Abstract:
A device for the transmission of electromagnetic signals, the device comprising: a conductive element at least one inducer, for inducing charge in said conductive element; a transmission circuit, for generation and transmission of electromagnetic signals; wherein said conductive element and said at least one inducer are movable, with respect to each other, between a plurality of relative positions; in a first position of said relative positions, said at least one inducer is arranged to induce a charge in said conductive element; in a second position of said relative positions, said conductive element is arranged to discharge; the conductive element is arranged to couple with the transmission circuit, in said first position and/or said second position, such that charging and/or discharging of said conductive element causes the transmission circuit to generate and transmit an electromagnetic signal; and the device is arranged such that movement of said device causes relative movement of said conductive element and said at least one inducer between said plurality of relative positions.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from United Kingdom Patent Application No. 1300927.9, filed on Jan. 18, 2013. The priority application is herein incorporated by reference in its entirety 
       BACKGROUND 
       [0002]    Energy harvesting is the process of capturing and storing small amounts of energy for use in a variety of application. One of the most common types of energy harvesting involves the transfer of energy from small movements in a device into electricity. For example, many watches capture user movement to power the watch mechanism. Energy harvesting using electrets has been proposed by Juji Suzuki, in his article Energy Harvesting from Vibration Using Polymer Electret (SUZUKI, Y. Energy Harvesting from Vibration Polymer Electret. International Symposium on Micro-Nano Mechatronics and Human Science. November 2008, pages 180 to 183). 
       SUMMARY 
       [0003]    A first aspect provides a device for the transmission of electromagnetic signals, the device comprising: a conductive element at least one inducer, for inducing charge in said conductive element; a transmission circuit, for generation and transmission of electromagnetic signals; wherein said conductive element and said at least one inducer are movable, with respect to each other, between a plurality of relative positions; in a first position of said relative positions, said at least one inducer is arranged to induce a charge in said conductive element; in a second position of said relative positions, said conductive element is arranged to discharge; the conductive element is arranged to couple with the transmission circuit, in said first position and/or said second position, such that charging and/or discharging of said conductive element causes the transmission circuit to generate and transmit an electromagnetic signal; and the device is arranged such that movement of said device causes relative movement of said conductive element and said at least one inducer between said plurality of relative positions. 
         [0004]    A second aspect provides a method of transmitting an electromagnetic signal using a device comprising: a conductive element; at least one inducer, for inducing charge in said conductive element; and a transmission circuit, for generation and transmission of electromagnetic signals; wherein said conductive element and said at least one inducer are movable, with respect to each other, between a plurality of relative positions; the method comprising: moving the device in a first direction to cause the conductive element and at least one inducer to move relatively closer to one another, thereby causing said at least one inducer to induce a charge in the conductive element; and moving the device in a second direction to cause the conductive element and at least one inducer to move relatively apart from one another, thereby causing the conductive element to discharge; wherein said steps of charging and/or discharging occur through said transmission circuit and cause the transmission circuit to generate and transmit an electromagnetic signal. 
         [0005]    Further features of embodiments are recited in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which: 
           [0007]      FIG. 1  is a perspective diagram of a device in accordance with a first embodiment; 
           [0008]      FIG. 2  is a side-view diagram of the device shown in  FIG. 1  in operation; 
           [0009]      FIG. 3  is a further side-view diagram of the device shown in  FIG. 1  in operation; 
           [0010]      FIG. 4  is a further side-view diagram of the device shown in  FIG. 1  in operation; 
           [0011]      FIG. 5  is a further side-view diagram of the device shown in  FIG. 1  in operation; 
           [0012]      FIG. 6  shows a device in accordance with a second embodiment; 
           [0013]      FIG. 7  shows a device in accordance with a third embodiment; and 
           [0014]      FIG. 8  shows a device in accordance with a fourth embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  is a simplified diagram of an RF (radio frequency) device  100  in accordance with an embodiment. The device  100  includes an electret  101  and a conductive plate  102 . The electret  101  and the conductive plate  102  are arranged in parallel. The conductive plate  102  is arranged to move between a first position in which it is in contact with the electret  101  and a second position in which it is separated from the elecret  101 . In  FIG. 1 , the conductive plate is shown between the first and second positions. The device  100  also includes a first contact  103 , a second contact  104  and an antenna  105 . Both contacts  103 ,  104  are coupled to the antenna  105 . The device  100  also includes a resonant circuit, which is not shown in  FIG. 1 . The resonant circuit is also coupled to the antenna  105 . The device  100  further includes a ground plane  106 , to which the electret  101  and the resonant circuit are coupled. 
         [0016]    When the device is physically shaken or moved, the conductive plate  102  moves between the first and second positions. The electret  101  is an insulating material with an implanted fixed charge. The electret  101  produces a strong electric field in the area through which the conductive plate  102  moves. As the conductive plate  102  moves, a charge is induced in the plate. In this sense, the conductive plate is an inducer. The conductive plate  102  discharges through the contacts  103 ,  104 , causing the resonant circuit to resonate, and an RF signal to be transmitted from the antenna  105 . All of the energy used to generate the signal is derived from the movement of the device. No energy is drawn from the electret itself. 
         [0017]      FIG. 2  is a more detailed diagram of the device  100  shown in  FIG. 1 . All of the components shown in  FIG. 1  are also shown in  FIG. 2 . In addition, resonant circuit  107  is shown in  FIG. 2 , coupled between the groundplane  106  and the antenna  105 . In  FIG. 2 , the conductive plate  102  is shown to be moveable in a direction perpendicular to the plane of the electret  101 . However, in alternative embodiments, the conductive plate  102  may move side-to-side or rotate relative to the electret, as will be described in more detail below. 
         [0018]    The electret  101  is positioned parallel and adjacent to the groundplane  106 . Here, the eletret  101  is positioned in contact with the groundplane  106 , and the groundplane  106  is a metal backplate. The electret  101  has an implanted negative charge. There is an induced positive charge in the groundplane  106 . This induced charge is a result of the electret charging process. In  FIG. 2 , the first contact  103  is located close to the electret  101 . The second contact  104  is situated close to the conductive plate&#39;s second position. When the conductive plate  102  is in the first position, it makes contact with the first contact  103  and the electret  101 . When the conductive plate  102  is in the second position, it makes contact with the second contact  104  and is separated from the electret  101 . 
         [0019]    As noted above, the device  100  is designed such that motion of the device results in the conductive plate  102  moving toward and away from the electret  101 . Initially, to a first order approximation, the entire electric field (E-field) is contained within the electret  101 . The E-field in the electret  101  is dependent on: the charge density, σ; the area, A; and the permittivity, ε. 
         [0000]    
       
         
           
             
               
                 
                   E 
                   = 
                   
                     
                       σ 
                        
                       
                           
                       
                        
                       A 
                     
                     ɛ 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0020]    The surface voltage of the electret  101  and the groundplane  106  is determined by the charge and the distance of separation between the electret  101  and the groundplane  106 . 
         [0000]        V=E.d   (2)
 
         [0000]    where d is the distance of separation. 
         [0021]      FIG. 3  shows the device  100  when the conductive plate  102  is in the first position and is in contact with the elecret  101  and first contact  103 . Charge redistributes between the conductive plate  102  and the groundplane  106  as they have to be at the same potential. Current i flows through the resonant circuit  107  and the antenna  105  radiates energy. Assuming that conductive plate  102  and the groundplane  106  are equidistant from the charge in the electret, 50% of the charge is transferred onto the conductive plate  102 . It should be noted that the conductive plate  102  and the electret  101  do not need to make contact. For example, in this embodiment, the conductive plate  102  and the electret  101  may move close to the charged electret, and make contact with the groundplane  106 . Such an arrangement would also be effective at charging the conductive plate  102 . 
         [0022]    If the direction of motion (of the device  100 ) is now reversed, the conductive plate  102  moves away from the electret  101 . The charge on the conductive plate  102  is captured as no circuit is made with the groundplane  106 . This is shown in  FIG. 4 . 
         [0023]    As the motion forces the conductive plate  102  and electret  101  apart, work is being done. The electric field between the conductive plate  102  and the electret  101  remains constant since the captured charge remains constant: 
         [0000]    
       
         
           
             
               
                 
                   E 
                   = 
                   
                     Q 
                     
                       ɛ 
                       0 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0024]    As described above, the electric field is constant but the separation (d) is increased. Therefore, the voltage on the conductive plate  102  increases since: 
         [0000]        V=E.d   (4)
 
         [0025]    In the second position, the conductive plate  102  contacts with the second contact  104  and makes a circuit with the groundplane  106 , as shown in  FIG. 5 . To a first approximation, the charge reverts to the initial state with all the charge residing in the groundplane  106 . The current i that results flows through the resonant circuit and results in radiation from the antenna  105 . 
         [0026]    The energy scavenging device  100  described above requires that a metal structure (the conductive plate  102 ) moves close to an electret (the electret  101 ) and that additional motion moves the now charged metal away from the electret. Once separated, the metal structure is connected to the groundplane discharging the metal structure. Accordingly, so long as these requirements are met, it is possible to design structures to scavenge energy from different types of motion, for example rotational and sliding motion. 
         [0027]      FIG. 6  shows a device  200  that could be used to scavenge energy from rotational movement. Sectors of metal backed electret  201 A,  201 B,  201 C are aligned with a rotating metal element  202  made up of similar sectors  202 A,  202 B,  202 C. A commutator  203  is used to make and break the required connections. As the rotating element sectors  202  and electret sectors  201  align, a connection is made, thereby charging the rotating element, as shown in the left-hand diagram. The rotation continues and the voltage on the element  202  is increased as the distance between the charged electret  201  and the rotating metal  202  is increased, as shown in the centre-most diagram. When the rotation elements  202  are maximally misaligned with the electret  201 , the commutator  203  remakes the contact and the charge flows back to the starting condition. At each contact current flows through a resonant circuit resulting in energy being radiated from the attached antenna. 
         [0028]    A thinner solution to that shown in  FIGS. 1 to 5 , in which the conductive plate slides over the charged electret, is shown in  FIG. 7 . The basic operating principle, however, remains the same.  FIG. 7  shows an RF (radio frequency) device  300  in accordance with an embodiment. The device  300  includes an electret  301  and a conductive plate  302 . The electret  301  and the conductive plate  302  are arranged in parallel. The conductive plate  302  is arranged to move between a first position in which it may be in contact with the electret  301  (as noted above, the conductive plate may be in contact with the electret, but is not required to be in contact with the electret) and a second position in which it is separated from the elecret  301 . The device  300  also includes a first contact  303 , a second contact  304  and an antenna  305 . Both contacts  303 ,  304  are coupled to the antenna  305 . The device  300  also includes a resonant circuit  307 . The resonant circuit is also coupled to the antenna  305 . The device  300  further includes a ground plane  306 , to which the electret  301  and the resonant circuit  307  are coupled. In use, the conductive plate slides from side-to-side. Other than the direction of movement, the device  300  operates in the same manner as that described above in connection with  FIGS. 1 to 5 . 
         [0029]      FIG. 8  is a more detailed diagram of an implementation of the device shown in  FIGS. 1 to 5 .  FIG. 8  shows a device  400 . The device  400  includes an electret coated aluminium plate  401 , which is equivalent to the electret  101 . The device  400  also includes the a disk  402 , which is equivalent to the conductive plate  102 . The disk  402  measures around 19 mm in diameter. The metal disk includes a copper spindle  403 . The copper spindle is axially mounted through the electret coated aluminium plate  401 . 
         [0030]    The device  400  also includes an uncoated metal plate  404 . The uncoated metal plate  404  and the electret coated aluminium plate  401  are connected by four supporting arms  405 A-D. The electret coated aluminium plate  401  and the uncoated metal plate  404  are both the same size and shape. They are each square in shape, and have a nominal thickness. Each supporting arm  405 A-D is positioned towards a respective corner of each plate. Each plate has a spindle supporting hole  406 A,  406 B towards its centre. The spindle  403  is supported through these holes such that the disk  402  may move back and forth along the axis of the spindle  403 . 
         [0031]    The device  400  also includes insulted contacts  407 A,  407 B. Insulated contact  407 A, is positioned on a side of the uncoated metal plate  404  opposite to the side of the electret  401 . Insulated contact  407 B is positioned on a side of the electret  401  opposite the side of the uncoated metal plate  404 . The disk  402  moves between two end positions. In a first position, the disk  402  contacts or moves adjacent to the electret  401 . In a second position, the disk  402  is positioned closer to the uncoated metal plate  404 . In the first position, the spindle  403  contacts with insulted conductor  407 B. In the second position, the spindle  403  contacts with insulated conductor  407 A. 
         [0032]    The device  400  also includes a dipole antenna  408 . The dipole antenna includes a first arm  409 A and a second arm  409 B. The first arm  409 A is coupled at one end to the insulated contacts  407 A,  407 B. The second arm  409 B is coupled at one end to the electret  401 . A tuning coil  410  is coupled between the first arm  409 A and the second arm  409 B of antenna  408 . 
         [0033]    In use, the disk  402  moves between the electret  401  and the uncoated metal plate  404 . Charge is induced into the disk  402  when it is positioned adjacent to the electret  401 . In that same position, current flows through the spindle  403  and causes the tuning coil  410  to resonate, and an RF signal is transmitted by the dipole antenna  408 . As the disk  402  moves away from electret  401 , it maintains a charge. This is then discharged through the spindle  403  when the disc  402  reaches the other discharge position, again causing the antenna to transmit an RF signal. 
         [0034]    Embodiments provide a means of achieving low cost communications and tagging without the need for a power supply or batteries. The pulse characteristics make it ideal for finding the direction of a tag. There are many possible applications including: emergency beacons; telemetry equipment; low-cost tagging; movement detection; low-data communication links; and position fixing/identifying. 
         [0035]    In the embodiment described in connection with  FIGS. 1 to 5 , the conductive plate  102  is arranged to move and the electret is fixed. In an alternative embodiment, the conductive plate may be fixed, and the electret may be arranged to move. 
         [0036]    In the embodiment described in connection with  FIGS. 1 to 5 , the electret is described as having a negative charge, and the conductive plate accordingly takes a positive charge. In an alternative embodiment, the electret may be positively charged, and the conductive plate may take a negative charge. 
         [0037]    In the embodiment described in connection with  FIGS. 1 to 5 , only a single electret is described. In an alternative embodiment, the device includes two electrets. The conductive plate is arranged to move between the two plates. The electrets would be oppositely charged in such an arrangement. Twice the amount of work would be required to move the conductor between two electrets, with the result that twice the energy would be scavenged and transmitted. 
         [0038]    In the embodiment described in connection with  FIGS. 1 to 5 , the conductive plate approaches the electret, in order for charge to induced in the plate. In an alternative embodiment, no groundplane is required. In such an embodiment, static charge induction is used to charge the conductor. 
         [0039]    In the embodiment described in connection with  FIGS. 1 to 5 , the conductive plate discharges through the resonant circuit when the plate is in contact with the electret. In an alternative embodiment, the conductive plate does not discharge at this point. Instead, the conductive plate only discharges when it is at the second discharge position. 
         [0040]    The above-described embodiments include a device which is suitable for RF transmission. It will be appreciated that such devices may also be arranged to operate at microwave frequencies. 
         [0041]    It will be appreciated that the afore-mentioned description is not limiting. Variations are possible without departing from the spirit and scope set forth in the claims. While particular combinations of features have been set forth in the description and claims, it will be appreciated that other combinations are possible within the scope of the claims.