JP2007170938A - Encoder - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To simplify a circuit constitution of an encoder, explaining it in detail, concerning a photoelectric encoder for detecting displacement of a moving object. <P>SOLUTION: A triangular wave is used for a signal f(t) used for modulation. In this case, a signal I(t) after modulation is observed repeatedly following the triangular wave in an optical sensor. In a detection device, cos(kωt)(k=1, 2, etc.) is used as a reference signal, and synchronous detection of a frequency component is executed. By this synchronous detection, each coefficient component of cos(ωt) and cos(2ωt) is acquired, and cos(x) and sin(x) included in each coefficient component are acquired. Since cos(x) and sin(x) are signals showing the displacement x, a scale relative to a head part, namely, relative displacement information in the X-axis direction of a moving body can be acquired from the values. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an encoder, and more particularly, to a photoelectric encoder that detects displacement of a moving object.

  Conventionally, a scale having a periodic pattern is irradiated with spot light that vibrates in the measurement direction according to a predetermined periodic signal, and based on a detection signal corresponding to the spot light state that varies depending on the irradiation position on the scale. An encoder that detects displacement information of a moving object (moving body) having a fixed scale is used (for example, see Patent Document 1). The detection signal is modulated by the vibration of the spot light, and when detecting the displacement information of the moving body, it is necessary to demodulate the signal.

  In the modulation-type optical encoder disclosed in Patent Document 1, the predetermined periodic signal for vibrating the spot light is a sine wave. In order to detect displacement information of the moving body from the signal modulated by the sine wave, a detection circuit for detecting both the amplitude and phase of the modulation signal is required, and the circuit configuration becomes complicated.

US Pat. No. 6,639,686

  According to a first aspect of the present invention, a scale having a pattern arranged in a predetermined direction; a detection of the scale pattern detected with a modulation signal that varies according to a periodic function including at least two frequency components An encoder comprising: a detection head that outputs a signal; and a detection device that detects relative position information between the scale and the detection head based on a phase or amplitude of a detection signal output from the detection head.

  According to this, a detection signal of a scale pattern modulated with respect to a predetermined direction by a modulation signal that varies according to a periodic function including at least two frequency components is detected. If a signal including at least two frequency components is used as a modulation signal instead of a signal that does not include one frequency component such as a sine wave, either the phase or the amplitude of the detection signal is Since the signal changes according to the phase of the signal before modulation, the relative displacement information of the scale with respect to the head can be detected based on one of them. As a result, since it is not necessary to detect both the phase and amplitude of the detection signal, the configuration of the detection circuit can be simplified.

  Hereinafter, an embodiment of the present invention will be described with reference to FIGS. FIG. 1 shows a schematic configuration of a main part of an encoder 100 according to an embodiment of the present invention. The encoder 100 irradiates a scale 20 fixed to a moving body (not shown) that can move in the X-axis direction, and detects displacement information of a moving object (moving body) based on the reflected light. It is an optical encoder.

  As shown in FIG. 1, the encoder 100 includes a head unit 16, a detection device 18, and a scale 20. The head unit 16 is a detection head for detecting relative displacement with respect to the scale 20, and the detection device 18 detects the relative displacement based on a signal including information on the relative displacement detected by the head unit 16. It is a device to detect. The scale 20 is a scale that serves as a scale for detecting the relative displacement.

  The head unit 16 includes a laser diode 3, a collimator lens 4, a beam splitter 6, an objective lens 7, a focus lens 8, and an optical sensor 9. The objective lens 7 is connected to the drive device 11 and is configured to be able to vibrate in the X-axis direction when driven by the drive device 11.

  The scale 20 is attached on a moving body (not shown). On the scale 20, the reflective grating 1 having periodicity in the moving direction (X-axis direction) of the moving body is formed. The grating 1 is, for example, an uneven surface type diffraction grating, and the surface shape thereof is a sine wave shape. Further, the pitch (period) p is the same in all sections. p is 50 μm or less, for example, 2 μm or 1.6 μm.

  A laser beam (wavelength is 640 nm, for example) emitted from the laser diode 3 is collimated by the collimator lens 4, passes through the beam splitter 6, and is condensed on the grating 1 on the scale by the objective lens 7. The driving device 11 generates a voltage (having a frequency of, for example, 1.5 KHz) that fluctuates according to a predetermined periodic signal input from the outside, and an internal piezoelectric element vibrates the objective lens 7 in the X-axis direction according to the voltage. ing. This vibration realizes beam deflection, and the laser beam vibrates in the X-axis direction on the grating 1 on the scale 20.

  The laser beam reflected on the grating 1 passes through the objective lens 7, is bent by the beam splitter 6, and is received by the optical sensor 9 via the focus lens 8. The optical sensor 9 is composed of a photodiode or the like. A current signal corresponding to the light reception result of the optical sensor 9 is converted into a voltage signal by an unillustrated IV converter. The voltage signal is sent to a detection device 18 (not shown).

  Thus, the encoder 100 outputs a laser beam toward the scale 20, receives the laser beam (beam probe) reflected by the grating 1 of the scale 20 by the optical sensor 9, and outputs a current signal corresponding to the light reception result. Output. In the encoder 100, the beam probe is vibrated in the X-axis direction according to a predetermined periodic signal. Therefore, the signal output from the optical sensor 9 is a signal including the signal component of the spatial angular frequency ω ′ of the diffraction grating on the scale 18 and the frequency component of a predetermined periodic signal. In other words, this output signal is a modulated signal obtained by modulating the signal of the spatial angular frequency ω ′ of the diffraction grating on the scale 18 with a predetermined periodic signal.

  As shown in FIG. 2, in the present embodiment, the predetermined periodic signal f (t) is a triangular wave. The triangular wave f (t) can be expressed by a Fourier series as shown by the following equation.

ここで、ωは、三角波に含まれる基本波成分の振動角周波数であり、ｔは時間である。上記式（１）からもわかるように、三角波は、少なくとも２つの周波数成分を含む周期信号である。

Here, ω is the vibration angular frequency of the fundamental wave component included in the triangular wave, and t is time. As can be seen from the above equation (1), the triangular wave is a periodic signal including at least two frequency components.

  When the predetermined periodic signal is a triangular wave, the optical sensor 9 observes a current signal I (t) such that the pattern signal corresponding to the pattern of the grating 1 that the beam probe hits is modulated by the triangular wave. . The current signal I (t) observed by the optical sensor 9 can be expanded in the Fourier series as shown by the following equation.

上記式（１）の右辺において、第１項は信号の直流（ＤＣ）成分であり、第２項は、変調周波数ωの基本波成分であり、第３項は、変調周波数ωの２次高調波成分であり、第４項は、変調周波数ωの３次高調波成分である。すなわち、三角波ｆ（ｔ）は、複数の周波数成分を含んでいる。

In the right side of the above equation (1), the first term is the direct current (DC) component of the signal, the second term is the fundamental component of the modulation frequency ω, and the third term is the second harmonic of the modulation frequency ω. The fourth term is a third harmonic component of the modulation frequency ω. That is, the triangular wave f (t) includes a plurality of frequency components.

  FIG. 2 shows the relationship between the pattern signal based on the pattern of the grating 1, the triangular wave f (t) used for modulation, and the detection signal I (t) detected by the optical sensor 9.

  The detection device 18 performs coherent detection of each frequency component using cos (kωt) (k = 1, 2,...) As a reference signal. By this synchronous detection, coefficient components of cos (ωt) and cos (2ωt) are obtained, and the only unknown components, cos (x) and sin (x), are obtained. Since cos (x) and sin (x) are signals representing the displacement x, the relative displacement information of the scale 18 relative to the head unit 20, that is, the moving body in the X-axis direction can be acquired from these values. .

  That is, when the periodic signal f (t) is a triangular wave represented by the above formula (1), synchronous detection based on the phase of the frequency component obtained by expanding the triangular wave f (t) by Fourier series is performed. The amplitude of the frequency component is detected, and the relative displacement of the scale 18 with respect to the head unit 20 is detected based on the magnitude of the amplitude. Therefore, in the present embodiment, it is not necessary to detect the phase of the modulated signal I (t) in the detection device 18, and it is not necessary to provide a circuit therefor, so that the circuit configuration of the detection device 18 is simplified. be able to.

  Then, when the relative displacement x fluctuates by a predetermined value (one period of the grating) or more, the detection device 18 adds the value of the up / down counter to which the counter value is incremented or decremented and the relative displacement x, Output as an output of the encoder 100.

  As described above in detail, according to the present embodiment, the pattern of the scale 20 that is modulated in the X-axis direction with the triangular wave f (t) that is the modulation source signal that varies according to the periodic function including at least two frequency components. Detection signal I (t) is detected. If a triangular wave f (t) including at least two frequency components is used as a modulation source signal instead of a signal including only one frequency component such as a sine wave, the modulation source signal I (t) is used as the modulation source signal I (t). Since the amplitude of the included frequency component changes according to the phase of the pattern signal before modulation, the relative displacement information of the scale 20 with respect to the head unit 16 can be detected based only on the amplitude. . As a result, since it is not necessary to detect both the phase and amplitude of the detection signal, the configuration of the detection device 18 can be simplified.

  In this embodiment, the beam probe is oscillated by vibrating the objective lens 7 by the driving device 11, but the present invention is not limited to this. For example, as shown in FIG. 3, on the optical path of the laser beam output from the laser diode 3, an acoustic optical device A / O that causes a change in diffraction angle or an electronic device E / that causes a change in refractive index. An optical device 5 such as O may be inserted, a triangular wave voltage signal or the like may be added to the optical device 5, and the diffraction angle and refractive index may be varied according to the triangular wave to oscillate the beam probe on the grating 1. Alternatively, a beam probe may be oscillated by arranging a mirror capable of rotational vibration on the optical path of the laser beam output from the laser diode 3 and vibrating the mirror in accordance with a triangular wave.

<Square wave>
Further, the predetermined periodic signal is not limited to a triangular wave. For example, a rectangular wave as shown in FIG. 4 may be used. In this case, as shown in FIG. 4, the modulated signal I (t) output from the optical sensor 9 is also a rectangular wave.

  In this case, the amplitude of the rectangular wave f (t) used for modulation is defined to be π / 2 with respect to the period of the original pattern signal. In this case, the amplitude of the modulated signal I (t) (rectangular wave) is in accordance with the signal level of the pattern signal before modulation at a place where the phase differs by π / 2. More specifically, the magnitude of the amplitude of the modulated rectangular wave I (t) can be considered that one corresponds to cos (x) and the other corresponds to sin (x). Therefore, the detection device 18 detects the amplitude of the rectangular wave I (t), and detects the relative displacement x based on the value (that is, cos (x), sin (x)).

  That is, when a rectangular wave is used as the predetermined periodic signal f (t), the modulated signal I (t) is also a rectangular wave, and the amplitude thereof is a signal representing the relative displacement x of the scale 20 with respect to the head unit 16. Become. Therefore, the detection device 18 only needs to detect the amplitude of the modulated signal I (t), so that it is not necessary to provide a circuit for detecting the phase, and the configuration of the detection device 18 is simplified. Can do.

  Further, if the rectangular wave f (t) used for the modulation is set to ¼ period of the original pattern signal, the amplitude level of the signal I (t) obtained as a result of the light reception is directly cos (x), Since sin (x), the detection of the relative displacement x in the detection device 18 becomes the easiest.

  Thus, an even function including at least two frequency components represented by a rectangular wave can be adopted as the predetermined periodic signal. When an even function is employed as the predetermined periodic signal f (t), the magnitude of the amplitude of the modulated signal I (t) changes according to the phase of the pattern signal before modulation. In 18, the relative displacement x of the scale 20 with respect to the head unit 16 is obtained by detecting only the amplitude. Therefore, since it is not necessary for the detection device 18 to include a circuit for detecting the phase of the modulated signal I (t), the configuration of the detection device 18 can be simplified.

  When an encoder having the configuration shown in FIG. 1, that is, an apparatus that vibrates the objective lens 7, that is, an encoder that mechanically vibrates a part of the apparatus is employed as the encoder, the rectangular wave f When the delay of the vibration with respect to (t) becomes large, a rectangular wave is suitably applied as the predetermined periodic signal f (t) by adopting the configuration shown in FIG. 3 as the configuration of the encoder. can do.

<Sawtooth wave>
Also, as shown in FIG. 5, the periodic signal f (t) used for modulation can be a sawtooth wave. Due to the sawtooth wave, the signal I (t) detected by the optical sensor 9 has a sine wave shape as shown in FIG.

  The amplitude of the sawtooth wave f (t) used for modulation is defined as 2π with respect to the period of the original pattern signal. In this case, the modulated signal I (t) is a continuous sine wave as shown by the following equation.

ここで、ωは、変調後の信号Ｉ（ｔ）の角周波数であり、ｔは時間である。この信号は、変位ｘに相当する位相情報を含んでいる。したがって、検出装置１８では、この変調後の信号Ｉ（ｔ）の位相を検出し、その位相に基づいて位置情報ｘを検出する。

Here, ω is the angular frequency of the modulated signal I (t), and t is time. This signal includes phase information corresponding to the displacement x. Therefore, the detection device 18 detects the phase of the modulated signal I (t) and detects the position information x based on the phase.

  That is, when a sawtooth wave is used as the periodic signal f (t) used for modulation, the modulated signal I (t) is a sine wave, and the phase thereof is information on the relative displacement x of the scale 20 with respect to the head unit 16. It becomes a signal containing. Therefore, since the detection device 18 only needs to detect the phase of the modulated signal I (t), a circuit for detecting the amplitude of the modulated signal I (t) is not required. The circuit configuration can be simplified.

  Further, if the sawtooth wave f (t) used for the modulation is set to one period of the original pattern signal, the phase of the received signal corresponds to the relative displacement x as it is. Detection of relative displacement x at is the easiest.

  Thus, an odd function can be adopted as the periodic signal f (t) used for modulation. In this case, only the phase of the modulated signal I (t) needs to be detected in order to detect the relative displacement x of the scale 20 with respect to the head unit 16.

  Even when a sawtooth wave is used as the periodic signal f (t) used for modulation, the detection apparatus monitors the phase information of the sawtooth wave, and, for example, 0, π, Only the amplitude (signal level) may be detected by monitoring the output of the light reception signal I (t) of the optical sensor 9 at the position 2π. That is, even if the signal f (t) used for modulation is an odd function, if it is a function that changes stepwise, only the signal level of the modulated signal I (t) is detected. Thus, it is possible to detect the relative displacement x.

  As described above, an even function signal typified by a triangular wave and a rectangular wave, an odd function signal typified by a sawtooth wave, and the like are employed as the modulation signal f (t). However, the present invention is not limited to this. Any periodic function including at least two frequency components can be used as the modulation signal f (t).

  Further, the present invention is not limited to the configuration of the optical system of the encoder 100 according to the above embodiment. For example, the optical sensor 9 need not be a photodiode, but may be a CCD, a photomultiplier, or the like. Moreover, the detection apparatus 18 may be comprised with hardware and may be comprised with software.

  Although the encoder according to the above embodiment is the encoder 100 that irradiates the grating 1 of the scale 20 with the spot light, the encoder to which the present invention is applicable is not limited to the encoder according to the above embodiment. For example, so-called diffracted light interference in which an index grating is disposed between the laser diode 3 and the scale 20 and a deflecting member that deflects each ± 1st-order diffraction grating generated in the index grating onto the scale 20 is disposed. The present invention can be applied to an encoder of a scheme, and can also be applied to a so-called shadow encoder. In short, the present invention is not limited to the modulation method of the encoder, and can be applied to any encoder of the modulation method.

  The encoder of the above embodiment is a linear encoder that detects the linear displacement of the moving body, but the present invention is naturally applicable to a rotary encoder that detects the amount of rotation of the rotating body.

  Note that the encoder of the present invention represented by the encoder 100 according to the above embodiment is particularly preferably used in the field of precision equipment. For example, it is also used for detecting position information of a stage on which a mask, a wafer, a liquid crystal substrate or the like is placed in an exposure apparatus, or position information of an optical system in the exposure apparatus. In addition to the exposure apparatus, the present invention can be applied to a positioning control technique in a precision instrument for manufacturing a micro device. It can also be applied to attitude control of a manipulator such as a transfer robot.

  In the above embodiment, the pattern formed on the scale 20 is a sinusoidal diffraction grating periodically arranged in the X-axis direction, but extends in a direction intersecting the X-axis direction, and X Any periodic pattern that is periodically arranged along the axial direction may be used. Furthermore, the pattern formed on the track of the scale 20 is not limited to a simple periodic pattern such as the grating 1, and may be a random pattern (for example, an M-sequence pattern).

  In the above-described embodiment, the encoder that includes the up / down counter that counts up the displacement of the moving body and outputs the counter value as the displacement information of the moving body has been described, but cos (4πx / p), sin An encoder that outputs (4πx / p) and the origin detection signal to the outside as they are may be used.

  The wavelength of the illumination light, the pitch of the grating 1 and the like can be changed as appropriate according to the required resolution.

  As described above, the present invention is not limited to the above-described embodiment, and can of course have various configurations without departing from the gist of the present invention.

  As described above, the encoder of the present invention is suitable for detecting position information of a moving object.

It is a schematic block diagram of the encoder which concerns on one Embodiment of this invention. It is a figure (the 1) which shows the mode of a modulation | alteration. It is a schematic block diagram of the encoder which concerns on other embodiment of this invention. It is a figure (the 2) which shows the mode of a modulation | alteration. FIG. 11 is a diagram (part 3) illustrating a state of modulation; FIG. 6 is a diagram (part 4) illustrating a state of modulation;

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Grating, 3 ... Laser diode, 4 ... Collimator lens, 5 ... Optical device, 6 ... Beam splitter, 7 ... Objective lens, 8 ... Focus lens, 9 ... Optical sensor, 11 ... Drive apparatus, 16 ... Head part, 18 ... detection device, 20 ... scale, 100 ... encoder.

Claims (5)

A scale having a pattern arranged in a predetermined direction;
A detection head that detects a pattern of the scale and outputs a detection signal modulated by a modulation signal that varies according to a periodic function including at least two frequency components;
An encoder comprising: a detection device that detects relative position information between the scale and the detection head based on a phase or amplitude of a detection signal output from the detection head. The periodic function is an even function;
The encoder according to claim 1, wherein the detection device detects the relative position information based on an amplitude of the detection signal. The periodic function is a square wave;
The pattern extends in a direction intersecting the predetermined direction, and is periodically arranged in the predetermined direction,
The encoder according to claim 2, wherein the amplitude of the periodic function is equal to a quarter period of the pattern. The periodic function is an odd function;
The encoder according to claim 1, wherein the detection device detects the relative position information based on a phase or an amplitude of the detection signal. The periodic function is a sawtooth wave;
The pattern extends in a direction intersecting the predetermined direction, and is periodically arranged in the predetermined direction,
The encoder according to claim 4, wherein the amplitude of the periodic function is equal to one period of the pattern.

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