CN116247831B - Concrete-air cross-medium wireless power transmission system and control method thereof - Google Patents

Abstract

The invention relates to the technical field of wireless power transmission, in particular to a concrete-air cross-medium wireless power transmission system and a control method thereof, wherein a receiving coil of the concrete-air cross-medium wireless power transmission system is arranged in concrete, a transmitting coil is arranged in air, and a variable capacitor is added into a primary circuit through designing an adjustable capacitor module so as to adjust the primary natural resonant frequency according to a disturbance observation method, so that both a primary side and a secondary side work in a resonant state, and the system keeps higher transmission efficiency; the invention also designs the frequency modulation tuning module to control the phase of the output voltage of the inverter to be consistent with the phase of the primary side current, so that the primary side circuit and the secondary side circuit of the WPT system are both resonant, the output power and the efficiency of the WPT system are improved, the higher output power and the higher efficiency can be still maintained when the concrete medium changes (such as humidity changes and temperature changes), and the electric equipment in the concrete can be charged stably, reliably and efficiently.

Description

A concrete-air cross-medium wireless power transmission system and control method thereof

Technical Field

The present invention relates to the technical field of wireless power transmission, and in particular to a concrete-air cross-medium wireless power transmission system and a control method for the concrete-air cross-medium wireless power transmission system.

Background technique

Environmental factors (wind, temperature, rain, earthquake, etc.) will constantly damage concrete buildings such as bridges, tunnels and buildings, and shorten the service life of buildings. In order to carry out targeted maintenance of buildings with structural defects, it is necessary to obtain internal damage data of the building, especially the moisture content and corrosion degree inside the building. In the early 1990s, many important long-span bridges were installed with structural health monitoring (SHM) systems, which not only monitor the changes in the bridge structure, but also monitor the impact of the environment on the bridge. Traditional structural monitoring systems rely on wired sensors to implement, but the wired sensor lines are complex, and long-term exposure to harsh external environments will cause cable damage, thereby affecting the reliability of the sensor. More importantly, the cables embedded in the bridge will accelerate water seepage inside the bridge and aggravate the internal corrosion of the bridge. In contrast, embedded wireless sensors are more suitable for SHM systems. At present, the lithium polymer battery power supply used by wireless sensors is easily limited by battery capacity and weight. Research on new power supply methods has become an urgent need in today's society.

Only when both the transmitting coil and the receiving coil are resonant can the wireless power transmission system perform efficient energy transmission through the coupling between the resonant coils. At this time, the output power and efficiency are very high. However, in the actual concrete-air cross-medium WPT system, due to the influence of the concrete electromagnetic parameters, the inherent resonant frequency of the transmitting coil and the receiving coil changes, resulting in large fluctuations in the output power and reducing the efficiency of the entire system.

Summary of the invention

The present invention provides a concrete-air cross-medium wireless power transmission system and a control method thereof, which solves the technical problem that a wireless power transmission system designed with a single medium used in a concrete-air cross-medium environment will cause the system to deviate from the resonant frequency, thereby significantly reducing the system output power and efficiency, making it difficult for the system to supply power stably and efficiently.

In order to solve the above technical problems, the present invention provides a concrete-air cross-medium wireless power transmission system, comprising a DC power supply, an inverter, a primary series resonant capacitor C _P , a transmitting coil L _P connected in sequence, and a receiving coil _LS , a secondary series resonant capacitor C _S , and an equivalent load _RL connected in sequence. The key points are:

The receiving coil _LS is installed in concrete, and the transmitting coil _LP is installed in the air;

The system also includes an adjustable capacitor module, which includes an efficiency calculation unit, a capacitor conduction angle calculation unit, and an adjustable capacitor circuit. The adjustable capacitor circuit is connected in series between the primary series resonant capacitor _CP and the transmitting coil _LP ; the efficiency calculation unit is used to calculate the system input power and output power and calculate the system efficiency; the capacitor conduction angle calculation unit is used to calculate the switch conduction angle α acting on the adjustable capacitor circuit according to the system efficiency, and the capacitance of the adjustable capacitor circuit connected to the circuit under the switch conduction angle α is equivalent to Wherein Ca _is the fixed capacitance in the adjustable capacitance circuit;

The system also includes a frequency modulation tuning module, which includes a current detection circuit, an inverter conduction angle calculation unit and a drive circuit. The current detection circuit is used to collect the output current of the inverter. The inverter conduction angle calculation unit is used to calculate the inverter conduction angle θ' according to the current collected by the current detection circuit. The drive circuit drives the inverter with the inverter conduction angle θ'.

Preferably, the inverter conduction angle calculation unit includes an orthogonal signal generator, a Park transformation module, a PI control module and a frequency phase angle generator;

The orthogonal signal generator is used to generate two corresponding voltage signals with equal amplitudes and a phase difference of 90° according to the current collected by the current detection circuit;

The Park transformation module is used to perform Park transformation on the two orthogonal voltage signals generated by the orthogonal signal generator to obtain a q-axis voltage and a d-axis voltage;

The PI control module is used to perform PI processing on the q-axis voltage obtained by the Park transformation module to obtain a frequency control amount;

The frequency phase angle generator is used to calculate the inverter conduction angle θ' according to the frequency control amount and the current frequency ω of the inverter and output it to the drive circuit.

Preferably, the adjustable capacitance circuit includes a fixed capacitance _Ca connected in series between the primary series resonant capacitance _CP and the transmitting coil _LP , and also includes a switch circuit connected in parallel at both ends of the fixed capacitance _Ca , wherein the switch circuit includes a first MOS transistor _S1 and a second MOS transistor _S2 ; an S pole of the first MOS transistor _S1 is connected to a common end of the primary series resonant capacitance _CP and the transmitting coil _LP , a D pole of the first MOS transistor _S1 is connected to a D pole of the second MOS transistor _S2 , an S pole of the second MOS transistor _S2 is connected to a common end of the fixed capacitance _Ca and the transmitting coil _LP , and a G pole of the first MOS transistor _S1 and a G pole of the second MOS transistor _S2 are connected to the capacitance conduction angle calculation unit.

Preferably, the capacitor conduction angle calculation unit calculates the switch conduction angle α using a perturbation observation method.

Preferably, the parameters of the system are determined by the following steps:

N1. Determine the system operating frequency f ₀ according to actual needs;

N2. According to actual needs, the shapes of the transmitting coil _LP and the receiving coil _LS are determined to be planar spiral coils, the radii of the copper cores of the windings are r _n1 and r _n2 respectively, and the radii of the windings are r _w1 and r _w2 respectively;

N3. Measure the inductance and internal resistance of the transmitting coil _LP and the receiving coil _LS , and determine the values of the primary series resonant capacitor _CP and the secondary series resonant capacitor _CS according to the resonance relationship;

N4. Determine the depth of embedding the receiving coil _LS in the concrete according to the building requirements;

N5. Calculate the parasitic capacitance C _PC generated by the concrete-air medium on the transmitting coil _LP and the parasitic capacitance C _SC generated on the receiving coil _LS ;

N6. Determine the value of the fixed capacitance _Ca according to the parasitic capacitance C _PC and the parasitic capacitance C _SC .

Preferably, in step N5, the parasitic capacitance C _PC is calculated by the following formula:

_εa is the relative dielectric constant of air, _εrp is the relative dielectric constant of the wire insulation layer of the transmitting coil _LP , _εo is the dielectric constant of vacuum, and _lP is the length of the winding of the transmitting coil _LP .

Preferably, in step N5, the parasitic capacitance C _SC is calculated by the following formula:

ε _c is the relative dielectric constant of concrete, ε _rs is the relative dielectric constant of the wire insulation layer of the receiving coil _LS , and _ls is the length of the winding of the receiving coil _LS .

The present invention also provides a control method for a concrete-air cross-medium wireless power transmission system, the key of which is that it includes:

The system is operated at an operating frequency of f ₀ and an initial switch conduction angle of α _0. After the system is stable, the input voltage and current of the system and the output voltage and current of the system are obtained and the efficiency of the system is calculated based on the obtained input voltage and current. The conduction angle of the switch tube of the adjustable capacitor module is controlled in real time by using the perturbation observation method with the goal of stabilizing the efficiency at a preset value. Furthermore, the input current of the system is obtained with the goal of making the output voltage and output current of the inverter in phase, and the conduction angle of the inverter is controlled by using PI control.

The present invention provides a concrete-air cross-medium wireless power transmission system and a control method thereof, and its beneficial effects are: a concrete-air cross-medium wireless power transmission system is designed, wherein the receiving coil _LS is installed in concrete, and the transmitting coil _LP is installed in the air, and a variable capacitor is added to the primary circuit by designing an adjustable capacitor module to adjust the natural resonance frequency of the primary side according to the perturbation observation method, so that both the primary side and the secondary side work in a resonant state, so that the system maintains a high transmission efficiency; the present invention also controls the phase of the inverter output voltage to be consistent with the phase of the primary current by designing a frequency modulation tuning module, so that both the primary side and the secondary side circuit of the WPT system resonate, and improve the output power and efficiency of the WPT system. In general, the present invention adopts a composite tuning control combining frequency modulation tuning and dynamic compensation tuning, which can maintain a high output power and efficiency when the concrete medium changes (such as humidity changes, temperature changes), has a wide range of applications, and is highly practical, and can charge electrical equipment in concrete stably, reliably, and efficiently.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is an architecture diagram of a concrete-air cross-medium wireless power transmission system provided by an embodiment of the present invention;

FIG. 2( a ) is a simulation diagram of the magnetic field around a receiving coil in the air provided by an embodiment of the present invention;

FIG2( b ) is a simulation diagram of the magnetic field around the receiving coil in concrete provided by an embodiment of the present invention;

FIG3 is a diagram of a partial circuit equivalent model of a system considering the influence of concrete provided by an embodiment of the present invention;

FIG4 is a schematic diagram of a frequency modulation and tuning control strategy provided by an embodiment of the present invention;

FIG5 is a flow chart of a frequency modulation and tuning control strategy provided by an embodiment of the present invention;

FIG6( a ) is a topological diagram of a variable capacitor provided in an embodiment of the present invention;

FIG6( b ) is a graph showing a change in variable capacitance according to an embodiment of the present invention;

FIG7 is a flow chart of a composite tuning control provided by an embodiment of the present invention;

FIG8( a ) is a waveform diagram of the output voltage and current of an inverter without frequency modulation and tuning control provided by an embodiment of the present invention;

FIG8( b ) is a waveform diagram of the inverter output voltage and current after frequency modulation and tuning control provided by an embodiment of the present invention;

FIG9( a ) is a waveform diagram of the primary current and the secondary current provided by an embodiment of the present invention without adopting dynamic compensation tuning control;

FIG9( b ) is a waveform diagram of the primary current and the secondary current after dynamic compensation tuning control is adopted according to an embodiment of the present invention;

FIG10( a ) is a graph showing the efficiency before and after composite tuning according to an embodiment of the present invention;

FIG10( b ) is a graph showing output power curves before and after composite tuning according to an embodiment of the present invention.

Detailed ways

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 are not to be construed as limitations of the present invention. The accompanying drawings are provided for reference and illustration only and do not constitute limitations on the scope of patent protection of the present invention, because many changes may be made to the present invention without departing from the spirit and scope of the present invention.

In order to realize high-efficiency and high-output power wireless power transmission in a concrete-air cross-medium scenario, an embodiment of the present invention first provides a concrete-air cross-medium wireless power transmission system, whose circuit topology is shown in FIG1 , including a DC power supply U _dc , an inverter, a primary-side series resonant capacitor C _P , and a transmitting coil L _P connected in sequence, and a receiving coil _LS , a secondary-side series resonant capacitor C _S , and an equivalent load _RL connected in sequence. The key point is that the system also includes an adjustable capacitor module and a frequency modulation tuning module for realizing a composite tuning control combining frequency modulation tuning and dynamic compensation tuning.

The adjustable capacitor module includes an efficiency calculation unit, a capacitor conduction angle calculation unit, and an adjustable capacitor circuit. The adjustable capacitor circuit is connected in series between the primary series resonant capacitor _CP and the transmitting coil _LP ; the efficiency calculation unit is used to calculate the system input power and output power and calculate the system efficiency; the capacitor conduction angle calculation unit is used to calculate the switch conduction angle α according to the system efficiency and act on the adjustable capacitor circuit.

The frequency modulation tuning module includes a current detection circuit, an inverter conduction angle calculation unit and a drive circuit. The current detection circuit is used to collect the output current of the inverter. The inverter conduction angle calculation unit is used to calculate the inverter conduction angle θ' according to the current collected by the current detection circuit. The drive circuit drives the inverter with the inverter conduction angle θ'.

In this example, the transmitting coil _LP and the receiving coil _LS are both wound with copper wire and present a planar spiral structure. Assuming that the radius of the wire wound with the coil is 1mm, the radius of the wound coil is 100mm, the transmitting coil is placed in the air and the receiving coil is embedded in concrete, the inductance of the transmitting coil and the receiving coil is about L = 23.6uH, and the resistance is R = 600mΩ. When the transmitting coil is placed in the air and the receiving coil is embedded in concrete, the coil placed in the air is less affected by the concrete, so the inductance and resistance are almost unchanged, while the coil embedded in the concrete is surrounded by a large amount of concrete medium, so the inductance and resistance will change. At this time, if the operating frequency remains unchanged, the secondary circuit will no longer resonate, and the output power and efficiency of the WPT system will be greatly reduced.

To further illustrate the impact of concrete medium on the WPT system, the magnetic field of the wireless power transmission system is simulated by simulation software. Comparing Figure 2(a) and Figure 2(b), it can be seen that the maximum value of the magnetic field strength around the receiving coil of the wireless power transmission system placed in the air is 50× ^10-6 T, while the maximum value of the magnetic field strength around the receiving coil embedded in concrete is reduced to 14× ^10-6 T. The reason for the reduction in the magnetic field strength around the receiving coil is that the concrete medium causes the wireless power transmission system to deviate from the resonant state. At the same time, because the electrical conductivity of concrete is greater than that of air, eddy current loss is generated in the concrete.

First of all, considering that compared with the WPT system in the air, the concrete-air trans-dielectric WPT system will be affected by the electromagnetic parameters of concrete, thereby changing the parameters of its original circuit. The equivalent circuit model of the concrete-air trans-dielectric WPT system is shown in Figure 3. The inductance of the transmitting coil and the receiving coil becomes the equivalent series inductance Leff and the equivalent series resistance ESR. The eddy current loss in the concrete is converted into the eddy current loss resistance R _loss in the circuit. The relative dielectric constant of the concrete will generate parasitic capacitance on the receiving coil and the transmitting coil. The parasitic capacitance in Figure 3 has been converted into the circuit and is not shown separately. The parasitic capacitance C _PC generated on the transmitting coil _LP is calculated by the following formula:

Among them, _εa is the relative dielectric constant of air, _εrp is the relative dielectric constant of the wire insulation layer of the transmitting coil _LP , _εo is the dielectric constant of vacuum, _lP is the length of the winding of the transmitting coil _LP , _rn1 is the radius of the copper core of the winding of the transmitting coil _LP , and _rw1 is the radius of the winding of the transmitting coil _LP .

The parasitic capacitance C _SC generated on the receiving coil _LS can be calculated by the following formula:

ε _c is the relative dielectric constant of concrete, ε _rs is the relative dielectric constant of the wire insulation layer of the receiving coil _LS , l _S is the length of the winding of the receiving coil _LS , r _n2 is the radius of the copper core of the winding of the receiving coil _LS , and r _w2 is the radius of the winding of the receiving coil _LS .

In order to overcome the problem of greatly reduced output power and efficiency of the WPT system caused by concrete medium, this example adopts a composite tuning control combining frequency modulation tuning and dynamic compensation tuning. The principle diagram and flow chart of the frequency modulation tuning control are shown in Figure 4 and Figure 5 respectively. It can be seen that the inverter conduction angle calculation unit includes an orthogonal signal generator, a Park transformation module, a PI control module and a frequency phase angle generator (FPG).

The orthogonal signal generator is used to generate two corresponding voltage signals with equal amplitude and 90° phase difference according to the current collected by the current detection circuit. The Park transformation module is used to perform Park transformation on the two orthogonal voltage signals generated by the orthogonal signal generator to obtain the q-axis voltage and the d-axis voltage. The PI control module is used to perform PI processing on the q-axis voltage obtained by the Park transformation module to obtain the frequency control amount. The frequency phase angle generator is used to calculate the inverter conduction angle θ' according to the frequency control amount and the current inverter frequency ω and output it to the drive circuit.

The primary current is collected and input into the orthogonal signal generator. The transfer function of the orthogonal signal generator is:

Assume the input signal is:

Where _Un , nω, _φn are the amplitude, angular frequency and phase of the nth harmonic of the input signal.

When the system is stable, we can get:

The orthogonal signal generator can generate two voltage signals U _α(n) and U _β(n) with equal amplitude and 90° phase difference. It can be found from the above formula that the orthogonal signal generator has a good suppression effect on high-order harmonics and is less affected by input signal distortion and interference.

The obtained orthogonal signal is related to the phase of the input current. After the orthogonal signal is processed by Parker transformation, it can be obtained:

From the above formula, it can be seen that the difference between the phase-locked loop output phase and the input phase can be expressed by the value of the q-axis voltage. When it is 0, the system input signal phase and output signal phase synchronization can be achieved, thereby realizing the resonance of the WPT system.

However, the receiving coil embedded in the concrete is more detuned due to the concrete medium, so it is necessary to add a variable capacitor to the primary circuit to adjust the natural resonant frequency of the primary circuit, and adjust the natural resonant frequency of the primary circuit according to the perturbation observation method, so that both the primary and secondary sides work in a resonant state. As shown in Figure 6(a), the adjustable capacitor circuit uses a PWM variable capacitor, including a fixed capacitor _Ca connected in series between the primary series resonant capacitor _CP and the transmitting coil _LP , and also includes a switch circuit connected in parallel at both ends of the fixed capacitor _Ca , the switch circuit includes a first MOS tube and a second MOS tube; the S pole of the first MOS tube is connected to the common end of the primary series resonant capacitor _CP and the transmitting coil _LP , the D pole of the first MOS tube is connected to the D pole of the second MOS tube, the S pole of the second MOS tube is connected to the common end of the fixed capacitor _Ca and the transmitting coil _LP , and the G pole of the first MOS tube and the G pole of the second MOS tube are connected to the capacitor conduction angle calculation unit.

The advantage of PWM variable capacitor is that it can continuously adjust the capacitance within a certain range. The equivalent capacitance of the capacitor combined with the primary series resonant capacitor _CP is:

As an example, a curve showing the change of the equivalent capacitance C _eq with the change of the conduction angle α of the switch tube is shown in FIG6( b ). It can be seen that the equivalent capacitance can be continuously adjusted within a certain range.

After determining the composition of the system, the system parameters can be determined by the following steps:

N1. Determine the system operating frequency f ₀ according to actual needs;

N2. According to actual needs, the shapes of the transmitting coil _LP and the receiving coil _LS are determined to be planar spiral coils, the radii of the copper cores of the windings are r _n1 and r _n2 respectively, and the radii of the windings are r _w1 and r _w2 respectively;

N3. Measure the inductance and internal resistance of the transmitting coil _LP and the receiving coil _LS , and determine the values of the primary series resonant capacitor _CP and the secondary series resonant capacitor _CS according to the resonance relationship;

N4. Determine the depth of the receiving coil _LS buried in the concrete according to the building requirements;

N5. Calculate the parasitic capacitance C _PC generated by the concrete-air medium on the transmitting coil _LP and the parasitic capacitance C _SC generated on the receiving coil _LS ;

N6. Determine the value of the fixed capacitor _Ca according to the parasitic capacitance C _PC and the parasitic capacitance C _SC .

Wherein, step N6 specifically includes the steps of:

N61. Determine the primary inductance variation range according to the parasitic capacitance C _PC , and determine the secondary inductance variation range according to the parasitic capacitance C _SC ;

N62. Calculate and determine the required range of change of the variable capacitor C _kb according to the range of change of the primary and secondary inductances;

N63. Calculate the variation range of the fixed capacitance _Ca according to the variation range of the variable capacitance C _kb and the range of the switch conduction angle α, and the maximum value of the variation range is _{Ca_max} ;

N64. Select a value in [1.2C _{a_max} , 2C _{a_max} ] as the parameter value of the fixed capacitance C _a .

By designing the parameter value of the fixed capacitor _Ca in steps N61 to N64, it is possible to avoid the fixed capacitor _Ca being too small to meet the adjustment requirements, or being too large to result in low adjustment sensitivity.

After determining the system parameters, it is necessary to control the system during operation, specifically: operate the system at an operating frequency of f ₀ and an initial switch conduction angle of α ₀ , and after the system is stable, obtain the input voltage and current of the system and the output voltage and current of the system and use them to calculate the efficiency of the system, and use the perturbation observation method to control the switch conduction angle of the adjustable capacitor module in real time with the goal of stabilizing the efficiency at a preset value; and obtain the input current of the system, and use PI control to control the conduction angle of the inverter with the goal of making the output voltage and output current of the inverter in phase. In the control of the system, the most important thing is to perform composite tuning control. Figure 7 is a flow chart of composite tuning control. The DC power supply converts DC into high-frequency AC through the inverter, and the electric energy is coupled to the receiving coil in the form of an electromagnetic field through the transmitting coil, and is finally consumed by the load. Through the combined action of frequency modulation tuning and variable capacitor tuning, the concrete medium can affect the WPT system and ensure that the system works in the optimal state.

To further illustrate the role of composite tuning control, this example uses Simulink to simulate the concrete-air cross-medium wireless power transmission system. During the simulation, the detuning phenomenon caused by the concrete medium in the actual situation is simulated by changing the value of the secondary inductance. The secondary inductance is 21.3uH~25.3uH. Figure 8(a) shows the output voltage and current waveform of the inverter without frequency tuning control. It can be seen from the figure that the inverter output current phase lags behind the voltage phase, indicating that the WPT system is inductive as a whole and the amplitude of the inverter output current is reduced. After frequency tuning, the inverter output voltage and current waveform is improved, as shown in Figure 8(b). The phase difference between the inverter output voltage and output current is 0, and the inverter output current amplitude increases. To determine whether the secondary circuit is in a resonant state, the phase difference between the primary current and the secondary current can be compared. When the WPT system is resonant, the primary current and the secondary current have a phase difference of 90°. Figure 9(a) is the waveform of the primary current and secondary circuit without dynamic compensation tuning. It can be seen from the figure that the phase difference between the primary current and the secondary current exceeds 90°, indicating that the secondary circuit is not in a resonant state at this time. Although the inverter input impedance phase is 0 at this time, the overall efficiency of the WPT system is not improved. After dynamic compensation tuning is adopted, the primary and secondary sides of the WPT system can be ensured to be in a resonant state. The waveforms of the primary current and the secondary current are shown in Figure 9(b).

Figure 10(a) and Figure 10(b) are the efficiency and output power curves of the WPT system before and after tuning. It can be seen that before tuning, the WPT has the highest efficiency only at the resonant frequency, but the efficiency will continue to decrease after the WPT is detuned. After tuning, the efficiency of the WPT system can be stabilized at a high level. At the same time, after using composite tuning, the output power of the WPT system is also improved.

In summary, the embodiment of the present invention provides a concrete-air cross-medium wireless power transmission system and a control method thereof, designs a concrete-air cross-medium wireless power transmission system, wherein the receiving coil _LS is installed in concrete, and the transmitting coil _LP is installed in the air, and a variable capacitor is added to the primary circuit by designing an adjustable capacitor module to adjust the natural resonant frequency of the primary according to the perturbation observation method, so that both the primary and secondary sides work in a resonant state, so that the system maintains a high transmission efficiency; the present invention also designs a frequency modulation tuning module to control the phase of the inverter output voltage to be consistent with the primary current phase, so that both the primary and secondary circuits of the WPT system resonate, thereby improving the output power and efficiency of the WPT system. In general, the present invention adopts a composite tuning control combining frequency modulation tuning and dynamic compensation tuning, which can maintain a high output power and efficiency when the concrete medium changes (such as humidity changes, temperature changes), has a wide range of applications, and is highly practical, and can charge electrical equipment in concrete stably, reliably, and efficiently.

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 (7)

1. The utility model provides a concrete-air strides medium wireless power transmission system, includes DC power supply, dc-to-ac converter, primary side series resonance electric capacity C _P, transmitting coil L _P that connect in proper order to and receiving coil L _S, secondary side series resonance electric capacity C _S, equivalent load R _L that connect in proper order, its characterized in that:

The receiving coil L _S is installed in concrete, and the transmitting coil L _P is installed in air;

The system also comprises an adjustable capacitance module, wherein the adjustable capacitance module comprises an efficiency calculation unit, a capacitance conduction angle calculation unit and an adjustable capacitance circuit, and the adjustable capacitance circuit is connected in series between the primary side series resonance capacitor C _P and the transmitting coil L _P; the efficiency calculation unit is used for calculating the input power and the output power of the system and calculating the efficiency of the system; the capacitance conduction angle calculation unit is used for calculating the conduction angle alpha of the switching tube according to the system efficiency and acting on the adjustable capacitance circuit, and the capacitance equivalent of the adjustable capacitance circuit connected to the circuit under the conduction angle alpha of the switching tube is as follows Wherein C _a is a fixed capacitance in the adjustable capacitance circuit;

The adjustable capacitance circuit comprises a fixed capacitance C _a connected in series between the primary side series resonance capacitance C _P and the transmitting coil L _P, and also comprises a switch circuit connected in parallel with two ends of the fixed capacitance C _a, wherein the switch circuit comprises a first MOS tube S ₁ and a second MOS tube S ₂; the S pole of the first MOS tube S ₁ is connected with the public end of the primary side series resonance capacitor C _P and the fixed capacitor C _a, the D pole of the first MOS tube S ₁ is connected with the D pole of the second MOS tube S ₂, the S pole of the second MOS tube S ₂ is connected with the public end of the fixed capacitor C _a and the transmitting coil L _P, and the G pole of the first MOS tube S ₁ and the G pole of the second MOS tube S ₂ are connected with the capacitor conduction angle calculation unit;

The system also comprises a frequency modulation tuning module, wherein the frequency modulation tuning module comprises a current detection circuit, an inversion conduction angle calculation unit and a driving circuit, the current detection circuit is used for collecting output current of the inverter, the inversion conduction angle calculation unit is used for calculating an inversion conduction angle theta 'according to the current collected by the current detection circuit, and the driving circuit drives the inverter by the inversion conduction angle theta';

The parameters of the system are determined by the following steps:

n1, determining the working frequency f ₀ of the system according to actual requirements;

N2, determining the shapes of the transmitting coil L _P and the receiving coil L _S as plane spiral coils according to actual requirements, wherein the radiuses of copper cores of windings are r _n1 and r _n2 respectively, and the radiuses of the windings are r _w1 and r _w2 respectively;

N3, measuring the inductance and the internal resistance of the transmitting coil L _P and the receiving coil L _S, and determining the values of a primary side series resonance capacitor C _P and a secondary side series resonance capacitor C _S according to the resonance relation;

n4, determining the depth of the receiving coil L _S buried in concrete according to building requirements;

N5, calculating a parasitic capacitance C _PC generated by the concrete-air medium on the transmitting coil L _P and a parasitic capacitance C _SC generated on the receiving coil L _S;

n6, determining the value of the fixed capacitance C _a from the parasitic capacitance C _PC and the parasitic capacitance C _SC.

2. A concrete-air cross-media wireless power transfer system as claimed in claim 1, wherein: the inversion conduction angle calculation unit comprises a quadrature signal generator, a Park conversion module, a PI control module and a frequency phase angle generator;

The quadrature signal generator is used for generating two corresponding voltage signals with equal amplitude and 90-degree phase difference according to the current acquired by the current detection circuit;

the Park conversion module is used for performing Park conversion on two orthogonal voltage signals generated by the orthogonal signal generator to obtain q-axis voltage and d-axis voltage;

The PI control module is used for performing PI processing according to the q-axis voltage obtained by the Park conversion module to obtain a frequency control quantity;

The frequency phase angle generator is used for calculating an inversion conduction angle theta 'according to the frequency control quantity and the current frequency omega of the inverter and outputting the inversion conduction angle theta' to the driving circuit.

3. A concrete-air cross-media wireless power transfer system as claimed in claim 2, wherein: the capacitance conduction angle calculation unit calculates the conduction angle alpha of the switching tube by adopting a disturbance observation method.

4. A concrete-air cross-medium wireless power transfer system according to claim 1, wherein in said step N5, the parasitic capacitance C _PC is calculated by:

epsilon _a is the relative dielectric constant of air, epsilon _rp is the relative dielectric constant of the wire insulation layer of the transmitting coil L _P, epsilon _o is the vacuum dielectric constant, and L _P is the length of the winding of the transmitting coil L _P.

5. The concrete-air cross-medium wireless power transfer system of claim 4, wherein in said step N5, the parasitic capacitance C _SC is calculated by:

Epsilon _c is the relative dielectric constant of concrete, epsilon _rs is the relative dielectric constant of the wire insulation layer of the receiving coil L _S, and L _S is the length of the winding of the receiving coil L _S.

6. The concrete-air cross-medium wireless power transmission system according to claim 1, wherein said step N6 specifically comprises the steps of:

n61, determining a primary side inductance change range according to the size of the parasitic capacitance C _PC, and determining a secondary side inductance change range according to the size of the parasitic capacitance C _SC;

N62, calculating and determining the change range of the required variable capacitor C _kb according to the change ranges of the primary side inductance and the secondary side inductance;

n63, calculating the variation range of the fixed capacitor C _a according to the variation range of the variable capacitor C _kb and the range of the conduction angle alpha of the switching tube, wherein the maximum value of the variation range is C _{a_max};

N64, a value is selected in [1.2C _{a_max},2C_{a_max} ] as the parameter value of the fixed capacitor C _a.

7. A control method of the concrete-air cross-medium wireless power transmission system according to any one of claims 1 to 6, comprising:

Operating the system with the working frequency f ₀ and the initial switch conduction angle alpha ₀, after the system is stable, acquiring the input voltage and current and the output voltage and current of the system, calculating the efficiency of the system, and controlling the conduction angle of a switch tube of the adjustable capacitor module in real time by adopting a disturbance observation method with the efficiency being stable at a preset value as a target; and acquiring input current of a system, aiming at making the output voltage and the output current of the inverter be in phase, and controlling the conduction angle of the inverter by adopting PI control.