JP7751781B2 - Ultrasonic non-contact power supply system, non-contact power transmitting device and non-contact power receiving device - Google Patents

Description

Article 30, paragraph 2 of the Patent Act applies. Publication name: Proceedings of the 2021 Industrial Applications Conference of the Institute of Electrical Engineers of Japan. Publication date: August 18, 2021. Publisher: The Institute of Electrical Engineers of Japan.

Article 30, paragraph 2 of the Patent Act applies. Publication name: IEICE Technical Report EE2020-35 (2021-01) Publication date: January 18, 2021 Publisher: Institute of Electronics, Information and Communication Engineers

Article 30, paragraph 2 of the Patent Act applies. Publication name: IEICE Technical Report EE2020-11 (2020-10) Publication date: September 30, 2020 Publisher: Institute of Electronics, Information and Communication Engineers

The present invention relates to a power supply system that uses ultrasound to transmit power to implantable medical devices without contact.

In recent years, wireless power transfer (WPT) systems, which transmit power from outside the body without contact, have been attracting attention as a method of powering batteries that power implanted medical devices such as pacemakers and artificial organs. Typical medical WPT methods include the electromagnetic induction method (magnetic field resonance method) using an induction coil pair and the electrostatic coupling method using parallel plates (electrode plates). These methods are capable of supplying relatively large amounts of power (up to several tens of watts).
However, the currently mainstream electromagnetic induction type WPT has problems such as health hazards such as skin burns and dizziness due to electromagnetically induced currents (eddy currents), and irritating effects that affect nerves and muscles, as well as dielectric loss that depends on the salt concentration of body fluids and blood, resulting in localized temperature increases within the body, and electromagnetic noise that can cause malfunctions in electronic devices inside the body, making it clear that it is not necessarily an appropriate means of powering medical devices implanted in the human body.

On the other hand, compared to electromagnetic induction WPT, WPT using ultrasonic vibrations has been proposed for medical use because it has no health effects from exposure to the magnetic and electric fields it generates, causes little irritation to nerves and sensory organs, and can be developed into bioenergy generation because even the micro-vibrations of organs can be used to generate electricity.It is expected to be put to practical use as a means of powering cochlear implants and implantable medical devices.
However, attenuation of sound waves due to obstacles such as the skeleton inside the body is an issue, making it difficult to achieve a power supply of several watts, which is the applicable range of the magnetic resonance method, and this is hindering the practical application of ultrasonic WPT.

One of the present inventors has already proposed an ultrasonic contactless power transfer system that uses a high-frequency inverter that makes full use of power semiconductor switches to enable frequency control and amplitude control, and that is more efficient and performs well underwater in environments where the surrounding environment changes significantly (see Patent Document 1). In the proposed ultrasonic contactless power transfer system, the power transmission unit is equipped with a resonance frequency tracking control means, which detects the conduction current of the ultrasonic transducer on the power transmission side and controls the operating frequency of the high-frequency inverter so that the voltage and current are in phase, in order to maximize the efficiency of electrical-acoustic conversion and maximize the output power generated by the ultrasonic transducer.
When using the proposed ultrasonic contactless power transfer system to transfer power wirelessly to the battery of an implantable medical device, the following problems arise. The first problem is that the impedance of the implantable medical device's battery (internal load) and the body tissue (insulator) between the external power transmitter and the internal power receiver is high, making it impossible to extract power directly from the high-frequency inverter of the external power transmitter. The second problem is the difficulty of deriving the values of the circuit parameters of the electrical equivalent circuit of the body tissue (insulator) between the power transmitter and the internal power receiver.

Japanese Patent Application Laid-Open No. 2017-220990

As mentioned above, while ultrasonic WPT overcomes the problems of electromagnetic induction WPT, it has problems such as a small transmitted power (a few milliwatts) and susceptibility to sound wave attenuation by bones, tissues, etc., and is therefore often limited to information-based applications such as monitoring implanted medical devices. For this reason, in order to commercialize and popularize ultrasonic WPT for power transmission, a power conversion circuit and piezoelectric module structure (combination) that can transmit higher power is required.
To achieve high-power transmission in ultrasonic WPT, it is necessary to solve the problem of not being able to extract power from the high-frequency inverter due to the high impedance of the battery (internal load) and body tissue (insulator).It is also necessary to solve the problem of difficulty in determining the circuit parameter values of the electrical equivalent circuit of the body tissue (insulator) during ultrasonic transmission and reception.

In light of this situation, the present invention aims to provide an ultrasonic contactless power transfer system that achieves more efficient, high-power transmission to the battery of an implantable medical device.

To solve the above problem, the ultrasonic contactless power transfer system of the present invention is composed of a contactless power transmitting device having the following features 1a to 1e and a contactless power receiving device having the following features 2a to 2c. The contactless power transmitting device is characterized in that the transmitting ultrasonic vibrator operates at an electrical system resonant frequency set based on the impedance characteristics combining three elements: the electrical impedance of the insulator located between the pair of transmitting ultrasonic vibrator and receiving ultrasonic vibrator, the impedance of the transmitting ultrasonic vibrator, and the impedance of the receiving ultrasonic vibrator; and the high-frequency inverter operates at the natural frequency of the AC circuit combining the impedance combining the three elements with the impedance of the resonant circuit and impedance matching circuit.

(Non-contact power transmission device)
1a) A high frequency inverter that converts the voltage supplied from a voltage source into an AC voltage of a predetermined high frequency.
1b) A resonant circuit connected to the output of a high frequency inverter.
1c) A power transmitting ultrasonic transducer using a piezoelectric element connected to a resonant circuit.
1d) A resonance frequency tracking control circuit that makes the mechanical resonance frequency inherent to the power transmitting ultrasonic vibrator substantially the same as the electrical resonance frequency and causes the operating frequency of the high frequency inverter to follow the electrical resonance frequency;
1e) Impedance matching circuits.

(Non-contact power receiving device)
2a) A power-receiving ultrasonic vibrator using a piezoelectric body that receives ultrasonic waves transmitted from a power-transmitting ultrasonic vibrator.
2b) A high-frequency rectifier circuit that rectifies the output voltage of the receiving ultrasonic vibrator.
2c) A battery connected to a high frequency rectifier circuit.

In the present invention, to improve transmitted power, a resonant circuit that takes into account the impedance characteristics of body tissue is provided in the external power transmitting device. This reduces the influence of body tissue impedance, which inhibits high-efficiency conversion of the piezoelectric forward and reverse effects, and operates the high-frequency inverter provided in the external power transmitting device at a high power factor, exciting the transmitting ultrasonic transducer with higher amplitude. The resonant circuit acts on the receiving ultrasonic transducer implanted in the body and the high-frequency rectifier circuit that supplies DC current to the battery, reducing the influence of body tissue and compensating for the charging current to the battery. This is one technical feature for improving transmitted power in the present invention, and to achieve this, it can be said that the present invention is characterized by the electrical circuit modeling of the transmitting and receiving ultrasonic transducers, as well as the body tissue (insulators) between the transmitting and receiving ultrasonic transducers.
Furthermore, when the impedance of the load (in this case, the battery of the implantable medical device) changes in an electrical circuit, the transmitted power is maximized when that impedance becomes the complex conjugate of the impedance of the power supply source of the power transmitter, but in the case of a high-frequency circuit, it is necessary to match the impedance characteristics of the transmission path as well. To achieve high-power transmission for greater power supply, the impedance of the body tissue is modeled in the electrical circuit, and the impedance of the power transmitter/receiver modules is calculated as a whole, including the electrical impedance of the ultrasonic transducers (power transmitter/receiver modules) on the transmitter and receiver sides. A resonant circuit is then installed on the transmitter side only to resonate with this circuit, enabling the high-frequency inverter to operate with high efficiency and low noise. This is another technical feature.
Furthermore, if the impedance is not matched, the target output may not be achieved, reflected waves or standing waves may occur in the transmission path, causing waveform distortion, or electric shock or radio interference may occur. Therefore, one of the technical features is the inclusion of an impedance matching circuit in the power transmission device.

The resonant frequency in the ultrasonic contactless power transfer system of the present invention is the frequency at which the absolute value of the impedance is minimum and the phase angle is zero in the impedance characteristics.

In the ultrasonic contactless power transfer system of the present invention, the high frequency inverter in the contactless power transmitting device is preferably configured as a voltage type half-bridge inverter, and the switch-on time ratios of the two switches are preferably approximately the same.
To strengthen the excitation energy of the piezoelectric element, a resonant circuit is connected to the high-frequency inverter. The power transmission/reception module and all electrical circuits on the power receiving side are modeled together, and the resonant circuit is installed only on the power transmitting side so that it resonates with the modeled electrical circuit.
This allows the low-impedance ultrasonic transducer on the power-transmitting side to operate as a current source, allowing high-frequency current to be input to the ultrasonic transducer on the power-transmitting side with higher efficiency.

In the ultrasonic contactless power supply system of the present invention, the contactless power receiving device preferably includes a voltage controller that inputs a battery voltage value and a command value and outputs a voltage control signal, and a receiving-side short-range wireless communication means that transmits the voltage control signal to the contactless power transmitting device, and the contactless power transmitting device preferably includes a transmitting-side short-range wireless communication means and a pulse modulation circuit that uses the voltage control signal received by short-range wireless communication as a command value to modulate the drive pulse of a high-frequency inverter whose clock frequency is a pulse signal obtained from a resonant frequency tracking control circuit.
Assuming a short distance (less than 10 cm) between the inside and outside of the body, a voltage control signal based on the voltage value of the battery inside the body and the command value is transmitted to the power transmission device via short-range wireless communication.

In the ultrasonic contactless power transfer system of the present invention, it is preferable that the ultrasonic vibrator on the power transmitting side is a bolt-clamped Langevin vibrator (BLT), and the ultrasonic vibrator on the power receiving side uses a piezoelectric thin film element. By combining a hybrid ultrasonic vibrator, such as a bolt-clamped Langevin vibrator (BLT) suitable for emitting powerful vibration energy on the power transmitting side installed outside the body and a thin, lightweight piezoelectric material (e.g., PZT) embedded inside the body, it is possible to introduce more powerful ultrasonic vibration energy into the body at low cost and to construct an ultrasonic contactless power transfer system that achieves higher power transmission.
A BLT (Bolt-Clamped Langevin Type Transducer) is a piezoelectric porcelain (PZT) vibrator sandwiched between two metal plates, which are fastened together with a bolt. PZT is lead zirconate titanate, a composite oxide (ceramic) of zirconium (Zr) and titanium (Ti). When pressure is applied, it becomes electrically polarized, and conversely, when voltage is applied, it expands and contracts. Because a compressive stress is applied in advance by the bolt, the BLT can achieve a very large stress amplitude, making it suitable for use as a powerful ultrasonic vibrator.

In the ultrasonic contactless power transfer system of the present invention, the high-frequency rectifier circuit in the contactless power receiving device is preferably a voltage multiplier rectifier circuit. Taking into account the high impedance inside the body, a voltage multiplier rectifier circuit, such as a voltage doubler rectifier circuit, is provided in the rectifier circuit of the power receiving device to achieve high-power transmission.

In the ultrasonic contactless power transfer system of the present invention, the insulator may be body tissue, and the contactless power receiving device may be implanted in the body or built into an implantable medical device. Here, the body tissue actually corresponds to bone, skin, fat, etc.
By utilizing an ultrasonic contactless power transfer system in implantable medical devices, power can be supplied from outside the body, eliminating the need for surgery to replace dead batteries in implantable medical devices and reducing the risks of surgical replacement (such as poor reconnection of pacemaker leads) and infections. Furthermore, the capacity of batteries in surgically implantable medical devices can be reduced, facilitating the miniaturization and weight reduction of the devices, potentially reducing the burden on the implant site and expanding the options for implantation locations. This is expected to expand the types and uses of implantable devices.

In the ultrasonic contactless power transfer system of the present invention, the impedance characteristics combining the three elements, including the electrical impedance of the insulator, may be measured using a phantom that represents the above-mentioned body tissue. For example, a medical phantom can be used as the phantom.

In the non-contact power receiving device constituting the ultrasonic non-contact power supply system of the present invention, the battery may have a first port connected to a high-frequency rectifier circuit and a second port connected to an energy harvesting device that harvests energy from an energy source in the surrounding environment.
An energy harvesting device is generally a device that obtains power by harvesting energy from sources such as sunlight, ambient light, vibrations, and heat, but in this invention, it refers to a device that obtains power by harvesting vibrational energy from body vibrations, assuming that the power receiving device is implanted in the body. A known device that extracts power from vibrations using MEMS (microelectromechanical systems) is installed in the power receiving device, and the battery of the power receiving device can be charged via two routes: a route supplied from a first port connected to a high-frequency rectifier circuit, and a route supplied from a second port connected to the energy harvesting device that harvests energy from vibrations.

A method for producing inductance and capacitance of a resonant circuit in a contactless power transmitting device constituting an ultrasonic contactless power feeding system of the present invention includes the following steps.
(Step 1) A step of measuring impedance characteristics that combine three elements: the electrical impedance of the insulator, the impedance of the power transmitting ultrasonic vibrator, and the impedance of the power receiving ultrasonic vibrator.
(Step 2) A step of calculating the circuit parameters of the resistance and capacitance of an equivalent circuit combining the three elements from the minimum absolute value of the impedance in the impedance characteristics and its phase.
(Step 3) Impedance matching step.
(Step 4) Setting the frequency at which the impedance is at a minimum to the resonant frequency of the resonant circuit.
(Step 5) Setting the Q value of the resonant circuit based on the resonant frequency.
(Step 6) Determine the inductance and capacitance values of the resonant circuit based on the resonant frequency and Q value.

This invention has the effect of achieving more efficient, high-power transmission for batteries in implantable medical devices. By introducing a resonant circuit connected to the high-frequency inverter of the power transmission device, the fundamental power factor on the transmission side can be improved.

Functional block diagram of an ultrasonic non-contact power transfer system (Example 1) Functional block diagram of a resonant frequency tracking control circuit in a power transmission device Flow diagram for setting circuit parameters of a resonant circuit in a power transmission device Circuit block diagram of an ultrasonic non-contact power transfer system (Example 1) Equivalent circuit diagram of three-element impedance Circuit diagram of ultrasonic contactless power transfer system (Example 1) Measurement circuit diagram of three-element impedance Three-element impedance measurement system configuration diagram Graph showing the measurement results of impedance characteristics Explanation of L-shaped simplified equivalent circuit Equivalent circuit diagram including resonant circuit and impedance matching circuit Simulation waveforms of circuit operation in an ultrasonic contactless power transfer system Functional block diagram of an ultrasonic non-contact power transfer system (Example 2) Circuit block diagram of an ultrasonic non-contact power transfer system (Example 2) Functional block diagram of an ultrasonic non-contact power transfer system (Embodiment 3) Circuit block diagram of an ultrasonic non-contact power transfer system (Embodiment 3) Functional block diagram of a power receiving device (Fourth embodiment)

An example of an embodiment of the present invention will be described in detail below, with reference to the drawings. Note that the scope of the present invention is not limited to the following examples and illustrated examples, and many modifications and variations are possible.

FIG. 1 shows functional blocks of an ultrasonic non-contact power transfer system according to an embodiment of the present invention.
As shown in FIG. 1 , the ultrasonic contactless power transfer system includes a power transmitter 1 and a power receiver 2. Power is supplied from the power transmitter 1 to the power receiver 2 in a contactless manner using ultrasonic transmission 3. The power transmitter 1 includes a high-frequency inverter 12 that converts a voltage supplied from a voltage source 11 into a predetermined high-frequency AC voltage, a resonant circuit 14 connected to the output of the high-frequency inverter 12, a power-transmitting ultrasonic transducer (piezoelectric element) 13 connected to the resonant circuit 14 via an impedance matching circuit 15, and a resonant frequency tracking control circuit 16 that adjusts the operating frequency of the high-frequency inverter to the mechanical resonant frequency of the power-transmitting ultrasonic transducer 13. The voltage source 11 may be any type that supplies a DC voltage, or may be one that converts an AC voltage to AC/DC. The power-transmitting ultrasonic transducer (piezoelectric element) 13 is a bolt-clamped Langevin transducer (BLT) capable of outputting powerful ultrasonic vibration energy. The resonance frequency tracking control circuit 16 makes the mechanical resonance frequency inherent to the power transmitting ultrasonic vibrator 13 approximately the same as the electrical resonance frequency, makes it follow the electrical resonance frequency, and sends an operating frequency to the switch drive circuit 19 of the high frequency inverter.
On the other hand, the power receiving device 2 is composed of a power receiving ultrasonic vibrator (piezoelectric element) 23 that receives the ultrasonic waves transmitted from the power transmitting ultrasonic vibrator 13, a high frequency rectifier circuit 22 that rectifies the output voltage of the power receiving ultrasonic vibrator, and a chargeable and dischargeable battery 21 connected to the output of the high frequency rectifier circuit 22.

A feature of the present invention is that in the power transmission device 1, the power transmission side ultrasonic vibrator 23 operates at an electrical system resonance frequency set based on the impedance characteristics obtained by combining three elements: the electrical impedance of the insulator 4 present between the pair of the power transmission side ultrasonic vibrator 13 and the power receiving side ultrasonic vibrator 23, the impedance of the power transmission side ultrasonic vibrator 13, and the impedance of the power receiving side ultrasonic vibrator 23; and the high frequency inverter 12 operates at the natural frequency of the AC circuit obtained by adding the impedance of the resonance circuit 14 and the impedance matching circuit 15 to the impedance obtained by combining the three elements.
The impedance of the insulator 4 is modeled as an electrical circuit, and the impedance of the power transmitting and receiving modules is calculated as a whole, including the electrical impedance of the power transmitting ultrasonic vibrator 13 and the power receiving ultrasonic vibrator 23. A resonant circuit is then installed on the power transmitting device 1 side only so that it resonates with this circuit, allowing the high frequency inverter 12 to operate with high efficiency and low noise.

Here, the resonance frequency tracking control circuit 16 in the power transmitting device 1 will be described with reference to the functional block diagram shown in FIG.
The resonant frequency tracking control 16 of the high-frequency inverter 12 operates the gate drive circuit 19 of the high-frequency inverter 12 in accordance with the terminal current of the power-transmitting-side ultrasonic vibrator 13, and causes the resonant frequency of the high-frequency inverter 12 to follow the mechanical resonant frequency specific to the power-transmitting-side ultrasonic vibrator 13.

Specifically, the current flowing through the power-transmitting-side ultrasonic transducer 13 is detected by a zero-current detector, and the detected terminal current is input to a phase difference detector. The output signal of the voltage-controlled oscillator is also considered to be the output voltage of the high-frequency inverter and input to the phase difference detector. The phase difference detector calculates the phase difference, and a downstream controller adjusts the high-frequency inverter voltage and the current flowing through the power-transmitting-side piezoelectric element so that they form a certain phase (a slight current delay phase required for soft switching). The voltage-controlled oscillator then outputs a clock frequency to the gate drive circuit of the high-frequency inverter 12. This allows the high-frequency inverter 12 to operate at the electrical resonance point where it generates its maximum output, while simultaneously bringing the terminal voltage and terminal current, which reflect the mechanical resonance frequency of the power-transmitting-side ultrasonic transducer 13, into phase, resulting in a load power factor of 1 and maximizing the mechanical output of the power-transmitting-side ultrasonic transducer 13. That is, the vibration velocity of the power-transmitting-side ultrasonic vibrator 13 and the terminal voltage of the power-transmitting-side ultrasonic vibrator 13 controlled by the high-frequency inverter 12 are adjusted to be in phase, maximizing the electromechanical energy conversion efficiency and enabling stable operation of the operating frequency of the high-frequency inverter 12 from rated load to no-load conditions. Note that a comparator that compares the phase θ with a reference value θ _ref , a proportional integrator (PI), and a limiter are used to adjust the output voltage of the high-frequency inverter 12 and control the phase of the vibrator displacement that changes depending on the usage environment of the power-transmitting-side ultrasonic vibrator 13. In addition, a low-pass filter is used as a noise filter to cut harmonic components.
The resonant circuit (hereinafter also referred to as a resonant tank) is not limited to a series resonant circuit connected in series to the high side of the high-frequency inverter 12 as shown in FIG. 2, but may be a parallel resonant circuit.

A method for creating the inductance and capacitance of the resonant circuit in the power transmission device will be described with reference to Figure 3. As will be described later, this method involves modeling the electrical circuit impedance of body tissue (insulators), theoretically deriving the resistance and reactance, and then determining the operating frequency of the high-frequency inverter that maximizes the transmitted power through a frequency characteristic test.
The fabrication method includes the following steps 1 to 6.
(Step 1) Measure the impedance characteristics, which are a combination of three elements: the electrical impedance of the insulator 4, the impedance of the power-transmitting-side ultrasonic vibrator 13, and the impedance of the power-receiving-side ultrasonic vibrator 23.
(Step 2) From the minimum absolute value of the impedance and its phase in the impedance characteristics obtained as a result of the measurement, the circuit parameters of the resistance and capacitance of the equivalent circuit combining the three elements are calculated.
(Step 3) The impedance matching circuit 15 is adjusted to match the impedances of the insulator 4, the power transmitting ultrasonic vibrator 13, and the power receiving ultrasonic vibrator 23.
(Step 4) The frequency at which the impedance is at its minimum is set to the resonant frequency of the resonant circuit.
(Step 5) Based on the resonant frequency, the Q value of the resonant circuit is set .
(Step 6) Based on the resonant frequency and the Q value, the values of the inductance and capacitance of the resonant circuit are determined.

The impedance matching circuit 15 can be a matching transformer or an LC circuit. Steps 1 to 6 above are explained below using specific circuit configurations (Figures 4 and 6).

Figure 4 shows the circuit configuration of the ultrasonic contactless power transfer system of the present invention. Figure 5 (1) shows a schematic diagram of the structure of the transmitting piezoelectric transducer (PT), which is the transmitting ultrasonic vibrator, the receiving piezoelectric transducer (PT), which is the receiving ultrasonic vibrator, and the insulator between them. The insulator uses an ultrasound phantom that mimics the human body as an insulator that simulates body tissue. The transmitting piezoelectric transducer (PT) is excited via a resonant circuit from a high-frequency inverter installed outside the body. Meanwhile, a high-frequency current is generated in the receiving PT, which is implanted inside the body, upon receiving the minute vibration energy transmitted from the insulator. This current then charges the battery of the implanted device via a high-frequency rectifier circuit.

Figure 6 shows an example of a high-frequency inverter and high-frequency rectifier circuit used in the ultrasonic contactless power transfer system of the present invention. A voltage-type half-bridge inverter is used for the transmitting-side high-frequency inverter, and is driven with equal switch-on ratios (approximately 50%). This allows the low-impedance transmitting-side PT to operate as a current source, more efficiently supplying high-frequency current to the PT. Furthermore, the series resonant tank effect of the inductor Ls and capacitor Cs installed in front of the transmitting-side PT supplies a low-distortion resonant current to the transmitting-side PT. Meanwhile, taking into account the high-impedance characteristics of the insulator and receiving-side PT, a full-wave voltage doubler rectifier circuit (VDC) is used on the receiving side.

In resonant tanks, which contribute to high-efficiency power transmission in ultrasonic contactless power transfer systems, the circuit parameters that make them up have a significant effect on the switching operation of the high-frequency inverter and high-frequency rectifier circuit. There is a method for estimating the internal impedance of a biological wireless power transfer module from the effective values and phase information of the voltage and current observed at the four terminals of a power transmitting/receiving PT, including a phantom. However, this estimation method, which is based on the effective values and phase difference of the voltage and current observed at the four terminals of a PT pair, makes it difficult to accurately determine circuit parameters that take into account not only the mechanical-mechanical coupling coefficient but also the mechanical material properties of the intervening insulator, such as the sound speed, attenuation, and hardness. Therefore, we newly introduced an electrical equivalent circuit for the somatic cell, and determined the equivalent circuit and parameters of a power transmitting/receiving module that combines a PT pair and a phantom from the impedance characteristics measured using a frequency analyzer.

Next, the electrical equivalent circuit of the transmitting and receiving module for the power transmitting and receiving PT including the phantom will be described.
The electrical equivalent circuit of the power transmitting and receiving module in the ultrasonic contactless power transfer system of the present invention is shown in Figure 5 (2). The damping capacitances C _dt and C _dr are admittances observed when the vibrating part of the PT is damped, and are not affected by vibration in an actual system. Here, the PT parameters are expressed by the following equations 1 to 3 using mass l _b , compliance C b , mechanical resistance r _b , and electromechanical conversion coefficient A.

The above PT parameters vary depending on the environment, such as air or water. In particular, r _L /A ² varies depending on the object present on the vibration plane of the PT. When operating in air, there are no obstacles to vibration and it can be assumed that there is a short circuit. However, when used in liquids such as water or the human body, the value becomes relatively large and is greatly affected by the physical parameters of the PT, making it difficult to derive a highly accurate value.

However, in Reference 1 (Y. Cheng, et.al., "Improving Power Delivery of CPT for
Biomedical Implants by Using Conjugate Impedance Matching", 2019 IEEE
Biomedical Circuits and Systems Conference (BioCAS),pp.1-4,
2019.) and Reference 2 (R. Erfani, et.al.,
"Modeling and Experimental Validation of a Capacitive Link for Wireless
Power Transfer to Biomedical Implants," in IEEE Transactions on Circuits
and Systems II: Express Briefs, vol. 65, no. 7, pp. 923-927, July 2018.), the electrical equivalent circuit of human tissue can be expressed as a symmetrical lattice circuit, as shown in the phantom part of Figure 5 (2). Each parameter can be defined by the following equations 4 to 7.

In the above equations 4 to 7, ε ₀ represents the permittivity of a vacuum, ε _rT the permittivity of the tissue, σ _T the conductivity of the tissue, d _T the thickness of the tissue, A _P the area of the contact surface at BLT, and L _T +L _P the diameter of the contact surface. Of these, the permittivity of the tissue ε _rT and the conductivity of the tissue σ _T vary greatly depending on the frequency band, and it is particularly difficult to uniquely determine these parameters because various tissues are intertwined in a complex manner within the body.
From the above viewpoint, Fig. 5(2) is transformed into a simplified T-type equivalent circuit as follows: First, the two complex impedances that make up the T-type circuit can be given by the following equations 8 and 9.

As a result, the phantom part of Fig. 5(2) can be expressed by the T-shaped equivalent circuit shown in Fig. 5(3). Furthermore, by substituting the series and parallel components in Fig. 5(3) all at once, Fig. 5(2) can be finally converted into the L-shaped simplified equivalent circuit shown in Fig. 5(4).
The converted L-shaped simplified equivalent circuit corresponds to a combination of three elements in step 1: the electrical impedance of the insulator, the impedance of the power transmitting ultrasonic vibrator, and the impedance of the power receiving ultrasonic vibrator.

(Impedance characteristic measurement)
In the above-mentioned L-shaped simplified equivalent circuit, the two complex impedances shown in Figure 5 (4) are measured while shorting and opening the terminals 2-4, and an equivalent circuit of the power transmitting and receiving module including the actual phantom is constructed.
Figure 8 shows the system configuration for the power transmission/reception module characteristic test. The power transmission/reception PT uses a bolted Langevin transducer (BLT) and an ultrasonic phantom (WRTMM06, OST) sandwiched between them to form the ultrasonic power transmission/reception module. The oscillator output built into the frequency response analyzer (FRA51615, NF circuit block) is amplified by a high-speed bipolar power supply (HSA42011, NF circuit block) to supply high-frequency current to the ultrasonic phantom. The impedance of the power transmission/reception module is measured by a frequency response analyzer via a shunt resistor (PA-001-0370, NF circuit block). The BLT is placed horizontally to effectively transmit vibration energy to the receiving BLT while suppressing ultrasonic vibration attenuation. Furthermore, ultrasonic gel is applied between the BLT and the phantom to improve ultrasonic vibration transmission.

Here, the internal impedance characteristics when terminal pair 2-4 in Fig. 8 is shorted and opened are shown in Fig. 9. From Fig. 9 (1) and (2), it can be seen that in both cases where terminal pair 2-4 is shorted and opened, the impedance is minimum at a frequency lower than the BLT mechanical vibration frequency of 40.3 kHz, that is, at 39.3 kHz which is in the capacitive region, and it can be seen that driving the inverter in this frequency band where the impedance is minimum will enable efficient operation.
Furthermore, from Figure 9 (1), when terminal pair 2-4 is shorted, the minimum absolute value of the impedance is 230 Ω and its phase is -24°, and from Figure 9 (2), when terminal pair 2-4 is open, the minimum absolute value of the impedance is 360 Ω and its phase is -47°. Based on these results, an equivalent circuit of the power transmitting and receiving module including an actual phantom is constructed.

The L-shaped simplified equivalent circuit shown in Figure 5 (4) can be expressed using resistance components _R1 and _R2 and capacitance components _C1 and _C2 as shown in Figure 10, and the circuit parameters for the L-shaped simplified equivalent circuit can be calculated from the relationship between the two impedances in the following equations 10 and 11 as follows: _R1 = 210 [Ω], _C1 = 43 [nF], _R2 = 35 [Ω], _C2 = 24 [nF].

(Resonant Tank Design Guidelines)
Using the circuit parameters in the above-mentioned L-shaped simplified equivalent circuit, a resonant tank to be applied to a high-frequency inverter is designed. Figure 11 (1) is a simplified equivalent circuit in which the DC voltage source _Vin and switches _Q1 and _Q2 in Figure 6 are replaced with a square wave voltage source _Vab , the ultrasonic power transmitting and receiving module part is replaced with Figure 10, and the voltage doubler rectifier circuit and load resistance _R0 are replaced with an AC equivalent resistance _Rac = _2R0 / ^π2 , to which an impedance matching transformer (MT) is added. This MT makes it possible to adjust the parameters of the series resonant tanks _Ls and _Cs . Here, if the parallel-connected _R2 , _C2 and _Rac in Figure 11 (1) are newly converted into series equivalents _R3 and _C3 , Figure 11 (1) can be converted as shown in Figure 11 (2).

Next, assuming that the turns ratio a of the matching transformer in Figure 11(2) is a = _w1 / _w2 and the value of the excitation inductance _Lm is sufficiently large, converting the secondary side of the transformer to the primary side results in Figure 11(3). Here, _Rmo and _Cmo in Figure 11(3) are a combination of _R1 , _R3 and _C1 , _C3 in Figure 11(2), and are expressed by the following equations 14 and 15. _Rmo is the equivalent load AC resistance, and _Cmo is the equivalent capacitance.

Furthermore, if the inductor and capacitor components in Fig. 11(3) are collectively defined as L ₀ and C ₀ , Fig. 11(1) can be finally simplified to Fig. 11(4), where L ₀ and C ₀ are expressed by the following equations 16 and 17.

The resonant frequency of the circuit is set to f _r = 39.3 kHz, at which the internal impedance of the ultrasonic power transmitting and receiving module is minimized, and the sharpness Q of the load resonance is expressed by the following equation 19: where ω _r is the resonant angular frequency. By providing the Q value, L ₀ or C ₀ is determined based on the resonant angular frequency, making it possible to design the series resonant tanks C _s and L _s , and further making it possible to adjust C _s and L _s using an impedance matching transformer. Here, the Q value is preferably set within 5 to 10, taking into account the voltage and current stresses on passive elements (capacitors, inductors) and power semiconductor switches.

(Circuit operation simulation results)
Based on the circuit parameters described above, we simulated and evaluated the circuit operation of an ultrasonic contactless power transfer system incorporating a resonant tank and configured with a half-bridge high-frequency inverter and rectifier circuit. The simulation circuit parameters are shown in Table 2 below. The switching frequency _fs was set to 39.3 kHz, the resonant frequency of the main circuit, and 45 kHz, a higher frequency band, i.e., the inductive region, and we confirmed the changes in each current amplitude and output _Po with and without ZVS (zero voltage soft switching) operation, as well as the usefulness of the designed resonant tank.

The resulting simulation waveforms are shown in Figure 12 (the current waveforms are scaled 100 times). The switching waveforms _vQ1 , _iQ1 , _vQ2 , and _iQ2 in Figure 12(1) show that ZVS operation of the power semiconductor switches _Q1 and _Q2 is achieved at 39.3 kHz. Furthermore, the inter-bridge voltage and current vab _{and iab} are nearly in phase, and the current _iPT1 flowing into the transmitting PT has a resonant waveform. This indicates that a low-distortion resonant current can be supplied to the transmitting PT due to the effect of the series resonant tank formed by the inductor _Ls and capacitor _Cs installed in the upstream stage of the transmitting PT. Furthermore, the waveforms of the receiving _PT voltage vPT2 before rectification and the output voltage _VRO show that the DC voltage _VRO is twice the peak value of _vPT2 due to the effect of the voltage doubler rectifier circuit.

On the other hand, the switching waveforms _vQ1 , _iQ1 , _vQ2 , and _iQ2 in Figure 12(2) show that at 45 kHz, the power transmitting/receiving module operates at a frequency significantly different from its resonant frequency. This increases the turn-off current of _Q1 and _Q2 , resulting in a high voltage rise rate dv/dt at turn-off. As a result, the high-frequency inverter operates in hard switching mode. Furthermore, the phase difference between the transmitting side _vab and _iab increases, deteriorating the power factor and reducing the amplitude of each current and the output _P0 .

These results confirm the usefulness of the equivalent circuit of the power transmitting/receiving module in the ultrasonic contactless power transfer system of the present invention and the operating frequency design of the high-frequency inverter based on it. Specifically, an ultrasonic phantom was inserted between two power transmitting/receiving PTs, and a power transmitting/receiving module was implemented and constructed. Analysis of frequency characteristics was performed, demonstrating that the resonant frequency of the electrical circuit system exists in the 39.3 kHz band, lower than the mechanical resonant frequency (40 kHz) of the PT alone. Furthermore, an open-circuit/short-circuit load test of the module was conducted to measure impedance and construct a resonant circuit. Simulation confirmed that the high-frequency inverter's drive frequency could be set to 39.3 kHz, achieving an output of 600 mW at the load end.

Figure 13 shows functional blocks of another embodiment of the ultrasonic contactless power transfer system of the present invention. In addition to the configuration of Example 1, the system shown in Figure 13 includes a voltage controller 26 that inputs the battery voltage value and a command value and outputs a voltage control signal to the power receiving device 2, and a short-range wireless communication circuit 27 that transmits the voltage control signal to the power transmitting device 1. The power transmitting device 1 also includes a short-range wireless communication circuit 17 and a pulse modulation circuit 18 that uses the voltage control signal received via short-range wireless communication to modulate the drive pulse of the high-frequency inverter 12, which uses the pulse signal obtained from the resonance frequency tracking control circuit 16 as its clock frequency. A switch drive circuit (gate drive circuit) 19 converts the digital signal from the pulse modulation circuit 18 into an analog signal and drives the high-frequency inverter by rapidly turning the power semiconductor switch on and off.

Referring to Figure 14, feedback of the voltage of the battery of the power receiving device 2 in the power transmitting device 1 of the ultrasonic contactless power transfer system will be described. The voltage controller 26 of the power receiving device 2 detects the voltage value of the battery 21, compares it with a command value (set value), and outputs a voltage control signal. This voltage control signal is fed back to the gate drive circuit 19 of the high-frequency inverter 12 of the power transmitting device 1 and is used to determine the switch-on ratio and on-off pattern of the switch of the high-frequency inverter 12. In other words, the gate drive pattern of the high-frequency inverter 12 is created by the output frequency of the resonance frequency tracking 16, and this pattern and its switch-on ratio change according to the voltage control signal that corresponds to the voltage value of the battery 21 of the power receiving device 2.
Here, the power transmitting device 1 and the power receiving device 2 are assumed to be non-contact but only a few centimeters apart, and voltage control signal information can be sent and received using short-range wireless communication such as NFC (Near Field Communication).

Figures 15 and 16 show functional and circuit block diagrams of another embodiment of the ultrasonic contactless power transfer system of the present invention. Unlike Example 1, the system shown in Figures 15 and 16 includes a resonant circuit 24 mounted in the power receiving device 2. The resonant circuit 24 provided in the power receiving device 2 has the effect of suppressing electromechanical conversion in the power transmitting/receiving PT pair and power attenuation due to the insulator, and increasing the high-frequency current in the power receiving ultrasonic vibrator 23, enabling more efficient transmission of power to the power receiving device's battery 21.

Figure 17 shows a functional block diagram of another embodiment of a power receiving device in the ultrasonic contactless power transfer system of the present invention. The power receiving device 2C shown in Figure 17 is designed to be implanted in the body and has a built-in energy harvesting device 29 that harvests vibrational energy using body vibrations. The battery 21 is provided with a line that supplies power from a first port connected to a high-frequency rectifier circuit 22, and a line that supplies power from a second port connected to the energy harvesting device 29. The energy harvesting device 29 generates power from vibrations using a piezoelectric element, which is smoothed and rectified via the rectifier circuit 28 and supplies the power to the battery 21.

The ultrasonic non-contact power supply system of the present invention is useful as a power supply system for implantable medical devices.

1, 1A Power transmitting device 2, 2A, 2B, 2C Power receiving device 3 Ultrasonic transmission 4 Insulator (body tissue)
6 Wireless signal 11 Voltage source 12 High frequency inverter 13 Power transmission side ultrasonic vibrator (power transmission side PT)
14, 24 Resonant circuit (resonant tank)
15 Impedance matching circuit 16 Resonance frequency tracking control circuit 17, 27 Short-range wireless communication circuit 18 Pulse modulation circuit 19 Switch drive circuit (gate drive circuit)
21 Battery 22 High-frequency rectifier circuit 23 Power-receiving ultrasonic vibrator (power-receiving PT)
26 Voltage control circuit 28 Rectifier circuit 29 Energy harvesting device

Claims (12)

a wireless power transmitting device including a high frequency inverter that converts a voltage supplied from a voltage source into a predetermined high frequency AC voltage, a resonance circuit connected to the output of the high frequency inverter, a power transmitting ultrasonic vibrator using a piezoelectric body connected to the resonance circuit, a resonance frequency tracking control circuit that makes the mechanical resonance frequency specific to the power transmitting ultrasonic vibrator substantially the same as the electrical resonance frequency and causes the operating frequency of the high frequency inverter to follow the electrical resonance frequency, and an impedance matching circuit;
A contactless power supply system comprising a power-receiving-side ultrasonic vibrator using a piezoelectric body that receives ultrasonic waves transmitted from the power-transmitting-side ultrasonic vibrator, a high-frequency rectifier circuit that rectifies an output voltage of the power-receiving-side ultrasonic vibrator, and a contactless power receiving device having a battery connected to the high-frequency rectifier circuit,
The non-contact power transmitting device
The power transmitting-side ultrasonic vibrator operates at the electrical system resonance frequency set based on the impedance characteristics combining three elements: the electrical impedance of an insulator present between the pair of the power transmitting-side ultrasonic vibrator and the power receiving-side ultrasonic vibrator, the impedance of the power transmitting-side ultrasonic vibrator, and the impedance of the power receiving-side ultrasonic vibrator;
An ultrasonic contactless power supply system characterized in that the high-frequency inverter operates at the natural frequency of an AC circuit obtained by adding the impedance of the resonant circuit and the impedance matching circuit to the impedance obtained by combining the three elements. The resonant frequency is
2. The ultrasonic contactless power transfer system according to claim 1, wherein the impedance characteristic is a frequency at which the absolute value of the impedance is a minimum and the phase angle is zero. The high-frequency inverter in the contactless power transmission device is
3. The ultrasonic contactless power transfer system according to claim 1, wherein the system is configured with a voltage-type half-bridge inverter, and the switch-on time ratios of the two switches are substantially the same. The non-contact power receiving device is
a voltage controller that receives a voltage value of the battery and a command value and outputs a voltage control signal; and a power receiving side short-range wireless communication means that transmits the voltage control signal,
The non-contact power transmitting device
power transmitting side short-range wireless communication means;
The ultrasonic contactless power supply system according to any one of claims 1 to 3, further comprising a pulse modulation circuit that uses the voltage control signal received by short-range wireless communication as a command value to modulate the drive pulse of the high-frequency inverter, which has a clock frequency as the pulse signal obtained from the resonance frequency tracking control circuit. The ultrasonic contactless power transfer system of any one of claims 1 to 4, characterized in that the contactless power receiving device further comprises a receiving-side resonant circuit connected to the receiving-side ultrasonic vibrator. The ultrasonic contactless power transfer system described in any one of claims 1 to 5, characterized in that the high-frequency rectifier circuit in the contactless power receiving device is a multi-voltage rectifier circuit. the power-transmitting-side ultrasonic transducer is a bolt-clamped Langevin transducer (BLT),
7. The ultrasonic non-contact power transfer system according to claim 1, wherein the power receiving ultrasonic vibrator uses a piezoelectric thin film element. The ultrasonic contactless power transfer system described in any one of claims 1 to 7, characterized in that the insulator is body tissue, and the contactless power receiving device is implanted in the body or built into an implantable medical device. The ultrasonic contactless power transfer system described in claim 8, characterized in that the impedance characteristics combining three elements including the electrical impedance of the insulator are measured using a phantom representing the body tissue. The non-contact power transmission device constituting the ultrasonic non-contact power supply system of any one of claims 1 to 9. The non-contact power receiving device constituting the ultrasonic non-contact power supply system according to any one of claims 1 to 9,
The battery
a first port connected to the high frequency rectifier circuit;
The contactless power receiving device further comprises a second port connected to an energy harvesting device that harvests energy from an energy source in the surrounding environment. A method for producing the inductance and capacitance of the resonant circuit in the contactless power transmission device constituting the ultrasonic contactless power transfer system according to any one of claims 1 to 9, comprising the following steps:
1) measuring impedance characteristics that combine three elements: the electrical impedance of the insulator, the impedance of the power-transmitting-side ultrasonic vibrator, and the impedance of the power-receiving-side ultrasonic vibrator;
2) calculating circuit parameters of resistance and capacitance of an equivalent circuit combining the three elements from the minimum absolute value of the impedance in the impedance characteristics and its phase;
3) Impedance matching step;
4) setting the frequency at which the impedance is at a minimum to the resonant frequency of the resonant circuit;
5) setting the Q value of the resonant circuit based on the resonant frequency;
6) determining the inductance and capacitance values of the resonant circuit based on the resonant frequency and Q value;