Field of the invention

The invention relates to a resonator for wireless resonant energy transfer system.

Description of the related art

Inductive energy transmission is the most common principle of wireless energy transmission to electric transport units (Ahn, S. and J. Kim, (2011), "Magnetic Field Design for Low EMF and High Efficiency Wireless Power Transfer System in On-Line Electric Vehicles", Proc. of EuCAP2011, 4148-4151, 2011; J. Kim, et al.,(2010), "Low Frequency Electromagnetic Field Reduction Techniques for the On-Line Electric Vehicle (OLEV)", IEEE International Symposium on Electromagnetic Compatibility, pp. 625-630). Core part of such system for wireless energy transmission consists of two coils (transmitting and receiving) wound on ferrite cores. These coils are interconnected via magnetic induction across the gap between them. Depending on the design of the coils and quality of alignment, this gap can change substantially - distance can increase up to 3 times in comparison with optimum position. This misalignment would entail a change in coils inductance. If the two coils are of different design, like having different number of turns, or having different cross-section of the magnetic circuit, the inductance of each coil will vary by different amounts. The resonant frequency of the two coils will drift from the initially calculated and set to oscillation generator, thus the efficiency of energy transmission will drop substantially.

In order to keep the wireless energy transmission system operating frequency stable, and to adjust transmitting and receiving oscillating circuits working point near full resonance it is necessary to compensate the mentioned inductance change, and fine-tune it according to the current alignment situation of the two coils. The approach used to change inductance, among others, is adjusted capacitance connection to the transmitting coil [D. M. Vilathgamuwa, J. P. K. Sampath (2015) "Wireless Power Transfer for Electric Vehicles - Present and Future Trends", "Plug In Electric Vehicles in Smart Grids", ISBN 978-981-287-298-2]. Other researchers (S. Aldhaher, Patrick C. K. Luk, J.F. Whidborne, (2014) "Electronic Tuning of Misaligned Coils In Wireless Power Transfer Systems", Power Electronics, IEEE Transactions on (Volume: 29, Issue: 11); T.C. Beh, "Automated impedance matching system for robust wireless power transfer via magnetic resonance coupling," M.S. thesis, Dept. Advance Energy, The University of Tokyo, Kashiwa, Chiba, 2011] offer relay switches as the operation instruments for the capacitances connection.

Several power converter topologies have been developed utilizing resonant switching techniques, such as, for example, U.S. Patent No. 7,924,577 "Two terminals quasi resonant tank circuit". It discloses a power converter having a transformer, a primary switch, an auxiliary switch, first and second resonance capacitors, and a secondary side rectification means.

PCT application publication No. WO2014/018968 "Variable capacitor for resonant power transfer systems" discloses a system and a design for tuning a wireless power transfer system. Said wireless power transfer system includes first and second switched capacitor circuits electrically connected to opposite poles of the inductor of a resonator in the wireless power system. The first and second switched capacitor circuits can be switched on and off with MOSFETS to change a capacitance of the circuits, and thus an effective capacitance of the resonator.

U.S. Patent No. 7,880,337 "High power wireless resonant energy transfer system" discloses a high power wireless resonant energy transfer system that transfers energy across an air gap.

Summary of the invention

A resonator as a multi-capacitor circuit for a wireless power transfer system, comprising an inductor adapted to receive or transmit wireless power, a main capacitor electrically connected to opposite poles of the inductor, at least two capacitor circuits electrically connected to the opposite poles of the inductor, each of the capacitor circuits comprising a capacitor in series with a respective switch and a respective diode in parallel with the switch, and a controller electrically connected to each capacitor circuit. The controller is configured to switch the switches on and off to change a capacitance of the capacitor circuits to adjust an effective capacitance of the resonator.

The switches are micro-controlled operated electronic switches with diodes in parallel to the switch use to connect multi -capacitor circuit to the transmitting coil, thus decreasing number of switches needed. Capacitors are connected to the inductor, a transmitting coil in case where a resonator is an energy transmitter, via switches, which are operated by the controller in accordance to feedback signal from the oscillating circuit. Additional capacitor circuits can be added to increase a sensitivity of the circuit.

The controller is measuring the duration of the first oscillation, calculate the drift of the resonance frequency, and connects appropriate summary capacitance to the inductor, thus adjusting the frequency to the set level. Switching capacitor circuits takes place when the voltage on the capacitors is at maximum. This allows to connect additional capacitor circuits without substantial current jumps in the circuit of the resonator, and diminishing transition processes in the circuit of the resonator induced by commutation. This allows to have high speed adjustment of resonance frequency. At the same time, this principle allows to commute circuits under high currencies.

The controller is configured to control a frequency of the inductor, thus tuning the frequency of the inductor to a such a resonance frequency to optimize a power transfer efficiency of the wireless power transfer system.

The resonator further comprises at least one auxiliary capacitor circuit with predetermined capacitance to be switched on and of by means of the controller to change a capacitance of the capacitor circuits.

The resonator is controlled based on the measurement of the natural oscillations of the oscillating circuits. Disconnecting the excitation generator creates the transient processes in the coil system, lasting less than 3 full oscillation periods. This signal is being measured, and the resonance frequency calculated.

Measurement speed is high. For example, at a oscillation frequency f=85 kHz, the time required to measure the natural frequency of the oscillating circuit was 35 μβ, and subsequential fine tuning using the described system took one oscillation period or 12 μβ. The proposed system of resonant frequency adjusting by using multi-capacitance circuit allows to compensate changes in the inductance of the coil system over a wider range of distances between them. Proper choice of tuning steps size and number allows to tune the resonant frequency with the required accuracy. Particular design and layout of microcontroller operated electronic switches together with multi-capacitance circuit allows to use it in medium power wireless energy transmission systems. Proposed design allows to almost instantly adjust the resonance frequency of coils, which positions it as more preferable then electromagnetic relay systems.

In the energy transmitting circuit the inductor is an energy transmitting coil. In the energy receiving circuit the inductor is energy receiving coil.

Brief description of the drawings

Fig. 1 - illustrates schematic diagram of a resonator R.

Fig. 2 - illustrates another embodiment of a resonator R. Fig. 3 - illustrates schematic diagram of a wireless power transfer system with resonator R in a transmitting circuit and in a receiving circuit.

Fig. 4 - illustrates another embodiment of a resonator R comprising five capacitor circuits. Detailed description of the invention

Fig. 1 illustrates one embodiment of the wireless energy transmission system comprising resonator R. A resonator R comprises an inductor L adapted to transmit wireless energy, a main capacitor C electrically connected to opposite poles of the inductor L, six capacitor circuits CC, CCl, CCn electrically connected to the opposite poles of the inductor L. Each of the capacitor circuits CC, CCl, CCn comprising a capacitor CX in series with a switch S and a diode D in parallel with the switch S. The resonator R also comprises a controller 1 electrically connected to each capacitor circuit CC, CCl, CCn. The controller 1 is configured to switch the switches S on and off to change a capacitance of the capacitor circuits CC, CCl, CCn to adjust an effective capacitance of the resonator R.

Fig. 2 illustrates another embodiment of the wireless energy transmission system comprising resonator R. A resonator R comprises an inductor LT adapted to transmit wireless energy, a main capacitor C7 electrically connected to opposite poles of the inductor LT, six capacitor circuits electrically connected to the opposite poles of the inductor LT. Main capacitor C7 has a capacitance of 175 nF. The inductor LT is a transmitting coil with a core with inductance of 13 μΗ.

Each of the capacitor circuits comprising a capacitor CI, C2, C3, C4, C5 and C6, where each is in series with a respective switch S and a respective diode D in parallel with said switch S. The capacitor CI has a capacitance of 1.6 nF, C2 has 3.3 nF, C3 has 6.6 nF, C4 has 13.2 nF, C5 has 26 nF and C6 has 52 nF. Additional capacitor circuits can be added having capacitors with capacitance of up to 102 nF with the step size 1.6 nF.

The resonator R also comprises a controller 1 electrically connected to each capacitor circuit. The controller 1 is configured to switch the switches S on and off to change a capacitance of the capacitor circuits to adjust an effective capacitance of the resonator R.

Energy receiving circuit has a receiving coil LR and a resistive load RL. The inductor LR is a receiving coil with a core with inductance of 100 μΗ. Resistive load RL can be a battery to be charged.

Experiments showed that the tuning setup allows to compensate drift of the transmitter coil LT inductance in the range between 13..20 μΗ, and consequently reduce the drift of resonance frequency with the step size 258 - 397 Hz. Excitation of the circuit was created using IGBT H-bridge 3, connected to square shape signal generator with output frequency in the range of 60 to 100 kHz. Power to the system was supplied from DC voltage source, where an output value was set to 800V.

Control of multi-capacitor circuit and wireless energy transfer between coils was performed by the controller 1, which is a micro-controller module assembled on processor dsPIC33FJ64GS610.

Transmitting coil LT was designed as the inductor - spiral coil with diameter of 450 mm, containing four turns of wire with cross section 25 mm ² . Litz wire was used to reduce losses introduced by skin effect. The coil is positioned on the nickel-zinc ferrite disk.

Tests of current (100 A) and voltage (800 V) on the transmitting coil LT showed that they are at the stated level - current in the coil reached 100 A, and voltage 800 V. The dependence of transmitting coil LT inductance from the distance h between transmitting LT and receiving LR coils can be changed in the range from 40 to 200 mm without losing energy. The results show that with the distance increase from 10 mm to 190 mm the inductance of transmitting coil LT decreases exponentially from 25 μΗ to 13 μΗ (decreases by 48%.). Trend line shape was close to exponential, and with distance increase from 10 mm up to 190 mm the resonance frequency increased from 67 to 93 kHz.

The resonator R is able to compensate inductance changes, and to keep the resonance frequency equal to the generator oscillation frequency at stated value f=85 kHz within the range of distances h between the coils from 40 to 200 mm. Frequency deviation from the stated value was less than 800 Hz.

At closer distances, less than 40 mm, a capacitor C7 with lower capacitance should be used. Fig. 3 illustrates one embodiment of a wireless power transfer system with resonator R in a transmitting circuit and in a receiving circuit. Given solution allows to change the capacitance of the resonator R in both circuits, therefore increasing effectiveness of tuning. Given system is very similar with disclosed systems in Fig. 1 and Fig. 2. The only difference is that the receiving circuit also has the resonator R.

Fig. 4 illustrated another embodiment of the resonator R having ten capacitor circuits with ten capacitors CI to CIO and one main capacitor C. Additional capacitor circuits can be added having capacitors with capacitance of up to 102 nF with the step size 1.6 nF.

While various implementations and embodiments of the resonator have been described, it will be apparent to those skilled in the art that many more are possible.