Foreign object detection for ferromagnetic wire-like objects
Systems and methods are described for improving foreign object detection for ferromagnetic wire-like objects. In particular, aspects include increasing FOD sensitivity using at least one inductive sense loop in response to detecting a change in resistance that is substantially equal to the change in reactance of a sense coil. By increasing the FOD sensitivity, objects such as ferromagnetic wire-like objects can be detected that are potentially hazardous objects. Detecting these potentially hazardous objects that were previously undetectable can reduce the chances of damage to surrounding materials or harm to humans.
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The present disclosure relates generally to wireless power transfer, and more specifically to devices, systems, and methods for detecting foreign ferromagnetic wire-like objects.
BACKGROUNDWireless Electronic Vehicle Charging (WEVC) from a ground-based unit to a vehicle-mounted pick-up unit generally requires foreign object detection (FOD). Conventional FOD systems can fail to detect certain small ferromagnetic objects having a length substantially longer than a thickness, such as nails, pins, steel wire pieces, staples, and so forth. These small objects have the potential of reaching hazardous temperatures if exposed to magnetic fields in the millitesla range and with an orientation substantially in a direction of the magnetic field.
Because of these challenges, inductive ferromagnetic sensing (IFS) has been used to detect ferromagnetic objects. The achievable sensitivity of IFS, however, is limited to paperclip-size objects, and are not sufficient for tiny steel wire pieces, fixing pins, nails, and so forth. Thermal sensing using an infrared (IR) camera is yet another alternative solution used to detect ferromagnetic objects. IR sensing, however, cannot accurately detect small hot objects shadowed by another cold object.
SUMMARYSystems and methods are described for improving foreign object detection for ferromagnetic wire-like objects. In particular, aspects include increasing FOD sensitivity using at least one inductive sense loop in response to detecting a change in resistance that is substantially equal to the change in reactance of a sense coil. By increasing the FOD sensitivity, objects such as ferromagnetic wire-like objects can be detected that are potentially hazardous objects. Detecting these potentially hazardous objects that were previously undetectable can reduce the chances of damage to surrounding materials or harm to humans.
Wirelessly transferring power involves transferring energy through electric fields, magnetic fields, electromagnetic fields, or otherwise using a transmitter to a receiver without the use of physical electrical conductors (e.g., power may be transferred through free space). The power output into an electro-magnetic field (e.g., a magnetic field) may be received, captured by, or coupled by a “receiving coupler” to achieve power transfer.
An electric vehicle is used herein to describe a remote system, an example of which is a vehicle that includes, as part of its locomotion capabilities, electrical power derived from a chargeable energy storage device (e.g., one or more rechargeable electrochemical cells or other type of battery). As non-limiting examples, some electric vehicles may be hybrid electric vehicles that include, besides electric motors, a traditional combustion engine for direct locomotion or to charge the vehicle's battery. Other electric vehicles may draw all locomotion ability from electrical power. An electric vehicle is not limited to an automobile and may include motorcycles, carts, scooters, and the like. By way of example and not limitation, a remote system is described herein in the form of an electric vehicle (EV). Furthermore, other remote systems that may be at least partially powered using a chargeable energy storage device are also contemplated (e.g., electronic devices such as personal computing devices and the like).
In some aspects, the electric vehicle coupler 116 may receive power when the electric vehicle coupler 116 is located in an energy field produced by the base system coupler 104a. The field corresponds to a region where energy output by the base system coupler 104a may be captured by an electric vehicle coupler 116. For example, the energy output by the base system coupler 104a may be at a level sufficient to charge or power the electric vehicle 112. In some cases, the field may correspond to the “near field” of the base system coupler 104a. The near-field may correspond to a region in which there are strong reactive fields resulting from the currents and charges in the base system coupler 104a that do not radiate power away from the base system coupler 104a. In some cases, the near-field may correspond to a region that is within about ½π of wavelength of the base system coupler 104a (and vice versa for the electric vehicle coupler 116) as will be further described below.
The local distribution center 130 may be configured to communicate with external sources (e.g., a power grid) via a communication backhaul 134, and with the base wireless charging system 102a via a communication link 108.
In some aspects the electric vehicle coupler 116 may be aligned with the base system coupler 104a and, therefore, disposed within a near-field region simply by the driver positioning the electric vehicle 112 such that the electric vehicle coupler 116 is sufficiently aligned relative to the base system coupler 104a. Alignment may be considered sufficient when an alignment error has fallen below a tolerable value. In other aspects, the driver may be given visual, auditory, or tactile feedback, or combinations thereof to determine when the electric vehicle 112 is appropriately positioned for wireless power transfer. In yet other aspects, the electric vehicle 112 may be positioned by an autopilot system, which may move the electric vehicle 112 until the sufficient alignment is achieved. This may be performed automatically and autonomously by the electric vehicle 112 with or without driver intervention. This may be possible for an electric vehicle 112 that is equipped with a servo steering wheel, radar sensors (e.g., ultrasonic sensors), and intelligence for safely maneuvering and adjusting the electric vehicle. In still other aspects, the electric vehicle and/or the base wireless charging system 102a may have functionality for mechanically displacing and moving the couplers 116 and 104a, respectively, relative to each other to more accurately orient or align them and develop sufficient and/or otherwise more efficient coupling between the couplers 116 and 104a.
The base wireless charging system 102a may be located in a variety of locations. As non-limiting examples, some suitable locations include a parking area at a home of the electric vehicle 112 owner, parking areas reserved for electric vehicle wireless charging modeled after conventional petroleum-based filling stations, stoplights, and other locations such as shopping centers and places of employment.
Charging electric vehicles wirelessly may provide numerous benefits. For example, charging may be performed automatically, virtually without driver intervention and manipulations thereby improving convenience to a user. There may also be no exposed electrical contacts and no mechanical wear out, thereby improving reliability of the wireless power transfer system 100. Safety may be improved since manipulations with cables and connectors may not be needed, and there may be no cables, plugs, or sockets to be exposed to moisture in an outdoor environment. In addition, there may be no visible or accessible sockets, cables, or plugs, thereby reducing potential vandalism of power charging devices. Further, since the electric vehicle 112 may be used as distributed storage devices to stabilize a power grid, a docking-to-grid solution may be used to increase availability of vehicles for Vehicle-to-Grid (V2G) operation.
The wireless power transfer system 100 may also provide aesthetic and non-impedimental advantages. For example, there may be no charge columns and cables that may be impedimental for vehicles and/or pedestrians.
As a further explanation of the vehicle-to-grid capability, the wireless power transmit and receive capabilities may be configured to be reciprocal such that either the base wireless charging system 102a transfers power to the electric vehicle 112 or the electric vehicle 112 transfers power to the base wireless charging system 102a. This capability may be useful to stabilize the power distribution grid by allowing electric vehicles to contribute power to the overall distribution system in times of energy shortfall caused by over demand or shortfall in renewable energy production (e.g., wind or solar).
In theory, absorbed power per surface area and thus final temperatures of ferromagnetic objects having a length l 216 substantially longer than a thickness d 218 (e.g., l>>d) are no function of their size as long as their thickness d 218 is significantly greater than a skin depth 220. Such or similar behavior can also be experimentally verified. These particular ferromagnetic objects can be referred to as “wire-like” objects, some examples of which include steel wire pieces, nails, fixing pins, and so on. The skin depth 220 refers to a penetration depth of the alternating magnetic field into the object, which relates to a volume through which electric current can flow through the object. For example, at 85 kilohertz (kHz), which is an example operating frequency of conventional WEVC systems, the skin depth of such “wire-like” objects may be in the order of 60 micrometers. This behavior of “wire-like” objects is in contrast to other form factor objects whose final temperature decreases as a function of their size, indicating that some objects too small to be detected by FOD may not exceed critical temperatures. This result is typical, however, for thicker objects, such as hexagonal nuts, bolts, and washers. Accordingly, the category of ferromagnetic objects having a length substantially greater than a diameter or thickness (e.g., a wire piece having a length of one centimeter (cm) or 20 millimeters (mm)) are considered potentially hazardous, but may be not detectable by conventional FOD systems.
Aspects assume FOD based on measuring complex impedance Z of the sense coil 204. In the presence of a conductive ferromagnetic object, the measured impedance of the sense coil 204 changes by an amount ΔZ representing a combination of a change in resistance ΔX of the sense coil 204 and a change in reactance ΔR of the sense coil 204, such that ΔZ=ΔR+jΔX, where j is a constant representing the unit imaginary number (e.g., the square root of −1). When a wire-like object is substantially smaller in size than that of the sense coil 204, the wire-like object changes the resistance X and reactance R of the sense coil 204 by substantially similar amounts (e.g., within five percent), which can be considered as substantially equal amounts, with negligible effects from the wire-like object's size and various material properties, such as permeability μr and conductivity σ. Consequently, this wire-like type of ferromagnetic object produces an impedance with a substantially equal amount of resistive and reactive parts, such that ΔX˜ΔR.
In the illustrated example, the sense coil 204 includes a reference impedance, which represents an impedance of the sense coil 204 when no ferromagnetic object is within sensing range (detection volume) of the sense coil 204. The reference impedance can be previously determined through calibration or a manual setting when no foreign object is present in the detection volume. In addition, the impedance is measured at a terminal 222 of the sense coil 204, intermittently or constantly. When the foreign object 208 comes in close proximity to the sense coil 204, the impedance of the sense coil 204 changes from the reference impedance by an amount ΔZ, as described above. The change in impedance can then be transformed to determine an amount of change in the resistance X (real component of the impedance) and reactance R (imaginary component of the impedance) of the sense coil 204.
In aspects, the foreign object 208 can be detected by the sense coil 204 when the foreign object is exposed to a biasing static magnetic field {right arrow over (B)}exp. The sense coil 204 may be excited by a sinusoidal signal source, e.g., signal source 210, at the voltage vs(t) 214 and the sense frequency fs, resulting in the sense current is(t) 414. The static magnetic field {right arrow over (B)}exp magnetically biases the foreign object 208. The foreign object's 450 electrical conductivity σ({right arrow over (B)}exp) 410 and magnetic permeability μ({right arrow over (B)}exp) 412 as apparent through inductive sensing are generally functions of the biasing static magnetic field {right arrow over (B)}exp. Since equivalent inductance and resistance are functions of σ({right arrow over (B)}exp) and μ({right arrow over (B)}exp), a presence of the foreign object 450 can be potentially detected by analyzing current is(t) in relation to source voltage vs(t) 515 and the strength of the static magnetic field {right arrow over (B)}exp.
The foreign object 208 may be detected based on “stimulated” inductive impedance and resistance sensing. This type of sensing is based on intermittent exposure of the foreign object 208 to the static magnetic field {right arrow over (B)}exp, which results in exposure intervals that toggle on and off and change the sense coil's 204 impedance. In at least one example, an equivalent resistance Rsc+ΔRsc(t) of the sense coil 204 can be measured constantly and recorded over a time period. According to the known magneto-impedance effect, both inductance and resistance of the sense coil 204 decrease during the toggled-on exposure interval, and increase during a toggled-off exposure interval. These characteristics may reveal the presence of a ferromagnetic foreign object.
For instance, at least one recorded time course of resistance Rsc+ΔRsc(t) is compared with an exposure time profile for the static magnetic field {right arrow over (B)}exp to determine the occurrence of the above-mentioned magneto-impedance effect. In some other aspects, this comparison is a correlation. For example, the recorded time course of the resistance Rsc+ΔRsc(t) or other sense coil 204 characteristic is correlated with the exposure time profile for the static magnetic field {right arrow over (B)}exp. In a further example, correlation is performed with at least one of a time-derivative, e.g., the first derivative d/dt (time gradient) of the recorded time course of at least one of an inductive sense coil's 204 characteristics.
In other aspects, the foreign object 208 may be exposed to a biasing time-varying (alternating) magnetic field {right arrow over (B)}exp(t). In some aspects, the biasing time-varying magnetic field {right arrow over (B)}exp(t) may be a low frequency magnetic field as generated by the inductive power transfer system. In such cases, means for generating a second time-varying magnetic field may include one or more inductive power transfer coils (e.g., transmit coil, receive coil, transmit/receive coil, or any combination thereof). The alternating magnetic field {right arrow over (B)}exp(t) may alternate with a frequency fIPT. As with the static magnetic field described above, the foreign object's 204 electrical conductivity σ({right arrow over (B)}exp) and magnetic permeability μ({right arrow over (B)}exp) vary in some relationship to the alternating magnetic field {right arrow over (B)}exp(t), so as to modulate current is(t), which is driven by the voltage vs(t) provided by the signal source 210.
The foreign object's 208 electrical conductivity σ({right arrow over (B)}exp) and magnetic permeability μ({right arrow over (B)}exp) are typically also functions of the object's temperature ϑ, and are thus also indirectly affected by the alternating magnetic field {right arrow over (B)}exp(t) via the Joule heating effect. However, this Joule heating effect is generally much weaker than the modulating effect due to exposure to the biasing alternating magnetic field, and changes due to the Joule heating effects are also orders of magnitude slower than the nearly instantaneous changes due to this modulating effect, depending on a thermal capacity of the foreign object 208 and a heating power. The presence of the foreign object 208 can potentially be detected by analyzing the current is(t) in relation to source voltage vs(t) and to the alternating magnetic field {right arrow over (B)}exp(t) signal by this modulating effect and, in some cases, also by the thermal effect.
Intermittent exposure of the foreign object 208 to the alternating magnetic field {right arrow over (B)}exp(t) results in exposure intervals of the sense coil 204 that toggle on and off and change the sense coil's 204 inductance and resistance. In the presence of the foreign object 208, the equivalent resistance and the equivalent inductance periodically vary with a frequency (modulating frequency) that is double that of the frequency fIPT of the alternating magnetic field {right arrow over (B)}exp(t). This frequency doubling effect indicates that this modulation of electromagnetic material properties is independent of the sign of the alternating magnetic field {right arrow over (B)}exp(t), and thus provides a rectifying effect. In addition, a short term average of the equivalent resistance instantaneously increases when the alternating magnetic field {right arrow over (B)}exp(t) is toggled on. This is an opposite effect from that previously described with respect to the ordinary magneto-impedance effect, where the equivalent resistance decreases with exposure to the static magnetic field. In addition, exposure to the alternating magnetic field {right arrow over (B)}exp(t) may heat the foreign object 208 due to the Joule heating effect. This temperature effect may correlate with a short term average of the equivalent resistance and of the short term average of the equivalent inductance.
For a sensing system using the sinusoidal sense voltage vs(t), this modulation effect can be generally observed in the time domain as an amplitude and phase modulation of the resulting current is(t). For some ferromagnetic objects (e.g. paper clips) this modulation effect may be highly non-linear, manifesting in a periodic sequence of short pulses, however, with a fundamental frequency that is twice that of the frequency fINPT of the alternating magnetic field {right arrow over (B)}exp(t). The degree of modulation depends on the impact of the foreign object 208 on the inductive sense coil's 204 equivalent inductance and equivalent resistance, as well as on the material and the orientation of the foreign object 208. In the frequency domain, this modulation can be observed as modulation harmonic side-bands up to several orders.
Consider now
It has been observed that a material from which an object is made, when present in a detection volume of the sense coil, can be determined based on an impedance change angle (e.g., the relationship between the change in the imaginary component, or reactance, and a change in the real component, or resistance) of the sense coil. Specifically, the material can be determined based on an angle that a line, connecting the complex impedance of the sense coil when no object is present (e.g., reference complex impedance) with the complex impedance of the sense coil when the object is present (e.g., current complex impedance), makes with a horizontal line extending through the complex impedance of the sense coil when no object is present.
Some materials cause an increase in both the reactance and the resistance of the sense coil, which results in an impedance change toward a first quadrant (e.g., between zero and 90 degrees of an impedance change plane represented by
By comparison, consider
To enhance FOD for the wire-like objects 308, the detection sensitivity 402 can be selectively increased as a ratio ΔX/ΔR of the change in resistance ΔX over the change in reactance ΔR approaches a value of one. Accordingly, a sensitivity function or gain function can be computed by the controller 206, where the function has a highest gain if ΔX/ΔR˜1, and where the gain progressively decreases as the ratio deviates (e.g., higher or lower) from one. To do this, a detection threshold can be reduced by an amount that is a function of the ratio ΔX/ΔR, trading off detection probability and false alarm probability. The trade-off, however, is minimal for ferromagnetic wire-like objects 308. For such objects, the detection threshold can be lowered to increase detection sensitivity without substantially increasing a false alarm rate. Accordingly, this threshold reduction may not be performed for all objects, but selectively for this particular category of wire-like objects.
In aspects, the sense coil 204 can be implemented as at least one separate inductive sense loop of a WEVC system. For example, an inductive power transfer (IPT) system of the WEVC system can include an IPT coil that is used to magnetically bias ferromagnetic foreign objects with a static magnetic field {right arrow over (B)}exp(t) or an alternating magnetic field {right arrow over (B)}exp(t). The sense coil 204 can be implemented to detect changes in impedance of the sense coil 204 caused by ferromagnetic foreign objects that are magnetically biased by the magnetic field generated by the IPT coil. In other aspects, the sense coil 204 can be implemented as the IPT coil of the IPT system.
The type of object present can be determined based essentially on a direction that the complex impedance of the sense coil shifts from the impedance of the sense coil when no objects are present in the detection volume. The type of object may be determined by a coarse determination of the direction of change in the reactance and the direction of change in the resistance of the sense coil. Alternatively, the type of object may be determined by performing an inverse tangent operation on a quotient of the amount of change in the reactance of the sense coil divided by the amount of change in the resistance of the sense coil and then determining a type of the object based on a result of the inverse tangent operation coinciding with a predetermined range of values corresponding to the type of the object. If the angle is determined to be approximately 45 degrees, then the detection sensitivity can be increased, as described above, to detect whether a ferromagnetic wire-like object or some non-critical object is present.
Example Procedures
The following discussion describes example procedures for improving foreign object detection for ferromagnetic wire-like objects. The example procedures may be employed in the system 100 of
At 504, an amount of change in a resistance of a sense coil is determined from a reference resistance value for the sense coil. For example, a reference resistance value for each sense coil may be previously determined through calibration or manual setting of the resistance for that sense coil when no foreign object is present in the detection volume. A current resistance of the sense coil is determined and compared to the calibrated resistance for that sense coil. The difference between these two values is an amount of change in the resistance of the sense coil from the reference resistance value for that sense coil.
At 506, a sensitivity of a detection circuit is increased if the amount of change in the resistance of the sense coil is substantially equal to the amount of change in the reactance of the sense coil. For example, a ratio of the amount of change in the reactance and the amount of change in the resistance can be computed. Then, a detection threshold can be reduced as the ratio approaches a value of one (e.g., 0.9, 0.96, 1.0, 1.05, 1.1, etc.). This ratio also represent the impedance change angle described above, which is equal to 45 degrees when the ratio equals one. Accordingly, as the ratio approaches the value of one, the impedance change angle approaches the value of 45 degrees (e.g., 44.0°, 44.5°, 45.0°, 45.5°, 46.0°, etc.). Accordingly, the detection threshold can be adjusted based on a function of the ratio of the amount of change in the reactance and the amount of change in the resistance of the sense coil.
At 508, a presence of an object is determined based on a relationship between the amount of change in the reactance of the sense coil and the amount of change in the resistance of the sense coil. For example, the type of object can be determined based on an angle formed between a horizontal line extending through a reference complex impedance of the sense coil, and a line connecting the reference complex impedance of the sense coil and a current complex impedance of the sense coil when the object is present. If the angle is approximately 45 degrees, then a ferromagnetic wire-like object may be detected.
Example Detection Circuits
The following discussion describes example detection circuits for improving foreign object detection for ferromagnetic wire-like objects in accordance with one or more aspects. The example detection circuits may be employed in the system 100 of
The circuit 600 further comprises an impedance analyzer 608 that is configured to receive, as inputs, outputs of the voltage sensor 615 and the current sensor 616. The impedance analyzer 608 is further configured to output an impedance Zs(t) determination to an evaluation & control unit 640 and to receive an IPT reference signal 624 for frequency and phase synchronization purposes from the IPT power source 622.
The evaluation & control unit 640 controls the sense frequency fs via an output 618 provided to the sense signal voltage source 612 and also controls the alternating magnetic field {right arrow over (B)}exp(t) via another output 642 provided to the IPT power source 622. The evaluation & control unit 640 additionally comprises an input to receive the IPT reference signal 624 from the IPT power source 622, which in addition to frequency and phase may also reflect a current level, a power level, or any other operational status of the IPT power source 622. The evaluation & control unit 640 also includes an output for providing a detection hypothesis Hi.
The series capacitor Cres 604 in combination with shunt inductor Lsh 606 serves as a high pass filter to attenuate the voltage induced in the inductive sensing coil 602 by the alternating magnetic field {right arrow over (B)}exp(t) at an IPT frequency fIPT. Attenuating this low frequency component may generally relax requirements on the voltage sensor 615, the current sensor 616, and the impedance analyzer 608, and may also reduce any non-linear distortion effects such as cross-modulation between any low and high frequency signal components. The capacitor Cres 604 may also lower a current component Iind, at the IPT frequency fIPT, induced in the inductive sensing coil 602, which will lower consequent Joule heating effects in the inductive sensing coil 602. Self-heating of the inductive sensing coil 602 may exert a disturbing effect (e.g., as a change in output of the inductive sensing coil 602) when sensing objects via a change of their temperature. The capacitor Cres 604 may also serve to fully or partially compensate for the reactance of the inductive sensing coil 602 at the sense frequency fs, as will be outlined in more detail below. The series resistor Rres 611 may serve to limit the sense current Is, e.g., if the circuit 600 is tuned to resonance (e.g., full compensation) to minimize an impedance at the measurement port 610.
In some aspects, the shunt inductor Lsh 606 may be omitted or reduced in physical size (e.g., for a lower current rating). In such aspects, the circuit 600 may actively cancel or minimize the IPT frequency fIPT voltage component at the measurement port 610 by using a signal voltage source (e.g., sense signal voltage source 612) that generates a high frequency sense signal superimposed over a low frequency compensation signal (e.g., at the IPT frequency fIPT). The circuit 600 may then adjust an amplitude and/or a phase of the low frequency compensation signal so as to minimize the IPT frequency voltage at the measurement port 610.
In some other aspects, the shunt inductor Lsh 606 may be an “air coil” not including any ferrite core. This may be required to avoid any modulation effect that may be produced by a ferrite core in the presence of the current component Iind. Since shunt inductor Lsh 606 is connected in parallel at the measurement port 610, even very weak modulation effects may seriously desensitize detection of the foreign object 650.
Selecting a suitable sense frequency requires particular attention. The fundamental frequency, the harmonic frequencies, and other switching noise of the IPT system may potentially interfere or cross-modulate with the sense signal thus desensitizing foreign object detection. With respect to the circuit 600 of
A quantity representative of the inductive sense coil's equivalent inductance and equivalent resistance may be the complex impedance Zs that is a function of time (e.g., is modulated) when the foreign object 650 is present. This notion of an impedance may be applied if the sense signal is a continuous sinusoidal wave and if the sense frequency fs is significantly higher than the modulation frequency (e.g., fINPT), which may hold for sense frequencies fs in the MHz range. For other excitations, the notion of an impedance may not be appropriate.
In some aspects, the time varying complex impedance Zs(t) may be obtained by sensing the voltage Vs and the current Is at the measurement port 610. In more detail, the complex impedance Zs(t) is obtained by filtering and envelope detection of the sensed voltage Vs and the sensed current Is in the filter & detector module 626 and the filter & detector module 628, respectively, to obtain the complex voltage envelope Vs(t) and the complex current envelope Is(t) (e.g., the modulation waveforms). The filter & detector module 626 may then output the complex voltage envelope Vs(t) and the filter & detector module 628 may then output the complex current envelope Is(t) to the module 630 that computes the quotient Zs(t)=Vs(t)/Is(t). The filters within the modules 626 and 628 may be matched filters that reduce noise with minimal distortion of the voltage Vs(t) and current Is(t) modulation waveforms. In addition, the filter & detector modules 626 and 628 may be frequency and/or phase synchronized to the alternating magnetic field {right arrow over (B)}exp(t) by receiving the IPT reference signal 624 from the IPT power source 622.
In some aspects, the complex impedance Zs(t) may be measured with a frequency fs, substantially at resonance, as essentially defined by the inductive sense coil's inductance Lsc (not shown in
The evaluation & control unit 640 may further process and compare the detected complex impedance waveforms Zs(t) against a reference waveform, also taking into account information received via the IPT reference signal 624, to finally select a detection hypothesis Hi. Such reference waveforms may be obtained in a system calibration process.
In some aspects, the evaluation & control unit 640 may be configured to determine the presence of the foreign object 650 based on detecting a predetermined level or amount of modulation and on other distinct characteristics in the detected complex impedance waveforms Zs(t). These characteristics (e.g., the modulation harmonics) may be analyzed in the time domain or in the frequency domain (e.g. by a Fourier series).
In some other aspects, the evaluation & control unit 640 may also correlate the determined complex impedance waveform Zs(t) with the duty cycles of the intermittent alternating magnetic field {right arrow over (B)}exp(t) to determine the presence of the foreign object 650 via the alternating magnetic field biasing and/or via the heating effect.
In this detection circuit 700 the sense signal voltage source (612 as shown in
The DAC 750 output voltage Vo and the resistor Rser 711 may be selected to act substantially as a constant alternating current (AC) source driving a quasi-constant sinusoidal current Is,1 into the inductive sensing coil that is selected by the multiplexer 754. Alternatively, the DAC 750 may provide a constant AC source output not requiring the series resistance Rser 711. Assuming a defined, constant sense current Is,1 , the sensed voltage Vs,i at the measurement port 728 of the i-th sense circuit (other circuits not shown) may be considered to directly reflect the impedance Zs,i.
Furthermore,
Each synchronous detector 768 (e.g., within the bank 758) provides an in phase mixer 766, a quadrature mixer 764, a low pass filter 762 and a complex (in-phase and quadrature) output 770 delivering a complex amplitude (e.g., representing a Fourier coefficient) denoted by Vs,n,k, where n refers to the n-th sense coil and k to the k-th order modulation harmonic, where |k|≤2K. The zero-order modulation harmonic Vs,n,O refers to the sense carrier signal component and is obtained by mixing Vs,i(t) with a sinusoidal quadrature waveform with frequency fs. The low pass filter 762 may be a Finite Impulse Response (FIR)-type filter dimensioned with respect to filtering requirements and detection time constraints. In a foreign object detection system using N inductive sensing coils and time multiplexing, available detection time for each sense coil reduces to 1/Nth that of an available detection time for the circuit 600 of
The numerical frequency waveforms (0°, 90°) input to the in-phase and quadrature mixers, respectively, are generated by a numerical frequency synthesizer 760 with frequencies. In some aspects, all frequency waveforms may be based on the sense frequency fs 718 received from the evaluation & control unit 740 and on the external IPT reference signal 724 received from IPT transfer system. In some aspects, the IPT reference signal 724 is derived from the residual IPT frequency voltage as it may be sensed at the measurement port 728 of the i-th sense circuit. The numerical frequency synthesizer 760 may include means for frequency and phase synchronizing an internal numerical oscillator to the external IPT reference signal 724.
The evaluation & control unit 740 may determine the presence of a potential object by comparing a detected set of complex outputs (Vs,n,K) against a set of reference values using an appropriate threshold. The reference values may be obtained in a process of system calibration. Moreover, the evaluation & control unit 740 may correlate time sequences of consecutively detected sets of outputs with the alternating magnetic field intervals, e.g., for detecting objects via the magnetic field biasing and/or heating effect. The zero-order coefficient Vs,n,O may be of particular relevance in such a correlation approach.
In some aspects, the real and imaginary components of the complex voltage Vs,n,O should reflect an inductive sensing coil's 702 equivalent resistance and equivalent inductance, respectively, requiring true mapping of the complex voltage. True voltage (e.g., impedance) mapping may provide most of the information for discriminating between potential objects and other disturbances. However, true voltage mapping may require the system to compensate for any phase error that is introduced by the analog circuitry between the DAC 750 and the ADC 752. This phase error may generally vary with frequency, with the selected inductive sensing coil 702, and also with the temperature and age of components within the circuit 700.
Therefore, in some aspects the evaluation & control unit 740 performs phase correction by the following procedure. First, the sense frequency fs 718 is tuned to minimize the magnitude of Vs,n,O. At this frequency the impedance at the measurement port 728 should, ideally, be purely resistive (zero phase), neglecting the effect of the shunt inductor Lsh 706, which allows the circuit 700 to determine phase error. Knowing the phase error, the circuit 700 may perform phase zeroing by rotating the voltage phasor Vs,n,O by an amount of measured phase error to cancel the measured phase error. This procedure may provide a sufficiently accurate phase calibration as needed for true voltage (e.g., impedance) mapping.
In some aspects, phase correction may also be required for the IPT frequency component (e.g., to be substantially in phase synchronicity with the voltage induced into the inductive sensing coil 702 at the IPT frequency fIPT). Such correction may be performed by the numerical frequency synthesizer 760 individually for each of the inductive sensing coils 702.
Ferrites used in the IPT coupler (not shown), in the shunt inductor Lsh 706, and other inherent non-linearity within the circuit 700 may produce a certain degree of impedance modulation. Such intrinsic modulation effects may be nullified in a calibration process.
Switching noise harmonic content produced by the IPT power source may be coupled into the inductive sensing coil 702 via the IPT coupler coil (not shown) and may fall on frequencies where foreign object detection is sensitive (e.g., the sense carrier frequency fs and modulation side-bands). Therefore, the circuit 700 may perform active interference avoidance by intelligently controlling the sense frequency fs in a manner effective to cause the IPT harmonics to remain outside of any of the sensitive frequency ranges, as defined by the bandwidth of each of the synchronous detectors 768, while remaining substantially at resonance. Conversely, the system may adjust the IPT frequency fIPT by a small amount.
Potential interference of an IPT frequency harmonic with any of the relevant modulation side-band harmonics can be identified by a passive detection performed by the bank of synchronous detectors 768 and the evaluation & control unit 740 when the high frequency sense signal (e.g., Vo driving Is,l at the DAC 750) is switched off. Any potential interference may be detected as an increased signal level at any of the detector 768 outputs. When interference is detected, the evaluation & control unit 740 may slightly adjust the sense frequency fs 718, slightly shifting the frequency of the numerical frequency synthesizer 760 until the interference disappears or drops below an acceptable threshold.
The IPT power source (not shown) may also generate broadband noise that cannot be mitigated by shifting the sense frequency fs. To some degree, the IPT power source may also modulate an impedance appearing at the measurement port 728. This may be explained by variations of the output impedance of the IPT power source (e.g., 622 with reference to
Therefore, the circuit 700 may implement a means to stabilize the output impedance and to reduce broadband noise in the sense frequency range. Such means may include high frequency filters (isolators) inserted between a power converter and the IPT coupler of the IPT system. These means may be considered an integral part of an IPT tuning and matching network. The means may further include measures in the IPT control system, and particularly in the generators of the pulse width modulated (PWM) drive waveforms, for smooth (transient-free) control and to minimize jitter. The above methods for reducing noise and impedance modulation may also apply to the IPT power sink (e.g., the IPT receiver, not shown), which may similarly disturb an inductive foreign object detection process.
The basic concept of detecting ferromagnetic metallic objects via alternating magnetic field biasing and by eddy current heating may also principally apply to an impulse response (pulse induction) detection technique, since the presence of a ferromagnetic object may also modify the impulse response of an inductive sensing coil, which may be a function of a strength of a biasing magnetic field and of the object's temperature. Such an impulse response detection technique may not need resonance tuning.
In an example, the foreign object 850 is detected by periodically applying a voltage pulse to the inductive sensing coil 802. Pulsing may be performed in synchronicity with the IPT frequency fIPT with a pulse rate equal to, or an integer multiple of, the IPT frequency fIPT and having a pulse phase adjusted to a suitable time instant with respect to an IPT reference signal 824, supplied to the evaluation & control unit 840 by the IPT power source 822, and with respect to switching transients that may be present in a real IPT system. This pulsing may be driven by exposure control signal 842 output from the evaluation & control unit 840. The evaluation & control unit 840 may be configured to determine a presence of the foreign object 850 based on consecutively sampled impulse responses by analyzing a degree of modulation (e.g., change), a degree of correlation with the alternating magnetic field exposure intervals, and other characteristics as sensed in the acquired time series.
Moreover, expansion of the impulse response approach to a plurality (array) of inductive sensing coils may not require a multiplexer in the sense pulse generating portion of the circuit 800, since a respective pulse switch 814 may individually provide a respective pulse to only one sense coil at a time. The DC voltage source 812 may be common to a number of sense circuits, while the shunt resistor Rsh 834, the voltage sensor 815, and the limiter 828 may be individual for each inductive sensing coil (e.g., coil 802). Multiplexing of the sensed voltages may be performed downstream (e.g., after) the limiter 828 or even downstream (e.g., after) the amplifier 830.
The following may apply to any ferromagnetic foreign object detection circuit as disclosed herein but is described in the context of the foreign object detection circuit as shown in
Depending on the system capabilities, the biasing and heating alternating magnetic field {right arrow over (B)}exp(t) may be generated by transferring real power to a load, e.g., to a battery of an electric vehicle. Alternatively, the magnetic field {right arrow over (B)}exp(t) may be generated by disconnecting the vehicle-side load and by generating substantially reactive power in the base IPT coupler 820 and/or in the vehicle IPT coupler (not shown).
The aspects disclosed herein may be applied more cautiously to prevent hazardous situations from occurring during the alternating magnetic field {right arrow over (B)}exp(t) exposure intervals. For example, the alternating magnetic field exposure {right arrow over (B)}exp(t) may start at a subcritical level. If no foreign object 850 is detected, the circuit 800 may successively increase (e.g., ramp up) the level of magnetic field {right arrow over (B)}exp(t) exposure from interval to interval.
Some objects, e.g., paper clips, may change their modulation response considerably with the level of the biasing alternating magnetic field {right arrow over (B)}exp(t). These effects, attributable to saturation, may reveal further information useful for determining a presence of the foreign object 850. Changes in a modulation response may also be observed when changing the frequency of the biasing alternating magnetic field {right arrow over (B)}exp(t). Using different levels for the biasing alternating magnetic field {right arrow over (B)}exp(t) may also help to discriminate between modulating effects as produced in the environment of an inductive sensing coil (802) e.g. by ferrite in the IPT coupler (not shown) and modulating effects as produced by ferromagnetic foreign objects, since these modulating effects may change differently when the level is altered. Therefore, in some aspects, foreign object detection is performed with at least one of a different level and a different frequency of the biasing alternating magnetic field {right arrow over (B)}exp(t).
The methods disclosed herein may be combined with at least one of a time differential approach (e.g., as previously discussed in connection with
Metallic structures in the environment of an inductive sense coil (e.g. the vehicle pad and vehicle underbody) may temporarily move. This may occur, for example, when a person enters or leaves the vehicle or when the vehicle is loaded or unloaded. These movements may seriously disturb inductive sensing based on the above-described processes. Such disturbances are expected to occur substantially simultaneously for a majority of inductive sensing coils of a foreign object detection array. Therefore, such a system may discontinue inductive sensing based on alternating magnetic field biasing and/or heating in the event that significant changes of an inductive sense coil's characteristic occur coincidentally on a majority of inductive sensing coils.
Example Wireless Power Transfer System
The front-end circuit 912 may include a filter circuit configured to filter out harmonics or other unwanted frequencies. The front-end circuit 912 may include a matching circuit configured to match the impedance of the transmitter 902 to the impedance of the power transmitting element 916. The front-end circuit 912 may include also a tuning circuit to create a resonant circuit with the power transmitting element 916. As a result of driving the power transmitting element 916, the power transmitting element 916 may generate a wireless field 920 to wirelessly output power at a level sufficient for charging a battery 922, or otherwise powering a load.
The transmitter 902 may further include a controller 924 operably coupled to the transmit circuitry 906 and configured to control one or more aspects of the transmit circuitry 906, or accomplish other operations relevant to managing the improved foreign object detection for ferromagnetic wire-like objects. The controller 924 may be a microcontroller or a processor. The controller 924 may be implemented as an application-specific integrated circuit (ASIC). The controller 924 may be operably connected, directly or indirectly, to each component of the transmit circuitry 906. The controller 924 may be further configured to receive information from each of the components of the transmit circuitry 906 and perform calculations based on the received information. The controller 924 may be configured to generate control signals (e.g., the control signal 914) for each of the components that may adjust the operation of that component. As such, the controller 924 may be configured to adjust or manage the inductive power transfer based on a result of the operations it performs. The transmitter 902 may further include a memory (not shown) configured to store data, for example, such as instructions for causing the controller 924 to perform particular functions, such as those related to management of wireless power transfer.
The receiver 904 may include receive circuitry 926 having a front-end circuit 928 and a rectifier circuit 930. The front-end circuit 928 may include matching circuitry configured to match the impedance of the receive circuitry 926 to the impedance of the power receiving element 932. The front-end circuit 928 may further include a tuning circuit to create a resonant circuit with the power receiving element 932. The rectifier circuit 930 may generate a DC power output from an AC power input to charge the battery 922, or provide power to some other load. The receiver 904 and the transmitter 902 may additionally communicate on a separate communication channel 934, e.g., Bluetooth™, ZigBee™, and cellular. The receiver 904 and the transmitter 902 may alternatively communicate via in-band signaling using characteristics of the wireless field 920.
Further, the receiver 904 may be configured to determine whether an amount of power transmitted by the transmitter 902 and received by the receiver 904 is appropriate for charging the battery 922 or powering a load. In certain embodiments, the transmitter 902 may be configured to generate a predominantly non-radiative field with a direct field coupling coefficient (k) for providing energy transfer. The receiver 904 may directly couple to the wireless field 920 and may generate an output power for storing or consumption by the battery 922 (or load), coupled to the output of the receive circuitry 926.
The receiver 904 may further include a controller 936 configured similarly to the transmit controller 924 as described above for one or more wireless power management aspects of the receiver 904. The receiver 904 may further include a memory (not shown) configured to store data, such as instructions for causing the controller 936 to perform particular functions, such as those related to foreign object detection for ferromagnetic wire-like objects. The transmitter 902 and receiver 904 may be separated by a distance and configured according to a mutual resonant relationship to minimize transmission losses between the transmitter 902 and the receiver 904.
The power transmitting element 916 and the power receiving element 932 may correspond to or be included as part of, respectively, the base wireless charging system 102a, 102b, the base system coupler 104a, 104b, and/or the electric vehicle coupler 116 that utilize techniques for improved foreign object detection for ferromagnetic wire-like objects described herein.
The various illustrative logical blocks, modules, circuits, and method steps described in connection with the aspects disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. The described functionality may be implemented in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the described aspects.
The various illustrative blocks, modules, and circuits described in connection with the aspects disclosed herein may be implemented or performed with a general purpose hardware processor, a Digital Signal Processor (DSP), an Application-Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose hardware processor may be a microprocessor, but in the alternative, the hardware processor may be any conventional processor, controller, microcontroller, or state machine. A hardware processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.
The steps of a method and functions described in connection with the aspects disclosed herein may be embodied directly in hardware, in a software module executed by a hardware processor, or in a combination of the two. If implemented in software, the functions may be stored on or transmitted as one or more instructions or code on a tangible, non-transitory, computer-readable medium. A software module may reside in Random Access Memory (RAM), flash memory, Read Only Memory (ROM), Electrically Programmable ROM (EPROM), Electrically Erasable Programmable ROM (EEPROM), registers, hard disk, a removable disk, a CD ROM, or any other form of storage medium known in the art. A storage medium is coupled to the hardware processor such that the hardware processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the hardware processor. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray™ disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. The hardware processor and the storage medium may reside in an ASIC.
Although subject matter has been described in language specific to structural features or methodological operations, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or operations described above, including not necessarily being limited to the organizations in which features are arranged or the orders in which operations are performed.
Claims
1. An apparatus for detecting an object, comprising:
- a sense coil; and
- a detection circuit configured to: determine an amount of change in a reactance of the sense coil from a reference reactance value for the sense coil; determine an amount of change in a resistance of the sense coil from a reference resistance value for the sense coil; and selectively increase a sensitivity of the detection circuit as by reducing a detection threshold by an amount that is a function of a ratio of the amount of change in the resistance reactance and the amount of change in the reactance resistance of the sense coil approaches a value of one, the sensitivity increased by reducing a detection threshold by an amount that is a function of the ratio as the ratio approaches a value by one.
2. The apparatus as described in claim 1, wherein the detection circuit is configured to determine a presence of the object based on a relationship between the amount of change in the reactance of the sense coil and the amount of change in the resistance of the sense coil.
3. The apparatus as described in claim 1, wherein the object is a conductive ferromagnetic object.
4. The apparatus as described in claim 1, wherein the sense coil and the detection circuit are implemented in a wireless electric vehicle charging system.
5. The apparatus as described in claim 1, wherein the sense coil is implemented as an inductive power transfer coil of an inductive power transfer system.
6. The apparatus as described in claim 1, wherein:
- the reference reactance value for the sense coil includes a reactance of the sense coil when no object is present; and
- the reference resistance value for the sense coil comprises a resistance of the sense coil when no object is present.
7. The apparatus as described in claim 1, wherein the detection threshold is reduced as the ratio approaches the value of one from approximately 0.9 or from approximately 1.1.
8. The apparatus as described in claim 1, wherein the detection circuit is further configured to determine an impedance change angle of the sense coil based on the amount of change in the resistance of the sense coil and the amount of change in the reactance of the sense coil, wherein the detection threshold is reduced as the impedance change angle approaches a value of 45 degrees from approximately 44 degrees or from approximately 46 degrees in a complex plane.
9. The apparatus as described in claim 1, wherein the amount of change in the resistance of the sense coil is determined to be within five percent of the amount of change in the reactance of the sense coil.
10. The apparatus as described in claim 1, wherein the detection circuit is further configured to:
- perform an inverse tangent operation on a quotient of the amount of change in the reactance of the sense coil divided by the amount of change in the resistance of the sense coil; and
- determine a type of the object based on a result of the inverse tangent operation coinciding with a predetermined range of values corresponding to the type of the object.
11. The apparatus as described in claim 10, wherein if the result of the inverse tangent operation comprises an angle that is approximately 45 degrees, the sensitivity of the detection circuit is increased to detect that the object is a ferromagnetic object or a non-critical object.
12. A method for detecting an object, the method comprising:
- detecting, by a detection circuit, an amount of change in a reactance of a sense coil from a reference reactance value for the sense coil;
- detecting, by the detection circuit, an amount of change in a resistance of the sense coil from a reference resistance value for the sense coil;
- selectively increasing a sensitivity of a detection circuit as by reducing a detection threshold by an amount that is a function of a ratio of the amount of change in the resistance reactance and the amount of change in the reactance resistance of the sense coil, the sensitivity of the detection circuit increased as the ratio approaches a value of one, the sensitivity increased by reducing a detection threshold by an amount that is a function of the ratio; and
- detecting a presence of the object based on a relationship between the amount of change in the reactance of the sense coil and the amount of change in the resistance of the sense coil.
13. The method as described in claim 12, wherein if the object is determined to have has a length that is substantially longer than a thickness of the object, the ratio of the amount of change in the reactance and the amount of change in the resistance of the sense coil approaches a value of one.
14. The method as described in claim 12, wherein if the object is a conductive ferromagnetic object, the ratio of the amount of change in the resistance and the amount of change in the reactance of the sense coil is within a range of approximately −1 to approximately +1.
15. The method as described in claim 12, wherein the detection circuit is configured to use at least two separate inductive sense loops.
16. The method as described in claim 12, further comprising ceasing inductive power transfer responsive to detecting the presence of the object.
17. The method as described in claim 12, wherein:
- the reference reactance value for the sense coil includes a reactance of the sense coil when no object is present; and
- the reference resistance value for the sense coil comprises a resistance of the sense coil when no object is present.
18. Non-transitory computer-readable media comprising code that, when executed, causes an apparatus for detecting an object to:
- determine that a ratio of an amount of change in reactance of a sense coil from a reference reactance value for the sense coil is substantially equal to an amount of change in resistance of the sense coil from a reference resistance value for the sense coil;
- adjust a sensitivity of a detection circuit based on the determination as a function of the determined ratio, the adjustment including a selective increase in the sensitivity of the detection circuit as a the ratio of the amount of change in the resistance of the sense coil and the amount of change in reactance of the sense coil approaches a value of one, the sensitivity increased based on a reduction to a detection threshold by an amount that is a function of the ratio; and
- detect a presence of the object based on a relationship between the amount of change in the reactance of the sense coil and the amount of change in the resistance of the sense coil.
19. The computer-readable media as described in claim 18, wherein if the object is determined to have has a length that is substantially longer than a thickness of the object, the ratio of the amount of change in the resistance and the amount of change in the reactance of the sense coil approaches a value of one.
20. The computer-readable media as described in claim 18, wherein if the object is a conductive ferromagnetic object, the ratio of the amount of change in the resistance and the amount of change in the reactance of the sense coil is within the range of approximately −1 to approximately +1.
21. The computer-readable media as described in claim 18, wherein the sense coil and the detection circuit are implemented in a wireless electronic vehicle charging system.
22. The computer-readable media as described in claim 18, wherein the sense coil comprises an inductive sensing coil that generates a magnetic field.
23. The computer-readable media as described in claim 18, wherein:
- the reference reactance value for the sense coil includes a reactance of the sense coil when no object is present; and
- the reference resistance value for the sense coil comprises a resistance of the sense coil when no object is present.
24. An apparatus for detecting an object, the apparatus comprising:
- means for determining that a ratio of an amount of change in a resistance of a sense coil from a reference resistance value of the sense coil is substantially equal to an amount of change in a reactance of the sense coil from a reference reactance value of the sense coil;
- means for selectively increasing a sensitivity of a detection circuit based on a determination that the amount of change in the resistance of the sense coil is substantially equal to the amount of change in the reactance of the sense coil, the sensitivity of the detection circuit increased as a ratio of the amount of change in the resistance of the sense coil and the amount of change in the reactance of the sense coil approaches a value of one, the sensitivity increased as the determined ratio approaches a value of one by reducing a detection threshold by an amount that is a function of the determined ratio; and
- means for detecting a presence of the object based on a relationship between the amount of change in the reactance of the sense coil and the amount of change in the resistance of the sense coil.
25. The apparatus as described in claim 24, further comprising means for determining that wherein if the object includes a length substantially longer than a thickness of the object, the ratio of the amount of change in the resistance and the amount of change in the reactance of the sense coil approaches a value of one.
26. The apparatus as described in claim 24, wherein if the object is a conductive ferromagnetic object, the ratio of the amount of change in the resistance and the amount of change in the reactance of the sense coil is within the range of approximately −1 to approximately +1.
27. The apparatus as described in claim 24, wherein the sense coil comprises an inductive sensing coil that generates a magnetic field.
28. The apparatus as described in claim 24, wherein the sense coil and the detection circuit are implemented in a wireless electronic vehicle charging system.
29. The apparatus as described in claim 24, wherein:
- the reference reactance value for the sense coil includes a reactance of the sense coil when no object is present; and
- the reference resistance value for the sense coil comprises a resistance of the sense coil when no object is present.
30. The method as described in claim 12, further comprising:
- performing an inverse tangent operation on a quotient of the amount of change in the reactance of the sense coil divided by the amount of change in the resistance of the sense coil;
- determining a type of the object based on a result of the inverse tangent operation coinciding with a predetermined range of values corresponding to the type of the object; and
- based on the result of the inverse tangent operation comprising an angle that is approximately 45 degrees, increasing the sensitivity of the detection circuit to detect that the object is a ferromagnetic object or a non-critical object.
31. An apparatus for detecting an object, the apparatus comprising:
- a wireless electric vehicle charging system including a sense coil and a detection circuit, the detection circuit configured to: determine an amount of change in a reactance of the sense coil from a reference reactance value for the sense coil; determine an amount of change in a resistance of the sense coil from a reference resistance value for the sense coil; and selectively increase a sensitivity of the detection circuit by reducing a detection threshold by an amount that is a function of a ratio of the amount of change in the reactance and the amount of change in the resistance of the sense coil, the sensitivity increased as the ratio approaches a value of one.
32. The apparatus as described in claim 31, wherein if the object has a length that is substantially longer than a thickness of the object, the ratio of the amount of change in the resistance and the amount of change in the reactance of the sense coil approaches a value of one.
33. The apparatus as described in claim 31, wherein if the object is a conductive ferromagnetic object, the ratio of the amount of change in the resistance and the amount of change in the reactance of the sense coil is within the range of approximately −1 to approximately +1.
34. The apparatus as described in claim 31, wherein the sense coil comprises an inductive sensing coil that generates a magnetic field.
35. The apparatus as described in claim 31, wherein:
- the reference reactance value for the sense coil includes a reactance of the sense coil when no object is present; and
- the reference resistance value for the sense coil comprises a resistance of the sense coil when no object is present.
36. The apparatus as described in claim 31, wherein the detection circuit is further configured to:
- perform an inverse tangent operation on a quotient of the amount of change in the reactance of the sense coil divided by the amount of change in the resistance of the sense coil; and
- determine a type of the object based on a result of the inverse tangent operation coinciding with a predetermined range of values corresponding to the type of the object, wherein if the result of the inverse tangent operation comprises an angle that is approximately degrees, the sensitivity of the detection circuit is increased to detect that the object is a ferromagnetic object or a non-critical object.
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Type: Grant
Filed: Nov 24, 2021
Date of Patent: Sep 5, 2023
Assignee: WiTricity Corporation (Watertown, MA)
Inventors: Hans Peter Widmer (Wohlenschwil), Andreas Daetwyler (Muhen), Lukas Sieber (Olten)
Primary Examiner: Kenneth Whittington
Application Number: 17/534,926
