1. Technical Field of the Invention
The present invention relates to an optical encoder provided by combining a light emitting element and a light receiving element. More specifically, the present invention relates to an optical encoder using a light guiding member (light guide) on the light receiving element.
2. Prior Art
Generally, an optical encoder has a rotary disk plate and a stationary mask plate that are closely disposed parallel to each other. A light emitting element and a light receiving element face to each other so as to sandwich the plates. An optical axis connecting the light emitting element and the light receiving element is disposed parallel to a rotary shaft of the rotary disk plate. The rotary disk plate rotates around the rotary shaft. The stationary mask plate is fixed to a body that holds the rotary shaft. Near an outside periphery of the rotary disk plate, a specified number of slits are radially formed at regular intervals. In this specification, a term xe2x80x9crotary slitxe2x80x9d may be used hereafter to represent a slit formed on the rotary disk plate. Since the number of rotary slits corresponds to the resolution, this specification may use the number of rotary slits to represent resolution P. The stationary mask plate also has one or more slits having almost the same shape as the rotary slits at the same intervals as for them. In this specification, a term xe2x80x9cstationary slitxe2x80x9d is used hereafter to represent a slit formed on the stationary mask plate. In some cases, the stationary slit may be referred to as an aperture in distinction from the slit formed on the rotary disk plate. The rotary slit and the stationary slit are formed by cutting out light blocking materials such as metal and resin. Alternatively, slits may be formed by using a material such as translucent glass or resin, forming a light blocking film on either surface thereof, and selectively etching the light blocking film. The width of the slit formed in this manner approximates to a half of the slit interval. According to the above-mentioned configuration, revolving the rotary disk plate changes a relative position between the rotary slit and the stationary slit. Light irradiated from the light emitting element is repeatedly blocked and passed to be an intermittent light flux and enters the light receiving element. The light receiving element converts the intermittent light flux into a cyclical electric signal for output. The cycle or frequency of the electric signal can be used to detect the number of revolutions of the rotary disk plate.
Such an optical encoder is disclosed in Japanese Patent Unexamined Publication No. 1996-005407, Japanese Patent Unexamined Publication No. 1997-196703, Japanese Patent Unexamined Publication No. 1998-332432, and earlier Japanese Patent Application No. 2002-150268 (not yet publicated)
In some cases, the optical encoder generates a plurality of light fluxes having intermittent phases deviated from each other. The light receiving element outputs corresponding electric signals having phases shifted from each other. When detecting a rotation direction as well as the number of revolutions, for example, the light receiving element outputs two electric signals having phases shifted from each other for 90 degrees. It is possible to detect a rotation direction of the rotary disk plate based on the relative phase relationship between 2-phase electric signals. When it is necessary to output electric signals with different phases, a plurality of stationary slits is disposed at specified angular intervals (nxc3x97Ap+Af) against a series of circumferentially arranged rotary slits (hereafter also referred to as a track in this specification). Here, n signifies a positive integer between 0 and resolution P, Ap an angular interval of the rotary slit, and Af an angle difference equivalent to the needed phase difference. In this case, when n=0, the stationary slits are disposed most closely. When n=P/2, the stationary slits are disposed most separately along the radial direction. Alternatively, there are disposed two series of radially separated stationary slits on the stationary mask plate. In this case, Af denotes an angular displacement between two series of stationary slits. By contrast, it may be preferable to provide the rotary disk plate with two concentric tracks and relatively shift positions of the rotary slits formed on each track as much as Af. In this case, the stationary slits corresponding to the tracks just need to be shifted from each other as much as nxc3x97Ap. Also in this case, when n=0, the stationary slits are disposed most closely. When n=P/2, the stationary slits are disposed most separately along the radial direction.
It is ideal to uneccentrically mount the rotary disk plate against the rotary shaft. Actually, however, the rotary disk plate is slightly eccentric against the rotary shaft due to various error factors. Accordingly, there is an eccentricity also on the circular track against the rotary shaft when the circular track is formed on the rotary disk plate and comprises a series of rotary slits. Such eccentricity causes a phase difference between electric signals to deviate from the preset phase difference in accordance with revolutions of the rotary disk plate, generating a so-called phase variation. The eccentricity of rotary slits causes a maximum phase variation under the condition of n=P/2 or a minimum phase variation under the condition of n=0 or P in outputs from each light receiving element. When there are two tracks of rotary slits or stationary slits separately along the radial direction, decreasing an interval between the tracks along the radial direction also decreases a phase variation. That is to say, narrowing an interval for disposing the light receiving elements decreases a phase variation in output electric signals. As a means for narrowing a spacing for disposing the light receiving elements, it is possible to accumulate a plurality of light receiving areas on the same chip. However, accumulating a plurality of light receiving areas on a single chip increases costs of the light receiving element. On the contrary, if we use a low-price, general-purpose product having one light receiving area per chip, it is impossible to dispose chips adjacently to each other. A remarkable phase variation results. It is difficult to provide high resolutions.
The present invention has been made in consideration of the above-mentioned problems of the prior art. It is therefore a major object of the present invention to provide a low-cost, high-resolution optical encoder capable of decreasing output phase variations due to eccentricity of a rotary slit. In addition, it is a minor object of the present invention to improve the efficient use of light fluxes guided from a light emitting element to a light receiving element.
To achieve these objects, there is provided an optical encoder comprising a rotary disk plate and a stationary mask plate disposed parallel to each other, and a light emitting element and a light receiving element disposed opposite to each other with respect to a set of the rotary disk plate and the stationary mask plate, wherein the rotary disk plate has slits formed along a circumferential direction thereof at a given interval, and rotates to intermittently pass light from the light emitting element through the slits such that the passed light has a periodical intensity variation. The stationary mask plate has apertures spatially shifted from each other, and splits the passed light by the apertures into at least two light fluxes with a given spacing, the two light fluxes having different phases of the periodical intensity variation due to the spatial shift of the apertures. There are provided at least two light receiving elements which receive the two light fluxes respectively, and generate at least two electric signals having a cycle corresponding to the periodical intensity variation of the light fluxes and having different electric phases corresponding to the different phases of the periodical intensity variations. The optical encoder further comprises a light guiding member provided between the stationary mask plate and the light receiving elements to guide the light fluxes from the stationary mask plate to the respective light receiving elements while expanding the spacing between the light fluxes such that the light receiving elements are spaced from each other at a distance greater than the spacing of the light fluxes. The light guiding member has an incident face to admit the light flux and an exit face to send the light flux to the light receiving element. At least the incident face is convexly curved to converge the light flux, thereby efficiently transmitting the light flux to the exit face.
Preferably, the exit face is also convexly curved to condense the light flux, thereby efficiently sending the light flux to the light receiving element.
In a specific form, the stationary mask plate has the apertures which are arranged along the circumferential direction of the rotary disk plate, and which are divided into two groups spaced from each other in a radial direction of the rotary disk plate, and the two groups of the apertures split the light intermittently passed through the rotary disk plate into the two light fluxes with leaving the spacing therebetween.
Alternatively, the stationary mask plate has the apertures which are arranged along the circumferential direction of the rotary disk plate, and which are divided into two groups spaced from each other in the circumferential direction, and the two groups of the apertures split the light intermittently passed through the rotary disk plate into the two light fluxes with leaving the spacing therebetween.
Expediently, the two light receiving elements are both accommodated inside an outer periphery of the rotary disk plate for miniaturization.
According to the general feature of the present invention, a compact optical rotary encoder is provided with a light guiding member (light guide) between a stationary slit and a light receiving element. The light guide admits a plurality of light fluxes with different phases in a narrow area, and then guides the light fluxes toward a plurality of light receiving elements separately disposed in a wider area. The stationary slits (apertures) for each output phase are disposed in as narrow an area as possible. The low-price light receiving elements are disposed in a wide area with a sufficient spacing. The light guide is inserted between the stationary slit disposed in a narrow area and the light receiving elements disposed in a wide area to guide the light fluxes to the light receiving elements. For example, the light guide is so shaped and structured that an incident light flux can repeat the total reflection and radiate. There is adjacently disposed a plurality of stationary slits having different spatial phases. From the stationary slits, the light guide guides light fluxes to the separately disposed light receiving elements with a minimal loss in the amount of received light. Using such light guide, it is possible to provide the low-price optical rotary encoder excellent in the phase accuracy of output signal waveforms.
In the compact optical rotary encoder according to the specific feature of the present invention, the light guiding member comprises a transparent component including an incident face to admit a light flux and an exit face to issue the light flux to each light receiving element. The total reflection surface is used for all parts of the light guiding member except the incident face and the exit face. At least the incident face is convexly curved to condense and efficiently guide light fluxes to the exit face. The incident face of the light guiding member (light guide) is shaped to be a convex lens so that diffused light fluxes from the light emitting element refract on the convex lens surface at the incidence and becomes almost parallel rays. In this manner, when the light flux forms an incident angle against the reflective surface between the incident face and the exit face, the incident angle becomes smaller than the critical angle. The light flux is efficiently guided to the exit face by stably repeating the total reflection. When the exit face of the light guide is convexly curved, it is possible to collect light fluxes after repeating the total reflection and send them to each light receiving element. This is useful when light fluxes radiated from the light guides diffuse before reaching the light receiving elements to decrease the amount of received light. Shaping the exit face into a convex lens makes it possible to reliably converge light fluxes at the light receiving element.