«» UK Patent Application .«» GB 2414120 uA
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`(43) Date of A Publication 16.11.2005
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`11.05.2004 Date of Filing:
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`(71) Applicant(s):
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`SplashpowerLimited
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`(Incorporated in the United Kingdom)
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`St John’s Innovation Centre,
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`Cowley Road, CAMBRIDGE, CB4 OWS,
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`United Kingdom
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`(21) Application No:
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`0410503.7
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`(72)
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`Inventor(s):
`Michael Craig Stevens
`Alexander Charles Knill
`John Robert Dunton
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`(51)
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`INT CL?:
`HO2H 3/44 3/12 , HO2J 7/02
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`(52) UK CL (Edition X ):
`H2K KSF K206 K360 K42Y K570 K779
`U1S $2058 $2078
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`(56) Documents Cited:
`EP 0867899 A2
`US 4218648 A
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`EP 0357829 A1
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`(58) Field of Search:
`UK CL (Edition W ) H2K
`INT CL? HO2H, Ho2J
`Other: EPODOC, WPI, JAPIO
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`(74) Agent and/or Addressfor Service:
`Haseltine Lake & Co
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`Imperial House, 15-19 Kingsway,
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`LONDON, WC2B 6UD,United Kingdom
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`(54) Abstract Title: Controlling inductive power transfer systems
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`(57) During a measurementperiod power transfer from primary unit 10, via driver 14 and primary coil 12 to
`secondary coil 32 of a separable secondary unit 30 is suspended, the decay of resonant energy stored in
`primary coil 12 is measured by means 12, and controller 16 restricts or stops power transfer depending on
`the decay measurement. Primary unit 10 is thereby shutdown when secondary unit 30 is not present or
`whena parasitic load (fig 4, 500) of a foreign object is in the vicinity of coil 12. The measurement may be
`based on the rate of decay of resonating energy and may involve comparing the decay measured in
`consecutive measurementperiods(fig 7), and may be augmented using a switchable dummy load (fig 4,
`40). The inductive power transfer system may be used to re-charge the battery of a portable device (fig 4,
`46).
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`32
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`30
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`Figure 1
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`Original Printed on Recycled Paper
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`Figure 1
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`Figure 2(B) |
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`Figure 3
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`Figure 5
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`Normal|Snub} Decay|Normal
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`Figure 6
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`1
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`2
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`time—-»
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`Figure 7
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`/ Operating
`nd Secondary devices
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`present and needing
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`power, and no
`[parasitic loads present/
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`YES
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`devices removed,
`or devices
`present stop
`needing power?
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`Parasitic load
`introduced?
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`YES
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`NO
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` Secondary
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`Parasitic load
`Sorc
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`levice(s
`removed and
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`introduced,or
`NO
`secondary
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`devicesalready
`devica(s)
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`present start
`introduced or
` needing power?
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`devices already Shutdown
`present
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` Parasitic load
`introduced?
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`Standby
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`No secondary devices
`present, or none that
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`Parasitic loads
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`Parasitic load
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`removed?
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`Figure 8
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`Fig. 9 (B)
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`Fig. 9 (C) ‘Gg
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`Fig. 9 (A)VALfo
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`Fig. 9 (D)
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`Fig. 9 (E)
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`2414120
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`CONTROLLING INDUCTIVE POWER TRANSFER SYSTEMS
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`The present invention relates to controlling inductive power transfer systems for
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`use, for example, to power portable electrical or electronic devices.
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`Inductive powertransfer systems suitable for powering portable devices may
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`consist of two parts:
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`e A primary unit havingat least one primary coil, through whichit drives an
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`alternating current, creating a time-varying magnetic flux.
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`e A secondary device, separable from the primary unit, containing a secondary coil.
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`Whenthe secondary coil is placed in proximity to the time-varying flux created
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`by the primary coil, the varying flux induces an alternating current in the
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`secondary coil, and thus power maybetransferred inductively from the primary
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`unit to the secondary device.
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`Generally, the secondary device supplies the transferred power to an external load, and
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`the secondary device maybecarried in or by a host object which includes the load. For
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`example the host object may bea portableelectrical or electronic device having a
`rechargeablebattery orcell. In this case the load may be a battery chargercircuit for
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`charging the battery or cell. Alternatively, the secondary device may be incorporated in
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`such a rechargeablecell or battery, together with a suitable battery charger circuit.
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`A class of such an inductive powertransfer systems is described in our United Kingdom
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`patent publication GB-A-2388716. A notable characteristic ofthis class of systemsis the
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`physically “open”nature of the magnetic system of the primary unit — a significant part of
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`the magnetic path is through air. This is necessary in order that the primary unit can
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`supply powerto different shapes and sizes of secondary device, and to multiple
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`secondary devices simultaneously. Another example of such an “open” system is
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`described in GB-A-2389720.
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`Such systems may suffer from some problems.Afirst problem is that the primary unit
`cannot be 100% efficient. For example, switching losses in the electronics and FPR losses
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`in the primary coil dissipate power even whenthere is no secondary device present, or
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`when no secondary devices that are present require charge. This wastes energy.
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`Preferably the primary unit should enter a low-power “standby mode”in this situation.
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`A second problem in such systems is that it is not possible to mechanically prevent
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`foreign objects from being placed into proximity with the primary coil, coupling to the
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`coil. Foreign objects made of metal will have eddy-currents induced therein. These eddy
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`currents tend to act to exclude the flux, but because the material has resistance, the
`flowing eddy currents will suffer ’R losses which will cause heating ofthe object. There
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`are two particular cases where this heating may besignificant:
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`e
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`Ifthe resistance of any metal is high, for example if it is impure or thin.
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`Ifthe material is ferromagnetic, for example steel. Such materials have high
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`permeability, encouraging a high flux density within the material, causing large
`eddy currents and therefore large IR losses.
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`In the present application, such foreign objects that cause powerdrain are termed
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`“parasitic loads”. Preferably the primary unit should enter a “shutdown mode” when
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`parasitic loads are present, to avoid heating them.
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`Various approaches to solve these two problems have been proposedin the priorart.
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`Solutions to the first problem, of not wasting power when no secondary device requires
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`charge, include:
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`e
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`In EP0533247 and US 6118249 the secondary device modulates its inductive load
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`during charging, causing a corresponding variation in the powertaken from the
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`primary unit. This indicates to the primary unit that it should stay out of the
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`standbystate.
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`e
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`In EP1022840 the primary unit varies the frequencyofits drive, and thus the
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`coupling factor to a tuned secondary unit. If the secondary unit is not taking
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`power, there is no difference in the power taken as the frequency is swept and
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`thus the primary unit goesinto a standbystate.
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`In US5536979 the primary unit simply measures the power flowingin the primary
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`coil, and enters a pulsing standbystate if this is below a threshold.
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`In US5896278 the primary unit contains detecting coils which have power
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`coupled back into them accordingto the position of the secondarydevice.If the
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`device is not present the primary unit enters a standby mode.
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`In US5952814 the secondary device has a mechanical protrusion which fits a slot
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`in the primary unit, activatingit.
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`In US6028413 the primary unit drives two coils, and there are a corresponding
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`two powerreceiving secondary coils in the secondary unit. The primary unit
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`measures the powerdelivered from each primary coil and enters standby mode if
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`it is below a threshold.
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`Solutions to the second problem,of parasitic loads, include:
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`As mentioned above, in EP1022840 the primary unit varies the frequency ofits
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`drive. In this system, the secondary deviceis tuned, so this frequency variation
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`will result in a variation of the powertaken from the primary unit. If the load is
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`instead a piece of metal, then varying the frequency will not have as mucheffect
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`and the primary unit will enter a shutdown state.
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`As mentioned above, in US5952814 a key in the secondary device activates the
`primary unit. The assumptionis that if a secondary deviceis present then this will
`physically exclude any foreign objects.
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`As mentioned above, in US6028413 the primary unit supplies powerto the
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`secondary device by driving two primary coils. If the amount of power supplied
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`by the twocoils is different, the primary unit assumesthat the load is nota valid
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`secondary device and enters shutdown mode.
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`These approachesall assume a 1:1 relationship between the primary unit and the
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`secondary device. Therefore they are not sufficient for systems such as those described in
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`GB-A-2388716 where more than one secondary device at a time may bepresent. For
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`example, they would not work whenthere are two secondary devices present, one
`requiring charge andthe othernot.
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`Someof these approachesalso assumethat the physicalor electrical presence of a valid
`secondarydevice implies that all foreign objects are physically excluded by the
`secondarydevice. This is not necessarily the case, particularly when the secondary
`devices may bepositioned arbitrarily in respect of the primaryunit, as in those described
`in GB-A-2388716.
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`According to a first aspect of the present invention, there is provided an inductive power
`transfer system comprising a primary unit, having a primary coil and electrical drive
`means connected to the primary coil for applying electrical drive signals thereto so as to
`generate an electromagnetic field, and the system also comprising at least one secondary
`device, separable from the primary unit and having a secondary coil adapted to couple
`with said field when the secondary device is in proximity to the primary unit so that
`powercan betransferred inductively from the primary unit to the secondary device
`without direct electrical conductive contacts therebetween, wherein the primary unit
`further comprises: control meansoperable to cause a circuit including said primary coil to
`operate, during a measurementperiod, in an undriven resonating condition in which the
`application of said drive signals to said primary coil by said electrical drive meansis
`suspended so that energy stored in said circuit decays over a course of said period; and
`decay measurement means operable to take one or more measuresof such energy decay
`during said period; said control meansbeing further operable, in dependence upon said
`one or more energy decay measures, to control the electrical drive meansso asto restrict
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`or stop inductive powertransfer from the primary unit.
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`Such a system is advantageous becauseit can addresseither or both the standby problem
`and the parasitic load detection problem listed above, in a robust and cost-effective
`manner, andis particularly advantageous in systems which may have multiple secondary
`devices present in different charge states, and/or whose open magnetic nature makesit
`easy for parasitic objects to couple to the primary coil.
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`In the present application, the term “ring-down”will be used to mean causing thecircuit
`(“resonanttank”) including the primary coil to operate in this undriven resonating
`condition.
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`During a ring-down, noenergyis being supplied to the primary coil, and so the decay of
`energy in the resonanttank is a measure of how muchenergyis being removed from it.
`The principal causes of energy loss are:
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`e Energy coupled into the secondary coil ofany secondary device present. This
`energy maybestored in a storage unit ofthe secondary device (if provided)
`and/or delivered to a load connected to the secondary device.
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`¢
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`Lossesto anyparasitic loads (foreign objects other than valid secondary devices)
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`present.
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`¢ Otherlosses in the primary unit or any secondary devices/host objects present.
`Theseother losses includeinefficiencies in the primary coil itself and any other
`components ofthe resonanttank (e.g. I’R losses in the copperofthe coil or
`effective series resistance of any resonating capacitor). They also include any
`magnetic losses in the primary and secondary units, for example magnetic
`hysteretic loop losses in any cores associated with the primary unit and/or
`secondary device.
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`In a preferred embodimentthe or one energy decay measure is a measureofa rate of such
`energy decay. In this case, the rate of energy decay is a measureofthe rate at which
`energy is being removed from the resonanttank.
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`The control means may employ the energy decay measures to detect when a substantial
`parasitic load is present in proximity to said primary unit, and restrict or stop inductive
`powertransfer from the primary unit followingthe detection of such a substantial
`parasitic load. For example, the energy decay rate may be compared with a shutdown
`threshold, andif the rate exceeds the threshold the power transfer is shut down.
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`Alternatively, or in addition, the control means may employthe energy decay measuresto
`detect when there is no secondary device present in proximity to the primary unit, and
`restrict or stop inductive power transfer from the primary unit when no such secondary
`device is detected.
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`Whencarrying out such detection the control means may employfirst compensation
`informationrelating to a load imposed on the primaryunit by losses in the primary unit
`itself so as to compensate for said losses of said primary unit. This prevents the detection
`from being affected by the inefficiencies in the primary coil itself and any other
`components of the resonant tank, as mentioned above.
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`The primary unit may further comprise calibration means for deriving part orall ofthe
`first compensation information from measurements taken by the primary unit whenit is
`effectively in electromagnetic isolation.
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`Asnoted above, dependingonits construction, the secondary device, and/or any host
`object which carries it, may suffer magnetic losses intrinsic to its construction, for
`example in magnetic core material, and other metal used in its construction. These so-
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`called “friendly parasitics” will be accounted for by the primary unit as a further parasitic
`loss, and if they sum to more than the threshold, the control means will shut the primary
`unit down. This may be avoided by providing a method of communicatingthe friendly
`parasitics of a device to the primary unit so that they may be accounted for and added to
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`the shutdown threshold.
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`Accordingly, whencarrying out such detection said control means may employ second
`compensation information relating to a parasitic load imposed on the primary unit by the
`secondary device so as to compensate for the parasitic load of the secondary device.
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`In this case, preferably the or each secondary device is operable to communicateits
`second compensation information directly to the primary unit or to communicate to the
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`primary unit other information from whichthe primary unit can derive the second
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`compensation information.
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`The secondary device may modulate a dummyload to communicate this other
`information, and the primary unit may derive the second compensation information by
`measuring the dummy load modulation.
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`This way of communicating the second compensation information also still works if
`multiple secondary devices are present. For example, assuming the secondary devices
`apply their dummyloadsat the same time, the primary unit will see a total dummy load
`equal to the sum ofthe individual dummy loadsofthe secondary devices. Thisis all that
`the primary unit needs to know for compensatingfor the parasitic loads of the secondary
`devices.
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`One wayof doingthis is to have a feedback resistor, whose value is proportional to the
`value ofthe friendly parasitics of the secondary device, which can be switched across the
`secondary coil, creating a dummyparasitic load. If this feedback resistor is always
`applied during certain ring-downs(for example during the third ring-down ofa set), the
`primary unit can then measurethe sum ofthe incremental loads resulting from the
`feedbackresistors in each secondary device present (whetherits load is drawing poweror
`not) and adjustits threshold accordingly. As described below, the same feedback resistor
`maybe used as a binary on/off signalling means to control standby mode.
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`Part orall of the first compensation information and/orpart orall of the second
`compensation information maybe informationstored in the primary unit during
`manufacture and/ortesting of the primary unit.
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`The system mayhave information varying means for varying oneor both ofthefirst and
`second compensation information when one or moreoperating conditions(for example,
`temperature) ofthe primary unit vary.
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`The decay measurement means maytake one or morefirst such energy decay measuresin
`a first such measurementperiod andtake one or more second such energy decay
`measures in a second measurementperiod. In this case the secondary device may
`comprise decay varying means which cause the secondary device to consume more
`energy in the first measurementperiod than in the second measurementperiod. The
`control meansin said primaryunit can then employthefirst and second energy decay
`measures to detect when a secondary device of the system is present in proximity to the
`primary unit. For example,if the control meansdetects a substantial difference between
`the first energy measures and the second energy measures, it may concludethat a
`secondary device must be present.
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`The decay varying means maycause the secondary device to impose a first dummy load
`on the primary unit in the first measurementperiod and to impose a second dummyload,
`different from the first dummyload, on said primary unit in the second measurement
`period. Oneofthe first and second dummyloads maybezero.
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`In this way, the secondary device may apply an additional dummy(or “feedback”) load
`only during somering-downsandnotduring others, or only during some part ofthe ring-
`down butnot others. This may be done for example by switching the feedbackresistor
`mentioned above across the secondary coil, creating a dummyparasitic load, as measured
`by the primary unit, which varies with time. In contrast, true parasitic loads (for example
`a piece of steel) will appear to be a constant parasitic load. If the primary unit control
`meanstakes similar load measurements between different ring-downs,or different parts
`of the same ring-down, it may put the drive meansinto a power-saving “standby” mode.
`This mode will be entered when no valid secondary device is present.
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`The decay varying means may bedisabled whenan actual load of the secondary device
`requires no power. Ifthe secondary device only presents a time-varying load during ring-
`down whenit requires power, then the primary unit will enter a standby mode even when
`there are secondary devices present, if they don’t require power.
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`A difference betweenthe first dummyload and the second dummyload may beset in
`dependence uponsaid parasitic load imposed on said primary unit by said secondary
`device. This is a convenient way to communicate the second compensation information
`to the primary unit.
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`The secondary device may be adapted to supply the powerit receives inductively from
`the primary unit to an actual Joad external to the secondary device. The load may be
`physically separable from the rest ofthe secondary device, for exampleit may be
`connected by a removable plug andsocket arrangement. Of course, the secondary device
`will generally include its own circuitry which will need some ofthe received powertoo.
`In somecasesthe load could beinternal to the secondary device.
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`The secondary device preferably comprises a storage means for storing energy received
`from the primary unit. The storage means, where provided, supplies stored energy to the
`actual load and/orcircuitry ofthe secondary device whenthecircuit is operating in the
`undriven resonating condition. This is not essential in all embodiments. Any load which
`can cope with short powerreductionsor interruptions will not require the storage means.
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`The secondary device may comprise load isolation means operable during the
`measurementperiod to prevent the supply to an actual load of the secondary device of
`any of the powerbeing received inductively by the secondary device from the primary
`unit. Ideally during a ring-down the storage meansofthe secondary device is of
`sufficient capacity, and sufficiently well-charged before the ring~-down commences,that
`the decay of energy in the storage means happens moreslowly than the decay of energy
`in the primary coil. If this cannot be guaranteed to be the case underall conditions, for
`example if the load is separable and with unknown characteristics, then the load isolation
`means can be included in the secondary device to disconnect the secondary device’s
`_
`storage means, and load, during the ring down,to ensurethat its electrical load is not
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`measured by the primary unit.
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`When the inductive powertransfer from the primary unit has been set to sucharestricted
`or stopped state, for example because the parasitic load becametoo high, the control
`means may causethe circuit to operate in the undriven resonating condition during a
`series of intermittent probing periods. In this case the decay measurement meanstakes
`one or more such energy decay measures during each said probing period. The control
`means employ the energy decay measures taken during the probing periods to determine
`whento end the restricted or stopped state. For example, the control means may
`periodically allow the drive to run for a short while in order to conduct a ring-down to
`measure the parasitic load and,if it is now below a threshold, resumethedrive.
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`Alternatively, when the inductive powertransfer from the primary unit has beenset into
`sucha restricted or stopped state, the control means may maintain thatstate until the
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`primary unit is reset by a user of the primary unit.
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`It may be advantageous to provide a clear means of synchronising the behaviour ofthe
`secondary device with that ofthe primary unit. For example, if the secondary device must
`isolate the load during ring-down,it may need a clear signal that such a ring-down is
`about to happen, since opening it once a ring-down is in process may betoo late. There
`are many possible meansof such synchronisation using a variety of communications
`means, but one convenient and reliable method is to use the power transfer channel
`between primary unit and secondary device by putting onto the primary coila signal
`which does not occur during normal operation. For example, the primary unit may change
`the amplitude, frequency or phaseofits drive.
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`The primary unit may comprise synchronising meansoperable to transmit a
`predetermined synchronising signal to the secondary device to synchronise operation of
`the secondary device with that of the primary unit.
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`The synchronising means maysendthe predetermined synchronising signal prior to
`causingthe circuit to operate in said undriven resonating condition.
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`In this case, the synchronising
`The primary unit may further comprise snubber means.
`means may switch the snubber meansinto the circuit after suspending the application of
`electrical drive signals-to said primary coil, so as to send the synchronising signal to the
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`In this way the primary coil is stopped very quickly by switching in
`secondary device.
`the snubber, which provides a very easily detectable synchronisingsignal.
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`The storage means(e.g. capacitor) of the secondary device maybe ofsignificant size and
`costif it is to provide sufficient capacity to supply the load during a ring-down. Therefore
`it is advantageousfor the ring-down to occur as quickly as possible. Measuring fewer
`cycles of the primary coil is one way to achieve this. This will reduce the accuracy ofthe
`system but this may be compensated for by averaging the results of several ring-downs.
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`The primary unit may further comprise resonant frequency increasing meansfor
`temporarily increasing a resonant frequencyofthe circuit during operation thereof in the
`undriven resonating condition. This is another way to shorten the ring-down time,
`because by increasing the resonant frequencyofthe primary coil during a ring-down,
`more cycles occur in less time.
`
`The control means preferably cause the electrical drive means to resume application of
`the drive signals to the primary coil in phase with any residual resonating energy within
`the circuit. After a ring-down is complete, the primary unit must resume normal
`operation as quickly as possible to prevent the secondary device storage means from
`discharging to the point where the supply to the load becomes inadequate. Restarting the
`drive to the primary coil in-phase with any residual resonating energy within the primary
`coil can ensure that the coil returns to nominal operating conditions as quickly as
`possible.
`
`The control means maycause the electrical drive means to temporarily boost the
`electrical drive signals prior to causing the circuit to operate in the undriven resonating
`condition. This can enhance the hold-up of the secondary device storage means, for any
`given such means, keeping the size and cost of such means to a minimum. Beforea ring-
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`GOOGLE AND SAMSUNG EXHIBIT 1013, 0018
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`GOOGLE AND SAMSUNG EXHIBIT 1013, 0018
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`down,the control means may cause a boostin the current in the primary coil for one or
`more cycles, thus temporarily raising the voltage in the secondary device storage means.
`This stores more energy in that means, enabling it to powerthe load for longer during
`ring-down.
`
`The control means may measure a natural resonant frequencyofthe circuit during the
`measurement period and employ the measured frequency to compensatefor an influence
`of changes in the natural resonant frequency on the energy decay measures. Because the
`resonant tank is not being actively driven during ring-down,the primary coil resonatesat
`the natural resonant frequency ofthe system. Measuring the natural resonant frequency
`during ring-down allowsa calculation ofthe inductance to be made,leading to a direct
`calculation oftotal powerloss. The inductance may change dueto the presence of
`secondary devices and/orparasitic loads.
`
`The control means may causethecircuit to operate in the undriven resonating condition
`during a series of different such measurement periods. In this case, the decay
`measurement means may take one or more such energy decay measuresin each said
`measurementperiod. The control means can then employ the energy decay measures
`taken in two or more different measurement periods to control said electrical drive
`means. For example, the contro] means can employ an averageofthe energy decay
`measures taken in different measurementperiods.
`
`Twoor more successive measurementperiods may be part of the same ring-down. This
`can give fast results. Alternatively, there may be just one measurementperiod in each
`ring-down. This gives slowerresults but it is possible to resumeapplication ofdrive
`signals to the primary coil between measurementperiods, making the power interruption
`to the secondary devices shorter. Also, the circuit conditions can be replicated at the start
`of each measurement period. The duration of each ring-down is preferably short
`compared to an interval between successive ring-downs.
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`GOOGLE AND SAMSUNG EXHIBIT 1013, 0019
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`In some systems, the primary coil may not driven continuously. It may, for example, be
`driven in pulses, for example in a pulse-width-modulated manner. In such systems, a
`partial “ring-down”is effectively happening wheneverthe primary coil is not being
`actively driven, perhaps once or more per cycle. Measurementof the natural resonant
`frequency and decayof the primary coil during these un-driven periods can provide the
`same functionality as that from the multiple-cycle ring-downs described above.
`
`The secondary device may have a timing means capable of changing the behaviour ofthe
`secondary device over time. For example, the timing means may detect when a second
`ring-down has happenedshortly aftera first ring-down,and, for example, switch in a
`conditional dummy(feedback) load only on such a shortly-following second ring-down.
`This may be useful for example in making interactions betweenthe primary unit and
`secondary device more deterministic. For example such a meanscould be used to make
`the standby detection scheme described above more reliable. For example,ifthe primary
`unit always conducts ring-downsin pairs a few millisecondsapart but repeated once
`every second, and the secondary device applies the feedback load only on the second of
`the two ring-downsofa pair, the primary unit may average several “first ring downs”,
`and separately average several “second ring-downs”, to improve the accuracy of
`measurement.
`
`The invention can operate even in systems which have morethan oneprimary coil. An
`example is systems which allow complete freedom ofposition or orientation of the
`secondary device by having two orthogonal coils, driven in quadrature, as in GB-A-
`2388716. In such systems there will be some coupling betweenthe two primary coils,
`which may makeit awkward to analyse a “ring-down” occurring simultaneously on both
`coils. In this case, the coils can be tested alternately. At the start of a test, both coils are
`halted, but then onlythefirst coil is allowed to ring-down while the second coil is held in
`a halted state to preventit from affecting the measurements onthefirst coil. During the
`nexttest the first coil is held halted while the second coil is allowed to ring-down.
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`GOOGLE AND SAMSUNG EXHIBIT 1013, 0020
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`GOOGLE AND SAMSUNG EXHIBIT 1013, 0020
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`Accordingly, in a preferred embodimentthe primary unit may havefirst and second such
`
`primary coils. A first such electrical drive meansis connectedto thefirst primary coil for
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`applying electrical drive signals thereto so as to generate a first electromagnetic field, and
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`a second suchelectrical drive meansis connected to the second primary coil for applying
`electrical drive signals thereto so as to generate a second electromagnetic field. The
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`;
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`control means causes the secondelectrical drive means to suspend application of
`electrical drive signals to the second primary coil whilst a first such circuit including the
`first primarycoil is in the undriven resonating condition and the decay measurement
`meansare taking one or more such measures of energy decayin thefirst circuit.
`Similarly the control means causesthefirst electrical drive means to suspend application
`ofelectrical drive signals to the first primary coil whilst a second such circuit including
`said second primary coilis in the undriven resonating condition and the decay
`measurement means are taking one or more such measuresof energy decay in the second
`circuit.
`
`The operating conditions ofthe primary unit after the ring-down is complete arelikely to
`be identical to those before the ring-down started.If the primary unit contains