WO2018111820A1

WO2018111820A1 - A charging pad and a method for detecting at least one receiver device

Info

Publication number: WO2018111820A1
Application number: PCT/US2017/065723
Authority: WO
Inventors: Arun Kumar RAGHUNATHAN; Deepak Aravind; Somakumar Ramachandrapanicker; Suma Memana Narayana Bhat
Original assignee: General Electric Company
Priority date: 2016-12-15
Filing date: 2017-12-12
Publication date: 2018-06-21

Abstract

A charging pad (104) includes a power drive unit (110) to generate a first low-amplitude signal having a first frequency and a second low-amplitude signal having a second frequency. Also, the charging pad (104) includes a transmitting unit (112) including at least one transmitter coil (118) to transmit the first low-amplitude signal and the second low-amplitude signal. Further, the charging pad (104) includes a controller (126) to determine a value change from a first value to a second value of at least one parameter of at least one of the first low-amplitude signal and the second low-amplitude signal, drive the power drive unit (110) to generate a first high-amplitude signal based on the value change of the at least one parameter of the first low-amplitude signal, and generate a second high-amplitude signal based on the value change of the at least one parameter of the second low-amplitude signal.

Description

A CHARGING PAD AND A METHOD FOR DETECTING AT LEAST

ONE RECEIVER DEVICE

BACKGROUND

[0001] Embodiments of the present invention relate generally to wireless power transfer systems and more particularly to a charging pad and a method for detecting at least one receiver device of a wireless power transfer system.

[0002] In a wireless power transfer system, a charging device is used to convert an input power to a transferrable power which is transmitted to charge one or more loads such as batteries in one or more receiver devices. The one or more receiver devices are compatible with one or more of the wireless frequency standards. For example, there are currently three competing frequency standards, namely the Alliance for Wireless Power (A4WP) standard, the Wireless Power Consortium (WPC) standard, and the Power Matters Alliance (PMA) standard. The WPC standard (Qi) may be defined in a frequency range of 100 kHz to 200 kHz. The PMA standard may be defined in a frequency range of 200 kHz to 400 kHz. Further, the A4WP standard may be defined at a frequency of about 7 MHz. A conventional charging device transmits the input power at only one frequency standard irrespective of the type of the receiver device. More specifically, the charging device cannot determine the wireless frequency standard to which the receiver device is compatible before transmitting the input power. Hence, the conventional charging device cannot be used to charge the one or more receiver devices operating at different frequency standards.

[0003] Accordingly, there is a need for an improved system and method for detecting one or more receiver devices and charging such one or more receiver devices operating at different frequency standards.

BRIEF DESCRIPTION [0004] In accordance with one embodiment of the present invention, a charging pad is disclosed. The charging pad includes a power drive unit configured to generate a first low-amplitude signal having a first frequency and a second low- amplitude signal having a second frequency different from the first frequency. Also, the charging pad includes a transmitting unit operatively coupled to the power drive unit. The transmitting unit includes at least one transmitter coil configured to transmit the first low-amplitude signal and the second low-amplitude signal. Further, the charging pad includes a controller operatively coupled to the at least one transmitter coil and the power drive unit. The controller is configured to determine a value change from a first value to a second value of at least one parameter of at least one of the first low-amplitude signal and the second low- amplitude signal. The controller is further configured to drive the power drive unit to generate a first high-amplitude signal having the first frequency if the value change of the at least one parameter of the first low-amplitude signal is greater than a threshold value. The controller is further configured to drive the power drive unit to generate a second high-amplitude signal having the second frequency if the value change of the at least one parameter of the second low-amplitude signal is greater than the threshold value.

[0005] In accordance with another embodiment of the present invention, a method for detecting at least one of a first receiver device and a second receiver device by a charging pad of a wireless power transfer system is disclosed. The method includes generating, by a power drive unit, a first low-amplitude signal having a first frequency and a second low-amplitude signal having a second frequency. Also, the method includes transmitting, by a transmitting unit, the first low-amplitude signal and the second low-amplitude signal. Further, the method includes determining, by a controller, a value change from a first value to a second value of at least one parameter of at least one of the first low-amplitude signal and the second low-amplitude signal. Additionally, the method includes detecting, by the controller, the first receiver device if the value change of the at least one parameter of the first low-amplitude signal is greater than a threshold value, and the second receiver device if the value change of the at least one parameter of the second low-amplitude signal is greater than the threshold value. Furthermore, the method includes driving the power drive unit to generate a first high-amplitude signal having the first frequency if the first receiver device is detected. Also, the method includes driving the power drive unit to generate a second high-amplitude signal having the second frequency if the second receiver device is detected.

DRAWINGS

[0006] These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

[0007] FIG. 1 is a diagrammatical representation of a wireless power transfer system in accordance with an embodiment of the present invention;

[0008] FIG. 2 is a diagrammatical representation of a wireless power transfer system in accordance with another embodiment of the present invention;

[0009] FIG. 3 is a perspective view of a charging pad having transmitter coils inductively coupled to receiver devices in accordance with an embodiment of the present invention; and

[0010] FIG. 4 is a flow chart illustrating a method for detecting one or more receiver devices in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

[0011] As will be described in detail hereinafter, embodiments of a charging pad and a method for detecting one or more wireless receiver devices are disclosed. In particular, the exemplary charging pad and the method disclosed herein detects the one or more wireless receiver devices to be charged, and accordingly transmits power at a corresponding frequency to the one or more detected wireless receiver devices.

[0012] Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this specification belongs. The terms "first", "second", and the like, as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The use of "including," "comprising" or "having" and variations thereof herein are meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms "connected" and "coupled" are not restricted to physical or mechanical connections or couplings, and can include electrical connections or couplings, whether direct or indirect. Furthermore, terms "circuit" and "circuitry" and "controlling unit" may include either a single component or a plurality of components, which are either active and/or passive and are connected or otherwise coupled together to provide the described function. In addition, the term operationally coupled as used herein includes wired coupling, wireless coupling, electrical coupling, magnetic coupling, radio communication, software based communication, or combinations thereof.

[0013] FIG. 1 is a diagrammatical representation of a wireless power transfer system 100 in accordance with an embodiment of the present invention.

[0014] In the illustrated embodiment, the wireless power transfer system 100 includes the power source 102, a charging pad 104, a first receiver device 106, and a second receiver device 108. It may be noted herein that the number of receiver devices may vary depending on the application. The receiver devices 106, 108 may include mobile devices, biomedical devices, portable consumer devices, or the like. For example, receiver devices 106, 108 may include cell phones, laptops, heating ventilation and air-conditioning (HVAC) units, or the like. In an automobile industry, for example, a vehicle includes one or more charging pads 104 that are used for supplying electrical power from the power source 102 to the receiver devices 106, 108. The charging pad 104 is referred to as an electrical enclosure upon which the one or more receiver devices may be placed for charging the one or more receiver devices. In one embodiment, the wireless power transfer system 100 may be referred to as a contactless power transfer system.

[0015] The first and second receiver devices 106, 108 are compatible with one of the wireless frequency standards. For example, one of the receiver devices 106, 108 may be compatible with Alliance for Wireless Power (A4WP) standard defined at a frequency of about 7 MHz. Similarly, another receiver device may be compatible with Wireless Power Consortium (WPC) standard (Qi) defined in a frequency range of 100 kHz to 200 kHz. In certain embodiments, one of the receiver devices 106, 108 may be compatible with Power Matters Alliance (PMA) standard defined in a frequency range of 200 kHz to 400 kHz. The other receiver device may be compatible with Air Fuel Alliance standard defined at a frequency of about 6.7 MHz. In the illustrated embodiment of the invention, the first receiver device 106 is considered to be compatible with a first frequency standard such as Air Fuel Alliance standard defined at a frequency of about 6.7 MHz. The first frequency standard is referred to as a high frequency standard. Similarly, the second receiver device 108 is considered to be compatible with a second frequency standard such as WPC standard defined in a frequency range of 100 kHz to 200 kHz. The second frequency standard is referred to as a low frequency standard. It should be noted herein that the receiver devices 106, 108 may be compatible with any frequency standard and are not limited to the frequency standards mentioned herein. Further, the use of any number of receiver devices that are compatible with any number of frequency standards may be envisioned.

[0016] In conventional power transfer systems that are compatible with different frequency standards, a charging device may not supply power to each of the receiver devices at the corresponding frequency standards. In one of the conventional power transfer systems, separate charging devices having separate converters and separate power exchange coils for each frequency standard, are employed to supply power to the corresponding receiver device. The use of such separate charging devices for each frequency standard may substantially increase cost of the conventional power transfer systems.

[0017] To overcome the problems/drawbacks associated with conventional systems, the exemplary power transfer system 100 includes the charging pad 104 configured to detect the receiver devices 106, 108 positioned proximate to or on the charging pad 104. In one embodiment, the receiver devices 106, 108 may be positioned at a predetermined distance from the charging pad 104. For example, the predetermined distance may be in a range from about 5 mm to 500 mm. The charging pad 104 is configured to charge the detected receiver devices 106, 108 at the corresponding frequency standard. [0018] The charging pad 104 includes a power drive unit 110, a transmitting unit 112, and a controller 126. The power drive unit 110 is electrically coupled to the power source 102 and the controller 126. The power source 102 is configured to supply input power to the power drive unit 110. In some embodiments, the input power may be in a range from about 0.1 W to 200W. In one embodiment, the power source 102 may be a part of the charging pad 104. In another embodiment, the power source 102 may be positioned external to the charging pad 104.

[0019] The power drive unit 110 is configured to generate low-amplitude signals having different frequencies to detect presence of the first receiver device 106 and/or the second receiver device 108. The power drive unit 110 is further configured to generate high-amplitude signals having frequencies that are compatible with the detected receiver devices 106, 108. In one example, the low- amplitude signals may be low-voltage signals and the high-amplitude signals may be high-voltage signals. Further, the magnitude of the high-voltage signals are in a range of about 5V to about 1000 V. Also, the magnitude of the low-voltage signals are in a range from about 0.1 % to 30 % of the high-voltage signals. In another example, the low-amplitude signals may be low-current signals and the high-amplitude signals may be high-current signals.

[0020] In particular, the power drive unit 110 is configured to generate a first low-amplitude signal 114 having the first frequency and a second low-amplitude signal 116 having the second frequency. In one embodiment, the first frequency may be in a range from about 1.5 MHz to about 100 MHz. The second frequency may be in a range from about 10 kHz to about 1 MHz. In one embodiment, the power drive unit 110 may include one or more oscillators such as piezo-electric oscillators, integrated circuit (IC)-based electronics oscillators, converters such as hybrid (H)-bridge converters, or a combination thereof.

[0021] The power drive unit 110 is further configured to transmit the first low- amplitude signal 114 having the first frequency and the second low-amplitude signal 116 having the second frequency to the transmitting unit 1 12. The transmitting unit 112 is configured to wirelessly transmit the first low-amplitude signal 114 having the first frequency and the second low-amplitude signal 116 having the second frequency to the first and second receiver devices 106, 108 respectively. In the illustrated embodiment, the transmitting unit 112 includes a single transmitter coil 118 electrically coupled to the power drive unit 110. The single transmitter coil 118 includes a first coil segment 120 and a second coil segment 122 inductively coupled to the first and second receiver devices 106, 108. If the transmitting unit 112 receives the first low-amplitude signal 114 having the first frequency, the first coil segment 120 of the transmitter coil 118 is excited to transmit the first low-amplitude signal 114 having the first frequency to the first receiver device 106. Similarly, if the transmitting unit 1 12 receives the second low-amplitude signal 116 having the second frequency, the second coil segment 122 of the transmitter coil 118 is excited to transmit the second low-amplitude signal 116 having the second frequency to the second receiver device 108. It should be noted herein that the number of coil segments of the single transmitter coil 118 may vary depending on the application.

[0022] Further, if the first receiver device 106 is positioned proximate to or on the charging pad 104, a first receiver coil 124 of the first receiver device receives the first low-amplitude signal 114 from the transmitter coil 118. As a result, a value change from a first value to a second value of one or more parameters of the first low-amplitude signal 114 occurs at the transmitter coil 118. The one or more parameters include current, voltage, and power. In the illustrated embodiment, a detector 125 is electrically coupled to the transmitter coil 118 to measure the voltage of the first low-amplitude signal 114. Further, the detector 125 is configured to communicate the measured voltage of the first low-amplitude signal 114 to the controller 126.

[0023] In one embodiment, if the first receiver device 106 is not disposed proximate to or on the charging pad 104, voltage of the first low-amplitude signal 114 across the transmitter coil 118 has the first value. Further, when the first receiver device 106 is positioned proximate to or on the charging pad 104, the voltage across the transmitter coil 118 increases from the first value to the second value. In such an embodiment, the transmitter coil 118 and the first receiver coil 124 are magnetically or inductively coupled to each other. [0024] The controller 126 is configured to determine a value change of the measured parameter (voltage) of the first low-amplitude signal 1 14. The controller 126 is configured to detect the first receiver device 106 based on the value change of the measured parameter of the first low-amplitude signal 1 14. Particularly, the controller 126 is configured to verify whether the value change of the measured parameter is greater than a threshold value. In one embodiment, the controller 126 is configured to determine whether the voltage of the first low-amplitude signal 1 14 across the transmitter coil 1 18 has increased from the first value to the second value. Further, the controller 126 is configured to verify whether the value change or difference between the first value and the second value is greater than the threshold value. If the value change of the measured parameter is greater than the threshold value, the controller 126 is configured to generate a first control signal 128. Further, the controller 126 is configured to transmit the first control signal 128 to drive the power drive unit 1 10 to generate a first high-amplitude signal 130 having the first frequency. In one embodiment, if the power drive unit 1 10 includes a H-bridge converter, the first control signal 128 includes switching pulses at the first frequency to operate the H-bridge converter at the first frequency. The H- bridge converter is configured to operate at the first frequency to convert the DC voltage signal of the input power to the first high-amplitude signal 130 having the first frequency.

[0025] Further, the power drive unit 1 10 is configured to transmit the first high- amplitude signal 130 having the first frequency to the first receiver device 106 via the single transmitter coil 1 18. More specifically, the first coil segment 120 of the transmitter coil 1 18 is excited at the first frequency to transmit the first high- amplitude signal 130 to the first receiver coil 124 of the first receiver device 106, resulting in charging one or more loads 140 such as batteries of the first receiver device 106.

[0026] The detector 125 is configured to measure the parameter (e.g. voltage) of the second low-amplitude signal 1 16. Further, the detector 125 is configured to communicate the measured parameter of the second low-amplitude signal 1 16 to the controller 126. If the second receiver device 108 is positioned proximate to or on the charging pad 104, a second receiver coil 132 of the second receiver device 108 receives the second low-amplitude signal 116 from the transmitter coil 118 of the charging pad 104. As a result, a value change from a first value to a second value of one or more parameters of the second low-amplitude signal 116 occurs at the transmitter coil 118. In such an embodiment, the transmitter coil 118 and the second receiver coil 132 are magnetically or inductively coupled to each other. If the second receiver device 108 is not disposed proximate to or on the charging pad 104, voltage of the second low-amplitude signal 116 across the transmitter coil 118 has the first value. Further, when the second receiver device is positioned proximate to or on the charging pad 104, the voltage across the transmitter coil 118 increases from the first value to the second value.

[0027] The controller 126 is configured to determine a value change of the measured parameter of the second low-amplitude signal 1 16. The controller 126 is configured to detect presence of the second receiver device 108 based on the value change of the measured parameter of the second low-amplitude signal 116. Particularly, the controller 126 is configured to verify whether the value change of the measured parameter is greater than the threshold value. In one embodiment, the controller 126 is configured to determine whether the voltage of the second low-amplitude signal 116 across the transmitter coil 118 has increased from the first value to the second value. Further, the controller 126 is configured to verify whether the value change or difference between the first value and the second value is greater than the threshold value. If the value change of the measured parameter is greater than the threshold value, the controller 126 is configured to generate a second control signal 134. Further, the controller 126 is configured to transmit the second control signal 134 to drive the power drive unit 110 to generate a second high-amplitude signal 136 having the second frequency. In one embodiment, if the power drive unit 110 includes the H-bridge converter, the second control signal 134 includes switching pulses at the second frequency to operate the H-bridge converter at the second frequency. The H-bridge converter is configured to operate at the second frequency to convert the DC voltage signal of the input power to the second high-amplitude signal 136 having the second frequency.

[0028] Further, the power drive unit 110 is configured to transmit the second high-amplitude signal 136 having the second frequency to the second receiver device 108 via the single transmitter coil 118. More specifically, the second coil segment 122 of the transmitter coil 118 is excited at the second frequency to transmit the second high-amplitude signal 136 to the second receiver coil 132 of the second receiver device 108 resulting in charging one or more loads 142 such as the batteries of the second receiver device 108.

[0029] Referring to FIG. 2, a diagrammatical representation of a wireless power transfer system 200 in accordance with another embodiment of the present invention is depicted. The wireless power transfer system 200 is similar to the wireless power transfer system 100 of FIG. 1, except that the transmitting unit 112 includes two transmitter coils 202, 204 configured to operate at two different frequencies. Particularly, the transmitting unit 112 includes a first transmitter coil 202 configured to operate at the first frequency and a second transmitter coil 204 configured to operate at the second frequency. The first transmitter coil 202 is configured to transmit the first low-amplitude signal 114 having the first frequency and the first high-amplitude signal 130 having the first frequency to the first receiver coil 124 of the first receiver device 106. In a similar manner, the second transmitter coil 204 is configured to transmit the second low-amplitude signal 116 having the second frequency and the second high-amplitude signal 136 having the second frequency to the second receiver coil 132 of the second receiver device 108.

[0030] Further, in the illustrated embodiment, the transmitting unit 1 12 includes a first detector 206 coupled to the first transmitter coil 202 and a second detector 208 coupled to the second transmitter coil 204. The first detector 206 is configured to measure at least one parameter of the first low-amplitude signal 114. Further, the first detector 206 is configured to transmit the at least one measured parameter of the first low-amplitude signal 114 to the controller 126. Similarly, the second detector 208 is configured to measure at least one parameter of the second low- amplitude signal 116. Further, the second detector 208 is configured to transmit the measured parameter of the second low-amplitude signal 116 to the controller 126. The function of the controller 126 to generate the first high-amplitude signal 130 and the second high-amplitude signal 136 based on detection of the receiver devices 106, 108 is similar to the embodiment of FIG. 1. [0031] Referring to FIG. 3, a perspective view of the charging pad 104 in accordance with an exemplary embodiment is depicted. For ease of understanding, the charging pad 104 is described with reference to the embodiment of FIG. 2. The charging pad 104 is referred to as an electrical enclosure on which the first and second receiver devices 106, 108 are placed for charging loads 140, 142 in the corresponding receiver devices 106, 108. The charging pad 104 includes a first layer 302 having one or more first transmitter coils 202 and a second layer 304 having one or more second transmitter coils 204. In one example, at least one of the first and second layers 302, 304 may be a printed circuit board or a non-metallic layer. Further, the one or more first transmitter coils 202 are inductively coupled to the first receiver device 106 and the one or more second transmitter coils 204 are inductively coupled to the second receiver device 108. In another embodiment, the charging pad 104 may include a single layer having the one or more first and second transmitter coils 202, 204 arranged in a desired pattern.

[0032] Referring to FIG. 4, a flow chart illustrating a method 400 for detecting one or more receiver devices in accordance with an embodiment of the present invention is depicted. At step 402, a power drive unit generates a first low- amplitude signal having a first frequency and a second low-amplitude signal having a second frequency. In one example, the first frequency may be in a range from about 1.5 MHz to about 100 MHz. The second frequency may be in a range from about 10 kHz to about 1 MHz. Further, the power drive unit transmits the first low-amplitude signal having the first frequency and the second low-amplitude signal having the second frequency to the transmitting unit.

[0033] Further, at step 404, the transmitting unit transmits the first low- amplitude signal and the second low-amplitude signal. Particularly, the transmitting unit includes one or more transmitter coils electrically coupled to the power drive unit. The one or more transmitter coils are used to wirelessly transmit the first low-amplitude signal having the first frequency and the second low- amplitude signal having the second frequency to the one or more receiver devices.

[0034] Subsequently, at step 406, the controller determines a value change from a first value to a second value of at least one parameter of at least one of the first low-amplitude signal and the second low-amplitude signal. In particular, if the one or more receiver devices are positioned proximate to or on the charging pad, the one or more receiver devices receives the corresponding low-amplitude signal from the one or more transmitter coils. As a result, a value change of one or more parameters of the corresponding low-amplitude signal occurs at the one or more transmitter coils. A detector that is electrically coupled to the one or more transmitter coils, is used to measure the at least one parameter (e.g. voltage) of the low-amplitude signal. The detector communicates the one or more measured parameters of the low-amplitude signal to the controller. The controller verifies whether the value change of the at least one measured parameter is greater than a threshold value. In one embodiment, the controller determines whether the voltage of the first low-amplitude signal across the one or more transmitter coils has increased from the first value to the second value. Further, the controller 126 verifies whether the value change or difference between the first value and the second value is greater than the threshold value.

[0035] At step 408, the controller detects the first receiver device if the value change of the at least one parameter of the first low-amplitude signal is greater than the threshold value. Similarly, the controller detects the second receiver device if the value change of the at least one parameter of the second low-amplitude signal is greater than the threshold value.

[0036] In addition, at step, 410, the controller drives the power drive unit to generate a first high-amplitude signal having the first frequency if the first receiver device is detected. The controller transmits a first control signal to drive the power drive unit to generate the first high-amplitude signal having the first frequency. Further, the power drive unit transmits the first high-amplitude signal having the first frequency to the corresponding receiver device via the one or more transmitter coils of the transmitting unit for charging one or more loads of the corresponding receiver device.

[0037] Further, at step, 412, the controller drives the power drive unit to generate a second high-amplitude signal having the second frequency if the second receiver device is detected. The controller transmits a second control signal to drive the power drive unit to generate the second high-amplitude signal having the second frequency. Further, the power drive unit transmits the second high-amplitude signal having the second frequency to another corresponding receiver device via the one or more transmitter coils of the transmitting unit. The second high- amplitude signal is used for charging one or more loads of the other corresponding receiver device.

[0038] While only certain features of the present disclosure have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the present disclosure.

Claims

CLAIMS:

1. A charging pad (104) comprising: a power drive unit (110) configured to generate a first low-amplitude signal having a first frequency and a second low-amplitude signal having a second frequency different from the first frequency; a transmitting unit (112) operatively coupled to the power drive unit (110), wherein the transmitting unit (112) comprises at least one transmitter coil (118, 202, 204) configured to transmit the first low-amplitude signal and the second low- amplitude signal; a controller (126) operatively coupled to the at least one transmitter coil and the power drive unit (110) and configured to: determine a value change from a first value to a second value of at least one parameter of at least one of the first low-amplitude signal and the second low-amplitude signal; drive the power drive unit (110) to generate a first high- amplitude signal having the first frequency if the value change of the at least one parameter of the first low-amplitude signal is greater than a threshold value; and drive the power drive unit (110) to generate a second high- amplitude signal having the second frequency if the value change of the at least one parameter of the second low-amplitude signal is greater than the threshold value.

2. The charging pad (104) of claim 1, further comprising a detector (125, 206, 208) operatively coupled to the at least one transmitter coil (118, 202, 204) and the controller (126), and configured to measure the at least one parameter of the first low-amplitude signal and the second low-amplitude signal.

3. The charging pad (104) of claim 2, wherein the detector (125, 206, 208) is configured to communicate the at least one parameter of the first low- amplitude signal and the second low-amplitude signal to the controller (126).

4. The charging pad (104) of claim 3, wherein the controller (126) is configured to: generate a first control signal if the value change of the at least one parameter of the first low-amplitude signal is greater than the threshold value; and generate a second control signal if the value change of the at least one parameter of the second low-amplitude signal is greater than the threshold value.

5. The charging pad (104) of claim 4, wherein the controller (126) is configured to: transmit the first control signal to the power drive unit (110) to generate the first high-amplitude signal having the first frequency; and transmit the second control signal to the power drive unit (110) to generate the second high-amplitude signal having the second frequency.

6. The charging pad (104) of claim 1, wherein the controller (126) is configured to detect a first receiver device (106) based on the value change of the at least one parameter of the first low-amplitude signal.

7. The charging pad (104) of claim 6, wherein the controller (126) is configured to determine the value change of the at least one parameter of the first low-amplitude signal if a first receiver coil (124) of the first receiver device (106) receives the first low-amplitude signal from the at least one transmitter coil (118, 202, 204) of the transmitting unit (112).

8. The charging pad (104) of claim 1, wherein the controller (126) is configured to detect a second receiver device (108) based on the value change of the at least one parameter of the second low-amplitude signal.

9. The charging pad (104) of claim 8, wherein the controller (126) is configured to determine the value change of the at least one parameter of the second low-amplitude signal if a second receiver coil (132) of the second receiver device (108) receives the second low-amplitude signal from the at least one transmitter coil (118. 202, 204) of the transmitting unit (112).

10. A method (400) for detecting at least one of a first receiver device (106) and a second receiver device (108) by a charging pad (104) of a wireless power transfer system (100), the method comprising: generating (402), by a power drive unit (110), a first low-amplitude signal having a first frequency and a second low-amplitude signal having a second frequency; transmitting (404), by a transmitting unit (1 12), the first low-amplitude signal and the second low-amplitude signal; determining (406), by a controller (126), a value change from a first value to a second value of at least one parameter of at least one of the first low-amplitude signal and the second low-amplitude signal; detecting (408), by the controller (126), the first receiver device (106) if the value change of the at least one parameter of the first low-amplitude signal is greater than a threshold value, and the second receiver device (108) if the value change of the at least one parameter of the second low-amplitude signal is greater than the threshold value; driving (410) the power drive unit (110) to generate a first high-amplitude signal having the first frequency if the first receiver device (106) is detected; and driving (412) the power drive unit (110) to generate a second high- amplitude signal having the second frequency if the second receiver device (108) is detected.

11. The method (400) of claim 10, further comprising: measuring, by a detector (125, 206, 208), the at least one parameter of the first low-amplitude signal and the second low-amplitude signal; and communicating the at least one parameter of the first low-amplitude signal and the second low-amplitude signal from the detector (125, 206, 208) to the controller (126).

12. The method (400) of claim 11, further comprising: generating, by the controller (126), a first control signal if the first receiver device (106) is detected; and generating, by the controller (126), a second control signal if the second receiver device (108) is detected.

13. The method (400) of claim 12, further comprising: transmitting, by the controller (126), the first control signal to the power drive unit (110) to generate the first high-amplitude signal having the first frequency; and transmitting, by the controller (126), the second control signal to the power drive unit (110) to generate the second high-amplitude signal having the second frequency.

14. The method (400) of claim 10, wherein the value change from the first value to the second value of the at least one parameter of the first low- amplitude signal occurs if a first receiver coil (124) of the first receiver device (106) receives the first low-amplitude signal from the transmitting unit.

15. The method (400) of claim 10, wherein the value change from the first value to the second value of the at least one parameter of the second low- amplitude signal occurs if a second receiver coil (132) of the second receiver device (108) receives the second low-amplitude signal from the transmitting unit.

16. The method (400) of claim 10, further comprising: transmitting, by the transmitting unit (112), the first high-amplitude signal having the first frequency to the first receiver device (106) operating at the first frequency; and transmitting, by the transmitting unit (112), the second high-amplitude signal having the second frequency to the second receiver device (108) operating at the second frequency.

17. The method (400) of claim 10, wherein the at least one parameter comprises voltage, current, and power of at least one of the first low-amplitude signal and the second low-amplitude signal.