JP6972768B2 - Varifocal mirror system - Google Patents

Description

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

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

For example, examples of the variable focus mirror include those shown in Patent Document 1. This variable focus mirror is formed on a silicon oxide film formed on a device layer, and is configured to have a piezoelectric film and an electrode for applying an electric field to the piezoelectric film. More specifically, the structure is such that 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, the device layer does not shrink, so that 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 becomes convex downward. Make a transformation. 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, the specified curvature variable range can be obtained within the specified voltage range. Design to. Specifically, with the intermediate electrode set to the ground potential, a rectangular wavy 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. 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 in the HUD, the variable focus mirror is driven by applying a rectangular wavy drive voltage of 72 Hz, and the deformation of 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 stage of using 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 period of time, the curvature with respect to the same voltage becomes large. For this reason, the characteristics of the variable focus mirror are out of the specification range, and there arises a problem that the distant display side is out of focus. It is considered that this is because the orientation direction of the crystal grains of the piezoelectric film rotates in the c-axis direction by driving for a long period of time.

A method for undoing the change in the orientation direction due to such long-term driving is also considered. Specifically, (1) a method of repeatedly using and recovering by generating an electric field in the opposite direction to a normal drive waveform with a unique drive waveform during use, and (2) reverse of a high voltage when the power is turned on, etc. There is a method of aligning the polarization with an electric field, and (3) a method of depolarizing with a DC voltage at any time when driving the variable focus mirror.

Japanese Unexamined Patent Publication No. 2017-032714

However, the method (1) cannot be adopted because the image is distorted when the HUD is used. Further, in the methods (2) and (3), since reverse polarization is frequently performed, stress is frequently generated at the grain boundaries of the piezoelectric film, causing cracks in the vicinity of the grain boundaries, and as a result, There are adverse effects such as accelerating the deterioration of characteristics such as a decrease in the piezoelectric constant and accelerating the dielectric breakdown life.

In view of the above points, the present invention does not affect the image of the HUD in the variable focus mirror system applied to the HUD, and reduces the frequency of reverse polarization so that the life of the piezoelectric film can be extended. The purpose is to do.

In order to achieve the above object, the variable focus mirror system according to claim 1 is formed on a base portion (11) constituting the membrane and the base portion, and is formed on the piezoelectric membranes (12b, 12d) and both sides of the piezoelectric membrane. A variable focus mirror (1) having an arranged electrodes (12a, 12c, 12e) and a mirror portion (12) for reflecting a light beam, and a predetermined space between the electrodes arranged on both sides of the piezoelectric film. It has a control unit (30) that bends the mirror portion into a spherical shape having a predetermined curvature by applying the voltage of the above, and a curvature detection unit (13, 20) that measures the curvature of the bent mirror portion. It also controls the variable focus mirror on the head-up display in the vehicle.

In such a variable focus mirror system, the control unit alternately switches between a first voltage (V1) and a second voltage (V2) larger than the first voltage when the variable focus mirror is driven. Is applied between the electrodes, and the mirror portion is bent at the first curvature (R1) by applying the first voltage, and the mirror portion is bent at the second curvature (R2) by applying the second voltage. 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 adjust the second voltage so that the curvature becomes the second curvature. Further, when the first voltage applied between the electrodes to bend the mirror portion with the first curvature is lower than the first threshold value (Th1), it is variable between the electrodes as reverse polarization when the power switch of the vehicle is turned off. By applying a DC voltage that generates an electric field opposite to that when the focal mirror is driven, the orientation direction of the piezoelectric film is adjusted until the first voltage reaches the second threshold (Th2) higher than the first threshold.

In this way, the depolarization of the variable focus mirror is performed when the power switch is off. Therefore, it is possible to perform reverse polarization so as not to affect the image of the HUD. Further, the depolarization is performed when the first voltage is lower than the first threshold value, and the first voltage reaches the second threshold value after the depolarization. Therefore, it is possible to prevent the reverse polarization from being performed frequently, and it is also possible to suppress the decrease in the life of the piezoelectric material.

The reference numerals in parentheses of each of the above means indicate an example of the correspondence with the specific means described in the embodiment described later.

It is sectional drawing of the variable focus mirror provided in the variable focus mirror system which concerns on 1st Embodiment. It is the whole block diagram of the variable focus mirror system at the time of driving. It is a waveform diagram of the voltage applied at the time of driving a variable focus mirror. It is a waveform diagram of the voltage applied at the time of the reverse polarization of a variable focus mirror. It is a figure which showed the direction of a crystal of a piezoelectric material, and how to call a surface. It is sectional drawing which showed the state when the variable focus mirror was bent. It is a figure which showed the relationship between the drive voltage and the curvature of a variable focus mirror. It is a figure which showed the orientation direction at the time of performing polarization at the time of manufacturing. It is a figure which showed the orientation direction which changed by long-term use. It is a figure which showed the relationship between the drive voltage which changed by use, and the curvature of a variable focus mirror. It is a figure which showed the relationship between the drive voltage which changed by use, and the curvature of a variable focus mirror. It is a figure which showed the approximate curve based on the log of the 1st voltage, the 1st threshold value and the 2nd threshold value. It is an overall block diagram of the variable focus mirror system at the time of depolarization. It is a figure which showed the approximate curve and the 2nd threshold value based on the log of the 1st voltage at the time of reverse polarization. It is a figure which showed the relationship between the driving voltage and the curvature of a variable focus mirror in the state a in which the 1st voltage is lower than the 1st threshold value. It is a figure which showed the relationship between the driving voltage and the curvature of a variable focus mirror in the state b in which the 1st voltage is larger than the 1st threshold value and is smaller than the 2nd threshold value. It is a figure which showed the relationship between the driving voltage and the curvature of a variable focal point mirror in the state c in which the 1st voltage becomes larger than the 2nd threshold value. It is a state transition diagram which showed the operation state of a control part together with the operation flow of a variable focus mirror system.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In each of the following embodiments, the parts that are the same or equal 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 the present embodiment is used as an LSM in a vehicle HUD to control a variable focus mirror. Hereinafter, the variable focus mirror system according to the present embodiment will be described with reference to FIGS. 1 to 13. The variable focus mirror 1 shown in FIG. 1 is a part constituting the mirror portion in the LSM, and is actually used to drive the support portion of the variable focus mirror 1 and the variable focus mirror 1 around the variable focus mirror 1. The LSM is configured by being provided with various drive units and the like.

As shown in FIG. 1, the variable focus mirror 1 is formed by using, for example, MEMS (Micro-Electro-Mechanical Systems) technology, and has a curvature in addition to a piezoelectric device constituting the mirror portion 12 on the base 11 in the substrate 10. It is configured to include a sensor 13 and the like.

The substrate 10 is composed of 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 bonded in this order.

The device layer 10a is made of, for example, Si or the like. The insulating film 10b is composed of, for example, an embedded oxide layer (hereinafter referred to as a BOX (abbreviation of Buried Oxide) layer) made of _{SiO 2.} The handle layer 10c is made 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 portion constituting the LSM including the variable focus mirror 1.

Specifically, the variable focus mirror 1 reflects the light beam irradiated to the LSM, and has, for example, a configuration in which the above-mentioned base portion 11, mirror portion 12, and curvature sensor 13 are provided in the central portion of the substrate 10. Has been done.

The base portion 11 constitutes a thin-walled membrane, and is a portion formed by circularly patterning the device layer 10a. In this embodiment, only the portion of the device layer 10a that becomes the base 11 is shown. 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 portion 12 are laminated in order on the upper surface of the base portion 11. The surface of the mirror portion 12 on the side opposite to the insulating film 15 is a reflective surface, and the light beam can be reflected by this reflective surface. Further, the curvature sensor 13 is arranged via the insulating film 15 on the upper surface of the base portion 11 at a position different from that of the mirror portion 12, specifically, at a position outside the mirror portion 12.

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

The base 11 has a flat upper surface, for example, the upper surface has a circular shape. A mirror portion 12 is laminated on the upper surface of the inner peripheral portion 11a of the base portion 11 via an insulating film 15. Then, the inner peripheral portion 11a of the base portion 11 is bent together with the reflecting surface when the mirror portion 12 is bent. By thinning the base 11, the sensitivity of the variable focus mirror 1 is increased.

On the other hand, the mirror portion 12 is not formed on the upper surface of the outer peripheral portion 11b located on the outer edge side of the inner peripheral portion 11a or on the outer side thereof, and the curvature sensor 13 is formed in that portion via the insulating film 15. ing. When the inner peripheral portion 11a is deformed with the deformation of the mirror portion 12, the outer edge side and the outer peripheral portion 11b of the inner peripheral portion 11a are also deformed. Based on this deformation, the curvature sensor 13 can detect the curvature.

The mirror portion 12 has a circular upper surface shape, and bends the reflecting surface into a spherical shape to change the focal position of the reflected light by the reflecting surface. The mirror portion 12 is composed of 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 laminated in this order.

The lower electrode 12a and the intermediate electrode 12c are formed of a single-layer film such as platinum (Pt) or strontium ruthenium oxide (hereinafter referred to as SRO) or a plurality of laminated films, and are formed as 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) or the like. The upper electrode 12e is made of, for example, Pt, SRO, or the like, and is a thin-film electrode. The surface of the upper electrode 12e itself may be a reflective surface, but a structure may be provided in which a film for forming the reflective surface is separately provided on the surface of the upper electrode 12e.

Although the lower electrode 12a, the intermediate electrode 12c, and the upper electrode 12e are not shown in FIG. 1, they are connected to the control device of the variable focus mirror 1 via wiring. By this control device, a voltage is applied to the lower electrode 12a and the upper electrode 12e, 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. As a result, 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 reflecting 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 composed of a piezo resistor connected in a Wheatstone bridge shape or the like. A voltage for sensing is applied to one connection point of the Wheatstone bridge, the opposite connection points are set as the ground potential, and the intermediate potential that is the potential of the remaining two connection points is output as a detection signal to make the potential difference between the two intermediate potentials. The curvature can be detected based on this.

When the curvature sensor 13 is configured by a piezo resistor, each piezo resistor may be configured by a material constituting a general resistor. In that case, it is preferable to set the formation direction and formation position of each piezo resistance in consideration of the relationship between the resistance coefficient in the piezo resistance and the crystallinity of the constituent material of the resistor.

As described above, the variable focus mirror 1 is configured. 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 is configured to include 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 inputs 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 composed of a well-known microcomputer equipped with a CPU, ROM, RAM, I / O, etc., and performs predetermined processing according to a program stored in the ROM or the like. Specifically, the control unit 30 controls the switch state in the switch unit 40, drives the variable focus mirror 1 through the switch unit 40 by applying a desired drive voltage, or drives 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 variable focus mirror 1, the control unit 30 alternately repeats the first voltage V1 and the second voltage V2 as the drive voltage as the lower electrode 12a and the lower electrode 12a. It is applied to the upper electrode 12e. As a result, 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 direction opposite to that at the time of 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 in the direction away from the c-axis side. The magnitude of the DC voltage applied at the time of depolarization is arbitrary, but the same voltage as the voltage applied at the time of polarization at the time of manufacturing is applied in the opposite direction. The voltage at the time of polarization is set to be as high as possible while being lower than the dielectric breakdown voltage with respect to the first piezoelectric film 12b and the second piezoelectric film 12d, so that a high polarization action is generated. Therefore, even at the time of reverse polarization, by setting the same voltage as at the time of polarization, it is possible to suppress the dielectric breakdown of the first piezoelectric film 12b and the second piezoelectric film 12d and to cause a high reverse polarization action.

For example, the control unit 30 monitors the first voltage V1 having the first curvature R1 and the second voltage V2 having 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 driving the variable focus mirror 1. Further, a voltage for performing reverse polarization is applied to the variable focus mirror 1 at a timing when the display by the HUD is not performed. An operation signal of 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), or a vehicle start switch in the case of an electric vehicle is input to the control unit 30. .. Based on this operation signal, the control unit 30 can detect that the timing is not displayed by the HUD.

Any circuit may be applied to the circuit of the part of the control unit 30 that outputs the drive voltage and the voltage at the time of depolarization, but in consideration of cost, it is configured by, for example, a unipolar circuit. ing.

The switch unit 40 has a configuration including 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. Further, 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. Subsequently, 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 has a substantially flat mirror portion 12 at the initial stage of manufacture. Here, the directions and planes of the crystals of the piezoelectric materials constituting the first piezoelectric film 12b and the second piezoelectric film 12d provided in the variable focus mirror 1 are shown as 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 in the <100> direction, a b-axis in the <010> direction, and a <001> direction. It has a c-axis, and has a (001) plane whose normal direction is the <001> direction and a (100) plane whose normal direction is the <100> direction. Since the piezoelectric film is a dielectric, the crystal grains are oriented, and when an electric field is applied, they are oriented in the c-axis direction as shown by the arrows in FIG. 4, so that they expand in the c-axis direction and a-axis. Shrink in the direction. At this time, since the device layer 10a does not shrink, the variable focus mirror 1 is deformed so as to be convex downward as shown in FIG. 5, assuming that 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 portion 12 of the variable focus mirror 1 deformed in this way is spherical, the surface can be used as a concave mirror.

Then, the resistance value of the piezo resistance provided in the curvature sensor 13 changes due to the deformation of the device layer 10a accompanying the deformation of the mirror portion 12 at this time, so that the detection signal of the curvature sensor 13 changes. Therefore, it is possible to detect the curvature of the variable focus mirror 1 based on the detection signal of the curvature sensor 13.

Regarding the variable focus 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 to perform polarization treatment, and after the polarization treatment, the specified curvature variable range is set within the specified voltage range. Design to be obtained. Specifically, with the intermediate electrode 12c set to the ground potential, a rectangular wave-shaped drive 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 variable focus mirror 1 has the first curvature R1, and the second voltage V2 is adjusted to the voltage when the variable focus mirror 1 has the second curvature R2 larger than the first curvature R1. Will be done. For example, in the operation of the HUD, the variable focus mirror 1 is driven by applying a rectangular wavy 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 variable focus mirror 1 have a relationship shown in FIG. 6, for example. As shown by the solid line in FIG. 6, at the initial use of the variable focus mirror 1, the first voltage V1 having the first curvature R1 and the second voltage V2 having the second curvature R2 are both 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 becomes large as shown by the broken line in FIG. Therefore, the characteristic of the variable focus mirror 1 is out of the specification range, and there arises a problem that the distant display side is out of focus. It is considered that this is because the orientation direction of the crystal grains deviated from the c-axis direction as shown in FIG. 7A during polarization is rotated in the c-axis direction as shown in FIG. 7B by long-term driving. ..

Therefore, in the variable focus mirror system of the present embodiment, the control unit 30 drives the variable focus mirror 1, and based on the detection result of the curvature in the sensor circuit 20, the changing first voltage V1 is set to RAM or the like. It is stored as a log in the memory, and reverse polarization is performed as necessary.

Specifically, as shown in FIG. 2, in the control unit 30, when driving the variable focus mirror 1, the first switch 41 is connected to the control unit 30 side, and the second switch 42 is on the ground potential side. Make it connected. Then, the control unit 30 outputs a rectangular wavy drive 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, a driving 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 42.

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 or 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. Then, 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 drive 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 the change in the orientation direction of the crystal grains of the piezoelectric material, as shown in FIG. 8A, and tentatively causes reverse polarization. If not done, the first voltage V1 can reach a negative voltage, as shown in FIG. 8B. It is conceivable to set the drive voltage output by the control unit 30 to a negative voltage, but in consideration of cost, it is preferable to use a unipolar circuit or the like, and it is not possible to generate a negative voltage.

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

Further, in the case of the present embodiment, the first threshold value Th1 is set based on the log of the first voltage V1 so that the value is set 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 period of 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, the circle mark is 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. The amount of decrease in the first voltage V1 with respect to the elapsed days increases. The change in the first voltage V1 with respect to the elapsed days can be expressed as an approximate curve passing through each of the plotted points by plotting the relationship between the elapsed days stored in the log and the first voltage V1.

Then, the first threshold value Th1 is set so that the reverse polarization is performed in the 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 set period, not when it is assumed that the HUD cannot be used, that is, when the first voltage V1 becomes 0. There is. For example, it is set as a set period that the "usable remaining days" becomes a predetermined number of days so that the depolarization is performed at the timing when the variable focus mirror 1 can be used for the remaining several days. Then, the value of the first voltage V1 on which the "usable remaining days" is assumed to be a predetermined number of days is set to the first threshold value Th1.

Therefore, while the variable focus mirror 1 is being driven, the first voltage V1 is monitored and the log is stored, the first threshold value Th1 is set based on the log, and the first voltage V1 being monitored is the first threshold value Th1. When it becomes lower, the reverse polarization is performed.

On the other hand, in the case of reverse polarization, as shown in FIG. 10, in the control unit 30, the first switch 41 is connected to the ground potential side and the second switch 42 is connected to the control unit 30 side. Then, the control unit 30 outputs a predetermined DC voltage as shown in FIG. 3B. As a result, 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 the reverse polarization, regardless of the timing, the orientation direction of the crystal grains of the piezoelectric material in the first piezoelectric film 12b and the second piezoelectric film 12d may be directed away from the c-axis and returned to the orientation direction at the time of manufacture. can. However, if depolarization occurs when using the HUD, it will affect the image of the HUD. Therefore, the first voltage V1 does not drop below the first threshold value Th1 and immediately reverse polarization, but the reverse polarization is performed when the power switch is turned off.

Further, with respect to the reverse polarization, the longer the time is taken or the higher the electric field is applied, the stronger the orientation direction of the piezoelectric material can be returned to the state at the time of manufacture. However, it is not preferable to perform the reverse polarization for a long time or to perform the reverse polarization at a high frequency because the life of the piezoelectric material is shortened.

Therefore, a second threshold value Th2 larger than the first threshold value Th1 is set. Then, after performing the depolarization for a predetermined time, the variable focus mirror 1 is driven, the first voltage V1 at that time is monitored, and if the first voltage V1 does not exceed the second threshold value Th2, the depolarization is performed again. If it exceeds, the reverse polarization is terminated. By providing the second threshold value Th2 in this way, the time for performing the reverse polarization can be limited to the time when the first voltage V1 exceeds the second threshold value Th2. Further, since the depolarization is performed to the extent that the first voltage V1 exceeds the second threshold value Th2, the variable focus mirror 1 can be driven until the first voltage V1 falls below the first threshold value Th1 again. By setting the second threshold value Th2 to a certain large value, it is possible to prevent the reverse polarization from being performed frequently, and it is also possible to suppress the decrease in the life of the piezoelectric material.

Specifically, the second threshold value 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 number of elapsed days is obtained. Then, as described above, when the first voltage V1 drops to the first threshold value Th1 and the reverse polarization is performed so as to reach the second threshold value Th2, the next depolarization is performed on the variable focus mirror 1. Is used 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 is expected to decrease from the second threshold value Th2 to the first threshold value Th1 is the period during which the variable focus mirror 1 can be used after the depolarization. Become. Therefore, a period during which the first voltage V1 is expected to decrease from the second threshold value Th2 to the first threshold value Th1 is obtained, and the period required for using the variable focus mirror 1 that can set this period to be equal to or less than the predetermined frequency of reverse polarization. Here, the second threshold value Th2 is set so as to be equal to or greater than the “number of days desired to be used”. For example, when the frequency of reverse polarization is set to be 30 days or more, the value of the second threshold value Th2 is set so that the variable focus mirror 1 can be driven without performing reverse polarization for 30 days. Regarding the usage 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 durability limit of the variable focus mirror 1 in the expected years of use of the vehicle. It is set to be less than. In this way, the second threshold value Th2 can be set based on the log of the first voltage V1.

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

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

As described above, when the state a shown in FIG. 12A, that is, the first voltage V1 is lower than the first threshold value Th1, the variable focus mirror 1 is depolarized. 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 value Th2 due to the reverse polarization, the reverse polarization ends. As a result, 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.

It is preferable that the above-mentioned "remaining usable days" and "desired days to be used" are set according to the usage status of the vehicle, the vehicle type, and the like. For example, it is considered that commercial vehicles and the like are used for a long time in a day, and the HUD is used for a long time. In particular, in the case of a vehicle such as a sightseeing bus in which the driver takes turns driving, the continuous use time of the HUD may be particularly long. In addition, the frequency of use of the vehicle differs depending on the user, and the usage time of the vehicle per day also differs. Therefore, by setting the "remaining number of days that can be used" and the "number of days that the vehicle wants to use" according to the usage status of the vehicle and the time of the vehicle type, it is possible to set a more generous number of days.

FIG. 13 is a state transition diagram showing the operation state of the control unit 30 together with the operation flow. As shown in this figure, the control unit 30 executes a determination routine for driving the variable focus mirror 1 or performing depolarization, a driving routine for the variable focus mirror 1, and a depolarization routine.

First, as a determination routine, in step S100, it is determined whether or not the power switch is off, and if an affirmative determination is made, the process proceeds to step S110 to stop the display by the HUD. Then, the process proceeds to step S120, and it is determined whether or not the execution start condition of the reverse polarization routine is satisfied. Here, regarding the execution start condition of the reverse polarization routine, it is determined that the start condition is satisfied 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 process shifts to the drive routine of 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-mentioned operation. At this time, as shown in the upper right of the drawing of the drive routine, in the control unit 30, for example, the first voltage V1 and the second voltage V2 are set based on the first voltage V1 and the second voltage V2 set at the time of the previous drive. The variable focus mirror 1 is driven by generating a rectangular wavy signal, that is, a voltage, which is repeated alternately. Then, based on the detection result of the curvature in the sensor circuit 20 during driving of the variable focus mirror 1, the first voltage V1 and the second voltage V1 are set by feedback control, and the driving of the variable focus mirror 1 is continued. Further, the change of the first voltage V1 at this time is stored as a log in a memory such as RAM, and the first threshold value Th1 and the second threshold value Th2 are set.

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

The reverse polarization routine is determined affirmatively in step S120 in the determination routine, that is, when the first voltage V1 is lower than the first threshold Th1 in the drive routine and the power switch is determined to be off in the determination routine. Will be executed.

In the depolarization routine, depolarization is performed with respect to 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 reaches the second threshold value Th2, and if it reaches, the reverse polarization is completed. Is going.

Specifically, as shown in the upper right of the figure of the reverse polarization routine, the control unit 30 performs reverse polarization with respect to the variable focus mirror 1 by generating a DC voltage. Then, when the depolarization for a predetermined time is completed, the variable focus mirror 1 is driven, and the first voltage V1 is monitored and stored in the log based on the detection result of the curvature in the sensor circuit 20. Further, after repeating such reverse polarization for a predetermined time and 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 approximate 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 depolarization 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 value Th2, and it is determined that the depolarization is completed. Will be done.

It is also conceivable that the user will start using the vehicle during the reverse polarization. In that case, it is necessary to display by HUD in preference to reverse polarization. Therefore, as shown in step S310, as interrupt monitoring, it is determined whether or not the power switch is turned off at predetermined control cycles. Then, the depolarization routine is repeated only when the power switch is off, and when the power switch is switched on, the processing of the depolarization routine is stopped and exits, 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, the depolarization of the variable focus mirror 1 is executed when the power switch is off. Therefore, it is possible to perform reverse polarization so as not to affect the image of the HUD. Further, when the first voltage V1 is lower than the first threshold value Th1, depolarization is performed, and after the depolarization, the first voltage V1 reaches the second threshold value Th2. Therefore, it is possible to prevent the reverse polarization from being performed frequently, and it is also possible to suppress the decrease in the life of the 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 value Th1 is set based on the log. This makes it possible to set the first threshold value according to the usage status of the vehicle, the 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. .. As a result, the first voltage V1 can be accurately reached to the second threshold value Th2 while reducing the number of repetitions of the reverse polarization.

(Other embodiments)
The present invention is not limited to the above-described embodiment, and can be appropriately modified within the scope of 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 value Th1 is set based on the log. However, this is given as an example of the case where the first threshold value Th1 is set according to the usage status of the vehicle, the vehicle type, and the like, and the first threshold value Th1 may be a predetermined constant value.

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

Further, in the above 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 laminated as the mirror portion 12 formed by arranging the electrodes on both sides of the piezoelectric film. The structure is taken as an example. That is, the lower electrodes 12a and the intermediate electrodes 12c are arranged on both sides of the first piezoelectric film 12b, and the intermediate electrodes 12c and the upper electrodes 12e are arranged on both sides of the second piezoelectric film 12d, so that the electrodes are arranged on both sides of the piezoelectric film. The structure is such that two structures are stacked. However, this is also only an example of the structure of the mirror portion 12. For example, the structure may be simply provided with only one structure in which electrodes are arranged on both sides of the piezoelectric film.

1 Varifocal 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)

It has a base (11) constituting a membrane, a piezoelectric film (12b, 12d) formed on the base, and electrodes (12a, 12c, 12e) arranged on both sides of the piezoelectric film, and has an optical beam. A variable focus mirror (1) comprising a mirror portion (12) for reflecting light
A control unit (30) that bends the mirror portion into a spherical shape having a predetermined curvature by applying a predetermined voltage between the electrodes arranged on both sides of the piezoelectric film.
A variable focus mirror system that controls a variable focus mirror in a head-up display in a vehicle having a curvature detecting unit (13, 20) for measuring the curvature of the bent mirror unit.
The control unit
When driving the variable focus mirror, a rectangular wavy driving voltage that alternately switches between a first voltage (V1) and a second voltage (V2) larger than the first voltage is applied between the electrodes, and the first voltage is applied. 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) while detecting with the curvature detecting portion. 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 that the curvature becomes the second curvature.
Further, when the first voltage applied between the electrodes in order to bend the mirror portion with the first curvature is lower than the first threshold value (Th1), when the power switch of the vehicle is turned off, it becomes reverse polarization. By applying a DC voltage between the electrodes that generates an electric field opposite to that when the variable focus mirror is driven, the piezoelectric voltage reaches a second threshold value (Th2) higher than the first threshold value. A variable focus mirror system that adjusts the orientation of the film. The control unit stores the first voltage as a log when the variable focus mirror is driven, obtains an approximate curve of change in the first voltage based on the log, and obtains an approximate curve of the change of the first voltage based on the log, and the variable is based on the approximate curve. The variable focus mirror system according to claim 1, wherein the first threshold value is set so that the remaining usable period in which the focus mirror can be driven is equal to or longer than the set period. The variable focus mirror system according to claim 2, wherein the control unit sets the set period based on the remaining usable period obtained according to the usage status of the vehicle or the vehicle type. Based on the approximate curve, the control unit obtains a period during which the first voltage that has reached the second threshold value due to the reverse polarization is expected to be lowered to the first threshold value again by driving the variable focus mirror. The variable focus mirror system according to claim 2 or 3, wherein the second threshold value is set so that the period is equal to or longer than the required period for using the variable focus mirror after the depolarization. The variable focus mirror system according to claim 4, wherein the control unit sets the usage request period based on the remaining usable period obtained according to the usage status of the vehicle or the vehicle type. In the control unit, the number of repetitions of the reverse polarization and the driving of the variable focus mirror after the reverse polarization is less than the number of repetitions which is the durability limit of the variable focus mirror in the expected years of use of the vehicle. The variable focus mirror system according to claim 5, which is set so as to be. The control unit drives the variable focus mirror after performing the reverse polarization for a predetermined time, and at the time of the drive, monitors the first voltage based on the detection result by the curvature detection unit and the first voltage. The log is stored a plurality of times, the remaining time of the reverse polarization applied until the first voltage reaches the second threshold value is estimated based on the multiple logs, and 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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