Abstract:
A wirelessly-powered device that uses a ferrite based antenna. The ferrite antenna can be tuned to reduce the amount of flux within the housing.

Description:
[0001]    This application claims priority from provisional application No. 61/030,987, filed Feb. 24, 2008, the entire contents of which disclosure is herewith incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    Our previous applications and provisional applications, including, but not limited to, U.S. patent application Ser. No. 12/018,069, filed Jan. 22, 2008, entitled “Wireless Apparatus and Methods”, the disclosure of which is herewith incorporated by reference, describe wireless transfer of power. The transmit and receiving antennas are preferably resonant antennas, which are substantially resonant, e.g., within 10% of resonance, 15% of resonance, or 20% of resonance. The antenna is preferably of a small size to allow it to fit into a mobile, handheld device where the available space for the antenna may be limited. An embodiment describes a high efficiency antenna for the specific characteristics and environment for the power being transmitted and received. Antenna theory suggests that a highly efficient but small antenna will typically have a narrow band of frequencies over which it will be efficient. The special antenna described herein may be particularly useful for this kind of power transfer. 
         [0003]    One embodiment uses an efficient power transfer between two antennas by storing energy in the near field of the transmitting antenna, rather than sending the energy into free space in the form of a travelling electromagnetic wave. This embodiment increases the quality factor (Q) of the antennas. This can reduce radiation resistance &lt;R r ) and loss resistance 
         [0004]    In one embodiment, two high-Q antennas are placed such that they react similarly to a loosely coupled transformer, with one antenna inducing power into the other. 
         [0005]    The antennas preferably have Qs that are greater than 200, although the receive antenna may have a lower Q caused by integration and damping. 
       SUMMARY 
       [0006]    The present application describes antennas for wireless power transfer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    In the Drawings: 
           [0008]      FIG. 1  shows a block diagram with equivalent circuits; 
           [0009]      FIG. 2  shows a measurement set up; 
           [0010]      FIG. 3  shows a first ferrite rod antenna with partial coils; 
           [0011]      FIG. 4  shows a second ferrite rod with a complete coil; 
           [0012]      FIG. 5  shows a plot of resonance frequency; and 
           [0013]      FIG. 6  shows a block diagram of the rod antenna in use. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    An embodiment uses ferrites in antennas for transmission and reception of magnetic flux used as wireless power. For example, ferrite materials usually include ceramics formed of MO—Fe 2 O 3 , where MO is a combination of divalent metals such as zinc, nickel, manganese and copper oxides. Common ferrites may include MnZn, NiZn and other Ni based ferrites. 
         [0015]    Ferrite structures concentrate magnetic flux lines into the structure, thereby creating a magnetic path/field with less interference and eddy current losses in device electronics. This in essence sucks in the magnetic flux lines, thereby improving the efficiency of the magnetic power distribution. An embodiment describes a ferrite rod-shaped antennas. These may provide compact solutions that are easy to integrate into certain kinds of packaging. Also, the properties of ferrites may 
         [0016]    The resonance frequency of Ferrite rod antennas may be easier to tune. In one embodiment, the tuning may be carried out by mechanically adjusting the position of the coil on the rod. 
         [0017]    However, Ferrite rod antennas may suffer from Q degradation at higher magnetic field strengths (higher receive power levels) due to increasing hysteresis losses in Ferrite material. The present application describes use of special ferrite antennas to carry out wireless transfer of power. 
         [0018]    The inventors realized that hysteresis losses in ferrite material may occur at higher power receive levels and higher magnetic field strengths. In addition, increasing the magnetic field strength may actually shift the resonance frequency, especially in certain materials where there are nonlinear B-H characteristics in the ferrites. In addition, harmonics emissions can be generated to in due to inherent nonlinearity. This nonlinearity becomes more important at lower Q factors. 
         [0019]    One aspect of the present system is to compare the performance of these antennas, at different power levels and other different characteristics. By doing this, information about the way these materials operate in different characteristics is analyzed. 
         [0020]    Ferrite Rod materials are normally used in communication receiver applications at small signal levels such as at or below 1 mW. No one has suggested using these materials at large levels, e.g. up to 2 W. In order to analyze the characteristics of these materials, measurement values and techniques are described herein. According to one embodiment, the measurement may be carried out at by using the antennas that transmit antenna, and assuming reciprocity as a receiving antenna. The tests increase the V and current, and determine the values of the result. 
         [0021]    According to one embodiment, the Q value is used to determine a limit for the amount of power applied. 
         [0022]    According to one embodiment, the characteristics of a ferrite Rod antenna are evaluated based on the following parameters
       Q-factor   Resonance frequency   Voltage across antenna coil   Antenna current   Inductance of antenna coil   Equivalent permeability of rod   Equivalent series resistance   Magnetic inductance in Ferrite rod   Measurement of tuning range that can be achieved by mechanically tuning of a ferrite rod       
 
         [0032]      FIG. 1  illustrates the ferrite Rod antenna  100  under test, where the system is formed of a ferrite Rod  102 , on which is wound two different sets of windings. The coupling windings  110  are connected to the electronic circuitry  112 . In this embodiment, the electronic circuitry may be transmitting circuitry, however it should be understood that the electronic circuitry can alternately be receiving circuitry. Accordingly, the circuitry  112  is referred to herein as power converting circuitry. The power circuitry  112  is formed of an AC part, for example and AC generator, with a matching impedance  116 . The matching impedance  116  is connected to a first wire  108  of the twisted-pair  111 . The second wire  109  of the twisted-pair  111  goes to ground. The two wires  108 ,  109  are collectively connected to a coupling windings  120 . Coupling winding  110  is located at a 1st place on the ferrite Rod  100  to. The coupling winding  110  is completely separated from the main winding  120 . Moreover, the number of windings of the coupling winding  110  may be ⅕ to 1/10 the number of windings of  120 . The important part is to induce magnetic flux into the ferrite Rod, without having the impedance of the inducement changed by any external characteristics. 
         [0033]    The main winding  120  is also in parallel with a main capacitor  125 . 
         [0034]    A number of different values within the  FIG. 1  embodiment may be measured. For example, these values may include 
         [0000]                                        U 0 :   Source voltage (e.m.f.) of LF power source   [V]       Z out :   Output (source) impedance of LF power source   [Ω]       U in :   Input voltage measured at antenna terminals a/b   [V]       I in :   Input current measured at antenna terminals a/b   [A]       Z in :   Input impedance measured at antenna terminals a/b   [Ω]       I A :   Antenna current (r.m.s.)   [A]       U c :   Voltage across antenna capacitance (r.m.s.)   [V]       P in :   Antenna input power   [W]       L:   Equivalent inductance of Ferrite rod antenna   [H]           (includes all reactive components except C)       C:   Capacitance required to achieve resonance frequency   [F]       R s :   Equivalent series resistance of Ferrite rod antenna   [Ω]           (includes all losses except source resistance)       U 0 ′:   Source voltage transformed into equivalent series circuit   [V]       R out ′:   Source resistance transformed into equivalent series circuit   [Ω]       Q UL :   Unloaded Q-factor       μ rod :   Effective relative permeability of Ferrite rod       B rod :   Computed magnetic flux density (induction) in Ferrite rod   [T]       N:   Number of turns       A Fe :   Ferrite cross sectional area   [m 2 ]                    
The different characteristics can also be determined from these values, as
 
         [0035]    2.2.2.2 Equations 
         [0036]    Resonance Frequency: 
         [0000]    
       
         
           
             
               
                 
                   
                     f 
                     res 
                   
                   = 
                   
                     1 
                     
                       2 
                        
                       π 
                        
                       
                         
                           L 
                           · 
                           C 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   2 
                    
                   
                     - 
                   
                    
                   1 
                 
               
             
           
         
       
     
         [0037]    Unloaded Q-Factor: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       Q 
                       UL 
                     
                     = 
                     
                       
                         
                           1 
                           
                             R 
                             s 
                           
                         
                          
                         
                           
                             L 
                             C 
                           
                         
                       
                       = 
                       
                         
                           2 
                            
                           π 
                            
                           
                               
                           
                            
                           f 
                            
                           
                               
                           
                            
                           L 
                         
                         
                           R 
                           s 
                         
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       Q 
                       UL 
                     
                     = 
                     
                       
                         2 
                          
                         
                           π 
                           · 
                           f 
                           · 
                           C 
                           · 
                           
                             U 
                             c 
                             2 
                           
                         
                       
                       
                         P 
                         
                           i 
                            
                           
                               
                           
                            
                           n 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   2 
                    
                   
                     - 
                   
                    
                   2 
                 
               
             
           
         
       
     
         [0038]    Input Power: 
         [0000]    follows 
         [0000]        P   in   =Re{U   in   ·I   in }  Equation 2-3 
         [0039]    Effective Relative Permeability of Ferrite Rod 
         [0000]    
       
         
           
             
               
                 
                   
                     μ 
                     rod 
                   
                   = 
                   
                     L 
                     
                       L 
                       air 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   2 
                    
                   
                     - 
                   
                    
                   4 
                 
               
             
           
         
       
     
         [0040]    Magnetic Flux Density (Inductance) in Ferrite Rod: 
         [0000]    
       
         
           
             
               
                 
                   
                     B 
                     rod 
                   
                   = 
                   
                     
                       U 
                       C 
                     
                     
                       π 
                       · 
                       
                         2 
                       
                       · 
                       N 
                       · 
                       
                         A 
                         Fe 
                       
                       · 
                       f 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   2 
                    
                   
                     - 
                   
                    
                   5 
                 
               
             
           
         
       
     
         [0041]      FIG. 2  illustrates the ways of measuring the different values, shown as channel  1 , channel  2  and Channel  3 . These different values can be measured as follows
       Oscilloscope: measures r.m.s. of U in  (CH 1 ), I in  (CH 2 ), U C  (CH 3 )   T1: Current transformer, toroid Epcos R16/T38, 25 turns   R1: Load resistor of T1(R1//R(CH 2 )=25 . . . 100 Ohm, 25 Ohm: 1 A current→1V at CH 2 )   AMP1: Amplifier arcus 100 W, voltage gain=33 (135 kHz)   R2: Load resistor of AMP1, 5 . . . 50 Ohm (needed for safety and stability of the amplifier)   T2: Isolation transformer 1:1 (2*40 turns bifilar, Epcos R16/T38 toroid) to prevent from ground loop interference   ATT1: Attenuator 50 Ohm, 10 . . . 20 dB to prevent from overload of AMP1   GEN1: RF signal generator (Rohde&amp;Schwarz SMG)         
         [0050]    According to a measurement procedure, the generator is started with −10 DBM of power, and at a frequency that is resonant to the calculated resonant frequency from the equation 2.1. At this resonant frequency, all of the signals U in , I in  and U c  are in phase so long as the polarities of channel  1  and Channel I mean channel  2  and Channel  3  is correct and the current channel (Ch 2 ) has a minimum value. 
         [0051]    The values of U in , I in  and U c  are measured at the resonant frequency. 
         [0052]    The remaining values are calculated. 
         [0053]    Table 1 represents the results for an “X” antenna made using ferrite materials. The measured values are used to calculate certain other values within this antenna. 
         [0054]    This antenna shown in  FIG. 3  has a length of 87 mm, and a diameter of 10 mm. The ferrite material used is Ferroxcube 4B2. The main coil of this antenna has 19 windings of main coil  300  for a total length of 20 mm of 300×0.4 mm wire. A three turn coupling coil  302  is connected to receive the magnetic resonant field from a generator  305 . The coupling coil  302  is spaced along the rod at 12 mm from the end of the main coil. A 55.17 nF 500V Mica capacitor  310  is used to form resonance. Q values are 
         [0055]    A number of measurements were carried out as shown in Table 1, where the left side of the table represents the inputs to the coil. Based on these inputs, and the equations noted above, the values on the right side of the table were calculated. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
             
             
               
                   
                 Input (measured) 
                 Calculation 
               
             
          
           
               
                 Meas 
                 f res 
                 U in 
                 I in 
                 Uc 
                 P in 
                 Z in 
                 L 
               
               
                 # 
                 kHz 
                 V rms 
                 mA rms 
                 V rms 
                 mW 
                 Ohm 
                 μH 
               
               
                   
               
               
                 8 
                 134.98 
                 0.00818 
                 0.1406 
                 0.0888 
                 0.0012 
                 58.179 
                 25.200 
               
               
                 7 
                 134.97 
                 0.0259 
                 0.511 
                 0.284 
                 0.0132 
                 50.685 
                 25.204 
               
               
                 6 
                 134.9 
                 0.0784 
                 1.67 
                 0.861 
                 0.131 
                 46.946 
                 25.230 
               
               
                 1 
                 134.920 
                 0.075 
                 1.450 
                 0.733 
                 0.109 
                 51.724 
                 25.222 
               
               
                 2 
                 134.752 
                 0.228 
                 5.270 
                 2.260 
                 1.202 
                 43.264 
                 25.285 
               
               
                 3 
                 134.294 
                 0.643 
                 18.440 
                 6.370 
                 11.857 
                 34.870 
                 25.458 
               
               
                 4 
                 133.113 
                 1.555 
                 68.070 
                 17.140 
                 105.849 
                 22.844 
                 25.912 
               
               
                 5 
                 131.011 
                 3.450 
                 244.400 
                 37.050 
                 843.180 
                 14.116 
                 26.750 
               
               
                   
               
             
          
           
               
                   
                 Calculation 
                   
               
             
          
           
               
                 Meas 
                 X 
                 Q UL 
                 I A 
                 R s 
                 μ rod 
                 B rod 
                 R p 
               
               
                 # 
                 Ohm 
                 U 
                 mA rms 
                 Ohm 
                 U 
                 mT peak 
                 Ohm 
               
               
                   
               
               
                 8 
                 21.372 
                 320.804 
                 4.155 
                 0.0666 
                 12.632 
                 0.099 
                 6856.3 
               
               
                 7 
                 21.374 
                 285.126 
                 13.287 
                 0.0750 
                 12.633 
                 0.318 
                 6094.2 
               
               
                 6 
                 21.385 
                 264.770 
                 40.262 
                 0.0808 
                 12.647 
                 0.963 
                 5662.1 
               
               
                 1 
                 21.382 
                 231.067 
                 34.282 
                 0.0925 
                 12.643 
                 0.820 
                 4940.6 
               
               
                 2 
                 21.408 
                 198.559 
                 105.567 
                 0.1078 
                 12.674 
                 2.531 
                 4250.8 
               
               
                 3 
                 21.481 
                 159.311 
                 296.537 
                 0.1348 
                 12.761 
                 7.159 
                 3422.2 
               
               
                 4 
                 21.672 
                 128.067 
                 790.886 
                 0.1692 
                 12.988 
                 19.434 
                 2775.5 
               
               
                 5 
                 22.020 
                 73.934 
                 1682.592 
                 0.2978 
                 13.408 
                 42.683 
                 1628.0 
               
               
                   
               
             
          
         
       
     
         [0056]    The table shows that the Q value stays greater than 100 up to a power level of approximately 100 mw. The 840 mw measurement showed a Q of  73 , and a resonant frequency that has shifted by almost 4 Khz from the value it shows at 10 −3  mw. Note again, as discussed 
         [0057]    According to one embodiment, therefore, the antenna is only operated in regions where it has specific values that are within the desired values of operation of the antenna, e.g, high enough Q, proper frequency, etc. 
         [0058]    A second embodiment used an antenna as shown in  FIG. 4 . This used a similar sized rod formed of similar material. Antenna  400  uses 75 turns of wire  405  and a two-turn coupling coil  410 , located over the main coil, at 25 mm from the end of the main coil. This antenna uses a 6.878 nF 400 V polypropylene capacitor  415 . 
         [0059]    Table 2 represents second measured and calculated results for the  FIG. 4  antenna. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Input (measured) 
                 Calculation 
                   
               
             
          
           
               
                 Meas 
                 f res 
                 U in 
                 I in 
                 Uc 
                 P in 
                 Z in 
                 L 
                 X 
                 Q UL 
                 I A 
                 R s 
                 μ rod 
                 B rod 
                 R p 
               
               
                 # 
                 kHz 
                 V rms 
                 mA rms 
                 V rms 
                 mW 
                 Ohm 
                 μH 
                 Ohm 
                 U 
                 mA rms 
                 Ohm 
                 U 
                 mT peak 
                 Ohm 
               
               
                   
               
             
          
           
               
                 1 
                 133.601 
                 0.0274 
                 0.38 
                 0.895 
                 0.0104 
                 72.105 
                 206.328 
                 173.200 
                 444.185 
                 5.187 
                 0.3889 
                 23.235 
                 0.258 
                 76932.9 
               
               
                 2 
                 133.541 
                 0.0828 
                 1.265 
                 2.684 
                 0.1047 
                 65.455 
                 206.514 
                 173.278 
                 396.918 
                 15.490 
                 0.4366 
                 23.256 
                 0.768 
                 68777.1 
               
               
                 3 
                 133.333 
                 0.2336 
                 4.462 
                 7.68 
                 1.042 
                 52.353 
                 207.159 
                 173.548 
                 326.062 
                 44.253 
                 0.5323 
                 23.329 
                 2.201 
                 58587.4 
               
               
                 4 
                 132.763 
                 0.610 
                 17.240 
                 19.710 
                 10.518 
                 35.389 
                 208.941 
                 174.293 
                 211.911 
                 113.085 
                 0.8225 
                 23.529 
                 5.673 
                 36934.7 
               
               
                 5 
                 131.504 
                 1.404 
                 65.100 
                 45.860 
                 91.400 
                 21.567 
                 212.961 
                 175.962 
                 130.768 
                 260.624 
                 1.3456 
                 23.982 
                 13.325 
                 23010.2 
               
               
                 6 
                 129.342 
                 2.882 
                 247.000 
                 94.650 
                 711.854 
                 11.668 
                 220.140 
                 178.903 
                 70.345 
                 529.057 
                 2.5432 
                 24.791 
                 27.962 
                 12584.9 
               
               
                 7 
                 127.234 
                 4.720 
                 652.000 
                 149.200 
                 3077.440 
                 7.239 
                 227.495 
                 181.867 
                 39.773 
                 820.378 
                 4.5726 
                 25.619 
                 44.807 
                 7233.5