Wireless electric energy coupling control method and system based on bicycle pile cooperation

Abstract

The invention provides a wireless electric energy coupling control method based on vehicle pile cooperation, and relates to the technical field of vehicle wireless charging, an electric vehicle wireless power transmission WPT system is of an LCC-S type compensation topological structure, and a DC-DC booster circuit is additionally arranged in front of an output load of the receiving end of the WPT system; the WPT system comprises a first receiver and a second receiver, the first receiver is of a double-pole double-coil structure, the second receiver is of a single-pole double-coil structure, the first receiver and the second receiver are decoupled, and two coils of the first receiver and two coils of the second receiver are arranged diagonally. According to the scheme, an LCC-S type compensation topological structure is improved, a DC-DC booster circuit is additionally arranged in front of an output load of a receiving end, and double-loop control parameters are optimized through a PSO algorithm, so that the charging power of the electric vehicle is stabilized; and the first receiver (double-pole double-coil) and the second receiver (single-pole double-coil) adopt a decoupling structure, and the coils are arranged diagonally, so that the energy transmission efficiency is effectively improved.

Description

Technical Field

[0001] The present invention relates to the technical field of wireless vehicle charging, and particularly to a wireless power coupling control method and system based on vehicle-pile cooperation. Background Art

[0002] As another major technological innovation in the field of new energy vehicles, wireless charging technology is gradually changing people's charging habits. It eliminates many inconveniences of traditional wired charging and provides strong support for the popularization and promotion of new energy vehicles.

[0003] In dynamic wireless charging technology in wireless charging technology, also known as "charging while driving", magnetic coupling wireless power transfer technology is usually adopted. By laying components such as transmitting coils, high-frequency inverters, and compensation networks under the original road surface, a power supply guide rail is formed. During the dynamic wireless charging process of an electric vehicle, the transmitting coil can identify and locate the position of the receiving coil. Once the transmitting coil approaches the receiving coil, it will automatically start, and transmit electric energy between the receiving coil and the transmitting coil through magnetic induction, so as to charge the on-vehicle battery pack. Such a charging system greatly reduces the capacity of the power battery equipped on the electric vehicle, saves energy and reduces emissions, and reduces the operating cost of the electric vehicle.

[0004] Since the vehicle is in a driving state during the dynamic wireless charging process, the relative position of the coupling coils is constantly changing, resulting in a change in the coupling coefficient and thus a continuous fluctuation in the charging power, reducing the energy transfer efficiency of the wireless power transfer (WPT) system of the electric vehicle. Summary of the Invention

[0005] The purpose of the present invention is to solve the problems in the above background art, and propose a wireless power coupling control method and system based on vehicle-pile cooperation.

[0006] The purpose of the present invention can be achieved through the following technical solutions: In the first aspect of the embodiments of the present invention, a wireless power coupling control method based on vehicle-pile cooperation is provided. The wireless power transfer (WPT) system of the electric vehicle is an LCC-S type compensation topology structure, and a DC-DC boost circuit is added before the output load at the receiving end of the WPT system; the WPT system includes a first receiver and a second receiver, the first receiver is a bipolar double-coil structure, the second receiver is a unipolar double-coil structure, the first receiver and the second receiver are decoupled, and the two coils of the first receiver and the two coils of the second receiver are arranged diagonally; Collect the inductor current and output voltage of the DC-DC boost circuit, and input the inductor current and output voltage as input signals into a PI controller and a PSO algorithm optimization module respectively; Optimize the parameters of the PI controller through the PSO algorithm; Determine the duty cycle of the switch in the output DC-DC boost circuit and output it to the PMW generator; The PMW generator generates a duty cycle control instruction to control the switch of the DC-DC boost circuit.

[0007] Optionally, the PSO algorithm takes minimizing the negative output power as the optimization objective function; the parameters of the PI controller optimized by the PSO algorithm include: Initialize the particle swarm parameters; each particle in the particle swarm includes a voltage loop proportionality, a voltage integral coefficient, a current loop proportionality, and a current integral coefficient; Substitute each particle into the preset voltage-current double closed-loop controller to determine the fitness, and iteratively update and optimize the parameters of the PI controller; the inner loop of the preset voltage-current double closed-loop controller is the current loop, the outer loop is the voltage loop, and it has voltage feedforward.

[0008] Optionally, the voltage loop PI controller:

[0009] where is the current reference signal, is the voltage loop proportionality, is the voltage reference signal, is the output voltage, is the voltage integral coefficient; Current loop PI controller:

[0010] is the duty cycle of the switch of the DC-DC boost circuit, is the current loop proportionality, is the inductor current, is the current integral coefficient.

[0011] Optionally, the transmitting end of the WPT system is composed of multiple transmitters with the same structure; each transmitter includes a first coil and a second coil, the first coil and the second coil share the same geometric center, and the coverage range of the second coil is included in the coverage range of the first coil; the first coil and the second coil are connected in reverse series.

[0012] Optionally, a third coil is deployed between every two adjacent transmitters, and the overlapping part of the coverage range of the first coil of each transmitter and the coverage range of the third coil accounts for one-half of the third coil; the first coil and the third coil are connected in the same direction in series.

[0013] In a second aspect of the embodiments of the present invention, a wireless power coupling control system based on vehicle-pile cooperation is further provided. The wireless power transfer (WPT) system for electric vehicles has an LCC-S type compensation topology structure, and a DC-DC boost circuit is added before the output load at the receiving end of the WPT system. The WPT system includes a first receiver and a second receiver. The first receiver has a bipolar double-coil structure, and the second receiver has a unipolar double-coil structure. The first receiver and the second receiver are decoupled, and the two coils of the first receiver and the two coils of the second receiver are arranged diagonally. The system includes: A data acquisition module, configured to acquire the inductor current and output voltage of the DC-DC boost circuit, and input the inductor current and output voltage as input signals into a PI controller and a PSO algorithm optimization module respectively; A PSO algorithm optimization module, configured to optimize the parameters of the PI controller through the PSO algorithm; A PI controller, configured to determine the duty cycle of the switch of the output DC-DC boost circuit and output it to a PMW generator; A PMW generator, configured to generate a duty cycle control instruction to control the switch of the DC-DC boost circuit.

[0014] Optionally, the PSO algorithm uses minimizing the negative output power as the optimization objective function. The PSO algorithm optimization module includes: An initialization module, configured to initialize the particle swarm parameters. Each particle in the particle swarm includes a voltage loop proportionality, a voltage integral coefficient, a current loop proportionality, and a current integral coefficient; An optimization update module, configured to substitute each particle into a preset voltage-current double closed-loop controller to determine the fitness, and iteratively update and optimize the parameters of the PI controller. The inner loop of the preset voltage-current double closed-loop controller is a current loop, the outer loop is a voltage loop, and it has a voltage feedforward.

[0015] Optionally, for the voltage loop PI controller:

[0016] where is the current reference signal, is the voltage loop proportionality, is the voltage reference signal, is the output voltage, is the voltage integral coefficient; For the current loop PI controller:

[0017] is the duty cycle of the switch of the DC-DC boost circuit, is the current loop proportionality, is the inductor current, Current integration coefficient.

[0018] Optionally, the transmitting end of the WPT system is composed of multiple transmitters with the same structure; each transmitter includes a first coil and a second coil, the first coil and the second coil share the same geometric center, and the coverage range of the second coil is included in the coverage range of the first coil; the first coil and the second coil are connected in reverse series.

[0019] Optionally, a third coil is deployed between every two adjacent transmitters, and the overlapping part of the coverage range of the first coil of each transmitter and the coverage range of the third coil accounts for one-half of the third coil; the first coil and the third coil are connected in the same direction in series.

[0020] Advantages of the present invention: The flowchart of a wireless power coupling control method based on vehicle-pile cooperation provided by an embodiment of the present invention. The wireless power transmission (WPT) system of the electric vehicle is an LCC-S type compensation topology structure, and a DC-DC boost circuit is added before the output load at the receiving end of the WPT system; the WPT system includes a first receiver and a second receiver. The first receiver is a bipolar double-coil structure, the second receiver is a unipolar double-coil structure, the first receiver and the second receiver are decoupled, and the two coils of the first receiver and the two coils of the second receiver are arranged diagonally; the inductor current and the output voltage of the DC-DC boost circuit are collected, and the inductor current and the output voltage are respectively input into a PI controller and a PSO algorithm optimization module as input signals; the parameters of the PI controller are optimized by the PSO algorithm; the duty cycle of the switch of the output DC-DC boost circuit is determined and output to a PMW generator; the PMW generator generates a duty cycle control instruction to control the switch of the DC-DC boost circuit.

[0021] The solution of the present invention improves the LCC-S type compensation topology structure by adding a DC-DC boost circuit before the output load at the receiving end, optimizes the double-loop control parameters through the PSO algorithm to achieve stable charging power of the electric vehicle; and the first receiver (bipolar double-coil) and the second receiver (unipolar double-coil) adopt a decoupled structure, and the coils are arranged diagonally, effectively improving the energy transmission efficiency. Description of the drawings

[0022] The present invention will be further described below with reference to the drawings.

[0023] Figure 1 It is a system block diagram of a wireless power coupling control system based on vehicle-pile cooperation provided by an embodiment of the present invention; Figure 2 It is a schematic diagram of the receiver of the WPT system provided by an embodiment of the present invention; Figure 3 It is a schematic diagram of a preset voltage-current double-loop controller provided by an embodiment of the present invention; Figure 4 Schematic diagram of the deployment of the transmitting coil of the WPT system provided by the embodiment of the present invention; Figure 5 Flow chart of a wireless power coupling control method based on vehicle-pile collaboration provided by the embodiment of the present invention. Detailed implementation manners

[0024] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0025] See Figure 1 , Figure 1 System block diagram of a wireless power coupling control system based on vehicle-pile collaboration provided by the embodiment of the present invention. The electric vehicle wireless power transfer WPT system is an LCC-S type compensation topology structure, and a DC-DC boost circuit is added before the output load at the receiving end of the WPT system; the WPT system includes a first receiver and a second receiver. The first receiver is a bipolar double-coil structure, the second receiver is a unipolar double-coil structure, the first receiver and the second receiver are decoupled, and the two coils of the first receiver and the two coils of the second receiver are arranged diagonally; the system includes: Data acquisition module, configured to acquire the inductor current and output voltage of the DC-DC boost circuit, and input the inductor current and output voltage as input signals into the PI controller and the PSO algorithm optimization module respectively; PSO algorithm optimization module, configured to optimize the parameters of the PI controller through the PSO algorithm, PI controller, configured to determine the duty cycle of the switch of the output DC-DC boost circuit and output it to the PMW generator; PMW generator, generating a duty cycle control instruction to control the switch of the DC-DC boost circuit.

[0026] Based on a wireless power coupling control system based on vehicle-pile collaboration provided by the embodiment of the present invention, the LCC-S type compensation topology structure is improved, a DC-DC boost circuit is added before the output load at the receiving end, and the double-loop control parameters are optimized through the PSO algorithm to achieve stable charging power of the electric vehicle; and the first receiver (bipolar double-coil) and the second receiver (unipolar double-coil) adopt a decoupled structure, and the coils are arranged diagonally, effectively improving the energy transmission efficiency.

[0027] In one implementation manner, see Figure 1, the charging coupling system of the WPT system includes a vehicle side and a pile side, both of which adopt a full-bridge structure. The pile side is the primary side and the transmitting end. The vehicle side consists of four switching tubes (P1 - P4) to form a full-bridge structure, and also includes a transmitting coil, a series capacitor Cp, a parallel capacitor Cf1, and a compensation inductor Lf1. The vehicle side is the secondary side and the receiving end, which consists of four diodes (VD1 - VD4) to form a full-bridge structure, and also includes receiving coils (the first receiver and the second receiver) on the secondary side, a series capacitor Cs, a parallel capacitor Cf2, a compensation inductor Lf2, a filter capacitor Co, and a load resistor RL. The inductor L, the switch S, the diode VD0, and the capacitor Cb form a DC-DC boost circuit.

[0028] The charging power equation of the WPT system is as follows: (1) Wherein, is the charging power, is the mutual inductance of the coupling coefficient, is the input voltage of the transmitting end, is the duty cycle of the boost circuit switch, is the inductance value of the compensation inductor, The resistance value of the load resistor.

[0029] According to formula (1), in order to suppress the charging power fluctuation caused by the continuous change of the coupling coefficient during the operation of the DWC system, the charging power can be controlled by adjusting the duty cycle of the boost circuit.

[0030] In one implementation, referring to Figure 2 , Figure 2 is a schematic diagram of the WPT system receiver provided by the embodiment of the present invention. The first receiver consists of coils 1 and 4, and coils 1 and 4 have opposite polarities to form a bipolar double-coil structure. The second receiver consists of coils 2 and 3, and coils 2 and 3 have the same polarity to form a unipolar double-coil structure. The mutual inductance between the first receiver and the transmitter plus the mutual inductance between the second receiver and the transmitter is the mutual inductance of the entire charging coupling system. Refer to the following formula: (2) Wherein, is the mutual inductance between the first receiver and the transmitter, The mutual inductance between the second receiver and the transmitter.

[0031] In one embodiment, the PSO algorithm takes minimizing the negative output power as the optimization objective function; the PSO algorithm optimization module includes: An initialization module for initializing the particle swarm parameters; each particle in the particle swarm includes a voltage loop ratio, a voltage integral coefficient, a current loop ratio, and a current integral coefficient; Optimization and update module, which is used to substitute each particle into a preset voltage-current double closed-loop controller to determine the fitness and iteratively update the parameters of the optimized PI controller; the inner loop of the preset voltage-current double closed-loop controller is the current loop, the outer loop is the voltage loop, and it has a voltage feedforward.

[0032] In one embodiment, refer to Figure 3 , Figure 3 which is a schematic diagram of the preset voltage-current double closed-loop controller provided by the embodiment of the present invention.

[0033] Voltage loop PI controller: (3) where is the current reference signal, is the voltage loop ratio, is the voltage reference signal, is the output voltage, is the voltage integral coefficient; Current loop PI controller: (4) is the duty cycle of the DC-DC boost circuit switch, is the current loop ratio, is the inductor current, is the current integral coefficient.

[0034] In one implementation, the outer loop voltage loop serves as the main control loop to generate the current reference signal, and the inner loop current loop serves as the slave control loop to quickly track the current command. The feedforward channel: directly compensates for the input voltage disturbance to improve the anti-interference ability. Figure 3 where V ref (s), I ref (s), V in (s), V L (s) and I L (s) are the Laplace transform forms of V ref , I ref , V in , V L and I L respectively. The integral characteristic of the output capacitor is 1 / sC, the feedback voltage divider is 1 / RL, Gv(s) is the voltage loop controller, Gi(s) is the current loop controller, and k is the voltage division ratio of the output voltage VL.

[0035] Determine the reference voltage signal according to the current charging mode of the vehicle , for example, if the vehicle is currently in the constant power charging mode, then (5) The required power set for the vehicle battery management system.

[0036] The vehicle is currently in the constant current charging mode, and the reference voltage signal is calculated according to the reference current set by the vehicle battery management system. : (6) If the vehicle is currently in the constant voltage charging mode, then the reference voltage signal is set. The rated voltage set for the vehicle battery management system; The reference voltage signal serves as the input of the current control loop; the reference voltage signal and the output voltage are compared through an adder to obtain the voltage error signal; the voltage error signal outputs the current reference signal through the current loop controller, and the current reference signal and the inductor current are compared through an adder to obtain the current error signal; the current error signal outputs the control signal through the current compensator, and the control signal adjusts the switch duty ratio.

[0037] In the voltage control loop, the output voltage obtains the output current through the feedback voltage dividing network 1 / RL and inputs it to the adder, where it is compared with the reference voltage to obtain the voltage error signal. The voltage error signal passes through the voltage loop controller to adjust the signal regulating switch duty ratio, thereby controlling the output voltage.

[0038] In one embodiment, refer to Figure 4 , the structural schematic diagram of the deployment of the transmitting end coil of the WPT system provided by the embodiment of the present invention. The transmitting end of the WPT system is composed of multiple transmitters with the same structure; each transmitter includes a first coil and a second coil. The first coil and the second coil share the same geometric center, and the coverage range of the second coil is included in the coverage range of the first coil; the first coil and the second coil are connected in reverse series; a third coil is deployed between every two adjacent transmitters, and the overlapping part of the coverage range of the first coil of each transmitter and the coverage range of the third coil accounts for one-half of the third coil; the first coil and the third coil are connected in the same direction in series.

[0039] In one implementation, the transmitting end of the WPT system can be deployed under any target road. When the vehicle is traveling on the target road, the transmitter under the road and the receiver on the vehicle can form a coupling coil for wireless charging of the electric vehicle.

[0040] Figure 4 In (a) is the distribution diagram of the first coil and the second coil. Figure 4In figure (b), it is the distribution diagram of the first coil and the third coil. The first coil serves as the main transmission coil, the second coil serves as the suppression transmission coil, and the third coil serves as the auxiliary transmission coil. Since the first coil and the second coil share the same geometric center, and the first coil and the second coil are connected in reverse series, the second coil can suppress the coupling coefficient when the receiver moves to the central part of the first coil; the third coil is located at the edge of the first coil and is connected in series with the first coil in the same direction, so the second coil can increase the coupling coefficient when the receiver moves to the edge part of the first coil. The combined deployment of the first coil, the second coil, and the third coil can avoid excessive changes in the coupling coefficient during vehicle movement, make the coupling coefficient relatively stable during the movement process, and can improve the anti-offset performance of mobile charging.

[0041] An embodiment of the present invention provides a wireless power coupling control method based on vehicle-pile cooperation. Refer to Figure 5 , Figure 5 which is a flowchart of a wireless power coupling control method based on vehicle-pile cooperation provided by an embodiment of the present invention.

[0042] S1, Collect the inductor current and output voltage of the DC-DC boost circuit, and input the inductor current and output voltage as input signals into the PI controller and the PSO algorithm optimization module respectively; S2, Optimize the parameters of the PI controller through the PSO algorithm; S3, Determine the duty cycle of the switch of the output DC-DC boost circuit and output it to the PMW generator; S4, The PMW generator generates a duty cycle control instruction to control the switch of the DC-DC boost circuit.

[0043] In one embodiment, the PSO algorithm takes minimizing the negative output power as the optimization objective function; optimizing the parameters of the PI controller through the PSO algorithm includes: Initialize the particle swarm parameters; each particle in the particle swarm includes a voltage loop ratio, a voltage integral coefficient, a current loop ratio, and a current integral coefficient; Substitute each particle into a preset voltage-current double closed-loop controller to determine the fitness, and iteratively update and optimize the parameters of the PI controller; the inner loop of the preset voltage-current double closed-loop controller is the current loop, the outer loop is the voltage loop, and it has voltage feedforward.

[0044] The above has described an embodiment of the present invention in detail, but the content described is only a preferred embodiment of the present invention and cannot be considered as limiting the scope of implementation of the present invention. All equivalent changes and improvements made according to the scope of the application of the present invention should still fall within the scope covered by the patent of the present invention.

Claims

1. A wireless power coupling control method based on vehicle-pile cooperation, characterized in that The wireless power transfer (WPT) system for electric vehicles has an LCC-S type compensation topology, and a DC-DC boost circuit is added before the output load at the receiving end of the WPT system; the WPT system includes a first receiver and a second receiver, the first receiver has a bipolar double-coil structure, the second receiver has a unipolar double-coil structure, the first receiver and the second receiver are decoupled, and the two coils of the first receiver and the two coils of the second receiver are arranged diagonally; Collect the inductor current and output voltage of the DC-DC boost circuit, and input the inductor current and output voltage as input signals into a PI controller and a PSO algorithm optimization module respectively; Optimize the parameters of the PI controller through the PSO algorithm; Determine the duty cycle of the switch of the output DC-DC boost circuit and output it to the PMW generator; The PMW generator generates a duty cycle control instruction to control the switch of the DC-DC boost circuit.

2. The wireless power coupling control method based on vehicle-pile cooperation according to claim 1, characterized in that The PSO algorithm uses minimizing the negative output power as the optimization objective function; optimizing the parameters of the PI controller through the PSO algorithm includes: Initialize the particle swarm parameters; each particle in the particle swarm includes a voltage loop ratio, a voltage integral coefficient, a current loop ratio, and a current integral coefficient; Substitute each particle into a preset voltage-current double closed-loop controller to determine the fitness, and iteratively update and optimize the parameters of the PI controller; the inner loop of the preset voltage-current double closed-loop controller is a current loop, the outer loop is a voltage loop, and it has voltage feedforward.

3. A wireless power coupling control method based on vehicle-pile cooperation according to claim 1, characterized in that, Voltage loop PI controller: I ref = Kpv(V ref - V L ) + Kiv∫(V ref - V L )dt where I ref is the current reference signal, Kpv is the voltage loop proportionality, V ref is the voltage reference signal, V L is the output voltage, and Kiv is the voltage integral coefficient; Current loop PI controller: D = Kpi(I ref - I L ) + Kii∫(I ref - I L )dt D is the duty cycle of the switch of the DC-DC boost circuit, Kpi is the current loop ratio, and I L is the inductor current, and Kii is the current integral coefficient.

4. A wireless power coupling control method based on vehicle-pile collaboration according to claim 1, characterized in that The transmitting end of the WPT system is composed of multiple transmitters with the same structure; each transmitter includes a first coil and a second coil, the first coil and the second coil share the same geometric center, and the coverage range of the second coil is included in the coverage range of the first coil; the first coil and the second coil are connected in reverse series.

5. A wireless power coupling control method based on vehicle-pile cooperation according to claim 4, characterized in that A third coil is deployed between every two adjacent transmitters, and the overlapping part of the coverage range of the first coil of each transmitter and the coverage range of the third coil accounts for one-half of the third coil; the first coil and the third coil are connected in the same direction in series.

6. A wireless power coupling control system based on vehicle-pile cooperation, characterized in that, The wireless power transfer (WPT) system for electric vehicles has an LCC-S type compensation topology, and a DC-DC boost circuit is added before the output load at the receiving end of the WPT system; the WPT system includes a first receiver and a second receiver, the first receiver has a bipolar double-coil structure, the second receiver has a unipolar double-coil structure, the first receiver and the second receiver are decoupled, and the two coils of the first receiver and the two coils of the second receiver are arranged diagonally; the system includes: A data acquisition module, which is used to collect the inductor current and output voltage of the DC-DC boost circuit, and input the inductor current and output voltage as input signals into a PI controller and a PSO algorithm optimization module respectively; A PSO algorithm optimization module, which is used to optimize the parameters of the PI controller through the PSO algorithm; A PI controller, which is used to determine the duty cycle of the switch of the output DC-DC boost circuit and output it to the PMW generator; A PMW generator, which generates a duty cycle control instruction to control the switch of the DC-DC boost circuit.

7. A wireless power coupling control system based on vehicle-pile cooperation according to claim 6, characterized in that The PSO algorithm takes minimizing the negative output power as the optimization objective function; the PSO algorithm optimization module includes: An initialization module for initializing the particle swarm parameters; each particle in the particle swarm includes a voltage loop ratio, a voltage integral coefficient, a current loop ratio, and a current integral coefficient; An optimization update module for substituting each particle into a preset voltage-current double closed-loop controller to determine the fitness and iteratively updating and optimizing the parameters of the PI controller; the inner loop of the preset voltage-current double closed-loop controller is a current loop, the outer loop is a voltage loop, and it has a voltage feedforward.

8. A wireless power coupling control system based on vehicle-pile cooperation according to claim 6, characterized in that Voltage loop PI controller: I ref = Kpv(V ref - V L ) + Kiv∫(V ref - V L )dt where I ref is the current reference signal, Kpv is the voltage loop proportionality, V ref is the voltage reference signal, V L is the output voltage, and Kiv is the voltage integral coefficient; Current loop PI controller: D = Kpi(I ref - I L ) + Kii∫(I ref - I L )dt D is the duty cycle of the switch of the DC-DC boost circuit, Kpi is the current loop ratio, and I L is the inductor current, and Kii is the current integral coefficient.

9. A wireless power coupling control system based on vehicle-pile cooperation according to claim 6, characterized in that The transmitting end of the WPT system consists of multiple transmitters with the same structure; each transmitter includes a first coil and a second coil, the first coil and the second coil share the same geometric center, and the coverage range of the second coil is included in the coverage range of the first coil; the first coil and the second coil are connected in reverse series.

10. A wireless power coupling control system based on vehicle-pile cooperation according to claim 9, characterized in that, A third coil is deployed between every two adjacent transmitters; the overlapping part of the coverage range of the first coil of each transmitter and the coverage range of the third coil accounts for one-half of the third coil; the first coil and the third coil are connected in the same direction in series.

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