Mirror rotation angle detection mechanism

Abstract: A mirror rotation angle detection mechanism is provided with a light emitting member therein to irradiate the light onto the reflection surface of a mirror which is rotated about an axis positioned substantially in the center of the reflection surface of itself. A light receiving element receives the light reflected by the reflection surface of the aforesaid mirror, and an angle detection circuit detects the rotation angle of the aforesaid mirror in accordance with the output of the aforesaid light receiving element. Preferably, a condenser lens may also be arranged between the aforesaid light emitting member and the aforesaid reflection surface.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a mirror rotation angle detection mechanism employed for the mirror rotation type tracking actuator and others of a high-speed access photoelectro-magnetic disc driving apparatus. Related Background Art In recent years, it has increasingly become necessary to detect precisely at a low cost the mirror rotation angle of a mirror rotation type tracking actuator in seek in order to speed up the access of a photoelectro-magnetic disc driving apparatus. A conventional apparatus of the kind is structured as shown in Fig. 1. In Fig. 1, the light ray 8 from an optical head is reflected by a mirror 1. The mirror 1, a mirror holder 2, and a coil 3 are rotatably held by a supporting member (not shown) to rotate within the space of a sheet with the reflection point C of the light ray on the surface of the mirror 1 as its substantial center. The coil 3 is wound around the mirror holder 2. A magnet 4 and a yoke 5 form a magnetic circuit. The mirror 1 is driven to rotate together with the mirror holder 2 and coil 3 when a current is applied to the coil 3. Photocoupler 63 and 64 are both reflective and output signals in accordance with the distance to a reflection surface 2a arranged on the mirror holder 2 by utilizing the property that the reflective luminous energy varies by distances. The reflective photocoupler 63 and the reflective photocoupler 64 are located against the mirror 1 at positions opposite to each other, and each of the output signal 63a and output signal 64a is inputted into a differential amplifier 7. The differential amplifier 7 picks up the differential between the output signal 63a and output signal 64a respectively of the reflective photocoupler 63 and photocoupler 64. The said differential is the difference between the two luminous energies, i.e., the difference in distances between the mirror surface 2a and the reflective coupler 63 and the reflective coupler 64, and is the rotation angle signal of the mirror which is rotated with a point C within the space of a sheet as its center. As the above describes, according to the conventional structure, a vibration mode which causes the mirror holder 2 to be in the translational motion in the direction indicated by arrow A as shown in Fig. 1, a rotational vibration mode with the center line B of the mirror 1 as its center, etc. are generated. Also, meanwhile, the reflective photocoupler 63 and reflective photocoupler 64, which function as rotation angle sensors, do not detect the rotation angle of the mirror 1 directly. In practice, the reflective photocouplers detect the distance between the mirror holder 2 and the reflection surface 2a, and at that, in manufacturing, the reflection surface 2a cannot necessarily be made in parallel with the reflection surface of mirror 1. Furthermore, the sensitivities of the reflective photocouplers 63 and 64 cannot necessarily be the same. Therefore, if there occurs the vibration mode which results in the translational motion in the directions A and further, the translational motion is deviated, the waveforms of the output signals of the differential amplifier 7 are influenced; thus making it impossible to perform a highly precise detection of the rotation angles. Also, when the rotational vibration mode is generated with the center line B as its center, the waveforms of output signals of the photocoupler 63, photocoupler 64, etc. are equally affected if the manufacturing precision of the reflection surface 2a of the mirror holder 2 or the structural precision of the supporting member is not extremely high. Then, a highly precise detection of rotation angles cannot be performed. The document FR-A-2618893 discloses apparatus having a double-sided mirror, the orientation of the mirror being measured by means of a beam reflected off the reverse side of the mirror. According to the present invention, there is provided a mirror rotation angle detection apparatus comprising: a mirror, having a reflection surface, which is rotatable about an axis substantially within the plane of the reflection surface; light emitting means for irradiating light onto the reverse side of the mirror; and light receiving means for receiving the light reflected by the mirror and generating an output which varies according to the rotation angle of the mirror; characterised in that the reflection surface comprises a thin layer of reflective material having substantially coplanar reflective opposite sides, in that the face of the mirror opposite the reflection surface is transmissive and in that the light receiving means is arranged to receive light reflected by the reverse side of the reflection surface. Thus, embodiments of the present invention may provide a high-precision mirror rotation angle detection mechanism. Also, a condenser lens 9 may preferably be arranged between the light emitting element 61 and the mirror 1. The mirror 1 is rotated about an axis on the surface thereof, i.e., the reflection surface 1a, while the controlling beam is reflected by the reverse side of the reflection surface 1a of the mirror 1, i.e., the substantial center of the rotation. Therefore, the influences of the translational motion vibration mode and rotational vibration mode due to the thickness of the mirror 1 as in the conventional art are small, thus making it possible to detect and follow the mirror rotation angle highly precisely. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view showing a conventional mirror rotation angle detection mechanism. Fig. 2 is a cross-sectional view showing an embodiment of the mirror rotation angle detection mechanism according to the present invention. Figs. 3A to 3C are views illustrating the operation of the mirror rotation angle detection mechanism shown in Fig. 2. Fig. 4 is a cross-sectional view showing another embodiment of the mirror rotation angle detection mechanism according to the present invention. Fig. 5 is a schematic view showing the optical head of an optical information recording and reproducing apparatus in which a mirror rotation angle detection mechanism of the present invention is incorporated. Fig. 6 is a cross-sectional view showing a mirror rotation angle detection mechanism which is structured to irradiate the light onto the reflection surface of the mirror from the right side thereof. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 2 is a view showing an embodiment of the present invention, in which the same reference marks as those appearing in Fig. 1 represent the same constituents. In Fig. 2, a reference numeral 61 designates a light emitting element for emitting a controlling beam; 62, a light receiving element for receiving the said beam; 1a, the reflection surface of a mirror 1; and 1b, the surface of the reverse side of the reflection surface 1a of the thick mirror 1. The light receiving element 62 is a two-piece photodiode or the like element, and as shown in Figs. 3A to 3C, the receiving part thereof is divided into two parts 62a and 62b, thereby enabling the difference in the luminous energies of these parts to be measured. The light ray 8 from an optical head is reflected by the mirror 1. The mirror 1 is rotatably supported by a supporting member (not shown) together with a mirror holder 2 and a coil 3 to be rotated about a point C as its center within a space of a sheet. The coil 3 is arranged in a magnetic circuit comprising a magnet 4 and a yoke 5 to enable the mirror 1, mirror holder 2, and coil 3 itself to be rotated by an electric current being applied to the coil 3. The irradiation light for the controlling beam from the light emitting element 61 is being transmitted through the transmitting surface 1b of the mirror 1 and reflected by the reverse side of the reflection surface 1a of the mirror 1. Thus, the irradiation light is received by the light receiving element 62. The reflection surface is obtained by bonding a substance for enhancing the reflecting power firmly to the surface of the base material of the mirror 1. However, since the thickness of such substance for enhancing the reflecting power is generally approximately 0.1 mm or less, the reflection surface 1a and the surface of the reverse side of the said reflection surface 1a are substantially on one and the same surface, and the center C of the rotation is substantially positioned on that surface. The outputs of the light receiving part 62a and light receiving part 62b of the light receiving 62 are inputted into an angle detection circuit 7. The beam on the light receiving element 62 is moved in accordance with the angles of the mirror 1 which vary by the rotation of the mirror 1. Therefore, the rotation angle signals of the mirror 1 are output by obtaining the differential between the outputs of the light receiving part 62a and light receiving part 62b of the two-piece photodiode in the angle detection circuit 7. Figs. 3A to 3C are views illustrating the operation of the present embodiment, in which the same reference numbers appearing in Fig. 2 represent the same constituents. Fig. 3A illustrates the case where the light receiving part 62a and light receiving part 62b of the two-piece photodiode of the light receiving element 62 are in contact at the interface in the direction perpendicular to the space of a sheet. Fig. 3B illustrates the case where the light receiving part 62a and light receiving part 62b of the two-piece photodiode of the light receiving element 62 are in contact at the interface within the space of sheet. Fig. 3C illustrates the case represented in Fig. 3A but still shows a state where the mirror 1 is in the translational motion in the direction A. In the case of Fig. 3A where the rotation is within the space of a sheet with the point C as its center, the luminous energies of the light receiving part 62a and light receiving part 62b vary by the rotation. Therefore, it is possible to measure the rotation within the space of a sheet with the point C as its center. Then, even when the rotational vibration mode is being generated with the line B perpendicular to the reflection surface of the mirror 1 as its center, the reflected light from the mirror 1 is vibrated on the boundary of the light receiving part 62a and light receiving part 62b. As a result, there is no influence exerted against the output by the rotation within the space of a sheet with the point C as its center. However, if the vibration mode is being generated, which may result in the translational motion in the directions A, there is a slight influence exerted against the output by the rotation within the space of a sheet with the point C as its center because the vibration occurs in the direction which generates a differential between the light receiving part 62a and light receiving part 62b as shown in Fig. 3C. Nevertheless, the aforesaid influence can be neglected if only the distance between the light emitting element 61 and the light receiving element 62 is made sufficiently close. In the case of Fig. 3B, the luminous energies of the light receiving part 62a and light receiving part 62b do not vary by the rotation when the rotation is within the space of a sheet with the point C as its center. Fig. 4 is a view showing another embodiment of the present invention, and the same reference marks appearing in Fig. 1 represent the same constituents. Here, a reference numeral 9 designates a condenser lens. Through the condenser lens 9, the irradiation light from the light emitting element 61 becomes a parallel luminous flux which is transmitted through the other surface 1b and reflected at the point C on the reverse side of the reflection surface of the mirror 1. Then, the reflected parallel luminous flux is converged into the light receiving element through the condenser lens 9. The two outputs respectively from the two-piece light receiving part 62a and light receiving part 62b of the light receiving element 62 are inputted into the angle detection circuit 7. The beam on the light receiving element 62 is moved by the rotation angles within the space of a sheet with the point C of the mirror 1 as its center while the arrangement of the light receiving element is in such a form that the light receiving part 62a and light receiving part 62b of the two-piece photodiode of the light receiving element 62 shown in Fig. 3A are in contact at the interface in the direction perpendicular to the space of a sheet. Consequently, the rotation angle signal is output by obtaining a differential between the outputs of the two-piece photodiode in the rotation angle detection circuit 7. At the time of the rotational vibration mode with the line B perpendicular to the reflection surface of the mirror 1 as its center, the reflected light from the mirror 1 is vibrated in the direction towards the boundary line of the light receiving part 62a and light receiving part 62b. As a result, the output by the rotation within the space of a sheet with the point C as its center is not influenced at all. Further, at the time of the vibration mode which may result in the translational motion in the direction A, the output by the rotation within the space of a sheet with the point C as its center is not influenced at all, either, because the light becomes the parallel luminous flux between the condenser lens 9 and the mirror 1. Fig. 5 is a view illustrating the operation of a mirror rotation actuator including the present invention and schematically shows the state where a mirror rotation angle detection mechanism according to the present invention is incorporated in an optical information recording and reproducing apparatus. Here, the same reference marks appearing in Fig. 2 represent the same constituents. In Fig. 5, a reference numeral 11 designates a light source for laser, etc.; 8, a beam generated by the light source 11; 12, an intermediate lens to converge the beam 8; 13, a poralized beam splitter; 14, a phase plate; 1, a mirror in a mirror rotation actuator; 16, an objective lens to converge the reflected beam 8 from the mirror 1; and 17, an information disc. The reference numeral 18 designates a light receiving element which receives the reflected light from the information disc 17. Also, a reference numeral 8a designates the controlling beam from the mirror rotation actuator shown in the first-embodiment already described, and a reference number 62 designates the light receiving element which receives the reflected light from the mirror of the mirror rotation actuator for the controlling beam 8a shown in the first embodiment. The beam generated by the light source 11 is converged by the intermediate lens 12 and reflected by the mirror 1 of the mirror rotation actuator after being transmitted through the polarized beam splitter 13 and the phase plate 14, and is converged by the objective lens 16 to irradiate the information disc 17. The beam 8 reflected by the information disc 17 is returned on the same path, reflected by the polarized beam splitter 13, and converted into an electrical signal by the light receiving element 18. At this juncture, the mirror 1 of the mirror rotation actuator is controlled to rotate by the system structured as in the first embodiment or the second embodiment, so that the beam spot of the beam 8 can accurately follow the information track of the information disc 17. In other words, if the mirror 1 is controlled to enable the controlling beam 8a to be irradiated always evenly onto the two light receiving parts 62a and 62b of the light receiving element 62, the mirror 1 is fixed at a given position irrespective of existing wind pressure and mechanical vibration. Hence, the recording can reliably be reproduced accurately. The objective lens 16 is vertically movable by the known technique to correct the focusing of the beam spot on the information disc 17. In the present embodiment, the light emitting element 61 and the light receiving element 62 are arranged on the reverse side of the reflection surface of the mirror 1 as shown in Fig. 2 and Fig. 4. Fig. 6 illustrates a case where the light emitting element 61 and the light receiving element 62 are arranged in the right side of the mirror 1, but which does not form part of the present invention. In Fig. 6, the same reference marks appearing in Fig. 1 represent the same constituents. In this case, the irradiation light from the light emitting element 61 for the controlling beam is reflected on the same surface as the light ray 8 from an optical head.

Claims: A mirror rotation angle detection apparatus comprising: a mirror (1), having a reflection surface (1a), which is rotatable about an axis substantially within the plane of the reflection surface (1a); light emitting means (61) for irradiating light onto the reverse side of the mirror; and light receiving means (62) for receiving the light reflected by the mirror and generating an output which varies according to the rotation angle of the mirror; characterised in that the reflection surface (1a) comprises a thin layer of reflective material having substantially coplanar reflective opposite sides, in that the face (1b) of the mirror (1) opposite the reflection surface (1a) is transmissive and in that the light receiving means (62) is arranged to receive light reflected by the reverse side of the reflection surface (1a). Apparatus as claimed in claim 1, including an angle detection circuit (7) for detecting the rotation angle of the reflection surface (1a) of said mirror (1) in accordance with the output of said light receiving means (62). Apparatus according to claim 1 or 2, further comprising a condenser lens (9) which is positioned between said light emitting means (1) and said reflection surface (1a). Apparatus according to any preceding claim, wherein the light receiving means (62) comprises two sensors (62a, 62b) and means (7) for providing said output in response to the difference between the luminous energies received by the two sensors.

Inventors: ISHII YUWA, KANEKO HISAHARU, ISHII, YUWA, KANEKO, HISAHARU
Applicants: NIPPON KOGAKU KK, NIKON CORPORATION
Application year: 1991
Main classification: G01D5
IPC codes: G11B7, G01D5