CN115693982A

CN115693982A - Metal foreign matter detection method of wireless charging transmitting terminal

Info

Publication number: CN115693982A
Application number: CN202310001176.8A
Authority: CN
Inventors: 陆钧; 王哲; 贺凡波; 葛俊杰; 马俊超
Original assignee: Jiangsu Yougan Science And Technology Co ltd; Hefei Yougan Technology Co ltd
Current assignee: Jiangsu Yougan Science And Technology Co ltd; Hefei Yougan Technology Co ltd
Priority date: 2023-01-03
Filing date: 2023-01-03
Publication date: 2023-02-03
Anticipated expiration: 2043-01-03
Also published as: CN115693982B

Abstract

The invention discloses a metal foreign matter detection method of a wireless charging transmitting terminal, which uses a power judgment method to judge whether metal foreign matter detection is needed or not, and enters a first metal foreign matter detection flow if the metal foreign matter detection is needed: and loading voltage on the excitation coil, stopping loading the voltage when the voltage reaches a preset value, and then recording a time point T1 when the current generated in each working transmitting unit reaches a peak current and a time point T2 when the current reaches an attenuation current, so as to obtain an attenuation time difference Ta = T2-T1, and when the attenuation time difference Ta is smaller than a standard time difference, judging that a corresponding working transmitting unit is located with a metal foreign body, otherwise, judging that no metal foreign body exists. The method determines whether metal foreign matter detection is needed or not through a power judgment method, and judges whether foreign matter exists or not through current oscillation time in the transmitting unit.

Description

Metal foreign matter detection method of wireless charging transmitting terminal

Technical Field

The invention relates to the field of wireless charging, in particular to a metal foreign matter detection method of a wireless charging transmitting terminal.

Background

Wireless charging is a technology that can directly transmit electric energy without physical contact, and a certain positional offset generally exists between a transmitting coil and a receiving coil of wireless charging. When a transmitting coil and a receiving coil have large offset, the transmission power, the efficiency and the like of the wireless charging system are obviously reduced; especially, when the offset exceeds a certain range, a situation that charging cannot be performed may even occur, which may affect the experience of the wireless charging technology and the popularization of the technology.

In addition, if a metal foreign object exists between the receiving coil and the transmitting coil, the efficiency of wireless charging is affected, and even the metal foreign object is heated, so that danger is generated.

Therefore, it is one of the key issues to be solved by the wireless charging technology to increase the offset range of wireless charging and improve the detection efficiency and accuracy of metal foreign objects.

Disclosure of Invention

The invention provides a metal foreign matter detection method of a wireless charging transmitting terminal. The metal foreign matter can be efficiently detected with low cost on the basis of providing a large offset range for the device to be charged.

In the method for detecting a metal foreign object at a wireless charging transmitting terminal of the invention, the wireless charging transmitting terminal comprises: an excitation coil; the transmitting coil group consists of a plurality of transmitting units connected in parallel, and each transmitting unit consists of a transmitting coil, a compensating capacitor and a control switch which are connected in series to form a loop; the exciting coil is coupled with the first transmitting unit; according to the position of the receiving coil, the corresponding transmitting unit is connected with a control switch and is used as a working transmitting unit to be coupled with the receiving coil; judging whether metal foreign matter detection is needed or not by using a power judgment method, starting wireless charging if not needed, and entering a first metal foreign matter detection flow if needed; the first metal foreign matter detection process comprises the following steps: and loading voltage on the excitation coil, stopping loading the voltage when the voltage reaches a preset value, and then recording a time point T1 when the current generated in each working transmitting unit reaches a peak current and a time point T2 when the current reaches an attenuation current, so as to obtain an attenuation time difference Ta = T2-T1, and when the attenuation time difference Ta is smaller than a standard time difference, judging that a metal foreign body exists at the position of the corresponding working transmitting unit, otherwise, judging that no metal foreign body exists.

Preferably, in the first metal foreign matter detection process, after it is determined that no metal foreign matter exists at the position of the working transmitting unit, a power determination method is used to determine whether metal foreign matter detection is required, wireless charging is started without the need, and a second metal foreign matter detection process is started if the need arises.

Preferably, the second metallic foreign matter is detected as: keeping a first control switch of the first transmitting unit closed, then sequentially switching on control switches of the non-working transmitting units, and loading a working frequency to an exciting coil

Of an alternating current signal of, wherein L _D3 For equivalent inductance after connected non-working unit, C _D3 The equivalent capacitance is the equivalent capacitance behind the accessed non-working unit; the detected phase difference is then compared for two parameters: the first parameter is as follows: a voltage of the alternating current signal applied to the exciting coil; and a second parameter: the first transmitting unit and the non-working transmitting unit which is connected in each time are connected in parallel to form the total current of a circuit; and if the value of the detected phase difference is within the threshold range, determining that no metal foreign matter exists, otherwise, determining that the metal foreign matter exists.

Preferably, the threshold range obtaining method is as follows:

presetting a first phase difference threshold phi _T And two threshold ranges are divided:

the first threshold value range is [0 DEG, phi ] _T ]；

The second threshold value range is [ phi ] _T ’，360°]Wherein phi _T ’=360°-Ф _T 。 _.

Preferably, the phase difference threshold value phi is preset _T The acquisition method comprises the following steps:

Ф _T ＝[（t3-t1）/(t2-t1)]*360°

wherein t1 is: a rising edge trigger time of a first cycle of a voltage of an alternating current signal applied to the excitation coil; t2 is: a rising edge trigger time of a second cycle of the voltage of the alternating current signal applied to the excitation coil; t3 is: the rising edge of the first cycle of the total current of the circuit after the transmitting unit is switched on triggers the time.

Preferably, the power determination method is: and judging whether the total transmission power of the working transmitting unit meets the rated transmission requirement, if so, detecting the metal foreign matters, and if not, detecting the metal foreign matters.

The method determines whether metal foreign matter detection is needed or not through a power judgment method, and judges whether foreign matter exists or not through current oscillation time in the transmitting unit.

In some embodiments, two metal foreign object detection methods are provided, the first metal foreign object detection process can quickly screen whether there is a metal foreign object, and the second metal foreign object detection process can find a metal foreign object that affects the operation of the transmitting coil assembly. One coil can be detected quickly and the other coil can be found out that the coil does not work due to foreign matters. The detection of the metal foreign bodies is completed from two aspects, and the high-efficiency and safe completion of wireless charging is guaranteed.

Drawings

Fig. 1 is a flow chart of a method for detecting a metal foreign object at a wireless charging transmitting terminal according to the present invention;

fig. 2 is a schematic diagram of a first metal foreign object detection flow in the metal foreign object detection method of the wireless charging transmitting terminal according to the present invention;

FIG. 3 is a diagram illustrating a corresponding structure of the method for detecting a metal foreign object at a wireless charging transmitter according to the present invention;

FIG. 4 is a schematic diagram of a receiving coil and a transmitting coil set in the method for detecting metallic foreign objects at a wireless charging transmitting terminal according to the present invention;

FIG. 5 is a schematic diagram of wirelessly charging an electronic device;

fig. 6 is a schematic diagram of one of the transmitting units of fig. 3.

Detailed Description

Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the drawings are illustrative only and should not be construed as limiting the invention.

The invention discloses a metal foreign matter detection method of a wireless charging transmitting terminal, and in order to better understand the working process of the method, the structure and the simple principle of wireless charging are explained first.

The method of the present application is applicable to a specific wireless charging transmitting terminal structure, and referring to fig. 3, fig. 4 and fig. 6, the wireless charging transmitting terminal (also referred to as transmitting apparatus) includes:

the excitation coil 1, the transmitting coil group 4 and other necessary components, such as a power supply 6, an operating circuit, an electric signal acquisition circuit 2, a controller 3 and the like. The working circuit comprises an inverter circuit 7 and a compensation circuit 8.

In wireless charging, the device to be charged has a receiving means (receiving coil 51) coupled with the transmitting coil L in the transmitting coil group 4.

With continued reference to fig. 3 and 6, the transmitting coil set 4 is composed of a plurality of transmitting units connected in parallel, each transmitting unit is composed of a transmitting coil L, a compensation capacitor C and a control switch S connected in series to form a loop, which may also be referred to as a transmitting loop, that is, the transmitting unit is formed by at least the above three parts. The inductance of the transmitting coil L is connected in series with the compensation capacitor C to form an LC transmitting loop, which may also be referred to as an LC transmitting unit. In order to distinguish the components within a plurality of different transmitting units, each transmitting unit may be numbered with the corresponding transmitting coil L, compensating capacitor C and control switch S included therein, e.g., the second transmitting unit includes the second transmitting coil L2, the second compensating capacitor C2 and the second control switch S2, etc. In FIG. 3, the transmitting coils in each transmitting unit are denoted by L1-Ln, the compensating capacitors in each transmitting unit are denoted by C1-Cn, and the control switches in each transmitting unit are denoted by S1-Sn.

One end of the compensation capacitor C of each transmitting unit is connected to each other, and the other end of the compensation capacitor C is also connected to each other, that is, all transmitting units are connected in parallel. I.e. a plurality of transmitting units are connected in parallel to form a transmitting coil set 4.

The control switch S of each transmitting unit is controlled by the controller 3, and the transmitting loop is switched on or off by switching on or off the control switch S, so that whether the transmitting unit works or not can be controlled. In operation, the receiving coil 51 is coupled to a part of the transmitting coils L of the transmitting units, and the transmitting units in which these transmitting coils L coupled to the receiving coil 51 are located, as the operating transmitting units, have their control switches S closed, so that they can supply wireless power to the receiving coil 51.

In the present application, the excitation coil 1 is coupled to the transmitting coil L of a transmitting unit, and for convenience of description, the transmitting unit is referred to as a first transmitting unit, namely, the first transmitting coil L1

How to determine which transmitting units can be used as working transmitting units. This will be explained in detail below.

Based on the above description, the structure of the wireless charging transmitting terminal can be known. Then, after the working transmitter units are selected, it is necessary to determine whether metallic foreign matter exists between the working transmitter units and the receiver coil 51.

Referring to fig. 1, a power determination method (how the power determination method is performed will be described in detail below) is used to determine whether metal foreign object detection is required, wireless charging is started without metal foreign object detection, and a first metal foreign object detection process is performed if metal foreign object detection is required.

The first metal foreign matter detection process mainly judges whether metal foreign matters exist or not through the attenuation time difference. After the detection is finished, if no foreign matter is found, the power judgment method is needed again to judge whether metal foreign matter detection is needed. If the metal foreign matter detection is still determined to be needed, the first metal foreign matter detection process is described, and the metal foreign matter above the working transmitting unit may not be detected. The reason may be that some non-operating transmitting units should be operating transmitting units, but are determined as non-operating transmitting units due to the presence of foreign objects above the non-operating transmitting units (the principle of the determination is described below), and therefore the non-operating transmitting units are checked when entering the second metal foreign object detection process.

The specific scheme is as follows:

the first step is as follows: and judging whether metal foreign matter detection is needed or not by using a power judgment method, starting wireless charging if metal foreign matter detection is not needed, and entering a first metal foreign matter detection flow if metal foreign matter detection is needed. And judging whether the total transmission power of the working transmitting unit meets the rated transmission requirement, if so, detecting the metal foreign matters, and if not, detecting the metal foreign matters. The power judgment method comprises the following steps: and judging whether the total transmission power of the working transmitting unit meets the rated transmission requirement, if so, detecting the metal foreign matters, and if not, detecting the metal foreign matters.

The first metal foreign object detection process may also be referred to as a correlation quality factor (Q value) detection process, and specifically includes:

when a voltage is applied to the excitation coil 1, the input to the excitation coil 1 is disconnected when the applied voltage rises to a predetermined value. In each voltage loading and load disconnection process, one working unit is switched in, namely the control switch S of one working unit is closed. The transmitting unit that is switched in as the voltage rises generates current by induction.

Because the emission unit is an undisturbed closed LC loop, energy freely oscillates in the loop of the unit, and the oscillation is expressed as induced current and gradually attenuates to 0 from a peak value, and the detection of the metal foreign matters can be realized by detecting the attenuation rule of the induced current.

Specifically, as shown in fig. 2, a peak current value I1 and an attenuation current value I2 are determined in advance, a time point T1 when the current I1 generated in the transmitting unit is recorded, and a time point T2 when the current I2 is generated are recorded, and an attenuation time difference Ta, ta = T2-T1, is obtained. The resulting change can determine the change in the transmit unit figure of merit. The quality factor of the transmitting coil is reduced due to the existence of the metal foreign matters on the transmitting coil of the transmitting unit, so that the attenuation is fast, and the attenuation time difference (T2-T1) is small. And comparing the measured attenuation time difference (T2-T1) with a standard time difference (the standard time difference can be the time difference without the metal foreign matters and can be generally detected and obtained in advance), and if the attenuation time difference is smaller than the standard time difference, namely the attenuation is faster, judging that the metal foreign matters exist above the starting ray ring.

If the first metal foreign matter detection process judges that foreign matters exist, an alarm or a prompt needs to be given out so as to manually clear the metal foreign matters.

The method can also be called as an oscillation attenuation method, because an independent closed loop is needed, if the existing two-coil structure is adopted, an LC loop for auxiliary detection needs to be added on a main circuit, the auxiliary LC loop is used independently through switching during detection, or a complex change-over switch is added in the main circuit, so that a system cuts off a power supply driving circuit, a load and other circuits which possibly generate interference during detection, and the method increases the complexity of the system and increases the cost of the system. Based on the application, the parallel loop formed by the transmitting coil group 4 is a relatively independent loop, and the system can realize functions without adding any other circuit or element.

And after the first metal foreign matter detection process, judging whether metal foreign matter detection is needed or not by using a power judgment method, starting wireless charging if not needed, and entering a second metal foreign matter detection process if needed.

For the second power utilization judging method, after the first metal foreign matter detection process judges that no metal foreign matter exists, the power utilization judging method is required to be used again for verification, and if the verification result is that metal foreign matter detection is not required, the subsequent wireless charging process can be started. If it is determined that the metal foreign matter detection is required, the second metal foreign matter detection process is determined to be retested.

The second metal foreign matter detection process mainly judges whether some transmitting units are misjudged as non-working transmitting units due to the existence of metal foreign matters, and the metal foreign matters at the positions of the transmitting units need to be detected and processed, so that the transmitting units are recovered to be working transmitting units.

The second metal foreign matter detection process is as follows:

keeping the first control switch S1 closed, disconnecting other control switches S, sequentially switching on the control switches S of the non-working transmitting units, and switching on the control switch S of only one non-working transmitting unit at a time. An alternating current signal (micro-power) is loaded on the exciting coil 1, so that the total inductive reactance and the total capacitive reactance in one non-working transmitting unit and the first transmitting unit which are connected in parallel are mutually counteracted, and the two transmitting units (transmitting loops in the two transmitting units) which are connected in parallel are resonated. The working frequency of the AC signal is determined by the parameters of the non-transmitting unit without the receiving coil 51 above, but the first transmitting unit with the receiving coil 51 above, i.e. the non-transmitting unit only has the inductance of the transmitting coil, while the transmitting coil L1 of the first transmitting unit has mutual inductance with the receiving coil 51 and the exciting coil 1, respectively, and the working frequency f is set according to the compensation capacitance values of the non-operating transmitting unit and the first transmitting unit ₃ Satisfy the requirement of

Wherein L is _D3 The inductance of the non-working transmitting coil and the first transmitting coil L1, the mutual inductance of the first transmitting coil L1, the receiving coil 51 and the exciting coil, and C are equivalent inductances of two transmitting units connected in parallel (namely, after the non-working unit is connected, the equivalent inductance of an equivalent circuit formed by the connected non-working unit and the first transmitting coil L1), and _D3 is the equivalent capacitance of two transmitting units connected in parallel. The transmitting means operating at a frequency f ₃ An ac input is applied to the excitation coil 1,the two transmitting units in parallel can be brought into resonance when no receiving coil 51 is present above the non-operative transmitting coil.

The detected phase difference Φ j of the following two parameters is then compared:

the first parameter is as follows: the voltage of the alternating current signal applied to the excitation coil 1.

And a second parameter: the first transmitting unit is connected with the non-working transmitting unit which is connected in parallel each time to form the total current of a circuit.

When the detected phase difference Φ j is within the threshold range, it is judged that the receiving coil 51 does not exist above the non-operating transmitting coil (because the operating frequency f at this time is ₃ The determination of (1) is based on the absence of the receiving coil 51 above the non-transmitting unit, therefore, within the threshold value, it indicates that there is no receiving coil 51 above and no foreign matter influence); when the detected phase difference phi j is not in the threshold range, the transmitting coil of the non-working transmitting unit is judged to have abnormal conditions such as metal foreign matters.

The threshold range acquisition mode is as follows:

the first threshold value range is [0 DEG, phi ] _T ]；

The second threshold value range is [ phi ] _T ’，360°]Wherein phi _T ’=360°-Ф _T ；

Presetting phase difference threshold phi _T The acquisition method comprises the following steps:

Ф _T ＝[（t3-t1）/(t2-t1)]*360°

wherein t1 is a rising edge trigger time of a first period of a voltage of an alternating current signal applied to the exciting coil 1;

t2 is: the rising edge trigger time of the second cycle of the voltage of the alternating current signal applied to the exciting coil 1;

t3 is: the rising edge of the first cycle of the total current of the circuit after the transmitting unit is switched on triggers the time.

The above three values are predetermined values, not values obtained during the detection process. I.e. the value detected after accessing a transmitting unit in normal working condition, and then the calculation is carried out based on the value.

The wireless charging system reports the abnormal condition of the metal foreign matter through the controller 3, and after the foreign matter is removed, the process of re-identifying the position of the receiving coil 51 on the transmitting coil group 4 is carried out, if the number of the receiving coils which can be used as working transmitting coils is increased after the foreign matter is removed, so that the total transmission power of the working transmitting coils meets the requirement, the wireless power transmission can be started; if the total transmission power of the operating transmitting coil still cannot meet the requirement and no metallic foreign object is found after the metallic foreign object detection, the number of the transmitting coils tightly coupled between the receiving coil 51 and the transmitting coil set 4 may be too small to initiate wireless charging, and the position of the receiving device needs to be changed to align with the transmitting coil set 4 to couple more transmitting coils or to start in a manner of reducing the transmitting power after the system confirmation.

Of course, since the exciting coil is disposed at the center of the transmitting coil set 4 in the present embodiment, when the receiving coil 51 is located above the first transmitting coil, another plurality of transmitting coils are generally covered, and the situation that the number of operating transmitting coils is too small is unlikely to occur. In addition, when the wireless charging system is designed according to the application, the transmittable power of each transmitting coil can be designed to be larger, and when abnormal conditions such as metal foreign matters occur, the requirement of the total transmission power can still be met under the condition that the number of the working transmitting coils is reduced due to the fact that the abnormal transmitting coils are disconnected. When the number of active transmitting coils exceeds the requirement of the total transmission power, the active transmitting units can also be selected to optimize the power transmission, for example, several active transmitting coils relatively concentrated together are selected, and the rest of the active transmitting units are disconnected to ensure that the electromagnetic field is more concentrated when the power is transmitted.

In the plurality of transmitting units, the transmitting coil L of one transmitting unit, which is referred to as a first transmitting unit, can be coupled to the exciting coil 1, and the corresponding transmitting coil is the first transmitting coil L1. Since there are a plurality of transmitting units, there are a plurality of corresponding transmitting coils L, and there may be only one exciting coil 1, and in terms of size, the size of one exciting coil 1 is equivalent to that of one transmitting coil L, it cannot be satisfied that one exciting coil 1 is coupled with the transmitting coils L in all transmitting units at the same time. The excitation coil 1 is located below the set of transmission coils 4, i.e. on the side relatively remote from the receiving coil 51.

For convenience of explanation, the transmitting unit in which the transmitting coil L coupled to the exciting coil 1 is located is named as a first transmitting unit. The first transmitting unit may be any one of the transmitting units, and preferably, is located at a central portion of the transmitting coil assembly 4.

The following is how to determine whether there is a receiving coil 51 above the transmitting unit, i.e. which transmitting units need to be the working transmitting units. It is possible to divide into four major steps, which are the basic steps, and in the following description, it is possible to interleave other steps among the four steps.

Step 1, an initial step may be counted, in which the excitation coil 1 is coupled to the first transmission coil L1 of the first transmission unit, the first control switch S1 is closed, and the other control switches S are opened. As explained above, the transmitting unit coupled with the excitation coil 1 is defined as the first transmitting unit.

Step 2, which may be referred to as an action step, is mainly an action of the control switches S, and on the basis of keeping the first control switch S1 closed, the other control switches S are closed in sequence, and one control switch S is closed each time. Typically in sequence. At each closing, the excitation coil 1 is loaded with a frequency of

Of the alternating current signal, the frequency being denoted as the operating frequency f ₂ The source of this frequency is described below. Wherein L is _D2 An equivalent inductance C consisting of the first transmitting unit, the transmitting unit in which the control switch S is closed each time, the exciting coil 1 and the receiving coil 51 _D2 Is the equivalent capacitance of the first transmitting unit and the transmitting unit where the control switch S is closed each time.

Taking the example of closing the second transmitting unit, the first transmitting unit and the second transmitting unit form a parallel circuit, and the first transmitting coil L in the circuit1 has mutual inductance with the exciting coil 1 and the receiving coil 51, and the second transmitting coil L2 has mutual inductance with the receiving coil 51. According to the principle of circuit analysis, the parallel circuit can be equivalently converted into an LC circuit, namely L _D2 The inductance is an inductance of an equivalent circuit, and is hereinafter referred to as an equivalent inductance; c _D2 The capacitance of the equivalent circuit is referred to as the equivalent capacitance hereinafter. Since the inductance of the transmitting coil L in each transmitting unit and the capacitance of the compensating capacitor C are known, L is mentioned above _D2 And C _D2 Are known.

For convenience of explanation, the transmitter unit to be identified is called the transmitter unit to be identified, that is, whether the receiver coil 51 is located at the position to be identified, each time the control switch S is closed.

Typically, each transmitting unit is substantially identical, and therefore, the equivalent inductance L is closed regardless of which transmitting unit' S control switch S is closed _D2 And an equivalent capacitance C _D2 The same value is also applied, so that the frequency of the ac signal applied to the excitation coil 1 is the same each time the control switch S is closed.

Of course, the parameters of each transmitter unit may be different, and these parameters may be known in advance, so that each time a different control switch S is closed, the corresponding adjustment frequency is sufficient.

Step 3, a comparison step, wherein step 3 is performed after each control switch S is closed in step 2, that is, step 3 is performed once after each control switch S is closed. The comparison in step 3 is a first phase difference Φ 1 between the following two parameters.

And (3) parameters III: a voltage of an alternating current signal applied to the exciting coil 1; and the fourth parameter is as follows: the first transmitting unit is connected in parallel with the transmitting unit where the control switch S is closed each time to form the total current of the circuit. Taking the second control switch S2 closed as an example, the data four is: the total current of the circuit formed by the first transmitting unit and the second transmitting unit in parallel. For the acquisition of the data 2, sampling resistors may be disposed at two ends of a parallel point of the parallel transmitting units (e.g., a parallel point of the first transmitting unit and the second transmitting unit), the electric signal collecting circuit 2 may input a current signal flowing through the sampling resistors to the controller 3 after processing of amplitude, waveform, and the like, and the controller 3 may measure a current phase angle.

And determining whether a receiving coil 51 is arranged at a position corresponding to the transmitting unit where the control switch S is positioned, which is closed each time, according to the value of the first phase difference phi 1, and if so, defining the corresponding transmitting unit as a working unit. The specific values and the judgment principles are explained in detail below.

Step 2 closes all the control switches S once, and step 3 determines whether there is a receiving coil 51 at the position corresponding to each transmitting unit. All the transmitting units judged to have the receiving coil 51 will be defined as the operating units.

And 4, a charging step, namely closing the control switches S of all the working units when wireless charging is carried out.

How to judge whether or not the receiver coil 51 is present based on the first phase difference Φ 1 is explained below, and the first threshold range and the second threshold range described above are also used.

And the first threshold value range is [0 DEG, phi ] _T ]It may be indicated that parameter four lags parameter three.

The second threshold value range is [ phi ] _T ’，360°]It may be shown that parameter four leads parameter three.

When the first phase difference Φ 1 is not within the two threshold ranges, it is determined that the receiving coil 51 is not present at the position corresponding to the transmitting unit where the control switch S is closed, and conversely, the receiving coil 51 is present.

When the control switch S is switched on the transmitting unit to be identified, the compensation capacitor C of the transmitting unit to be identified is communicated with the transmitting coil L to form a series circuit. The excitation coil 1 is loaded with a signal (ac signal of micropower), the working frequency of the signal is set, and one transmitting unit to be identified and the first transmitting unit can be in a resonance state when the receiving coil 51 is arranged above the transmitting unit to be identified and the first transmitting unit, that is, the equivalent inductive reactance and the equivalent capacitive reactance in the parallel transmitting unit to be identified and the first transmitting unit are mutually offset, and the two parallel transmitting units resonate.

Operating frequency of AC signalIt can be determined according to the parameters of the transmitting end that when the transmitting coil of the transmitting unit to be identified and the transmitting coil of the first transmitting unit have the receiving coil 51 above, the transmitting coil L of the transmitting unit to be identified and the receiving coil 51 have mutual inductance, and the transmitting coil L1 of the first transmitting unit respectively has mutual inductance with the receiving coil 51 and the exciting coil 1, the working frequency f is made to be the same according to the compensation capacitance of the transmitting unit to be identified and the first transmitting unit ₂ Satisfy the requirement of

At the transmitting end at the operating frequency f ₂ The signal applied to the excitation coil 1, which is also an ac signal, can bring the two parallel transmitting units into resonance.

The first transmitting unit is located in the central region of the transmitting coil set 4 consisting of all transmitting units. In this way, it is generally ensured that the first transmitting unit is always in the vicinity of the receiving coil 51 when the device to be charged is placed. Taking wireless charging of a mobile phone or other electronic devices as an example, as shown in fig. 5, an outer frame a is an area where the mobile phone is placed, an inner frame B is an area where the transmitting coil set 4 is located, and a receiving coil 51 (also shown by a dotted line in fig. 5) of the mobile phone (shown by a dotted line in fig. 5) is generally located in a central area of the mobile phone, so that the receiving coil 51 and the first transmitting unit can be coupled as long as the mobile phone is placed in the outer frame a.

In the above, the transmitting unit where the transmitting coil L to be coupled with the exciting coil 1 is located is named as a first transmitting unit, it can be seen that the first transmitting unit is artificially defined, and in combination with the description in the previous paragraph, the transmitting unit located in the central area of the transmitting coil group 4 is selected, and the transmitting unit is coupled with the exciting coil 1 to be the first transmitting unit, so that the receiving coil 51 at the location of the first transmitting unit can be ensured.

Of course, in some embodiments, the first transmitting unit is not necessarily arranged in the central region of the transmitting coil set 4, and whether there is the receiving coil 51 above the first transmitting unit needs to be determined by the following method.

The process is carried out before step 2, already in step 1The first control switch S1 is closed and then the excitation coil 1 is supplied with a frequency of

Of the alternating current signal, the frequency being denoted as the operating frequency f ₁ . Wherein L is _D1 The equivalent inductance of the first transmitting unit includes the inductance of the first transmitting coil L1 and the mutual inductance with the exciting coil and the receiving coil, C _D1 Is the equivalent capacitance of the first transmitting unit.

Comparing a second phase difference Φ 2 between:

and V, parameters: the voltage of the alternating current signal applied to the excitation coil 1. The content of the parameter is identical to that of the parameter 1, and the specific numerical values are not necessarily the same. And a sixth parameter: current of the first transmitting unit.

And determining whether a receiving coil 51 exists at the position corresponding to the first transmitting unit according to the value of the second phase difference phi 2. The judgment method is the same as the judgment method of the first phase difference Φ 1, and is compared with two divided threshold ranges, and when the second phase difference Φ 2 is within the two threshold ranges, it is judged that the receiving coil 51 is provided above the first transmitting unit.

If the position corresponding to the first transmitting unit has the receiving coil 51, the step 2 is carried out, and the first transmitting unit is defined as a working unit; if there is no receiving coil 51, the step 2 is proceeded to, but the first transmitting unit is not defined as the working unit.

Although step 2 is performed regardless of whether the receiving coil 51 is present or not, the frequency of the ac signal applied to the exciting coil 1 may change in step 2. From the formulation point of view, it is still

However, since there is no receiving coil above the first transmitting unit, each time the control switch S is closed, the equivalent circuit formed by the connected transmitting unit, the first transmitting unit, the exciting coil 1 and the receiving coil 51 is changed, and thus the corresponding equivalent inductances are also changed.The expression for frequency is not changed, but the values in the formula are changed.

For convenience, we default to the first radiation unit being arranged in the center of the transmitting coil set 4, and as long as the device to be charged is placed, the receiving coil 51 is ensured above it, so that the above-mentioned variation of the values can be avoided. Of course, the first transmitting unit may be always located at a position where there is no receiving coil 51, for example, the first transmitting unit is placed outside the transmitting coil set 4, that is, outside the inner frame B, and there is no receiving coil 51 located above the first transmitting unit, so that the expression of the above formula is not changed, but the corresponding value is a changed value, because there are few receiving coils above the first transmitting unit, and therefore the parameter value of the equivalent circuit changes, resulting in a change in the final value.

In the prior art, when identifying the position of the receiving coil 51, the wireless charging system with multiple transmitting coils is generally determined by measuring the reflected impedance generated in the transmitting device by the receiving device, or by measuring the mutual inductance between the transmitting coil and the receiving coil 51, because in the measuring process, current needs to be loaded in the transmitting device and the receiving device and parameters of the current need to be detected, and the current needs to flow through a power conversion circuit and a load in the receiving device, and the like, and the measurement result is influenced by different factors of the load and the like. In order to solve this problem, the prior art would short-circuit the load in the receiving device when identifying the position of the receiving coil, so as to avoid the influence caused by the load end. For example, patent CN114050668B connects two points at the output end of an impedance matching circuit in a receiving apparatus when measuring the mutual inductance value M of a coil, and disconnects a rectifying and smoothing circuit and a load end at the rear end by short-circuiting the two points.

In this embodiment, the transmitting coil set 4 is an independent loop formed by a plurality of transmitting units connected in parallel, the equivalent impedance of the 2 transmitting units connected in parallel is related to the mutual inductance of the exciting coil 1 and the receiving coil 51 in addition to the inductance and the compensation capacitance of the reflecting coil itself, the mutual inductance of the first transmitting coil L1 and the exciting coil 1 is a fixed value, and the mutual inductance of the receiving coil 51 and other transmitting coils is a target parameter to be detected, and the result is reflected on the phase difference between the voltage of the transmitting device and the current of the receiving device, and during the phase difference measurement, the parallel operating transmitting units are not affected by the power conversion circuit, the load and the like of the receiving device. Therefore, the beneficial advantage of this embodiment is that it is not necessary to add a switching circuit on one side of the receiving device, i.e. it is not necessary to modify the circuit of the receiving device, so that on one hand, the number of components of the system can be reduced, and the cost and complexity can be reduced, and on the other hand, the wireless charging receiving device has better compatibility with the existing circuit with a mature design or the wireless charging receiving device already used in a large scale, such as a mobile phone.

After the identification of all the transmitting units is completed, it is necessary to determine whether the total transmission power of all the working units meets the requirement of system power transmission, specifically, it is determined according to the transmission power that each transmitting coil can carry and the number of the transmitting coils that can be used as working transmitting coils (the transmitting coils in the working units are the working transmitting coils), the transmission power of each transmitting coil is determined by the minimum working voltage that occurs when it is working and the maximum current-carrying capacity of the transmitting coil, the total transmission power of all the working transmitting coils is greater than the maximum transmission power of the system, so that power transmission can be started, otherwise, the power transmission cannot be started, or the power transmission is started in a manner of reducing the transmission power.

After the wireless charging meets the charging working condition, the control switch S of the transmitting unit where the working transmitting coil is located is turned on, for example, as shown in fig. 3, on the premise of keeping the control switch of the first transmitting unit turned on, the control switches of the xth transmitting unit to the yth transmitting unit are turned on, the transmitting device loads the micropower alternating current signal on the exciting coil 1, and the working frequency of the alternating current signal is set as the working frequency f during the wireless charging power transmission ₄ Then, the fourth phase difference Φ 4 is determined from the following parameter 7 and parameter 8.

And a seventh parameter: the total current of the operating transmitting coil.

And eight parameters: the voltage applied to the excitation coil 1.

For example, if at the operating frequency f ₄ The exciting coil 1 is loaded with an alternating current signal, and the working transmitting unit in the transmitting coil group 4 is positionedThe resonance state, the recorded phase difference Φ 4 should be approximately 0.

After the steps are completed, the wireless charging system starts power transmission, and the direct current of the power supply is converted into the working frequency f through the inverter circuit ₄ The high-frequency alternating current of (1), which flows through the compensation circuit, generating a high-frequency alternating electromagnetic field around the exciting coil; since the first transmitting coil L1 is tightly coupled to the exciting coil 1, and is within the range of the high-frequency alternating magnetic field, the first transmitting coil L1 will generate an induced voltage, generate a current in the first transmitting unit, and through the parallel working transmitting units, generate an induced high-frequency alternating electromagnetic field on the first transmitting coil L1 and the remaining working transmitting coils (for example, the transmitting coils Lx to Ly in fig. 3), the receiving coil 51 induces the alternating electromagnetic field generated by the exciting coil 1 and the working transmitting coils to generate an induced voltage, and the induced voltage is converted into a direct current by the rectifying and filtering circuit after passing through the compensation circuit, so as to charge loads such as a battery. The controllers of the receiving device and the transmitting device interact the charging requirement of the load and the control parameters of the two sides through the communication link, and the transmitting device adjusts the charging requirement.

In the power transmission process of wireless charging, a high-frequency alternating electromagnetic field is generated above the transmitting coil set 4, and whether new metal foreign matters exist between the transmitting coil set 4 and the receiving coil 51 needs to be continuously detected. The presence of metallic foreign bodies on the surface of the transmitting coil assembly 4 may degrade the quality factor of the transmitting coil assembly 4, resulting in a reduction of the transmission capability, and while keeping the output power at the load side unchanged, an increase in the input power is exhibited at the transmitting device, even if the difference between the input power and the output power becomes large. Normally the difference between the input power and the output power is due to system losses in the power conversion, which power difference can theoretically be obtained in advance according to the transmission efficiency. Therefore, in the prior art, the metal foreign matter is often identified by detecting the change of the power difference value, that is, when the change of the difference value exceeds the threshold value set accordingly, the change of the difference value can be considered as the increase of the power loss due to the occurrence of the metal foreign matter, and accordingly, the metal foreign matter can be considered to be present on the transmitting coil group 4.

In the application, whether metal foreign matters appear or not can be monitored in the wireless charging process through the method. However, for some metal foreign bodies with smaller volume, the proportion of the energy lost in the total energy of wireless transmission is smaller, the change of the power difference caused by the metal foreign bodies is smaller relative to the total transmission power, and the metal foreign bodies cannot be detected by adopting a power difference detection mode. In addition, the power conversion circuit for wireless charging is also affected by voltage, frequency, temperature and other factors, which may cause some fluctuation changes of the difference between the input power and the output power, and in addition, the error of power measurement, the offset between the coils and other factors may also cause the failure of the change mode of the detected power difference.

In order to make up for the deficiency of the mode of detecting the change of the power difference value, the method further detects the metal foreign matters by adopting the mode of detecting the phase angle difference under the condition that the fluctuation of the power difference value is small, namely the change of the power difference value is in the set threshold range, so that the identification precision is improved, and the identification blind area is reduced. Specifically, the fifth phase difference Φ 5 of the following two parameters is compared:

nine parameters: the voltage of the alternating current signal applied to the excitation coil 1.

Ginseng, dozens of: the electric signal acquisition circuit detects the total current signals of the first transmitting unit and all the working units which are connected in parallel.

The fifth phase difference Φ 5 is different from the reference of comparison of the above detected phase difference Φ j and the first phase difference Φ 1, so a new threshold range is introduced, which is determined as follows:

let the pre-recorded reference phase angle difference be phi _S Let the previously correspondingly determined threshold deviation be phi _T Both values are base values that are tested and recorded in advance. Then correspondingly determining the new threshold value range as phi _B ~Ф _A Wherein phi _A =Ф _S +Ф _T ，Ф _B =Ф _S -Ф _T . At this time phi _S Not equal to 0 deg. and 360 deg., nor close to 0 deg. and 360 deg., we refer to this range as the first range. When reference phase angle difference phi _S At or near 0 and 360, Φ may occur _A Greater than 360 DEG or phi _B Less than 0 deg.. When phi _A >At 360 deg., let phi _A ^’ =Ф _A 360 °, the corresponding new threshold ranges become 2, where the first new threshold range is 0

~ Ф _A ^’ And the second new threshold range is phi _B ~ 360

We refer to these two ranges as the second range. When phi _B <At 0 deg., let phi _B ^’ =Ф _B +360 °, the corresponding first and second new threshold ranges have changed, specifically: the first new threshold range becomes 0

~ Ф _A The second new threshold range becomes Φ _B ^’ ~ 360

. When the fifth phase difference Φ 5 is within the new threshold range (included in the first new threshold range or the second new threshold range), it is determined that the wireless charging system is operating normally, and no metal foreign object is present above the transmitting coil group 4; otherwise, judging that a metal foreign body appears above the working transmitting coil. The new threshold range is phi _A And phi _B The values of (a) are related and only one of the three ranges mentioned above will occur.

For example, [ phi ] _S =10°、Ф _T (= 2 °), can know [ phi ] _A =12°、Ф _B =8 °, new threshold range is [8 °,10 ° ]]I.e. the first range.

If, phi _S =359°、Ф _T (= 2 °), can know [ phi ] _A =361°、Ф _B =357°、Ф _A ^’ =1 °, new threshold range becomes two, first new threshold range [0 °,1 ° ]]Second new threshold range [357 °,360 ° ]]I.e., the second range.

If, phi _S =1°、Ф _T (= 2 °), can know [ phi ] _A =3°、Ф _B =-1°、Ф _B ^’ =359 °, the first new threshold range becomes [0 °,3 ° ]]The second new threshold range becomes 359 °,360 °]I.e., a third range.

Only one of the three ranges mentioned above will occur.

To ensure the safety of charging transmission, the wireless charging system should stop power transmission immediately when a metal foreign object is found on the surface of the working transmitting coil assembly 4. After stopping power transmission, the transmitting coil where the metallic foreign object is located can be further detected, and the control switch of the loop where the transmitting coil above which the metallic foreign object is located is closed (the position of the metallic foreign object can be judged by the method for detecting the phase difference phi j).

If the number of the remaining working transmitting coils can still meet the requirement of the total transmission power, the power transmission can be resumed through the remaining working transmitting units; if the number of the remaining working transmitting coils cannot meet the requirement of the total transmission power, the position of the transmitting coil where the metal foreign matters exist is reported through the controller, wireless power transmission can be started after the foreign matters are removed, namely, power transmission is started again after position recognition of the receiving coil 51 and detection of the metal foreign matters are completed again according to the steps.

In the prior art, in the power transmission process of a wireless charging system, a power conversion circuit, a load and the like in the system change due to system adjustment, a charging state and the like, so that the impedance of the system also changes, and therefore if the method of changing the phase difference is adopted, the system cannot distinguish whether the change is caused by a metal foreign object or the influence caused by the impedance change of other circuits in the system. In order to solve the problem, in the prior art, in addition to detecting a change in a power difference value, the detection of the metal foreign object in the power transmission process of the wireless charging system is mainly realized by adding an auxiliary detection coil array on the surface of a transmission coil. For example, patent CN111086401a discloses a wireless charging system, and a detection device, a detection method, and a charging method thereof, which adopt a coil matrix independent of a transmitting coil and a receiving coil to detect a metal foreign object in order to avoid interference of a system power conversion circuit, a load, and the like, and obtain higher detection accuracy by detecting impedance changes of the detection coil in the coil matrix, thereby making up for the defect that a small-volume metal foreign object cannot be detected by adopting a method of detecting a power difference.

In the application, the transmitting coil group 4 is an independent loop formed by connecting a plurality of transmitting units in parallel, the equivalent impedance of the transmitting unit working in parallel is related to the mutual inductance of the exciting coil and the receiving coil besides the inductance and the compensating capacitance of the reflecting coil, but the position of the receiving coil is fixed in the charging process, so the mutual inductance of the transmitting unit and the receiving coil is also a fixed value, the equivalent impedance of the transmitting unit working in parallel is not changed during power transmission, the phase difference of voltage and current cannot be influenced by a power conversion circuit, a load and the like of a receiving device in the working process, and the circuit structure of the embodiment can accurately and dynamically detect the metal foreign matters in the working process.

In the detection process of the metal foreign body, the parallel working transmitting unit is equivalent to the auxiliary detection coil array, but is a part of the system for realizing power transmission, and plays roles of energy relay and transmission enhancement. On the other hand, the leading and lagging states of the voltage and the current can be distinguished by adopting a phase difference mode, namely the real part and the imaginary part of the equivalent impedance are distinguished, and the method is more accurate than a mode of simply measuring the module value of the equivalent impedance.

In the embodiment, the exciting coil 1 is arranged at the center of the transmitting coil set 4, so that when the receiving device is placed above the transmitting coil set 4, the exciting coil can be covered, namely, the electromagnetic field emitted by the exciting coil can be covered, and the outward leakage of the electromagnetic field can be reduced. In practice, the equivalent circuit of the wireless charging system is the same, since the transmitting units of the transmitting coil set 4 are connected in parallel, changing the position of the exciting coil, which is coupled with one or more other transmitting coils of the transmitting coil set 4. In other words, the excitation coil does not necessarily need to be arranged in the center of the transmission coil set 4, and the arrangement for changing the position thereof does not affect the system characteristics and the corresponding control manner.

In the prior art, a scheme of switching multiple transmitting coils is often adopted to expand a chargeable area, such as a three-coil charging structure of a mobile phone, where the three coils refer to a structure in which three transmitting coils are identified and then only one transmitting coil is selected for power transmission, and actually, the structure is a two-coil structure. Although this method expands the chargeable area to some extent, in many cases, the receiving coil is not perfectly aligned with any one of the coils, and only one of the coils with the best coupling is selected to transmit power, so that the best charging performance is mostly not obtained during each charging. And this kind of condition can be avoided in the design of this application, and this application is through setting up a plurality of transmitting coil effective increase transmission area territories to guarantee that receiving coil can and transmitting coil between obtain bigger coupling, can obtain better charging performance under the prerequisite that has improved receiving arrangement's positional deviation tolerance. On the other hand, in the scheme of the application, only one set of power supply driving circuit is adopted by connecting the transmitting units in parallel, and a plurality of sets of power supply driving units are not required, so that the number of components, the cost and the control complexity of the system are reduced. According to the scheme, the power supply driving circuit is not connected to the transmitting unit group connected in parallel through the relay three-coil structure, so that the influence of the power supply driving circuit and the receiving device is equivalently isolated, the phase difference can be detected without a complex switch switching mechanism, accordingly, the position of the receiving coil can be identified, metal foreign bodies can be identified before power transmission and in the transmission process, and better compatibility of the receiving device can be obtained.

The electric signal acquisition circuit 2 can acquire the electric signal of any transmitting unit, such as current and voltage, and total current of all transmitting units connected in parallel, and the controller 3 is communicated with the electric signal acquisition circuit 2 and can control the on-off relation of any control switch S according to the acquired current information.

Referring to fig. 3 and 4, the receiving apparatus, also called receiving end, generally includes a receiving coil 51, a receiving end controller 52, a receiving end compensation circuit 53, a receiving end rectifying and filtering circuit 54, a battery as a load 55, and the like, when the receiving apparatus is located above the transmitting coil set 4, the exciting coil 1, the transmitting coil set 4, and the receiving coil 51 above the transmitting coil set 4 constitute a three-coil structure with a relay coil, and the transmitting coils L in the plurality of transmitting units are used as the relay coil between the exciting coil 1 and the receiving coil 51.

For the wireless charging system, the relay coil in the three-coil structure plays a role of increasing the resonant current, has stronger transmission capability, can obtain a longer transmission distance, and can realize higher transmission efficiency and higher transmission power compared with the conventional two-coil structure with only receiving and transmitting. When the wireless power transmission device works, the position of the receiving coil 51 and the corresponding transmitting unit control the switch S to be closed, so that the transmitting unit is conducted, and the wireless power transmission work can be completed. I.e. the receiving coil 51 can be coupled to the transmitting coil L in the transmitting unit. For convenience of explanation, the position of the receiving coil 51 and the corresponding transmitting unit are referred to as a working transmitting unit.

The power supply is used as an external device and can be a direct current power supply or an alternating current power supply. When a direct-current power supply is selected, the input direct current is connected to the inverter circuit, the input direct current is converted into high-frequency alternating current through the inverter circuit and then is output to the input end of the compensation circuit, and the high-frequency alternating current is loaded to the exciting coil 1 through the compensation circuit. When the alternating current power supply is selected, the alternating current power supply is used as input, a first-stage rectification conversion circuit is added at the front end of the inverter circuit, the output of the alternating current power supply is converted into direct current after rectification conversion, and then the direct current is input to the input end of the inverter circuit.

The controller of the receiving device performs information interaction with the controller 3 of the transmitting terminal through a wireless communication link, and the receiving device sends the charging requirement to the transmitting terminal.

The transmitting coils are arranged in an array mode, the transmitting coils are closely and parallelly arranged on a plane, the outer shapes of the transmitting coils are regular shapes, such as one or combination of a circle, a square, a rectangle, a hexagon and the like, and the transmitting coils can be arranged in a mode that partial areas of the adjacent transmitting coils are overlapped except for parallel arrangement.

The present invention has been described in detail with reference to the embodiments shown in the drawings, and it is therefore intended that the present invention not be limited to the exact forms and details shown and described, but that various changes and modifications can be made without departing from the spirit and scope of the invention.

Claims

1. A method for detecting metal foreign matters at a wireless charging transmitting terminal is characterized in that,

the wireless transmitting terminal that charges includes:

an excitation coil (1);

the transmitting coil group (4) consists of a plurality of transmitting units connected in parallel, and each transmitting unit consists of a transmitting coil, a compensation capacitor and a control switch which are connected in series to form a loop;

the excitation coil (1) is coupled with a first transmitting unit;

according to the position of the receiving coil (51), the corresponding transmitting unit is connected with a control switch and is coupled with the receiving coil (51) as a working transmitting unit;

judging whether metal foreign matter detection is needed or not by using a power judgment method, starting wireless charging if not needed, and entering a first metal foreign matter detection process if needed;

the first metal foreign matter detection process comprises the following steps: and loading voltage on the excitation coil (1), stopping loading the voltage when the voltage reaches a preset value, and then recording a time point T1 when the current generated in each working transmitting unit reaches a peak current and a time point T2 when the current reaches an attenuation current so as to obtain an attenuation time difference Ta = T2-T1, and when the attenuation time difference Ta is smaller than a standard time difference, judging that a metal foreign body exists at the position of the corresponding working transmitting unit, otherwise, judging that no metal foreign body exists.

2. The method for detecting metallic foreign objects at a wireless charging transmitter according to claim 1,

in the first metal foreign matter detection process, after the working transmitting unit is judged to be not provided with the metal foreign matter, a power judgment method is used for judging whether the metal foreign matter detection is needed or not, wireless charging is started if the metal foreign matter detection is not needed, and a second metal foreign matter detection process is started if the metal foreign matter detection is needed.

3. The method for detecting metallic foreign objects at a wireless charging transmitter according to claim 2,

the second metallic foreign matter is detected as:

keeping a first control switch of the first transmitting unit closed, then sequentially switching on control switches of the non-working transmitting units, and loading a working frequency of

Of an alternating current signal of, wherein L _D3 For equivalent inductance after connected non-working unit, C _D3 The equivalent capacitance is the equivalent capacitance behind the accessed non-working unit;

the detected phase difference is then compared for two parameters:

the first parameter is as follows: a voltage of the alternating current signal applied to the exciting coil (1);

and a second parameter: the first transmitting unit and the non-working transmitting unit which is connected in each time are connected in parallel to form the total current of a circuit;

and if the value of the detected phase difference is within the threshold range, determining that no metal foreign matter exists, otherwise, determining that the metal foreign matter exists.

4. The method for detecting metallic foreign objects at a wireless charging transmitter according to claim 3,

the threshold range acquisition mode is as follows:

the first threshold value range is [0 DEG, phi ] _T ]；

The second threshold value range is [ phi ] _T ’，360°]Wherein phi _T ’=360°-Ф _T 。

5. The method for detecting metallic foreign objects at a wireless charging transmitter according to claim 4,

Ф _T ＝[（t3-t1）/(t2-t1)]*360°

wherein t1 is: -a rising edge trigger time of a first period of the voltage of the alternating signal applied to the excitation coil (1);

t2 is: the rising edge trigger time of the second cycle of the voltage of the alternating current signal applied to the excitation coil (1);

6. The method for detecting metallic foreign objects at a wireless charging transmitter according to any one of claims 1 to 5,

the power judgment method comprises the following steps: and judging whether the total transmission power of the working transmitting unit meets the rated transmission requirement, if so, detecting the metal foreign matters, and if not, detecting the metal foreign matters.