JP2019039960A - Variable focus mirror system - Google Patents

Abstract

To prolong the life of a piezoelectric film without affecting the image of an HUD and by reducing the frequency of reverse polarization.SOLUTION: Reverse polarization is executed on a variable focus mirror 1 when a power source switch is in the off-state. This allows the reverse polarization to be done so that the image of an HUD will not be affected. Also, a first voltage V1 is allowed to reach a second threshold value Th2 after the reverse polarization, while reverse polarization is to be done when the first voltage V1 becomes lower than a first threshold value Th1. It thus becomes possible to prevent reverse polarization from being done too frequently and to suppress reduction in the life of the piezoelectric film.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a variable focus mirror system that adjusts the curvature of a variable focus mirror.

  Conventionally, development of a laser scan type head-up display (hereinafter referred to as HUD) has been promoted. In a HUD laser scanning module (hereinafter referred to as LSM), laser scanning is performed by adjusting the laser spot diameter. For this reason, the laser spot diameter is adjusted by providing the HUD with a variable focus mirror that changes the focal position of the reflected light.

  For example, as the variable focus mirror, one shown in Patent Document 1 can be cited. This variable focus mirror is formed on a silicon oxide film formed on the device layer, and has a configuration including a piezoelectric film and an electrode for applying an electric field to the piezoelectric film. More specifically, the lower electrode, the first piezoelectric film, the intermediate electrode, the second piezoelectric film, and the upper electrode are sequentially formed on the silicon oxide film.

  The piezoelectric film provided in the variable focus mirror is made of a dielectric, and the crystal grains have an orientation direction. When an electric field is applied, the crystal grains are oriented in the c-axis direction, so that they expand in the c-axis direction and shrink in the a-axis direction. At this time, since the device layer is not reduced, the center position of the variable focus mirror protrudes on the device layer side, that is, if the variable focus mirror is on the upper side and the device layer is on the lower side, the variable focus mirror is convex downward. Deform. Since the variable focus mirror deformed in this way has a spherical shape, its outermost surface can be used as a concave mirror.

  In the variable focus mirror, in order to align the orientation direction of the crystal grains of the piezoelectric film at the time of manufacture, an electric field is applied in the c-axis direction to perform polarization treatment, and after the polarization treatment, a specified curvature variable range can be obtained within a specified voltage range. To design. Specifically, a rectangular-wave drive voltage in which the first voltage V1 and the second voltage V2 are alternately switched is applied to the upper electrode and the lower electrode while the intermediate electrode is at the ground potential. The first voltage V1 is adjusted to the voltage when the variable focus mirror has the first curvature R1, and the second voltage V2 is adjusted to the voltage when the variable focus mirror has the second curvature R2 larger than the first curvature R1. . For example, in the operation with HUD, the variable focus mirror is driven by applying a rectangular-wave drive voltage of 72 Hz, and the deformation with the first curvature R1 and the second curvature R2 is alternately repeated.

  Here, the drive voltage and the curvature of the variable focus mirror have the following relationship. That is, at the initial use of the variable focus mirror, both the first voltage V1 having the first curvature R1 and the second voltage V2 having the second curvature R2 are positive voltages. However, when the variable focus mirror is driven for a long time, the curvature with respect to the same voltage increases. For this reason, the characteristics of the variable focus mirror are out of the specification range, and there arises a problem that the far display side is out of focus. This is considered to be because the orientation direction of the crystal grains of the piezoelectric film rotates in the c-axis direction by long-term driving.

  A technique for reversing the change in orientation direction due to such long-term driving is also considered. Specifically, (1) A method of generating a reverse electric field with a unique drive waveform in the normal drive waveform during use and repeating use and recovery, (2) Reverse of high voltage at power-on, etc. There are a method of aligning polarization by an electric field, and a method of (3) reverse polarization by a DC voltage as needed when driving the variable focus mirror.

JP 2017-032714 A

  However, the method (1) cannot be adopted because the image is disturbed when the method is performed by HUD. In addition, since the methods (2) and (3) frequently perform reverse polarization, stress is frequently generated in the crystal grain boundaries of the piezoelectric film, causing cracks in the vicinity of the grain boundaries. Detrimental effects such as quick deterioration of characteristics such as a decrease in piezoelectric constant and shortening of the dielectric breakdown life occur.

  In view of the above points, the present invention allows a variable focus mirror system applied to a HUD to have no effect on the HUD image and to increase the lifetime of the piezoelectric film by reducing the frequency of reverse polarization. The purpose is to do.

  In order to achieve the above object, a variable focus mirror system according to claim 1 is formed on a base (11) constituting the membrane and on the base, and on the piezoelectric films (12b, 12d) and both sides of the piezoelectric film. A variable focus mirror (1) having electrodes (12a, 12c, 12e) and a mirror part (12) for reflecting a light beam; and a predetermined gap between the electrodes arranged on both sides of the piezoelectric film. A control unit (30) for bending the mirror part into a spherical shape with a predetermined curvature and a curvature detection part (13, 20) for measuring the curvature of the bent mirror part. Then, the variable focus mirror in the head-up display in the vehicle is controlled.

  In such a varifocal mirror system, the control unit, when driving the varifocal mirror, has a rectangular-wave drive voltage that alternately switches between a first voltage (V1) and a second voltage (V2) that is greater than the first voltage. Is applied between the electrodes, the first voltage is applied to bend the mirror portion with the first curvature (R1) and the second voltage is applied to bend the mirror portion with the second curvature (R2). Based on the detection result of the curvature detection unit, feedback control is performed to adjust the first voltage so that the curvature of the mirror unit becomes the first curvature and to adjust the second voltage so as to become the second curvature. Furthermore, when the first voltage falls below the first threshold (Th1), when the power switch of the vehicle is turned off, a DC voltage that generates an electric field opposite to that at the time of driving the variable focus mirror is applied between the electrodes as reverse polarization. Thus, the orientation direction of the piezoelectric film is adjusted until the first voltage reaches the second threshold (Th2) higher than the first threshold.

  Thus, the reverse polarization of the variable focus mirror is executed when the power switch is off. For this reason, it is possible to perform reverse polarization without affecting the HUD image. In addition, reverse polarization is performed when the first voltage falls below the first threshold, and the first voltage reaches the second threshold after reverse polarization. For this reason, it becomes possible not to perform reverse polarization with high frequency, and it also becomes possible to suppress the lifetime reduction of a piezoelectric material.

  In addition, the code | symbol in the bracket | parenthesis of each said means shows an example of a corresponding relationship with the specific means as described in embodiment mentioned later.

It is sectional drawing of the variable focus mirror with which the variable focus mirror system concerning 1st Embodiment is equipped. 1 is an overall configuration diagram of a variable focus mirror system during driving. FIG. It is a wave form diagram of the voltage applied at the time of the drive of a variable focus mirror. It is a wave form diagram of the voltage applied at the time of reverse polarization of a variable focus mirror. It is the figure which showed the direction of the crystal | crystallization of a piezoelectric material, and the name of a surface. It is sectional drawing which showed the mode when a variable focus mirror was bent. It is the figure which showed the relationship between a drive voltage and the curvature of a variable focus mirror. It is the figure which showed the orientation direction when polarization was performed at the time of manufacture. It is the figure which showed the orientation direction changed by long-term use. It is the figure which showed the relationship between the drive voltage changed by use and the curvature of a variable focus mirror. It is the figure which showed the relationship between the drive voltage changed by use and the curvature of a variable focus mirror. It is the figure which showed the approximated curve based on the log of a 1st voltage, and the 1st threshold value and the 2nd threshold value. It is a whole block diagram of the variable focus mirror system at the time of reverse polarization. It is the figure which showed the approximated curve and 2nd threshold value based on the log of the 1st voltage at the time of reverse polarization. It is the figure which showed the relationship between the drive voltage in the state a in which the 1st voltage fell below the 1st threshold value, and the curvature of a variable focus mirror. It is the figure which showed the relationship between the drive voltage in the state b in which a 1st voltage is larger than a 1st threshold value and smaller than a 2nd threshold value, and the curvature of a variable focus mirror. It is the figure which showed the relationship between the drive voltage in the state c in which the 1st voltage became larger than a 2nd threshold value, and the curvature of a variable focus mirror. It is the state transition diagram which showed the operation state of the control part with the flow of operation | movement of a variable focus mirror system.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following embodiments, parts that are the same or equivalent to each other will be described with the same reference numerals.

(First embodiment)
The variable focus mirror system according to the first embodiment will be described. The variable focus mirror system of this embodiment is used as an LSM in a vehicle HUD and controls the variable focus mirror. Hereinafter, the variable focus mirror system according to the present embodiment will be described with reference to FIGS. The variable focus mirror 1 shown in FIG. 1 is a part constituting the mirror part in the LSM. In practice, the variable focus mirror 1 support unit and the variable focus mirror 1 are driven around the variable focus mirror 1. The LSM is configured by providing various driving units and the like.

  As shown in FIG. 1, the varifocal mirror 1 is formed by using, for example, a MEMS (Micro-Electro-Mechanical Systems) technique, and has a curvature in addition to a piezoelectric device that constitutes a mirror portion 12 on a base portion 11 in a substrate 10. The sensor 13 and the like are provided.

  The substrate 10 is constituted by an SOI (abbreviation of Silicon on Insulator) substrate having a structure in which a device layer 10a, an insulating film 10b, and a handle layer 10c are sequentially bonded.

The device layer 10a is made of, for example, Si. The insulating film 10b is composed of a buried oxide layer (hereinafter referred to as a BOX (abbreviated as “Buried Oxide”) layer) made of, for example, SiO ₂ . The handle layer 10c is composed of a silicon substrate or the like. The device layer 10a, the insulating film 10b, and the handle layer 10c are patterned in a shape corresponding to each part constituting the LSM including the variable focus mirror 1.

  Specifically, the varifocal mirror 1 reflects a light beam applied to the LSM. For example, the varifocal mirror 1 includes the base 11, the mirror 12, and the curvature sensor 13 at the center of the substrate 10. Has been.

  The base 11 constitutes a thin membrane and is a portion formed by patterning the device layer 10a into a circle. In the present embodiment, only the portion that becomes the base portion 11 of the device layer 10a is illustrated. A mirror portion 12 is formed on the upper surface of the base portion 11, and an insulating film 15 is formed between the base portion 11 and the mirror portion 12. That is, the insulating film 15 and the mirror part 12 are sequentially laminated on the upper surface of the base part 11. A surface of the mirror portion 12 opposite to the insulating film 15 is a reflecting surface, and the light beam is reflected by the reflecting surface. Further, the curvature sensor 13 is disposed on the upper surface of the base portion 11 via the insulating film 15 at a position different from the mirror portion 12, specifically, at a position outside the mirror portion 12.

  As shown in FIG. 1, the handle layer 10 c and the insulating film 10 b are removed from the inner peripheral portion 11 a of the base 11. Further, the handle layer 10c and the insulating film 10b are left on the outer peripheral portion 11b of the base portion 11.

  The base 11 has an upper surface that is a flat surface, and the upper surface has a circular shape, for example. On the upper surface of the inner peripheral portion 11 a of the base portion 11, the mirror portion 12 is laminated via the insulating film 15. And the inner peripheral part 11a of the base part 11 is bent with a reflective surface, when the mirror part 12 bends. By making the base 11 thinner, the sensitivity of the variable focus mirror 1 is increased.

  On the other hand, the mirror portion 12 is not formed on the outer peripheral side of the inner peripheral portion 11a or the outer peripheral portion 11b located outside the inner peripheral portion 11a, and the curvature sensor 13 is formed through the insulating film 15 in that portion. ing. When the inner peripheral part 11a is deformed along with the deformation of the mirror part 12, the outer edge side of the inner peripheral part 11a and the outer peripheral part 11b are also deformed. Based on this deformation, curvature detection by the curvature sensor 13 is performed.

  The mirror unit 12 has a circular top surface and changes the focal position of reflected light by the reflecting surface by bending the reflecting surface into a spherical shape. The mirror unit 12 is configured by a piezoelectric element having a structure in which a lower electrode 12a, a first piezoelectric film 12b, an intermediate electrode 12c, a second piezoelectric film 12d, and an upper electrode 12e are sequentially stacked.

  The lower electrode 12a and the intermediate electrode 12c are formed of a single layer film or a plurality of laminated films such as platinum (Pt) and strontium ruthenium oxide (hereinafter referred to as SRO), and are thin film electrodes. The first piezoelectric film 12b and the second piezoelectric film 12d are made of, for example, lead zirconate titanate (hereinafter referred to as PZT). The upper electrode 12e is made of, for example, Pt, SRO, etc., and is a thin film electrode. Note that the surface of the upper electrode 12e itself may be a reflective surface, but a structure in which a film for forming the reflective surface is additionally provided on the surface of the upper electrode 12e may be employed.

  Although not shown in FIG. 1, the lower electrode 12a, the intermediate electrode 12c, and the upper electrode 12e are each connected to the control device of the variable focus mirror 1 through wiring. By this control device, voltage is applied to the lower electrode 12a and the upper electrode 12e and the intermediate electrode 12c is set to the ground potential, and a potential difference is generated between the lower electrode 12a and the upper electrode 12e and the intermediate electrode 12c. Thereby, a piezoelectric effect is generated, and the first piezoelectric film 12b and the second piezoelectric film 12d are deformed. Along with this, the base portion 11 is bent together with the mirror portion 12, the reflection surface is bent, and the focal position of the reflected light can be changed.

  The curvature sensor 13 constitutes a detection signal according to the curvature of the variable focus mirror 1, that is, according to the deformation of the base 11, and the curvature of the variable focus mirror 1 can be detected based on the detection signal of the curvature sensor 13. It has become. For example, the curvature sensor 13 is configured by a piezoresistor connected in a Wheatstone bridge shape. A sensing voltage is applied to one connection point of the Wheatstone bridge, the opposite connection point is set as a ground potential, and an intermediate potential that is the potential of the remaining two connection points is output as a detection signal. Based on this, the curvature can be detected.

  In addition, when the curvature sensor 13 is comprised by a piezoresistor, what is necessary is just to comprise about each piezoresistor with the material which comprises a general resistor. In that case, it is preferable to set the formation direction and position of each piezoresistor in consideration of the relationship between the resistance coefficient in the piezoresistor and the crystallinity of the constituent material of the resistor.

  The variable focus mirror 1 is configured as described above. FIG. 2 is a diagram showing a system configuration of a variable focus mirror system having such a variable focus mirror 1. As shown in this figure, the variable focus mirror system includes a sensor circuit 20, a control unit 30, and a switch unit 40 as a control device in addition to the variable focus mirror 1 described above.

  The sensor circuit 20 receives the detection signal of the curvature sensor 13 and detects the curvature of the variable focus mirror 1 based on the input detection signal. Then, the sensor circuit 20 transmits the detected curvature of the variable focus mirror 1 to the control unit 30.

  The control unit 30 is configured by a known microcomputer including a CPU, ROM, RAM, I / O, and the like, and performs predetermined processing according to a program stored in the ROM. Specifically, the control unit 30 controls the switch state in the switch unit 40 and drives the variable focus mirror 1 through the switch unit 40 by applying a desired drive voltage, or keeps the variable focus mirror 1 for a long time. Reverse polarization is performed to bring the change in curvature when driven closer to the original state. When driving the varifocal mirror 1, the control unit 30 applies a rectangular wave voltage that alternately repeats the first voltage V1 and the second voltage V2 as shown in FIG. Applied to the upper electrode 12e. Thereby, the curvature of the variable focus mirror 1 is adjusted to the first curvature R1 and the second curvature R2. Further, when performing reverse polarization, the control unit 30 applies a constant DC voltage as shown in FIG. 3B to the intermediate electrode 12c. As a result, an electric field in the opposite direction to that during polarization is applied to the first piezoelectric film 12b and the second piezoelectric film 12d, and the orientation direction of the crystal grains is directed away from the c-axis side. The magnitude of the DC voltage applied during reverse polarization is arbitrary, but the same voltage as that applied during polarization during manufacture is applied in the reverse direction. The voltage at the time of polarization is made as high as possible while lowering the dielectric breakdown voltage with respect to the first piezoelectric film 12b and the second piezoelectric film 12d, so that a high polarization action occurs. For this reason, even at the time of reverse polarization, by setting the same voltage as that at the time of polarization, it is possible to cause a high reverse polarization action while suppressing the dielectric breakdown of the first piezoelectric film 12b and the second piezoelectric film 12d.

  For example, the control unit 30 monitors the first voltage V1 at which the curvature of the variable focus mirror 1 becomes the first curvature R1 and the second voltage V2 at which the second curvature R2 becomes the second curvature R2 based on the detection result of the sensor circuit 20. At the same time, the first voltage V1 and the second voltage V2 are applied when the variable focus mirror 1 is driven. In addition, a voltage for performing reverse polarization is applied to the varifocal mirror 1 at a timing when display by the HUD is not performed. The controller 30 receives operation signals from a vehicle power switch, for example, an accessory switch (hereinafter referred to as an ACC switch), an ignition switch (hereinafter referred to as an IG switch), an electric vehicle, or the like. . Based on this operation signal, the control unit 30 can detect that it is the timing at which display by HUD is not performed.

  It should be noted that any circuit may be applied to the circuit of the control unit 30 that outputs the drive voltage and the voltage at the time of reverse polarization. For example, a unipolar circuit is used in consideration of cost. ing.

  The switch unit 40 includes a first switch 41 and a second switch 42. The first switch 41 switches the voltage applied to the lower electrode 12a and the upper electrode 12e, and the second switch 42 switches the voltage applied to the intermediate electrode 12c. When the lower electrode 12a and the upper electrode 12e or the intermediate electrode 12c are connected to the control unit 30 by the first switch 41 and the second switch 42, a desired voltage is applied from the control unit 30 to each electrode. When the lower electrode 12a and the upper electrode 12e or the intermediate electrode 12c are not connected to the control unit 30 by the first switch 41 or the second switch 42, each electrode is set to the ground potential.

  As described above, the variable focus mirror system according to the present embodiment is configured. Next, the operation and the like of the variable focus mirror system configured as described above will be described.

  First, the basic operation of the variable focus mirror 1 in the variable focus mirror system will be described.

  First, as shown in FIG. 1, the variable focus mirror 1 is substantially flat at the beginning of manufacture. Here, the directions of the crystal directions and the surfaces of the piezoelectric material constituting the first piezoelectric film 12b and the second piezoelectric film 12d provided in the variable focus mirror 1 are shown in FIG. FIG. 4 shows a case where, for example, PZT is used as the piezoelectric material.

  As shown in this figure, the crystals constituting the first piezoelectric film 12b and the second piezoelectric film 12d have an a axis that is a <100> direction, a b axis that is a <010> direction, and a <001> direction. And the (001) plane having the <001> direction as a normal vector and the (100) plane having the <100> direction as a normal vector. Since the piezoelectric film is a dielectric, the crystal grains are oriented, and when an electric field is applied, the piezoelectric film is oriented in the c-axis direction as indicated by the arrow in FIG. Shrink in the direction. At this time, since the device layer 10a is not reduced, the variable focus mirror 1 is deformed so as to protrude downward as shown in FIG. 5 when the mirror portion 12 is on the upper side and the device layer 10a is on the lower side. . Since the surface of the mirror section 12 has a spherical shape, the deformable variable focus mirror 1 thus deformed can be used as a concave mirror.

  Then, due to the deformation of the device layer 10a accompanying the deformation of the mirror part 12 at this time, the resistance value of the piezoresistor provided in the curvature sensor 13 changes, so that the detection signal of the curvature sensor 13 changes. For this reason, the curvature of the variable focus mirror 1 can be detected based on the detection signal of the curvature sensor 13.

  For the varifocal mirror 1, in order to align the orientation direction of the crystal grains of the piezoelectric material at the time of manufacture, an electric field is applied in the c-axis direction and polarization processing is performed. Design to be obtained. Specifically, in the state where the intermediate electrode 12c is set to the ground potential, a rectangular wave driving voltage in which the first voltage V1 and the second voltage V2 are alternately switched is applied to the lower electrode 12a and the upper electrode 12e. The first voltage V1 is adjusted to the voltage when the varifocal mirror 1 has the first curvature R1, and the second voltage V2 is adjusted to the voltage when the varifocal mirror 1 has the second curvature R2 larger than the first curvature R1. Is done. For example, in operation in HUD, the variable focus mirror 1 is driven by applying a rectangular-wave-like drive voltage of 72 Hz, and the deformation of the first curvature R1 and the second curvature R2 is alternately repeated.

  The drive voltage and the curvature of the varifocal mirror 1 have the relationship shown in FIG. 6, for example. As shown by a solid line in FIG. 6, at the initial use of the variable focus mirror 1, both the first voltage V1 having the first curvature R1 and the second voltage V2 having the second curvature R2 are positive voltages. . However, when the variable focus mirror 1 is driven for a long period of time, the curvature with respect to the same voltage increases as shown by the broken line in FIG. For this reason, the characteristic of the variable focus mirror 1 falls outside the specification range, and there arises a problem that the far display side is out of focus. This is considered to be because during polarization, the crystal grain orientation direction shifted from the c-axis direction as shown in FIG. 7A is rotated in the c-axis direction as shown in FIG. 7B by long-term driving. .

  Therefore, in the varifocal mirror system of the present embodiment, the controller 30 drives the varifocal mirror 1 and changes the first voltage V1 that changes based on the detection result of the curvature in the sensor circuit 20 to a RAM or the like. Store as a log in memory and perform reverse polarization if necessary.

  Specifically, as shown in FIG. 2, when the control unit 30 drives the variable focus mirror 1, the first switch 41 is connected to the control unit 30 side, and the second switch 42 is set to the ground potential side. Make it connected. Then, the control unit 30 outputs a rectangular wave driving voltage in which the first voltage V1 having the first curvature R1 and the second voltage V2 having the second curvature R2 are alternately and repeatedly switched as shown in FIG. 3A. As a result, the drive voltage is applied to the lower electrode 12a and the upper electrode 12e through the first switch 41, and the intermediate electrode 12c is set to the ground potential through the second switch.

  At this time, the control unit 30 monitors the curvature of the variable focus mirror 1 through the sensor circuit 20, and feedback-controls the drive voltage so that the curvature of the variable focus mirror 1 becomes the first curvature R1 and the second curvature R2. The first voltage V1 having the first curvature R1 and the second voltage V2 having the second curvature R2 are set. The first voltage V1 at this time is stored as a log. As the log of the first voltage V1, the value of the first voltage V1 and the date and time at that time or the total driving time of the variable focus mirror 1 are stored.

  Here, when the variable focus mirror 1 is driven for a long period of time, the first voltage V1 gradually decreases due to a change in the orientation direction of the crystal grains of the piezoelectric material, as shown in FIG. 8A. If not, as shown in FIG. 8B, the first voltage V1 may reach a negative voltage. Although it is conceivable that the drive voltage output from the control unit 30 is a negative voltage, it is preferable to adopt a unipolar circuit or the like in consideration of cost, and a negative voltage cannot be generated.

  For this reason, when the first voltage V1 becomes less than the first threshold value Th1, the first threshold value Th1 that is a determination threshold value for performing the reverse polarization is set so that the reverse polarization is performed before the first voltage V1 becomes 0. The reverse polarization is performed.

  Further, in the case of the present embodiment, the first threshold Th1 is set based on the log of the first voltage V1, and is set to a value according to the usage status of the HUD.

  Specifically, as shown in FIG. 9, when the variable focus mirror 1 is driven for a long time, the first voltage V1 gradually decreases, and this is stored in a memory or the like as a log of the first voltage V1. To go. In FIG. 9, a circle indicates the value of the first voltage V1 stored as a log. As shown in this figure, the first voltage V1 gradually decreases according to the number of days that the variable focus mirror 1 has been used.

  The method of changing the first voltage V1 with respect to the number of days elapsed at this time differs depending on the frequency of use of the HUD by the user, in other words, the frequency of use of the vehicle. For example, the longer the usage time of the vehicle per day, The amount of decrease in the first voltage V1 with respect to the elapsed days increases. About the change of the 1st voltage V1 with respect to this elapsed days, if the relationship between the elapsed days memorize | stored in the log and the 1st voltage V1 is plotted, it can represent as an approximated curve which passes through each plotted point.

  Then, the first threshold value Th1 is set so that reverse polarization is performed in a state before the first voltage V1 becomes a negative voltage and the HUD cannot be used. Here, the first threshold value Th1 is set so that the remaining usable period is equal to or longer than the predetermined setting period, not when the HUD is assumed to be unusable, that is, when the first voltage V1 becomes zero. Yes. For example, the setting period is set such that the “remaining usable days” is a predetermined number of days so that reverse polarization is performed at the timing when the variable focus mirror 1 can be used for the remaining days. Then, the value of the first voltage V1 that is assumed that the “remaining usable days” becomes a predetermined number of days is set to the first threshold Th1.

  Accordingly, the first voltage V1 is monitored while the variable focus mirror 1 is being driven and the log is stored, the first threshold Th1 is set based on the log, and the first voltage V1 being monitored is set to the first threshold Th1. When it is further lowered, reverse polarization is performed.

  On the other hand, during reverse polarization, as shown in FIG. 10, the control unit 30 causes the first switch 41 to be connected to the ground potential side and the second switch 42 to be connected to the control unit 30 side. Then, the control unit 30 outputs a predetermined DC voltage as shown in FIG. 3B. Thus, a DC voltage is applied to the intermediate electrode 12c through the second switch 42, and the lower electrode 12a and the upper electrode 12e are set to the ground potential through the first switch 41.

  Regarding reverse polarization, at any timing, the orientation direction of the crystal grains of the piezoelectric material in the first piezoelectric film 12b and the second piezoelectric film 12d is directed away from the c-axis and returned to the orientation direction at the time of manufacture. it can. However, if reverse polarization is performed when using the HUD, the HUD image will be affected. For this reason, the reverse polarization is not performed immediately after the first voltage V1 falls below the first threshold Th1, but the reverse polarization is performed when the power switch is turned off.

  Furthermore, the reverse polarization can be more strongly returned to the state at the time of manufacture as the time is increased or the higher electric field is applied. However, it is not preferable to perform reverse polarization for a long period of time or to perform reverse polarization at a high frequency because the life of the piezoelectric material is reduced.

  For this reason, a second threshold value Th2 larger than the first threshold value Th1 is set. Then, after performing reverse polarization for a predetermined time, the variable focus mirror 1 is driven, and the first voltage V1 at that time is monitored. If the first voltage V1 does not exceed the second threshold Th2, reverse polarization is performed again. If so, reverse polarization is terminated. Thus, by providing the second threshold Th2, the time for performing reverse polarization can be limited until the first voltage V1 exceeds the second threshold Th2. Further, since the reverse polarization is performed to the extent that the first voltage V1 exceeds the second threshold Th2, the variable focus mirror 1 can be driven until the first voltage V1 drops below the first threshold Th1 again. Then, by setting the second threshold Th2 to a relatively large value, it is possible to prevent reverse polarization from being performed at a high frequency, and it is possible to suppress a decrease in the lifetime of the piezoelectric material.

  Specifically, the second threshold Th2 can be set based on the log of the first voltage V1. As described above, based on the log of the first voltage V1, an approximate curve when the first voltage V1 changes according to the elapsed days is obtained. As described above, when the first voltage V1 decreases to the first threshold value Th1, the reverse polarization is performed to reach the second threshold value Th2. This is a time when the first voltage V1 drops to the first threshold Th1 again. Therefore, as shown in FIG. 9, the period during which the first voltage V1 decreases from the second threshold Th2 to the first threshold Th1 is a period during which the variable focus mirror 1 can be used after reverse polarization. Become. For this reason, a period during which the first voltage V1 is assumed to decrease from the second threshold Th2 to the first threshold Th1 is obtained, and the use request period of the varifocal mirror 1 that can make this period equal to or less than a predetermined frequency of reverse polarization, Here, the second threshold Th2 is set so as to be equal to or greater than “the number of days that the user wants to use”. For example, when the frequency of reverse polarization is 30 days or more, the value of the second threshold Th2 is set so that the variable focus mirror 1 can be driven without performing reverse polarization for 30 days. Regarding the use request period at this time, the number of repetitions of the reverse polarization and the drive in which the normal variable focus mirror 1 is driven after the reverse polarization becomes the endurance limit of the variable focus mirror 1 in the expected service life of the vehicle. It is set to be less. In this way, the second threshold Th2 can be set based on the log of the first voltage V1.

  Further, when reverse polarization is performed by setting the second threshold Th2, the first voltage V1 is monitored by driving the variable focus mirror 1 after performing reverse polarization for a predetermined time, and the first voltage V1 is the second voltage V1. The reverse polarization is repeated until the threshold Th2 is exceeded. The remaining time of reverse polarization at this time can also be estimated by storing a log of the first voltage V1 at the time of monitoring, and the time for performing reverse polarization can be set based on the estimated remaining time.

  Specifically, as shown in FIG. 11, an approximate curve is calculated based on a log of the first voltage V1 after performing reverse polarization for a predetermined time a plurality of times, for example, a few times. Based on the approximate curve, the remaining time required until the first voltage V1 reaches the second threshold Th2 is estimated, and the reverse polarization is continued until the remaining time elapses. Thereafter, the reverse polarization is terminated and the first voltage V1 is monitored again. By applying such a method, the first voltage V1 can accurately reach the second threshold Th2 while reducing the number of repetitions of reverse polarization.

  As described above, when the state a shown in FIG. 12A, that is, the first voltage V <b> 1 falls below the first threshold Th <b> 1, reverse polarization is performed on the variable focus mirror 1. Further, due to the reverse polarization, even if the state b shown in FIG. 12B, that is, the first voltage V1 becomes larger than the first threshold value Th1, the reverse polarization is repeated until the second threshold value Th2 is reached. Then, when the state c shown in FIG. 12C and the first voltage V1 reach the second threshold Th2 due to the reverse polarization, the reverse polarization ends. Thereby, even if the variable focus mirror 1 is driven, the first voltage V1 does not become a negative voltage, and the curvature of the variable focus mirror 1 can be accurately set to the first curvature R1.

  The above-mentioned “remaining usable days” and “desired days” are preferably set in accordance with the use state of the vehicle and the vehicle type. For example, it is considered that a commercial vehicle or the like is used for a long time and the HUD is used for a long time. In particular, in the case of a vehicle such as a sightseeing bus where the driver drives in shifts, the continuous use time of the HUD can be long. Moreover, the use frequency of a vehicle changes with users, and the use time of the vehicle per day differs. For this reason, it is possible to set more days by setting “remaining usable days” and “number of days to be used” according to the use state of the vehicle and the time of the vehicle type.

  FIG. 13 is a state transition diagram showing the operation state of the control unit 30 together with the flow of the above operation. As shown in this figure, the control unit 30 executes a routine for determining whether to drive the variable focus mirror 1 or reverse polarization, a drive routine for the variable focus mirror 1, and a reverse polarization routine.

  First, as a determination routine, in step S100, it is determined whether or not the power switch is off. If an affirmative determination is made, the process proceeds to step S110 to stop the display by HUD. Then, the process proceeds to step S120, and it is determined whether or not a condition for starting execution of the reverse polarization routine is satisfied. Here, the execution start condition for the reverse polarization routine is determined to satisfy the start condition when predetermined information is stored in the drive routine of the variable focus mirror 1 described later. On the other hand, if a negative determination is made in step S100, the routine proceeds to the driving routine for the variable focus mirror 1.

  In the drive routine of the variable focus mirror 1, basically, the variable focus mirror 1 is driven according to the display on the HUD, and the variable focus mirror 1 performs the above-described operation. At this time, as shown in the upper right in the drawing of the driving routine, the control unit 30 determines the first voltage V1 and the second voltage V2 based on the first voltage V1 and the second voltage V2 set at the time of the previous driving, for example. The varifocal mirror 1 is driven by generating an alternating rectangular wave signal, that is, a voltage. Then, the first voltage V1 and the second voltage V1 are set by feedback control based on the detection result of the curvature of the sensor circuit 20 during driving of the variable focus mirror 1, and the drive of the variable focus mirror 1 is continued. Further, the change in the first voltage V1 at this time is stored as a log in a memory such as a RAM, and the first threshold Th1 and the second threshold Th2 are set.

  In such a driving routine, as shown in step S200, the control unit 30 determines whether or not the first voltage V1 is lower than the first threshold value Th1, as the necessity determination of the reverse polarization. If an affirmative determination is made in step S200, since reverse polarization is necessary, the process proceeds to step S210 and information to that effect is stored. This information is information indicating that the reverse polarization routine execution start condition is satisfied. If this information is stored, it is determined in step S120 in the determination routine that the reverse polarization routine execution start condition is satisfied. Is done. If a negative determination is made in step S200, the process proceeds to step S220 to reset information indicating that the reverse polarization routine execution start condition is satisfied.

  The reverse polarization routine is determined when step S120 in the determination routine is affirmative, that is, when the first voltage V1 falls below the first threshold Th1 in the drive routine and the power switch is determined to be off in the determination routine. Executed.

  In the reverse polarization routine, reverse polarization is performed on the variable focus mirror 1 based on the log of the first voltage V1 stored in the drive routine. Then, as shown in step S300, it is determined whether or not the first voltage V1 when the variable focus mirror 1 is driven after the reverse polarization has reached the second threshold Th2, and if so, the reverse polarization is completed. Is going.

  Specifically, as shown in the upper right of the diagram of the reverse polarization routine, the control unit 30 performs reverse polarization on the variable focus mirror 1 by generating a DC voltage. When the reverse polarization for a predetermined time is finished, the varifocal mirror 1 is driven, and the first voltage V1 is monitored and stored in a log based on the detection result of the curvature in the sensor circuit 20. Further, after performing the reverse polarization for the predetermined time and the monitoring of the first voltage V1 a plurality of times, an approximate curve is calculated based on the first voltage V1 stored in the log, and the first curve is calculated based on the approximate curve. The remaining time of reverse polarization until the 1 voltage V1 reaches the second threshold Th2 is calculated. Then, after continuing the reverse polarization for the remaining time, the variable focus mirror 1 is driven again, and it is confirmed whether or not the first time V1 has reached the second threshold Th2, and it is determined that the reverse polarization is completed. Is done.

  It is also conceivable that the user starts using the vehicle during reverse polarization. In that case, it is necessary to perform display by HUD in preference to reverse polarization. Therefore, as shown in step S310, it is determined whether or not the power switch is turned off every predetermined control period as interrupt monitoring. Then, the reverse polarization routine is repeated only when the power switch is off, and when the power switch is turned on, the reverse polarization routine is stopped and exited, and the process shifts to the drive routine of the variable focus mirror 1. It has become.

  As described above, in the variable focus mirror system according to the present embodiment, reverse polarization of the variable focus mirror 1 is executed when the power switch is off. For this reason, it is possible to perform reverse polarization without affecting the HUD image. Further, when the first voltage V1 is lower than the first threshold value Th1, reverse polarization is performed, and after the reverse polarization, the first voltage V1 reaches the second threshold value Th2. For this reason, it becomes possible not to perform reverse polarization with high frequency, and it also becomes possible to suppress the lifetime reduction of a piezoelectric material.

  Further, the first voltage V1 is stored in a log while the variable focus mirror 1 is being driven, and the first threshold Th1 is set based on the log. As a result, the first threshold value can be set according to the vehicle usage status, vehicle type, and the like.

  Further, after performing reverse polarization for a predetermined time, the variable focus mirror 1 is driven to monitor the first voltage V1 and store the log, and to estimate the remaining time of reverse polarization based on the log. . Thereby, the first voltage V1 can accurately reach the second threshold Th2 while reducing the number of repetitions of reverse polarization.

(Other embodiments)
The present invention is not limited to the embodiment described above, and can be appropriately changed within the scope described in the claims.

  For example, in the above embodiment, the first voltage V1 is stored in a log while the variable focus mirror 1 is being driven, and the first threshold Th1 is set based on the log. However, this is given as an example when the first threshold value Th1 is set according to the use state of the vehicle, the vehicle type, etc., and the first threshold value Th1 may be set to a predetermined constant value.

  Similarly, in the above embodiment, the second threshold Th2 is also set based on the log of the first voltage V1. However, this is also given as an example in the case where the second threshold Th2 is set in accordance with the use status of the vehicle, the vehicle type, etc., and the second threshold Th2 may be set to a predetermined constant value.

  In the above-described embodiment, the lower electrode 12a, the first piezoelectric film 12b, the intermediate electrode 12c, the second piezoelectric film 12d, and the upper electrode 12e are stacked as the mirror part 12 configured by arranging electrodes on both surfaces of the piezoelectric film. An example of such a structure is given. That is, the lower electrode 12a and the intermediate electrode 12c are disposed on both surfaces of the first piezoelectric film 12b, and the intermediate electrode 12c and the upper electrode 12e are disposed on both surfaces of the second piezoelectric film 12d, so that the electrodes are disposed on both surfaces of the piezoelectric film. The structure in which the two are arranged is a two-layer structure. However, this is only an example of the structure of the mirror section 12. For example, a structure having only one structure in which electrodes are arranged on both sides of the piezoelectric film may be used.

DESCRIPTION OF SYMBOLS 1 Variable focus mirror 12 Mirror part 12a Lower electrode 12b Piezoelectric film 12c Intermediate electrode 12d Piezoelectric film 12e Upper electrode 13 Curvature sensor 30 Control part 40 Switch part

Claims (7)

A base (11) constituting the membrane; a piezoelectric film (12b, 12d) formed on the base; and electrodes (12a, 12c, 12e) disposed on both sides of the piezoelectric film; A variable focus mirror (1) comprising a mirror section (12) for reflecting
A control unit (30) for bending the mirror unit into a spherical shape with a predetermined curvature by applying a predetermined voltage between electrodes disposed on both sides of the piezoelectric film;
A variable focus mirror system for controlling the variable focus mirror in a head-up display in a vehicle having a curvature detection unit (13, 20) for measuring the curvature of the bent mirror unit,
The controller is
When driving the varifocal mirror, a rectangular-wave drive voltage that alternately switches between a first voltage (V1) and a second voltage (V2) that is higher than the first voltage is applied between the electrodes, and the first voltage is applied. Is detected by the curvature detection unit while bending the mirror part with the second curvature (R2) by applying the second voltage and bending the mirror part with the first curvature (R1). Based on the result, feedback control is performed to adjust the first voltage so that the curvature of the mirror portion becomes the first curvature and adjust the second voltage so as to become the second curvature,
Further, when the first voltage falls below the first threshold (Th1), an electric field opposite to that during driving of the variable focus mirror is generated between the electrodes as reverse polarization when the vehicle power switch is turned off. A varifocal mirror system that adjusts the orientation direction of the piezoelectric film by applying a DC voltage until the first voltage reaches a second threshold (Th2) that is higher than the first threshold.   The control unit stores the first voltage when the variable focus mirror is driven as a log, obtains an approximate curve of the change in the first voltage based on the log, and determines the variable based on the approximate curve. The variable focus mirror system according to claim 1, wherein the first threshold value is set so that a remaining usable period during which the focus mirror can be driven is equal to or longer than a set period.   The varifocal mirror system according to claim 2, wherein the control unit sets the setting period based on the remaining usable period determined according to a use situation or a vehicle type of the vehicle.   The control unit obtains a period during which it is assumed that the first voltage that has reached the second threshold value due to the reverse polarization decreases to the first threshold value again by driving the variable focus mirror, based on the approximate curve. 4. The variable focus mirror system according to claim 2, wherein the second threshold value is set so that the period is equal to or longer than a use request period of the variable focus mirror after the reverse polarization.   The varifocal mirror system according to claim 4, wherein the control unit sets the use request period based on the remaining usable period obtained according to a use state or a vehicle type of the vehicle.   The control unit is configured such that the usage request period is less than the number of repetitions in which the number of repetitions of the reverse polarization and the driving of the variable focus mirror after the reverse polarization becomes a durability limit of the variable focus mirror in the expected service life of the vehicle. The variable focus mirror system according to claim 5, wherein the variable focus mirror system is set to be   The control unit drives the variable focus mirror after performing the reverse polarization for a predetermined time, and monitors the first voltage based on a detection result of the curvature detection unit and drives the first voltage during the driving. Storing the log of a plurality of times, estimating the remaining time of the reverse polarization until the first voltage reaches the second threshold based on the plurality of logs, and until the remaining time elapses The variable focus mirror system according to any one of claims 1 to 4, wherein the reverse polarization is continued.

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