JP2007147283A - Encoder - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the modulation efficiency of an incident angle modulation type encoder of a diffraction interference mode. <P>SOLUTION: Mirrors 14A and 14B are disposed for guiding +1 order diffracted light and -1 order diffracted light so that the direction of varying the tilting of the incident light with respect to an index diffraction grating 13 become the same as the direction of varying the tilting, with respect to a moving diffraction grating 15, of reinterfered (+1, -1) diffracted light and (-1, +1) order diffracted light of the diffracted light generated by the moving diffraction grating 15. Thus, modulation of the phase of the complex amplitude of the diffracted light due to the optical path difference between the +1 order diffracted light and -1 order diffracted light from the index diffraction grating 13, and modulation of the phase of the complex amplitude of the diffracted light due to displacement of the incident positions of these diffracted lights on the moving diffraction grating 15, have the same polarity, which cause the phase difference between the (+1, -1) order diffracted light and (-1, +1) order diffracted light reinterfered by the action of the moving diffraction grating 15. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical encoder, and more specifically, irradiates light to a diffraction grating having a grating arranged orthogonal to the moving direction, and detects a change in intensity of light transmitted or reflected by the diffraction grating, The present invention relates to an encoder that detects a relative movement amount of a diffraction grating.

  Conventionally, as a general optical encoder, a diffraction grating having a grating that moves with a moving body and is formed at equal intervals perpendicular to the moving direction, and an irradiation optical system that irradiates the diffraction grating with two coherent light beams And a detector that detects the intensity change of the interference light by interfering with positive and negative diffracted lights of the same order diffracted by the diffraction grating, and detects the movement amount of the diffraction grating based on the intensity change of the interference light A diffraction interference type encoder is known (for example, Patent Document 1).

  In recent years, an optical encoder with improved S / N ratio and improved detection accuracy has been proposed (for example, Patent Document 2). The encoder includes a scale having a grating arranged along the moving direction of the moving body, and a probe that vibrates the beam along the arrangement direction of the grating on the scale. By oscillating in the direction, a signal including information on the relative position of the scale with the vibration center of the beam as a reference position is output. Then, the relative position between the beam and the scale is obtained by comparing the signal output from the probe with the drive signal for vibrating the beam.

  Recently, an improvement in the S / N ratio is also required for the above-described diffraction interference type encoder.

JP 2003-35570 A US Pat. No. 6,639,686

  According to a first aspect of the present invention, there is provided an encoder having a separation optical system that separates incident light into a plurality of lights; and an optical system that guides the plurality of separated lights onto a scale. A changing device that changes an incident angle of the incident light; and an optical member that changes an incident angle of the plurality of lights with respect to the scale in the same direction as the changing direction of the incident angle of the incident light by the changing device. This is an encoder.

  According to this, since the optical member for changing the incident angles of the plurality of lights generated by separating the incident light by the separation optical system is provided in the same direction as the incident light changing direction, the incident light is provided. Modulation of the phase of the complex amplitude of the diffracted light due to the optical path difference between the two diffracted light caused by the change of the phase, and modulation of the phase of the complex amplitude of the diffracted light due to the relative displacement of the incident position on the scale of the diffracted light However, their modulation directions can always be the same so that they do not cancel each other. As a result, the modulation efficiency of the interference signal indicating the interference state of the two diffracted lights can always be kept high.

  From a second viewpoint, the present invention provides an encoder having a separation optical system that separates incident light into a plurality of lights, and an optical system that irradiates the separated plurality of lights onto a scale. The optical system irradiates the plurality of lights on the scale from the same direction as the incident direction of the incident light with respect to the separation optical system. It is an encoder characterized by doing.

  According to this, the phase of the complex amplitude of the diffracted light due to the optical path difference between the two diffracted lights caused by the change of the incident light and the relative displacement of the incident position on the scale of those diffracted lights The modulation directions can always be the same so that the modulation of the phase of the complex amplitude does not cancel each other. As a result, the modulation efficiency of the interference signal indicating the interference state of the two diffracted lights can always be kept high.

  Hereinafter, an embodiment of the present invention will be described with reference to FIGS. FIG. 1 schematically shows a configuration of a main part of an encoder 100 according to an embodiment of the present invention. The encoder 100 is a so-called diffraction interference type encoder, and is a linear encoder that detects a moving direction, a moving amount, or a displacement of a moving body (for example, a stage) that moves in a predetermined direction.

  As shown in FIG. 1, the encoder 100 includes a light source 11, a collimator lens 12, a diffractive optical element (index diffraction grating) 13, mirrors 14A and 14B, a scale (moving diffraction grating) 15, and a light receiving element. 16, an actuator 17, and a vibrating mirror 18.

  The light source 11 is, for example, a light source that emits coherent light, for example, laser light having a wavelength λ = 850 nm in the + X direction. The collimator lens 12 disposed on the + X side of the light source 11 converts the laser light emitted from the light source 11 into parallel light.

  The oscillating mirror 18 reflects parallel light from the collimator lens 12 incident from the −X direction. The reflection direction of the parallel light is approximately the −Z direction, but strictly speaking, it is determined by the direction of the reflecting surface that varies due to the rotational vibration of the vibrating mirror 18. The operation of the encoder 100 accompanying the rotational vibration of the vibration mirror 18 will be described in detail later.

  The parallel light reflected by the vibration mirror 18 enters the index diffraction grating 13. As the index diffraction grating 13, for example, a transmission type phase grating can be adopted. The grating pitch p of the index diffraction grating 13 is set to 50 μm or less, for example, about 8 μm.

  The index diffraction grating 13 generates a plurality of diffracted lights based on the incident parallel lights. FIG. 1 shows ± first-order diffracted light generated at the center of the index diffraction grating 13 in the X-axis direction among the diffracted lights. The diffracted light emitted to the + X side indicated by the solid line is the + 1st order diffracted light, and the diffracted light emitted to the −X side indicated by the dotted line is the −1st order diffracted light. Note that the emission angles of these diffracted lights are determined by the pitch p of the index diffraction grating 13 and the wavelength λ of the laser light. As the index diffraction grating 13, the difference in the unevenness of the grating of the index diffraction grating 13 may be set to λ / 2 in order to cause a phase difference in the light passing through each grating and eliminate the 0th order light. .

  The mirrors 14A and 14B are arranged between the index diffraction grating 13 and the moving diffraction grating 15 so that the reflection surfaces thereof face each other. The + 1st order diffracted light generated by the index diffraction grating 13 is reflected by the mirror 14A, is further reflected by the mirror 14B, and enters the moving diffraction grating 15. On the other hand, the −1st order diffracted light generated by the index diffraction grating 13 is reflected by the mirror 14B, then further reflected by the mirror 14A, and is incident on the moving diffraction grating 15. In the present embodiment, the ± first-order diffracted light emitted from the same position of the index diffraction grating 13 is reflected twice by the mirrors 14A and 14B once, and then intersects at the same position on the moving diffraction grating 15. Has been. That is, the ± first-order diffracted light separated by the index diffraction grating 13 is incident on the same position of the moving diffraction grating 15.

  Similar to the index diffraction grating 13, the moving diffraction grating 15 is a transmissive phase grating. The grating pitch p of the moving diffraction grating 15 is the same as that of the index diffraction grating 13, and is set to 50 μm or less, for example, about 8 μm. In the moving diffraction grating 15, diffracted light of ± first-order diffracted light emitted from the index diffraction grating 13 is generated by the diffraction action. The outgoing angle of this diffracted light is determined by the incident angle of ± first-order diffracted light incident on the moving diffraction grating 15, the pitch p of the moving diffraction grating 15, and the wavelength λ of the laser light.

  The moving diffraction grating 15 diffracts the + 1st order diffracted light emitted from the index diffraction grating 13 to emit −1st order diffracted light, and diffracts the −1st order diffracted light emitted from the index diffraction grating 13 to emit + 1st order diffracted light. The ± first-order diffracted light emitted from the moving diffraction grating 15 travels toward the −Z side and enters the light receiving element 16 while interfering with each other. As a result, the light receiving element 16 outputs a current signal (interference signal) I indicating the interference intensity of the light (interference light) with which the ± first-order diffracted lights interfere with each other. In the following description, out of the interference light incident on the light receiving element 16, the −1st order diffracted light generated when the + 1st order diffracted light generated by the index diffraction grating 13 enters the moving diffraction grating 15 is expressed as (+1, −1). The + 1st order diffracted light generated when the -1st order diffracted light generated by the index diffraction grating 13 is incident on the moving diffraction grating 15 is referred to as (−1, +1) th order diffracted light.

<Basic principle of displacement detection>
By the way, the light source 11, the collimator lens 12, the index diffraction grating 13, the mirrors 14A and 14B, and the light receiving element 16 are fixed to a fixed object (not shown) serving as a reference for displacement measurement, and their positions are constant. . The combination of the light source 11, the collimator lens 12, the index diffraction grating 13, the mirrors 14A and 14B, and the light receiving element 16 is referred to as a detection unit for the moving diffraction grating 15. On the other hand, the moving diffraction grating 15 is fixed to a moving body (not shown) and is displaced together with the moving body. This moving body is a displacement measurement target. The displacement direction is the same as the direction in which the moving diffraction grating 15 is arranged. Here, the principle of detecting the displacement of the moving body will be described. 2A and 2A are diagrams for explaining the basic detection principle of the displacement of the moving body.

  First, it is assumed that the moving diffraction grating 15 is at the position shown in FIG. In this case, the ± first-order diffracted light generated at the point A of the index diffraction grating 13 is incident on the point B of the moving diffraction grating 15. Here, as the moving body moves in the + X direction, the moving diffraction grating 13 moves from the position shown in FIG. 2A (the position of X = 0) to the position shown in FIG. 2B by + x. Suppose you move. In this case, the ± first-order diffracted light generated at the point A of the index diffraction grating 13 is incident on the position B ′ moved by x in the −X direction from the point B.

  After this movement, the complex amplitude of the (+1, −1) order diffracted light generated in the moving diffraction grating 15 and reaching the light receiving surface of the light receiving element 16 is expressed as exp [2jπx / p] when the amplitude is normalized. Similarly, the complex amplitude of the (−1, +1) -order diffracted light is expressed as exp [−2jπx / p] when the amplitude is also normalized and expressed. Here, j is a unit complex number. That is, according to the relative displacement x of the moving diffraction grating 15 with respect to the index diffraction grating 13, the phase of the complex amplitude of the (+1, −1) order diffracted light and the phase of the complex amplitude of the (−1, +1) order diffracted light are: It changes in the opposite direction.

In this case, the interference signal I received by the light receiving element 16 is a signal having a value represented by the following equation, assuming that there is no modulation as described later.
I = | exp [−2jπx / p] + exp [2jπx / p] | ²

  That is, when the moving diffraction grating 15 moves in the X-axis direction as the moving body moves, the relative displacement between the index diffraction grating 13 and the moving diffraction grating 15 changes, and the light receiving element 16 receives the light accordingly. The phase difference between the (+1, −1) order diffracted light and the (−1, +1) order diffracted light changes, and their interference state changes. Therefore, a detection device (not shown) detects the relative displacement x of the moving diffraction grating 15 with respect to the index diffraction grating 13 based on the intensity change of the modulated interference signal I obtained from the light receiving element 16. This is the displacement of the moving body in the X-axis direction.

<About modulation>
By the way, the interference signal I from the light receiving element 16 is modulated by the vibration of the vibrating mirror 18. Below, the basic principle of the modulation will be described.

  Returning to FIG. 1, the vibrating mirror 18 has one end connected to a rotation axis parallel to the Y axis, and is rotatable about the Y axis around the rotation axis. The other end of the vibration mirror 18 is connected to the actuator 17. The actuator 17 is composed of, for example, a piezo element and can vibrate with a vibration width of several μm and a frequency of about 20 kHz to 30 kHz. The actuator 17 controls the mirror surface of the oscillating mirror so that the oscillating mirror 18 rotates about the rotation axis in the XZ plane according to a predetermined periodic signal, for example, a sine wave signal, under the control of a control device (not shown). Reciprocates in a direction perpendicular to As a result, the oscillating mirror 18 rotates and vibrates with a predetermined amplitude around the Y axis.

  Due to the rotational vibration of the oscillating mirror 18, the incident angle of the parallel light with respect to the index diffraction grating 13 varies periodically. As a result, the emission angle of ± first-order diffracted light generated in the index diffraction grating 13 is also modulated.

<Modulation by shifting the incident position of the diffraction grating>
FIG. 3 shows how the diffracted light generated in the index diffraction grating 13 changes according to the modulation of the incident angle of the incident light on the index diffraction grating 13. As shown in FIG. 3, when the light incident on the index diffraction grating 13 is inclined to the + X side with respect to the Z axis, the ± first-order diffracted light generated at the point A on the index diffraction grating 13 is With respect to the moving diffraction grating 15, as indicated by (solid line), it enters a point C shifted from the point B by −x to the −X side.

  On the other hand, FIG. 4 shows a state where the moving diffraction grating 15 is actually moved from the point B ′ further to the + X side by Δx. In this case, as shown in FIG. 4, the ± 1st-order diffracted light emitted from the point A of the index diffraction grating 13 is incident on a point C further away from the point B ′ by Δx on the −X side. That is, the emission position of ± first-order diffracted light emitted from the index diffraction grating 13 and the incident position of ± first-order diffracted light on the moving diffraction grating 15 when the incident angle of the incident light on the index diffraction grating 13 is modulated. The relationship is the same when the incident angle of the incident light to the index diffraction grating 13 is modulated and when the moving diffraction grating 15 is moved.

  In the case shown in FIG. 4, the complex amplitude of the (+1, −1) order diffracted light detected by the light receiving element 16 is exp [2jπ (x + Δx) / p], and the complex amplitude of the (−1, +1) order diffracted light. Becomes exp [−2jπ (x + Δx) / p]. That is, the phase of the complex amplitude of the (+1, −1) order diffracted light and the phase of the complex amplitude of the (−1, +1) order diffracted light have the same magnitude and change in the opposite direction. Therefore, even in the case shown in FIG. 3, the phase of the complex amplitude of the (+1, −1) order diffracted light and the phase of the complex amplitude of the (−1, +1) order diffracted light are the same as in the case shown in FIG. , They will be modulated in the opposite direction with the same magnitude.

<Modulation by optical path length>
FIG. 5 shows a change in the optical path difference of ± first-order diffracted light generated in the index diffraction grating 13. FIG. 5 shows the optical path of ± first-order diffracted light until it reaches the moving diffraction grating 15 when the mirrors 14A and 14B are not arranged. As shown in FIG. 5, the optical path of the + 1st order diffracted light guided to the moving diffraction grating 15 is such that the incident light on the index diffraction grating 13 is incident on the + X side of the incident light on the index diffraction grating 13. The vertical, that is, not tilted, becomes shorter, and the phase of the complex amplitude on the moving diffraction grating 15 advances. As a result, (+1, −1) next time generated in the moving diffraction grating 15 The phase of the complex amplitude of the folded light also advances. Further, the optical path of the −1st order diffracted light guided to the moving diffraction grating 15 is perpendicular, that is, the incident light to the index diffraction grating 13 is inclined, in contrast to the case where the incident light to the index diffraction grating 13 is inclined to the + X side. In the absence, the phase of the complex amplitude on the moving diffraction grating 15 is delayed, and as a result, the phase of the complex amplitude of the (−1, +1) -order diffracted light generated in the moving diffraction grating 15 is delayed. Will also be delayed.

  As described above, when the incident angle of the incident light on the index diffraction grating 13 is inclined, the phase of the complex amplitude of the two diffracted lights (± first-order diffracted light) re-interfering with the moving diffraction grating 15 changes. . The cause of the phase change is a pseudo-generation that occurs due to a deviation between the emission position where ± 1st-order diffracted light is emitted from the index diffraction grating 13 and the incident position where ± 1st-order diffracted light is incident on the moving diffraction grating 15. There are a change in the incident position on the moving diffraction grating 15 and a change in the optical path length of the ± first-order diffracted light from the index diffraction grating 13 to the movement diffraction grating 15.

  More specifically, when the incident angle of the incident light on the index diffraction grating 13 is inclined in the + X direction, the complex amplitude of the (+1, −1) order diffracted light received by the light receiving element 16 is incident on the moving diffraction grating 15. The phase is advanced by the shift of the position, and the phase is further advanced by the change of the optical path length of the + 1st order diffracted light. The complex amplitude of the (−1, +1) th order diffracted light received by the light receiving element 16 is the moving diffraction grating 15. The phase is delayed due to the shift of the incident position on the light, and the phase is further delayed due to the change in the optical path length of the −1st order diffracted light.

  In other words, when the incident angle of the incident light on the index diffraction grating 13 is inclined in the + X direction, the phase of the complex amplitude of the (+1, −1) order diffracted light incident on the light receiving element 16 is the moving diffraction grating 15. The phase of the complex amplitude of the (+1, −1) order diffracted light is changed by the change of the incident position on the moving diffraction grating 15 as well as the change of the optical path length of the + 1st order diffracted light. The delay is also caused by a change in the optical path length of the first-order diffracted light.

  This is the same even when the incident angle of the incident light incident on the index diffraction grating 13 is incident on the −X side. Even in this case, the phase of the complex amplitude of the (+1, −1) order diffracted light incident on the light receiving element 16 is delayed both by the change of the optical path length of the + 1st order diffracted light and by the change of the incident position on the moving diffraction grating 15. Thus, the phase of the complex amplitude of the (−1, +1) th order diffracted light is delayed both by the change of the optical path length of the −1st order diffracted light and by the change of the incident position on the moving diffraction grating 15.

  As described above, according to the present embodiment, the component of the phase change due to the optical path length of the ± 1st order diffracted light and the component of the phase change due to the change of the incident position of the ± 1st order diffracted light on the moving diffraction grating 15 are: They always have the same sign, and are mutually intensified without canceling each other. Conversely, between the diffracted lights, that is, between the (+1, −1) order diffracted light and the (−1, +1) order diffracted light, the opposite sign is obtained. Become. Further, as described above, the incident angle of the incident light incident on the index diffraction grating 13 periodically varies due to the rotational vibration of the vibrating mirror 18. As a result, the change in the phase difference between the (+1, −1) order diffracted light and the (−1, +1) order diffracted light to be modulated becomes large, and as a result, the interference signal I indicating the interference state of these diffracted lights. Thus, good modulation efficiency can be obtained.

  In the encoder 100 according to the present embodiment, high modulation efficiency is realized by reflecting ± first-order diffracted light twice each by the mirror 14A and the mirror 14B. Here, for example, consider a case where the number of reflections of diffracted light by the mirrors 14A and 14B is one. In this case, for example, if the incident light on the index diffraction grating 13 is tilted to the + X side, the ± 1st order diffracted light emission position of the index diffraction grating 13 and the ± 1st order diffracted light incident position on the moving diffraction grating 15 are Contrary to the encoder 100 according to the present embodiment, it is shifted by Δx in the + X direction. As a result, the phase of the complex amplitude of the (−1, +1) order diffracted light is delayed, and the phase of the complex amplitude of the (+1, −1) order diffracted light is advanced. That is, in this case, the phase fluctuation due to the optical path length of the ± first-order diffracted light and the phase fluctuation due to the diffraction grating are opposite to each other and cancel each other, and (+1, −1) -order diffracted light, (− The amount of change in the phase of the complex amplitude of (1, +1) is reduced, and as a result, the modulation efficiency of the interference signal I is lowered.

  In addition, as a result of intensive studies by the present inventors, the interference signal I in the case where the ± first-order diffracted light emitted from the index diffraction grating 13 is reflected only once and the case where the ± first-order diffracted light is reflected twice each. When the ± 1st order diffracted light is reflected only once, when the incident angle changes once, the amount of change in the phase difference between the two diffracted lights becomes 0.002 rad. On the other hand, in the case where the ± first-order diffracted light is reflected twice, when the incident angle changes by 1 degree, the amount of change in the phase difference between the two diffracted lights becomes 9 rad, and sufficient modulation efficiency is obtained for the encoder 100 according to the present embodiment. It has been proven that

  FIG. 6 shows the relationship between the incident angle of ± first-order diffracted light incident on the index diffraction grating 13 and the incident angle of diffracted light incident on the moving diffraction grating 15 in the encoder 100 according to the present embodiment. . In FIG. 6, in order to simplify the explanation, the X-axis direction shift of the incident position of the diffracted light on the moving diffraction grating 15 caused by the change in the incident angle of the ± first-order diffracted light incident on the index diffraction grating 13 is shown. Are omitted in the figure.

  As indicated by the dotted line in FIG. 6, when the incident angle of the incident light incident on the index diffraction grating 13 changes to the + X side with respect to the Z axis, the incidence of ± first-order diffracted light incident on the moving diffraction grating 15 The angle also changes to the + X side. Further, as indicated by a one-dot chain line in FIG. 6, when the incident angle of the incident light incident on the index diffraction grating 13 changes to the −X side, the incident angle of the ± first-order diffracted light incident on the moving diffraction grating 15. Also changes to the -X side. That is, in the encoder 100 according to the present embodiment, the direction in which the incident angle of incident light changes with respect to the index diffraction grating 13 is the same as the direction in which the incident angle of each diffracted light incident on the moving diffraction grating 15 changes. It is set to become. In other words, the incident direction incident on the index diffraction grating 13 and the incident direction of each diffracted light incident on the moving diffraction grating 15 are set to be the same direction. In this way, as described above, the phase change due to the optical path length of the ± 1st-order diffracted light and the phase change due to the change of the incident position of the ± 1st-order diffracted light on the moving diffraction grating 15 always have the same sign and cancel out. They will strengthen each other without being met.

  The detection device (not shown) that has received the modulated interference signal I obtained from the light receiving element 16 uses the vibration frequency ω (described above) of the vibration mirror 18 driven by the actuator for this interference signal I with sufficient modulation efficiency. The synchronous detection is performed using a sine wave signal that is an integral multiple of the modulation frequency ω. By this synchronous detection, the zero-order component, the first-order component, the second-order component, the third-order component, the fourth-order component,...

  In this detection apparatus, a sine signal (sin (4πx / p) representing a displacement x with a phase shifted by 90 degrees from a specific frequency component of a time change of the interference signal I, for example, a primary component and a secondary component of the modulation frequency ω. )) And a cosine signal (cos (4πx / p)). That is, according to the encoder 100 according to the present embodiment, the time signal from the light receiving element 16 is changed with time even though only one light receiving element 16 is provided. Both the cosine signal and the cosine signal can be obtained.

  In the detection device, among the detected components, the zero-order component can be used for light amount control of the light source 11, and the ratio between the primary component and the tertiary component or the secondary component and the tertiary component. The ratio may be used for fluctuation control (that is, modulation degree control) of the incident angle of the incident light on the index diffraction grating 13 by the rotation signal of the vibrating mirror 18 by the actuator 17.

  As described above, by changing the incident angle of the incident light on the index diffraction grating 13, the modulation frequency of the phase of the complex amplitude of ± first-order diffracted light due to the change of the incident position on the moving diffraction grating 15, and ± 1 next time Since the modulation frequency of the phase of the complex amplitude of each diffracted light due to the change in the optical path length of the folded light is the same (both are ω), the detection device can also detect the sine signal and the cosine signal synchronously at the same frequency ω. it can.

  As described above in detail, according to the encoder 100 according to the present embodiment, the incident light on the index diffraction grating 13 is inclined with respect to the index diffraction grating 13 and ± 1 next time incident on the moving diffraction grating 15. Mirrors 14A and 14B for guiding the + 1st order diffracted light and the −1st order diffracted light are arranged so that the direction in which the folded light is inclined is the same. Thus, the + 1st order diffracted light from the index diffraction grating 13 and − which causes the phase difference between the (+1, −1) order diffracted light and the (−1, +1) th order diffracted light that re-interfer by the action of the moving diffraction grating 15. The modulation direction of the complex amplitude phase of each diffracted light due to the optical path difference of the first-order diffracted light is the same as the modulation direction of the complex amplitude phase of the diffracted light due to the shift of the incident position of the diffracted light on the moving diffraction grating 15. Direction. As a result, the fluctuation of the phase difference between the (+1, −1) order diffracted light and the (−1, +1) order diffracted light can be increased, and the interference is a signal indicating the interference result of these diffracted lights. Good modulation efficiency can be obtained for the signal I.

  Due to this action, the encoder 100 can be variously designed in design. For example, even if the incident angle of the incident light on the index diffraction grating 13 is small, sufficient modulation efficiency can be obtained. Therefore, the vibration amplitude of the actuator 17 is reduced and the vibration amplitude for vibrating the vibration mirror 18 is reduced. be able to. As a result, the driving voltage for driving the actuator 17 can be reduced. As a result, it is possible to improve the durability thereof, and it is possible to realize reduction of noise at the carrier frequency.

  In the present embodiment, the two mirrors 14A and 14B are arranged so as to face each other, and ± first-order diffracted light generated by the index diffraction grating 14 using the two mirrors 14A and 14B is transferred onto the moving diffraction grating 13. However, a mirror may be placed at each position where the diffracted light is reflected. That is, four mirrors may be individually arranged at the reflection position of the diffracted light.

  In the present embodiment, when the ± first-order diffracted light generated in the index diffraction grating 13 is guided to the moving diffraction grating 15, the number of reflections reflected by the mirrors 14A and 14B is set to two. However, in the present invention, The number of reflections is not limited to two. The point is that the number of reflections is such that the direction in which the incident angle of the incident light changes with respect to the index diffraction grating 13 is the same as the direction in which the incident angle of each diffracted light incident on the moving diffraction grating 15 changes. Good.

  In the above embodiment, the mirrors 14A and 14B are employed as the optical system for guiding the ± first-order diffracted light generated by the index diffraction grating 13 onto the moving diffraction grating 15, but instead of these, prisms, beam splitters, or A lens may be used. For example, a lens group that forms an erect image on the moving diffraction grating 15 instead of the inverted image of the index diffraction grating 13 may be disposed between the index diffraction grating 13 and the moving diffraction grating 15.

  That is, in order to guide the ± first-order diffracted light generated in the index diffraction grating 13 onto the moving diffraction grating 15, not only a reflection optical system using a mirror but also a refractive optical system that refracts the diffracted light is used. it can. In short, in the present invention, the ± first-order diffracted light generated in the index diffraction grating 13 is deflected a plurality of times, and then the direction in which the incident angle of incident light changes with respect to the index diffraction grating 13 and the moving diffraction grating 15 An optical system that guides each diffracted light may be used so that the direction in which the incident angle of each diffracted light incident on the light beam changes is the same.

  In the above embodiment, a piezoelectric element is used as the actuator 17, but a magnetostrictive element, a voice coil motor, a rotary motor, or the like may be used. That is, the present invention is not limited to actuators. In addition, for example, a vibrating mirror having a surface coated with a reflective film may be used.

  Moreover, in the said embodiment, although the incident angle of the incident light which injects into the index diffraction grating 13 was changed by the rotational vibration by the vibration mirror 18, this invention is not limited to this.

  FIG. 7 shows a schematic configuration of an encoder 101 as another encoder to which the present invention can be suitably applied. As shown in FIG. 7, the encoder 101 is different from the encoder 100 according to the above embodiment in that a light source 11 ′ is employed instead of the modulation mechanism of the modulation mirror 18 and the actuator 17.

  The light source 11 'is a point light source array in which a plurality of point light sources are arranged at a predetermined pitch. In this light source 11 ′, point light sources are arranged in a line in the X-axis direction, and the arrangement pitch is set to be sufficiently smaller than the pitch of the index diffraction grating 13 and the like. A surface emitting laser light source can be adopted as each point light source.

  These point light sources are driven by a light source driving device (not shown). The light source driving device selectively switches a light source to be lit among a plurality of point light sources of the light source 11 ′, and periodically varies the lighting position. The laser light emitted from any one of the point light sources is converted into parallel light by the collimator lens 12 and then enters the index diffraction grating 13, and is index diffracted according to the position of the point light source that emits light. The incident angle of the parallel light incident on the light 13 can be changed. Therefore, if the light emission patterns of a plurality of point light sources have periodicity, the incident angle is modulated similarly to the encoder 100 according to the above embodiment, and the same as the encoder 100 according to the above embodiment. High modulation efficiency can be obtained.

  Since such switching of the lighting position can be performed at a higher speed than changing the position and orientation of the object as in the case of the rotational vibration of the oscillating mirror 18, the modulation frequency of the periodic modulation is related to the above embodiment. It can be higher than the encoder, for example, it can be set to the MHz level.

  In addition, the incident light is incident on the diffraction grating by using a combination of a spatial filter such as a liquid crystal and a condensing lens as well as an encoder that can change the incident angle with a light source having a plurality of point light sources. You may make it employ | adopt the encoder which changes the incident angle of light. If the position at which the laser light passes through the spatial filter is changed in the X-axis direction according to a predetermined periodic signal, and the laser light that has passed through the spatial filter is refracted by the condenser lens, the incident light incident on the diffraction grating The incident angle can be changed.

  Further, the position of the light source itself may be periodically changed in the X-axis direction using an actuator such as a piezo element. In addition, an optical element that can freely change the refractive index of the transmitted light, such as an acousto-optic element (AOM) or an electro-optic element (EOM), is disposed between the light source and the index diffraction grating. The incident angle of the incident light with respect to the index diffraction grating may be changed by supplying a signal or the like that periodically changes the refractive index to the optical element.

  In the above-described embodiment, the incident angle of incident light is changed with respect to the index diffraction grating 13, but instead, the index diffraction grating 13 is rotated and oscillated around the Y axis, and its posture is periodically changed. You may make it make it. Even in this case, the incident angle of the incident light with respect to the index diffraction grating 13 changes periodically, and high modulation efficiency can be obtained as in the encoder 100 according to the above embodiment.

  Further, the arrangement relationship between the index diffraction grating 13 and the moving diffraction grating 15 may be reversed. That is, a diffraction grating that receives parallel light modulated by the vibrating mirror 18 may be a moving diffraction grating, and a diffraction grating that receives diffraction light generated by the diffraction grating may be an index diffraction grating. Further, the number of diffraction gratings is not limited to two and may be one. In this case, the encoder is of a type in which diffracted light generated with respect to incident light incident on a predetermined location on a single reflective diffraction grating is incident again on a different location of the diffraction grating. . Moreover, the encoder provided with three or more diffraction gratings may be sufficient. In the above embodiment, the detection unit is fixed to a fixed object and the moving diffraction grating 15 is fixed to a moving body. However, the moving diffraction grating 15 and the detection unit only have to move relatively. The detection unit may be fixed to the moving body by fixing the moving diffraction grating 15 to a fixed object (referred to as a fixed diffraction grating when fixed to the fixed object). Further, although the description has been given using the moving diffraction grating as the scale, a scale in which a pattern in which light transmitting portions and light shielding portions are alternately arranged may be used.

  In the above embodiment, the index diffraction grating 13 and the moving diffraction grating 15 are transmissive phase gratings. However, the index diffraction grating 13 may be a reflective diffraction grating. FIG. 8 shows an encoder 102 as an example of an encoder when the index diffraction grating 13 is a reflection type diffraction grating. Since this encoder 102 further includes prisms 19A and 19B, the number of reflections of ± first-order diffracted light reflected by the mirror 14A and the mirror 14B is one.

  Since the index diffraction grating 13 ′ is a reflection type diffraction grating, as shown in FIG. 8, ± first-order diffracted light generated by the index diffraction grating 13 ′ is emitted to the + Z side. The prism 19A emits the −1st order diffracted light after being totally reflected twice, and the prism 19B emits the + 1st order diffracted light after being totally reflected twice. The −1st order diffraction grating emitted from the prism 19 </ b> A is reflected once by the mirror 14 </ b> A and enters the moving diffraction grating 15. Further, the + 1st order diffraction grating emitted from the prism 19B is reflected by the mirror 14B, and enters the position substantially the same as the incident position of the -1st order diffracted light of the moving diffraction grating 15. That is, the encoder 102 uses an optical system that reflects the diffracted light generated by the index diffraction grating 13 three times and enters the moving diffraction grating 13.

  Even with such an encoder 102, high modulation efficiency can be obtained as in the encoder 100 according to the above-described embodiment. The moving diffraction grating 15 can also be a reflection type diffraction grating. In this case, if the light receiving element 16 is disposed on the + Z side of the moving diffraction grating 15 with the light receiving surface thereof set as the -Z side. Good.

  The pitches of the index diffraction grating 13 and the moving diffraction grating 15 are not necessarily the same, but the exit directions of the diffracted lights generated by the index diffracted light 13 and the moving diffraction grating 15 are the light wavelength λ, Since it is determined by the pitch, the positional relationship between the optical system between the index diffraction grating 13 and the moving diffraction grating 15, the light receiving element 16, and the like is appropriately determined by the pitch of these diffraction gratings. For example, when the pitches of the index diffraction grating 13 and the moving diffraction grating 15 are different, the reflection surface of the mirror 14A and the reflection surface of the mirror 14B are not arranged in parallel to each other, but according to the ratio of the pitches. Will be placed.

  Thus, the optical system in the encoder can be variously modified. For the sake of space, in the embodiment and the like, all the optical elements of the optical system in the encoder are all arranged in the XZ plane. However, the invention is not limited to this, and the diffracted light generated by the index diffraction grating 13 is moved to the moving diffraction grating 15. It goes without saying that various optical elements of the optical system guided upward may be arranged three-dimensionally.

  In the above embodiment, ± 1st-order diffracted light is used as measurement light, but the present invention is not limited to this. Further, re-interference light of higher-order diffracted light may be used as measurement light, or between diffracted lights of different orders such as 0th order and nth order (or -nth order), + nth order and + (m + n) th order. Re-interference light may be used as measurement light. In the above embodiment, an index diffraction grating is used as the diffractive optical element. However, a beam splitter that splits coherent light emitted from the light source into two may be used. Even when a beam splitter is used, each of the two lights divided by the beam splitter may be guided to the same position on the moving diffraction grating 15 to cause interference.

  In the above embodiment, the index diffraction grating 13 and the moving diffraction grating 15 are phase gratings, but it is needless to say that they may be amplitude type diffraction gratings.

  In the above embodiment, the periodic signal followed by the vibration of the actuator 17 that vibrates the modulation mirror 18 is a sine wave. However, this periodic signal may be a triangular wave or a sawtooth wave. In short, the modulation signal may be a predetermined periodic signal.

  The encoder 100 according to the present embodiment is a linear encoder that detects displacement in one axial direction of a moving body, but the present invention can also be applied to a rotary encoder that detects the amount of rotation of a rotating body. Of course.

  The wavelength of the laser beam and the value of the pitch p of each diffraction grating in the above embodiment are merely examples, and are appropriately determined according to the resolution required for the encoder. The smaller the pitch p of the diffraction grating, the higher the resolution of the encoder.

  As described above, the encoder of the present invention is suitable for detecting the displacement of the moving body.

It is a figure which shows schematic structure of the encoder which concerns on one Embodiment of this invention. FIG. 2A is a diagram (part 1) for explaining a basic detection principle of displacement of a moving body, and FIG. 2B is a diagram for explaining a basic detection principle of displacement of the moving body. FIG. It is a figure which shows the mode of a diffracted light when the incident angle of the incident light which injects into an index diffraction grating is modulated. It is a figure which shows the state which the moving diffraction grating actually moved only (DELTA) x to the -X side. It is a figure which shows the modulation | alteration of the phase of diffracted light by the change of optical path length. It is a figure for demonstrating the relationship between the incident light which injects into an index diffraction grating, and the diffracted light which injects into a movement diffraction grating. It is a figure which shows the modification (the 1) of an encoder. It is a figure which shows the modification (the 2) of an encoder.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11, 11 '... Light source, 12 ... Collimator lens, 13 ... Index diffraction grating, 14A, 14B ... Mirror, 15 ... Moving diffraction grating, 16 ... Light receiving element, 17 ... Actuator, 18 ... Vibrating mirror, 19A, 19B ... Prism, 100, 101, 102. Encoder.

Claims (13)

A separation optical system for separating incident light into a plurality of lights;
An encoder having an optical system for guiding the separated plurality of lights onto a scale;
A changing device for changing an incident angle of the incident light with respect to the separation optical system;
An encoder comprising: an optical member that changes an incident angle of the plurality of lights with respect to the scale in the same direction as a changing direction of the incident angle of the incident light by the changing device.   The encoder according to claim 1, wherein the optical member deflects each of the plurality of lights separated by the separation optical system a plurality of times. The optical member is
The encoder according to claim 1, wherein the encoder forms part of the optical system. The separation optical system includes:
The encoder according to claim 1, further comprising a diffractive optical element having a diffraction grating. The changing device is:
The encoder according to claim 4, wherein an incident angle of the incident light is changed by periodically changing an attitude of the diffractive optical element. A light source that emits coherent light as the incident light;
The changing device is disposed between the light source and the diffractive optical element, and deflects the light emitted from the light source with respect to the diffractive optical element, and a drive mechanism that drives the deflecting element. The encoder according to claim 4, further comprising: A plurality of point light sources that emit coherent light as the incident light and are arranged along the arrangement direction of the grating of the diffractive optical element;
5. The encoder according to claim 4, wherein the changing device controls the plurality of point light sources according to a predetermined periodic signal so as to alternatively emit the light from the plurality of point light sources. A separation optical system that separates incident light into a plurality of lights;
An encoder having an optical system for irradiating the separated light onto the scale;
A changing device that periodically changes the incident direction of the incident light with respect to the separation optical system;
The encoder is characterized in that the optical system irradiates the plurality of lights on the scale from the same direction as the incident direction of the incident light with respect to the separation optical system. The optical system is
The encoder according to claim 8, further comprising a deflecting optical member that deflects each of the plurality of lights separated by the separation optical system a plurality of times.   The encoder according to claim 8 or 9, wherein the separation optical system includes a diffractive optical element having a diffraction grating.   The encoder according to claim 10, wherein the changing device changes the incident angle of the incident light by periodically changing the posture of the diffractive optical element. A light source that emits coherent light as the incident light;
The changing device is disposed between the light source and the diffractive optical element, and deflects the light emitted from the light source with respect to the diffractive optical element, and a drive mechanism that drives the deflecting element. The encoder according to claim 10, further comprising: A plurality of point light sources that emit coherent light as the incident light and are arranged along the arrangement direction of the grating of the diffractive optical element;
The encoder according to claim 10, wherein the changing device controls the plurality of point light sources according to a predetermined periodic signal so as to selectively emit the light from the plurality of point light sources.

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