable to resonant frequencies of different bands, such as by varying the resiliency of the shell. Thus, even in an environment which exhibits wide frequency variations, at least one vibratory structure in a harvesting device can be driven at a resonant frequency. 
   Additionally, a number of different design variations may be incorporated into an energy harvesting device incorporating principles of the invention. In one such alternative shown in  FIG. 15 , a suspended structure  232  includes a resilient outer shell  234  which defines a channel  236 . The channel  236  is filled with a fluid  238 . A number of beads  240  are located within the channel  236 . In this embodiment, the beads  240  are positively buoyant in the fluid  238 . Thus, when the suspended structure  123  is vibrated, the heavier fluid  238  is biased toward areas which define antinodes while the beads  240  are forced toward the nodes N. 
   Moreover, while the vibratory members of the resonators of  FIGS. 12 and 15  are shown to extend over several wavelengths of the resonant frequency of the vibratory members, vibratory members in alternative embodiments may extend over less than one wavelength of the resonant frequency of the vibratory member. By way of example, the spring  126  of the resonator  118  of  FIG. 2  may be designed such that the transfer arms  122  and  124  are always separated by a distance of ½ the wavelength of the resonant frequency of the spring  126  over the range of frequencies used to generate electrical energy. Such designs are useful when incorporating piezoelectric components because the antinode is predefined. Thus, the piezoelectric component may be pre-positioned at the location of the vibratory member exhibiting the greatest flexure. Of course, the resonators described herein are not limited to use with piezoelectric components. For example, in alternative embodiments, coil and magnet components, capacitive components, charged beams and magneto structures may be used. 
   While the present invention has been illustrated by the description of exemplary system components, and while the various components have been described in considerable detail, applicant does not intend to restrict or in any limit the scope of the appended claims to such detail. Additional advantages and modifications will also readily appear to those skilled in the art. The invention in its broadest aspects is therefore not limited to the specific details, implementations, or illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant&#39;s general inventive concept.