Abstract:
The invention relates to an electricity generator having recovery of mechanical vibration energy, including a bistable system suitable for adopting one stable position or another relative to two ends in response to being energized in a predetermined direction (F EXT ); at least one monostable oscillating system suitable for periodically engaging with at least one of the two ends, in response to said energizing; and a device for converting the movement of the element of the bistable system into electric power.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a generator capable of generating electric energy from mechanical vibrations. It especially aims at the generation of electric energy by recovery of the mechanical vibration energy generated by certain pieces of equipment, for example, air conditioning ducts of a building, operating computers, industrial machines, etc. 
       DISCUSSION OF PRIOR ART 
       [0002]    To power certain low-consumption electronic systems, for example, occupancy or temperature sensors in a building, it has been provided to use generators capable of converting the mechanical energy of environment vibrations into electric energy. This especially enable to form fully stand-alone wireless sensors, which are thus easier to install and do not require repeated expensive battery replacement operations. More generally, the recovery of vibration energy has obvious practical, economical, and environmental advantages. 
         [0003]      FIG. 1  is a very simplified side view illustrating the operating principle of a mechanical vibration energy recovery electric generator  1  of monostable type. 
         [0004]    Generator  1  comprises a protection and support package  3 , of generally parallelepipedal shape, capable of being assembled on a vibrating surface. In this example, external vibrations are capable of exciting package  3  in a way which can be schematically accounted for by a force of orientation F EXT , substantially orthogonal to the lower surface of the package. Package  3  contains a spring blade  5 , attached by one its ends to a lateral surface of the package, and having its other end left free. At rest, blade  5  is substantially orthogonal to orientation F EXT  of the external excitation. A mass  7  is attached to blade  5 , on the side of its free end, to form a monostable mass-spring type oscillating system. 
         [0005]    Under the effect of external mechanical vibrations, transmitted by package  3 , blade  5  and mass  7  oscillate up and down around an equilibrium position. In the drawing, the equilibrium position is shown in a full line and the oscillating motion is illustrated by dotted lines. 
         [0006]    A mechanoelectrical converter, not shown, is provided to convert the motion of blade  5  and/or of ground  7  into electric energy. It may be a piezoelectric-type converter. 
         [0007]    A disadvantage of such an energy recovery device is that it only has a good efficiency when the frequency of the external vibrations corresponds to the resonance frequency of the oscillating system formed by mass  7  and blade  5 . Now, the spectrum of the vibration frequencies present in the environment is generally quite wide and may be variable. Thus, generator  1  only recovers a small fraction of the mechanical energy of environment vibrations. 
         [0008]    Further, vibration frequencies currently present in the environment are generally quite low, that is, they do not exceed from a few tens to a few hundreds of Hz. Now, the current tendency to miniaturize electronic device results in decreasing the dimensions of generator  1 , and especially the dimensions of the oscillating system formed by blade  5  and mass  7 . This inevitably results in an increase of the resonance frequency of the oscillating system. Thus, such an energy recovery device has a poor mechanoelectrical conversion efficiency. 
         [0009]      FIG. 2  is a very simplified side view illustrating the operating principle of a mechanical vibration energy recovery electric generator  11  of bistable type. 
         [0010]    Like generator  1  of  FIG. 1 , generator  11  comprises a package  13  capable of being excited by external vibrations symbolized by a force of orientation F EXT , orthogonal to the lower surface of the package. Package  13  contains a spring blade  15  having its two ends bearing, in compression, against two opposite lateral surfaces of the package. The two bearing points of blade  15  on package  13  are located along an axis substantially orthogonal to orientation F EXT  of the vibrations. A mass  17  is attached to blade  15 , substantially at its center. Blade  15 , in compression between its two ends, defines with mass  17  a non-linear or bistable system which may, under the effect of external vibrations, switch from one to the other of two stable equilibrium positions (respectively shown in a full line and in a dotted line in the drawing). This system may also oscillate around each of the two equilibrium positions. 
         [0011]    A mechanoelectrical converter, not shown, is provided to convert motions into electric energy. 
         [0012]    Jumps from one to the other of the equilibrium positions are directly linked to the amount of mechanical vibration energy transmitted by the package, and not to the frequency of the vibrations. Such jumps do not necessarily occur periodically. 
         [0013]    Thus, generator  11 , of bistable type, has the advantage over generator  1  of  FIG. 1 , of monostable type, of more widely exploiting the vibration frequency spectrum of the environment. 
         [0014]    However, the energy level of external vibrations must be sufficient to cause, as often as possible, jumps from one equilibrium position to the other. Below a given vibration energy level, such jumps do not occur. In this case, the generation of electric energy is only linked to the oscillations around one of the two equilibrium positions, and only the fraction of vibration energy corresponding to the resonance frequency of the device is recovered. The efficiency of generator  11  is then much lower. 
       SUMMARY 
       [0015]    Thus, an object of an embodiment of the present invention is to provide a mechanical vibration energy recovery electric generator overcoming all or part of the disadvantages of current generators. 
         [0016]    An object of an embodiment of the present invention is to provide such a generator which has a greater efficiency than current converters. 
         [0017]    An object of an embodiment of the present invention is to provide such a generator capable of operating from a wide vibration frequency range. 
         [0018]    An object of an embodiment of the present invention is to provide such a generator having an efficiency which remains good even when the energy level of the vibration is low. 
         [0019]    Thus, an embodiment of the present invention provides a mechanical vibration energy recovery electric generator, comprising: a bistable system capable of taking one or other of two stable positions with respect to two ends, as a response to an excitation of given orientation; at least one monostable oscillating system capable of periodically acting on one at least of the two ends, as a response to said excitation; and a converter of the motion of an element of the bistable system into electric energy. 
         [0020]    According to an embodiment of the present invention, the monostable oscillating system comprises: a first ring-shaped spring blade, attached at a first point to a support; and a first mass, attached to the first blade at a second point of the ring opposite to the first point, the axis formed by the first and second points being non-orthogonal to said orientation; and the bistable system comprises: a second spring blade, in compression between two points of the ring forming said ends, said ends belonging to a chord of the ring which is not parallel to said orientation; and a second mass attached to the second blade. 
         [0021]    According to an embodiment of the present invention, the converter comprises at least one piezoelectric element arranged on the second blade. 
         [0022]    According to an embodiment of the present invention, the generator comprises a plurality of bistable systems arranged so that the monostable system is capable of simultaneously acting on each of the bistable systems. 
         [0023]    According to an embodiment of the present invention, the monostable oscillating system comprises: first and second arc-shaped spring blades, in front of each other, each blade being attached by its ends to a rigid support package, the point of attachment of each blade being arranged along a line which is not orthogonal to said orientation; and first and second masses, each mass being attached to an intermediate point of one of the blades via a rigid connection which is not parallel to said orientation; and the bistable system comprises: a third spring blade in compression between the first and second blades, the bearing points of the third blade on the first and second blades forming said ends, such bearing points being arranged along a line which is not parallel to said orientation; and a third mass, attached to the third blade. 
         [0024]    According to an embodiment of the present invention, the converter comprises at least one piezoelectric element arranged on said third blade. 
         [0025]    According to an embodiment of the present invention, the bistable system comprises: a beam having a cross-shaped section with terminations forming said ends, the beam being attached to a support by a first termination and being arranged parallel to said orientation; and a spring system for maintaining the beam in compression between its terminations so that it is capable of taking one or the other of two stable positions corresponding to the deformation of the beam by buckling according to one or the other of two opposite rotation directions; and the monostable oscillating system comprises, on the side of a second termination of the beam, at least one mass capable of acting on the spring system, as a response to said excitation. 
         [0026]    According to an embodiment of the present invention, the spring system comprises a resilient core thoroughly crossing the beam lengthwise, this core being attached, under tension, to each of the beam terminations. 
         [0027]    According to an embodiment of the present invention, said mass is connected to the second termination of the beam via a rigid arm orthogonal to said orientation. 
         [0028]    According to an embodiment of the present invention, the converter comprises at least one piezoelectric element arranged on the beam. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    The foregoing and other objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which: 
           [0030]      FIG. 1 , previously described, is a very simplified side view of a mechanical vibration energy recovery electric generator, of monostable type; 
           [0031]      FIG. 2 , previously described, is a very simplified side view of a mechanical vibration energy recovery electric generator, of bistable type; 
           [0032]      FIG. 3  is a side view schematically showing an embodiment of a mechanical vibration energy recovery electric generator; 
           [0033]      FIG. 4  is a side view schematically showing another embodiment of a mechanical vibration energy recovery electric generator; and 
           [0034]      FIG. 5  is a perspective view schematically showing another embodiment of a mechanical vibration energy recovery electric generator. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, the various drawings are not to scale. 
         [0036]      FIG. 3  is a side view schematically showing an embodiment of a mechanical vibration energy recovery electric generator  21 . 
         [0037]    Generator  21  comprises a protection and support package  23 , for example, of generally parallelepipedal shape, capable of being assembled on a vibrating surface. In this example, the external vibrations are capable of exerting, on package  23 , an excitation F EXT  substantially orthogonal to the lower surface of the package. 
         [0038]    Package  23  contains a monostable oscillating system comprising a ring-shaped spring blade  25 , and a mass  27 . Ring  25  is attached, at one point, to a surface of package  23 , here the lower surface. Mass  27  is attached to ring  25  at a point of the ring substantially opposite to the point of attachment of the ring to the package. Thus, the point of attachment of the ring to the package and the point of attachment of the mass to the ring define a line substantially parallel to orientation F EXT  of the external excitation. More generally, it will be ascertained that this line is not orthogonal to orientation F EXT  of the external vibrations. 
         [0039]    Under the effect of external mechanical vibrations, ring  25  and mass  27  oscillate up and down around an equilibrium position. In the drawing, a low position is illustrated by dotted lines. 
         [0040]    The system further comprises a bistable system coupled to the monostable system, comprising a spring blade  29  in compression between its two ends. In this example, the ends of blade  29  bear against two opposite points of ring  25 , and these two bearing points define a line substantially orthogonal to orientation F EXT  of the external vibrations. More generally, it will be ascertained that the bearing points of blade  29  on ring  25  define a line which is not parallel to orientation F EXT  of the vibrations. Ring  25  exerts on blade  29  a compressive force F COMP , maintaining blade  29  in compression between its two ends. A mass  31  is attached to blade  29 , substantially at its center in this example. 
         [0041]    Blade  29  and mass  31 , define a bistable system of the type described in relation with  FIG. 2 . This system may, under the effect of external vibrations, switch from one to the other of two equilibrium positions, and/or oscillate around each of the two equilibrium positions. 
         [0042]    In generator  21 , the bistable system is coupled to the monostable oscillating system in such a way that the value of force F COMP , which maintains blade  29  of the bistable system in compression, varies periodically, substantially at the monostable system resonance frequency. Indeed, when the monostable system oscillates, the points of attachment of blade  29  to ring  25  tend to draw away from each other, and then to come closer to each other, inducing a decrease of the force exerted on the ends of blade  29 , followed by an increase thereof. 
         [0043]    When compressive force F COMP  varies, the minimum vibration energy level necessary to trigger the switching of the bistable system from one equilibrium position to the other varies accordingly. 
         [0044]    The inventor has observed that, when compressive force Fcomp varies periodically, the minimum external excitation level necessary to cause a given number of position jumps of the bistable system within a given time interval significantly decreases with respect to the same in a conventional bistable system where F COMP  is constant (for equivalent average values of F COMP ). 
         [0045]    An advantage of the provided device is that the mechanical energy of the external vibration itself enables to periodically vary force F COMP . Thus, it is not necessary to provide an external actuator provided with a dedicated power supply. 
         [0046]    A mechanoelectrical converter, not shown, is provided to convert the motion of blade  29  and/or of ground  31  into electric energy. It may be a converter of piezoelectric type, for example piezoelectric elements placed on blade  29 , towards the ends thereof, capable of undergoing the deformations of the blade on switching from one to the other of the equilibrium positions, and during linear oscillations around one or the other of the equilibrium positions. It may be also be a converter of electromagnetic or electrostatic type. 
         [0047]    As an example, ring  25  may be formed from a steel ring having a width approximately ranging between 5 and 50 mm, for example, on the order of 20 mm, and a thickness approximately ranging from 100 to 500 μm, for example, on the order of 200 μm. The ring may be substantially circular at rest and have a diameter approximately ranging from 50 to 100 mm, for example, on the order of 70 mm. Mass  27  may have a weight approximately ranging from 5 to 25 g, for example on the order of 10 g. Blade  29  of the bistable system may be formed from a steel ring having a width approximately ranging from 5 to 50 mm, for example, on the order of 20 mm, and having a thickness approximately ranging from 50 to 250 μm, for example, on the order of 75 μm. Mass  31  may have a weight approximately ranging from 2 to 15 g, for example, on the order of 5 g. Masses  27  and  31  may be respectively attached to blade  31  and to ring  25  by gluing. Of course, such dimensions are given as an example only. In practice, the system dimensions may range between a few tenths of a mm and a few tens of cm. 
         [0048]    According to an alternative embodiment, not shown, it is provided to arrange, against the walls of ring  25 , several bistable systems such as that formed by blade  29  and mass  31 . 
         [0049]      FIG. 4  is a side view schematically showing another embodiment of a mechanical vibration energy recovery electric generator  41 . 
         [0050]    Like generator  21  of  FIG. 3 , generator  41  comprises a rigid support package  43  capable of being excited by external vibrations of orientation F EXT , orthogonal to the lower surface of the package. 
         [0051]    Package  43  contains a double monostable oscillating system comprising two spring blades  45   a  and  45   b , each being arc-shaped. Blades  45   a  and  45   b  face each other. Each blade has one of its ends attached to the upper surface of the package and its other end attached to the lower surface of the package. In this example, the points of attachment of each blade to the package define a line substantially parallel to orientation F EXT  of the external vibrations. Masses  47   a  and  47   b  are respectively attached to blades  45   a  and  45   b , via rigid connections  49   a  and  49   b . In this example, rigid connections  49   a  and  49   b  are orthogonal to orientation F EXT  of the external vibrations, and are connected to blades  45   a  and  45   b  at their centers  50   a  and  50   b.    
         [0052]    Under the effect of external mechanical vibrations, spring blades  45   a  and  45   b  deform, and masses  47   a  and  47   b  oscillate around an equilibrium position, which causes a rotational oscillation of points  50   a  and  50   b , around an axis substantially orthogonal to the plane of  FIG. 4 , as well as a substantially vertical oscillation of points  50   a  and  50   b . The oscillating motion is illustrated by dotted lines in the drawing. Spring blades  45   a  and  45   b , associated with masses  47   a  and  47   b  via rigid connections  49   a  and  49   b , form a double monostable system. 
         [0053]    Package  43  further contains a bistable system comprising a spring blade  51 , having its ends respectively attached to blades  45   a  and  45   b , preferably at points  50   a  and  50   b . The spacing between blades  45   a  and  45   b  is such that blade  51  is maintained in compression between its two ends. A mass  53  is attached to blade  51 , for example, at its center. 
         [0054]    When the double monostable system oscillates, the action of the deformation of blades  45   a  and  45   b  on each end of blade  51  of the bistable system can be modeled by a moment M. Moment M periodically varies, substantially at the resonance frequency of the monostable system. 
         [0055]    When moment M varies, the minimum vibration energy level necessary to trigger the switching of the bistable system from one equilibrium position to the other varies simultaneously. Thus, like for generator  21  of  FIG. 3 , the minimum excitation level necessary to cause position jumps of the bistable system is much lower than for a conventional bistable system. Similarly, for a given external excitation level, the number of position jumps within a given time interval may be greater than for a conventional bistable system. 
         [0056]    A mechanoelectrical converter, not shown, is provided to convert the motion of blade  51  and/or of ground  53  into electric energy. 
         [0057]    As a variation, an electric generator similar to generator  41  of  FIG. 4 , however comprising a simple monostable system acting on a single one of the ends of blade  51 , the other end being connected to a fixed point, may be provided. 
         [0058]      FIG. 5  is a perspective view schematically showing another embodiment of a mechanical vibration energy recovery electric generator  61 . 
         [0059]    Generator  61  comprises a support package  63  capable of being excited by external vibrations of orientation F EXT , orthogonal to the lower surface of the package. For more clarity, only the lower surface of package  63  is shown in the drawing. 
         [0060]    Package  63  contains a beam with a cross-shaped section  65 . Beam  65  is attached by one of its ends to a surface of package  63 , here the lower surface, so that the beam is substantially parallel to orientation F EXT  of the external vibrations. Although, in this example, beam  65  is a beam with a cross-shaped section, the invention is not limited to this specific case, and it may be a beam with a section in the shape of a star having any other number of branches (greater than or equal to three). 
         [0061]    A spring system, shown by hatchings, is provided to maintain the beam in compression between its ends. In this example, it is a resilient central core  67  (or prestressed spring) which thoroughly crosses the beam lengthwise, this central core being connected, under tension, to the lower and upper ends of the beam. 
         [0062]    Beam  65  is submitted, between its ends, to a substantially vertical compressive force F COMP , resulting from the tension of spring  67 . Spring  67  is selected to be such that force F COMP  is sufficiently large to cause a buckling (not shown) of the beam according to one or the other of two opposite rotation directions (around the central axis of the beam). 
         [0063]    As a result of this buckling, beam  65  is submitted, between its ends, in addition to compressive force F COMP , to a torsion force, where this torsion force can be modeled by a moment M around an axis substantially parallel to the central axis of the beam. 
         [0064]    Further, a mass system  71  is connected to the free end of the beam (here, the upper end). In this embodiment, on the upper end side of the beams, a rigid arm  69 , extending along a direction orthogonal to the central axis of the beam, is attached to each branch of the beam, thus prolonging the branch to which it is attached. The end of each arm  69 , towards the outside of the beam, has a mass  71  attached thereto. 
         [0065]    Beam  65 , in compression and in torsion between its ends, forms a bistable system capable of switching, under the effect of an external excitation, from one to the other of two stable positions, the two stable positions corresponding to the beam buckling according to one and the other of the two rotation directions. In particular, beam  65  is capable of passing from one to the other of the two buckling positions under the effect of a rotational excitation of its ends. 
         [0066]    Further, spring  67  and mass system  71 , which is solidly attached to the upper end of spring  67 , define a monostable oscillating system. Under the effect of outer mechanical vibrations, masses  71  oscillate up and down around an equilibrium position. During such oscillations, the torsion (buckling) of beam  65  varies around one of the beam buckling positions. Since masses  71  are offset with respect to the central axis of the beam, linear torsion oscillations of beam  65  cause a rotating motion of masses  71  around the central axis of the beam. Thus, the oscillations of masses  71  cause a periodic variation of compressive force F COMP  and of torsion moment M, to which the ends of beam  65  are submitted. 
         [0067]    This results, as for the embodiments described in relation with  FIGS. 3 and 4 , in a significant decrease of the minimum level of vibration energy necessary to trigger the switching from one to the other of the equilibrium positions of the bistable system. Similarly, to trigger a given number of position jumps within a given time interval, the required vibration energy level is lower than for a conventional bistable system. 
         [0068]    A mechanoelectrical converter, not shown, is provided to convert the rotating motions of the beam and the branch deformations into electric energy. 
         [0069]    An advantage of the provided vibration energy recovery electric generators is that they have a greater efficiency than current generators. 
         [0070]    Another advantage of the provided vibration energy recovery electric generators is that they exploit a wide range of vibration frequencies. Further, their efficiency remains high, even when the vibration energy level is low. 
         [0071]    Various embodiments of a vibration energy recovery electric generator have been described hereabove. More generally, an aspect of the present invention is to provide a mechanical vibration energy recovery electric generator comprising a bistable system capable of taking one or the other of two stable positions as a response to a given excitation, and a monostable oscillating system capable of periodically acting on the bistable system, under the effect of this same excitation, to vary the vibration energy level necessary to trigger the switching of the bistable system from one of its equilibrium positions to the other. 
         [0072]    Specific embodiments of the present invention have been described. Various alterations, modifications, and improvements will readily occur to those skilled in the art. 
         [0073]    In particular, in each of the above-described embodiments, the oscillating structure is attached to the support and protection package at one or several attachment points. It will be within the abilities of those skilled in the art to provide alternative embodiments in which the number and the position of the attachment points differ from those in the above-discussed examples. 
         [0074]    Further, the invention is not limited to a parallelepipedal geometry of the generator package, such as mentioned hereabove.