Antenna device

An antenna device includes: a transmitting unit which is connected to a control unit of an in-vehicle device mounted at a vehicle; and a transmission antenna connected to the transmitting unit. The transmitting unit operates the transmission antenna based on a binary signal and a carrier signal from the control unit. The transmitting unit includes: a duty ratio controller that modifies the binary signal to a duty ratio signal having a prescribed duty ratio and outputs the duty ratio signal; and a driving circuit that supplies an energizing current to the transmission antenna based on the carrier signal. The duty ratio controller changes intensity of the signal transmitted from the transmission antenna by changing the energizing current according to the duty ratio signal so as to form a desired communication range.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an antenna device of an in-vehicle device that is used in a communication system for performing unlock/lock or the like of a vehicle door between an in-vehicle device mounted at the vehicle and a portable device carried with a user. More specifically, the present invention relates to an antenna device that forms an arrival range (hereinafter, referred to as a communication range) of a transmission request signal that is transmitted in order to detect the existence of the portable device.

2. Description of the Related Art

Recently, there is popularized so-called, a smart entry system for performing unlock and lock or the like of a vehicle door only when a user approaches the vehicle or departs from the vehicle while carrying a potable device. Because the smart entry system can unlock and lock the vehicle door without a mechanical key, it is excellent in convenience.

According to this system, the in-vehicle device mounted at the vehicle outputs a transmission request signal through an antenna device. The portable device that receives this transmission request signal sends a reply signal to the in-vehicle device. The in-vehicle device that receives the reply signal controls a door actuator to unlock and lock the vehicle door.

The above-mentioned in-vehicle device is provided with a plurality of antenna devices. The antenna devices include:

an antenna device having a transmission antenna for an outside of the vehicle that is disposed at a transmitting unit and, for example, in a door handle of each vehicle door; and

an antenna device having a transmission antenna for an inside of the door that is disposed in the vicinity of the transmitting unit and, for example, an instrument panel.

The transmitting unit is driven by a control unit of the in-vehicle device in the antenna device. The transmitting unit outputs the transmission request signal to a predetermined communication range through the transmission antenna.

Formation of the communication range in a conventional antenna device used in this system will be demonstrated with reference toFIG. 8andFIG. 9.

FIG. 8is a block diagram of the conventional antenna device.FIG. 9is waveform diagrams demonstrating an operation of the conventional antenna device.

Referring toFIG. 8, in transmitting unit51of antenna device50, binary signal Sa is input from a control unit of an in-vehicle device (not shown) to modulation unit52formed with an AND circuit through input terminal56, and carrier signal Sb is input from the control unit of the in-vehicle device to modulation unit52through input terminal54. Binary signal Sa is a signal having a duty ratio of 50% that repeats High (H)/Low (L) shown inFIG. 9. Carrier signal Sb is a carrier signal that forms a pulse string shown inFIG. 9. Modulation unit52modulates carrier signal Sb by binary signal Sa and outputs modulated signal Sf shown inFIG. 9.

InFIG. 8, driving circuit57is formed with connecting in series a pair of power transistors between power supply Vd and earth (GND). First power transistor121on power supply Vd side is P channel FET, and second power transistor122on the GND side is N channel FET. Moreover, first power transistor121and second power transistor122are provided with parasitic diodes121aand122ain parallel, respectively.

Modulated signal Sf is input from modulation unit52to first power transistor121and second power transistor122of driving circuit57, respectively.

InFIG. 8, transmission antenna55is formed so that coil55aand capacitor55bis connected to each other in series. One end of transmission antenna55is connected to a middle point124between first power transistor121and second power transistor122through wiring152, terminal58, and resistance53which is disposed at transmitting unit51. The other end of transmission antenna55is connected to GND on the circuit side through wiring154and terminal59. That is, transmission antenna55is connected to second power transistor122in parallel.

Resistance value Ra of resistance53, inductance La of coil55aand capacitance Ca of capacitor55bare referred to as antenna constants. Transmission antenna55has Q factor indicating strength of a prescribed resonance that is decided by the antenna constant. This Q factor is proportional to La/Ra of the antenna constant, and when the value of La is made constant, it has the characteristic of Q∝1/Ra. Generally, it is performed to reduce a winding number of a coil and to form the transmission antenna in order to cheapen transmission antenna55. The Q factor of the conventional art transmission antenna55is relatively small, for instance, Q=10.

Antenna device50is configured such that transmission antenna55is connected to transmitting unit51as described above.

According to the above-mentioned configuration, modulation unit52controls ON/OFF state of driving circuit57by modulated signal Sf in antenna device50. As a result, antenna current Ie shown inFIG. 9flows to transmission antenna55. Transmission antenna55transmits intensity of the transmission request signal according to antenna current Ie and forms the communication range that is substantially in proportion to the size of antenna current Ie.

That is, in t1(t-ON) period (during energizing) where binary signal Sa is H and modulated signal Sf repeats H/L, modulation unit52alternately controls ON/OFF state of first power transistor121and second power transistor122. For this reason, transmission antenna55becomes in the energizing state. At this time, as shown in the waveform of positive polarity envelope ofFIG. 9, since Q factor of transmission antenna55is Q=10 which is relatively small, antenna current Ie becomes energizing current91that is saturated to the maximum current soon after rising.

In t2(t-OFF) period (during non-energizing the current) where binary signal Sa is L and modulated signal Sf is also L, modulation unit52controls only power transistor122at ON state. For this reason, transmission antenna55becomes in the non-energizing state. At this time, antenna current Ie is consumed by resistance53and becomes non-energizing current92that converges to zero soon after falling.

As described above, since Q factor of transmission antenna55is small in any case of the energizing current91and the non-energizing current92, antenna current Ie of transmission antenna55has the characteristic that is immediately saturated or converged. In antenna device50, energizing current91is changed by varying resistance Ra of the antenna constant, and the communication range that is substantially in proportion to the maximum value is formed.

That is, in antenna device50, since the maximum value of the energizing current91flowing into transmission antenna55is changed by resistance Ra of the antenna constant, as shown inFIG. 9, large energizing current J1flows into transmission antenna55, when R is small. Moreover, small energizing current J2flows into transmission antenna55, when R is large. For this reason, for example, the desired communication range is formed at the inside or outside of the vehicle in proportion to the size of the energizing current91that flows into each transmission antenna55through transmission antenna55arranged in the door handle or the vicinity of the instrument panel.

For example, Japanese Patent Unexamined Publication No. 2002-47835 is known as information of a conventional art document that relates to the above-mentioned technology.

According to the conventional art antenna device as described above, the formation of the communication range is performed with varying resistance value Ra in the resistance of the antenna device. Accordingly, the individual communication range, which differs depending on the arrangement position of the transmission antenna, vehicle model or the like, is set by varying resistance Ra of each antenna device.

It is complicate to set the communication range by varying this resistance value Ra. That is, every time the communication range is measured by using an experiment vehicle or the like, operation that attaches again resistance with soldering iron is accompanied. Furthermore, the communication range is changed when the arrangement position of the transmission antenna or the vehicle design etc. are varied between from the experiment vehicle to a finished vehicle. Therefore, similar operation is performed in each case of those changes.

An universal article is generally used as the resistance. The resistance value is decided within the range of, for example, 5Ω to 12Ω, and the range is changed gradually into 4.9 Ω, 5.6 Ω, 6.8Ω, . . . , according to JIS standard or the like. Therefore, the formation of the communication range is difficult when such a resistance as 5.3Ω that is not included in the JIS standard is necessary. Accordingly, the formation of the communication range with a good accuracy is difficult.

SUMMARY OF THE INVENTION

An antenna device according to the present invention has a structure as follows.

An antenna device includes: a transmitting unit which is connected to a control unit of an in-vehicle device mounted at a vehicle; and a transmission antenna connected to the transmitting unit. The transmitting unit operates the transmission antenna based on a binary signal and a carrier signal from the control unit. The transmitting unit includes: a duty ratio controller that modifies the binary signal to a duty ratio signal having a prescribed duty ratio and outputs the duty ratio signal; and a driving circuit that supplies an energizing current to the transmission antenna based on the carrier signal. The duty ratio controller changes intensity of the signal transmitted from the transmission antenna by changing the energizing current according to the duty ratio signal so as to form a desired communication range.

According to the antenna device of the present invention having the above-mentioned configuration, a communication range of the antenna device is set without changing the resistance of the antenna constant, and a desired communication range is set with a good accuracy.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be now described with reference toFIG. 1andFIG. 2.

First Embodiment

FIG. 1is a block diagram of antenna device according a first embodiment of the present invention.FIG. 2is waveform diagrams demonstrating an operation of antenna device according to the first embodiment of the present invention.

Duty ratio controller1includes duty ratio control unit1aand storage unit1b. Storage unit1bstores duty ratio information on a plurality of duty ratios in advance. Duty ratio control unit1acontrols such that binary signal Sa of the duty ratio 50% shown inFIG. 2becomes desired duty ratio signal Sa1shown inFIG. 2, according to the duty ratio information selected from storage unit1b. Binary signal Sa is input from a control unit (not shown) of the in-vehicle device to duty ratio control unit1athrough inputting terminal16of transmitting unit12.

Binary signal Sa is a signal of a cycle T having a duty ratio of 50% to which each period t0of High (H)/Low (L) is equal. Meanwhile, duty ratio signal Sa1is formed base on duty ratio information, and is a signal of a cycle T having a prescribed duty ratio that is decided by the ratio of a period t1of H and a period t2of L.

Driving circuit4is formed with first power transistor21and second power transistor22serving as a pair of switching element that is connected in series between power supply Vd and earth (GND). Here, first power transistor21on power supply Vd side is P channel FET, and second power transistor22on the GND side is N channel FET. Moreover, first power transistor21and second power transistor22are provided with parasitic diodes21aand22ain parallel, respectively.

In driving circuit4, carrier signal Sb that forms a pulse string shown inFIG. 2is input to first power transistor21and second power transistor22, respectively from a control unit (not shown) of the in-vehicle device through input terminal14of transmitting unit12. First power transistor21and second power transistor22are ON/OFF controlled by carrier signal Sb.

Switching circuit7is formed with third power transistor23. Third power transistor23is N channel FET and includes parasite diode23ain parallel. Duty ratio signal Sa1shown inFIG. 2is input to third power transistor23from duty ratio controller1, and third power transistor23is ON/OFF controlled by duty ratio signal Sa1.

Transmission antenna5includes coil5aand capacitor5bthat are connected to each other in series. One end of transmission antenna5is connected to middle point28between first power transistor21and second power transistor22through wiring15, terminal18, and resistance26which is disposed at transmitting unit12. The other end of transmission antenna5is connected to third power transistor23through wiring17and terminal20, and connected to GND through third power transistor23. That is, transmission antenna5is connected between driving circuit4and switching circuit7.

Resistance26, coil5a, and capacitor5bhave resistance value Ra, inductance La, and capacitor Ca, respectively. Ra, La, and Ca are referred to as antenna constants. Transmission antenna5has Q factor indicating strength of a prescribed resonance that is decided by the antenna constant. In order to obtain a prescribed Q factor, transmission antenna5has coil5awith a lot of winding numbers based on the relational expression of Q∝La/Ra. For this reason, this Q factor has relatively large value within the range of Q=40 to 220.

Resistance6forms an attenuation circuit. Resistance6is connected between third power transistor23and middle point28of first power transistor21and second power transistor22. Accordingly, resistance6is connected to a series connection body of resistance26and transmission antenna5in parallel. Furthermore, resistance6may be connected to transmission antenna5in parallel.

According to the above-mentioned configuration, in antenna device10, duty ratio controller1controls ON/OFF state of switching circuit7by using duty ratio signal Sa1. At the same time, the control unit (not shown) of the in-vehicle device controls ON/OFF state of driving circuit4by using carrier signal Sb. As a result, antenna current Ie shown inFIG. 2flows to transmission antenna5having a prescribed Q factor. Antenna device10transmits intensify of the transmission request signal according to antenna current Ie and forms the communication range that is substantially in proportion to the size of antenna current Ie. Antenna current Ie, which is controlled by switching circuit7and flows to transmission antenna5, changes depending on an energizing time to transmission antenna5.

The waveform of positive polarity envelope of antenna current Ie is shown inFIG. 2.

That is, in t-ON period (during energizing) where duty ratio signal Sa1is H and carrier signal Sb repeats H/L, duty ratio controller1controls third power transistor23to ON state. At this time, since first power transistor21and second power transistor22are alternately ON/OFF controlled by carrier signal Sb, transmission antenna5becomes in the energizing state. Q factor of transmission antenna5has a relatively large value within the range of Q=40 to 220. Therefore, as shown inFIG. 2, antenna current Ie flows to transmission antenna5without saturating at once after rising, where antenna current Ie serves as energizing current201of the energizing state having a waveform of a positive polarity envelope that represents a substantial straight shape from a substantial parabola.

In t2(t-OFF) period (during non-energizing the current) where duty ratio signal Sa1is L, duty ratio controller1controls third power transistor23to OFF state. For this reason, transmission antenna5becomes in the non-energizing state regardless of alternately ON/OFF controlling of first power transistor21and second power transistor22as carrier signal Sb repeats H/L. Therefore, antenna current Ie becomes non-energizing current202of non-energizing state that converges to zero soon after falling.

A loop-shaped passage of this non-energizing current202is formed with transmission antenna5and resistance6serving as an attenuation circuit connected to transmission antenna5in parallel, and non-energizing current202is consumed and attenuated with this resistance6which has resistance value much larger than resistance26, thereby being rapidly converged to zero.

As described above, since Q factor of transmission antenna5is relatively large, antenna current Ie of transmission antenna5has the characteristic that represents a substantial straight shape from a substantial parabola without saturating immediately after rising of energizing current201.

Antenna device10uses the rising characteristic of energizing current201at t-ON period (during energizing) where duty ratio signal Sa1is H, and antenna device10changes the maximum value of energizing current501by varying the duty ratio of duty ratio signal Sa1. Antenna device10transmits intensity of the signal based on energizing current201in which the maximum value is changed, as a transmission request signal. For this reason, for example, the desired communication range is formed at the inside or outside of the vehicle in proportion to the size of energizing current201that flows into transmission antenna5arranged in the door handle or the vicinity of the instrument panel.

Specifically, the communication range of antenna device10is formed as follows.

For example, when Q factor of transmission antenna5is40and duty ratio controller1selects duty ratio information “60” on storage unit1b, the positive polarity envelope in the energizing current201of antenna current Ie shows the characteristic in which the rising represents a substantial parabola without saturating, as shown inFIG. 2.

It considers the case where the communication range is formed with selecting duty ratio information “60” on storage unit1bdue to duty ratio controller1, when Q factor is larger, for example, Q factor is about 220. In this case, the positive polarity envelope in the energizing current201of antenna current Ie shows the characteristic in which the rising is substantially in inverse proportion to Q factor to become small inclination θ, and represents a substantial straight shape, as shown inFIG. 2.

Accordingly, as shown inFIG. 2, when Q factor of transmission antenna5is40and the duty ratio of duty ratio signal Sa1is 60%, antenna device10can set the maximum value of energizing current201to current Ix. In addition, when Q factor of transmission antenna5is 40 and the duty ratio of duty ratio signal Sa1is 40%, antenna device10can set the maximum value of energizing current201to current Iy.

Meanwhile, when Q factor of transmission antenna5is220and the duty ratio of duty ratio signal Sa1is 60%, antenna device10can set the maximum value of energizing current201to current Ix. In addition, when Q factor of transmission antenna5is 220 and the duty ratio of duty ratio signal Sa1is 50%, antenna device10can set the maximum value of energizing current201, where Ix>Iy.

As described above, duty ratio controller1changes the maximum value of energizing current201of transmission antenna5by varying the duty ratio of duty ratio signal Sa1. For this reason, transmitting unit12transmits intensity of the transmission request signal based on energizing current201from transmission antenna5and forms the desired communication range that is substantially in proportion to this current.

Antenna device10can store duty ratio information in storage unit1bas a value distinguished in detail, for example, 53% and 53.5%. Therefore, since in antenna device10, duty ratio controller1selects the detailed duty ratio information of storage unit1bby program manipulation of duty ratio control unit1aand thereby the maximum value of the energizing current201of transmission antenna5is minutely changed, it is possible to set the communication range having a good accuracy.

It is preferable that the practicable duty ratio of this duty ratio signal Sa1is set in the range of 40% to 60% so as to ensure transmission time of the transmission request signal.

Moreover, it is preferable that Q factor of transmission antenna5is in the range of 40 to 220. When Q factor is less than 40, the rising characteristic of energizing current201becomes closer to that of energizing current91of the conventional art shown inFIG. 9. When Q factor becomes much smaller than 40, the rising of energizing current201is immediately saturated. Therefore, even though the duty ratio is changed somewhat, since the change in the antenna current is small, it is difficult to use in practice.

Meanwhile, when Q factor is more than220, since the rising characteristic of energizing current201shows that the inclination θ becomes further small to have a gently inclined straight, there is a practicality. However, the winding number of the coil is need to further increase from the relational expression of Q∝La/Ra to further enlarge Q factor. Moreover, since it becomes easy to be influenced by the wiring resistance of wirings15and17to reduce resistance Ra having the value of several ohms, there is a limit to reduce resistance Ra. Accordingly, it is difficult to use in practice.

As described above, according to an embodiment of the present invention, the maximum value of energizing current201that flows into transmission antenna5can be adjusted by varying the duty ratio of duty ratio signal Sa1formed with duty ratio controller1, so that the desired communication range can be formed with using transmission antenna5having a prescribed Q factor. In duty ratio controller1, duty ratio signal Sa1is set by selecting from the value distinguished in detail. For this reason, it is possible to obtain antenna device10in which the communication range having a good accuracy is set.

In addition, the range where the rising characteristic is useful, that is, the maximum value of energizing current201can be effectively changed with the duty ratio of duty ratio signal Sa1by adjusting Q factor of transmission antenna5to the range of about 40 to 220.

Furthermore, non-energizing current202can be adjusted to zero in a short time by providing the attenuation circuit that attenuates non-energizing current202of transmission antenna5. As a result, it is possible to maintain communication performance without changing transmission speed of the transmission request signal. The attenuation circuit can be configured at a low price by forming with resistance6.

Second Embodiment

In a second embodiment of the present invention, the same reference numerals can be denoted to the same component as in the first embodiment of the present invention and the detailed description will be simplified.

FIG. 3is a block diagram of an antenna device according to the second embodiment of the present invention. Transmitting unit31further includes current detecting circuit32that detects antenna current Ie in addition to elements of transmitting unit12of the first embodiment of the present invention.

Current detecting circuit32includes resistance34, amplifier36, and low-pass filter38. Resistance34is inserted between third power transistor23and GND. Amplifier36amplifies the voltage generated in resistance34by the flowing of antenna current Ie. Low-pass filter38is configured with resistance38aand capacitor38b. Low-pass filter38smoothes the output signal of amplifier36. Moreover, antenna device30feedbacks analog detecting signal Si that varies depending on antenna current Ie to duty ratio controller1.

According to the above-mentioned configuration, in duty ratio controller1, duty ratio control unit1arecognizes as a digital signal by converting detecting signal Si proportional to antenna current Ie into AD. At the same time, duty ratio controller1controls the duty ratio of duty ratio signal Sa1by comparing this digital signal with current reference value Is stored in storage unit1bbeforehand, such that antenna current Ie and current reference value Is may be equal to each other, that is, Si=Is.

Therefore, antenna device30forms the desired communication range by properly selecting current reference value Is, and performs a feedback control so that antenna current Ie and current reference value Is may be always equal to each other.

One example of the above-mentioned feedback control is as follows.

Duty ratio controller1changes the duty ratio of duty ratio signal Sa1at regular intervals, and operates transmission antenna5in a prescribed number. Duty ratio controller1selects and decides the duty ratio having a minimum difference with current reference value Is among two or more detecting signals Si obtained by above-mentioned operation. Since antenna current Ie flowing into transmission antenna5is controlled by duty ratio signal Sa1of the decided duty ratio, constant antenna current Ie can be secured, and the communication range can be constantly maintained.

According to this embodiment of the present invention, current detecting circuit32is provided, and duty ratio controller1feedbacks detecting signal Si so that antenna current Ie and current reference value Is are equal to each other and controls transmission antenna5. For this reason, it is possible to obtain stable antenna device30in which the deviation of the circuit characteristic or the communication range that varies in response to influence on, for example, parameter deviation, secular variation, and temperature change of transmission antenna5is small in addition to the effect according to the first embodiment of the present invention.

According to this embodiment of the present invention, it is demonstrated that storage unit1bstores current reference value Is. However, the present invention is not limited to this, and conversion data information of detection signal Si previously stored and the duty ratio may be used in place of current reference value Is.

Third Embodiment

FIG. 4is a block diagram of an antenna device according to a third embodiment of the present invention.FIG. 5is waveform diagrams demonstrating an operation of this antenna device.

FIG. 6is a block diagram of another antenna device according to the third embodiment of the present invention.

In the third embodiment of the present invention, the same reference numerals can be denoted to the same component as in the first and second embodiments of the present invention, and the detailed description will be simplified.

Duty ratio controller1has the same components as the duty ratio controller demonstrated in the first and second embodiments of the present invention. In a word, as described in the first embodiment of the present invention, duty ratio controller1controls such that binary signal Sa of the duty ratio 50% shown inFIG. 5becomes desired duty ratio signal Sa1shown inFIG. 5. Binary signal Sa is the same signal as binary signal Sa described in the first embodiment. In short, binary signal Sa is input from a control unit (not shown) of the in-vehicle device to duty ratio control unit1athrough inputting terminal16of transmitting unit120.

Modulation unit2is formed with AND circuit. Duty ratio signal Sa1is input to one input terminal of modulation unit2, and carrier signal Sb shown inFIG. 5is input to the other input terminal of modulation unit2from the control unit (not shown) of the in-vehicle device. Modulated signal Sc shown inFIG. 5is output from the above-mentioned two signals.

Here, carrier signal Sb is a signal that forms the pulse string of carrier frequency f0. Furthermore, modulated signal Sc has the same duty ratio as duty ratio signal Sa1.

Signal combining unit3includes logic circuit of inverter3aand OR circuit3b. Signal combining unit3outputs combined signal Sc1shown inFIG. 5combining modulated signal Sc to be input with duty ratio signal Sa1. This combined signal Sc1also has the same duty ratio as duty ratio signal Sa1.

Driving circuit4has the same configuration as the driving circuit of the first and second embodiments of the present invention. Generally, this circuit is referred to as a half bridge.

In driving circuit4, combined signal Sc1is input to first power transistor21, and modulated signal Sc is input to second power transistor22, respectively. First power transistor21and second power transistor22are ON/OFF controlled by combined signal Sc1and modulated signal Sc.

Transmission antenna5has the same configuration as the transmission antenna of the first and second embodiments of the present invention. One end of transmission antenna5is connected to middle point28between first power transistor21and second power transistor22through wiring15, terminal18, and resistance26which is disposed at transmitting unit120.

The other end of transmission antenna5is connected to GND of transmitting unit120through wiring17and terminal20.

Like the first and second embodiments of the present invention, resistance26, coil5a, and capacitor5bhave resistance value Ra, inductance La, and capacitor Ca, respectively.

Here, transmission antenna5has Q factor that is relatively large value within the range of Q=40 to 220, as described in the first and second embodiments of the present invention.

According to the above-mentioned configuration, antenna device40uses transmission antenna5having a prescribed Q factor and uses the rising characteristic of the energizing current of transmission antenna5decided by Q factor.

That is, duty ratio controller1changes the maximum value of energizing current of transmission antenna5by varying duty ratio signal Sa1. For this reason, the signal according to this current is output from transmission antenna5, as a transmission request signal. Accordingly, transmission antenna5forms the communication range that is substantially in proportion to the size of this current.

For example, it will be described the example in which duty ratio controller1selects duty ratio information “60” of storage unit1b, outputs duty ratio signal Sa1of the duty ratio 60% from binary signal Sa of the duty ratio 50%, and forms the communication range.

First, duty ratio controller1selects the duty ratio information “60”. For this reason, combined signal Sc1input to first power transistor21and modulated signal Sc input to second power transistor22have t1(t-ON) period and t2(t-OFF) period by cycle T, and is formed to the signal of the duty ratio 60% whose t1/T is 0.6.

First power transistor21is ON/OFF controlled by combined signal Sc1ofFIG. 5, and second power transistor22is ON/OFF controlled by modulated signal Sc ofFIG. 5. Therefore, antenna current Ie shown inFIG. 5flows to transmission antenna5.

Moreover, when combined signal Sc1and modulated signal Sc are L, first power transistor21is ON controlled, and second power transistor22is OFF controlled. Meanwhile, when combined signal Sc1and modulated signal Sc are H, first power transistor21is OFF controlled, and second power transistor22is ON controlled.

Accordingly, in t-ON period where combined signal Sc1and modulated signal Sc repeat H/L, first power transistor21and second power transistor22are alternately ON/OFF controlled. For this reason, energizing current501in the energizing state flows to transmission antenna5.

In t2(t-OFF) period where combined signal Sc1is H and modulated signal Sc is L, first power transistor21and second power transistor22are OFF controlled. For this reason, non-energizing current502in the non-energizing state flows to transmission antenna5.

Antenna current Ie is formed by an alternately continued current in energizing current501and non-energizing current502.

For example, when Q factor of transmission antenna5becomes approximately 40, as shown inFIG. 5, the positive polarity envelope in the energizing current501of antenna current Ie shows the characteristic in which the rising represents a substantial parabola without saturating, like the first embodiment of the present invention.

When Q factor of transmission antenna5is larger (e.g., Q factor is about 220), since antenna current Ie is substantially in inverse proportion to Q factor to become small inclination θ of the rising, the rising characteristic of energizing current501represents a substantial straight.

Accordingly, in t-ON (t1) period where Q factor of transmission antenna5is 40 and the duty ratio of duty ratio signal Sa1is 60%, the maximum value of energizing current501flowing to transmission antenna5can be set to current Ix. In addition, when Q factor of transmission antenna5is 40 and the duty ratio of duty ratio signal Sa1is 40%, the maximum value of energizing current501can be set to current Iy, where Ix>Iy.

Furthermore, in t-ON period where Q factor of transmission antenna5is 220 and the duty ratio of duty ratio signal Sa1is 60%, the maximum value of energizing current501flowing to transmission antenna5can be set to current Ix. In addition, when Q factor of transmission antenna5is 220 and the duty ratio of duty ratio signal Sa1is 50%, the maximum value of energizing current501can be set to current Iy.

That is, energizing current501can be set to current Ix in the duty ratio 60% when Q factor is 40, and energizing current501can be set to current Iy in the duty ratio 40% when Q factor is 40. Moreover, energizing current501can be set to current Ix in the duty ratio 60% when Q factor is 220, and energizing current501can be set to current Iy in the duty ratio 50% when Q factor is 220.

As described above, duty ratio controller1changes the maximum value of energizing current501of transmission antenna5by varying the duty ratio of duty ratio signal Sa1. For this reason, it forms the desired communication range that is substantially in proportion to this current.

Therefore, as described in the first embodiment of the present invention, since detailed duty ratio information such as duty ratio 53% is stored in storage unit1bto be selected, it is possible to accurately adjust the formation of the communication range.

It is preferable that the practicable duty ratio of this duty ratio signal Sa1is set in the range of 40% to 60% so as to ensure transmission time of the transmission request signal.

Moreover, as described reason in the first embodiment of the present invention, it is preferable that Q factor of transmission antenna5is in the range of 40 to 220.

It is preferable to shorten the falling time of non-energizing current502in t-OFF period so as to adjust the non-energizing current to zero in prescribed cycle T.

In the above t-OFF period, combined signal Sc1input to first power transistor21is set to H by the operation of signal combining unit3, and modulated signal Sc input to second power transistor22is set to L by the operation of signal combining unit3. As a result, both first power transistor21and second power transistor22are OFF controlled.

For the passage of non-energizing current502in t-OFF period, when non-energizing current502flows in a positive direction, that is, in an arrow direction Ie shown inFIG. 4, non-energizing current502flows through a path that again returns to transmission antenna5via GND and parasitic diode22aof second power transistor22from transmission antenna5. Meanwhile, when non-energizing current502flows in a negative direction, non-energizing current502flows through a path that connects power supply Vd via transmission antenna5and parasitic diode21aof first power transistor21from GND.

For the passage and the path of non-energizing current502, when non-energizing current502flows in the positive direction or in the negative direction, for convenience, it is defined that the attenuation circuit is connected with transmission antenna5in parallel.

Non-energizing current502in t-OFF period passes through parasitic diodes21aand22aof the attenuation circuit by the operation of signal combining unit3in the passage of both the positive direction and the negative direction. Accordingly, non-energizing current is consumed in parasitic diodes21aand22a, and non-energizing current502ofFIG. 5rapidly attenuates and converges to zero, as shown in positive polarity envelope503ofFIG. 5.

Therefore, non-energizing current502is adjusted to zero in prescribed cycle T. That is, the transmission speed of the transmission request signal does not decrease, since it is not necessary to lengthen cycle T.

As described above, according to this embodiment of the present invention, since antenna device40adjusts the maximum value of energizing current501that flows into transmission antenna5having a prescribed Q factor by varying the duty ratio of duty ratio signal Sa1formed with duty ratio controller1, the desired communication range can be formed.

Therefore, it is possible to obtain the antenna device that can form the communication range having a good accuracy by setting the duty ratio of duty ratio signal Sa1in detail.

The range where the rising characteristic is useful, that is, the maximum value of energizing current501can be changed at the duty ratio of duty ratio signal Sa1by adjusting Q factor of transmission antenna5to the range of about 40 to 220.

Furthermore, even when Q factor of transmission antenna5is largely set, non-energizing current502can be adjusted to zero in a short time by providing the attenuation circuit that attenuates non-energizing current502of transmission antenna5. As a result, it is possible to maintain communication performance without changing the transmission speed of the transmission request signal.

The path of the attenuation circuit is formed, where parasitic diodes21aand22aare included. That is, since other added parts are not needed, it is possible to form at a low price. This parasitic diode is inevitably formed in FET structure and is not parts other than FET.

According to this embodiment of the present invention, it is demonstrated that the passage of non-energizing current502of transmission antenna5passes through parasitic diodes21aand22a. However, it is not limited thereto, and for example, the passage may be formed such that the non-energizing current of the transmission antenna passes through the resistance by connecting the resistance to transmission antenna5ofFIG. 4in parallel.

Driving circuit4is made a half bridge, but it is not limited thereto. For example, as shown inFIG. 6, by providing driving circuit4ain driving circuit4in parallel, antenna device60may be configured such that a full bridge is formed with these four power transistors, and transmission antenna5is connected to middle points between one pair of the power transistors, respectively.

As shown inFIG. 6, transmitting unit35of antenna device60includes another driving circuit4a, another inverter circuit33, and second signal combining unit13which is another signal combining unit in addition to driving part120shown inFIG. 4.

Second modulated signal Sd, where modulated signal Sc is inversed to second modulated signal Sd by inverter circuit33, is input to third power transistor230of driving circuit4a. Second combined signal Sd1formed by combining second modulation signal Sd with duty ratio signal Sa1and second signal combining unit13is input to fourth power transistor240. Here, third power transistor230and fourth power transistor240have parasitic diodes230aand240a, respectively.

Therefore, first power transistor21is ON/OFF controlled by combined signal Sc1. Second power transistor22is ON/OFF controlled by modulated signal Sc. Third power transistor230is ON/OFF controlled by second modulated signal Sd. Fourth power transistor240is ON/OFF controlled by second combined signal Sd1. For this reason, antenna current Ie flows to transmission antenna5.

This configuration can be formed so that the characteristic of antenna current Ie is the same as that of the half bridge by forming transmission antenna5to a prescribed Q factor. Accordingly, it is possible to control output power of transmission antenna5by changing the maximum of energizing current501depending on the duty ratio of duty ratio signal Sa1. For this reason, antenna device60can form the desired communication range.

The above-mentioned full bridge can be used for high electric power compared with the half bridge. In other words, when the full bridge is connected to the same power supply Vd as the half bridge, since energizing current501of transmission antenna5can be enlarged, a wider communication range can be easily formed.

Fourth Embodiment

FIG. 7is a block diagram of an antenna device according to the fourth embodiment of the present invention.

In a fourth embodiment of the present invention, the same reference numerals can be denoted to the same component as in the first to third embodiments of the present invention and the detailed description will be simplified.

Transmitting unit41of antenna device70according to the fourth embodiment of the present invention further includes current detecting circuit42that detects antenna current Ie, in addition to transmitting unit120of the third embodiment described above.

Current detecting circuit42includes resistance44that is inserted between transmission antenna5and GND, amplifier46that amplifies the voltage generated in resistance44when antenna current Ie flows to resistance44, and low-pass filter48that smoothes the output of amplifier46. Low-pass filter48is formed with resistance48aand capacitor48b. Detecting signal Si1of analog current, which varies depending on antenna current Ie, is fed back to duty ratio controller1.

According to the above-mentioned configuration, in duty ratio controller1, duty ratio control unit1arecognizes detecting signal Si1proportional to antenna current Ie as a digital signal by AD-converting. At the same time, duty ratio controller1controls the duty ratio of duty ratio signal Sa1by comparing this digital signal with current reference value Is1stored in storage unit1bbeforehand, such that antenna current Ie and current reference value Is1may be equal to each other, that is, Si1=Is1.

Therefore, antenna device70forms the desired communication range by properly selecting current reference value Is1and performs a feedback control so that antenna current Ie and current reference value Is1are equal to each other.

One example of the above-mentioned feedback control is as follows.

Duty ratio controller1changes the duty ratio of duty ratio signal Sa1at regular intervals, and operates transmission antenna5in a prescribed number. Next, duty ratio controller1selects and decides the duty ratio having a minimum difference with current reference value Is1among two or more detecting signals Si1obtained by this. Since antenna current Ie flowing in transmission antenna5is controlled by duty ratio signal Sa1of selected duty ratio, antenna current Ie can be constantly maintained. Therefore, the constant antenna current Ie can be secured, so that the constant communication range can be maintained.

According to this embodiment of the present invention, current detecting circuit42is provided, and duty ratio controller1controls transmission antenna5by performing feedback detecting signal Si1so that antenna current Ie and current reference value Is1are equal to each other. For this reason, it is possible to obtain stable antenna device40in which the deviation of the communication range that varies in response to influence on, for example, circuit characteristics or parameter deviation, secular variation, and temperature change of transmission antenna5is small in addition to the effect according to the third embodiment of the present invention.

According to this embodiment of the present invention, it is demonstrated that storage unit1bstores current reference value Is1. However, it is not limited thereto, and for example, conversion data information of detection signal Si1detected previously and the duty ratio may be used in place of current reference value Is1.

The transmitting unit includes a duty ratio controller. The duty ratio controller controls a binary signal such that the binary signal becomes a duty ratio signal having a prescribed duty ratio and outputs the duty ratio signal, the binary signal being input from the control unit of the in-vehicle device to the transmitting unit. An energizing current is supplied to the transmission antenna based on the duty ratio signal and a carrier signal that is input from the control unit of the in-vehicle device to the transmitting unit. The duty ratio controller changes intensity of the signal transmitted from the transmission antenna by changing the energizing current according to the change of a prescribed duty ratio and forms a prescribed communication range.

According to any embodiments described above, it is demonstrated that the duty ratio controller, the modulating unit, and the signal combining unit, etc. are configured with hardware that combines a plurality of electronic parts. However, these elements may be configured not hardware but one microcomputer.

The antenna device according to the present invention can form the desired communication range having a high accuracy without changing resistance Ra of antenna constant. Therefore, it is useful to the antenna device that is used in the system that can unlock/lock the vehicle door.