Wireless charging system based on double layers and multiple coils and efficiency improving method thereof

Abstract

The invention relates to the technical field of wireless power transmission, and particularly discloses a wireless charging system based on double layers and multiple coils and an efficiency improving method thereof.The system comprises a transmitting terminal and is characterized in that the transmitting terminal comprises a plurality of transmitting sub-coils which are arranged in an upper-layer and lower-layer staggered overlapping mode; wherein the plurality of transmitting sub-coils on the lower layer are mutually connected in series to share the first inverter, the plurality of transmitting sub-coils on the upper layer are mutually connected in series to share the second inverter, and each transmitting sub-coil is correspondingly provided with a short circuit switch. The method has the advantages that the coupler generates a uniform magnetic field through a double-layer staggered and stacked transmitting coil structure, so that the dislocation tolerance of a system is improved, the problem of a charging weak coupling area caused by a single layer of multiple coils is solved, the optimal efficiency condition is determined through mutual inductance identification in the control process, the activation strategy of the coils is determined, and the control accuracy is improved. And the primary side voltage is changed by adjusting the phase shift angle of the inverter, so that the system realizes constant power output.

Description

Technical Field

[0001] The present invention relates to the field of wireless power transmission technology, and in particular to a wireless charging system based on a double-layer multi-coil and a method for improving its efficiency. Background Art

[0002] With the development of industrial automation and the improvement of people's daily life, automated guided vehicles (AGVs) can be seen everywhere in industry and daily life. Wireless power transfer (WPT) is an emerging charging technology for AGVs. It has many advantages such as safety, convenience, and automation, and has attracted more and more attention in recent years. Due to the randomness of the AGV's docking position, an offset may occur between the transmitting coil and the receiving coil, resulting in a sharp drop in the coupling coefficient. In this case, the efficiency will be reduced, and even the rated power cannot be output. At present, there are various solutions to solve the problem of difficult alignment of the coupling mechanism, such as using DD coils, single-layer multi-transmitting coils, hybrid array structures, etc. to improve the offset performance of the coupler structure. However, it is difficult for a single-layer multi-transmitting coil to achieve full coverage of the charging area, and the system still has the problem of low efficiency in the weak coupling area. Summary of the invention

[0003] In view of this, the present invention first provides a wireless charging system based on a double-layer multi-coil to solve the problem of low efficiency caused by the weak coupling area of ​​the existing single-layer multi-transmitting coil.

[0004] In order to achieve the above object, the specific technical solutions adopted by the present invention are as follows:

[0005] A wireless charging system based on a double-layer multi-coil includes a transmitting end. The key point is that the transmitting end includes multiple transmitting sub-coils and is arranged in an upper and lower layer staggered and overlapped manner, wherein: the multiple transmitting sub-coils in the lower layer are connected in series to share a first inverter, and the multiple transmitting sub-coils in the upper layer are connected in series to share a second inverter, and each transmitting sub-coil is correspondingly provided with a short-circuit switch.

[0006] Optionally, the first inverter and the second inverter are connected in parallel to a DC power supply.

[0007] Optionally, each transmitting sub-coil and a primary compensation capacitor form a series resonance module, and each of the series resonance modules is correspondingly provided with a short-circuit switch.

[0008] Optionally, the transmitting end is provided with 7 transmitting sub-coils on the lower layer and 6 transmitting sub-coils on the upper layer, and each transmitting sub-coil has the same structure.

[0009] Optionally, it further includes a receiving end, which is provided with a receiving coil, a secondary side compensation capacitor, a rectifier and filter module and an electrical load, wherein the receiving coil and the secondary side compensation capacitor form a series resonant circuit.

[0010] Optionally, the transmitting end is also provided with a control module, and the control module is provided with a load detection module and a DC power supply output parameter detection module. The load detection module obtains the load equivalent resistance by detecting the secondary side output voltage and the secondary side output current. The DC power supply output parameter detection module is used to detect the DC power supply voltage, the first inverter input current and the second inverter input current. The control module controls the first inverter phase, the second inverter phase and the disconnection strategy of each short-circuit switch according to the load equivalent resistance, the DC power supply voltage, the first inverter input current and the second inverter input current, so as to achieve optimal efficiency wireless charging with minimum loss as the goal and constant power output as the constraint condition.

[0011] Based on the above system, the present invention also provides a method for improving the efficiency of a wireless charging system based on a double-layer multi-coil, the key of which is that it includes the following steps:

[0012] S1: System initialization. When only one short-circuit switch is disconnected, the load detection module detects the secondary output voltage and secondary output current to obtain the load equivalent resistance R. eq ;

[0013] S2: When the short-circuit switch corresponding to each transmitting sub-coil is disconnected independently, the DC power supply output parameter detection module is used to collect the DC power supply voltage U in , the first inverter input current I in1 and the second inverter input current I in2 , and calculate the mutual inductance of each transmitting sub-coil when it is connected separately, and sort the lower layer transmitting sub-coils and the upper layer transmitting sub-coils according to the mutual inductance;

[0014] S3: In the lower layer transmitting sub-coils and the upper layer transmitting sub-coils, the transmitting sub-coils with the largest mutual inductance are respectively taken as the first ones, and the number of transmitting coils is increased one by one according to the arrangement order obtained in step S2 to calculate the efficiency evaluation index, and the number of connected transmitting sub-coils in each layer when the efficiency evaluation index is the largest is determined; wherein: the efficiency evaluation index is the ratio of the square of the sum of mutual inductances to the number of activated coils;

[0015] S4: According to the arrangement order obtained in step S2 and the access number of each layer of transmitting sub-coils when the efficiency evaluation index is maximized determined in step S3, the short-circuit switch state corresponding to each layer of transmitting sub-coils is controlled to activate the target coil.

[0016] Furthermore, the above method also includes the steps of:

[0017] S5: By adjusting the inverter phase shift angle to change the primary voltage, the system can achieve constant power output, including:

[0018] S51: Initialize constant output power P target , error range, angle variable Δθ and first inverter phase shift angle α;

[0019] S52: According to cosα / cosβ=M 1s / M 2s Calculate the second inverter phase shift angle β, where M 1s is the mutual inductance between the multiple transmitting sub-coils and the receiving coils in the lower layer, M 2s is the mutual inductance between the multiple transmitting sub-coils and the receiving coils in the upper layer;

[0020] S53: Calculate the primary voltage U according to the input DC power supply voltage and the phase shift angle of each inverter 1 ,U 2 size;

[0021] S54, according to the primary voltage U 1 ,U 2 The output power P is calculated by the magnitude and current mutual inductance out , and the obtained output power P out and constant output power P target Compare and determine whether it is within the error ε range. If not, adjust the first inverter phase shift angle α according to the angle variable Δθ, and return to step S52 to execute in a loop.

[0022] Furthermore, the system follows:

[0023]

[0024] Calculate the mutual inductance between the lower layer i-th transmitting sub-coil and the upper layer j-th transmitting sub-coil and the receiving coil, R p is the equivalent internal resistance of a single transmitting sub-coil.

[0025] according to

[0026]

[0027]

[0028] The mutual inductances between the lower-layer connected transmitting sub-coil and the upper-layer connected transmitting sub-coil and the receiving coil are obtained respectively, K 1 is the number of connected lower-layer transmitting sub-coils, K 2 The number of upper transmitting sub-coils connected.

[0029] Further, in step S54, according to:

[0030] Calculate the output power P out , ω is the system operating angular frequency.

[0031] The remarkable effects of the present invention are:

[0032] The present invention proposes a wireless charging system based on a double-layer multi-coil and a method for improving its efficiency. Firstly, a double-layer staggered stacked transmitting coil structure is used to enable a coupler to generate a uniform magnetic field, thereby improving the system's tolerance to misalignment and compensating for the problem of a weakly coupled charging area caused by a single-layer multi-coil. During the control process, the optimal efficiency condition is determined by mutual inductance identification, thereby determining the activation strategy of the coil, and by adjusting the inverter phase shift angle and changing the primary voltage, the system achieves a constant power output. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 is a circuit diagram of a double-layer multi-coil wireless charging system of the present invention;

[0034] Figure 2 is an equivalent circuit topology diagram of the double-layer multi-coil wireless charging system of the present invention;

[0035] Figure 3 is a schematic diagram of the structure of a transmitting coil provided in a specific embodiment;

[0036] Figure 4 The efficiency of the present invention varies with different G 1 , G 2 And the changing three-dimensional picture;

[0037] Figure 5 is a control block diagram of the double-layer multi-coil wireless charging system of the present invention;

[0038] Figure 6 is a flow chart of the coil activation strategy of the present invention;

[0039] Figure 7 It is a flow chart of changing the phase shift angle to achieve constant output power according to the present invention. DETAILED DESCRIPTION

[0040] The following specifically illustrates the implementation mode of the present invention in conjunction with the accompanying drawings. The embodiments are provided for illustrative purposes only and cannot be understood as limiting the present invention. The accompanying drawings are provided for reference and illustration only and do not constitute a limitation on the scope of patent protection of the present invention, because many changes can be made to the present invention without departing from the spirit and scope of the present invention.

[0041] like Figure 1-Figure 3As shown, the present embodiment provides a wireless charging system based on double-layer multi-coils, including a DC power supply, an inverter, a coupling mechanism, a compensation network, a rectifier, and a load. Among them: the transmitting end of the coupling mechanism includes a plurality of transmitting sub-coils and is arranged in an interlaced and overlapping manner of the upper and lower layers, the plurality of transmitting sub-coils in the lower layer are connected in series to share a first inverter, the plurality of transmitting sub-coils in the upper layer are connected in series to share a second inverter, and each transmitting sub-coil is correspondingly provided with a short-circuit switch.

[0042] In this example, the first inverter and the second inverter are connected in parallel to the DC power supply, each transmitting sub-coil and a primary compensation capacitor form a series resonance module, and each series resonance module is provided with a corresponding short-circuit switch. Figure 3 It can be seen that the transmitter has 7 transmitting sub-coils on the lower layer and 6 transmitting sub-coils on the upper layer, and each transmitting sub-coil has the same structure. In addition, the receiving coil and the secondary side compensation capacitor of the receiving end also form a series resonant circuit, so that both the primary side and the secondary side form an SS type topology structure.

[0043] Combination Figure 1 It can be seen that U in is the DC power supply voltage, I in1 ,I in2 is the DC input current flowing into the two inverters, U 1 , U 2 is the primary AC voltage, I p1 ,I p2 is the primary AC current, R 1 , R 2 is the equivalent internal resistance of the coil, M 1s 、M 2s Respectively represent the mutual inductance from the lower coil and upper coil to the receiving coil, M 12 represents the cross coupling between the two layers of transmitting coils, U s is the AC voltage on the secondary side, I out is the secondary AC current, U L ,I L are the load voltage and current, R eq For AC equivalent load, coil self-inductance, compensation capacitor, filter capacitor, internal resistance are L p , C f , R p Indicates that C Σ1 ,C Σ2 Represents the compensation capacitor, S 1 -S 4 , S 5 -S 8 The MOSFET tube that constitutes the inverter bridge, D 1 -D 4To form the diodes of the rectifier bridge, the reference direction of the current in each coil and the direction of the same-named end of the inductor have been marked.

[0044] For the above system, fundamental wave analysis is widely used. This method is based on Fourier transform and decomposes the waveform into fundamental wave and other harmonics. Since the harmonic content is relatively small, it is usually ignored. Figure 2 The equivalent circuit structure shown in Figure 1 is Figure 2 The circuit relationship shown in the figure establishes the following equation:

[0045]

[0046] Among them, X 1 =X L1 -X CΣ1 =ωL 1 -1 / ωC Σ1 , X 2 =X L2 -X CΣ2 =ωL 2 -1 / ωC Σ2 , X s =X Ls -X Cs =ωL s -1 / ωC s .L 1 =K 1 L p , L 2 =K 2 L p ,R 1 =K 1 R p , R 2 =K 2 R p ,K 1 ,K 2 M is the number of activated coils in each layer. 1s 、M 2s is the mutual inductance of each layer of transmitting coil and receiving coil, which can be expressed by formula (2):

[0047]

[0048] The relationship between the primary voltage and the DC input voltage satisfies the following equation:

[0049]

[0050] Where α and β are the phase shift angles of the two inverters. When the system is in a resonant state, the angular frequency ω should satisfy the relationship described in equation (4):

[0051]

[0052] R eq is the equivalent resistance of the rectifier and the load, satisfying the relationship described in equation (5):

[0053]

[0054] The coupling coefficient between the transmitting coils is much smaller than the coupling coefficient between the transmitting coil and the receiving coil, so M is ignored. 12 To simplify the calculation. Combining the above analysis and solving the equation, we can get the primary current I p1 ,I p2 and the secondary current I out expression.

[0055]

[0056] The ratio of the primary current I p1 / I p2 Satisfies the relationship described in formula (7):

[0057]

[0058] Furthermore, the expression of output power is as follows:

[0059]

[0060] The efficiency expression is defined as in formula (9):

[0061]

[0062] Solve for the optimal conditions of efficiency and consider the constant output power (P target ) constraints, and solve the efficiency optimization problem by constructing the Lagrangian function.

[0063] Lλ=K1Ip 1 2 Rp+K 2 Ip 2 2 Rp+Iout 2 Rs+λ(Iout 2 Req-Ptarget) (10)

[0064] Depend on Figure 2 It is easy to know that the secondary current and the primary current satisfy the following equation:

[0065] (jωM 1s I p1 +jωM 2s I p2 )=(R eq +R s )I out(11)

[0066] Substituting it into (10), eliminating I out , so we have:

[0067]

[0068] I p1 ,I p2 ,λ finds the first-order derivative, that is The solution process is as follows:

[0069]

[0070] Combining the three equations in (13), the primary current relationship under the optimal efficiency condition should be:

[0071]

[0072] The current ratio at this time is:

[0073]

[0074] Combining (3)(7)(15), the relationship between the inverter phase shift angle and mutual inductance can be obtained:

[0075] cosα / cosβ=M 1s / M 2s (16)

[0076] Formula (16) is used as a reference to adjust the phase shift angle so that the system can achieve a constant output power, which is also a prerequisite for optimal efficiency.

[0077] Definition G f It is the ratio of the square of the sum of the mutual inductances to the number of activated coils.

[0078]

[0079] M f-i is the mutual inductance between the i-th transmitting sub-coil and the receiving coil in the f-th layer transmitting coil, K f is the number of transmitting sub-coils connected to the f-th layer transmitting coil, M fs 2 represents the square of the sum of the mutual inductances of the transmitting sub-coils connected to the fth layer, from Figure 4 It can be seen that G 1 , G 2 The relationship between G and efficiency is proportional. f The larger the value, the higher the efficiency. 1 , G 2 When both take the maximum value, the system efficiency is highest.

[0080] Therefore, if Figure 5 As shown, a wireless charging system based on a double-layer multi-coil proposed in this embodiment is further provided with a control module at the transmitting end, and the control module is provided with a load detection module and a DC power supply output parameter detection module. The load detection module obtains a load equivalent resistance by detecting the secondary side output voltage and the secondary side output current. The DC power supply output parameter detection module is used to detect the DC power supply voltage, the first inverter input current and the second inverter input current. The control module controls the first inverter phase, the second inverter phase and the disconnection strategy of each short-circuit switch according to the load equivalent resistance, the DC power supply voltage, the first inverter input current and the second inverter input current, so as to achieve optimal efficiency wireless charging with minimum loss as the goal and constant power output as the constraint condition.

[0081] When controlling specifically, Figure 6 and Figure 7 It can be seen that this embodiment first proposes a method for improving the efficiency of a wireless charging system based on a double-layer multi-coil, comprising the following steps:

[0082] S1: System initialization. When only one short-circuit switch is disconnected, the load detection module detects the secondary output voltage and secondary output current to obtain the load equivalent resistance R. eq ;

[0083] S2: When the short-circuit switch corresponding to each transmitting sub-coil is disconnected independently, the DC power supply output parameter detection module is used to collect the DC power supply voltage U in , the first inverter input current I in1 and the second inverter input current I in2 , and calculate the mutual inductance of each transmitting sub-coil when it is connected separately, which are respectively denoted as M 1-i and M 2-j , sort the lower-layer transmitting sub-coils and the upper-layer transmitting sub-coils respectively according to the mutual inductance;

[0084] S3: In the lower layer transmitting sub-coil and the upper layer transmitting sub-coil, the transmitting sub-coil with the largest mutual inductance is respectively taken as the head, and the number of transmitting coils is increased one by one according to the arrangement order obtained in step S2 to calculate the efficiency evaluation index, and the number of connected transmitting sub-coils of each layer is determined when the efficiency evaluation index is the largest; wherein: the efficiency evaluation index is the ratio of the square of the sum of mutual inductances to the number of activated coils, which can be specifically calculated according to formula (17); thus, G is obtained 1max and G 2max ;

[0085] S4: According to the arrangement order obtained in step S2 and the number of connected transmitting sub-coils in each layer when the efficiency evaluation index is maximized as determined in step S3, the short-circuit switch state corresponding to each transmitting sub-coil is controlled to activate the target coil;

[0086] S5: The primary voltage is changed by adjusting the inverter phase shift angle to achieve constant power output of the system.

[0087] Combination Figure 7 It can be seen that the specific steps of step S5 include:

[0088] S51: Initialize constant output power P target , error range, angle variable Δθ and first inverter phase shift angle α;

[0089] S52: According to cosα / cosβ=M 1s / M 2s Calculate the second inverter phase shift angle β, where M 1s is the mutual inductance between the multiple transmitting sub-coils and the receiving coils in the lower layer, M 2s is the mutual inductance between the multiple transmitting sub-coils and the receiving coils in the upper layer;

[0090] S53: Calculate the primary voltage U according to the input DC power supply voltage and the phase shift angle of each inverter 1 ,U 2 size;

[0091] S54, according to the primary voltage U 1 ,U 2 The output power P is calculated by the magnitude and current mutual inductance out , and the obtained output power P out and constant output power P target Compare and determine whether it is within the error ε range. If not, adjust the first inverter phase shift angle α according to the angle variable Δθ, and return to step S52 to execute in a loop.

[0092] In specific implementation, both step S2 and step S5 may be performed as follows:

[0093]

[0094]

[0095] The mutual inductance between the i-th transmitting sub-coil in the lower layer and the j-th transmitting sub-coil in the upper layer and the receiving coil is calculated respectively.

[0096] In addition, combined with the above analysis, step S54 can be performed as follows:

[0097] Calculate the output power P out。

[0098] In order to illustrate the feasibility of the above system and method, the following experimental verification is carried out:

[0099] The magnetic coupling mechanism includes a double-layer multi-transmitting coil and a receiving coil. The outer radius of the transmitting coil is 55mm, the number of turns is 10, and the outer radius of the receiving coil is 124.23mm, the number of turns is 20. On this basis, a wireless charging system experimental platform with a transmission distance of 40mm was built. The main equipment includes a DC power supply, DSP, inverter, magnetic coupling mechanism, rectifier, electronic load and bidirectional switch, and the PWM wave frequency is set to 85kHz.

[0100] The experiment takes the receiving coil at (0,55,40) mm as an example. Figure 3 For the coil structure shown, TX1 and TX7 of the first layer and TX1 and TX6 of the second layer should be activated. In this case, the measured mutual inductance value M 1s ,M 2s By adjusting the phase angles α and β to 30° and 12°, the ratio of the primary current to the mutual inductance and the number of activated transmitting coils is roughly equal. The system can achieve energy transmission with a power of 500W and a maximum efficiency of 91% when the coupling mechanism is not aligned. The experimental results show the effectiveness of the proposed method to improve the efficiency of the double-layer multi-coil wireless charging system.

[0101] In summary, in order to improve the transmission efficiency and anti-offset performance of the wireless charging system of the automatic traction vehicle, the present invention proposes a wireless charging system based on a double-layer multi-coil and its efficiency improvement method. First, based on the equivalent circuit model of the double-layer multi-coil wireless charging system of the SS topology, the primary and secondary current expressions are derived, and then the power and efficiency expressions are obtained. With the goal of minimizing losses and constant output power, the Lagrangian function is constructed to solve the optimal efficiency condition. When the ratio of the mutual inductance ratio to the number of activated coils is equal to the primary current ratio, the system can achieve maximum efficiency transmission. Based on the optimal efficiency condition, the mutual inductance is identified, the activation strategy of the coil is determined, and the inverter phase shift angle is adjusted so that the primary voltage ratio tracks the mutual inductance ratio, thereby enabling the system to achieve constant power output.

[0102] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A wireless charging system based on double-layer multi-coils, comprising a transmitting end, characterized in that: The transmitting end includes a plurality of transmitting sub-coils and are arranged in an interlaced and overlapping manner in two layers, wherein: the plurality of transmitting sub-coils in the lower layer are connected in series to share a first inverter, the plurality of transmitting sub-coils in the upper layer are connected in series to share a second inverter, and each transmitting sub-coil is correspondingly provided with a short-circuit switch.

2. The double-layer multi-coil wireless charging system according to claim 1, characterized in that: The first inverter and the second inverter are connected to a DC power supply in parallel.

3. The double-layer multi-coil wireless charging system according to claim 1, characterized in that: Each transmitting sub-coil and a primary compensation capacitor form a series resonance module, and each of the series resonance modules is correspondingly provided with a short-circuit switch.

4. The double-layer multi-coil wireless charging system according to any one of claims 1 to 3, characterized in that: The transmitting end is provided with 7 transmitting sub-coils at the lower layer and 6 transmitting sub-coils at the upper layer, and each transmitting sub-coil has the same structure.

5. The double-layer multi-coil wireless charging system according to claim 4, characterized in that: It also includes a receiving end, which is provided with a receiving coil, a secondary side compensation capacitor, a rectifier filter module and an electrical load, wherein the receiving coil and the secondary side compensation capacitor form a series resonant circuit.

6. The double-layer multi-coil wireless charging system according to claim 5, characterized in that: The transmitting end is also provided with a control module, and the control module is provided with a load detection module and a DC power supply output parameter detection module. The load detection module obtains a load equivalent resistance by detecting the secondary side output voltage and the secondary side output current. The DC power supply output parameter detection module is used to detect the DC power supply voltage, the first inverter input current and the second inverter input current. The control module controls the first inverter phase, the second inverter phase and the disconnection strategy of each short-circuit switch according to the load equivalent resistance, the DC power supply voltage, the first inverter input current and the second inverter input current, so as to achieve optimal efficiency wireless charging with minimum loss as the goal and constant power output as the constraint condition.

7. A method for improving the efficiency of a wireless charging system based on a double-layer multi-coil as claimed in claim 6, characterized in that: The following steps are involved: S1: System initialization. When only one short-circuit switch is disconnected, the load detection module detects the secondary output voltage and secondary output current to obtain the load equivalent resistance R. eq ; S2: When the short-circuit switch corresponding to each transmitting sub-coil is disconnected independently, the DC power supply output parameter detection module is used to collect the DC power supply voltage U in , the first inverter input current I in1 and the second inverter input current I in2 , and calculate the mutual inductance of each transmitting sub-coil when it is connected separately, and sort the lower layer transmitting sub-coils and the upper layer transmitting sub-coils according to the mutual inductance; S3: In the lower layer transmitting sub-coils and the upper layer transmitting sub-coils, the transmitting sub-coils with the largest mutual inductance are respectively taken as the first ones, and the number of transmitting coils is increased one by one according to the arrangement order obtained in step S2 to calculate the efficiency evaluation index, and the number of connected transmitting sub-coils in each layer when the efficiency evaluation index is the largest is determined; wherein: the efficiency evaluation index is the ratio of the square of the sum of mutual inductances to the number of activated coils; S4: According to the arrangement order obtained in step S2 and the access number of each layer of transmitting sub-coils when the efficiency evaluation index is maximized determined in step S3, the short-circuit switch state corresponding to each layer of transmitting sub-coils is controlled to activate the target coil.

8. The method for improving the efficiency of a wireless charging system based on a double-layer multi-coil according to claim 7, characterized in that: Also includes the steps: S5: By adjusting the inverter phase shift angle to change the primary voltage, the system can achieve constant power output, including: S51: Initialize constant output power P target , error range, angle variable Δθ and first inverter phase shift angle α; S52: According to cosα / cosβ=M 1s / M 2s Calculate the second inverter phase shift angle β, where M 1s is the mutual inductance between the multiple transmitting sub-coils and the receiving coils in the lower layer, M 2s is the mutual inductance between the multiple transmitting sub-coils and the receiving coils in the upper layer; S53: Calculate the magnitude of the primary voltages U1 and U2 according to the input DC power supply voltage and the phase shift angles of each inverter; S54, calculate the output power P according to the magnitude of the primary voltage U1, U2 and the current mutual inductance out , and the obtained output power P out and constant output power P target Compare and determine whether it is within the error ε range. If not, adjust the first inverter phase shift angle α according to the angle variable Δθ, and return to step S52 to execute in a loop.

9. The method for improving the efficiency of a wireless charging system based on a double-layer multi-coil according to claim 7 or 8, characterized in that: according to: Calculate the mutual inductance between the lower layer i-th transmitting sub-coil and the upper layer j-th transmitting sub-coil and the receiving coil, R p is the equivalent internal resistance of a single transmitting sub-coil; according to: The mutual inductances between the lower-layer connected transmitting sub-coils and the upper-layer connected transmitting sub-coils and the receiving coils are obtained respectively, K1 is the number of connected transmitting sub-coils in the lower layer, and K2 is the number of connected transmitting sub-coils in the upper layer.

10. The method for improving the efficiency of a wireless charging system based on a double-layer multi-coil according to claim 9, characterized in that: In step S54, according to: Calculate the output power P out , ω is the system operating angular frequency.

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