Patent Description:
Optical planetariums known in the related art can be of a classical four-axis type or a modem three-axis type such as the ones as disclosed in, for example, <CIT>. The disadvantages of such solutions, as well as the ones relating to other known solution dealing with multiaxial composite control, are discussed hereafter.

A configuration of a representative optical planetarium projector which is currently most widespread is illustrated in <FIG>. An optical planetarium <NUM> includes a north-sky projector 111a and a south-sky projector 111b, each having a hemispherical shape, as a projector <NUM> for projecting fixed stars. The north-sky projector 111a and the south-sky projector 111b are provided so as to face away from each other. The north-sky projector 111a and the south-sky projector 111b are referred to as "star ball <NUM>". The star ball <NUM> is rotatable about a diumal axis <NUM>. The star ball <NUM> is supported on a fork mount <NUM> so as to be rotatable about a latitude axis <NUM> that is set horizontally and is orthogonal to the diumal axis <NUM>. When the star ball <NUM> is rotated about the latitude axis <NUM>, an inclination angle between the diurnal axis <NUM> and a horizontal plane can be changed as desired.

On the star ball <NUM>, stars projected from the star ball <NUM> are arranged on coordinates referring equatorial coordinates based on a rotation axis of the Earth so that the diumal axis <NUM> points to current north celestial pole and south celestial pole. Thus, when the star ball <NUM> is rotated about the diumal axis <NUM>, diumal motion of stars is reproduced on a dome screen. Further, when the star ball <NUM> is rotated about the latitude axis <NUM>, the optical planetarium <NUM> can reproduce a starry sky seen from an observation site at any latitude on the Earth.

Instead of a traditional concentrical arrangement of audience seats toward a center of a dome, a unidirectional arrangement of audience seats has become a mainstream for recent planetariums. Such an arrangement allows easy alignment of a line of sight of a presenter and lines of sight of audience. Thus, the presenter can give correct presentation on celestial bodies and objects to all the audience. Meanwhile, the presenter and the audience face forward in a specific direction, and thus have a difficulty in seeing a side opposite to a front side. For example, when the south is set as the front side, the presenter and the audience are both required to tum around for explanation of celestial bodies or objects in the north. To eliminate such inconvenience, the optical planetarium <NUM> has an azimuth axis <NUM> about which the optical planetarium <NUM> is allowed to rotate as a whole in a horizontal direction. The type capable of freely changing a direction corresponding to the front side in such a manner has been widespread. In recent years, as described above, the optical planetarium <NUM> having three axes, that is, the diumal axis <NUM>, the latitude axis <NUM>, and the azimuth axis <NUM> for reproducing movement of celestial bodies such as fixed stars has been commonly used. Such a type of optical planetarium <NUM> is referred to as "modem three-axis type.

Hitherto, an optical planetarium <NUM> as illustrated in <FIG> has been commonly used. The optical planetarium <NUM> has a precession axis <NUM> in addition to a diumal axis <NUM> and a latitude axis <NUM> so as to reproduce a change in the rotation axis of the Earth over a long period of time, which is referred to as precession motion. Such a type of optical planetarium <NUM> is referred to as "classical three-axis type. " A north-sky star ball 211a and a south-sky star ball 211b are configured to project a starry sky not referring the coordinates based on the rotation axis of the Earth but referring ecliptic coordinates based on an orbital plane of the Earth. The north-sky star ball 211a and the south-sky star ball 211b are configured so as to be rotatable about the precession axis <NUM>. The precession axis <NUM> is held by a precession-axis holder <NUM>, and the precession-axis holder <NUM> as a whole is rotatable about the diumal axis <NUM>. The precession-axis holder <NUM> keeps an angle between the diumal axis <NUM> and the precession axis <NUM> at about <NUM>°. This angle is an inclination angle of the equator of the Earth with respect to the ecliptic plane. The diurnal axis <NUM> is held by the latitude axis <NUM>. As in the case of the modem three-axis type, a starry sky seen from an observation site at any latitude can be reproduced by rotating about the latitude axis <NUM>. The classical three-axis type optical planetarium <NUM> can reproduce not only current movement of stars in which a current northem pole star (α Ursae Minoris: Polaris) is as a northem pole star but also movement of stars, for example, in the future after <NUM>,<NUM> years from today in which Vega in the constellation of Lyra is as a northem pole star, by rotating about the precession axis <NUM>.

Further, there is also known an optical planetarium <NUM>, which corresponds to a classical three-axis type additionally having an azimuth axis <NUM> for horizontally rotating the optical planetarium <NUM> as a whole. Such a type of optical planetarium <NUM> is referred to as "classical four-axis type.

In the classical three-axis type and classical four-axis type optical planetariums <NUM>, the north-sky star ball 211a and the south-sky star ball 211b are installed on the precession-axis holder <NUM> in an inclined manner, which may result in a larger device size. The optical planetarium <NUM> having a large size may obstruct views of the audience. Thus, in recent years, the classical three-axis type and classical four-axis type optical planetariums <NUM> have been less used, and the modem three-axis type optical planetarium <NUM> has mainly been used. The modem three-axis type allows the star ball <NUM> to be provided in a compact manner, and thus the optical planetarium <NUM> does not obstruct the views of the audience.

The modem three-axis optical planetarium <NUM> is not able to reproduce precession motion etc. by rotation about one axis. Thus, angles about three axes are controlled by a computer so as to virtually reproduce the precession motion etc. through a three-axis composite motion. In theory, the star ball <NUM> can have any attitude angle by combining rotations about three axes, that is, the diumal axis <NUM>, the latitude axis <NUM>, and the azimuth axis <NUM>. Not only the precession motion but also rotational motion about any point on the celestial sphere can be reproduced by accurately controlling the angles about the three axes. Accordingly, for example, a starry sky which can be seen from a planet other than the Earth can be reproduced.

In the multiaxial composite control using the modem three-axial type, when two axes come closer to each other, for example, when the diumal axis <NUM> is aligned with the azimuth axis <NUM>, the degree of freedom in motion is reduced, which may result in failing to appropriately reproduce a starry sky. With four-axis composite motion using the classical four-axis type, such a reduction in the degree of freedom in motion is prevented. Thus, a starry sky can be reproduced with high accuracy. However, the classical four-axis type optical planetarium <NUM> becomes complex and is increased in size as compared to the modem three-axis type.

The present invention has an object to allow a device to be compact and allow a wide variety of starry skies to be appropriately reproduced in an optical planetarium.

According to one aspect of the present invention, there is provided an optical planetarium, including: a projector configured to project images of stars; a diumal-axis support mechanism configured to allow the projector to rotate about a diumal axis; a latitude-axis support mechanism configured to allow the projector to rotate about a latitude axis that is orthogonal to the diumal axis; an azimuth-axis support mechanism configured to allow the projector to rotate about an azimuth axis that is set vertically; and a latitude-axis inclination angle changing mechanism configured to allow an angle between the latitude axis and a horizontal plane to be changed within a predetermined range.

According to the present invention, it is possible to allow the device to be compact and allow a wide variety of starry skies to be appropriately reproduced in the optical planetarium.

One embodiment is described with reference to the drawings. An optical planetarium according to this embodiment is a projector with four axes achieved by additionally providing a swing axis for inclining a latitude axis within a given range to a modem three-axis type optical planetarium. Four axes allow appropriate reproduction of precession motion or rotational motion about any point on a celestial sphere, such as a starry sky that can be seen from a planet other than the Earth. Further, a configuration according to this embodiment allows a device to be compact. A type of optical planetarium according to this embodiment is referred to as "new four-axis type".

An outline of a configuration example of an optical planetarium <NUM> according to this embodiment is illustrated in <FIG>. As illustrated in <FIG>, like the related-art modem three-axis type optical planetarium <NUM>, the optical planetarium <NUM> according to this embodiment includes a projector <NUM> configured to project images of stars. In the example illustrated in <FIG>, the projector <NUM> includes a star ball <NUM>. The star ball <NUM> includes a north-sky projector 11a and a south-sky projector 11b, each having a hemispherical shape. The north-sky projector 11a and the south-sky projector 11b are mounted so as to face away from each other. The star ball <NUM> is supported by a diumal-axis support mechanism <NUM> so as to be rotatable about a diumal axis <NUM>. The diumal-axis support mechanism <NUM> includes, for example, a shaft member, a bearing, a motor, a reduction gear, and an encoder. The shaft member extends along the diumal axis <NUM>. The bearing receives the shaft member. The motor and the reduction gear rotate the shaft member. The encoder detects a rotational angle. A configuration of the diumal-axis support mechanism <NUM> is not limited to that described above, and may be any configuration. For example, the diumal-axis support mechanism <NUM> may include an annular rail and a slider. The slider is provided on a peripheral edge portion of a disc plate, and slides relative to the rail.

The star ball <NUM> is supported on a fork mount <NUM> by a latitude-axis support mechanism <NUM> so as to be rotatable about a latitude axis <NUM> that is orthogonal to the diurnal axis <NUM>. The latitude-axis support mechanism <NUM> may also have a variety of configurations. The latitude-axis support mechanism <NUM> includes, for example, a shaft member, a bearing, a motor, a reduction gear, and an encoder. The optical planetarium <NUM> is configured to be able to freely change an inclination angle between the diumal axis <NUM> and a horizontal plane through rotation of the star ball <NUM> about the latitude axis <NUM>.

Like the related-art modem three-axis type optical planetarium <NUM>, on the star ball <NUM>, stars projected from the star ball <NUM> are arranged on coordinates referring equatorial coordinates based on a rotation axis of the Earth. When the star ball <NUM> rotates about the diumal axis <NUM>, diumal motion of stars is reproduced on a dome screen. Further, when the star ball <NUM> rotates about the latitude axis <NUM> that lies horizontally, a starry sky seen from an observation site at any latitude on the Earth can be reproduced.

Further, the optical planetarium <NUM> is configured to rotate about an azimuth axis <NUM> that is set vertically so that the optical planetarium <NUM> is rotated in a horizontal direction as a whole. Specifically, the fork mount <NUM> is supported by an azimuth-axis support mechanism <NUM> so as to be rotatable about the azimuth axis <NUM> with respect to a base <NUM> fixed on a ground. The azimuth-axis support mechanism <NUM> may also have a variety of configurations. The azimuth-axis support mechanism <NUM> includes, for example, a shaft member, a bearing, a motor, a reduction gear, and an encoder. When the fork mount <NUM> rotates about the azimuth axis <NUM>, a direction corresponding to a front side can be changed as desired. The diumal axis <NUM>, the latitude axis <NUM>, and the azimuth axis <NUM> are set so as to cross each other at a center of the star ball <NUM>.

The optical planetarium <NUM> according to this embodiment further includes a latitude-axis inclination angle changing mechanism <NUM> configured to be able to change an angle between the latitude axis <NUM> and the horizontal plane within a predetermined range. In particular, the optical planetarium <NUM> according to this embodiment has a swing axis <NUM> about which the latitude axis <NUM> rotates. The swing axis <NUM> is set so as to be orthogonal to the azimuth axis <NUM> and the latitude axis <NUM> at the center of the star ball <NUM>.

The fork mount <NUM> in this embodiment has an arc-like shape sharing a center with the star ball <NUM> so as to allow the latitude axis <NUM> to rotate about the swing axis <NUM>. A rail <NUM> having an arc-like shape that shares a center with the star ball <NUM> is provided on a support part <NUM> which is provided on the azimuth-axis support mechanism <NUM> and rotates about the azimuth axis <NUM>. The fork mount <NUM> includes a slider <NUM> configured to slide on the rail <NUM>. The fork mount <NUM> is supported on the support part <NUM> provided on the azimuth-axis support mechanism <NUM> through intermediation of the rail <NUM> and the slider <NUM>. It is preferred that cross-roller bearings be provided to a portion corresponding to the rail <NUM> and the slider <NUM>. A rack gear <NUM> having an arc-like shape is provided to the fork mount <NUM> so as to extend along the rail <NUM> having an arc-like shape and the slider <NUM>. A pinion gear <NUM> that drives the rack gear <NUM> is provided in the support part <NUM>. The pinion gear <NUM> is driven by a swing-axis motor <NUM>. An inclination angle of the fork mount <NUM> is controlled through an operation of the swing-axis motor <NUM> as desired. A configuration of the latitude-axis inclination angle changing mechanism <NUM> described here is an example. Other configurations may be used for the latitude-axis inclination angle changing mechanism <NUM> as long as the latitude-axis inclination angle changing mechanism <NUM> has the same functions. A movable range of the fork mount <NUM> about the swing axis <NUM> is, for example, ±<NUM>°, but is not limited thereto. When the movable range is ±<NUM>°, the star ball <NUM> can rotate within a range of ±<NUM>° about the swing axis <NUM> passing through the center of the star ball <NUM>.

As illustrated in <FIG>, in a planetarium system <NUM> according to this embodiment, the optical planetarium <NUM> described above is arranged at a center of a dome <NUM> having a hemispherical shape. The dome <NUM> has an inner surface serving as a screen <NUM>. The optical planetarium <NUM> projects images of fixed stars onto the screen <NUM> on the inner surface of the dome <NUM>. An operation of the optical planetarium <NUM> for the projection is controlled by a controller <NUM>. The control includes that of the rotation of the star ball <NUM> about the diumal axis <NUM>, the latitude axis <NUM>, the azimuth axis <NUM>, and the swing axis <NUM>. The controller <NUM> includes a computer. The controller <NUM> includes an integrated circuit such as a central processing unit (CPU), an application specific integrated circuit (ASIC), or a field programmable gate array (FPGA). Further, the controller <NUM> includes, for example, a read only memory (ROM), a random access memory (RAM), a storage, an input device, a display device, and various interfaces. The controller <NUM> operates in accordance with a program or hardware.

An operation of the new four-axis type optical planetarium <NUM> according to this embodiment is now described in comparison with an operation of the related-art modem three-axis type optical planetarium <NUM>, which has been described with reference to <FIG>.

In both of the star ball <NUM> of the new four-axis type optical planetarium <NUM> according to this embodiment and the star ball <NUM> of the related-art modem three-axis type optical planetarium <NUM>, stars are arranged on coordinates based on the equatorial coordinates having a current rotation axis of the Earth as a reference. Specifically, for example, a star that is currently located at a celestial north pole is arranged on the diumal axis <NUM> in the north-sky projector 11a. More specifically, a northem pole star (α Ursae Minoris: Polaris) is arranged in the vicinity of the celestial north pole, that is, in the vicinity of the diurnal axis <NUM> in the north-sky projector 11a with an offset of less than <NUM>° from the celestial north pole.

It is now supposed, as an example, a case in which diumal motion of a starry sky in the future after <NUM>,<NUM> years from today is reproduced. In this case, in a north sky, Vega in the constellation of Lyra is located in the vicinity of the celestial north pole in a north sky although Vega is not located exactly at the celestial north pole. When viewed from a geographical point of a northem latitude of about <NUM>°, a Vega <NUM> is seen at an altitude of substantially <NUM>° in the north sky and hardly moves from there as illustrated in <FIG>. A Polaris <NUM> and the Vega <NUM> are away from each other by about <NUM>°, and thus the Polaris <NUM> rises in a northeastern sky and passes near a zenith <NUM> at a distance of <NUM>° away from the zenith to the north. Other stars also make diumal motion about the Vega <NUM>.

In reality, Polaris is not located exactly at the celestial north pole even today, and Vega may not be located exactly at the celestial north pole in the future after <NUM>,<NUM> years. For description, however, it is assumed that Polaris is located at the celestial north pole today, Vega is located at the celestial north pole in the future after <NUM>,<NUM> years, and Polaris passes near the zenith.

Consideration is now made on movement of a starry sky, which can be seen from a geographical point at a northem latitude of <NUM>° in the future after <NUM>,<NUM> years, is reproduced through three-axis composite motion with use of the related-art modem three-axis type optical planetarium <NUM>. In this case, the diumal axis <NUM> of the optical planetarium <NUM> points toward the Polaris <NUM>. An angle about the azimuth axis <NUM> changes in accordance with a direction toward the Polaris <NUM>. When the Polaris <NUM> is sufficiently far away from the zenith <NUM>, an angular velocity about the azimuth axis <NUM> is small, causing no problem.

When the Polaris <NUM> passes near the zenith <NUM>, however, an azimuth angle of the Polaris <NUM> rapidly changes as indicted by arrows in <FIG> although the diumal motion is not so fast. Accordingly, it is required that the rotation about the azimuth axis <NUM> be performed at high speed. When the Polaris <NUM> passes exactly through the zenith <NUM>, a velocity of the passage instantaneously becomes infinite. A point at which the velocity becomes infinite is referred to as "singular point". When the Polaris <NUM> approaches the zenith <NUM>, it means that an angle between the diumal axis <NUM> and the azimuth axis <NUM> decreases. When an altitude of the Polaris <NUM> above the horizon is referred to as "a°", the angle between the diurnal axis <NUM> and the azimuth axis <NUM> is (<NUM>-a)°. In other words, the phenomenon occurs at the singular point due to a decrease in degree of freedom in the motion, which is caused by alignment between the azimuth axis <NUM> and the diumal axis <NUM>. This phenomenon is equivalent to a so-called "gimbal lock" state. In an actual operation of the optical planetarium <NUM>, a maximum speed or a maximum acceleration of rotation about a rotation axis is limited. Thus, the optical planetarium <NUM> fails to precisely follow ideal movement and causes unnatural movement.

With the classical four-axis type optical planetarium <NUM> which has been described with reference to <FIG>, the above-mentioned problem can be solved through four-axis composite motion. However, the classical four-axis type optical planetarium <NUM> has a large size, and thus may, for example, obstruct views of audience.

On the other hand, in the new four-axis type optical planetarium <NUM> according to this embodiment, rotation about four axes, that is, the diumal axis <NUM>, the latitude axis <NUM>, the azimuth axis <NUM>, and the swing axis <NUM> is controlled so as to prevent the diumal axis <NUM> and the azimuth axis <NUM> from coming excessively close to each other. In this manner, the above-mentioned problem which may occur in the related-art modem three-axis type optical planetarium can be prevented.

The new four-axis type optical planetarium <NUM> according to this embodiment has two control modes, that is, a swing-axis fixed mode and an azimuth-axis fixed mode. In the swing-axis fixed mode, an angle about the swing axis <NUM> is fixed to a predetermined value, and angles about the diumal axis <NUM>, the latitude axis <NUM>, and the azimuth axis <NUM> are calculated so as to control a posture of the star ball <NUM>. When the angle about the swing axis <NUM> is <NUM>° and the latitude axis <NUM> lies horizontally, the swing-axis fixed mode is the same as that in the related-art modem three-axis type optical planetarium. In the azimuth-axis fixed mode, an angle about the azimuth axis <NUM> is fixed to a predetermined value, and the angles about the diumal axis <NUM>, the latitude axis <NUM>, and the swing axis <NUM> are calculated so as to control the posture of the star ball <NUM>. It is known that the angles about the axes can be mathematically calculated by, for example, a matrix calculation in each of the modes. The description of the calculation is omitted here.

Generally, the control is performed in the swing-axis fixed mode in which the angle about the swing axis <NUM> is fixed to <NUM>°, which is an initial value. On the other hand, when the diumal axis <NUM> and the azimuth axis <NUM> are close to each other and the angle between the diurnal axis <NUM> and the azimuth axis <NUM> is smaller than a predetermined range, for example, when the altitude of the Polaris <NUM> exceeds <NUM>°, the control mode is switched to the azimuth-axis fixed mode. Further, in the azimuth-axis fixed mode, when the diumal axis <NUM> and the azimuth axis <NUM> are away from each other and the angle between the diumal axis <NUM> and the azimuth axis <NUM> is larger than a predetermined range, for example, when the altitude of the Polaris <NUM> is lower than <NUM>°, the control mode is switched to the swing-axis fixed mode.

Switching between the swing-axis fixed mode and the azimuth-axis fixed mode is not always performed based only on the angle between the diumal axis <NUM> and the azimuth axis <NUM> as a reference. For example, switching between the swing-axis fixed mode and the azimuth-axis fixed mode may be performed so that an angular velocity or an angular acceleration of the rotation about the diumal axis <NUM> or the azimuth axis <NUM> is less than a predetermined value. Further, switching between the swing-axis fixed mode and the azimuth-axis fixed mode may be performed through use of both of the angle between the diumal axis <NUM> and the azimuth axis <NUM> and the angular velocity or the angular acceleration of the rotation about the diumal axis <NUM> or the azimuth axis <NUM>.

It is preferred that the optical planetarium have a transition mode in which both of the rotation about the azimuth axis <NUM> and the rotation about the swing axis <NUM> have an angular velocity being provided so that the switching between the above-mentioned modes is performed smoothly. Specifically, when switching from the swing-axis fixed mode to the azimuth-axis fixed mode is performed, the angular velocity about the azimuth axis <NUM> is gradually decelerated in the transition mode. When switching from the azimuth-axis fixed mode to the swing-axis fixed mode is performed, the angular velocity about the swing axis <NUM> is gradually decelerated in the transition mode. Further, when switching from the azimuth-axis fixed mode to the swing-axis fixed mode is performed, it is preferred that the angle about the swing axis <NUM> be gently returned to <NUM>°, which is the initial value. Further, there is a case in which an angle about the swing axis <NUM> is required to be larger or smaller than limit values of the device, e.g., ±<NUM>°, depending on a control state. In this case, the angle about the swing axis <NUM> is fixed to a maximum value of the angle, and the control mode is switched to the swing-axis fixed mode. Also in this case, it is preferred that the angular velocity about the swing axis <NUM> be gradually decreased so as to stop the rotation before the angle reaches ±<NUM>°.

An example of the control according to this embodiment is described with reference to a flowchart of <FIG>.

In Step S1, the controller <NUM> controls the operation of the optical planetarium <NUM> in the swing-axis fixed mode. Specifically, the controller <NUM> fixes the angle about the swing axis <NUM> and calculates the angles about the diumal axis <NUM>, the latitude axis <NUM>, and the azimuth axis <NUM> so as to reproduce a target starry sky. The controller <NUM> controls the rotation about the diumal axis <NUM>, the latitude axis <NUM>, and the azimuth axis <NUM> based on the obtained values.

In Step S2, the controller <NUM> determines whether or not the angle between the diumal axis <NUM> and the horizontal plane is larger than a predetermined first value, for example, <NUM>°. When the angle between the diumal axis <NUM> and the horizontal plane is not larger than the first value, the process retums to Step S1. Then, the control in the swing-axis fixed mode is continued. On the other hand, when the angle between the diumal axis <NUM> and the horizontal plane is larger than the first value, the process proceeds to Step S3.

In Step S3, the controller <NUM> controls the operation of the optical planetarium <NUM> in the transition mode so as to switch the control for the operation of the optical planetarium <NUM> to that in the azimuth-axis fixed mode. Specifically, the controller <NUM> determines the angle about the azimuth axis <NUM> so as to gradually decrease the angular velocity of the rotation about the azimuth axis <NUM> and calculates the angles about the diumal axis <NUM>, the latitude axis <NUM>, and the swing axis <NUM> for the reproduction of the target starry sky based on the angle about the azimuth axis <NUM>. The controller <NUM> controls the rotation about the diumal axis <NUM>, the latitude axis <NUM>, the azimuth axis <NUM>, and the swing axis <NUM> based on the obtained values.

In Step S4, the controller <NUM> determines whether or not the angular velocity of the rotation about the azimuth axis <NUM> has become equal to zero, specifically, the rotation about the azimuth axis <NUM> has stopped. When the rotation about the azimuth axis <NUM> has not stopped, the process retums to Step S3. Then, the control in the transition mode is continued. On the other hand, when the rotation about the azimuth axis <NUM> has stopped, the process proceeds to Step S5.

In Step S5, the controller <NUM> controls the operation of the optical planetarium <NUM> in the azimuth-axis fixed mode. Specifically, the controller <NUM> fixes the angle about the azimuth axis <NUM> under the state in which the rotation about the azimuth axis <NUM> is stopped, and calculates the angles about the diumal axis <NUM>, the latitude axis <NUM>, and the swing axis <NUM> so as to reproduce a target starry sky. The controller <NUM> controls the rotation about the diumal axis <NUM>, the latitude axis <NUM>, and the swing axis <NUM> based on the obtained values.

In Step S6, the controller <NUM> makes a determination regarding whether or not the rotation about the swing axis <NUM> is to be restricted. There is a restriction on the rotation about the swing axis <NUM> such that the rotation is allowed only in the range of, for example, ±<NUM>°. Thus, when the angle about the swing axis <NUM> is likely to reach ±<NUM>°, the control in the azimuth-axis fixed mode cannot be continued. Consequently, the control mode is required to be switched to the swing-axis fixed mode. Accordingly, the controller4 determines whether or not the angle about the swing axis <NUM> is likely to reach, for example, ±<NUM>°. For example, the controller4 compares a limit value, which is set to a smaller absolute value as a current angular velocity about the swing axis <NUM> increases, with a current angle about the swing axis <NUM>. When the current angle about the swing axis <NUM> exceeds the limit value, the controller <NUM> determines that the rotation about the swing axis <NUM> is to be restricted.

In Step S7, the controller <NUM> determines whether or not the rotation about the swing axis <NUM> is to be restricted based on the determination made in Step S6. When the rotation about the swing axis <NUM> is to be restricted, the process proceeds to Step S9. On the other hand, when the rotation about the swing axis <NUM> is not to be restricted, the process proceeds to Step S8.

In Step S8, the controller <NUM> determines whether or not the angle between the diumal axis <NUM> and the horizontal plane is smaller than a predetermined second value, for example, <NUM>°. When the angle between the diumal axis <NUM> and the horizontal plane is not smaller than the second value, the process retums to Step S5. Then, the control in the azimuth-axis fixed mode is continued. On the other hand, when the angle between the diumal axis <NUM> and the horizontal plane is smaller than the second value, the process proceeds to Step S9. The second value may be set to a value smaller than the first value so that switching between the swing-axis fixed mode and the azimuth-axis fixed mode is not repeatedly performed at short intervals.

In Step S9, the controller <NUM> controls the operation of the optical planetarium <NUM> in the transition mode so that the control mode of the operation of the optical planetarium <NUM> is switched to the swing-axis fixed mode. Specifically, the controller4 determines the angle about the swing axis <NUM> so as to gradually decrease the angular velocity of the rotation about the swing axis <NUM> while making the angle about the swing axis <NUM> closer to a target value. Then, the controller <NUM> calculates the angles about the diumal axis <NUM>, the latitude axis <NUM>, and the azimuth axis <NUM> for the reproduction of the target starry sky based on the angle about the swing axis <NUM>. The controller <NUM> controls the rotation about the diumal axis <NUM>, the latitude axis <NUM>, the azimuth axis <NUM>, and the swing axis <NUM> based on the obtained values. In this case, when it is determined in Step S7 that the rotation about the swing axis <NUM> is likely to reach the limit thereof, the target value of the angle about the swing axis <NUM> is set to the limit value, for example, ±<NUM>°. On the other hand, when it is determined in Step S8 that the angle between the diumal axis <NUM> and the horizontal plane is smaller than the second value, the target value of the angle about the swing axis <NUM> is set to <NUM>° corresponding to an initial state.

In Step S10, the controller <NUM> determines whether or not the angular velocity of the rotation about the swing axis <NUM> has become equal to zero and the angle about the swing axis <NUM> has become equal to the target value, specifically, whether or not the rotation about the swing axis <NUM> has stopped at the target value. When the rotation about the swing axis <NUM> has not stopped at the target value, the process retums to Step S9. Then, the control in the transition mode is continued. On the other hand, when the rotation about the swing axis <NUM> has stopped at the target value, the process retums to Step S1. Then, the control in the swing-axis fixed mode is performed again. In the case in which the angle about the swing axis <NUM> is fixed to, for example, ±<NUM>°, and the control is performed in the swing-axis fixed mode, when the angle between the diumal axis <NUM> and the horizontal plane has become sufficiently small, the angle about the swing axis <NUM> is gradually returned to <NUM>°. The above-mentioned operation is repeated.

Description has been made of the example in which the control is performed so as to prevent the angle between the diurnal axis <NUM> and the horizontal plane from becoming larger than the predetermined value, i.e., to prevent the angle between the diurnal axis <NUM> and the azimuth axis <NUM> from becoming smaller than the predetermined value. However, a control method is not limited to that described above. Similarly, the angles about the diumal axis <NUM>, the latitude axis <NUM>, the azimuth axis <NUM>, and the swing axis <NUM> may be controlled so as to keep the angular velocity or the angular acceleration of the rotation about the diumal axis <NUM> or the azimuth axis <NUM> to a value smaller than a predetermined value. Specifically, switching between the swing-axis fixed mode and the azimuth-axis fixed mode may be performed in accordance with the angular velocity or the angular acceleration of the rotation about the diumal axis <NUM> or the azimuth axis <NUM>. Also in this case, similar effects are obtained.

Description is now made of a behavior of the related-art modem three-axis type optical planetarium <NUM> and a behavior of the new four-axis type optical planetarium <NUM> according to this embodiment when the diumal motion of a starry sky in the future after <NUM>,<NUM> years from today, which has been described with reference to <FIG>, is reproduced.

<FIG> is a graph for showing angles about the diumal axis <NUM>, the latitude axis <NUM>, and the azimuth axis <NUM> with respect to elapsed time when the diumal motion is reproduced with use of the related-art modem three-axis type optical planetarium <NUM>. At about <NUM> seconds when the angle about the latitude axis <NUM> is approximately <NUM>°, the Polaris <NUM> passes near the zenith <NUM>. It is understood that the rotation about the diumal axis <NUM> and the rotation about the azimuth axis <NUM> rapidly become faster around the time when the Polaris <NUM> passes near the zenith <NUM>.

On the other hand, <FIG> is a graph for showing angles about the diumal axis <NUM>, the latitude axis <NUM>, the azimuth axis <NUM>, and the swing axis <NUM> with respect to elapsed time when the similar diumal motion is reproduced with use of the new four-axis type optical planetarium <NUM> according to this embodiment. Like the case of <FIG>, at about <NUM> seconds, the Polaris <NUM> passes near the zenith <NUM>. It is understood that, with the new four-axis type optical planetarium <NUM> according to this embodiment, the rotation about the diumal axis <NUM> and the rotation about the azimuth axis <NUM> do not rapidly become faster even when the Polaris <NUM> passes near the zenith <NUM>. It is also understood that the rotation about each of the axes is smooth under other conditions.

As described above, the new four-axis type optical planetarium <NUM> according to this embodiment can operate in two modes, that is, the swing-axis fixed mode and the azimuth-axis fixed mode. In the azimuth-axis fixed mode, the optical planetarium <NUM> allows Polaris to smoothly move near the zenith through a combination of the rotation about the swing axis <NUM> and the rotation about the latitude axis <NUM>. Thus, the problem which may otherwise occur at the singular point is not caused. As a result, the optical planetarium <NUM> can smoothly and appropriately reproduce the movement of stars without causing a sudden rise in rotation velocity about the axis, such as the diumal axis <NUM> or the azimuth axis <NUM>, which may otherwise occur only with the swing-axis fixed mode.

In order to allow smooth and appropriate reproduction of any starry sky, the number of axes about which the optical planetarium is rotatable is only required to be increased. Thus, as described above, similar effects are obtained with use of the classical four-axis type. Meanwhile, as described above, the classical four-axis optical planetarium <NUM> has a large device size. In contrast, in the new four-axis type optical planetarium <NUM> according to this embodiment, the swing axis <NUM> can be added by forming the fork mount <NUM> in an arc-like shape and mounting the roller bearings along the rail having an arc-like shape. Thus, the optical planetarium <NUM> is not increased in size as a whole. According to this embodiment, the optical planetarium <NUM> that is as small and lightweight as the modem three-axis type optical planetarium can be achieved even though the optical planetarium <NUM> is of the four-axis type.

Another method is conceivable as a method of adding an axis to the modem three-axis type optical planetarium. For example, it is conceivable to add an axis so that the azimuth axis can be swung by inclining all the parts placed on the base as a whole. With such a configuration, however, an azimuth-axis inclination angle changing mechanism is required to be provided to the base, resulting in a larger device size. Accordingly, in this embodiment, a four-axis type device is achieved by fixing the azimuth axis vertically and allowing the latitude axis to be inclined. In particular, as described above in the embodiment, it is preferred that the latitude-axis inclination angle changing mechanism <NUM> have the following configuration in view of reduction in device size. That is, the swing axis <NUM> around which the latitude axis <NUM> rotates is set so as to be orthogonal to the azimuth axis <NUM> and the latitude axis <NUM> and to pass through the center of the star ball <NUM>. The fork mount <NUM> that supports the star ball <NUM> so that the star ball <NUM> is rotatable about the latitude axis <NUM> is provided in a plane containing the latitude axis <NUM> and the azimuth axis <NUM>. The latitude-axis inclination angle changing mechanism <NUM> includes the slider <NUM> provided to the fork mount <NUM>. The slider <NUM> slides on the rail <NUM> provided in an arc-like shape with the swing axis <NUM> as a center.

Description has been made of the reproduction of a starry sky that can be seen from a geographical point at a northem latitude of <NUM>° in the future after <NUM>,<NUM> years as an example. A similar operation is performed to reproduce other motions of celestial bodies. Specifically, when Polaris arranged in the vicinity of the diumal axis passes near the zenith or the nadir, although the problem described above may occur with the modem three-axis type optical planetarium, the new four-axis type optical planetarium according to this embodiment can prevent the problem. The new four-axis type optical planetarium <NUM> according to this embodiment operates in the same manner under various conditions, for example, in a case in which a starry sky that can be seen from space is to be projected, and the new four-axis type optical planetarium <NUM> always enables projection of an appropriate starry sky in such cases.

Claim 1:
An optical planetarium (<NUM>), comprising:
a projector (<NUM>) configured to project images of stars;
a diumal-axis support mechanism (<NUM>) configured to allow the projector (<NUM>) to rotate about a diumal axis (<NUM>);
a latitude-axis support mechanism (<NUM>) configured to allow the projector (<NUM>) to rotate about a latitude axis (<NUM>) that is orthogonal to the diumal axis (<NUM>);
an azimuth-axis support mechanism (<NUM>) configured to allow the projector (<NUM>) to rotate about an azimuth axis (<NUM>) that is set vertically;
characterized in that the optical planetarium (<NUM>) further comprises:
a latitude-axis inclination angle changing mechanism (<NUM>) configured to allow an angle between the latitude axis (<NUM>) and a horizontal plane to be changed within a predetermined range.