Transmitter and microwave power transmission system

Abstract

Provided is a transmitter which can maximize the efficiency of rectification in a power-receiving electronic device irrespective of the distance therebetween. The transmitter transmits electric power to the electronic device through use of a microwave. The electronic device receives the microwave, converts the microwave to DC power, and uses the DC power as operation power. The transmitter includes a carrier wave generator for generating a carrier wave; a modulating signal generator for generating a modulating signal whose level increases, decreases, or increases and then decreases during one cycle; an amplitude modulator for amplitude-modulating the carrier wave generated by the carrier wave generator by, the modulating signal output from the modulating signal generator and for amplifying the modulated carrier wave and outputting the amplified, modulated carrier wave as a transmission signal; and an antenna for radiating the transmission signal output from the amplitude modulator into a space as the microwave.

Claims

1. A transmitter which transmits electric power to an electronic device through wireless transmission of a microwave, the electronic device being configured to receive the microwave, convert the microwave to DC power through use of a rectifying circuit, and use the DC power as operation power, the transmitter comprising: a carrier wave generator for generating a carrier wave; a modulating signal generator for generating a modulating signal whose level increases, decreases, or increases and then decreases during one cycle; an amplitude modulator for amplitude-modulating the carrier wave generated by the carrier wave generator by the modulating signal output from the modulating signal generator and for amplifying; an antenna for radiating a transmission signal which is amplitude-modulated and output from the amplitude modulator into a space as the microwave.

2. The transmitter according to claim 1, wherein maximum and minimum values of amplitude of the transmission signal are determined such that, when input power levels of the rectifying circuit of the electronic device which correspond to the maximum and minimum values, respectively, are defined as a maximum input power level and a minimum input power level, respectively, an optimum input power level at which the rectifying circuit has a maximum rectification efficiency falls within a range between the minimum input power level and the maximum input power level.

3. The transmitter according to claim 2, wherein the maximum value of the amplitude of the transmission signal is determined such that the maximum input power level at the time when the electronic device is located at a maximum distance exceeds the optimum input power level, the maximum distance being a maximum value of a distance from the transmitter within an operable range which is determined with respect to the transmitter and within which the electronic device must operate.

4. The transmitter according to claim 2, wherein the modulating signal generator generates the modulating signal which varies the amplitude of the transmission signal in accordance with a periodic function such that the amplitude of the transmission signal varies continuously and monotonously between the minimum value and the maximum value.

5. The transmitter according to claim 3, wherein the modulating signal generator generates the modulating signal which varies the amplitude of the transmission signal in accordance with a periodic function such that the amplitude of the transmission signal varies continuously and monotonously between the minimum value and the maximum value.

6. The transmitter according to claim 1, wherein the modulating signal is a sawtooth wave, a triangular wave, or a sinusoidal wave.

7. The transmitter according to claim 1, wherein the length of one cycle of the modulating signal is 0.1 sec to 2 sec.

8. The transmitter according to claim 1, wherein the microwave radiated from the antenna is a microwave whose duration is equal to or greater than the one cycle of the modulating signal and is radiated intermittently.

9. The transmitter according to claim. 1, wherein the electronic device is an electronic device which has no battery and has a rectenna and which can perform a wakeup operation through use of electric power produced as a result of rectification of the microwave over a period corresponding to one cycle of the modulating signal.

10. The transmitter according to claim 2, wherein the electronic device is an electronic device which has no battery and has a rectenna and which can perform a wakeup operation through use of electric power produced as a result of rectification of the microwave over a period corresponding to one cycle of the modulating signal.

11. The transmitter according to claim 1, wherein the electronic device is a device which transmits a response signal to the transmitter through use of electric power produced as a result of rectification of the microwave over a period corresponding to one cycle of the modulating signal.

12. A microwave power transmission system comprising: a transmitter for transmitting electric power to an electronic device through wireless transmission of a microwave; and an electronic device being configured to receive the microwave, convert the microwave to DC power through use of a rectifying circuit, and use the DC power as operation power; wherein the transmitter comprising: a carrier wave generator for generating a carrier wave; a modulating signal generator for generating a modulating signal whose level increases, decreases, or increases and then decreases during one cycle; an amplitude modulator for amplitude-modulating the carrier wave generated by the carrier wave generator by the modulating signal output from the modulating signal generator and for amplifying; an antenna for radiating a transmission signal which is amplitude-modulated and output from the amplitude modulator into a space as the microwave; and wherein a relative distance of the electronic device with respect to the transmitter varies as a result of movement of the electronic device, and the electronic device receives and rectifies the microwave output from the transmitter and activates a device provided therein through use of electric power produced as a result of rectification of the microwave.

13. The microwave power transmission system according to claim 12, wherein maximum and minimum values of amplitude of the transmission signal are determined such that, when input power levels of the rectifying circuit of the electronic device which correspond to the maximum and minimum values, respectively, are defined as a maximum input power level and a minimum input power level, respectively, an optimum input power level at which the rectifying circuit has a maximum rectification efficiency falls within a range between the minimum input power level and the maximum input power level.

14. The microwave power transmission system according to claim 13, wherein the maximum value of the amplitude of the transmission signal is determined such that the maximum input power level at the time when the electronic device is located at a maximum distance exceeds the optimum input power level, the maximum distance being a maximum value of a distance from the transmitter within an operable range which is determined with respect to the transmitter and within which the electronic device must operate.

15. A microwave power transmission system comprising: a transmitter for transmitting electric power to an electronic device through wireless transmission of a microwave; and a plurality of electronic devices being configured to receive the microwave, convert the microwave to DC power through use of a rectifying circuit, and use the DC power as operation power; wherein the transmitter comprising: a carrier wave generator for generating a carrier wave; a modulating signal generator for generating a modulating signal whose level increases, decreases, or increases and then decreases during one cycle; an amplitude modulator for amplitude-modulating the carrier wave generated by the carrier wave generator by the modulating signal output from the modulating signal generator and for amplifying; an antenna for radiating a transmission signal which is amplitude-modulated and output from the amplitude modulator into a space as the microwave; and wherein respective relative distances of the plurality of electronic devices with respect to the transmitter differ from one another, and each of the plurality of electronic devices receives and rectifies the microwave output from the transmitter and activates a device provided therein through use of electric power produced as a result of rectification of the microwave.

16. The microwave power transmission system according to claim 15, wherein maximum and minimum values of amplitude of the transmission signal are determined such that, when input power levels of the rectifying circuit of each of the plurality of electronic devices which correspond to the maximum and minimum values, respectively, are defined as a maximum input power level and a minimum input power level, respectively, an optimum input power level at which the rectifying circuit has a maximum rectification efficiency falls within a range between the minimum input power level and the maximum input power level.

17. The microwave power transmission system according to claim 16, wherein the maximum value of the amplitude of the transmission signal is determined such that the maximum input power level at the time when each of the plurality of electronic devices is located at a maximum distance exceeds the optimum input power level, the maximum distance being a maximum value of a distance from the transmitter within an operable range which is determined with respect to the transmitter and within which each of the plurality of electronic devices must operate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a block diagram of a power transmission system composed of a transmitter according to one embodiment of the present invention and an electronic device to which electric power is supplied from the transmitter;

(2) FIG. 2 is a waveform chart showing an example of a modulating signal used in the transmitter according to the embodiment;

(3) FIG. 3 is a waveform chart showing an example of a transmission signal obtained by modulating a carrier wave in the transmitter according to the embodiment;

(4) FIG. 4 is a circuit diagram of a rectifying circuit of the electronic device used in the system of the embodiment;

(5) FIG. 5 is a characteristic diagram showing the relation between rectification efficiency and input power level of the rectifying circuit of the electronic device used in the system of the embodiment;

(6) FIG. 6 is an explanatory diagram showing the operation principle of the system of the embodiment;

(7) FIG. 7 is a waveform chart showing another amplitude modulation of microwave in another embodiment; and

(8) FIG. 8 is a waveform chart showing the output of the rectifying circuit in the embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(9) Embodiments of the present invention will be described with reference to the drawings; however, the present invention is not limited to the embodiments.

First Embodiment

(10) FIG. 1 shows the configuration of a transmitter 10 according to a first embodiment of the present invention and the configuration of a power transmission system according to the first embodiment which is composed of the transmitter 10 and an electronic device 30 which receives a microwave from the transmitter 10. An assumed example of the power transmission system of the present embodiment is a system which is composed of a transmitter 10 mounted on a car and an electronic key 30 possessed by the owner of the car.

(11) The transmitter 10 includes a carrier wave generator 11; a modulating signal generator 12; a mixer 13 serving as an amplitude modulator which amplitude-modulates a carrier wave of a microwave band output from the carrier wave generator 11 by a modulating signal output from the modulating signal generator 12 and amplifies the modulated carrier wave; and a transmission antenna 14 which radiates, as a transmission signal S.sub.1, the amplitude-modulated microwave output from the mixer 13. Also, the transmitter 10 includes a receiving antenna 16 for receiving a response signal S.sub.2 from the electronic device 30; and a demodulation circuit 15 for demodulating a signal output from the receiving antenna 16. The carrier wave generator 11 generates a carrier wave of 2.45 GHz. The frequency of the carrier wave may be a frequency in bands use of which is permitted by the Radio Act, such as 5.8 GHz, 24 GHz, etc.

(12) As shown in FIG. 2, the modulating signal output from the modulating signal generator 12 is a sawtooth wave. The magnitude of the modulating signal varies in accordance with a linear function such that the magnitude increases linearly from the 0 level to the maximum level. A.sub.max during a period of one cycle T.sub.c. The modulating signal generator 12 repeatedly and intermittently outputs, as one packet, the modulating signal of three cycles. Therefore, as shown in FIG. 3, the transmission signal output from the mixer 13 has an intermittent oscillation waveform produced as a result of amplitude modulation of the microwave of 2.45 GHz by the sawtooth wave. The time length of one cycle is 1 sec. Although the time length of the cycle depends on the required responsiveness of the electronic device 30, preferably, the time length of the cycle is 0.5 sec to 2 sec. The transmission signal S.sub.1 which is amplitude-modulated, is radiated from the transmission antenna 14 into the air. Since the maximum value of the amplitude of the transmission signal S.sub.1; namely, the maximum transmission power level, corresponds to the maximum level A.sub.max of the modulating signal, in the following description, A.sub.max is also used to denote the maximum transmission power level. Notably, the minimum value of the amplitude of the transmission signal; i.e., the minimum transmission power level, is not limited to the 0 level, and may be any level.

(13) The electronic device 30 includes a receiving antenna 34; a rectifying circuit 31 for rectifying the signal received by the receiving antenna 34; a receiving circuit 32 and an ID (key) information transmitter 33 which are activated by DC power output from the rectifying circuit 31; and a transmission antenna 35. The receiving circuit 32 wakes up upon reception of the DC power output from the rectifying circuit 31 and activates the ID (key) information transmitter 33. The ID (key) information transmitter 33 code-modulates a microwave (carrier) using the ID of the owner's vehicle, and transmits the response signal S.sub.2 which is code-modulated from the transmission antenna 35 toward the transmitter 10. The transmitter 10 receives the response signal S.sub.2 through the receiving antenna 16 and demodulates the response signal S.sub.2 by the demodulation circuit 15 to thereby decode the ID. When the decoded ID coincides with an ID registered in the vehicle, a signal indicating that the decoded ID coincides with the registered ID is output to a controller for controlling doors of the vehicle. Upon reception of that signal, the controller performs an operation; for example, unlocks the doors. Notably, the electronic device 30 includes no battery. The electronic device 30 may include a capacitor for holding the DC power produced as a result of rectification of the received transmission signal.

(14) In the above-described power transmission system, the rectifying circuit 31 of the electronic device 30 is configured as shown in FIG. 4. FIG. 4 shows a rectenna example in which a well known single shunt rectifying circuit is used. The rectenna has a line 40 connected to the receiving antenna 34, and a Schottky barrier diode 37 whose cathode is connected to the line 40 at a point A. Also, the rectenna has a /4 stub 38 for the fundamental wave which is connected to the line 40 at a point B, and a /4 stub 41 for the second harmonic which is connected to the line 40 at the point B. Also, a stub 39 for matching for reflection prevention which is connected to the line 40 to be located between the point A and the receiving antenna 34. The line length between the point A at which the Schottky barrier diode 37 is connected to the line 40 and the point B at which the /4 stubs 38 and 41 are connected to the line 40 is /4 for the fundamental wave. This configuration equivalently realizes full-wave rectification.

(15) Since the rectifying circuit 31 having the above-described configuration includes the Schottky barrier diode 37, which is a non-linear element, the input impedance varies in accordance with the input power level. Accordingly, the rectifying circuit 31 has a characteristic as shown in FIG. 5; i.e., its rectification efficiency varies in accordance with the input power level of the rectifying circuit 31. Namely, there exists an optimum input power level P.sub.opt which provides the maximum rectification efficiency E.sub.max. There exists an optimum input power level range P.sub.opt in which the maximum rectification efficiency E.sub.max or a rectification efficiency close to the maximum rectification efficiency is obtained.

(16) Under the assumption that the power level of the microwave output from the transmitter 10 is maintained at the maximum transmission power level A.sub.max, the reception power level of the microwave received by the electronic device 30 decreases in accordance with an increase in the relative distance of the electronic device 30 with respect to the transmitter 10. FIG. 6 shows the relation between the maximum input power level P.sub.max of the rectifying circuit 31 and the distance of the electronic device 30 from the transmitter 10 for the case where the transmission signal is transmitted from the transmitter 30 at the maximum transmission power level A.sub.max. The optimum input power level range P.sub.opt shown in FIG. 6 is the optimal range P.sub.opt shown in FIG. 5 and including an input power level which provides the maximum rectification efficiency.

(17) In an assumed case where the transmission signal is not amplitude-modulated, when the electronic device 30 is located in a distance range within which the input power level of the rectifying circuit 31 does not fall within the optimum input power level range P.sub.opt, the maximum rectification efficiency is not realized.

(18) In view of the forgoing, in the present embodiment, the transmission power level, which is the amplitude of the transmission signal, is increased linearly from the 0 level to the maximum transmission power level A.sub.max during one cycle. Accordingly, the input power level of the rectifying circuit 31 also increases linearly from the 0 level to the maximum power level P.sub.max during one cycle. As shown in FIG. 6, when the electronic device 30 is located at the position corresponding to a distance L.sub.1, L.sub.x, or L.sub.2, during one cycle, the input power level increases by P.sub.1, P.sub.x, or P.sub.2 with elapse of time. The input power level certainly exceeds the optimal power level P.sub.opt when the input power level increases during one cycle. Accordingly, when the electronic device 30 is located at a position corresponding to any of these distances, the rectifying circuit 31 certainly has the maximum rectification efficiency E.sub.max.

(19) Meanwhile, in the case where the electronic device 30 is located at a position corresponding to the distance at which the maximum input power level P.sub.max does not exceed the upper limit level of the optimum input power level range P.sub.opt, the input power level does not cross the entire optimum input power level range P.sub.opt during one cycle during which the input power level increases. Accordingly, in this case, the maximum rectification efficiency is rot realized. A distance at which the maximum input power level P.sub.max becomes equal to the upper limit level of the optimum input power level range P.sub.opt is denoted by L.sub.max. In this case, when the distance of the electronic device 30 from the transmitter 10 exceeds the distance L.sub.max, the maximum rectification efficiency is not realized. In other words, there exists the maximum distance L.sub.max at which the maximum rectification efficiency is realized. There exists a distance range within which use of the electronic device 30 is expected in the power transmission system of the present embodiment. The maximum distance L.sub.max must be set fall outside this distance range.

(20) As described above, in the present embodiment, the maximum rectification efficiency can be realized even when the relative distance of the electronic device 30 with respect to the transmitter 10 changes. Accordingly, the DC power obtained in the electronic device 30 can be constantly maintained at the maximum value irrespective of the relative distance. In the case where the electronic device 30 is an electronic tag including no battery, it becomes possible to wake up the electronic tag without fail.

Second Embodiment

(21) FIG. 7 shows an example of another amplitude modulation of the transmission signal S.sub.1. In the example, the modulating signal is a sinusoidal wave of 0.5 Hz. The half cycle of the sinusoidal wave corresponds to one cycle of the modulating signal. The transmission power level varies with time as shown in FIG. 8. The input power level of the rectifying circuit 31 becomes equal to an attenuated power level at which the transmission power level varying with the above-described sinusoidal wave is attenuated in accordance with the relative distance of the electronic device 30. As shown in FIG. 8, in the case where the electronic device 30 is located at a position corresponding to a distance L.sub.1, the input power level of the rectifying circuit 31 becomes the optimum input power level P.sub.opt at time T.sub.1 at which the transmission power level becomes a level P.sub.1. Similarly, at time T.sub.4, the input power level becomes the optimum input power level P.sub.opt. Accordingly, during one cycle, the maximum rectification efficiency is realized two times. The obtained rectified power is the total of the rectified powers obtained at time T.sub.1 and T.sub.4, respectively.

(22) In the case where the electronic device 30 is located at a position corresponding to a distance L.sub.2 greater than the distance L.sub.1, the input power level of the rectifying circuit 31 becomes the optimum input power level P.sub.opt at time T.sub.2, later than time T.sub.1, at which the transmission power level becomes a level P.sub.2. Similarly, at time T.sub.3 which is earlier than time T.sub.4, the input power level becomes the optimum input power level P.sub.opt. Accordingly, during one cycle, the maximum rectification efficiency is realized two times. The obtained rectified power is the total of the rectified powers obtained at time T.sub.2 and T.sub.3, respectively. As described above, in the present embodiment as well, the maximum rectification efficiency can be realized even when the relative distance of the electronic device 30 with respect to the transmitter 10 changes. Accordingly, the DC power obtained in the electronic device 30 can be constantly maintained at the maximum value irrespective of the relative distance. In the case where the electronic device 30 is an electronic tag including no battery, it becomes possible to wake up the electronic tag without fail.

(23) Notably, in the above-described first and second embodiments, the transmitter 10 may be configured as follows. After the reception of the response signal S.sub.2 from the electronic device 30, the transmitter 10 continuously radiates the microwave while maintaining the transmission power level at the reception timing (desirably, a timing earlier than the reception timing by a time corresponding to a propagation delay time of a circuit, etc.). The transmission power level at that timing is the power level which realizes the maximum rectification efficiency. Accordingly, after that timing, more efficient power transmission becomes possible.

(24) The present invention can be applied to radio (wireless) power transmission for activation of RF-ID tags, IC tags, and electronic keys which have no battery.