Cleaning apparatus, imaging unit including cleaning apparatus, and cleaning method

A cleaning apparatus includes an imaging section, a protective cover in a field of view of the imaging section, a vibrator to vibrate the protective cover, a piezoelectric driver to drive the vibrator, a cleaning liquid discharger to discharge a cleaner onto a surface of the protective cover, and a signal processing circuit to control the piezoelectric driver. The signal processing circuit controls the piezoelectric driver such that the protective cover is vibrated at a resonant frequency for a predetermined period after the cleaning liquid is caused to be discharged and controls the piezoelectric driver such that, after the predetermined period, vibration of the protective cover is stopped or the protective cover is vibrated at a frequency other than the resonant frequency.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a cleaning apparatus, an imaging unit including the cleaning apparatus, and a cleaning method.

2. Description of the Related Art

An imaging unit has been provided at a front portion or a rear portion of a vehicle to record an image captured by the imaging unit or to control a safety device using the captured image. Since such an imaging unit is often provided outside the vehicle, foreign matter such as raindrops, mud, and dust may adhere to a light-transmissive body (lens or protective glass) that covers an outside of an imaging element. When foreign matter adheres to the light-transmissive body, the foreign matter that has adhered is reflected in an image captured by the imaging unit and a clear image cannot be obtained.

Thus, an imaging unit provided with a function of removing raindrops adhering to a light-transmissive body covering an outside of an imaging element has been developed (Japanese Unexamined Patent Application Publication No. 2012-138768). In the imaging unit disclosed in Japanese Unexamined Patent Application Publication No. 2012-138768, a dome-shaped cover is disposed in front of the imaging element, and a piezoelectric ceramic vibrator is disposed on a cylindrical portion at an extended position of the dome-shaped cover. Thus, in the imaging unit, when raindrops adhere to the dome-shaped cover, the piezoelectric ceramic vibrator can be vibrated to remove the raindrops adhering to the dome-shaped cover.

In the imaging unit described in Japanese Unexamined Patent Application Publication No. 2012-138768, when raindrops adhering to the dome-shaped cover are to be removed by vibration, after excitation is started, the piezoelectric ceramic vibrator is rapidly vibrated at up to a highest frequency. Thereafter, in the piezoelectric ceramic vibrator, the frequency is repeatedly swept along a sawtooth wave in which a maximum point gradually decreases in a vicinity of a frequency of good efficiency and is set to 0 (zero) at an end of the excitation. Alternatively, in the piezoelectric ceramic vibrator, the frequency is swept in a monotonically decreasing manner from a vicinity of a resonant frequency in the configuration including the cylindrical portion and the dome-shaped cover and is set to 0 (zero) at an end of the excitation.

That is, in the imaging unit described in Japanese Unexamined Patent Application Publication No. 2012-138768, when raindrops adhering to the dome-shaped cover are to be removed by vibration, the frequency is swept only in the above-described pattern, and vibration is always performed in the same vibration pattern regardless of a type of foreign matter. Furthermore, in the imaging unit, since there is no method of removing foreign matter adhering to the dome-shaped cover other than vibrating the dome-shaped cover, there has been a possibility that foreign matter adhering to the dome-shaped cover (light-transmissive body) cannot be removed depending on a type of foreign matter.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide cleaning apparatuses that are each able to effectively clean foreign matter adhering to a light-transmissive body, imaging units each including such cleaning apparatuses, and cleaning methods.

A cleaning apparatus according to a preferred embodiment of the present disclosure includes a holder to hold an imaging element, a light-transmissive body in a field of view of the imaging element, a vibrator to vibrate the light-transmissive body, a driver to drive the vibrator, a discharger to discharge a cleaner onto a surface of the light-transmissive body, and a controller to control the driver and the discharger, in which the controller is configured or programmed to control the driver such that the light-transmissive body is vibrated at a resonant frequency for a predetermined period after the discharger is caused to discharge the cleaner and control the driver such that, after the predetermined period, vibration of the light-transmissive body is stopped or the light-transmissive body is vibrated at a frequency other than the resonant frequency.

An imaging unit according to a preferred embodiment of the present disclosure includes the cleaning apparatus described above.

A cleaning method according to a preferred embodiment of the present disclosure is a cleaning method for cleaning a surface of a light-transmissive body by a cleaning apparatus that includes a holder to hold an imaging element, the light-transmissive body in a field of view of the imaging element, a vibrator to vibrate the light-transmissive body, a driver to drive the vibrator, a discharger to discharge a cleaner onto the surface of the light-transmissive body, and a controller to control the driver and the discharger, the cleaning method including causing the discharger to discharge the cleaner, controlling the driver such that the light-transmissive body is vibrated at a resonant frequency for a predetermined period after the cleaner is caused to be discharged, and controlling the driver such that, after the predetermined period, vibration of the light-transmissive body is stopped or the light-transmissive body is vibrated at a frequency other than the resonant frequency.

According to preferred embodiments of the present invention, since the controller controls the driver such that the light-transmissive body is vibrated at the resonant frequency for the predetermined period after causing the cleaner to be discharged and then controls the driver such that vibration of the light-transmissive body is stopped or the light-transmissive body is vibrated at a frequency other than the resonant frequency, it is possible to effectively clean foreign matter adhering to the light-transmissive body.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, imaging units according to preferred embodiments of the present invention will be described in detail with reference to the drawings. Note that, in the drawings, the same reference numeral denotes the same or corresponding elements and portions.

Hereinafter, an imaging unit according to Preferred Embodiment 1 will be described with reference to the drawings.FIG.1is a perspective view of an imaging unit100according to Preferred Embodiment 1.FIG.2is a sectional view of the imaging unit100according to Preferred Embodiment 1. The imaging unit100includes a housing1, a protective cover2, which is transparent and provided on one surface of the housing1, a cleaning nozzle3including an opening31for discharging a cleaning liquid onto a protective cover2, a vibrator12for vibrating the protective cover2, and an imaging section5provided inside the protective cover2. Note that, a configuration including the housing1, the protective cover2, the cleaning nozzle3, and the vibrator12and excluding the imaging section5from the imaging unit100defines a cleaning apparatus for cleaning foreign matter (adhering matter) adhering to an imaging range of the imaging section5.

As illustrated inFIG.2, the imaging section5is supported by a main body member4with a cylindrical or substantially cylindrical shape and fixed to a base plate4a. The base plate4ais fixed to a portion of the housing1. Thus, the housing1defines and functions as a holder that holds the imaging section5via the main body member4and the base plate4a. Note that, the holder is not limited to the structure illustrated inFIG.2as long as the imaging section5can be held.

A circuit6including an imaging element is built in the imaging section5. A lens module7is fixed in an imaging direction of the imaging section5. Note that, the imaging element is, for example, a charge coupled device (CCD) image sensor, a complementary MOS (CMOS) image sensor, or the like. The lens module7includes a cylindrical body, and a plurality of lenses9are provided therein. Note that, the structure of the imaging section5is not particularly limited as long as an object to be imaged positioned in front of the lens9can be imaged.

The housing1has a rectangular or substantially rectangular tube shape and is made of, for example, metal or a synthetic resin. Note that, the housing1may have another shape, such as a cylindrical or substantially cylindrical shape, for example. The base plate4ais fixed to one end side of the housing1, and the protective cover2and the vibrator12are provided on the other end side of the housing1.

The vibrator12has a cylindrical or substantially cylindrical shape. The vibrator12includes a first cylindrical member13having a cylindrical or substantially cylindrical shape, a second cylindrical member14having a cylindrical or substantially cylindrical shape, and a piezoelectric vibrator15having a cylindrical or substantially cylindrical shape. The cylindrical piezoelectric vibrator15includes two cylindrical or substantially cylindrical piezoelectric plates16and17. In the two piezoelectric plates16and17, a polarization direction of one piezoelectric plate is opposite to a polarization direction of the other piezoelectric plate in a thickness direction.

Note that, in the present preferred embodiment, the vibrator and the piezoelectric vibrator may each have, for example, a rectangular or substantially rectangular tube shape, instead of the cylindrical or substantially cylindrical shape. Preferably, a cylindrical or ring shape is used.

The piezoelectric plates16and17are each made of, for example, PZT based piezoelectric ceramics. However, other piezoelectric ceramics such as, for example, (K, Na)NbO3may be used. Further, a piezoelectric single crystal such as, for example, LiTaO3may be used. Electrodes (not illustrated) are provided on both surfaces of the piezoelectric plates16and17. This electrode has a laminated structure of, for example, Ag/NiCu/NiCr.

The first cylindrical member13having a cylindrical or substantially cylindrical shape is fixed to a lower surface of the piezoelectric vibrator15. The first cylindrical member13is made of metal, for example. As the metal, for example, duralumin, stainless, kovar, or the like can be used. However, the first cylindrical member13may be made of a semiconductor such as, for example, Si having conductivity.

The piezoelectric vibrator15is sandwiched between a portion of the first cylindrical member13and a portion of the second cylindrical member14. The first cylindrical member13and the second cylindrical member14are each made of, for example, metal and have conductivity. By applying an AC electric field to an electrode of each of the piezoelectric plates16and17, the piezoelectric vibrator15can be longitudinally or laterally vibrated. A female screw portion is provided on an inner peripheral surface of a part of the second cylindrical member14. Thereby, the first cylindrical member13is screwed into the second cylindrical member14, and the first cylindrical member13is fixed to the second cylindrical member14. By this screwing operation, a portion of the first cylindrical member13and a portion of the second cylindrical member14are pressed to contact an upper surface and a lower surface of the piezoelectric vibrator15.

Accordingly, vibration generated in the piezoelectric vibrator15efficiently vibrates the vibrator12. In the present preferred embodiment, the vibrator12is efficiently excited by a longitudinal effect or a lateral effect.

On the other hand, the second cylindrical member14includes a flange portion14bprojecting outward. The flange portion14bis located on and fixed to a recessed portion of the housing1.

A flange portion14cprojecting outward is provided at an end of the second cylindrical member14. A portion extending between the flange portion14band the flange portion14cis a thin-walled portion14a. A thickness of the thin-walled portion14ais less than a thickness of the first cylindrical member13. Thus, the thin-walled portion14a, which is cylindrical or substantially cylindrical shape, is largely displaced by vibration of the vibrator12. Due to the presence of the thin-walled portion14a, vibration, particularly amplitude, can be increased.

The protective cover2is fixed to the flange portion14c. The protective cover2defines and functions as a light-transmissive body that transmits light from an object to be imaged. The protective cover2includes an opening opened in one direction. An end of the opening is joined to the flange portion14c. This joining is made using, for example, an adhesive or a brazing material. Further, for example, thermal compression bonding, anodic bonding, or the like may be used.

The protective cover2has a dome shape extending from the end joined to the flange portion14c. In the present preferred embodiment, for example, the dome shape is a hemispherical shape. Note that, the imaging section5has a viewing angle of, for example, about 170°. However, the dome shape is not limited to the hemispherical shape. A shape in which a cylinder extends from a hemisphere, or a curved surface smaller than a hemisphere may be used, for example. In addition, a flat plate shape, instead of the dome shape, may also be used, for example. An entirety or substantially an entirety of the protective cover2has translucency. In the present preferred embodiment, the protective cover2is made of glass, for example. However, instead of glass, for example, transparent plastic, or the like may be used. Alternatively, for example, translucent ceramics may be used. Depending on the application, it is preferable to use tempered glass. Thus, the strength can be improved. Further, in the case of glass, a coating layer made of DLC or the like, for example, may be provided on a surface in order to improve the strength.

The lens module7and the imaging section5described above are disposed inside the protective cover2. An external object to be imaged is photographed through the protective cover2.

The housing1is provided with the cleaning nozzle3including the opening31for discharging a cleaning liquid onto the protective cover2. The cleaning nozzle3has a cylindrical or substantially cylindrical shape, is supplied with the cleaning liquid from an end portion opposite to an end portion in which the opening31is provided, and discharges the cleaning liquid from the opening31to an end of the protective cover2. A tip of the cleaning nozzle3is outside an imaging range (field of view) of the imaging section5, and the opening31is not at a position that is reflected in an image of the imaging section5. InFIG.2, a flow of the cleaning liquid is indicated by arrows. The cleaning nozzle3functions as a discharger that discharges the cleaning liquid. In the present preferred embodiment, the configuration is illustrated in which one cleaning nozzle3is provided in the housing1, but a configuration may be adopted in which a plurality of cleaning nozzles3are provided in the housing1.

Next, control of the cleaning apparatus will be described with reference toFIG.3.FIG.3is a block diagram for explaining control of the cleaning apparatus of the imaging unit100according to Preferred Embodiment 1.

The imaging unit100includes the imaging section5, a signal processing circuit20, a piezoelectric driver30, a piezoelectric device40, a cleaning liquid discharger50, a cleaning driver60, an impedance detection unit70, and a power supply circuit80. The signal processing circuit20is a controller that processes an imaging signal from the imaging section5and supplies a control signal to the piezoelectric driver30and the cleaning driver60. As the cleaning liquid discharger50, a configuration in which a cleaning liquid is discharged from the opening31of the cleaning nozzle3is illustrated as one block.

The signal processing circuit20includes, for example, a central processing unit (CPU) as a control center, a read only memory (ROM) storing a program, control data, and the like for operating the CPU, a random access memory (RAM) defining and functioning as a work area of the CPU, an input/output interface for maintaining signal consistency with peripheral devices, and the like.

The piezoelectric driver30generates an AC output signal having a frequency f and a voltage V, according to a control signal from the signal processing circuit20and a drive voltage. The piezoelectric device40is defined by the vibrator12including the piezoelectric vibrator15illustrated inFIG.2, and when an AC output signal is applied to the piezoelectric vibrator15, the vibrator12and the protective cover2vibrates to remove foreign matter.

Further, the signal processing circuit20can generate a control signal for discharging a cleaning liquid to the protective cover2to perform cleaning. The cleaning driver60controls, based on a control signal from the signal processing circuit20, the cleaning liquid discharger50to discharge a cleaning liquid onto the protective cover2.

The impedance detector70monitors a current of the piezoelectric driver30, when an AC output signal is applied to the piezoelectric vibrator15to operate the piezoelectric device40. Thus, the signal processing circuit20can perform, based on impedance detected by the impedance detector70, tracking control (feedback control) such that the protective cover2is always vibrated at a resonant frequency. Note that, in order to vibrate the protective cover2at the resonant frequency, it is necessary to cause the piezoelectric vibrator15to vibrate the vibrator12and the protective cover2at the resonant frequency.

Next, how a cleaning liquid discharged onto a surface of the protective cover2changes when the protective cover2is vibrated at the resonant frequency will be described with reference toFIG.4.FIG.4is a diagram for explaining a state of a cleaning liquid discharged onto the surface of the protective cover2. The protective cover2includes water-repellent coating or the like on the surface thereof in some cases and has water repellency. Thus, when about 2 ml of a cleaning liquid W is discharged onto the surface of the protective cover2, the water repellency of the protective cover2causes a contact angle between the cleaning liquid W and the protective cover2to be about 90 degrees or greater, and a contact area between the cleaning liquid W and the protective cover2to be about 0.02 cm2, for example.

On the other hand, when the protective cover2, onto the surface of which the cleaning liquid W is discharged, is vibrated at the resonant frequency, the cleaning liquid W and the protective cover2resonate with each other to reduce surface tension of the cleaning liquid W, and the cleaning liquid W easily spreads out. That is, when vibration energy of the protective cover2is applied to the cleaning liquid W, the cleaning liquid W easily spreads out on the surface of the protective cover2, and a state changes as though the surface of the protective cover2is hydrophilic (pseudo-hydrophilic). Specifically, when about 2 ml of the cleaning liquid W is discharged onto the surface of the protective cover2and the protective cover2is vibrated at the resonant frequency, the contact angle between the cleaning liquid W and the protective cover2becomes about 6 degrees due to the pseudo-hydrophilicity of the protective cover2, and the cleaning liquid W spreads out to have the contact area of about 0.28 cm2, for example, between the cleaning liquid W and the protective cover2. That is, vibrating the protective cover2at the resonant frequency makes it possible to spread out the cleaning liquid W until the contact area increases about 14 times.

Further, a relationship between the contact angle, which is formed between the cleaning liquid W and the protective cover2, and a drive voltage of the piezoelectric device40will be described with reference toFIG.5.FIG.5is a diagram for explaining the relationship between the contact angle, which is formed between the cleaning liquid W and the protective cover2, and the drive voltage of the piezoelectric device40. As illustrated inFIG.5, when the protective cover2, onto the surface of which the cleaning liquid W is discharged, is vibrated at the resonant frequency, a magnitude of vibration (magnitude of amplitude) can be increased by increasing the drive voltage of the piezoelectric device40. Thus, since the vibration energy of the protective cover2applied to the cleaning liquid W also increases, the cleaning liquid W more easily spreads out on the surface of the protective cover2.

Specifically, when the drive voltage of the piezoelectric device40is low, the contact angle between the cleaning liquid W and the protective cover2is about 50 degrees, for example. On the other hand, when the drive voltage of the piezoelectric device40is high, the contact angle between the cleaning liquid W and the protective cover2is about 6 degrees, for example.

Next, operation of the cleaning apparatus of the imaging unit will be described based on a flowchart.FIG.6is a flowchart for explaining operation of the cleaning apparatus of the imaging unit according to Preferred Embodiment 1. First, the signal processing circuit20determines that foreign matter adheres to the surface of the protective cover2(step S11). The signal processing circuit20can determine whether or not foreign matter adheres to the surface of the protective cover2, based on a change over time in a value (for example, a current value) related to impedance and detected by the impedance detector70when the piezoelectric device40is operated or a change over time in an image captured by the imaging element.

Note that, the signal processing circuit20may also determine whether or not foreign matter adheres to the surface of the protective cover2by combining the change over time in the value related to the impedance and detected by the impedance detector70and the change over time in the image captured by the imaging element. In addition, the signal processing circuit20operates as a determination unit that determines foreign matter adhering to the surface of the protective cover2.

Further, the signal processing circuit20may specify a type of the foreign matter adhering to the surface of the protective cover2, based on an analysis result (brightness integrated value) of the image captured by the imaging element. Accordingly, the signal processing circuit20can determine that the foreign matter adhering to the surface of the protective cover2is opaque matter such as, for example, mud.

When no foreign matter adheres to the surface of the protective cover2(NO in step S11), the processing returns to step S11and the signal processing circuit20monitors adhering of foreign matter to the surface of the protective cover2. On the other hand, when the foreign matter adheres to the surface of the protective cover2(YES in step S11), the signal processing circuit20drives the cleaning driver60to cause a cleaning liquid to be discharged (step S12).

In order to make the surface of the protective cover2pseudo-hydrophilic, the signal processing circuit20causes the protective cover2to vibrate at the resonant frequency when the cleaning liquid W is discharged onto the surface (step S13). By making the surface of the protective cover2pseudo-hydrophilic, the discharged cleaning liquid W easily spreads out on the surface of the protective cover2. Thus, even when an amount of the cleaning liquid W to be discharged onto the surface of the protective cover2is small, it is possible to remove the foreign matter adhering to the surface of the protective cover2.

A state in which a small amount of the cleaning liquid W is discharged to remove foreign matter adhering to the surface of the protective cover2will be described with reference toFIG.7.FIG.7is a diagram for explaining a state in which foreign matter adhering to the surface of the protective cover2is removed with a small amount of the cleaning liquid W. First, inFIG.7, foreign matters S1and S2different in size adhere to the surface of the protective cover2. A small amount (for example, about 0.5 ml) of the cleaning liquid W is discharged onto the surface of the protective cover2. When the protective cover2is vibrated not at the resonant frequency, only a portion of the foreign matters S1and the S2can be wetted with the small amount of the cleaning liquid W due to water repellency of the protective cover2.

However, when the protective cover2is vibrated at the resonant frequency, the surface of the protective cover2becomes pseudo-hydrophilic, and even the small amount of the cleaning liquid W can wet the foreign matters S1and S2entirely.FIG.7illustrates a state in which the cleaning liquid W spreading out on the surface of the protective cover2covers the foreign matters S1and S2entirely.

FIG.8is a graph showing an amount of the cleaning liquid W necessary for removing foreign matter in each of a case where the protective cover2is vibrated not at the resonant frequency and a case where the protective cover2is vibrated at the resonant frequency. InFIG.8, when the protective cover2is vibrated not at the resonant frequency to discharge the cleaning liquid W, the amount of the cleaning liquid W necessary for removing the foreign matter is about 15.1 ml, for example. On the other hand, when the protective cover2is vibrated at the resonant frequency to discharge the cleaning liquid W, the amount of the cleaning liquid W necessary for removing the foreign matter is about 0.6 ml, for example.

That is, about 14.5 ml of the cleaning liquid W can be reduced by the protective cover2being vibrated at the resonant frequency. In particular, in an imaging unit provided in a vehicle, a tank storing the cleaning liquid W may not be increased in size in some cases, and when an amount of the cleaning liquid W to be used can be reduced with the same cleaning power, the imaging unit can be reduced in size.

Referring back toFIG.6, the signal processing circuit20determines whether a predetermined period (for example, two to three seconds) elapses after the protective cover2is vibrated at the resonant frequency and the cleaning liquid W is discharged onto the surface (step S14). Note that, the signal processing circuit20may perform control such that, when a type of the foreign matter adhering to the surface of the protective cover2is specified based on an analysis result of an image captured by the imaging element, the signal processing circuit20changes the predetermined period and causes the protective cover2to be vibrated at the resonant frequency for a period according to the type of foreign matter.

When the predetermined period does not elapse after the cleaning liquid W is discharged onto the surface (NO in step S14), the processing returns to step S13and the signal processing circuit20continues the process of causing the protective cover2to be vibrated at the resonant frequency. On the other hand, when the predetermined period elapses after the cleaning liquid W is discharged onto the surface (YES in step S14), the signal processing circuit20causes the protective cover2to be vibrated at a frequency other than the resonant frequency (step S15).

By vibrating the protective cover2at a frequency other than the resonant frequency, the state of the surface of the protective cover2being pseudo-hydrophilic can be returned to the state of water repellency. That is, the imaging unit100can vibrate the protective cover2at a frequency other than the resonant frequency to flick off the foreign matter adhering to the surface of the protective cover2together with the cleaning liquid W. Thus, the imaging unit100can more strongly clean off the foreign matter adhering to the protective cover2. Note that, the signal processing circuit20may stop vibration of the protective cover2, instead of causing the protective cover2to be vibrated at a frequency other than the resonant frequency. Since the surface of the protective cover2is water-repellent, the foreign matter adhering to the surface of the protective cover2can be flicked off together with the cleaning liquid W, also when the vibration of the protective cover2is stopped.

As described above, in order to make it easy for the cleaning liquid W to spread out on the surface of the protective cover2, it is preferable that the signal processing circuit20continues to cause the protective cover2to be vibrated at the resonant frequency for the predetermined period after the cleaning liquid W is discharged onto the surface. However, since the state of the protective cover2changes, for example, the foreign matter adhering to the surface of the protective cover2is removed during the predetermined period, the resonant frequency may change. Thus, there was a case in which, even when the signal processing circuit20causes the protective cover2to be vibrated at the resonant frequency at the beginning of the predetermined period, the protective cover2is vibrated at a frequency other than the resonant frequency of the protective cover2later.

Thus, the imaging unit100performs, based on the impedance detected by the impedance detector70, tracking control such that the protective cover2is always vibrated at the resonant frequency.FIG.9is a graph for explaining the tracking control for vibrating the protective cover2at the resonant frequency.

First, the foreign matters S1and S2different in size adhere to the surface of the protective cover2. In this case, the signal processing circuit20causes the protective cover2to be vibrated at a resonant frequency FHa at which impedance shown in the graph of frequency and impedance inFIG.9rapidly changes.

Next, the foreign matter S1is removed from the surface of the protective cover2at timing when a time t1elapses after the protective cover2is vibrated at the resonant frequency FHa. When the foreign matter S1is removed from the surface of the protective cover2and the state of the protective cover2changes, the resonant frequency of the protective cover2changes to be higher from the resonant frequency FHa to a resonant frequency FHb.

When the change in the resonant frequency of the protective cover2from the resonant frequency FHa to the resonant frequency FHb is detected based on the impedance detected by the impedance detector70, the signal processing circuit20performs the tracking control for vibrating the protective cover2at the resonant frequency FHb higher than the resonant frequency FHa.

As shown in a graph of frequency and time inFIG.9, when the change in the resonant frequency of the protective cover2with respect to time is a change as a waveform F2, the signal processing circuit20causes the piezoelectric driver30to vibrate the piezoelectric device40in accordance with the change. It can be seen that the change in the frequency with respect to time when the piezoelectric driver30causes the piezoelectric device40to vibrate is like a waveform F1and that the piezoelectric device40vibrates so as to track the change in the frequency along the waveform F2.

As described above, the imaging unit100according to Preferred Embodiment 1 includes the cleaning apparatus. This cleaning apparatus is configured to include the housing1for holding the imaging section5, the protective cover2disposed in the field of view of the imaging section5, the vibrator12to vibrate the protective cover2, the piezoelectric driver30to drive the vibrator12, the cleaning liquid discharger50to discharge a cleaner onto the surface of the protective cover2, and the signal processing circuit20to control the piezoelectric driver30. The signal processing circuit20controls the piezoelectric driver30such that the protective cover2is vibrated at the resonant frequency for the predetermined period after the cleaning liquid discharger50is caused to discharge the cleaning liquid W and controls the piezoelectric driver30such that, after the predetermined period, vibration of the protective cover2is stopped or the protective cover2is vibrated at a frequency other than the resonant frequency.

Thus, in the cleaning apparatus according to Preferred Embodiment 1, the signal processing circuit20controls the piezoelectric driver30such that the protective cover2is vibrated at the resonant frequency for the predetermined period after the cleaning liquid is caused to be discharged and then controls the piezoelectric driver30such that vibration of the protective cover2is stopped or the protective cover2is vibrated at a frequency other than the resonant frequency, and thus the foreign matter adhering to the protective cover2can be effectively cleaned.

Note that, a cleaning method of cleaning the surface of protective cover2by the cleaning apparatus of the imaging unit100according to Preferred Embodiment 1 includes a step of causing the cleaning liquid discharger50to discharge a cleaning liquid, a step of controlling the piezoelectric driver30such that the protective cover2is vibrated at a resonant frequency for a predetermined period after the cleaning liquid is caused to be discharged, and a step of controlling the piezoelectric driver30such that, after the predetermined period, vibration of the protective cover2is stopped or the protective cover2is vibrated at a frequency other than the resonant frequency.

Further, the signal processing circuit20may be capable of changing a drive voltage of the piezoelectric driver30in the predetermined period. This makes it possible to change the drive voltage of the piezoelectric driver30and to adjust a contact angle of the cleaning liquid discharged onto the surface of the protective cover2, thus making it possible to reduce an amount of the cleaning liquid to be discharged.

Furthermore, the impedance detector70to detect a value related to the impedance of the piezoelectric device40driven by the piezoelectric driver30may be further included, and the signal processing circuit20may perform, based on the impedance detected by the impedance detector70, tracking control such that the protective cover2is always vibrated at the resonant frequency. Thus, even when a state of the surface of the protective cover2changes during the predetermined period, the protective cover2can always be vibrated at the resonant frequency, and the cleaning liquid discharged onto the surface of the protective cover2can be easily spread out.

In addition, the signal processing circuit20may operate as a determiner to determine foreign matter adhering to the surface of the protective cover2. When determining that foreign matter adheres to the surface of the protective cover2, the signal processing circuit20may cause the cleaning liquid discharger50to discharge the cleaning liquid. Thus, when the foreign matter adheres to the surface of the protective cover2, the cleaning liquid can be discharged from the cleaning liquid discharger50.

Further, the signal processing circuit20may determine, based on at least one piece of information of a change over time in a value related to the impedance and detected by the impedance detector70and a change over time in an image captured by the imaging element, that foreign matter adheres to the surface of the protective cover2. Thus, the signal processing circuit20can discriminate the foreign matter adhering to the surface of the protective cover2without a sensor to be separately provided for detecting the foreign matter adhering to the surface of the protective cover2.

Further, when a type of the foreign matter adhering to the surface of the protective cover2is specified based on an analysis result of the image captured by the imaging element, the signal processing circuit20may change the predetermined period, for which the protective cover2is vibrated at the resonant frequency, according to the type of foreign matter. Accordingly, the signal processing circuit20can vibrate the protective cover2at the resonant frequency for an optimum period according to the type of the foreign matter adhering to the surface of the protective cover2.

In the cleaning apparatus according to Preferred Embodiment 1, the resonant frequency of the protective cover2may change due to temperature. Thus, in a cleaning apparatus according to Preferred Embodiment 2 of the present invention, a configuration in which a temperature detector that measures temperature is provided will be described.

FIG.10is a block diagram for explaining control of a cleaning apparatus of an imaging unit200according to Preferred Embodiment 2. The imaging unit200includes the imaging section5, the signal processing circuit20, the piezoelectric driver30, the piezoelectric device40, the cleaning liquid discharger50, the cleaning driver60, the impedance detector70, the power supply circuit80, and a temperature detector90. The imaging unit200has the same or substantially the same configuration as that of the imaging unit100illustrated inFIG.3, except that the temperature detector90is added, and the same or corresponding elements are denoted by the same reference numerals and detailed description thereof will not be repeated.

The temperature detector90can measure a temperature of the imaging unit200, for example, in a vicinity of the vibrator or the protective cover2. It is sufficient that the temperature detector90can output the measured temperature to the signal processing circuit20, and, for example, a known temperature sensor or temperature measurement device may be used.

The signal processing circuit20uses information on the temperature measured by the temperature detector90to control the piezoelectric driver30such that the protective cover2is vibrated at the resonant frequency. For example, when the temperature is changed from about −40° C. to about 85° C., the resonant frequency of the protective cover2decreases as the temperature increases. Thus, the signal processing circuit20controls the frequency, at which the protective cover2is vibrated in consideration of not only the impedance detected by the impedance detector70but also the measurement result of the temperature detector90. Compared with a case where tracking control is performed, based only on the impedance detected by the impedance detector70, such that the protective cover2is vibrated at the resonant frequency, accuracy of tracking a change in the resonant frequency of the protective cover2is improved in the case where the tracking control is performed in consideration of the temperature measured by the temperature detector90.

As described above, the cleaning apparatus according to Preferred Embodiment 2 may further include the temperature detector90that measures a temperature of the piezoelectric device40or the protective cover2, and the signal processing circuit20may control a frequency at which the protective cover2is vibrated, based on a measurement result of the temperature detector90. Thus, the signal processing circuit20can accurately perform tracking such that the protective cover2is vibrated at the resonant frequency.

In the imaging units according to the above-described preferred embodiments, although a configuration of the imaging section5is not particularly described in detail, the imaging section5may include, for example, a camera, a LiDAR, a Rader, or the like.

The imaging units according to the above-described preferred embodiments have been described to be configured such that one cleaning nozzle3is provided in the housing1as illustrated inFIG.1, but the present invention is not limited thereto, and a configuration may be provided in which a plurality of the cleaning nozzles3are provided in the housing1.

The imaging units according to the above-described preferred embodiments have been described to be configured such that, as illustrated inFIG.2, the vibrator12includes the first cylindrical member13having the cylindrical or substantially cylindrical shape, the second cylindrical member14having the cylindrical or substantially cylindrical shape, and the piezoelectric vibrator15having the cylindrical or substantially cylindrical shape, but the present invention is not limited thereto, and another configuration may be provided as long as the protective cover2can be vibrated at the resonant frequency in the configuration.

The imaging units according to the above-described Preferred embodiments are applicable not only to an imaging unit provided in a vehicle, but also to an imaging unit for an application in which a light-transmissive body disposed in a field of view of an imaging element needs to be cleaned.

The description has been provided that, in the imaging units according to the above-described preferred embodiments, as information for determining that foreign matter adheres to the surface of the protective cover2, there is a change over time in an image captured by the imaging section5, for example, a change over time in a brightness integral value of an image captured by the imaging section5. However, the present invention is not limited thereto, and for example, as a change over time in an image captured by the imaging section5, a blur of an edge of the captured image may be evaluated by a frequency spectrum of image processing to determine, based on a change over time in the frequency spectrum, that foreign matter adheres to the surface of the protective cover2.

Specifically, when raindrops adhere as foreign matter to the surface of the protective cover2, in an image captured by the imaging section5, a blur occurs at an edge of the image as compared with a case where no raindrop adheres thereto, and frequency power of a frequency spectrum increases at a lower frequency as compared with an image in a state where no blur occurs. Thus, when the frequency at which the frequency power increases in the frequency spectrum changes to be low, the signal processing circuit20can determine that raindrops as foreign matter adhere to the surface of the protective cover2. By the signal processing circuit20determining, in combination with the change over time in the frequency spectrum, that foreign matter adheres to the surface of the protective cover2, accuracy can be further improved.