Device, system, and method for detecting foreign matter

Abstract

To provide a detection device, a detection system, and a detection method for detecting a foreign object.SOLUTION: A detection device (100) includes a detection mat (102) having a plurality of detection coils (106) and at least one pair of groups of the detection coils (106). One pair of these groups of detection coils (106) includes a first group of detection coils (106) and a second group of detection coils (106). The first group of detection coils (106) includes a first impedance value, and the second group of detection coils (106) includes a second impedance value. The detection device (100) includes one or more drive subsystems (112) and a comparison subsystem (114).SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] FIELD OF THE DISCLOSURE Embodiments herein relate to a detection apparatus, and more particularly, embodiments herein relate to a detection apparatus for detecting foreign objects in an operating environment of a system. [Background technology]

[0002] Conventional wireless power transfer (WPT) systems operate on the principle of inductive power transfer, which transfers energy wirelessly from one device to another. Typically, in a WPT system, a primary power device, such as a transmitter, wirelessly transfers power to a secondary power device, called a receiver. The transmitter and receiver include respective coils that are placed in operative proximity during operation of the WPT system. When a current passes through the transmitter coil, a magnetic field is generated, inducing an electromotive force in the receiver coil, which transfers power wirelessly from the transmitter to the receiver.

[0003] In such WPT systems, if any metal object, such as a metal can or aluminum foil, is present between the transmitter coil and the receiver coil during wireless power transmission, this metal object may be unnecessarily heated due to eddy currents, which may result in safety hazards, including fire safety issues, and may also unintentionally affect the efficiency of the wireless power transmission process or even interrupt the entire process altogether.

[0004] There are various methods for detecting foreign objects in a WPT system. These methods include detecting the presence of a foreign object by frequency variations in the current in the primary coil, based on measurements of the power drawn from the primary coil, or detecting an imbalanced difference in the current and voltage in the primary coil. Some detection methods may also have some delay in detecting a foreign object after wireless power transmission begins. During this time, the foreign object continues to consume power, which results in wasted power. Summary of the Invention [Means for solving the problem]

[0005] In one embodiment, a sensing apparatus includes a sensing mat having a plurality of sensing coils and at least one pair of sensing coils, the pair of sensing coils including first and second groups of sensing coils. The first group of sensing coils includes a first impedance value and the second group of sensing coils includes a second impedance value. The sensing apparatus further includes one or more driving subsystems and a comparison subsystem. The driving subsystem is operably coupled to the sensing mat and configured to excite the at least one pair of sensing coils. The comparison subsystem is operably coupled to the sensing mat and configured to receive a differential current signal from the at least one pair of sensing coils, where the comparison subsystem is configured to generate a control signal based on the differential current signal.

[0006] In another embodiment, a detection system includes a detection device and a wireless power transfer (WPT) system. The detection device includes a detection mat having a plurality of detection coils and at least one pair of detection coils, the at least one pair of detection coils including first and second groups of detection coils. The first group of detection coils includes a first impedance value, and the second group of detection coils includes a second impedance value. The detection device further includes one or more driving subsystems and a comparison subsystem. The driving subsystem is operably coupled to the detection mat and configured to excite the at least one pair of detection coils. The comparison subsystem is operably coupled to the detection mat and configured to receive a differential current signal from the at least one pair of detection coils, where the comparison subsystem is configured to generate a control signal based on the differential current signal. The WPT system includes a primary power source configured to provide power in the form of an alternating current (AC) voltage signal. The WPT system further includes a transmitter unit having at least one transmitter coil, the transmitter unit configured to receive the AC voltage signal from the primary power source. The transmitter unit is also configured to generate a primary magnetic field in response to the received AC voltage signal, and the detection mat is operably coupled to the transmitter unit. The WPT system further includes a receiver unit including at least one receiver coil and configured to receive at least a portion of the primary magnetic field generated by the transmitter unit. The WPT system further includes a control unit operably coupled to the detection device and configured to receive a control signal from the detection device and control a power supply to the transmitter unit based on the control signal.

[0007] In yet another embodiment, a method includes utilizing a sensing device including a sensing mat having a plurality of sensing coils and at least one pair of sensing coils, the pair of sensing coils including first and second groups of sensing coils. The first group of sensing coils includes a first impedance value and the second group of sensing coils includes a second impedance value. The method further includes exciting the at least one pair of sensing coils and measuring a differential current signal from the at least one pair of sensing coils. The method further includes generating a control signal based on the differential current signal and transmitting the control signal. [Brief description of the drawings]

[0008] These and other features and aspects of the present invention will be more effectively understood when the following detailed description is read in conjunction with the accompanying drawings, in which like reference characters represent like parts throughout.

[0009] [Figure 1] 1 is a schematic diagram of an exemplary detection device for detecting the presence of a foreign object, according to an aspect of the present disclosure. [Figure 2A] 1 is a schematic diagram illustrating an example detection coil pair, according to an aspect of the present disclosure. [Figure 2B] 1 is a schematic diagram illustrating an example detection coil pair, according to an aspect of the present disclosure. [Figure 3A] 1 is a schematic diagram illustrating a portion of a sensing apparatus including a pair of sensing coils coupled to a drive subsystem, according to an aspect of the present disclosure. [Figure 3B] 1 is a schematic diagram illustrating a portion of a sensing apparatus including a pair of sensing coils coupled to a drive subsystem, according to an aspect of the present disclosure. [Figure 3C] 1 is a schematic diagram illustrating a portion of a detection apparatus having one or more drive subsystems coupled to a pair of detection coils according to an embodiment of the present disclosure. [Figure 3D]1 is a schematic diagram illustrating a portion of a detection apparatus having one or more drive subsystems coupled to a pair of detection coils according to an embodiment of the present disclosure. [Figure 3E] 1 is a schematic diagram illustrating a portion of a detection apparatus having one or more drive subsystems coupled to a pair of detection coils according to an embodiment of the present disclosure. [Figure 4] 1 is a schematic diagram of an exemplary detection system having a detection device for detecting the presence of a foreign object in an operating environment of a WPT system according to an embodiment of the present disclosure. [Diagram 5] 1 is a schematic diagram illustrating an exemplary comparison subsystem of a detection device according to aspects of the present disclosure. [Figure 6] 1 is an exemplary method for detecting the presence of a foreign object in an operating environment of a WPT system according to an embodiment of the present disclosure. [Figure 7] 1 is an exemplary method for detecting the presence of a foreign object in an operating environment of a WPT system according to an embodiment of the present disclosure. [Figure 8] 1 is a cross-sectional view of a detection mat according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The embodiments herein relate to a detection device, a detection system, and a method for detecting foreign objects. In certain embodiments, the detection device can be used to detect foreign objects in a wireless power transfer (WPT) system. As used herein, the term "foreign object" refers to an object that is conductive or has detectable magnetic permeability and is not part of the system but is incidentally present in the operating environment of the system. Non-limiting examples of foreign objects include ferrous objects, tin cans, or other conductive or ferrous objects. In one example, a can or a set of keys is considered a foreign object in the operating environment of the WPT system.

[0011] As used herein, the term "primary magnetic field" refers to a magnetic field excited by a transmitter unit, e.g., a transmitter unit of the present WPT system. The term "operating environment" is defined by the primary magnetic field in the present system, where the primary magnetic field of the transmitter unit is detectably present and can detectably interact with foreign objects. As an example, foreign objects present in the operating environment of the present WPT system may experience an increase in temperature due to interaction with the primary magnetic field.

[0012] As used herein, the term "detector coil group" refers to a single detector coil or to two or more coils, where the detector coils in each group are electrically coupled to each other, and the term "detector coil group pair" refers to two such detector coil groups.

[0013] Some electric vehicles (EVs) and hybrid vehicles are charged by wireless or inductive power transfer using a WPT system. This WPT system mainly consists of two parts: a transmitter unit that is part of the charging station and a receiver unit located inside the electric vehicle (EV). The transmitter unit of the charging station is inductively coupled to the receiver unit of the EV. Typically, when the receiver coil of the EV is brought into the operating environment of the transmitter coil and power is supplied to the transmitter unit, power begins to be transferred between the coil of the transmitter unit and the coil of the receiver unit.

[0014] If a foreign object made of conductive or steel material is accidentally placed in the operating environment of the WPT, the foreign object may interact with the magnetic field generated by the transmitter unit. As a result, the foreign object may accidentally intercept the magnetic field and draw power from the transmitter unit through the magnetic field. Thus, the foreign object draws power, resulting in power waste and unnecessary heating of the foreign object. If this is not dealt with in a timely manner or if it goes unnoticed, the temperature of the foreign object may rise to a point where handling the foreign object may be dangerous and may also pose a safety risk. Therefore, it is desirable to detect the presence of the foreign object in order to prevent power waste and avoid risks in a dangerous environment.

[0015] 1 shows a detection device 100 comprising a detection mat 102 having a plurality of detection coils 106 and at least one pair of the groups of detection coils 106 (not shown in FIG. 1). The at least one pair of the groups of detection coils 106 includes a first group of detection coils 106 and a second group of detection coils 106, the first group of detection coils 106 having a first impedance value and the second group of detection coils 106 having a second impedance value. The detection device 100 further comprises one or more drive subsystems 112 operably coupled to the detection mat 102 and configured to excite the at least one pair of the groups of detection coils 106. Furthermore, the detection device 100 comprises a comparison subsystem 114 operably coupled to the detection mat 102 and configured to receive a differential current signal from the at least one pair of the groups of detection coils 106. The comparison subsystem 114 is configured to generate a control signal based on the differential current signal.

[0016] In a particular embodiment, the detection device 100 is configured to detect the presence of a foreign object in the operating environment of a wireless power transfer (WPT) system (not shown in FIG. 1). The presence of a foreign object in the operating environment of the WPT system may cause a variation in the impedance value of one or more of the detection coils 106. The variation in the impedance value of the detection coils 106 is used to confirm the presence of the foreign object. The set of detection coils 106 shall be described in more detail with respect to FIGS. 2A-2B and 3A-3B.

[0017] Furthermore, the detection mat 102 may include several geometric locations, also referred to as "detection regions" 104. The detection regions 104 define discrete regions of the detection mat 102, each of which includes one or more groups of detection coils 106. It should be noted that the detection regions 104 may not be physically separated or isolated from one another, where the detection regions 104 may be electrically insulated from one another. Also, the detection regions 104 may have a geometric or non-geometric shape.

[0018] The detection mat 102 may be a flexible mat, a conformable mat, a rigid mat, a plug-and-play mat, a stand-alone mat, or a combination thereof. It should be noted that the surface area of ​​the detection mat 102 may be equal to or greater than the surface area of ​​the transmitter unit on which the detection mat 102 is disposed. The substrate of the detection mat 102 may be made of an electrically insulating material. The detection mat 102 may further include a mechanical wear-resistant material to withstand the operation of an electric vehicle thereon. The detection mat 102 may further be designed for outdoor applications, and thus may be designed to withstand temperature or humidity, and may be resistant to water ingress.

[0019] In some embodiments, the detection coil 106 may be disposed on the substrate of the detection mat 102 or may be embedded in the substrate of the detection mat 102 for user safety and aesthetics. In some other embodiments, the detection coil 106 may be printed, molded, woven, or additively manufactured onto the substrate of the detection mat 102.

[0020] The detection coils 106 are electromagnetic coils. Although the illustrated embodiment of FIG. 1 uses a detection coil 106 for illustrative purposes, each of the detection coils 106 may include a spiral or other coil phase. Additionally, each detection area 104 may have one or more detection coils 106. In certain embodiments, a group of detection coils 106 may include various coil phases, shapes, and geometries of the detection coils 106. Non-limiting examples of shapes of the detection coils 106 include helical coils, spiral coils, and non-circular coils. Different detection coils 106 or groups of detection coils 106 provided on the detection mat 102 may have the same impedance value or may have different impedance values.

[0021] Each pair of groups of detection coils 106 includes two groups that are arranged in the same or different geometric locations on the detection mat 102 to form a pair of groups of detection coils 106. As an example, the two groups in the pair may be arranged in the same detection area 104 or in two different detection areas 104. Non-limiting examples of each group of detection coils 106 include concentric coils, adjacently arranged coils, and parallel arrangements of coils. Furthermore, one or more groups of detection coils 106 may be arranged in a single plane or in two or more layers. The detection coils 106 in a group of detection coils 106 may be connected in series, parallel, or a combination of series and parallel connections.

[0022] In some embodiments, a pair of the group of detection coils 106 includes concentric coils. In a non-limiting example, the inductance value of one of the two concentric coils is different from the inductance value of the other concentric coil. As an example, the inductance value of one concentric coil of the pair of concentric coils may be represented as L, while the inductance value of the other concentric coil of the pair of concentric coils may be represented as nL.

[0023] The driving subsystem 112 includes one or more driving units and a driving controller. The driving subsystem 112 is operatively coupled to the detection mat 102 and configured to excite one or more pairs of the group of detection coils 106. The driving subsystem 112 may be configured to intermittently or periodically excite a designated group of detection coils 106 in the detection device 100, where the designated group of detection coils forms one or more pairs of the group of detection coils 106. The driving subsystem 112 may be configured to excite pairs of the group of detection coils 106 belonging to the same or different detection areas 104. In a particular embodiment, the driving subsystem 112 may be configured to excite groups of detection coils 106 that are symmetrically arranged with respect to the primary magnetic field (not shown in FIG. 1) of the WPT system.

[0024] In one example, the drive subsystem 112 may be configured to simultaneously or sequentially excite pairs of the detection coils 106. Additionally or alternatively, the drive subsystem 112 may be configured to simultaneously or sequentially excite two or more pairs of the detection coils 106 in the detection mat 102. In one example, the drive subsystem 112 may be configured to scan the detection mat 102 for the presence of a foreign object. Detection of the presence of a foreign object may be completed before initiating wireless power transmission of a device (such as an EV). Furthermore, detection of the presence of a foreign object may also be performed periodically or intermittently during the process of wireless power transmission of the device (such as an EV).

[0025] In one example, as detailed with respect to Figures 3A-3E, each pair of the group of detection coils 106 may be coupled to a respective drive subsystem, and in another example, two or more pairs of the group of detection coils 106 may be coupled to different drive units that share a common drive controller.

[0026] In some embodiments, the drive unit of the drive subsystem 112 is configured to provide a voltage signal to one or more groups of the sensing coils 106, and the drive controller is configured to control the operation of the drive unit. To this end, the drive controller turns on the drive subsystem 112 to excite one or more groups of the sensing coils 106, and turns off the drive subsystem 112 when the group of the sensing coils 106 does not need to be excited. In certain embodiments, the drive subsystem 112 includes one or more of an inverter, a converter, a linear amplifier, an electronic switch, or a combination thereof. The switching operation of the drive subsystem 112 may be performed by an electronic switch, such as a bidirectional switch, a semiconductor switch, a mechanical switch, such as a relay, a contractor, or a combination thereof. In certain embodiments, the drive subsystem 112 may draw power from a primary power source configured to provide power to a transmitter unit of the WPT system. Alternatively, the drive subsystem 112 may have a separate power source, such as, but not limited to, a battery.

[0027] It should be noted that during operation, if a foreign object is present on the detection mat 102, it may be inductively coupled to one or more detection coils 106, thereby affecting the resulting impedance value of those particular groups of detection coils 106. Furthermore, the specific impedance values ​​of the individual groups of detection coils 106 in the detection device 100 may also be the same or different in the absence of a foreign object. Furthermore, the specific impedance values ​​of the two groups in the pair of groups of detection coils 106 may be the same or different in the absence of a foreign object. In some of these embodiments, the detection mat 102 further includes a compensator element (not shown in FIG. 1 ) operably coupled to at least one of the pair of groups of detection coils 106 to result in similar resulting impedance values ​​from each of the two groups in the pair of groups of detection coils 106 in the absence of a foreign object. In some other embodiments, a drive controller may be used to provide voltage compensation to one of the groups in the pair of groups of detection coils 106. As an example, the drive controller of the drive subsystem 112 supplies different voltage signals to the two groups in a pair of detection coils 106 such that, in the absence of a foreign object, the two groups in the pair of detection coils 106 are driven by the same current of similar magnitude and phase.

[0028] The comparison subsystem 114 is operatively coupled to the detection mat 102 and configured to receive one or more differential current signals from the detection mat 102. The comparison subsystem 114 is configured to determine a detection voltage signal based on the differential current signals. Note that in the absence of a foreign object, the value of the differential current signal may be very small and undetectable. Also, due to inductive coupling between the foreign object and the one or more detection coils 106, the value of the differential current signal may be a non-zero value that is detectable in the presence of a foreign object.

[0029] The detection device 100 may be a stand-alone device that can be used to detect the presence of a foreign object in a system, such as, but not limited to, a WPT system. To that end, the detection device 100 may be used with an existing WPT system to detect the presence of a foreign object before and / or during wireless power transmission. In addition to the detection mat 102, the drive subsystem 112, and the comparison subsystem 114, the detection device 100 may further include a communication unit (not shown in FIG. 1 ) that allows the detection device 100 to communicate with the system in which it is used. As an example, the communication unit may enable the detection device 100 to communicate with the WPT system in accordance with Society of Automotive Engineers (SAE) standards. The communication unit may be used to communicate a detection voltage signal to a control unit of the WPT system to determine whether to continue powering a transmitter unit of the WPT system.

[0030] Advantageously, the detection device 100 of the present technology is configured to detect the presence of even relatively small foreign objects in the operating environment of the present WPT system, which in one example may have a surface area of ​​2 cm2 or more.

[0031] 2A-2B, in certain embodiments, one or more pairs of detection coil groups of a detection device may be excited to obtain a differential current signal from the detection mat, where the differential current signal is used to determine the presence or absence of a foreign object, and each of the two groups of detection coil pairs may include one or more detection coils.

[0032] In certain embodiments, the first and second groups in the pair of detection coil groups are arranged symmetrically with respect to the primary magnetic field. Furthermore, the first and second groups in the pair of detection coil groups are arranged in the same geometrical area on the detection mat. In some embodiments, the first and second groups in the pair of detection coil groups include a structurally similar arrangement with respect to the detection coils. Also, in certain embodiments, the first and second impedance values ​​of the groups in the pair of detection coil groups are similar when no foreign objects are present in the operating environment of the WPT system. As used herein, similar impedance values ​​refer to impedance values ​​that have a variation of ±0.5% or less of each other. As an example, the impedance value of the first group may be within minus 0.5% of the impedance value of the second group, where the first and second groups form the pair of detection coil groups.

[0033] In some other embodiments, the first and second impedance values ​​are different in the absence of a foreign object. In some of these embodiments, the detection mat may include a compensator element operably coupled to one of the first or second groups of the pair of detection coils to compensate for differences in impedance values ​​of the first and second groups of detection coils 106. In some embodiments, the first and second impedance values ​​are different in the absence of a foreign object, and the one or more drive subsystems further include a drive controller configured to provide different voltage signals to the first and second groups of the pair of detection coils such that the first and second groups of the pair of detection coils are driven by the same current of similar magnitude and phase in the absence of a foreign object.

[0034] As described with respect to FIG. 2A, the detection coils in a particular pair of detection coils are arranged symmetrically with respect to the primary magnetic field, and thus are exposed to similar primary magnetic field strengths. As described with respect to FIG. 2B, the groups in a pair of detection coils may be arranged in the same geometric area in the detection mat, and thus the primary magnetic field strengths may be similar for these groups. In some embodiments, each group in a pair of detection coils includes the same impedance value when there is no foreign object in the operating environment of the WPT system. In some other embodiments, the two groups in the pair of detection coils have different impedance values. In these embodiments, the two groups in the pair of detection coils may provide a negligible and undetectable amount of differential current in conjunction with the compensator element or with the assistance of the drive controller, but in the presence of a foreign object, one or more detection coils in a group of the pair of detection coils are electrically coupled with the foreign object to change the net impedance of the detection coil group, resulting in a difference in the current drawn by the two groups in the pair of detection coils. As a result, the differential current, which is the difference between the input current of one of the two groups in the pair and the input current from the other group in that pair of detection coil groups, reaches a detectable value.

[0035] 2A illustrates an exemplary detection mat 200 including a plurality of detection regions 202 having groups 204 of detection coils 205. Although the non-limiting illustrated embodiment illustrates the detection mat 200 as having a 4x4 array of detection regions 202 and each detection region 202 as having four groups 204 of detection coils 205, the number of detection regions 202 in the detection mat 200 and the number of groups 204 in each detection region 202 may vary and is for illustrative purposes only. Additionally, although the illustrated example illustrates the detection mat 200 as having 16 detection regions 202, the number of detection regions 202 in a detection mat, such as the detection mat 200, may be one or more. Each group 204 may include one or more detection coils 205. Furthermore, the spiral shape of the detection coils 205 in the group 204 is for illustrative purposes only and the shape of the detection coils 205 may be, but is not limited to, circular, non-circular, spiral, helical, or combinations thereof. The detection mat 200 may be disposed on a transmitter unit (not shown in FIG. 2A). Furthermore, the dotted circle 206 represents a transmitter coil of a transmitter unit disposed below the detection mat 200.

[0036] Any two groups 204 of detector coils likely to be exposed to a similar strength of the primary magnetic field may be paired to form a pair of detector coil groups for detecting the presence of a foreign object. Some possible pairs of groups 204 of detector coils 205 are represented by n-n'. As an example, the group 204 referred to by reference number 208 is arranged symmetrically with respect to the primary magnetic field of the transmitter coil 206 or transmitter unit, forming a pair 212 of detector coil groups 208. Similarly, group 210 is arranged symmetrically with respect to the primary magnetic field of the transmitter unit, forming another pair 214 of detector coil groups 210. It should be noted that other pairs of groups 204 of detector coils 205 other than 212, 214, n-n' are also possible.

[0037] In a particular embodiment, when the detection mat 200 is disposed on the transmitter unit, the detection coils 205 in one or more pairs of the group 204 may be excited to detect the presence of a foreign object in the operating environment of the WPT system (not shown in FIG. 2A). As an example, one of the two pairs 212 or 214 may be excited by a driving subsystem. Furthermore, when the impedance values ​​of the two groups in the group pairs are different, a compensator element 218 may be operably coupled to at least one of the groups 204 in the pair of groups 204 of the detection coils 205 so that the resulting impedance values ​​from each of the two groups 204 in the pair of groups of the detection coils 205 are similar when no foreign object is present.

[0038] FIG. 2B shows a detection mat 220 with multiple detection areas 224 with two or more detection coils 222. The detection devices 220 may be located on a transmitter unit (not shown in FIG. 2B). In the illustrated embodiment, the dashed circle 225 represents a transmitter coil of a transmitter unit located under the detection devices 220. Each of the areas 224 also includes multiple groups of concentric detection coils 222 located in each detection area 224. It is contemplated within the scope of the present application that there may be a greater number of detection coils 222 per area 224 or that the shapes and geometries of the detection coils 222 may be different, and the embodiment shown in FIG. 2B is a representative embodiment presented for illustrative purposes only. The pairs 226 of groups 228 and 230 of detection coils 222 are formed in the illustrated manner. The individual groups 228 and 230 are located in the same geometrical area such that the detection coils 222 of the groups 228 and 230 are exposed to similar primary magnetic field strengths.

[0039] 3A shows a portion 300 of the detection device herein. The portion 300 includes a pair 302 of groups 304 and 306 of detection coils 308 arranged on a detection mat of the detection device. In the embodiment contemplated herein, both of the two groups 304 and 306 of the detection coils 308 are excited by a single drive subsystem 310. The drive subsystem 310 includes a drive unit 311 and a drive controller 314. Also, both of the groups 304 and 306 of the detection coils 308 are excited using the single drive unit 311 of the drive subsystem 310. The drive unit 311 includes an inverter / converter 312, capacitors 309 and 311, and one or more compensator elements 315 and 317. It is noted that the capacitance values ​​of the capacitors 309 and 311 may be fixed or variable. The drive controller 314 of the drive subsystem 310 is configured to control the excitation voltage to the two groups 304 and 306 of the detection coils 308. As shown, the detection coils 308 in each group 304 and 306 may be coupled to one another in series, parallel, or both. Capacitors 309 and 311 are coupled to these groups 304 and 306 and form resonant circuits with the detection coils 308 of groups 304 and 306. Capacitors 309 and 311 help to adjust the current driven in the detection coils 308 and therefore modify the strength of the magnetic field used to detect foreign objects on the detection mat of the detection apparatus.

[0040] One or more detection coils 308 in the vicinity of a foreign object may be electromagnetically coupled to the foreign object, and this inductive coupling may result in a variation in the value of the current drawn by the detection coils 308. In one embodiment, the two groups 304 and 306 of the pair 302 of detection coils 308 may have detectably similar impedance values ​​in the absence of a foreign object. To detect the presence of a foreign object, a differential current signal of the pair 302 is calculated based on the feedback current signal I1 318 of the group 304 and the continuing current signal I2 320 of the group 306, or vice versa. In another embodiment, when the two groups 304 and 306 have detectably different impedances in the absence of a foreign object, one or more compensator elements 315 and 317 are used to ensure that the currents drawn by the two groups are similar in the absence of a foreign object. Non-limiting examples of the compensator elements 315 and 317 may include inductance, resistance, capacitance in series or parallel or both forms.

[0041] 3B shows another embodiment in which a portion 330 of the detection device herein comprises a pair 332 of a first group 334 and a second group 336 of detection coils 338 arranged on a detection mat of the detection device. The two groups 334 and 336 are coupled to a drive subsystem 340. The drive subsystem 340 includes two drive units 342 and 344 configured to excite the first group 334 and the second group 336, respectively. The two drive units 342 and 344 include inverters 348 and 350, DC / DC converters 352 and 354, and capacitors 333 and 335. Although not shown, one or both drive units 342 and 344 may further include one or more compensator elements. Capacitors 333 and 335 coupled to these groups 334 and 336 form resonant circuits with the detection coils 338 of the groups 334 and 336 to help regulate the current driven in the coils and therefore the strength of the magnetic field at the detection mat. Inverters 348 and 350 receive inputs from two DC / DC converters 352 and 354 driving the two groups 334 and 336, which may have similar or different impedance values ​​in the absence of a foreign object.

[0042] The drive subsystem 340 further includes a drive controller 346 that may be configured to compensate for any differences in current values ​​of the first and second groups 334 and 336, even if the impedance values ​​of the first and second groups 334 and 336 differ. In embodiments where the groups 334 and 336 have different impedance values, the drive controller 346 may be configured to adjust the currents used to drive the two groups 334 and 336 in one or more ways. In one embodiment, the drive controller 346 may be configured to vary the voltage signals V1 and V2 of the inverters 348 and 350 by controlling the DC / DC stage converters 352 and 354. In another embodiment, the duty cycle of the inverters 348 and 350 may be changed without changing the operating frequency of the inverters 348 and 350, such that the first and second groups 334 and 336 of the detection coils 338 are driven by the same currents of similar magnitude and phase when no foreign object is present. The differential current signal of the pair 332 is calculated based on the feedback current signal I1 356 of the first group 334 and the continuing current signal I2 358 of the second group 336, or vice versa.

[0043] 3C-3E, which show an embodiment of an operable coupling between the drive subsystem and the detection coil groups. During detection operation, the detection coil groups are coupled to one or more drive subsystems of the detection system and draw current from one or more drive units of the drive subsystem. If the impedance values ​​of both groups in the pair of groups are similar, the current drawn by the two groups in the pair of detection coil groups will have a small differential current value in the absence of a foreign object. However, if the impedances of the two groups in the pair of detection coil groups are different, the drive controller compensates one or both groups in the pair of detection coils to ensure that the two groups are driven by similar current signals in the absence of a foreign object. The drive controller further controls the excitation voltage, excitation frequency, and excitation time of the detection coils. The drive controller turns on the drive units to excite the detection coil groups and turns off the drive units when there is no need to excite the detection coil groups. The drive controller may use electronic circuitry to control these drive units. Non-limiting examples of electronic circuits include microcontrollers, field-programmable gate arrays (FPGAs), semiconductor devices, logic gates, device drivers, oscillators and timers, or combinations thereof.

[0044] 3C illustrates a portion 360 of a sensing apparatus (not shown in FIG. 3C) having a drive subsystem 362 coupled to a plurality 364 of sensing coil groups 366 disposed on a sensing mat. The drive subsystem 362 includes a plurality of drive units 368, where each drive unit 368 is coupled to a corresponding group 366 of sensing coils. The drive subsystem 362 further includes a drive controller 370 controlling the operation of the drive units 368.

[0045] 3D illustrates a portion 380 of a sensing apparatus (not shown in FIG. 3D ) having a drive subsystem 382 coupled to a plurality 384 of groups 386 of sensing coils disposed on a sensing mat. The drive subsystem 382 includes a plurality of drive units 387. The drive subsystem 382 further includes a drive controller 389 that controls the operation of the drive units 387. Pairs 388 of the sensing coil groups are coupled to the drive subsystem 382 such that each pair 388 is coupled to a single drive unit 387.

[0046] FIG 3E shows a portion 390 of the sensing apparatus (not shown in FIG 3E) using two drive subsystems 391 and 392 coupled to a plurality 393 of sensing coils 394. The sensing coils are disposed on a sensing mat. Pairs 395 and 397 of sensing coils 394 are coupled to drive subsystems 391 and 392, respectively. Drive subsystem 391 includes a drive unit 399 and a drive controller 401, and drive subsystem 392 includes a drive unit 403 and a drive controller 405.

[0047] 4 illustrates an exemplary detection system 400 for detecting the presence of a foreign object in an operating environment 402 of a WPT system 404. The system 400 includes a detection device 406 and a WPT system 404. The detection device 406 includes a detection mat 408 having a plurality of detection coils and at least one pair of detection coils for detecting the presence of a foreign object in the operating environment 402 of the WPT system 404.

[0048] The detection device 406 further includes a drive subsystem 410. The drive subsystem 410 includes a drive unit 411 and a drive controller 412. Although not shown, the detection system 400 may include more than one drive subsystem 410. Furthermore, the drive subsystem 410 may include multiple drive units 411, or multiple drive controllers 412, or both. The drive subsystem 410 is operably coupled to the detection mat 408 and configured to excite two or more detection coil groups in the detection mat 408. The detection device 406 further includes a comparison subsystem 414 operably coupled to the detection mat 408 and configured to receive a differential current signal from the detection mat. Furthermore, the comparison subsystem 414 is configured to provide a control signal based on the differential current signal.

[0049] The WPT system 404 of the detection system 400 includes a primary power source 418 configured to provide power in the form of an alternating current (AC) voltage signal to a transmitter unit 420 of the WPT system 404. The transmitter unit 420 includes at least one transmitter coil (not shown in FIG. 4 ). The transmitter unit 420 is configured to receive the AC voltage signal from the primary power source 418 and generate a primary magnetic field or operating environment 402 in response to the received AC voltage signal.

[0050] The detection mat 408 is operatively coupled to the transmitter unit 420. Specifically, the detection mat 408 is disposed on the transmitter unit 420. In some embodiments, the detection mat 408 is disposed directly on the surface 422 of the transmitter unit 420. In some other embodiments, the detection mat 408 is disposed above the transmitter unit 408 such that the detection mat 408 is disposed within the operating environment of the WPT system 404. The detection mat 408 may be disposed and aligned with respect to the transmitter coil of the transmitter unit 420. The detection mat 408 may be disposed such that the transmitter coil is geometrically symmetrically disposed with respect to the detection mat 408. Furthermore, the detection mat 408 may be a stand-alone mat 408, a plug-and-play mat, and may have one or more of a flexible structure, a conformal structure, or a rigid structure.

[0051] The WPT system 404 further includes a receiver unit 424 having at least one receiver coil configured to receive at least a portion of the primary magnetic field 402 generated by the transmitter unit 420. The receiver unit 424 may be disposed on an underside 426 of a device that needs to be charged. In the illustrated embodiment, the receiver unit 424 is disposed on an underside 426 of an electric vehicle (EV) 416 that is charged using the WPT system 404. The EV 416 may include an external power source, such as a battery 417. The WPT system 404 further includes a control unit 426, which controls the operation of the primary power source 418 and is operably coupled to the detection device 406. The control unit 426 receives a control signal from the detection device 406. The control unit 426 also controls the supply of power from the primary power source 418 to the transmitter unit 420 based on the control signal.

[0052] In a particular embodiment, the detection device 406 comprises a communication unit 428 operably coupled to the comparison subsystem 414 and configured to receive a control signal from the comparison subsystem 414. Furthermore, this communication unit 428 is configured to communicate with the control unit 426 of the WPT system 404. In a particular embodiment, this communication unit 428 can transmit a control signal to a user of the EV 416 or an operator of the respective EV charging station to communicate the presence and / or absence of a foreign object, for example by using a cellular network.

[0053] The differential current signal received by the comparison subsystem 414 from the detection coils of the detection mat 408 is converted into a voltage signal called a detection voltage signal. In some embodiments, the detection voltage signal is compared to a threshold or threshold voltage signal. The threshold or threshold may be defined by a user based on, for example, the parameters and design of the detection device 406. If the value of the detection voltage signal is greater than the value of the threshold voltage signal, it indicates that the currents drawn by the two groups in the pair of detection coils are different, which indicates the presence of a foreign object in the operating environment of the WPT system 404. Alternatively, if the value of the voltage signal is less than or equal to the value of the threshold voltage signal, it indicates that the currents drawn by the two groups in the pair of detection coils are suitably similar, which indicates that there is no foreign object in the operating environment of the WPT system 404 or that there is a very small foreign object (such as a coil) that will not adversely affect the operation of the WPT.

[0054] Further, in an embodiment where the value of the detected voltage signal is greater than the value of the threshold voltage signal, a control signal can be communicated by the detection device 406 to the WPT system 404 to shut down the power supply to the transmitter unit 420. In some embodiments, the communication unit 428 communicates with the control unit 426, which then shuts down the power supply from the primary power source 418 to the transmitter unit 420. In another embodiment, when the transmitter unit 420 is configured with an array of coils, the control signal communicated by the detection device 406 to the control unit 426 causes the control unit 426 to excite only a subset of the array of coils in the transmitter unit 420, thereby avoiding the area where the foreign object is detected.

[0055] FIG. 5 illustrates an exemplary comparison subsystem 500 operatively coupled to a pair 502 of detection coil groups 504 and 506 of a detection mat. It is noted that the illustrated example of FIG. 5 is a non-limiting example of measuring differential currents, and other methods of measuring differentials may also be used herein, including sensing the individual currents and detecting the difference between the individual currents using electronic circuitry, such as, but not limited to, a differential amplifier. The differential current signal from the pair 502 may be determined by using a continuing current signal to one group and a feedback current signal from the other group in the pair of groups. In the illustrated embodiment, a feedback current signal I1 508 from group 504 and a continuing current signal I2 510 from group 506 are used to derive the differential current signal for the pair 502 of groups 504 and 506.

[0056] As the two current signals 508 and 510 pass through the magnetic core 512, the difference between these current signals 508 and 510 creates a magnetic flux linkage within the core, thereby inducing a corresponding sensed voltage signal, generally indicated by reference numeral 516, in an electrical winding 514 wound around the magnetic core 512. This sensed voltage is proportional to the difference between the current signals 508 and 510 under magnetically non-saturated core conditions, and represents a measure of the differential current between the two groups 504 and 506 in the group pair 502.

[0057] The comparison subsystem 500 further includes a rectifier 518 that receives and rectifies the detected voltage signal. An optional filter 520 filters the voltage signal before transmitting it to a comparator 522. In one example, the filter 520 is configured to filter out high frequency components from the measurement. The comparator 522 compares the filtered voltage signal to a threshold voltage signal, and in response, a control signal is communicated to a WPT system, such as the WPT system 404 of FIG. 4.

[0058] 6 is a flow chart 600 of an exemplary method for detecting the presence of a foreign object in an operating environment of a system. In one example, the system may be a WPT system. In block 602, a detection device is utilized by placing a detection mat of the detection device on a transmitter unit of the WPT system. The detection mat may be placed directly on the transmitter unit or may be placed above the transmitter unit such that it is not in physical contact with the transmitter unit. The detection mat includes a plurality of detection coils and at least one pair of detection coil groups, the at least one pair of detection coil groups including a first group of detection coils and a second group of detection coils, the first group of detection coils including a first impedance value and the second group of detection coils including a second impedance value.

[0059] At least one pair of detection coils are excited in block 604. In some embodiments, the groups in a pair may be symmetrically positioned with respect to the primary magnetic field. In some other embodiments, the groups in a pair are positioned at the same geometric location. In a particular embodiment, the groups in a pair may be positioned in two different detection regions, which may be selected such that the two detection regions in the pair of detection regions are symmetrically positioned with respect to the magnetic field of the transmitter unit.

[0060] In certain embodiments, more than one pair of detection coils may be energized. In one example, different pairs of detection coils may be energized simultaneously to detect the presence of a foreign object. In one embodiment, the different pairs of detection coils may be selected such that the entire surface of the detection mat is energized to detect the presence of a foreign object. In some embodiments, different pairs may be energized at different instances in time. In another embodiment, more than one pair may be energized simultaneously. In one embodiment, the different pairs of detection coils may be energized in a chronological order.

[0061] The pairs of the groups may be energized prior to and during the initiation of wireless power transmission operation of the WPT system to the receiver unit. The pairs of the groups may be energized intermittently or at regular intervals. The detection coils corresponding to the various pairs of detection regions are energized at various instances in time. Different voltage signals are provided to the detection coils of the pairs of detection coils with different impedance values ​​such that the detection coils of the pairs are driven by the same currents of similar magnitude and phase when no foreign object is present.

[0062] At block 606, a differential current signal of the excited detection coil is determined. In one embodiment, the differential current signal is determined using the continuing current signal of one of the pair of detection coils and the return current signal of the other of the pair of detection coils. In an embodiment where more than one pair is excited simultaneously, corresponding individual differential current signals may be determined to detect the presence of a foreign object. Based on the differential current signals, the presence or absence of a foreign object in the operating environment of the WPT system is determined.

[0063] A control signal is then generated based on the differential current signal at block 608. The generated control signal is transmitted at block 610. In one example, the control signal is transmitted to a control unit of the WPT system. In some embodiments, power supply to a transmitter unit of the WPT system is continued, adjusted, or stopped based on the control signal received by the control unit of the WPT system.

[0064] 7 is a method flow chart 700 of a method for controlling power supply to a transmitter unit based on a control signal. At block 702, a detection voltage signal proportional to the differential current signal is generated, for example using an electrical winding and a magnetic core of a comparison subsystem. At block 704, the detection voltage signal is compared to a threshold. In some embodiments, the threshold may be a threshold value or a threshold voltage signal. The threshold voltage signal may be a predetermined voltage signal based on the specifications of the WPT system.

[0065] If the detection voltage signal is found to be greater than the threshold voltage signal in decision block 706, a control signal is sent to the WPT system to not start, adjust, or stop powering the transmitter unit (block 708). Alternatively, if the detection voltage signal is less than the threshold voltage signal in decision block 706, a control signal is sent to the WPT system indicating that no communication will be made to the WPT system, or that powering the transmitter unit will continue or start (block 710).

[0066] This detection may be performed in real time or near real time. Near real time detection may be performed such that a control signal is generated and communicated within microseconds to milliseconds from the start of detection. In a particular embodiment, the detection mat may be scanned for any foreign objects at regular intervals, for example once per second. Upon detection of a foreign object, a communication signal is sent to the transmitter unit.

[0067] 8 is a cross-sectional view of a detection mat 800 according to aspects of the present disclosure. In some embodiments, the detection mat 800 may be a stand-alone structure that is removably coupled to a transmitter unit, such as the transmitter unit 420 of FIG. 4. In one example, the detection mat 800 may be a plug-and-play structure. The detection mat 800 includes a substrate 802 having one or more printed circuit boards (PCBs) 804. The substrate 802 may include an electrically insulating material. In some embodiments, the substrate 802 may itself be a printed circuit board (PCB).

[0068] Additionally, the group 806 of sensing coils 808 may be disposed on the substrate 802. In one embodiment, the group 806 of sensing coils 808 may be disposed on the respective PCB 804. In some embodiments, the sensing coils 808 may be disposed on the substrate 802 or may be embedded in the substrate 802 for user safety and aesthetics. The sensing coils 808 may also be printed on a flexible or regular printed circuit board. In certain embodiments, the sensing coils 808 may be printed, molded, woven, or additively manufactured on the substrate 808 or PCB 804. Note that the sensing coils 808 may each be small and therefore wound in a fine gauge wire.

[0069] A cover layer 810 may be disposed on the detection coil 808. The cover layer 810 has a first side 812 and a second side 814. In certain embodiments, the detection coil 808 may be disposed directly on the second side 814 of the cover layer 810. In some of these embodiments, the detection coil 808 may not be disposed on the PCB 804. Additionally, suitable electronic circuitry 816 may be provided on the substrate 802 to operatively couple the detection mat 800 to the drive and comparison subsystems of the detection apparatus.

[0070] The substrate 802 and the cover layer 810 may include flexible materials, rigid materials, or a combination thereof. The cover layer 810 includes a thermally conductive and electrically insulating (TCEI) material. In one embodiment, the thermally conductive and electrically insulating (TCEI) material may include an elastomer or a thermoplastic with wear-resistant fillers. In one embodiment, the elastomer may be silicone rubber. The fillers may be TCEI fillers such as aluminum oxide, aluminum nitride, beryllium oxide, boron nitride, graphene oxide, silicon carbide, and silicon nitride. Similarly, the thermoplastics may be polyolefins, polycarbonates, poly(methyl-methacrylate) (PMMA), and polyesters. In a particular embodiment, the substrate 802 and the cover layer 810 may be foldable together with the detection coil 808. In one embodiment, the detection mat 800 may be a conformable structure. To that end, when the sensing mat 800 is placed over the transmitter unit, the sensing mat 800 may be configured to generally conform to the gradients and curvatures of the surface of the transmitter unit. In some embodiments, the cover layer 810 may form an enclosure around the substrate 802, the sensing coil 808, and possibly around the electronic circuitry 816.

[0071] In one embodiment, the detection mat 800 may be integrated with a standard SAE transmitter system. In a particular embodiment, the dimensions of the detection mat 800 may be in the range of about 0.5 m to about 2.2 m. The detection mat 800 may be appropriately sized to cover the surface area of ​​the transmitter coil. In some examples, the length of the detection mat 800 may be in the range of about 0.5 m to about 2.2 m, and the width may be in the range of about 0.5 m to about 2.2 m. The thickness of the detection mat 800 may be in the range of about 1 mm to about 20 mm. In one embodiment, the detection mat 800 may be a one-piece structure. In another embodiment, the detection mat 800 may be formed by integrating independent individual parts.

[0072] Advantageously, the device, system, and method according to the present technology are simple in structure and do not require any complicated assembly. Also, the present technology is very sensitive and can detect the presence of a foreign object immediately after wireless power transmission is initiated. This helps to minimize power waste. The embodiments herein provide a sensitive, simple, and accurate system for detecting a foreign object located in the operating environment of the present WPT system. Moreover, the detection device herein can be deployed in an existing WPT system with minimal or no modifications or adjustments required to the existing WPT system. Advantageously, the device, system, and method herein realize a safe working environment during wireless power transmission of an electric vehicle (EV), which is realized by stopping the wireless power transmission of the EV upon detection of a foreign object in the operating environment of the present WPT system, or by issuing an alarm upon detection of the presence of a foreign object.

[0073] While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art, and it is therefore to be understood that the appended claims are intended to cover all such modifications and changes which are within the scope of the invention.

Claims

1. a detection mat (102) having a plurality of detection coils (106) and at least one pair of groups of the detection coils (106), the at least one pair of groups of the detection coils (106) including a first group of the detection coils (106) and a second group of the detection coils (106), the first group of the detection coils (106) including a first impedance value and the second group of the detection coils (106) including a second impedance value; one or more drive subsystems (112) operatively coupled to the detection mat (102) and configured to excite at least one pair of the group of detection coils (106); a comparison subsystem (114) operably coupled to the detection mat (102) and configured to receive differential current signals from at least one pair of the group of detection coils (106), the comparison subsystem (114) being configured to generate a control signal based on the differential current signals; A detection device (100).

2. 2. The detection device (100) of claim 1, wherein the first and second groups of detection coils (106) are arranged symmetrically with respect to a primary magnetic field of a wireless power transfer (WPT) system, and the detection device (100) is configured to detect the presence of a foreign object in an operating environment of the WPT system based on the differential current signal.

3. The detection apparatus (100) of claim 1 , wherein the first and second ones of the groups of detection coils (106) include structurally similar arrangements of the detection coils (106).

4. 2. The detection device (100) of claim 1, wherein the first and second ones of the groups of detection coils (106) are arranged in the same geometrical area on the detection mat (102).

5. The detection device (100) of claim 1 , wherein the first and second impedance values ​​are similar to one another.

6. 2. The detection device (100) of claim 1, wherein the first and second impedance values ​​are different from each other, and the detection mat (102) further includes a compensator element operably coupled to one of the first or second groups of detection coils (106) for compensating for the difference in impedance values ​​in the first and second groups of detection coils (106).

7. 2. The detection device (100) of claim 1, wherein the first and second impedance values ​​are different from each other, and the one or more drive subsystems (112) further include a drive controller configured to supply different voltage signals to the first and second groups of the detection coils (106) such that the first and second groups of the detection coils (106) are driven by the same current of similar magnitude and phase.

8. 2. The detection device (100) of claim 1, wherein the one or more drive subsystems (112) include two drive units respectively coupled to the first and second groups of the detection coils (106) and at least one drive controller, the drive controller being coupled to the drive units.

9. The detection apparatus (100) of claim 1, wherein the plurality of detection coils (106) comprises a plurality of detection regions, each of the plurality of detection regions comprising one or more groups of the detection coils (106).

10. The detection apparatus (100) of claim 9, wherein at least a portion of the one or more drive subsystems (112) are configured to be coupled to two or more of the plurality of detection regions.

11. The sensing device (100) of claim 10, wherein sensing regions in the plurality of sensing regions corresponding to a pair of sensing regions include structurally similar arrangements with respect to the sensing coil (106).

12. The detection device (100) of claim 1, wherein the detection mat (102) is a flexible mat, a conformable mat, a rigid mat, or a plug-and-play mat.

13. 2. The detection device (100) of claim 1, wherein the detection coils (106) in the group of detection coils (106) are coupled in series, parallel, or a combination of series and parallel connections, and the plurality of detection coils (106) include concentric coils, adjacently arranged coils, and parallel arrangements of coils, and the plurality of detection coils (106) are arranged in one or more layers.

14. The comparison subsystem (114) an electrical winding and magnetic core configured to receive a differential current signal from the first and second groups of detection coils (106) and generate a detection voltage signal based on the differential current signal; a comparator configured to compare the detection voltage signal with a threshold voltage signal to generate the control signal; The detection device (100) of claim 1, comprising:

15. A detection device (406), a detection mat (408) having a plurality of detection coils (106) and at least one pair of groups of the detection coils (106), the at least one pair of groups of the detection coils (106) including a first group of the detection coils (106) and a second group of the detection coils (106), the first group of the detection coils (106) including a first impedance value and the second group of the detection coils (106) including a second impedance value; one or more drive subsystems (410) operably coupled to the detection mat (408) and configured to excite at least one pair of the group of detection coils (106); a comparison subsystem (414) operably coupled to the detection mat (102) and configured to receive differential current signals from at least one pair of the group of detection coils (106), the comparison subsystem being configured to generate a control signal based on the differential current signals; A detection device (406) comprising:

1. A wireless power transfer (WPT) system, comprising: a primary power source configured to provide power in the form of an alternating current (AC) voltage signal; a transmitter unit including at least one transmitter coil, the transmitter unit configured to receive the AC voltage signal from the primary power source, the transmitter unit configured to generate a primary magnetic field in response to the received AC voltage signal, and the detection mat (102) operably coupled to the transmitter unit; a receiver unit including at least one receiver coil and configured to receive at least a portion of the primary magnetic field generated by the transmitter unit; a control unit operatively coupled to the detection device; and receiving the control signal from the detection device; and a control unit configured to control a power supply to the transmitter unit based on the control signal; A wireless power transfer (WPT) system including: A detection system (400) comprising:

16. The detection system (400) of claim 15, wherein the drive subsystem is configured to intermittently or periodically excite the detection device.

17. The detection system (400) of claim 15, wherein the one or more drive subsystems further include a communication unit configured to communicate the control signal to the control unit.

18. Utilizing (602) a sensing device comprising a sensing mat having a plurality of sensing coils and at least one pair of the sensing coils, the at least one pair of the sensing coils including a first group of the sensing coils and a second group of the sensing coils, the first group of the sensing coils including a first impedance value and the second group of the sensing coils including a second impedance value; exciting at least one pair of the detection coils (604); Measuring (606) a differential current signal from at least one pair of the detection coils; generating (608) a control signal based on the differential current signal; Transmitting the control signal (610); The method (600).

19. 20. The method (600) of claim 18, further comprising controlling (700) a power supply to a transmitter unit based on the control signal.

20. generating a sensed voltage signal based on the differential current signal (702); comparing the detected voltage signal with a threshold voltage signal to generate the control signal (704); 20. The method of claim 19, further comprising:

21. 20. The method (600) of claim 18, wherein exciting (604) at least one pair of the detection coils comprises exciting the at least one pair of the detection coils intermittently or at periodic intervals.

22. 20. The method of claim 18, wherein exciting at least one pair of the detection coils comprises exciting at different instances in time the detection coils corresponding to different pairs of the detection coils.

23. 20. The method of claim 18, wherein exciting at least one pair of the detection coils includes providing different voltage signals to the detection coils in the pair of detection coils with different impedance values ​​such that the detection coils in the pair are driven by the same current of similar magnitude and phase.

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