U.S. patent application number 15/320606 was filed with the patent office on 2017-06-08 for foreign object detection in inductive power transfer field.
This patent application is currently assigned to PowerbyProxi Limited. The applicant listed for this patent is PowerbyProxi Limited. Invention is credited to Sander VOCKE, Tom VOCKE.
Application Number | 20170163100 15/320606 |
Document ID | / |
Family ID | 54935837 |
Filed Date | 2017-06-08 |
United States Patent
Application |
20170163100 |
Kind Code |
A1 |
VOCKE; Sander ; et
al. |
June 8, 2017 |
FOREIGN OBJECT DETECTION IN INDUCTIVE POWER TRANSFER FIELD
Abstract
A method for determining the presence of a foreign object in an
inductive power transfer field in which control circuitry of an
inductive power system performs the steps of: providing power to a
direct current to alternating current converter; providing power
from the converter to a transmitter coil in the inductive power
transfer field; waiting for the current in the transmitter coil to
stabilize; estimating the reactive power in the transmitter coil;
estimating the real power in the transmitter coil; and using the
estimated reactive power and estimated real power to determine
whether a foreign object is present.
Inventors: |
VOCKE; Sander; (Freemans
Bay, Auckland, NZ) ; VOCKE; Tom; (Freemans Bay,
Auckland, NZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
PowerbyProxi Limited |
Freemans Bay, Auckland |
|
NZ |
|
|
Assignee: |
PowerbyProxi Limited
Freemans Bay, Auckland
NZ
|
Family ID: |
54935837 |
Appl. No.: |
15/320606 |
Filed: |
June 11, 2015 |
PCT Filed: |
June 11, 2015 |
PCT NO: |
PCT/NZ2015/050072 |
371 Date: |
December 20, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02J 50/60 20160201;
H02J 50/12 20160201 |
International
Class: |
H02J 50/60 20060101
H02J050/60; H02J 50/12 20060101 H02J050/12; H02J 50/90 20060101
H02J050/90; H02J 5/00 20060101 H02J005/00; H02J 7/00 20060101
H02J007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 20, 2014 |
NZ |
626547 |
Claims
1.-44. (canceled)
45. A method for determining the presence of a foreign object in an
inductive power transfer field in which control circuitry of an
inductive power system performs the steps of: providing power to a
direct current to alternating current converter; providing power
from the converter to a transmitter coil in the inductive power
transfer field; waiting for the current in the transmitter coil to
stabilize; estimating the reactive power in the transmitter coil;
estimating the real power in the transmitter coil; and using the
estimated reactive power and estimated real power to determine
whether a foreign object is present.
46. A method for determining the presence of a foreign object in an
inductive power transfer field as claimed in claim 45 further
comprising the steps of: estimating the real power by sensing the
direct current into the converter and combining this value with the
value of the power supply voltage; and/or estimating the reactive
power by sensing the peak alternating current through the
transmitter coil.
47. A method for determining the presence of a foreign object in an
inductive power transfer field as claimed in claim 46 further
comprising the step of estimating the reactive power as the sensed
alternating current through the transmitter coil combined with the
peak voltage on the transmitter coil.
48. A method for determining the presence of a foreign object in an
inductive power transfer field as claimed in claim 45 further
comprising the steps of: sampling the instantaneous current and
voltage waveforms through the transmitter coil; multiplying the
sampled instantaneous current and voltage waveforms, integrating
the product of the multiplication and estimating the real power as
the result of the integration divided by the time sampled; and
calculating the root mean square values of the sampled voltage and
current waveforms and estimating apparent power as the product of
the root mean square current and root mean square voltage, and
estimating the reactive power based on the estimated real power and
estimated apparent power.
49. A method for determining the presence of a foreign object in an
inductive power transfer field as claimed in claim 45 further
comprising the steps of: sensing the amplitude of the voltage
through the transmitter coil, sensing the amplitude of the current
through the transmitter coil, and sensing the phase difference
between the voltage and current waveforms; and/or estimating the
real power from the amplitude of the voltage and current through
the transmitter coil and the cosine of the phase difference between
the voltage and current; and/or estimating the reactive power from
the amplitude of the voltage and current through the transmitter
coil and the sine of the phase difference between the voltage and
current waveforms.
50. A method for determining the presence of a foreign object in an
inductive power transfer field as claimed in claim 45 further
comprising the step of sensing the real and reactive powers at a
plurality of frequencies.
51. A method for determining the presence of a foreign object in an
inductive power transfer field as claimed in claim 50 further
comprising the steps of combining the estimates into a data set and
determining at least one characteristic of the data set.
52. A method for determining the presence of a foreign object in an
inductive power transfer field as claimed in claim 51, wherein the
characteristic is one of centre, size or average value.
53. A method for determining the presence of a foreign object in an
inductive power transfer field as claimed in claim 51 further
comprising the step of classifying whether the measurement relates
to a foreign object depending on the data set.
54. A method for determining the presence of a foreign object in an
inductive power transfer field as claimed in claim 45, further
comprising the steps of: storing calibration values of real and
reactive power for each transmitter coil when no receiver is
present and no foreign object is present; and/or storing
calibration values of real and reactive power for each transmitter
coil at a plurality of frequencies; and/or comparing the estimated
real and reactive power to the calibration values.
55. A method for determining the presence of a foreign object in an
inductive power transfer field as claimed in claim 45 further
comprising the step of bringing a receiver into proximity with the
inductive power transfer field prior to the step of providing power
to a transmitter coil.
56. A method for determining the presence of a foreign object in an
inductive power transfer field as claimed in claim 55 wherein the
receiver is adapted to run a start-up sequence in which no current
is drawn for a predetermined period or until a signal is sent from
the transmitter.
57. A method for determining the presence of a foreign object in an
inductive power transfer field as claimed in claim 55 further
comprising the step of waiting for the receiver current and/or
voltage to reach a steady state prior to estimating the real and
reactive power.
58. A method for determining the presence of a foreign object in an
inductive power transfer field as claimed in claim 56 further
comprising the step of estimating the real and reactive power when
the receiver is drawing substantially no current.
59. An inductive power transfer device configured to execute the
method of claim 45.
Description
FIELD OF THE INVENTION
[0001] The present invention is in the field of inductive power
transfer systems. More particularly, although not exclusively, the
invention relates to a method and system for detecting foreign
objects present on an inductive power transfer surface and in
particular to foreign objects located between an inductive power
transfer surface and a receiver.
BACKGROUND OF THE INVENTION
[0002] Inductive power transfer systems are used to wirelessly
provide power from a transmitter device to a receiver. This
technology is now being used in wireless charging pads for handheld
devices. Typically, a primary side or transmitter generates a
time-varying magnetic field with a transmitting coil or coils. This
magnetic field induces an alternating current in a suitable
receiving coil or coils that can then be used to charge a battery,
or power a device or other load. In some instances, the transmitter
coil(s) or the receiver coil(s) may be connected to capacitors to
create a resonant circuit, which can increase power throughput or
efficiency at the corresponding resonant frequency.
[0003] A common problem with inductive power transfer systems is
controlling when the transmitter should be powered and when the
transmitter should be switched off. A further problem arises when a
non-receiver (a foreign object) is brought into the range of the
transmitter, and an unwanted current (and therefore heat) is
induced therein. These non-receivers are typically known as
parasitic loads. Further, a conducting foreign object may be
located between the transmitter and a compatible receiver.
Transmitting in this instance may result in damage to the
transmitter and/or receiver.
[0004] Automatic systems for the detection of foreign objects have
been described in the conventional art. For example: [0005] systems
that rely on an additional coil in the transmitter side for foreign
object detection [0006] systems that use a detection circuit to
detect a higher harmonic of the transmitter coil frequency [0007]
systems incorporated into a receiver
[0008] Many of the systems for foreign object detection rely on
additional detection circuitry. The drawback of this is that it
adds cost and bulk to inductive power transfer systems. Many of the
systems for foreign object detection also rely on power transfer
occurring in a resonant circuit and may not be effective in
non-resonant power transfer. Receiver based systems rely on every
possible receiver being equipped for foreign object detection.
Further receiver based systems can only detect foreign objects
between a transmitter and the receiver.
[0009] It is an object of the invention to provide an improved or
alternative method and system for foreign object detection in an
inductive power transfer field, or to at least provide the public
with a useful choice.
SUMMARY OF THE INVENTION
[0010] According to one exemplary embodiment there is provided a
method for determining the presence of a foreign object in an
inductive power transfer field in which control circuitry of an
inductive power system performs the steps of: providing power to a
direct current to alternating current converter, providing power
from the converter to a transmitter coil in the inductive power
transfer field, waiting for the current in the transmitter coil to
stabilize, estimating the reactive power in the transmitter coil,
estimating the real power in the transmitter coil, and using the
estimated reactive power and estimated real power to determine
whether a foreign object is present.
[0011] According to another exemplary embodiment there is provided
an inductive power transfer device comprising: a converter adapted
to be electrically connected to a power supply and adapted to
output alternating current to a transmitter coil, a controller for
controlling the frequency of the converter output alternating
current, at least one transmitter coil adapted to receive
alternating current from the converter and further adapted to
generate a time-varying magnetic field with predetermined frequency
and strength, at least one sensor adapted to sense features of the
inductive power transfer device voltage and current from which
estimates of real and reactive power though the transmitter coil
can be made and provide sensor output to the controller, said
controller configured to: control the supply of power to the
transmitter coil, receive signals from the sensor as to current
flow through the transmitter coil, determine when the current in
the transmitter coil has reached a steady state condition, receive
sensor output from the sensor of features of the inductive power
transfer device voltage and current, estimate the real and reactive
power in the transmitter coil from the sensor output received from
the sensor, and determine whether a foreign object is present based
on the estimated real and reactive power.
[0012] It is acknowledged that the terms "comprise", "comprises"
and "comprising" may, under varying jurisdictions, be attributed
with either an exclusive or an inclusive meaning. For the purpose
of this specification, and unless otherwise noted, these terms are
intended to have an inclusive meaning--i.e. they will be taken to
mean an inclusion of the listed components which the use directly
references, and possibly also of other non-specified components or
elements.
[0013] Reference to any prior art in this specification does not
constitute an admission that such prior art forms part of the
common general knowledge.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The accompanying drawings which are incorporated in and
constitute part of the specification, illustrate embodiments of the
invention and, together with the general description of the
invention given above, and the detailed description of embodiments
given below, serve to explain the principles of the invention.
[0015] FIG. 1 shows a block diagram of an inductive power transfer
system;
[0016] FIG. 2 shows the resulting shapes of four frequency sweeps
with and without a foreign object and/or a receiver present;
and
[0017] FIG. 3 shows the results gained when the centre of the
shapes B, C and D depicted in FIG. 2, weighted by the total area of
the respective shapes, are plotted.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0018] Embodiments of the present invention relate to method for
detecting or identifying a foreign object in an inductive power
transfer system. FIG. 1 is a block diagram shown a general
representation of an inductive power transfer system 1. The
inductive power transfer system includes a transmitter 2 and a
receiver 3. The transmitter includes a DC-AC converter 4 that is
electrically connected to an appropriate power supply 5, such as
Mains power. In FIG. 1 this is shown as converter 4 being connected
to a DC-DC converter 6 that is in turn connected to the DC power
supply 5. The converter 4 may be a non-resonant half bridge
converter or other converter adapted for the particular inductive
power transfer system, such as a push-pull converter. The converter
4 is configured to output an alternating current of desired
frequency and amplitude to one or more transmitting inductors 7.
The voltage of the output of the converter may also be regulated by
the converter 4, the DC-DC converter or combination of both.
[0019] The transmitting inductor(s) 7 may be a suitable
configuration of one or more coils or other electrically reactive
components which provide an inductance, depending on the
characteristics of the magnetic field that are required in a
particular application and the particular geometry of the
transmitter. In some inductive power transfer system, the
transmitting inductors may be connected to reactive components,
such as capacitors, (not shown) to create a resonant circuit. The
transmitting coil(s) receive alternating current from the converter
4 and generate a time-varying magnetic field. The frequency and
strength of the magnetic field are controlled by the controller.
Each transmitter coil may be individually operated.
[0020] FIG. 1 also shows control circuitry or a controller 8 of the
transmitter 2. The controller can be directly or indirectly
connected to the various components (blocks) of the transmitter.
The controller receives inputs from the connected parts of the
transmitter and produces an output that controls the way those
parts of the transmitter operate. The controller may include or
have access to electronic storage 9. In preferred embodiments the
electronic storage is an on-board memory. The controller may be a
programmable logic controller that is programmed to perform
different computational tasks depending on the requirements of the
inductive power transfer system.
[0021] In addition to the features of the inductive power transfer
system 1 outlined thus far, FIG. 1 further shows representations of
sensors 10a and 10b for sensing particular operating
characteristics of the transmitter 2. It is understood that the
sensors 10a and 10b may be connected to other or different parts of
the transmitter than as depicted, be provided as a single
integrated sensor, or as more than two distributed sensors,
depending on the characteristics being sensed. The invention is not
limited in this respect. In FIG. 1, the sensor 10a is shown
connected to the junction between the DC-DC converter 6 and the
DC-AC converter 4, which is appropriate for measuring the current
supplied to the converters, and the sensor 10a is shown connected
to the transmitting inductor(s) 7, which is appropriate for
measuring the current through, or voltage over, the inductor(s) 7.
However, the sensors 10a and 10b may be alternatively or
additionally adapted to sense the required characteristics
elsewhere in the transmitter system.
[0022] In embodiments of the invention discussed later sensors for
making different types of measurements are discussed. Those skilled
in the art appreciate that there are many possible types of sensors
that are adapted for the sensing described and the invention is not
limited in this respect. It will be understood that an appropriate
sensor will be used for the sensing depending upon the required
functionality.
[0023] Returning to FIG. 1, the receiver 3 includes one or more
receiving inductors 11 provided as a suitable configuration of one
or more coils or other electrically reactive components which
provide an inductance, that are suitably connected to receiver
circuitry 12 that in turn supplies received power to a load 13. The
load may be, for example, a rechargeable battery. The receiver
circuitry converts the induced current into a form that is
appropriate for the load. In some inductive power transfer systems,
the receiving inductors may be connected to reactive components,
such as capacitors, (not shown) to create a resonant circuit.
[0024] The receiver may also include control circuitry either as
part of the receiver circuitry 12 or as one or more separate
components (not shown) for disabling current to flow into the
receiver load, effectively "disconnecting" the load from the system
1. This control circuitry may also generate a start-up sequence
including a time delay prior to enabling current to flow into the
load, or wait for a signal from the transmitter prior to enabling
current to flow into the load. This functionality may be
implemented by any suitable means, such as a series electronic
switching device between the load and the rest of the system.
[0025] There will now be described several embodiments of methods
for detecting foreign objects in an inductive power transfer field,
or for detecting receivers. Although these methods will be
described in relation to the inductive power transfer system 1
described in relation to FIG. 1, it will be understood that the
methods may be adapted to work with any number of appropriate
inductive power transfer system configurations, and similarly
inductive power transfer systems may be adapted to work with these
methods, and the invention is not limited in this respect. In this
description the following definitions apply: `real power` is
defined as the total average power being dissipated in the system,
plus any power flowing into the receiver's load; `reactive power`
is defined as the average power that is flowing back and forth
between energy storage devices (such as capacitors and inductors)
in the system, without being dissipated; and `apparent power` is
defined as the product of the root mean square current going into
the system and the root mean square voltage going into the system.
In all of these definitions, the "system" is anything after the
point at which a measurement is being made, e.g., when measurement
is at the input of the DC-AC converter, anything that comes before
is not included in the "system".
[0026] Further, in this description the following is to be
understood. In a non-resonant "hard-driven" inductive power
transmitter circuit the voltage on the transmit coil is
substantially a square wave and the current through the coil is
substantially a triangular wave. The difference between a
non-resonant and a resonant inductive power transmitter circuit, as
the terms are commonly used in the field of inductive power
transfer (as opposed to their strict technical definitions), is as
follows. A series circuit having a transmitter conductor and a
capacitor may be resonant or non-resonant depending on the values
of the inductor and the capacitor, and the driving frequency. In a
resonant transmitter circuit, the reactance of the inductor and
capacitor are the same order of magnitude resulting in
substantially sinusoidal waveforms with a variable phase
difference. In a non-resonant transmitter circuit, the reactance of
the capacitor is of a lower order of magnitude than that of the
inductor, and the resulting waveforms resemble a square wave
(voltage) and a triangular wave (current) with no fixed phase
relationship.
Real and Reactive Power: Measurements
[0027] FIG. 2 shows the results in curves or shapes (these terms
are used synonymously and interchangeably herein) A, B, C and D of
estimating real and reactive power at a plurality of frequencies
with and without foreign objects present. Prior to obtaining
measurements a calibration value for both real and reactive power
is stored. The origin of the graph represents a 0% offset from the
calibration value for that frequency. If all frequencies measured
have the same values as the respective calibration measurements
then all the measurement points are at the origin. The x axis is
the percentile difference between the real power and its respective
calibration measurement at that frequency. The y axis is the
percentile difference between the reactive power and its respective
calibration measurement at that frequency. A plurality of
measurements can be made at a plurality of frequencies. Each of the
data sets are plotted with their points connected by a curve. Thus
every curve is a data set of measurements at different frequencies.
Every point in the curve is a measurement at a different frequency,
but with the same placement of objects in the field. The points are
connected by a curve in ascending order of frequency. In curve D of
FIG. 2, D.sub.first and D.sub.last represent the lowest and highest
measurement frequency for curve D, respectively.
[0028] In FIG. 2: [0029] curve A shows a data set that may
typically be measured when doing the real and reactive power
measurements at a plurality of frequencies, with no objects
whatsoever in the transmitter field. As expected, the measurements
are virtually equal to their respective calibration values (which
is why the points appear close to 0% displacement from their
calibrations on both axes); [0030] curve B is an example of a curve
that may be measured when a receiver is in the transmitter field.
In the case observed, the result is that the reactive power is
lower than the calibration value by various amounts at different
frequencies, and the real power is either lower or higher,
depending on the frequency. These offsets are highly dependent on
the specific ferrites, coil designs and circuits used, their
frequency responses and the exact receiver positioning; and [0031]
curves C and D are examples of curves that may be measured when a
foreign metal object is positioned between the receiver and the
transmit coil(s).
[0032] In these cases, they can be seen to have the same general
shape as curve B, but the average real and reactive powers are
higher. Also, the difference in real and reactive powers over
frequency may be increased. These differences are highly dependent
on the specific ferrites, coil designs and circuits used, their
frequency responses and their exact positioning--and the material,
geometry and positioning of the foreign metal object as well. Curve
D corresponds to a bigger foreign object than curve C, resulting in
bigger differences in average and size compared to curve B, than
curve C.
[0033] It can clearly be seen from FIG. 2 that the curves for
foreign objects are offset from those for non-foreign objects in
both spread size and average value. The offset difference is in
part dependent on the degree of overlap of the foreign object on
the coil(s), for example, when the foreign object only partially
overlaps a coil the difference is reduced.
[0034] FIG. 3 shows the results gained when characteristics of the
shapes from FIG. 2 are used for foreign object determination. Using
the data sets associated with these shapes, the averages and sizes
of the shapes can be evaluated. Averages are then the average
percentile offset from the respective calibration. Sizes are the
difference between maximum and minimum offset from the respective
calibration. This is evaluated separately for the real and reactive
parts so the results are plotted on separate axes in FIG. 3. The
averages are weighted by the sizes. Curves B, C and D in FIG. 2 are
each represented by a single point in FIG. 3 as points B, C and D,
respectively. These points can then be compared to a threshold
determined or pre-set for foreign object detection. The dotted line
in FIG. 3 is an example of a threshold that would, in the case
presented, separate the foreign object situations (points C and D)
from the non-foreign object situation (point B).
[0035] This method could also be used for foreign object detection
in the absence of a receiver. Based on these measured relationships
and characteristics of foreign objects and IPT receivers, different
measurement regimes may be used to detect the presence of foreign
objects. Examples of such regimes are now described.
Real and Reactive Power: Average and Peak Current
[0036] A transmitter may begin foreign object detection when the
presence of a potential receiver is detected. A receiver may be
brought into proximity with the transmitter prior to commencing
foreign object detection. The detection of the presence of a
potential receiver can be performed in a number of ways, for
example, the various methods and systems described in PCT
Publication No. WO 2013/165261, the entire contents of which are
hereby incorporated by reference, may be used.
[0037] In embodiments of the invention the receiver is configured
to commence a charging start-up mode when the receiver detects the
presence of a transmitter and prior to being charged by the
transmitter. In this mode the receiver runs in a substantially
no-load condition, as the receiver keeps the output load
disconnected. In this condition there may still be the load of the
receiver controller circuitry itself. This condition may continue
for a set period or until a signal is received from the transmitter
to cease the start-up mode. The transmitter performs foreign object
detection during the start-up mode. The transmitter may send the
signal once foreign object detection is completed and if no foreign
object is detected. Once the predetermined period expires, or a
signal is received from the transmitter, the receiver begins
charging and the load is connected to the receiver coil(s).
[0038] When the transmitter is in foreign object detection mode,
the controller controls the converter so that it supplies the
inductor with alternating current at one or more test frequencies.
Typically the test frequencies are of the same order of magnitude
as (and may include) the power transfer frequency used by the
inductive power transfer system.
[0039] Upon supplying the current at the test frequency(s) the
controller waits for a predetermined period for the current to
stabilize within the transmitter coils in foreign object detection
mode. The controller may also wait for current to stabilize within
the receiver. For example, the predetermined period is typically
approximately 50 milliseconds.
[0040] The sensor 10a is configured to sense the average direct
current into the converter 4 (prior to DC to AC conversion). The
output from the sensor is provided to the controller. The product
of the average direct current into the DC to AC converter 4 with
the power supply voltage provides an estimate of the average real
power in the transmitter coil.
[0041] The controller controls one or more sensors (e.g., the
sensor 10b) to sense the peak alternating current through the
transmitter coils and the peak voltage across the transmitter
coils. In some embodiments the peak voltage may be fixed, and
therefore the known peak voltage need not be sensed.
[0042] The controller combines these values into an estimate of the
reactive power. In a preferred embodiment the reactive power in the
transmitter coil is estimated as
P REACTIVE = 1 3 I pk V pk , Equation ( 1 ) ##EQU00001##
where I.sub.pk is the peak current through the transmitter coil(s)
in Amperes and V.sub.pk is the peak voltage across the transmitter
coil in Volts. For example, the controller may be a microprocessor,
an FPGA, other digital logic device or an analog discrete
multiplier/integrator.
[0043] The estimated reactive power provides a close estimate of
the reactive power as the current relating to reactive power is
dominant over the current relating to real power through the
transmitter coil where the current is measured. This holds as long
as little or no real power is being drawn by the receiver.
[0044] The real and reactive power estimates may be performed at
one test frequency or over a plurality or sweep of frequencies and
combined into a data set for further analysis. The plurality of
frequencies are not limited to the charging or operating frequency
of the transmitter.
[0045] When the receiver is in a substantially no-load condition
the real and reactive power of the receiver will depend on the
receiver's mechanical or material design (for example,
proportionate use of ferromagnetic material, e.g., ferrite, metal,
etc.), position (for example, distance from and degree of overlap
with the transmitter coil), the receiver circuitry itself and the
presence of any foreign object(s). As opposed to during charging,
when no load is present (i.e., disconnected) the real and reactive
power are not substantially dependent on the receiver's load.
[0046] Once the real power and the reactive power in the
transmitter coil(s) are estimated the controller determines whether
a foreign object is present. This determination can be by any
suitable means, including but not limited to: comparing estimated
values to threshold values stored in electronic storage, comparing
the estimated values to calibration values stored in electronic
storage, determining a shape of the estimated values or a
characteristic of the estimated values and comparing this to a
threshold or expected value. While the data set may be viewed as a
shape (see FIG. 2; discussed in detail later) the transmitter
controller mathematically evaluates the data set to determine a
characteristic of the data set. For example, average value and
centre can be determined directly from the data set. Those skilled
in the art will be aware that other characteristics can be
determined directly from the data set.
[0047] In an embodiment calibration values of real and reactive
power for each transmitter coil with no load and with no foreign
object present are stored in the inductive power transfer system
memory and accessed by the controller for comparison purposes.
Calibration values for real and reactive power for each transmitter
coil may be stored in the electronic storage for a plurality of
frequencies. Preferably these frequencies include the test
frequencies. The presence of a foreign object may be determined by
comparing the estimated real and reactive power through the
transmitter coil(s) to the calibration values.
[0048] If a foreign object is assumed to be metal then the
difference between the estimate points and their respective
calibration values or expected non-foreign object values would
increase with the foreign object being introduced. However, there
are other influences that make the situation more complex. One of
these influences is the receiver ferrite. At low receiver loads,
the ferrite may cancel out the change in reactive and/or real power
caused by foreign metal. The influence for the receiver ferrite at
low loads may be just as large, or larger than, that of the metal
(foreign object) in between. It also means that the curve seen when
multiple frequencies are measured is not always linear or close to
linear. Ferrite, or more generally ferromagnetic material is
typically provided in the receiver in association with the receiver
coil(s) in order to augment the induced magnetic field and increase
the amount of power coupled from the transmitter coil(s).
[0049] Repeating the real and reactive power estimates over a
plurality of frequencies allows the response to be more closely
determined. As previously stated the estimates can be combined into
a data set for further analysis. The estimates over a plurality of
frequencies may form a curve. The controller may evaluate the data
set and find a characteristic of the shape to determine whether or
not a foreign object is present. For example, the size, average
value or centre of the shape as evaluated from the data set.
Real and Reactive Power: Waveform Sampling and Processing
[0050] A transmitter may begin foreign object detection when the
presence of a potential receiver is detected. A receiver may be
brought into proximity with the transmitter prior to commencing
foreign object detection. In embodiments of the invention the
receiver is configured to commence a charging start-up mode when
the receiver detects the presence of a transmitter and prior to
being charged by the transmitter. In this mode the receiver runs in
a substantially no-load condition, as the receiver keeps the output
load disconnected. In this condition there may still be the load of
the receiver controller circuitry itself. This condition may
continue for a set period or until a signal is received from the
transmitter to cease the start-up mode. The transmitter performs
foreign object detection during the start-up mode. The transmitter
may send the signal once foreign object detection is completed and
if no foreign object is detected. Once the predetermined period
expires, or a signal is received from the transmitter, the receiver
begins charging and the load is connected to the receiver
coil(s).
[0051] When the transmitter is in foreign object detection mode the
controller controls the converter so that it supplies the inductor
with alternating current at one or more test frequencies. Typically
the test frequencies are of the same order of magnitude as (and may
include) the power transfer frequency used by the inductive power
transfer system.
[0052] Upon supplying the current at the test frequency(s) the
controller waits for a predetermined period for the current to
stabilize within the transmitter coils in foreign object detection
mode. The controller may also wait for current to stabilize within
the receiver. The predetermined period is typically approximately
50 milliseconds.
[0053] The controller controls one or more sensors (e.g., the
sensor 10b) to sample the instantaneous voltage and current through
the transmitter coil. The output from the sensor(s) is provided to
the controller. Multiple samples of the instantaneous voltage over
the transmitter coil(s) and the total instantaneous current through
the transmitter coil(s) are taken for every cycle of their periodic
waveforms, and stored. This process is referred to herein as
"sampling the waveforms". At least one entire period of the voltage
waveform and one entire period of the current waveform are sampled
by the sensor(s). Either an integer number of periods of the
current and voltage waveforms is sampled or a large number of
periods of the voltage and current waveforms are sampled so that an
integer number of periods is not essential.
[0054] To estimate the real power in the transmitter coil, the
controller multiplies the voltage and current waveforms together
and the product of the multiplication is integrated. The real power
is estimated as the result of the integration divided by the time
over which the waveform was sampled.
[0055] To estimate the reactive power in the transmitter coil(s)
the controller first determines an estimate of the apparent power.
The apparent power is the product of the root mean square (RMS)
values of the current and voltage. The controller calculates the
root mean square values of the sampled voltage and current
waveforms. One skilled in the art will appreciate that these values
are easily calculated from the sampled voltage and current
waveforms using integration or other suitable techniques. The
estimated reactive power has a Pythagorean relationship with the
estimated apparent power and the estimated real power, as in the
following equation (with all powers in Watts):
P.sub.REACTIVE= {square root over
(P.sub.APPARENT.sup.2-P.sub.REAL.sup.2)} Equation (2).
[0056] In embodiments the real and reactive power estimates are
performed at one test frequency or over a plurality or sweep of
frequencies and combined into a data set for further analysis. The
plurality of frequencies are not limited to the charging or
operating frequency of the transmitter.
[0057] When the receiver is in a substantially no-load condition
the real and reactive power of the receiver will depend on the
receiver's mechanical or material design (for example,
proportionate use of ferromagnetic material, e.g., ferrite, metal,
etc.), position (for example, distance from and degree of overlap
with the transmitter coil), receiver circuitry and presence of any
foreign object(s). As opposed to during charging, when no load is
present (i.e., disconnected), the real and reactive power are not
substantially dependent on the receiver's load.
[0058] Once the real power and the reactive power in the
transmitter coil(s) are estimated the controller determines whether
a foreign object is present. This determination can be by any
suitable means, including but not limited to: comparing estimated
values to threshold values stored in electronic storage, comparing
the estimated values to calibration values stored in electronic
storage, determining a shape of the estimated values or a
characteristic of the estimated values and comparing this to a
threshold or expected value. While the data set may be viewed as a
shape (see FIG. 2) the transmitter controller mathematically
evaluates the data set to determine a characteristic of the data
set. For example, average value and centre can be determined
directly from the data set. Those skilled in the art will be aware
that other characteristics can be determined directly from the data
set.
[0059] In an embodiment calibration values of real and reactive
power for each transmitter coil with no load and with no foreign
object present are stored in the inductive power transfer system
memory and accessed by the controller for comparison purposes.
Calibration values for real and reactive power for each transmitter
coil may be stored in the electronic storage for a plurality of
frequencies. Preferably these frequencies include the test
frequencies. The presence of a foreign object may be determined by
comparing the estimated real and reactive power through the
transmitter coil(s) to the calibration values.
[0060] If a foreign object is assumed to be metal then the
difference between the estimate points and their respective
calibration values or expected non-foreign object values would
increase with the foreign object being introduced. However, there
are other influences that make the situation more complex. One of
these influences is the receiver ferrite. At low receiver loads,
the ferrite may cancel out the change in the reactive and/or real
power caused by foreign metal. The influence of the ferrite at low
loads may be just as large, or larger than, that of the metal
(foreign object) in between. It also means that the curve seen when
multiple frequencies are measured is not always linear or close to
linear. Ferrite, or more generally ferromagnetic material is
typically provided in the receiver in association with the receiver
coil(s) in order to augment the induced magnetic field and increase
the amount of power coupled from the transmitter coil(s).
[0061] Repeating the real and reactive power estimates over a
plurality of frequencies allows the response to be more closely
determined. As previously stated the estimates can be combined into
a data set for further analysis. The estimates over a plurality of
frequencies may form a shape. The controller may evaluate the data
set and find a characteristic of the shape to determine whether or
not a foreign object is present. For example the size, average
value or centre of the shape as evaluated from the data set.
Real and Reactive Power: Amplitude and Phase Measurement
[0062] A transmitter may begin foreign object detection when the
presence of a potential receiver is detected. A receiver may be
brought into proximity with the transmitter prior to commencing
foreign object detection. In embodiments of the invention the
receiver is configured to commence a start-up mode that commences
when the receiver detects the presence of a transmitter and prior
to being charged by the transmitter. In this mode the receiver runs
in a substantially no-load condition, as the receiver keeps the
output load disconnected. In this condition there may still be the
load of the receiver controller circuitry itself. This condition
may continue for a set period or until a signal is received from
the transmitter to cease the start-up mode. The transmitter
performs foreign object detection during the start-up mode. The
transmitter may send the signal once foreign object detection is
completed and if no foreign object is detected. Once the
predetermined period expires, or a signal is received from the
transmitter, the receiver begins charging and the load is connected
to the receiver coil(s).
[0063] When the transmitter is in foreign object detection mode the
controller controls the converter so that it supplies the inductor
with alternating current at one or more test frequencies. Typically
the test frequencies are of the same order of magnitude as (and may
include) the power transfer frequency used by the inductive power
transfer system.
[0064] Upon supplying the current at the test frequency(s) the
controller waits for a predetermined period for the current to
stabilize within the transmitter coils in foreign object detection
mode. The controller may also wait for current to stabilize within
the receiver. The predetermined period is typically approximately
50 milliseconds.
[0065] The controller controls the sensor(s) to sense the
amplitudes of the voltage and current through the transmitter
coil(s) undergoing foreign object detection. The controller further
controls a (further) sensor to sense the phase difference between
the voltage and current waveforms.
[0066] In a resonant system it is likely that the voltage and
current waveforms through the transmitter coil will be
substantially sinusoidal. In this case the real power in Watts in
the transmitter coil can be estimated by the controller as:
P.sub.REAL=I.sub.pkV.sub.pk cos(.theta.) Equation (3),
where V.sub.pk is the peak voltage over the transmitter coil(s) in
Volts, I.sub.pk is the peak total current through the transmitter
coil(s) in Amperes and .theta. is the phase difference between the
voltage and current waveforms in radians.
[0067] The reactive power in the transmitter coil in Watts can be
estimated by the controller as:
P.sub.REACTIVE=I.sub.pkV.sub.pk sin(.theta.) Equation (4),
where the symbols are the same quantities as in the previous
equation.
[0068] In practice the voltage and current waveforms may not be
substantially sinusoidal. In this case Equations (3) and (4) for
real and reactive power may require adjustment. One skilled in the
art will be able to make suitable adjustments to the given formulas
to account for changes in waveforms.
[0069] In embodiments the real and reactive power estimates are
performed at one test frequency or over a plurality or sweep of
frequencies and combined into a data set for further analysis. The
plurality of frequencies are not limited to the charging or
operating frequency of the transmitter.
[0070] When the receiver is in a substantially no-load condition
the real and reactive power of the receiver will depend on the
receiver's mechanical or material design (for example,
proportionate use of ferromagnetic material, e.g., ferrite, metal,
etc.), position (for example, distance from and degree of overlap
with the transmitter coil), receiver circuitry and presence of any
foreign object(s). As opposed to during charging, when no load is
present (i.e., disconnected), the real and reactive power are now
not substantially dependent on the receiver's load.
[0071] Once the real power and the reactive power in the
transmitter coil(s) are estimated the controller determines whether
a foreign object is present. This determination can be by any
suitable means, including but not limited to: comparing estimated
values to threshold values stored in electronic storage, comparing
the estimated values to calibration values stored in electronic
storage, determining a shape of the estimated values or a
characteristic of the estimated values and comparing this to a
threshold or expected value. While the data set may be viewed as a
shape (see FIG. 2) the transmitter controller mathematically
evaluates the data set to determine a characteristic of the data
set. For example, average value and centre can be determined
directly from the data set. Those skilled in the art will be aware
that other characteristics can be determined directly from the data
set.
[0072] In an embodiment calibration values of real and reactive
power for each transmitter coil with no load and with no foreign
object present are stored in the inductive power transfer system
memory and accessed by the controller for comparison purposes.
Calibration values for real and reactive power for each transmitter
coil may be stored in the electronic storage for a plurality of
frequencies. Preferably these frequencies include the test
frequencies. The presence of a foreign object may be determined by
comparing the estimated real and reactive power through the
transmitter coil(s) to the calibration values.
[0073] If a foreign object is assumed to be metal then the
difference between the estimate points and their respective
calibration values or expected non-foreign object values would
increase with the foreign object being introduced. However, there
are other influences that make the situation more complex. One of
these influences is the receiver ferrite. At low receiver loads,
the ferrite may cancel out the change in reactive and/or real power
caused by foreign metal. The influence of the receiver ferrite at
low loads may be just as large, or larger than, that of the metal
(foreign object) in between. It also means that the curve seen when
multiple frequencies are measured is not always linear or close to
linear. Ferrite, or more generally ferromagnetic material is
typically provided in the receiver in association with the receiver
coil(s) in order to augment the induced magnetic field and increase
the amount of power coupled from the transmitter coil(s).
[0074] Repeating the real and reactive power estimates over a
plurality of frequencies allows the response to be more closely
determined. As previously stated the estimates can be combined into
a data set for further analysis. The estimates over a plurality of
frequencies may form a shape. The controller may evaluate the data
set and find a characteristic of the shape to determine whether or
not a foreign object is present. For example the size, average
value or centre of the shape as evaluated from the data set.
[0075] It will be appreciated by one skilled in the art that there
are numerous methods that may be used to estimate the real and
reactive power through the transmitter coil(s). The embodiments
described above are not intended to limit the invention. Other
methods for estimating the real and reactive power may be used.
[0076] Further, whilst the description herein relates to performing
foreign object or non-IPT receiver detection employing control
circuitry of the IPT transmitter, the foreign object detection can
be equally performed by control circuitry of the IPT receiver. In
either case, the control circuitry of either or both of the IPT
transmitter and receiver may be configured to ensure that inductive
power transfer is only performed if no foreign object is detected,
or if the type or location of foreign object detected is determined
to not cause a potential problem in the inductive power
transfer.
[0077] Further still, in the exemplary embodiments described the
foreign object or non-IPT receiver detection is performed during a
start-up or otherwise non-charging or power transfer stage of the
operation of the IPT system or receiver. Those skilled in the art
understand that the various methods described could be adapted to
be performed at a particular time or times (e.g., intermittently)
during charging or power transfer through temporary entry into a
non-charging or power transfer stage so as to ensure that foreign
object conditions have not changes since charging or power transfer
has begun. Alternatively, or additionally, the methods described
may be adapted to be performed during charging or power
transfer.
[0078] Furthermore, the different exemplary methods of foreign
object or non-receiver detection described may be performed
singularly or in combination, either as a standalone foreign object
detection test regime or in conjunction with one or more other
foreign object detection tests in the applicable IPT system. Such
singular or expanded foreign object detection test regimes could be
performed in IPT systems using inductive coupling of single
transmitter and receiver coils, so-called 1:1 systems or using
inductive coupling of plural transmitter and receiver coils,
so-called N:N systems.
[0079] It should be noted that in this specification the words
sensing and measuring are applied interchangeably to the sensors.
These terms are not meant to be limiting.
[0080] While the present invention has been illustrated by the
description of the embodiments thereof, and while the embodiments
have been described in detail, it is not the intention of the
Applicant to restrict or in any way limit the scope of the appended
claims to such detail. Additional advantages and modifications will
readily appear to those skilled in the art. Therefore, the
invention in its broader aspects is not limited to the specific
details, representative apparatus and method, and illustrative
examples shown and described. Accordingly, departures may be made
from such details without departure from the spirit or scope of the
Applicant's general inventive concept.
* * * * *