DE4447617C2 - Moving body displacement measuring appts. - Google Patents

Abstract

The apparatus has a light source (11) which produces a coherent, parallel beam with a known wavelength lambda and dia. D and a fixed and a movable diffraction plate (13,14) with grids of equal grid interval p and main diffraction components of order +/- 1. The diffraction plates are arranged in an optical path of the parallel beam so that the beam is diffracted first at the grid in the fixed plate and then at the gird in the movable plate. An optical detector (16) detects a light quantity produced by interference of the diffraction components produced by the successive diffractions. A displacement of the movable plate is detected from the detected light quantity.

Description

The present invention relates to a device for acquiring a reference position when measuring the position of a movable body to a given object.

Fig. 1 shows a known optical encoder for measuring a position or an angle, wherein reference numeral 301 denotes a light source, reference numeral 302 a turntable, which has an A / B phase signal region, in the evenly spaced slots or a grid 306 are circumferentially arranged and includes a Z-phase signal region in which only one slot 307 is circumferentially arranged. Reference numeral 303 denotes a fixed disk with an A / B phase signal region in which slots or a grid 308 are arranged at the same pitch as on the turntable and a Z phase signal region with only one slot 309 is provided on the circumference. Reference numeral 304 denotes a photo sensor for detecting light which passes through the turntable 302 and the fixed plate 303 . When detecting light that passes through the A / B phase signal region of the turntable and the fixed plate, a signal (A / B phase signal) is detected in accordance with an angle of the turntable, while when detecting light that passes through passes through the Z-phase signal regions, a signal (Z-phase signal) is detected which indicates an origin of the turntable.

However, there is a problem with the aforementioned known method that the degree of modulation of the A / B phase signal is disturbed. This problem is explained below. Fig. 2 shows a basic structure of a known optical encoder used for measuring a position or an angle of a moving body (Japanese Patent Laid-Open No. 257419/1991). In Fig. 2, reference numeral 11 denotes a light source containing a laser diode or a light emitting diode of relatively high coherence; the reference numeral 12 denotes a collimator lens for parallel alignment of light emitted by the light source 11 , the reference numeral 13 a fixed diffraction plate which has a grating with a section with a rectangular wave-like shape and perpendicular to an optical axis of the parallel aligned light or collimated light which is emitted from the lens 12 , reference numeral 14 is a movable diffraction plate having a section of a rectangular wave shape which is arranged perpendicular to the optical axis, the movable diffraction plate being capable of being perpendicular to move (up and down in the drawing). The gratings of the fixed and rotating diffraction plates have the same period.

In addition, a step difference "d" with respect to the height between the upper and lower edges of the fixed and movable diffraction plates 13 , 14 has the following relationship to the wavelength λ of the light source 11 :

| n - n ₀ | × d = (λ / 2) × (1 + 2 m) (equation 1),

where m = 0, ± 1, ± 2,. , ., A refractive index n of the material of the fixed and rotatable diffraction plates 13, 14 denote ₀ and n is a refractive index of a medium between the plates 13 and 14, respectively. Furthermore, in Fig. 2, reference numeral 105 denotes a condenser lens for concentrating light which passes through the movable diffraction plate 14 , while reference numeral 105 denotes a photosensor which converts a diffracted image which has been concentrated by the lens 105 into an electrical signal . The movable diffraction plate 14 is, for example, attached to a rotating rotation device, while the fixed diffraction plate 13 is attached in a stationary manner. A rotation amount or the like of the rotation device can be obtained by causing the movable diffraction plate 14 to be offset from the fixed diffraction plate 13 from an output signal of the photosensor 16 .

An operation of the known optical coding device which has the structure described above is described below. First, light emitted by an optical source 11 is aligned or collimated in parallel by a collimator lens 12 . Then the light strikes the solid diffraction plate substantially perpendicularly. The difference "d" in height between the upper and lower edges of the fixed diffraction plate 13 is designed to give the relationship in accordance with Equation 1 as described above. In this case, it is known that the components of the order even numbered diffracted light - including 0 - become 0 and that a maximum of diffracted light energy is concentrated at orders ± 1 (approximately 40% each). Therefore, the light entering the fixed diffraction plate 13 is bent from the plate 13 and enters as diffracted light 110 and the +1 order light as diffracted from 111 -1 order. This diffracted light 110 , 111 enters the movable diffraction plate 14 and exits as diffracted light. Analogous to the light diffracted by the fixed diffraction plate 13 , the diffracted light of the movable diffraction plate 14 has 0-components of diffracted light of the orders with even numbers including 0, and a maximum of energy is on refracted light of the orders ± 1 (approximately 40% each ) concentrated.

Refracted light emerging from the movable diffraction plate 14 is expressed as (n, m), where n denotes a diffraction order of the fixed diffraction plate 13 and m denotes a diffraction order of the movable diffraction plate 14 . Then diffracted light passes through the movable diffraction plate 14 parallel to the optical axis including diffracted light 121 with (+1, -1), diffracted light 122 with (-1, +1), diffracted light with (-3, +3), diffracted Light with (+3, -3) etc. through. Diffracted light of orders 3 or more is not shown in FIG. 2 for reasons of clarity. When the movable diffraction plate 14 is moved at a constant speed perpendicular to the optical axis, the phase of the diffracted light changes with orders of more than 3 relative to that of the order 0. It is known that an optical intensity of a light by interference from diffracted Light (+ k, -k) with which (-k, + k) can be obtained with a sine wave of a frequency of k / p (or a frequency that is k times the base frequency 1 / k), where p denotes a grid spacing of a grid which is arranged in the movable diffraction plate 14 . Therefore, the diffracted light interferes with (+1, -1) and (-1, +1), which makes up a substantial part of the amount of light, and an output signal in the form of a sine wave is obtained which has a frequency twice that Base frequency of the fixed and movable diffraction plates 13 and 14 corresponds. Therefore, methods known in the prior art make it possible to precisely detect a position by using the components of twice the frequency.

However, there is a problem in the prior art that the degree of modulation is disturbed. The refracted light from (+1, -1) and (-1, +1) is concentrated by the condenser lens 105 to use the light efficiently as described above. Then a minimum optical intensity, which is detected by the photosensor 16 , does not become 0, or the degree of modulation is disturbed.

This is explained with reference to Fig. 3, in which λ is the wavelength of the light from the light source 11 , D is the beam size, p is the grating distance of the fixed and movable diffraction plates 13 , 14 and reference numeral 105 denotes a Fourier transform lens with a focal length f. In addition, Δx denotes an offset amount of the movable diffraction plate 14 and α a difference angle λ / p of refracted light of order 1. Since α is sufficiently small, sinα = tanα = α applies. A shape of a portion of the grating of the fixed and movable diffraction plates 13 , 14 is expressed as a complex amplitude for simplicity

cos (kαx) = {exp (ikαx) + exp (-ikαx)} / 2,

and the refracted light of the orders ± 1 is approximated as light aligned in parallel. Subsequently, a complex amplitude of refracted light of order +1 in the movable diffraction plate 14 is expressed as Aϕexp (-ikαx), while that of order -1 is expressed as Aϕexp (+ ikαx), where ϕ = exp (-ikgcosα) and A denotes an amplitude of an incident beam. A complex amplitude f1 of refracted light of order +1 on the movable diffraction plate 14 is expressed as follows:

f1 (x) = Aϕ / 2 { _e -ikαΔx + e - ikα (2x - Δx) (Equation 2).

Similarly, a complex amplitude f2 of diffracted light in order +1 on the movable diffraction plate 14 is expressed as follows:

f2 (x) = AΘ / 2 { _e ikα (2x - Δx) + _e ikαΔx} (Equation 3).

Then a divergence of the diffracted light with +1 on the movable diffraction plate 14 : (-D / 2 - gα, D / 2 - gα). If equation 2 is Fourier-transformed in this area, a following equation 4 is therefore achieved in which ω = 2πx (fλ).

Accordingly, since a divergence of the diffracted light of -1 on the movable diffraction plate 14 becomes (-D / 2 + gα, D / 2 + gα), when Equation 3 is Fourier transformed in this area, a following equation becomes 5 scored. Therefore, a complex amplitude detected by the photosensor 16 corresponds to Equation 6.

In Equation 6, the first expression refers to diffracted light from (+1, -1) and (-1, +1); the second term refers to diffracted light with (+1, +1), and the third term refers to diffracted light with (-1, -1).

Next, an effect around the optical axis of the second and third expressions will be explained. In equation 6, if ω = Δx = 0, the amplitude of the first term AX. On the other hand, the amplitudes would be from the second and third expression

A | sin (kαD) | / (2kα) ≦ A / (2kα) = Ap / (4π).

If D = 0.5 mm and p = 10 µm, (are the first expression / the second Expression) and (the third expression / the second expression) 0.0016 or less.  

Therefore, the second and third printouts are sufficiently small and negligible in the vicinity of the optical axis, and only the first printout is detected by the photosensor 16 . Then only the first expression in Equation 6 is considered below.

Fig. 4 shows an amplitude distribution on the photosensor 14 when an offset amount Δx of the movable diffraction plate 14 is 0 or the output signal intensity is maximum. It was found in Fig. 4 that the amplitude on the optical axis is maximum. The calculation assumes that λ = 633 nm, g = 2 mm, f = 5 mm, p = 10 µm and D = 0.5 mm. On the other hand, Fig. 5 shows an amplitude distribution when the intensity is minimum (kaΔx = π / 2 or Ax = p / 4), wherein the amplitude with the maximum amplitude in Fig. 4 is normalized. In this case, the first expression in Equation 6 becomes Equation 7:

Figure 5 and Equation 7 show that light exists outside the optical axis. If such a light exists, the degree of modulation is disturbed. Fig. 6 shows the degree of modulation when the above-mentioned values are used and the size of the photo sensor is 55 0 µm. The degree of modulation is defined as (output signal intensity - minimum intensity) / (maximum intensity - minimum intensity).

To avoid such an effect, a pin hole or the like can be provided to shade light outside the optical axis. However, a beam size containing a substantial amount of the light in Fig. 5 is only 12 µm small, and it is therefore necessary to make the pin hole so small that it is a few µm smaller than the beam size. In this case, however, light is also detected and the degree of modulation is impaired. In addition, the positioning of the pin hole relative to the optical axis is difficult. Furthermore, if such a small pin hole is used, the amount of light lost is large, a signal obtained from the photosensor is weak, and the apparatus is susceptible to being affected by noise.

Another problem of known methods is explained below. If a geometric center of the turntable deviates from its center of rotation, errors in the A / B phase signals are accumulated. This problem is explained by using a model shown in Fig. 7, numeral 51 denotes a light source, numeral 52 a collimator lens for parallel alignment of light emitted from the light source, numeral 53 a turntable be evenly spaced slots on the circumference, reference numeral 54 is a fixed plate with slots which are the same distance as the slots of the turntable, and reference numeral 55 denotes a photosensor for detecting light which passes through the turntable 53 and the fixed plate 54 .

When the turntable 53 is rotated, the positions of the slit openings of the turntable 53 change relative to those of the fixed plate 54 , so that an amount of light received by the photosensor changes in accordance with the change in the relative positions. FIGS. 8A and 8B show a variation of the output signal of the photo sensor 55 in this case. If the gap distance is large enough not to cause diffraction, the output signal changes as shown in Fig. 8A. If the gap distance with respect to the distance between the turntable 53 and the fixed plate 54 is small, a waveform of an output signal of the photo sensor 55 is affected by the diffraction at the slits, and the edges of the waveform are rounded to be changed like a sine wave as shown in Fig. 8B.

Equation 8 shows the output signal of the photo sensor 55 when the signal waveform is approximated as a sine wave.

y = Asin (NΘ) + B (Equation 8),

where A is a signal amplitude, B is a dc component of the signal, N is a number of slots formed in the turntable 43 , and Θ is a rotation angle.

The above-mentioned accumulated errors caused by the eccentricity of the turntable 53 are explained with reference to FIG. 9, which illustrates an irradiation position of a beam and the trace of the beam on the turntable 53 . If an eccentricity quantity exists between a rotation center 60 and a center 61 of the turntable 53 , a rotation angle of a specific point 62 or an angle Θ relative to the rotation center 60 is different from an angle Θ _a relative to the center 61 of the turntable 53 . If r denotes a distance between the rotation angle 60 and the photosensor 55 , then δ = Θ - Θ _a = (∈ / r) cosΘ. Since the output signal of the photo sensor 55 depends on the angle Θ _a from the center of the turntable 61 , the output signal is expressed according to equation 9:

y = asin (NΘ _a ) + B
= Asin {N (Θ + (∈ / r) cosΘ)} + B (Equation 9).

When a rotation angle is changed from zero to Θ, the number of pulses of a photo sensor 55 is expressed as follows:

N (Θ + (∈ / r) cosΘ) / (2π) (Equation 10).

Then, when a rotation angle is changed from α to β, the number of pulses of the photosensor 55 is expressed as follows:

N (β - α + (∈ / r) (cosβ - cosα)) / (2π) (Equation 11).

Therefore, a difference in the number of pulses relative to a true number of pulses N (β - α) or an accumulated error of the signals is expressed as follows:

N (∈ / r) (cosβ - cosα) / (2π)
= {N∈ / (πr)} sin {(β + α) / 2} sin {β - α) / 2} (Equation 12).

A maximum accumulated error occurs when α = 0 and β = π, and it is N∈ / (πr) pulses. For example, the number of pulses per Revolution 10,000, the position r of the photosensor is from the center of rotation 20 mm and the eccentricity size is ∈ 10 µm, this amounts to accumulated errors on 1.6 pulses. This is too much for an encoder of 10,000 pulses, and the encoder cannot be used practically become.

The accumulated errors can be reduced if r is increased or the eccentricity quantity ∈ is reduced. However, in order to reduce the accumulated errors to 0.1 pulses or less, r must be more than 320 mm, and the size of the encoder becomes very large. If the eccentricity quantity ∈ is reduced, it must be reduced to a value of less than 0.6 µm, and this makes the construction of the turntable 53 very difficult.

Therefore, in prior art methods for realizing a high resolution encoder, two photosensors are placed at two symmetrical points with respect to the center of rotation of the turntable 53 and an arithmetic mean of the optical intensities detected by the photosensors is used to detect the errors that have occurred avoid. The principle of this method is explained below.

When the eccentricity of the turntable occurs, the output signals of the two photosensors are expressed according to Equations 13 and 14 using Equation 9.

y1 = A1sin {N (Θ + (∈ / r) cosΘ)} + B1 (Equation 13).

y2 = A2sin {N (Θ - π + (∈ / r) cos (Θ - π))} + B2
= A2sin {N (Θ - (∈ / r) cosΘ)} + B2 (Equation 14).

If it is assumed for the sake of simplicity that A1 = A2 = A and B1 = B2 = B, an arithmetic mean of these two output signals according to equation 15 is achieved.

y = y1 + y2
= 2Asin (NΘ) cos {(N∈ / r) cosΘ} + 2B (Equation 15).

Equation 15 shows that errors are not due to accumulated errors because the Effect of eccentricity in the expression on the period of the pulse signals disappears.

Equation 15, however, clearly shows that an amplitude of the signal obtained with cos {(N∈ / r) cosΘ} is multiplied, and this means that the signal amplitude changed with the angle of rotation if there is an eccentricity ∈. If | N∈ / r | <π applies, there exists a part in which the signal amplitude during a rotation of the turntable becomes 0. Therefore it is for use in an encoder required that | N∈ / r | <π applies. For example, if N = 10,000 and r = 20 mm applies, it is necessary that ∈ is 6.3 µm or less. Then to a To produce compact coding device with high resolution, the turntable be built very precisely, which increases the costs. Since the exzen tricity or the axis must also be reduced due to a load the axle is also large to increase its weight, and the areas of use  are limited for the coding device to be used.

Another problem with known methods is described below; it is that the precision of the position detection of the origin of the Encoder according to the change in the intensity of the light source is affected. This problem is explained below. It is known to be a Detect the position of a component without physical contact. For example is disclosed as disclosed in Japanese Patent Application Laid-Open No. 44,202 / 1990 Body illuminated with a light to display its image with a video camera, and it becomes a position by binarizing the output of a linear Array sensors detected. In addition, to a reference position of a moving To detect the body, a slit is provided in the moving body, and the Body is illuminated. A light passing through the slot is used by Fotosenso received and their output signals are binarized.

An example of a known position determination will be explained with reference to Figs. 10 and 11A, 11B. Fig. 10 is a view of a known positioning apparatus, wherein numeral 251 denotes a light source and numeral 252 a moving body. A slot 253 is provided in the moving body 252 . Reference numeral 254 denotes a photosensor. The moving body 252 is arranged between the light source 251 and the photo sensor 254 and it moves perpendicular to an axis between the light source 251 and the photo sensor 254 .

An operation of this apparatus is described below. FIG. 11A shows a light beam 255, which extends through the slot 253 in the moving body 252 to photo sensor 254th The moving body is assumed to move from left to right along an x-axis. Therefore, the light beam 255 sweeps over the photosensor 254 according to the movement of the moving body 252 . Then, the output of the photo sensor 254 has a waveform as shown in Fig. 11B. In order to counter the influence of scattered light or the like entering the photosensor 254 , a suitable threshold value is set for converting the output signal into a binary signal. In this way, a reference position signal of the moving body can be obtained.

However, there is the following problem: when the intensity of the light emitted from the light source fluctuates, an influence that corresponds to the fluctuation of the threshold value finally occurs, and the pulse width of the reference position signal and the position of the signal edges are changed. Therefore the precision of the position detection is impaired. In addition, with a reduced beam size in the photosensor 254, a change in the output signal of the photosensor 254 becomes large with a change in the moving body 252 . Therefore, the deterioration in the precision of the position detection due to the noise of stray light and electrical noise decreases. However, if the size of the slit 253 is reduced too much to reduce the beam size, diffraction takes place and the beam size at the photosensor 254 increases in the opposite direction. In addition, as the slit size is reduced, an amount of light received by the photosensor 254 decreases and errors due to noise increase. A gap between the slit 253 and the photosensor 254 can be narrowed to avoid the diffraction effects. If the gap is narrowed, however, there is a possibility that the moving body may touch the photosensor and damage it. In order to generate a pulse of the narrow width previously described, the width of the photo sensor 254 and the beam size must also be reduced. The aforementioned problems of contact between the moving body 252 and the photosensor 254 and deterioration in the precision of the position determination due to noise also take place in this case.

DE 26 17 797 A1 discloses in FIGS. 1 and 5 devices for optically measuring the position and movement of an object. Due to the size of the diaphragm and the geometric dimensions of the photodetectors, these devices are not suitable for high-precision measurement of a reference position. Rather, the devices are suitable for determining the position within a small path from the ratio of the difference to the sum of the photo currents emitted by the two photodiodes.

DD 276 012 A3 relates to a device for obtaining reference points during mechanical movements, especially for incremental ones Measuring systems in precision machine or precision device construction. As A double diode is used for the photoelectric receiver. In the known device stirs the light moved by the photo sensors from one The gap from the optics on the moving sled to the photo sensors is mapped. The known device has an arrangement of optics the moving object, which has a light spot over two neighboring photosenso ren directs, the reference position is reached when the light spot Photo sensors irradiated symmetrically. This reference position is derived from the Difference signal derived from the photo sensors. The establishment of the reference point extraction is used in an incremental measuring system. To the moving object, a focused optic is arranged, which is a beam of light Determination of a reference position via a differential diode steers.

DE 27 51 757 B2 discloses a photoelectric device for fixing a zero position. This known device is a Apparatus for detecting a reference position of a movable body relative to one given object, with a light source for generating a coherent, parallel light beam or coherent, collimated light beam, a movable body which is aligned by an optical path of the parallel can pass perpendicular to the parallel aligned beam, a first cylindrical lens attached to the movable body that the parallel aligned beam, a differential photodiode on the predefined object for detecting a quantity of light only of that beam was concentrated by the cylindrical lens, the differential photodiode one  Separator that is smaller than a diameter of the concentrated beam in Direction of movement of the movable body, and the differential photodiode Width that is larger than the diameter of the concentrated beam, and an evaluation circuit for generating a difference signal between the output signals of the differential photodiode, a position of the movable body can be determined according to the difference signal.

It is an object of the present invention to provide a device that a Generated signal of a reference position with a predetermined pulse width, the of changes in light intensity from that emitted by a light source Light is unaffected and has a precision that of signal noise is not so affected.

According to the invention, a coherent, parallel beam hits one moving body, which is aligned by an optical path of the parallel Beam moves perpendicular to the parallel aligned beam. In this case concentrates a first condenser or focusing optical element, such as a condenser lens attached to the movable body is the parallel beam. First and second photo sensors only capture an amount of light of the beam concentrated by the first condenser means. The first and second photo sensors are spaced from each other, the is smaller than a diameter of the parallel aligned beam in a movement direction of the movable body while having a width that is larger than the diameter of the parallel aligned beam. A signal ver working means generates a difference signal between the output signals of the first and second photo sensors, and a position of the movable body can can be determined according to the difference signal. In this way, a reference point for detecting the position of a moving body precisely determined become. A second condenser is also preferred on the fixed movable body, and third and fourth photosensor means and Photo sensors detect a quantity of light only from that beam which of that  second condenser is aligned accordingly in parallel. Then one second difference signal between the output signals from the third and fourth photo sensors achieved, and a position of the movable body can according to a light source for generating pulse signals from the first and second difference signals can be determined.

An advantage of the present invention is that a position of a moving body can be detected optically without changes in intensity to be influenced by the light emitted by a light source.

These and other objects and features of the present invention based on the following description of the preferred embodiments of the Invention with reference to the accompanying drawings clearly. Show it:

Fig. 1 is an illustration of a prior art optical encoder;

Fig. 2 is an illustration of a known optical encoder;

Fig. 3 is a representation of a model of the optical encoder;

Fig. 4 is a graph showing an amplitude distribution of a photo sensor used in the model;

Fig. 5 is a graph of an amplitude distribution of a photo sensor used in the model;

Fig. 6 is a graph showing the degree of modulation in the model;

Fig. 7 is an illustration of a known optical encoder;

. 8A and 8B are graphs of signal waveforms of a prior art optical encoder;

Fig. 9 is an illustration of the eccentricity of a turntable of the optical encoder;

Fig. 10 is a view of a known positioning apparatus;

Fig. Signals 11A and 11B are representations of the apparatus and a graph of its output;

FIG. 12 is a perspective view of a Positionsmeßapparates a first embodiment of the invention;

FIG. 13 is a block diagram of EMBODIMENT OF A signal processing means of this off;

Fig. 14 is an illustration of waveforms of each part of the signal processing means of this embodiment;

FIG. 15 is an illustration of a second embodiment of the invention;

FIG. 16 is an illustration of a photo-sensor of this embodiment;

FIG. 17 is an illustration of a third embodiment of the invention;

FIG. 18 is an illustration of a photo-sensor of this embodiment;

Fig. 19 is a perspective view of a fourth embodiment, and

FIG. 20 is an illustration of a photo-sensor of this embodiment;

FIG. 21 shows an optical path from the light source 1 to the mirror 5 from FIG. 19, and

Fig. 22 is an optical path from the mirror 6 to the photo sensors 8 of Fig. 19.

Referring to the drawings, in which like or corresponding parts in different representations and embodiments of the invention with the same Reference numerals are provided, is explained below.

First embodiment

A first embodiment of the invention will be explained with reference to FIGS. 12 to 14. Fig. 12 is a perspective view of the apparatus according to the first embodiment of the invention. A laser light source 201 is disposed at a position which is located before the focal point of a collimator lens 202 for collimating a light emitted from the light source 201 light. The z-axis is assumed to coincide with an optical axis of the collimator lens 202 . A movable body 203 moves perpendicular to the optical axis through the emitted light. The x-axis is assumed to be a direction along which the movable body 203 moves.

A first condenser lens 216 and a second condenser lens 217 are arranged on the movable body 203 in the y direction perpendicular to the optical axis and allow light to pass through from the collimator lens 202 . A shading section 223 is provided on the movable body 203 and shades the light incident from the collimator lens 202 outside the entrance pupil of the condenser lenses 216 and 217 to prevent light from the collimator lens 202 on photo sensors 218 , 219 , 220 and 221 in one Area in which the movable body 203 moves.

The first photo sensor 218 and the second photo sensor 219 are arranged on a path of a focal point of the first condenser lens 216 outside a path of a focal point of the second condenser lens 217 . The distance between the first and second photo sensors 218 and 219 in the x direction of the movable body 203 is smaller than the size of the beam concentrated by the condenser lens 216 on the photo sensors 218 and 219 . The third photo sensor 220 and the fourth photo sensor 221 are arranged on a path of a focal point of the second condenser lens 217 outside a path of a focal point of the first condenser lens 216 . The distance between the third and fourth photosensors 220 and 221 in the x direction of the movable body 203 is smaller than a size of the beam concentrated by the condenser lens 217 on the photosensors.

As previously mentioned, the distance between the first and second photosensors 218 and 219 in the moving direction of the movable body 203 is smaller than the size of the beam concentrated by the condenser lens 216 on the photosensors 218 and 219 , and the distance between the third and fourth photosensors 220 and 221 in the direction of movement of the movable body 203 is smaller than the size of the beam concentrated by the condenser lens 217 on the photosensors. In addition, a distance takes between a position in which the output signals of the first and second photo sensors 218 and 219 correspond to each other along the moving direction of the movable body 203 and a position in which the output signals of the third and fourth photo sensors along a moving direction of the movable body Body 203 correspond to each other, a value that is different from a predetermined value ver, which corresponds to a distance between a center of the first condenser lens 216 and that of the second condenser lens 217 . Therefore, if part of the beam aligned in parallel by the first condenser lens 216 is on the first or second photosensor 218 , 219 , part of the beam aligned in parallel by the second condenser lens 217 is on the third or fourth photosensor 220 , 221 .

Reference numeral 222 denotes a signal processing means which receives output signals from the photo sensors 218 , 219 , 220 and 221 and outputs a position detection signal. Fig. 13 is a block diagram of a structure of the signal processing means 222. A sum signal generator circuit 224 generates a sum signal C1 of an output signal A1 of the photosensor 218 and an output signal B1 of the photosensor 219 , while a difference signal generator circuit 225 generates a difference signal D1 of the signals A1 and B1. A binaryization circuit 226 generates a binary signal E1 of the signal C1. Furthermore, a binarization circuit 227 receives the signal D1 and performs binarization using a hysteresis characteristic to output a signal F1. A Summensi gnalgeneratorkreis 228 generates a sum signal C2 from an output signal A2 of the photo sensor 220 and an output signal B2 of the photosensor 221, while a differential signal generator circuit 229 generates a difference signal D2 from the signals A2 and B2. A binarization circuit 230 outputs a binary signal E2 of the signal C2. In addition, a binarization circuit 231 receives the signal D2 and performs binarization using a hysteresis characteristic to generate a signal F2. An exclusive OR gate 223 receives signals F1 and F2 and generates an exclusive OR signal J. An AND gate 233 receives signals E1 and E2 and generates an AND signal K.

An operation of the apparatus described above will now be explained. As seen in FIG. 12, light hits a focal point of the condenser lens when light exits the collimator lens 202 and enters the entrance pupil of the condenser lens 216 , 217 when the movable body 203 moves. The focal point lies on an axis which runs parallel to the light emerging from the collimator lens 202 and extends through a center thereof. Therefore, an offset amount of the movable body 203 in the x-axis direction corresponds to an offset amount of the focal points of the condenser lenses 216 and 217 . The photo sensors 218 and 219 lie on a track of the focal point of the condenser lens 217 . Then the concentrated beam sweeps over the photosensors as the movable body 203 moves.

In the following, the generation of a reference position signal from the signals A1, A2, B1 and B2 will be explained with reference to FIG. 14, which shows waveforms of the signal processor 222 when the movable body 203 moves in the positive direction. Signals A1 and B1 are generated by the photo sensors 218 and 219 when the concentrated beam generated by the condenser lens 216 moves from the photo sensor 218 to the photo sensor 219 . Then the difference signal D1 changes from a negative value to a positive value. When the binarization is performed with a hysteresis characteristic to prevent noise contained in the signal D1, a signal F1 is generated. Analogously, signals A1 and B1 are generated by the photo sensors 220 and 221 when the concentrated beam generated by the condenser lens 217 moves from the photo sensor 220 to the photo sensor 221 . A signal F2 is then generated by binarizing the signal D2. A movement distance of the body 203 from a rising edge of the signal F1 to that of the signal F2 or from a rising edge of the signal F2 to that of the signal F1 is given as the difference between a distance from a position of the moving body 203 in which the output signals of the first and second photo sensors 218 and 219 correspond to each other in a moving direction of the body 203 , to a position of the moving body 203 where the output signals from the third and fourth photo sensors 220 and 221 correspond to each other, and a distance from a center of one of the first condenser lens 216 concentrated beam and that of the beam concentrated by the second condenser lens 217 . Therefore, if the beam concentrated by the first condenser lens 216 and the second condenser lens 217 is on the photosensor, a signal J of predetermined width can be obtained as a reference position signal of the movable body 203 by using an exclusive OR operation of the signals F1 and F2.

Next, a sum signal C1 from signals A1 and B1 and a sum signal C2 from signals A2 and B2 are binarized to obtain signals E1 and E2. It is determined from the signals E1 and E2 that the concentrated beam is located at the photosensors. Then, by using an AND operation with the signal J, a position detection signal K of the movable body 203 is obtained. Since an edge of the signal K is generated at a zero point of the difference signals D1 and D2, it is not impaired by changes in intensity of the light source 201 . In addition, because the signal D1 is a differential signal of the signals A1 and B1 and the signal D2 is a differential signal of the signals A2 and B2, common-mode interferences in the signals - such as signal noise due to stray light on the photosensors - cancel each other out. A signal change rate at a zero point of the signals D1, D2 with respect to an offset of the movable body 203 in the direction of the x-axis becomes approximately twice as large as that of the signals A1, B1, A2 or B2 alone. Therefore, an error of the reference position signal due to signal noise can be reduced. The signal K is thus used as a correct reference signal with a predetermined width.

As previously described, the first and second concentrated rays are at this embodiment from the light emitted by the light source with two Generates lenses which are arranged on the movable body and they will from the first and second photo sensors or from the third and fourth Photo sensors determined. It then becomes a pulse signal corresponding to one Difference signal between the output signals of the first and second photos sors and that of the output signals of the third and fourth photosensors. It can therefore a signal of a reference position with a predetermined pulse can be achieved without changing the light intensity of the Light source emitted light to be affected, the signal being a Precision that is not so affected by signal noise.

A Fresnel zone plate of the amplitude type or phase type can be used in place of the condenser lenses 216 , 217 . A shading element 223 is used in the embodiment to prevent light from the collimator lens 202 outside the entrance pupil of the condenser lens from reaching the photosensors. However, a lens with a focal point outside the optical axis can be used as the condenser lens 216 , 217 to create the focal point outside the optical path or to ensure that light emitted by the collimator lens does not illuminate the photosensors directly. The photo sensors 218 , 219 , 220 and 221 are located at a focal point on the back of the condenser lens 216 , 217 in this embodiment to reduce the size of the concentrated beam. However, they can also be placed out of focus. Because a center of the beam concentrated by the condenser lens 216 , 217 is arranged on a line which runs parallel to the light emitted by the collimator lens 202 and passes through the centers of the condenser lens 216 , 217 , an offset quantity of the movable body 203 in x corresponds to Direction of that of the center of the concentrated beam. The photo sensors 218 , 219 , 220 and 221 are arranged in a plane which is arranged perpendicular to the z-axis in the aforementioned example. However, they can also be arranged in a plane which is inclined with respect to the z-axis. Although the photo sensors 218 , 219 , 220 and 221 are arranged in the same plane in the aforementioned example, they can also be arranged in different planes. The light source 201 may be a light emitting diode or the like instead of the laser light source.

Second embodiment

Fig. 15 shows a basic structure of a second embodiment of the invention, and Fig. 16 shows a structure of an optical detection section. First, a structure of an A / B phase signal output section is explained. In Fig. 15, a light source 11 includes a laser diode or a light-emitting diode with a relatively high coherence. A collimator lens 12 aligns light which is emitted by a light source 11 in parallel. A fixed diffraction plate 313 has a grating with a rectangular wave section and is arranged perpendicular to an optical axis of the parallel aligned light. A movable diffraction plate 314 has a grating with a rectangular wave section and is movable in the x direction perpendicular to the optical axis. The grating of the fixed diffraction plate 313 has the same period as that of the movable diffraction plate 314 . A step difference "d" with respect to the height between the upper and lower edges of the fixed and movable diffraction plates 313 and 314 satisfies Equation 1 as previously explained. A condenser lens 15 aligns light that passes through the movable diffraction plate 314 in parallel. The input pupil of lens 15 is limited to a range within D - 2gλ / p, where "D" denotes the size of the beam that was collimated by collimator lens 12 , "g" is a distance between fixed diffraction plate 313 and movable diffraction plate 314 , λ the wavelength of the light and "p" a grating distance of the gratings arranged in the diffraction plates. The distance between the diffraction plates 313 and 314 is set so that g <pD / (2λ).

An optical detection section 316 , shown in detail in FIG. 16, includes photo sensors 16 , 218 , 219 , 220 and 221 . In contrast to the optical detection section shown in FIG. 12, the photo sensor 16 is included, which converts a diffracted image concentrated by the lens 15 into an electrical signal.

Next, a Z-phase signal output section will be explained. The first photo sensor 218 and the second photo sensor 219 are not arranged on a trace of a focal point of the Fresnel zone plate 217 . The distance between the first and second photosensors 218 and 219 in the moving direction of the movable diffraction plate 314 is smaller than a size of the beam that was concentrated on the photosensors 218 and 219 by the condenser lens 216 . The third photo sensor 220 and the fourth photo sensor 221 are arranged on a trace of a focal point of the Fresnel zone plate 217 outside a trace of a focal point of a Fresnel zone plate 216 . The distance between the third and fourth photosensors 220 and 221 in the moving direction of the movable diffraction plate 314 is smaller than a size of the beam which is concentrated by the Fresnel zone plate 217 on the photosensors.

The width of the photo sensors 218 and 219 in the x-axis direction is larger than a size of the beam concentrated by the Fresnel zone plate 216 on the photo sensors, while a width of the photo sensors 220 and 221 in the x-axis direction is larger than a size of that of the Fresnel Zone plates 217 on the photosensors ren concentrated beam.

As previously explained, the distance between the first and second photo sensors 218 and 220 in the x-axis direction is smaller than a diameter of a beam concentrated by the Fresnel zone plate 216 at least on the photo sensors, while the distance between the first and second photo sensors 218 and 220 is less than a diameter of a beam concentrated by Fresnel zone plate 217 onto the photo sensors. Moreover, a distance is Zvi rule a position in which the output signals of the first and second photo sensors along a direction of movement of the movable diffraction plate correspond and 314 to each other to a position in which the output signals of the third and fourth photosensors along a moving direction of the moving cash diffraction plate 314 to each other correspond to a distance between a center of the first Fresnel zone plate 216 and that of the second Fresnel zone plate 217 different from a predetermined value. When a portion of the beam concentrated by the first Fresnel zone plate 216 hits the first or second photo sensor 218 , 219 , a portion of the beam concentrated by the second Fresnel zone plate 217 hits the third or fourth photo sensor 220 , 221 .

Reference numeral 222 denotes a signal processor which receives output signals from the photo sensors 218 , 219 , 220 and 221 and generates a position detection signal. A shading section 223 is provided on the movable diffraction plate 314 and shades light emerging from the collimator lens 12 outside the entrance pupil of the Fresnel zone plate 216 and 217 to prevent the light emerging from the collimator lens 12 from the photo sensors 218 , 219 , 220 and 221 in a region where the movable diffraction plate 314 moves.

The signal processor 222 is shown in Fig. 13 and has already been described. The sum signal generator circuit 224 generates a sum signal C1 from an output signal A1 of the photosensor 218 and an output signal B1 of the photosensor 219 , while the difference signal generator circuit 225 generates a difference signal D1 of the signals A1 and B1. The binaryization circuit 226 generates a binary signal E1 of the signal C1. In addition, the binarization circuit 227 receives the signal D1 and binarizes using a hysteresis characteristic to output the signal F1. The sum signal generator circuit 228 generates a sum signal C2 of an output signal A2 of the photosensor 220 and an output signal B2 of the photosensor 221 , while the difference signal generator circuit 229 generates a difference signal D2 of the signals A2 and B2. The binarization circuit 230 outputs a binary signal E2 of the signal C2. Furthermore, the binarization circuit 231 receives the signal D2 and performs binarization using a hysteresis characteristic to generate a signal F2. Exclusive OR gate 232 receives signals F1 and F2 and generates exclusive OR signal J.

An AND gate 233 receives signals E1 and E2 and generates an AND signal K.

An operation of the A / B phase signal output section will first be explained with reference to the apparatus described above. A light emitted by the light source 11 is aligned in parallel by the lens 12 and strikes the fixed diffraction plate 313 substantially perpendicularly. Since the step difference "d" with respect to the height between the upper and lower edges of the fixed diffraction plate 13 has the relationship according to equation 1 as described above, a maximum of energy is concentrated in diffracted light of orders ± 1. Therefore, the light entering the fixed diffraction plate 313 is diffracted and exits as diffracted light. The diffracted light enters the movable diffraction plate 314 and exits as diffracted light. Analogous to the fixed diffraction plate 313 , a maximum of energy of the diffracted light, which emerges from the movable diffraction plate 314 , is concentrated in diffracted light of the orders ± 1. The input pupil of lens 15 is limited to a range within D - 2gλ / p, as previously described.

The region limited to the D - 2gλ / p region will be explained with reference to FIG. 12. A diffraction angle of diffracted light 110 of order +1 is λ / p. If λ / p is sufficiently small, diffracted light of order + 1 is only diffracted by gλ / p at a point that is a distance "g" away. Similarly, diffracted light 111 of order -1 is also diffracted by gλ / p. In the drawing, a region delimited by solid lines denotes a region with diffracted light in order +1, while a region enclosed by a broken line denotes a region with diffracted light of order -1. Therefore, a region that is imaged with D - 2gλ / p denotes a region in which diffracted light of the orders ± 1 interferes with each other. In this region, light diffracted by the movable diffraction plate 314 is concentrated parallel to the optical axis by the lens 15 . The light concentrated by the lens 15 is detected by a photosensor 16 .

In the above-described apparatus, an output signal with a frequency which is twice that of the prior art is achieved. In addition, the degree of modulation can be greatly improved as explained in the first embodiment. Fig. 13 shows a modulation characteristic for this case. The degree of modulation in this case is 1.00. Therefore, the position can be determined precisely.

Although the opening is limited by the entrance pupil of the lens 15 in this embodiment, it is clear that corresponding advantages can be obtained by using a restriction with a pin hole or the like of the same size.

Next, the Z-phase signal output section will be explained. As shown in Fig. 12, when the collimator lens 202 hits the entrance pupil of Fresnel zone plates 216 and 217 , when the movable diffraction plate 314 moves, light strikes the focal points of the Fresnel zone plates. The focal point is on an axis which is arranged parallel to the light emerging from the collimator lens and extends through a center of the Fresnel zone plate. Therefore, an offset amount of the movable diffraction plate 314 in the x-axis direction corresponds to an offset amount of the focal points of the Fresnel zone plates 216 and 217 . The photo sensors 218 and 219 are arranged on a trace of the focal point of the Fresnel zone plate 216 , while the photo sensors 220 and 221 are arranged on a trace of the focal point of the Fresnel zone plate 217 . Then the concentrated beam sweeps across the photosensors as the movable diffraction plate 314 moves.

Next, the generation of a reference position signal from the signals A1, A2, B1 and B2 will be explained with reference to Fig. 22, which shows waveforms of the signal processing means 222 when the movable body 203 moves in the positive direction. When the concentrated beam generated by Fresnel zone plate 216 moves from photosensor 218 to photosensor 219 , signals A1 and A2 are generated by photosensors 218 and 219 . The difference signal D1 then changes from a negative value to a positive value. If the binarization is carried out with a hysteresis characteristic in order to avoid signal noise contained in the signal D1, a signal F1 is achieved. Analogously, signals A1 and B2 are obtained from the photo sensors 220 and 221 when the beam generated and concentrated by the Fresnel zone plate 217 moves from the photo sensor 220 to the photo sensor 221 . Then a signal F2 is obtained by binarizing the signal D2.

A moving distance of the movable diffraction plate 314 from a rising edge of the signal F1 to that of the signal F2 or from a rising edge of the signal F2 to that of the signal F1 is given as a difference between a distance from a position of the movable diffraction plate 314 in which the output signals the first and second photosensors correspond to each other in the moving direction of the movable diffraction plate 314 , to a position of the movable diffraction plate 314 in which the output signals of the third and fourth photosensors correspond to each other, and a distance from a center of the one concentrated by the first Fresnel zone plate 216 Beam to that of the beam concentrated by the second Fresnel zone plate 217 . Therefore, when the beam concentrated by the first Fresnel zone plate 216 and the second Fresnel zone plate 217 is at the photosensors, a signal F of the width described above can be obtained as a reference position signal by performing an exclusive OR operation of the signals F1 and F2 become.

Next, a sum signal C1 from signals A1 and B1 and a sum signal C2 from signals A2 and B2 are binarized to obtain signals E1 and E2. It is determined from the signals E1 and E2 that the concentrated beam is located on the photosensors. By using a logical product operation with the signal J, a position detection signal K of the movable diffraction plate 314 is then achieved. Furthermore, since an edge of the signal K is obtained at a zero point of the difference signals D1 and D2, it is not influenced by changes in the intensity of the light source 201 . Since the signal D1 is a difference signal from the signals A1 and B1 and the signal D2 is a difference signal from the signals A2 and B2, signal noise of the same phase in the signals, such as noise due to stray light on the photosensors, cancels one another. A signal change rate at the zeros of the signals D1, D2 against an offset of the movable diffraction plate 314 in the x-axis direction becomes approximately twice as large as that of the individual signals A1, B1, A2 or B2. Therefore, an error of the reference position signal due to noise can be reduced. Therefore, the signal K is used as a correct reference signal with the previously described width.

The photo sensors 218 , 219 , 220 and 221 are arranged at a rear focal point of the Fresnel zone plates 216 , 217 in this embodiment to reduce the size of the concentrated beam. However, they can also be placed out of focus. The reason for this is that a center of the concentrated beam of the focusing lenses 216 , 217 is on a straight line which is parallel to the light which emerges from the collimator lens 202 and passes through the centers of the Fresnel zone plates 216 , 217 , wherein an amount of displacement of the movable diffraction plates 314 in the x direction corresponds to that of the center of the concentrated beam. The photo sensors 218 , 219 , 220 and 221 are arranged in a plane which is perpendicular to the z-axis in the aforementioned example. However, they can also be arranged in a plane inclined with respect to the z-axis. Although the photo sensors 218 , 219 , 220 and 221 are arranged in the same plane in the aforementioned example, they can also be arranged in different planes. The light source 1 may be a light emitting diode or the like instead of the laser light source.

In this embodiment, the degree of modulation is good and an angle signal is obtained precisely because the input pupil of the condenser lens 15 is limited to the range D - 2gλ / p. As described above, the first and second concentrated beams are generated from the light formed by the light source 11 with two Fresnel zone plates 216 , 217 which are arranged on the movable body 314 , and they are generated by the first and second photosensors 218 and 219 and the third and fourth photosensors 220 and 221 . Then, a pulse signal is generated in accordance with the difference signal between the output signals of the first and second photo sensors 218 and 219 and that of the output signals of the third and fourth photo sensors 220 and 221 . Therefore, a signal of a reference position with a predetermined pulse width can be obtained without changing the intensity of light emitted from a light source 11 and with a precision that is not so affected by noise. In addition, because the diffraction plate 314 and the Fresnel zone plates 216 , 217 can be manufactured with a punch, the productivity is high and the cost is low.

Third embodiment

FIG. 17 shows a basic structure of a third embodiment of the invention, and FIG. 18 shows a structure of an optical detection section 217 . First, a structure of an A / B phase signal output section corresponding to that of the third embodiment shown in Fig. 15 will be explained. A light source 11 emits a coherent light, and a collimator lens 12 aligns the light in parallel. A fixed diffraction plate 313 has a grating with a rectangular wave section and is arranged perpendicular to an optical axis of the parallel aligned light. A movable diffraction plate 314 has a grating with a rectangular wave section and is movable in the x direction perpendicular to the optical axis. The grating of the fixed diffraction plate 313 has a period identical to that of the movable diffraction plate 314 .

A step difference "d" with respect to the height between the upper and lower edges of the fixed and movable diffraction plates 313 , 314 has a relationship according to equation 1, as previously explained.

The optical detection section 317 shown in Fig. 18 includes photo sensors 106 , 218 , 219 , 220 and 221 . The photosensor 106 is placed at a distance of pD / (2λ) - g or more from a movable diffraction plate 314 , where D is a size of a beam aligned in parallel by the collimator lens 12 , g is a distance between a fixed diffraction plate 313 and the movable diffraction plate 314 , λ denotes the wavelength of the light and p denotes a grating distance of the diffraction plates. The photo sensor 106 receives light which is limited in width to D - 2gλ / p. This embodiment differs from the second embodiment in the point that this embodiment does not use a condenser lens 15 used in the second embodiment.

Next, a Z-phase signal output section will be explained. A first photo sensor 218 and a second photo sensor 219 are not arranged on a trace of a focal point of the Fresnel zone plate 217 . A distance between the first and second photosensors 218 and 219 in the moving direction of the movable diffraction plate 314 is smaller than a size of the beam concentrated by the condenser lens 216 on the photosensors 218 and 219 . A third photo sensor 220 and a fourth photo sensor 221 are arranged on a trace of a focal point of the Fresnel zone plate 217 outside a trace of a focal point of the Fresnel zone plate 216 . A distance between the third and fourth photosensors 220 and 221 in the direction of movement of the movable diffraction plate 314 is smaller than a size of the beam concentrated by the Fresnel zone plate 217 on the photosensors. The width of the photo sensors 218 and 219 in the x-axis direction is larger than a size of the beam concentrated by the Fresnel zone plate 216 on the photo sensors, while the width of the photo sensors 220 and 221 in the x-axis direction is larger is a size of the beam concentrated by the Fresnel zone plate 217 onto the photosensors.

As previously explained, the distance between the first and second photosensors 218 and 220 in the x-axis direction is smaller than a diameter of a beam concentrated by the Fresnel zone plate 216 at least on the photosensors, while the distance between the first and second Photo sensors 218 and 220 is smaller than a diameter of a beam concentrated by Fresnel zone plate 217 on the photo sensors. Further, a distance between a position in which the output signals of the first and second photo sensors along a direction of movement of the movable diffraction plate 314 are equal to each other, and a position in which the output signals of the third and fourth photosensors along a moving direction of the moving cash diffraction plate 314 to each other become the same by a predetermined value from a distance between a center of the first Fresnel zone plate 216 and that of the second Fresnel zone plate 217 . When a portion of the beam concentrated by the first Fresnel zone plate 216 is at the first or second photosensor 218 , 219 , a portion of the beam from the second Fresnel zone plate 217 is on the third or fourth photosensor 220 , 221 . Reference numeral 222 denotes a signal processor which receives output signals from the photo sensors 218 , 219 , 220 and 221 and outputs a position detection signal. A shading section 223 is disposed on the movable diffraction plate 314 and shades light emitted from the collimator lens 12 and incident outside the entrance pupil of the Fresnel zone plates 216 and 217 to prevent light emitted from the collimator lens 12 from the photo sensors 218 , 219 , 220, and 221 are illuminated in a region where the movable diffraction plates 314 move.

The signal processor 222 is shown in Fig. 13 and has already been explained. The sum signal generator circuit 224 generates a sum signal C1 of an output signal A1 of the photosensor 218 and an output signal B1 of the photosensor 219 , while the difference signal generator circuit 225 generates a difference signal D1 of the signals A1 and B1. The binarization circuit 226 generates a binary signal E1 of the signal C1. Furthermore, the binarization circuit 227 receives the signal D1 and performs binarization using a hysteresis characteristic to output a signal F1. The hysteresis is chosen so that it has a value that is greater than a maximum amplitude of the noise contained in the signal G1. The sum signal generator circuit 228 generates a sum signal C2 of an output signal A2 of the photosensor 220 and an output signal B2 of the photosensor 221 , while the difference signal generator circuit 229 generates a difference signal D2 of the signals A2 and B2. The binarization circuit 230 generates a binary signal E2 of the signal C2. Furthermore, the binarization circuit 231 receives the signal D2 and performs binarization using a hysteresis characteristic to output a signal F2. The hysteresis is chosen to have a value greater than a maximum amplitude of the noise contained in the signal D2. Exclusive OR gate 232 receives signals F1 and F2 and generates an exclusive OR signal J. An AND gate 233 receives signals E1 and E2 and generates an AND signal K.

With respect to the above-described apparatus an operation of the A / B-phase signal output section with reference to Fig. 15 and Fig. 15 is first explained. A light emitted by the light source 11 is aligned in parallel by the lens 12 and strikes the fixed diffraction plate 313 substantially vertically. The light incident on the fixed diffraction plate 313 is diffracted and emerges as diffracted light.

The light emerging from the movable diffraction plate 314 is expressed as (n, m), where n denotes an order of diffraction of the fixed diffraction plate 313 and m denotes an order of diffraction of the movable diffraction plate 314 . An area marked with "a" shows an area with (-1, +1) and (+ 1, -1); an area labeled "b" shows an area labeled (-1, +1), and an area labeled "c" shows an area labeled (+1, -1); an area labeled "d" indicates an area labeled (-1, -1), and an area labeled "e" indicates an area labeled (+1, +1). Diffracted light of order 3 or more is not shown in FIG. 15 for reasons of clarity. A light receiving plane of a photosensor 106 is selected such that a distance from the diffraction plate is 314 pD / (2λ) - g or more and limits the light to a region within D - 2gλ / p. Therefore, the photosensor 106 detects the light only in the area "a".

Next, the Z-phase signal output section will be explained. As shown in FIG. 12, light entering collimator lens 202 into the entrance pupil of Fresnel zone plates 216 and 217 as movable diffraction plate 314 moves strikes the focal points of Fresnel zone plates. The focal point is on an axis that runs parallel to the light emitted by the collimator lens and extends through a center of the Fresnel zone plate. Therefore, an offset amount of the movable diffraction plate 314 in the x-axis direction corresponds to an offset amount of the focal points of the Fresnel zone plates 216 and 217 . The photo sensors 218 and 219 are arranged on a track of the focal point of the Fresnel zone plate 216 , while the photo sensors 220 and 221 are arranged on a track of the focal point of the Fresnel zone plate 217 . Then the concentrated beam sweeps across the photosensors as the movable diffraction plate 314 moves.

Next, the generation of a reference position signal from the signals A1, A2, B1 and B2 will be explained with reference to Fig. 14, which shows waveforms in the signal processing means 222 when the movable body 203 moves in the positive direction. When the concentrated beam generated by Fresnel zone plate 216 moves from photosensor 218 to photosensor 219 , signals A1 and B1 are obtained from photosensors 218 and 219 . Then the difference signal D1 changes from a negative value to a positive value. If the binarization is performed with a hysteresis characteristic in order to avoid noise contained in the signal D1, a signal F1 is obtained. Similarly, signals A2 and B2 are obtained from photosensors 220 and 221 when the beam generated by Fresnel zone plates 217 moves from photosensor 220 to photosensor 221 . Then a signal F2 is obtained by binarizing the signal D2.

A moving distance of the movable diffraction plate 314 from a rising edge of the signal F1 to that of the signal F2 or from a rising edge of the signal F2 to that of the signal F1 is given as a difference between a distance from a position of the movable diffraction plate 314 at which the output signals of the first and second photosensors correspond to each other in a moving direction of the movable diffraction plate 314 , to a position of the movable diffraction plate 314 in which the outputs of the third and fourth photosensors correspond to each other, and a distance from a center of the beam concentrated by the first Fresnel zone plate 216 and that of the beam concentrated by the second Fresnel zone plate 217 . Therefore, when the beam concentrated by the first Fresnel zone plate 216 and the second Fresnel zone plate 217 is at the photosensors, a signal F having a width as mentioned above can be obtained as a reference position signal by using an exclusive OR operation of the signals F1 and F2 become.

Next, a sum signal C1 of signals A1 and B1 and a sum signal C2 of signals A2 and B2 are binarized to obtain signals E1 and E2. It is determined from the signals E1 and E2 that the concentrated beam is located on the photosensors. Then, by performing an AND operation on the J signal, a position detection signal K of the movable diffraction plate 314 is obtained. Because an edge of the signal K is obtained at a zero point of the difference signals D1 and D2, it is not influenced by changes in intensity of the light source 201 . In addition, because the signal D1 is a differential signal of the signals A1 and B1 and the signal D2 is a differential signal of the signals A2 and B2, signal noises of the same phase contained in the signals, such as noise due to stray light at the photosensors, cancel each other out . A signal change rate of the signals D1, D2 at a zero point against an offset of the movable diffraction plate 314 in the x-axis direction becomes approximately twice as large as that of the signals A1, B1, A2 or B2 alone. Therefore, an error of the reference position signal due to noise can be reduced. Therefore, the signal K is used as a correct reference signal with a predetermined width.

In this embodiment, the degree of modulation is improved in that only the area is used in the diffracted light of orders ± 1 with each other interfere. Because a condenser lens is not needed, the number of Components reduced, and the weight of the encoder can be be dropped off; be discontinued; be deducted; be dismissed. In addition, since the diffraction plates and the Fresnel Zone plates can be made simultaneously with a punch that High productivity and low costs.

The photo sensors 218 , 219 , 220 and 221 are placed at a focal point on the back of the Fresnel zone plates 216 , 217 in this embodiment to reduce the size of the concentrated beam. However, they can also be placed out of focus. The reason for this is that a center of the beam aligned in parallel by the focusing lenses 216 and 217 is located on a straight line which runs parallel to the light emitted by the collimator lens 202 and extends through the center of the Fresnel zone plates 216 , 217 , wherein an offset size of the movable diffraction plates 314 in the x direction corresponds to that of the center of the concentrated beam. The photo sensors 218 , 219 , 220 and 221 are arranged in a plane running perpendicular to the z-axis in the aforementioned example. However, they can also be arranged in a plane inclined with respect to the z-axis. In addition, although the photo sensors 218 , 219 , 220 and 221 are arranged on the same level in the aforementioned example, they can also be arranged on different levels. The light source 11 may be a light emitting diode or the like instead of the laser light source.

Fourth embodiment

Fig. 19 shows a basic structure of a fourth embodiment of the invention, and Fig. 20 shows a structure of a photo sensor. First, a structure of an A / B phase signal output section is explained. A collimator lens 2 aligns light that is emitted by a coherent light source 1 with a wavelength λ in parallel. A first, fixed plate 3 has a grating with a rectangular wave section which transmits diffracted light of orders ± 1 only, and a turntable 324 has a grating with a rectangular shaft section which transmits diffracted light only orders of ± 1 of the same diffraction angle as that the first fixed plate 3 . The turntable 324 has circumferential gaps in radial directions and includes first and second Fresnel zone plates 216 and 217 . Mirrors 5 , 6 direct light emerging from the turntable 324 to a position symmetrical with respect to the center of rotation. A second fixed plate 7 has a grating with a rectangular wave section which transmits diffracted light of only the orders ± 1 with the same diffraction angle as that of the first fixed plate 3 . An optical detection section 318 contains photo sensors 8 , 218 , 219 , 220 and 221 , as can be seen in detail in FIG. 20.

A Z phase signal output section is explained below. The first photo sensor 218 and the second photo sensor 219 are arranged on a trace of a focal point of the Fresnel zone plate 216 and not on a trace of a focal point of the Fresnel zone plate 217 . A distance between the first and second photosensors 218 and 219 in the direction of movement of the turntable 324 is smaller than a size of the beam aligned in parallel by the condenser lens 216 onto the photosensors 218 and 219 . The third photo sensor 220 and the fourth photo sensor 221 are arranged on a trace of a focal point of the Fresnel zone plate 217 outside a trace of a focal point of the Fresnel zone plate 216 . A distance between the third and fourth photo sensors 220 and 221 in the direction of movement of the turntable 324 is smaller than a size of the beam aligned in parallel by the Fresnel zone plate 217 on the photo sensors. A width of the photo sensors 218 and 219 in the x-axis direction is larger than a size of the beam aligned in parallel from the Fresnel zone plate 216 to the photosensors, while a width of the photo sensors 220 and 221 in the x-axis direction is larger, as a size of the beam parallel from Fresnel zone plate 217 to the photosensors. A distance between the first and second photosensors 218 and 220 in the x-axis direction is smaller than a diameter of a beam aligned at least in parallel by the Fresnel zone plate 216 onto the photosensors, while a distance between the first and second photosensors 218 , 220 is smaller than a diameter of a beam aligned in parallel by the Fresnel zone plate 217 onto the photosensors. A distance between a position in which the output signals of the first and second photo sensors 218 , 219 correspond to one another along a direction of rotation of the rotary plate 324 and a position in which the output signals of the third and fourth photo sensors 220 , 221 correspond to one another along a direction of rotation of the rotary plate 324 is different from a distance between a center of the first Fresnel zone plate 216 and that of the second Fresnel zone plate 217 by a predetermined value. When a portion of the beam aligned in parallel from the first Fresnel zone plate 216 is on the first or second photosensor 218 , 219 , a portion of the beam aligned in parallel by the second Fresnel zone plate 217 is on the third or fourth photosensor 220 , 221 .

Reference numeral 222 denotes a signal processor which receives output signals from the photo sensors 218 , 219 , 220 and 221 and outputs a position detection signal. A shading section 223 is provided on the turntable 324 and shades the light exiting the collimator lens 12 outside the entrance pupil of the Fresnel zone plates 216 and 217 to prevent the light emitted from the collimator lens 12 from the photosensors 218 , 219 , 220 and 221 exposed in a region where the turntable 324 is moving.

The signal processor 222 is shown in Fig. 13 and has already been explained. The sum signal generator circuit 224 generates a sum signal C1 of an output signal A1 of the photosensor 218 and an output signal B1 of the photosensor 219 , while the difference signal generator circuit 225 generates a difference signal D1 of the signals A1 and B1. The binarization circuit 226 generates a binary signal E1 of the signal C1. In addition, the binarization circuit 227 receives the signal D1 and performs binarization using a hysteresis characteristic to output a signal F1. The hysteresis is chosen so that it has a value that is greater than a maximum amplitude of the noise contained in the signal D 1. The sum signal generator circuit 228 generates a sum signal C2 of an output signal A2 of the photosensor 220 and an output signal B2 of the photosensor 221 , while the difference signal generator circuit 229 generates a difference signal D2 of the signals A2 and B2. A binarization circuit 230 generates a binary signal E2 of the signal C2. Furthermore, the binarization circuit 231 receives the signal D2 and performs binarization using a hysteresis characteristic to output a signal F2. The hysteresis is selected so that it has a value that is greater than a maximum amplitude of the signal noise contained in the signal D2. Exclusive OR gate 232 receives signals F1 and F2 and generates exclusive OR signal J. An AND gate 233 receives signals E1 and E2 and generates an AND signal K.

With respect to the embodiment made as explained above, an operation of the A / B phase signal output section will first be explained below. In FIG. 19, an optical path from the light source 1 to the mirror 5 on an xy plane from a positive x direction corresponds to that in FIG. 21. The operation will now be explained using FIG. 21. The light emitted by the light source 1 is aligned in parallel by the collimator lens 2 and strikes the first, fixed plate 3 . The light is separated from the first, fixed plate 3 into diffracted light of the orders ± 1. The diffracted light of the orders ± 1, which leaves the first fixed plate 3 , strikes the turntable 324 and is separated into the diffracted light of the orders ± 1. Since the diffraction angles of diffracted light of the order ± 1 on the first, fixed plate 3 and the turntable 324 are identical, a luminous flux 31 is parallel to a luminous flux 32 . The luminous flux 31 denotes a luminous flux which is diffracted by the first, fixed plate 3 in the positive x-direction and further diffracted in the negative x-direction by the turntable 324 , while the Li 13397 00070 552 001000280000000200012000285911328600040 0002004447617 00004 13278chtstrom 32 denotes a luminous flux which is bent by the first, fixed plate 3 in the negative y-direction and is further bent in the positive y-direction by the turntable 324 .

It is known that when a grating moves relative to an incoming light, the phase of the diffracted light of orders ± 1 increases or decreases. This means that the phase of diffracted light of the order +1, which is diffracted along a direction of movement of the grating with respect to an optical axis of the incoming light, is accelerated by 2πx (λp), where p is a grating distance of the grating and λ is the wavelength of the light. On the other hand, the phase of diffracted light of the order +1, which is diffracted along a direction opposite to the direction of movement of the grating with respect to an optical axis of the incident light, slows by 2πx (λp). Therefore, complex amplitudes of optical currents 31 and 32 are expressed as in Equation 19, where θ denotes an angle of rotation of the turntable 324 , wherein a moving direction of the grating of the turntable 324 is taken as a positive direction of the angle of rotation θ in Fig. 21, N is a divisor of the grating in the turntable 324 and r denotes a distance from a center of the turntable 324 to a point at which a beam hits the turntable 324 . The currents 31 and 32 are subjected to phase modulation by the turntable 324 and are directed by the mirror 5 into a position symmetrical with respect to the center of rotation of the turntable 324 .

Fig. 22 relates to an optical path from the mirror 6 to the photo sensor 8 to an xz plane of a positive y-direction in Fig. 19. Therefore, this embodiment is described with reference to FIG. 22 explained. The light fluxes 31 and 32 emanating from the mirror 6 are parallel and are separated from the second fixed plate 7 into diffracted light of the orders ± 1. The current 31 is diffracted by the second, fixed plate 7 in a negative x direction and further by the turntable 324 in a positive x direction in order to achieve a current 33 . The current 32 is diffracted by the second fixed grating 7 in a positive x direction and further by the turntable 324 in a negative x direction in order to achieve a current 34 . Streams 33 and 34 are parallel to and overlaid to create interference. When the turntable 324 rotates in a positive direction, complex amplitudes of the currents 33 and 34 are expressed according to equation (20) because the direction of movement of the turntable 324 has a negative x direction in FIG. 7. In addition, an interference light amplitude of the currents 31 and 32 is calculated according to equation (21). The photosensor 8 detects an optical intensity, which is expressed in equation (21), from which it follows that 4N pulses per revolution can be detected or four times the number of divisions of the grating can be achieved in the turntable.

Accumulated errors are next explained when an eccentricity quantity ∈ of the turntable 324 occurs. If the turntable 324 has an eccentricity, a rotation angle θ deviates from a rotation angle θ _a with respect to the rotation center of the turntable 324 . The angle of rotation θ _a is expressed as θ _a = θ + (∈ / r) cosθ, as can be seen from the drawing. Therefore, the complex amplitudes of streams 31 and 32 are expressed as in equation (22). With respect to the point symmetrical about a certain point with regard to the center of rotation of the turntable 324 :
θ _a = θ - (∈ / r) cosθ. Therefore, the complex amplitudes of currents 33 and 34 are expressed as in equation (23). An interference intensity of the currents 33 and 34 or an optical intensity received by the photosensor 8 is expressed in equation (24). Since equation (24) has no expression containing an eccentricity quantity ∈, it was found that there were no accumulated errors due to eccentricity, and it was also found that there were no changes in intensity.

Next, the Z-phase signal output section will be explained with reference to FIG. 12. As shown in FIG. 12, the light reaches a focal point of the Fresnel zone plate when light emerging from the collimator lens 202 hits the entrance pupil of the Fresnel zone plates 216 and 217 when the turntable 324 rotates. The focal point is on an axis which runs parallel to the light emerging from the collimator lens and runs through a center of the Fresnel zone plate. Therefore, an offset size of the turntable 324 in the x-axis direction corresponds to an offset size of the focal points of the Fresnel zone plates 216 and 217 . The photo sensors 218 and 219 are located on a path of the focal point of the Fresnel zone plate 217 . Then the concentrated beam sweeps across the photosensors as the turntable 324 rotates.

Next, the generation of a reference position signal from the signals A1, A2, B1 and B2 will be explained with reference to Fig. 14, which shows waveforms of the signal processing means 222 when the turntable 324 moves in the positive direction. When the concentrated beam generated by Fresnel zone plate 216 moves from photosensor 218 to photosensor 219 , signals A1 and B1 are generated by photosensors 218 and 219 . Then the difference signal D1 changes from a negative to a positive value. If the binarization is carried out with a hysteresis characteristic in order to avoid signal noise contained in the signal D1, a signal F1 is achieved. Analogously, signals A2 and B2 are obtained from the photo sensors 220 and 221 when the concentrated beam generated by the Fresnel zone plate 217 moves from the photo sensor 220 to the photo sensor 221 . Then a signal F2 is obtained by binarizing the signal D2.

A movement distance of the turntable 324 from a rising edge of the signal F1 to that of the signal F2 or from a rising edge of the signal F2 to that of the signal F1 is given as a difference between a distance from a position of the turntable 324 at which the output signals of the first and second photosensors correspond in a direction of movement of the turntable 324 , a position of the body where the outputs of the third and fourth photosensors correspond to each other, and a distance from a center of the beam concentrated by the first Fresnel zone plate 216 and that of the second Fresnel zone plate 217 concentrated beam. Therefore, if the beam concentrated by the first and second Fresnel zone plates 216 and 217 is on the photosensors, a signal F of a predetermined width can be used as a reference position signal of the turntable 324 by using an exclusive OR operation of the signals F1 and F2 be achieved.

Next, an OR signal C1 of signals A1 and B1 and an OR signal C2 of signals A2 and B2 are binarized to obtain signals E1 and E2. It is determined from the signals E1 and E2 that the concentrated beam is located at the photosensors. A position detection signal K of the turntable 324 is then achieved by using an AND operation with the signal J. Furthermore, because an edge of the signal K is achieved at a zero point of the difference signal D1 and D2, it is not influenced by changes in the intensity of the light source 201 . Since signal D1 is a difference signal between signals A1 and B1 and signal D2 is a difference signal between signals A2 and B2, common-mode interferences in the signals, such as noise due to stray light from the photo sensors, cancel each other out. A signal change rate at a zero point of the signals D1 and D2 against an offset of the turntable 324 in the x-axis direction becomes approximately twice as large as that of the signals A1, B1, A2 or B2 alone. Therefore, an error of the reference position signal due to noise can be reduced. The signal K is therefore used as a correct reference signal with a predetermined width.

The photo sensors 218 , 219 , 220 and 221 are placed at a focal point on the back of the Fresnel zone plate 216 , 217 to reduce the size of the concentrated beam. However, they can also be placed out of focus. This is based on the fact that a center of the concentrated beam of the Fresnel zone plate 216 , 217 is located on a straight line which runs parallel to the light emerging from the collimator lens 202 and passes through the center of the Fresnel zone plate 216 , 217 , an offset quantity of the turntable 324 in the x direction corresponds to that of the center of the concentrated beam. The photo sensors 218 , 219 , 220 and 221 are arranged in a plane which is perpendicular to the z-axis in the previously mentioned example. However, they can also be arranged in a plane which is inclined with respect to the z axis. In addition, although photo sensors 218 , 219 , 220, and 221 are arranged in the same plane in the aforementioned example, they can also be arranged in different planes. The light source may be a light emitting diode or the like instead of the laser light source.

A distance between the first fixed plate 3 and the turntable 324 is increased to completely separate refracted light of orders ± 1 on the turntable 324, and it is set to a distance between the first fixed plate 3 and the turntable 324 by currents only 334 to be received by the photosensor 8 , so that signals with a higher degree of modulation can be obtained.

As previously explained, in this embodiment the diffracted light of orders ± 1, which is parallel to each other due to the first fixed plate 3 and the turntable 324 with gratings, is subjected to phase modulation. Next, it is further subjected to phase modulation at positions symmetrical with respect to the center of rotation of the turntable 324 . Then it is interfered with on the second fixed plate. Therefore, even if the turntable 324 has an eccentricity, an angle can be detected precisely without being greatly influenced by the changes in the optical intensity and without accumulated errors. Since a precise construction and a precise adjustment of the turntable are not necessary, the costs can be reduced, a coding device with compact outer dimensions can be manufactured and the resolution can be improved. In addition, an encoder is not so much affected by an eccentricity of an axis due to a load, and a compact axis can be manufactured. In addition, since the diffraction plate 324 and the Fresnel zone plates 216 , 217 can be manufactured with a punch, the productivity is high and the cost is low. In embodiments 2-4, Fresnel zone plates are used to generate a concentrated beam. However, a condenser lens can also be used instead of the Fresnel zone plate. On the other hand, in the aforementioned embodiments in which a condenser lens is used, a Fresnel zone plate can be used instead of a condenser lens to generate a concentrated beam.

Claims (7)

1. Device for detecting a reference position when measuring the position of a movable body to a predetermined object, with
a light source ( 201 ) for generating a coherent, parallel beam;
a movable body ( 203 ; 314 ; 324 ) that travels perpendicular to the parallel beam in a particular direction;
a first condenser ( 216 ) which is arranged on the moving body ( 203 ; 314 ; 324 ) and focuses the parallel aligned beam;
a first and second photo sensor ( 218 ; 219 ) arranged next to one another on the predetermined object in the direction of movement for detecting only that light which is focused by the first condenser, the first and second photo sensors ( 218 ; 219 ) being at a distance from one another which is smaller than the diameter of the focused beam in the moving direction of the moving body ( 203 ; 314 ; 324 ), and the first and second photo sensors ( 218 ; 219 ) have a width larger than the diameter of the focused beam; and
a first signal processing for generating a first difference signal from the output signals of the first and the second photo sensor, from which a first position signal for the moving body is determined;
marked by
a second condenser ( 217 ) which is arranged on the moving body ( 203 ; 314 ; 324 ) and focuses the parallel aligned beam;
a third and a fourth photo sensor ( 220 , 221 ) arranged next to one another on the predetermined object in the direction of movement for detecting only that light which has been focused by the second condenser, the third and fourth photo sensors ( 220 ; 221 ) being at a distance from one another is smaller than the diameter of the focused beam in the direction of movement of the moving body ( 203 ; 314 ; 324 ), and the third and fourth photo sensors ( 220 ; 221 ) have a width which is larger than the diameter of the focused beam;
a second signal processing ( 222 ) for generating a second difference signal from the output signals of the third and fourth photosensors ( 220 ; 221 ), from which a second position signal for the moving body is determined, and a pulse signal from the first and second difference signals and one Position signal as a reference position for the moving body ( 203 ; 314 ; 324 ) according to the pulse signal. 2. Device according to claim 1, characterized in that at least one of the condensers ( 216 ; 217 ) contains a condenser lens. 3. Device according to one of claims 1 or 2, characterized in that at least one of the condensers ( 216 ; 217 ) has a Fresnel zone plate. 4. The device according to claim 1, characterized in that
a fixed diffraction plate ( 3 ; 313 ) is provided and the movable body ( 203 ; 314 , 324 ) contains a movable diffraction plate, each containing a grating with a grating spacing ("p") with main diffraction components of the orders ± 1, which are parallel to each other in a distance ("g") in an optical path of the parallel beam perpendicular to the optical axis of the parallel beam so that the parallel beam is successively diffracted from the gratings in the fixed and movable diffraction plates ( 313 ; 314 ) becomes; and that continues
optical detection means ( 316 ; 318 ) is provided for detecting an amount of light substantially caused by interference of the diffraction components of the orders ± 1, the diffraction components being generated by successive diffraction at the first ( 3 ; 313 ) and the second diffraction plate;
the first and second condensers ( 216 ; 217 ) being attached to the movable diffraction plate and the coherent, parallel beam having a wavelength (λ) and a diameter ("D"); and
an offset of the movable diffraction plate can be determined from the detected amount of light. 5. The device according to claim 4, characterized in that the optical detection means ( 316 ; 318 ) contains a condenser lens ( 15 ) with an entrance pupil size within a range of D - 2gλ / p, the condenser lens ( 15 ) being able to move through the fixed and movable Light diffuses through diffraction plates, and the optical detection means ( 316 ) includes a photosensor ( 16 ) for detecting light concentrated by the condenser lens ( 15 ). 6. The device according to claim 4, characterized in that the optical detection means ( 316 ; 318 ) includes a photosensor ( 16 ) located at a position away from the fixed and movable diffraction plate of D / 2λ - g or more in an area within of D - 2gλ / p is arranged around the optical axis. 7. The device according to claim 1, characterized in that
a first fixed diffraction plate ( 313 ) is provided which contains a first grating with main diffraction components of order ± 1;
the moveable body ( 202 ; 314 ; 324 ) includes a turntable that can rotate about a center of rotation, the moveable diffraction plate including a second grating with an annular shape that is symmetrical with respect to the center of rotation, the second grating having the same spacing like that of the first grating containing main diffraction components of orders ± 1, the first fixed diffraction plate ( 3 ; 313 ) and the turntable being arranged parallel to each other in an optical path of the parallel beam perpendicular to the optical axis of the parallel beam, so that the parallel beam is successively diffracted at the first and second gratings;
optical steering means ( 5 , 6 ) is provided for directing light which passes through the first and second gratings to a second fixed diffraction plate ( 7 ) at a position symmetrical with respect to the center of rotation of the turntable; and
the second fixed diffraction plate ( 7 ) is provided with a third grating which is arranged parallel to the turntable, the third grating having main diffraction components of the order ± 1 and a diffraction angle which corresponds to that of the diffraction components of the first fixed diffraction plate ( 3 ; 313 ) , the third grating has the same grating spacing as that of the first fixed diffraction plate ( 3 ; 313 ) and the second fixed diffraction plate ( 7 ) is arranged in an optical path of the optical steering means ( 5 , 6 ) perpendicular to the optical axis, so that from the optical steering means ( 5 , 6 ) directed light is sequentially diffracted by the second and third gratings;
optical detection means ( 316 ; 318 ) is provided for detecting an amount of light substantially generated by interference of diffraction components of orders ± 1, diffraction components being generated by successive diffraction on the first, second and third gratings; and
the first and second condensers ( 204 ; 216 ; 217 ) are attached to the turntable in an optical path of the optical steering means to the optical detection means ( 316 ; 318 ).