Wireless power transfer system with large coupling tolerance based on dual-frequency detuning

Abstract

A wireless power transmission system with a wide range of coupling tolerance based on dual-frequency detuning. First, the system has two LC series resonant and two LC parallel resonant branches. By designing the parameters of each branch component, the entire system can operate in resonant and detuned states at different frequency points, where detuning can be divided into inductive detuning and capacitive detuning; then, the system parameters of the dual-frequency compensation network are designed according to the two preset resonant frequencies, so that the system can present different SS compensated resonant states and power transmission characteristics at different operating frequencies; finally, since the square wave input voltage contains all odd harmonics with the switching frequency as the fundamental frequency, the system can be excited by the fundamental frequency and triple frequency components of the square wave voltage, so the system power can superimpose two detuned SS power transmission characteristics to achieve the function of resisting coupling changes. The system can improve the ability of the wireless power transmission system to resist coupling changes without additional feedback control or primary-secondary communication.

Description

Technical Field

[0001] The present invention belongs to the technical field of wireless power transmission, and in particular relates to a wireless power transmission system with a wide range of coupling tolerance based on dual-frequency detuning. Background Art

[0002] Wireless power transmission technology has attracted more and more attention due to its safety, reliability and low maintenance cost, and has been widely used in mobile devices, electric vehicles, implantable medical devices, underwater robots and other fields. Compared with the traditional wiring power transmission technology, this technology is more flexible, safe and reliable, and can achieve short and medium distance power transmission between power supply equipment and power consumption equipment. It has the advantages of strong versatility and high convenience. However, as the relative position between the transmitting and receiving coils changes, the coupling coefficient between them will change dramatically, resulting in a sharp fluctuation in the transmission power. The application prospects of unstable wireless power transmission with changes in relative position are very slim.

[0003] Therefore, the present invention proposes a wireless power transmission system based on dual-frequency detuning, aiming to achieve relatively stable power transmission within a wide range of coupling fluctuations without closed-loop control. Summary of the invention

[0004] The present invention discloses a wireless power transmission system with a wide range of coupling tolerance based on dual-frequency detuning. The system has the same compensation network added to both the primary and secondary sides. By designing the compensation network, the entire system can exhibit different resonant states under voltage excitations of different frequencies, and for each frequency alone, the system can be simplified into an SS-compensated wireless power transmission system. The system selects a square wave voltage input with a duty cycle of 50%, and by combining the power transmission characteristics of different resonant states of the SS compensation network, relatively stable power transmission can be achieved when the receiving coil position is offset. Therefore, this new dual-frequency detuning system has good robustness to changes in self-inductance and mutual inductance caused by coil position offset.

[0005] To achieve the above object, the technical solution of the present invention is as follows:

[0006] The wireless power transmission system with a large coupling tolerance based on dual-frequency detuning has the following specific steps:

[0007] S1, according to the structural relationship of LC-LC topology, a combination circuit of four LC branches is designed to obtain the topological structure of the dual-frequency detuning compensation network;

[0008] S2, by selecting two different operating frequencies and according to the constraints of the primary-secondary detuning rate, the component parameters of the four LC branches are designed;

[0009] S3, by selecting a square wave voltage input with a duty cycle of 50%, a wireless power transmission system capable of transmitting energy through two frequency channels is obtained, and the system has a power transmission characteristic that can resist coupling changes caused by position offset.

[0010] As a further improvement of the present invention, the topology structure of the dual-frequency detuning compensation network described in S1 is designed, and the specific process is as follows:

[0011] In terms of overall structure, the system includes an inverter for generating high-frequency square wave voltage, a primary-secondary compensation network, a rectifier circuit for converting AC to DC, and a load;

[0012] V in,n , n=1,3,5…represents the nth harmonic voltage, M represents the mutual inductance between the primary and secondary coils, V load Represents the load voltage. The structure of the original secondary compensation network is exactly the same, with a parallel LC connected in series with a series LC. L1, C1, L p ,C p It forms the resonant network of the primary side, L2, C2, L s ,C s The resonant network of the secondary side is formed. Through parameter design, the whole system is designed as a dual-frequency bandpass filter, that is, the whole system is low-impedance for the input voltage of the base frequency and the triple frequency, and is high-impedance for the input voltage of other frequencies.

[0013] The whole system presents different resonant states under different frequency excitations. And the excitations of different frequencies serve as the main channels for energy transmission in low coupling and high coupling situations respectively. By superimposing the power of different frequencies, the quasi-constant power output of the whole system can be achieved within a large coupling range.

[0014] As a further improvement of the present invention, as described in S2, by selecting two different operating frequencies, according to the constraints of the primary-secondary-side detuning rate, the detuning rates of different frequencies of the primary-secondary side are designed, and the specific process is as follows:

[0015] As mentioned above, the primary and secondary sides of the system are designed to be detuned. In order to characterize the degree of detuning, the detuning rate is defined as the ratio of the system detuning impedance to the coil inductance:

[0016]

[0017] where ω n ,α n ,β n , n=1,3 respectively represent the fundamental frequency of the square wave voltage, the triple frequency angular frequency, the detuning rate of the primary circuit at the fundamental frequency and triple frequency, and the detuning rate of the secondary circuit at the fundamental frequency and triple frequency. A detuning rate of 0 represents complete resonance;

[0018] Assuming that only one frequency is considered, the entire system is equivalent to an SS-compensated wireless power transmission system, and the calculation formula for the equivalent capacitance value of the system is:

[0019]

[0020] Solving equation (2), the equivalent capacitance of the primary and secondary sides at different frequencies is:

[0021]

[0022] The rectifier and its subsequent load circuit are equivalent to a pure resistor, and the resistance expression is:

[0023]

[0024] As a further improvement of the present invention, by selecting a square wave voltage input with a duty cycle of 50%, a wireless power transmission system capable of transmitting energy through two frequency channels is obtained, and the system has a power transmission characteristic that can resist coupling changes caused by position offset. The specific process is as follows:

[0025] 1) System mathematical model

[0026] Based on the equivalent circuit of the dual-frequency detuned wireless power transmission system, for different frequency input voltages, the entire system is simplified to SS compensation. In order to simplify the calculation, the parasitic parameters of the system components are ignored, and the system mathematical model is:

[0027]

[0028] Among them, V in,n , n = 1, 3 is the fundamental frequency and triple frequency component of the inverter output square wave voltage, I pn , I sn , n = 1, 3 are the primary and secondary coil currents under base frequency and triple frequency voltage excitation respectively. Equation (5) describes the electrical characteristics of this system under dual frequency excitation. The primary and secondary coil currents are:

[0029]

[0030] 2) Detuning solution

[0031] The design idea of ​​the dual-frequency detuning scheme is that the proposed system is in different detuning states at two frequencies. Therefore, the whole system can combine the power transfer characteristics of the SS-compensated WPT system in different detuning states and achieve smooth power transfer for a wide range of coupling changes. The system is excited by a square wave with a duty cycle of 50%, and the root mean square (RMS) values ​​of the first and third harmonic voltages are defined as:

[0032]

[0033] In order to achieve zero voltage switching (Zero Voltage Switching) of the switch, the system should be designed as inductive detuning. However, considering the solution domain problem of the four detuning rates, the proposed system is set as inductive detuning and capacitive detuning under the first and third harmonic excitations, respectively. In addition, as shown in Equation (7), the power transmission capability of the fundamental wave is much stronger than that of the third harmonic. The voltage gain at the two frequencies can be calculated as:

[0034]

[0035] in, G v,n n=1,3 represent the voltage gain of the fundamental frequency and triple frequency respectively. From formula (8), we can know that if α n , β n If the signs of the two are the same, the voltage gain is greater than when the signs are opposite. This is because after the impedance of the receiving end is mapped to the transmitting end, its detuned impedance type is flipped. If the detuned impedance type of the receiving end is the same as that of the transmitting end, the detuned impedance of the transmitting end will be reduced, making power transmission easier. Therefore, in order to make the fundamental and third harmonic output powers at the same power level, the primary and secondary sides are set to inductive and capacitive detuning at the fundamental frequency, respectively, and are set to capacitive detuning at the triple frequency, as shown in equation (9).

[0036] α1>0,β1<0,α3<0,β3<0 (9)

[0037] 3) Coupling tolerance analysis

[0038] The coupling change of the system is mainly caused by the change of mutual inductance M, which is reflected in the change of the relative position of the coils at the transmitting end and the receiving end. Therefore, the coupling tolerance of the system can be regarded as the ability of the system to resist the position deviation of the coil. Based on formula (6), the output power expression at different frequencies is:

[0039]

[0040] Where: P out,n , n = 1, 3 represent the fundamental frequency and triple frequency output power of the system respectively. In order to have a deeper understanding of the influence of the coupling coefficient on the output power of the system, the output power is derived to find the extreme point:

[0041]

[0042] k n , n = 1, 3 represent the output power extreme points of the base frequency and triple frequency respectively. It can be seen from formula (11) that the power extreme point is positively correlated with the degree of detuning of the primary and secondary sides. The higher the degree of detuning, the larger the power extreme point k. The output power of the entire system is the superposition of the output power of the base frequency and triple frequency:

[0043] P out =P out，1 +P out，3 (12).

[0044] As a further improvement of the present invention, the design of the detuning rate described in S3 is a key factor in whether the output power of the entire system can be stable, and the following three conditions need to be met:

[0045] 1. Reasonably set the values ​​of k1 and k3: k1 is designed to be in a strong coupling state and k3 is designed to be in a weak coupling state, that is, the triple frequency component of the square wave output can transmit energy in a weak coupling state, and when the system is in a strong coupling state, the fundamental frequency component becomes the main channel for energy transmission. In the present invention, in order to present a smoother change trend of the fundamental frequency output power, k1 is set to be greater than 1; and k3 is set to a weak coupling state. The total output power curve is shown in FIG. Figure 4 As shown. Figure 4 , the smooth output power coupling range is from k min to k max In order to maintain the stability of power transmission, the constraint condition of detuning rate is:

[0046]

[0047] 2. The output power of the base frequency and the triple frequency needs to be designed to be at the same order of magnitude through parameters to ensure the relative stability of the system power transmission. When the system is in a weakly coupled state:

[0048] P out_rated =P out3,k3 +P out1,k3 (14)

[0049] P out_rated is the rated output power of the system, P out3,k3 , P out1,k3 They are the output power of the system triple frequency and fundamental frequency when the coupling coefficient is k3, and when the system is in a strong coupling state:

[0050] P out_rated =P out3,kmax +P out1,kmax (15)

[0051] P out3,kmax , P out1,kmax They are the system triple frequency and fundamental frequency with a coupling coefficient of k. max Output power at time, k max Belongs to the range of strong coupling;

[0052] 3. In the entire coupling variation range, the establishment of condition 2 ensures that the output power of the two coupling endpoints is similar. In order to reduce the power fluctuation in the intermediate coupling range, the design of the detuning rate still needs to satisfy equation (16):

[0053]

[0054] Here, ε is a positive number that tends to 0.

[0055] The wireless power transmission system based on dual-frequency detuning proposed by the present invention can reduce the influence of coupling changes on the transmission power, and it can provide an approximately constant power transmission curve. The system uses the same compensation network on the primary and secondary sides. These resonant cavities can exhibit different characteristics under the excitation of different frequencies. The system is equivalent to SS inductive and capacitive detuned wireless power transmission systems when the operating frequencies are f1 and f3, respectively. The input of the system is a square wave voltage input with a duty cycle of 50%, whose fundamental frequency is f1 and contains all odd multiple components of f1. The system can be regarded as a bandpass filter, and the higher harmonics except the fundamental frequency and the triple frequency are suppressed. The system can resist the influence of coupling changes, significantly reduce the fluctuation of output power, and provide constant output power under a wide range of coupling changes. Compared with other types of wireless power transmission systems, the system has a more compact coil structure and a higher degree of positional freedom, and can achieve relatively stable output power without the use of additional sensors and closed-loop controllers. The design of the system is simple and easy to understand and implement. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 Topological diagram of the system of the present invention;

[0057] Figure 2 The system equivalent circuit diagram of the present invention;

[0058] Figure 3 A bar graph comparing the output voltages of the base frequency and triple frequency of the present invention;

[0059] Figure 4 The output power of the present invention changes with the calculated curve of the coupling coefficient;

[0060] Figure 5 The output power of the embodiment of the present invention varies with the coupling coefficient. DETAILED DESCRIPTION

[0061] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments:

[0062] The design and application of the system of the present invention includes the following specific steps:

[0063] In order to verify the feasibility and effectiveness of the wireless power transmission system based on dual-frequency detuning proposed by the invention, the present invention is carried out on Figure 1 The system shown is simulated, designed and verified, with a MATLAB Simulink model as a simulation embodiment, and the results are explained in detail based on simulation and mathematical analysis.

[0064] After selecting f1 and f3 as 50kHz and 150kHz respectively, the detuning rates of the primary and secondary sides at different frequencies are selected as follows: α1 = 0.8, α2 = -0.13, β1 = -1, β2 = -0.59. The simulation component parameters that meet these four detuning rates are shown in Table 1:

[0065] Table 1 System component parameters

[0066]

[0067] A. Output voltage FFT analysis:

[0068] Frequency selection and detuning design are one of the key characteristics of the system proposed in the present invention. The operating frequency of the present invention is set to f1, and its triple frequency is f3. In order to gain a deeper understanding of the operating characteristics of the system at different frequencies, the FFT analysis tool in the "powergui" module of MATLAB Simulink is used to measure the amplitude of the output voltage base frequency and triple frequency output voltage of the designed system. The results are shown in the figure. Figure 3 shown.

[0069] like Figure 3 As shown in the figure, when the system model is in a weak coupling state, the system transmits power simultaneously by the base frequency and the triple frequency. According to the analysis in Section 2, the system model can be equivalent to SS compensation circuits of different frequencies, and the output voltage extreme value of the triple frequency is set in the weak coupling state. When the system is in a strong coupling state, the output voltage of the triple frequency gradually decreases while the output voltage of the base frequency gradually increases and becomes the main component of the system output voltage. The output voltage extreme point of the base frequency is set in the strong coupling state.

[0070] The FFT results of the output voltage prove that the proposed dual-frequency detuned system model has the power transmission characteristics of both inductive detuning and capacitive detuning of the SS compensation network, ensuring the reliable operation of the system under two different frequency drives.

[0071] B. Power transmission characteristics:

[0072] Since the system can work under two different frequency characteristics, the power transfer characteristics of the SS compensation network in different resonant states are used to improve the system performance when the coupling changes. In order to verify the feasibility of large-scale quasi-constant power transmission, the power transfer characteristics of the system model are measured and recorded using MATLAB Simulink.

[0073] In this system model, a 50k square wave voltage is selected as the input, an open-loop controller is used, and the amplitude of the input voltage source is pre-set according to Table 1. The mutual inductance of the system is represented by the coupling coefficient k, which ranges from 0 to 1. The output power and AC-AC efficiency measurement results of the embodiment are shown in Figure 5 shown.

[0074] like Figure 5 As shown in Figure 1, when the coupling coefficient k changes from 0.32 to 0.68, the system provides relatively constant power, and the maximum output power fluctuation does not exceed 5%. The output power of the system is maintained at 130W. This is because the power extreme value of the triple frequency component is designed in the weak coupling section. Figure 5 In the coupling section shown, the output power of the triple frequency gradually decreases. The output power extreme point of the fundamental frequency is designed at the strong coupling point. Figure 5 In the coupling section shown, the output power of the fundamental frequency gradually increases and can basically compensate for the reduced part of the triple frequency output power, thereby achieving quasi-constant power output with a wide range of coupling changes. Figure 5 It is also shown that the efficiency of the system increases with the increase of the coupling coefficient k. The maximum AC-AC efficiency exceeds 89% in the relatively constant output power region.

[0075] In an embodiment, the change in the coupling coefficient k is caused by the change in the relative position of the transmitting coil and the receiving coil. Through the design of dual-frequency and detuning rate, the entire system presents different resonant states under the excitation of different frequencies. Compared with the traditional compensation topology, the system has better robustness to coupling changes. The wireless power transmission system based on dual-frequency detuning shown in the present invention combines the power transmission characteristics of different resonant states of the SS compensation network under different coupling conditions, and only requires one set of coils. Compared with other types of wireless power transmission systems, this system not only has a more compact coil structure and higher spatial freedom, but also can achieve more stable power transmission under a larger coupling coefficient variation range.

[0076] The above description is only a preferred embodiment of the present invention and does not constitute any other form of limitation to the present invention. Any modification or equivalent change made based on the technical essence of the present invention still falls within the scope of protection required by the present invention.

Claims

1. A wireless power transmission system with a wide range of coupling tolerance based on dual-frequency detuning, characterized in that: The specific steps are as follows: S1, according to the structural relationship of LC-LC topology, a combination circuit of four LC branches is designed to obtain the topological structure of the dual-frequency detuning compensation network; The topology of the dual-frequency detuning compensation network described in S1 is as follows: In terms of overall structure, the system includes an inverter for generating high-frequency square wave voltage, a primary-secondary compensation network, a rectifier circuit for converting AC to DC, and a load; V in,n , n=1,3,5…represents the nth harmonic voltage, M represents the mutual inductance between the primary and secondary coils, V load Represents the load voltage. The structure of the original secondary compensation network is exactly the same, with a parallel LC connected in series with a series LC. L1, C1, L p ,C p It forms the resonant network of the primary side, L2, C2, L s ,C s The resonant network of the secondary side is formed. Through parameter design, the whole system is designed as a dual-frequency bandpass filter, that is, the whole system is low-impedance for the input voltage of the base frequency and the triple frequency, and is high-impedance for the input voltage of other frequencies. S2, by selecting two different operating frequencies and according to the constraints of the primary-secondary detuning rate, the component parameters of the four LC branches are designed; As described in S2, by selecting two different operating frequencies and according to the constraints of the primary-secondary-side detuning rate, the detuning rates of different frequencies of the primary-secondary side are designed. The specific process is as follows: As mentioned above, the primary and secondary sides of the system are designed to be detuned. In order to characterize the degree of detuning, the detuning rate is defined as the ratio of the system detuning impedance to the coil inductance: where ω n , n = 1, 3 represent the fundamental frequency of the square wave voltage, the angular frequency of the triple frequency component, α n ,β n , n=1,3 represent the detuning rate of the primary circuit at the fundamental frequency and triple frequency and the detuning rate of the secondary circuit at the fundamental frequency and triple frequency respectively, and a detuning rate of 0 represents complete resonance; Assuming that only one frequency is considered, the entire system is equivalent to an SS-compensated wireless power transmission system, and the calculation formula for the equivalent capacitance value of the system is: Where j is the imaginary unit, j 2 =-1, solve equation (2), the equivalent capacitance of the primary and secondary sides of SS at different frequencies is: Among them, C ep,n C es,n n=1,3 represent the equivalent SS compensation capacitors of the primary and secondary sides at the base frequency and triple frequency respectively. The rectifier and its subsequent load circuit are equivalent to a pure resistor, and the resistance expression is: S3, by selecting a square wave voltage input with a duty cycle of 50%, a wireless power transmission system capable of transmitting energy through two frequency channels is obtained, and the system has a power transmission characteristic that can resist coupling changes caused by position offset; As described in S3, by selecting a square wave voltage input with a duty cycle of 50%, a wireless power transmission system capable of transmitting energy through two frequency channels is obtained, and the system has a power transmission characteristic that can resist coupling changes caused by position offset. The specific process is as follows: 1) System mathematical model Based on the equivalent circuit of the dual-frequency detuned wireless power transmission system, for different frequency input voltages, the entire system is simplified to SS compensation. In order to simplify the calculation, the parasitic parameters of the system components are ignored, and the system mathematical model is: Among them, V in,n , n = 1, 3 is the fundamental frequency and triple frequency component of the inverter output square wave voltage, I pn , I sn , n = 1, 3 are the primary and secondary coil currents under the fundamental frequency and triple frequency voltage excitation respectively. Equation (5) describes the electrical characteristics of this system under dual frequency excitation. The primary and secondary coil currents are: 2) Detuning solution The design idea of ​​the dual-frequency detuning scheme is that the proposed system is in different detuning states at two frequencies. Therefore, the whole system can combine the power transfer characteristics of the SS-compensated WPT system in different detuning states and achieve smooth power transfer for a wide range of coupling changes. The system is excited by a square wave with a duty cycle of 50%. The root mean square (RMS) values ​​of the first and third harmonic voltages are defined as: In order to achieve zero voltage switching of the switch, the system should be designed as inductive detuning. However, considering the solution domain problem of the four detuning rates, the proposed system is set as inductive detuning and capacitive detuning under the first and third harmonic excitations, respectively. In addition, as shown in Equation (7), the power transmission capability of the fundamental wave is much stronger than that of the third harmonic. The voltage gain at the two frequencies can be calculated as: in, G v,n n=1,3 represent the voltage gain of the fundamental frequency and the triple frequency respectively. From formula (8), we can know that if α n , β n The signs of the two are the same, then the voltage gain is greater than the case where the signs are opposite. This is because after the impedance of the receiving end is mapped to the transmitting end, its detuned impedance type is reversed. If the detuned impedance types of the receiving end and the transmitting end are consistent, the detuned impedance of the transmitting end will be reduced, making power transmission easier. Therefore, in order to make the fundamental wave and the third harmonic output power at the same power level, the primary and secondary sides are set to inductive and capacitive detuning at the fundamental frequency, respectively, and are set to capacitive detuning at the triple frequency, as shown in formula (9); α1＞0,β1＜0,α3＜0,β3＜0 (9) 3) Coupling tolerance analysis The coupling change of the system is mainly caused by the change of mutual inductance M, which is reflected in the change of the relative position of the coils at the transmitting end and the receiving end. Therefore, the coupling tolerance of the system can be regarded as the ability of the system to resist the position deviation of the coil. Based on formula (6), the output power expression at different frequencies is: Where: P out,n , n = 1, 3 represent the fundamental frequency and triple frequency output power of the system respectively. In order to have a deeper understanding of the influence of the coupling coefficient on the output power of the system, the output power is derived to find the extreme point: k n , n = 1, 3 represent the output power extreme points of the base frequency and triple frequency respectively. It can be seen from formula (11) that the power extreme point is positively correlated with the degree of detuning of the primary and secondary sides. The higher the degree of detuning, the larger the power extreme point k. The output power of the entire system is the superposition of the output power of the base frequency and triple frequency: P out =P out，1 +P out，3 (12); The design of the detuning rate described in S3 is a key factor in whether the output power of the entire system can be stable, and the following three conditions must be met:

1. Reasonably set the values ​​of k1 and k3: k1 is designed to be in a strong coupling state and k3 is designed to be in a weak coupling state, that is, the triple frequency component of the square wave output can transmit energy in a weak coupling state, and when the system is in a strong coupling state, the fundamental frequency component becomes the main channel for energy transmission. In order to present a smoother change trend of the fundamental frequency output power, k1 is set to be greater than 1; and k3 is set to a weak coupling state, and the smooth output power coupling range is from k min to k max , in order to maintain the stability of power transmission, the constraint condition of detuning rate is:

2. The output power of the base frequency and the triple frequency needs to be designed to be at the same order of magnitude through parameters to ensure the relative stability of the system power transmission. When the system is in a weakly coupled state: P out_rated =P out3,k3 +P out1,k3 (14) P out_rated is the rated output power of the system, P out3,k3 , P out1,k3 They are the output power of the system triple frequency and fundamental frequency when the coupling coefficient is k3, and when the system is in a strong coupling state: P out_rated =P out3,kmax +P out1,kmax (15) P out3,kmax , P out1,kmax They are the system triple frequency and fundamental frequency with a coupling coefficient of k. max Output power at time, k max Belongs to the range of strong coupling; 3. In the entire coupling variation range, the establishment of condition 2 ensures that the output power of the two coupling endpoints is similar. In order to reduce the power fluctuation in the intermediate coupling range, the design of the detuning rate still needs to satisfy equation (16): Here, ε is a positive number that tends to 0.

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