GB2201509A

GB2201509A - Displacement measuring apparatus capable of forming an output signal of substantially constant amplitude

Info

Publication number: GB2201509A
Application number: GB08725143A
Authority: GB
Inventors: Tetsuharu Nishimura; Koh Ishizuka; Masaaki Tsukiji; Chikara Sato; Susumu Kozuki
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 1986-10-31
Filing date: 1987-10-27
Publication date: 1988-09-01
Anticipated expiration: 2007-10-27
Also published as: DE3736704A1; GB8725143D0; DE3736704C2; JPS63115012A; GB2201509B

Description

i 2e,o 1509 4 1

1 TITLE OF THE INVENTION

Displacement Measuring Apparatus Capable-of Forming an Output Signal of Substantially Constant Amplitude BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a displacement measuring apparatus for detecting the displacement such as rotation or movement of an object to be measured or measuring the amount of.displacement of such object.

Also, the present invention relates particularly to a displacement measuring apparatus in which a coherent light beam is caused to enter a diffraction grating provided on an object to be mqasured and diffractedlights from the diffraction grating are caused to interfere with each other to thereby form interference fringes and any change in the light and shade of theinterference fringes is detected to thereby detect the displacement of the object to be measured or measur ethe amount of displacement of such object.

In the present invention, there is disclosed a displacement measuring apparatus which is capable of effecting correction on a signal produced by photo- receptor means when any change in the light and shade of the interference fringes is detected and outputting a signal of substantially constant amplitude.

1 2,2 o 15 0 9 1 1 Related Background Art

Photoelectric rotary encoders or linear encoders have heretofore been often utilized as displacement measuring apparatuses which effect detection of the amount of movement of a moving object in an industrial machine tool, detection.-of the rotation, movement, position, etc..of robot arms and detection of the amount of rotation, the speed of rotation, etc. of a rotational mechanism..

Of these, there have been proposed various displacement measuring apparatuses of the diffraction type wherein a diffraction grating is provided on the object to be measured and a diffracted light produced from said diffraction grating-is utilized to find the amount of displacement such as the amount of movement or the amount of rotation of the object to be measured. Such displacement measuring apparatuses, because of their relatively great ease of highly accurate measurement, are often used particularly for precision machines such as 9C machine tools and semiconductor printing apparatuses.

Specific examples of the displacement measuring apparatus of the described diffraction type are shown, for example, U.S. Patents No.. 3, 726,595, No. 3,738,,758, No. 3,756,723, No. 3,891,321, No. 4,629,886 and No. 4,676,645, Japanese Laid-Open -Patent Applications.,No. 191906/1983 and No. 191907/1983.

w 1 i 1 1 1 In the displacement measuring apparatus of the diffraction type, diffracted lights produced from the diffraction grating are caused to interfere with each other to form interference fringes, and the light and shade of the interference fringes is counted by photoreceptor means to thereby obtain an interference signal regarding displacement.

Accordingly, when the output of the light source is fluctuated by an environmental change such as a temperature change or the transmittance of the diffraction grating (in the case of a reflection diffraction grating, the reflectance thereof) is not uniform or the thickness of the line width of the transmitting portion or the reflecting portion is not uniform if an amplitude type diffraction grating is employed, the output value E regarding the interference fringes from the photoreceptor means is output as an unstable waveform as shown in Figure 1A of the accompanying drawings.

Particularly, the diffraction grating is ready to cause irregularity of etching during its manufacture, and it is very difficult to improve the non-uniformity of the line width in the entire measurement area (in the case of a phase type diffraction grating, the shape of'the level difference or the like), and this tendency - 4 remarkably presents itself.

When due tothe causes as mentioned above, the output value from the photoreceptor means fluctuates as shown in Figure 1A and becomes lower than the slice level-of the comparator in the subsequent counter circuit, it becomes impossible to accurately count the output waveform.

Even if the output value exceeds the slice level, the center level of amplitude is unstable and therefore, the widths of the "H" level and "L" level of the output of the comparator become unstable as shown in Figure 1B of the accompanying drawings. Thus, signal processing such As electrical division in the subsequent electrical circuit becomes difficult and it becomes very difficult to accomplish measurement of displacement at high accuracy and high resolution.

SUMMARY OF THE INVENTION

It is the object of the present invention to eliminate the above-noted problems peculiar to the prior art and to provide a displacement measuring apparatus which is always capable of accomplishing highly accurate measurement of displacement.

To achieve the above object, a displac.ement measuring apparatus in accordance with the present invention has means foz directing a light to-a diffraction grating formed ar, an object to be measured, optical V 1 means for forming interference fringes from a diffracted light produced by said diffraction grating, means for photo-electrically converting said interference fringes and detecting an-interference signal, means for detecting the intensity of said diffracted light and forming a reference signal, and means using said reference signal to form.a signal of constant. amplitude from said interference signal,'and is characterized in that the displacement of-said object to be measured is measured 10 on the basis of said signal.of constant amplitude.

According to a preferred form of the present invention, said interference fringes are provided bysuperposing diffracted lights of particular order, e.g. lst-order, produced by the diffraction grating one upon the other. This superposition of the diffracted lights leads to a preferable result regarding the intensity and the light-and-shade ratio of the interference fringes.

The formation of said reference signal is accomplished by receiving and photoelectrically convert- ing at least one diffracted light by a photoreceptor in a state in which only the intensity of the diffracted light can be detected independently of the displacement p of the object to be measured even if said diffracted light interferes with other diffracted light.

According to the present invention, in order that a reference signal may be formed from a diffracted light produced by the diffraction grating, not only the 1 i 1 output fluctuation of a light source itself which supplies a light, but also the fluctuation of the intensity of the diffracted light attributable to characteristic.of the diffraction grating itself can be accurately monitored",,--Thereby, a signal of constant amplitude regarding the displacement of the object to be measured can be formed.. from the interference signal.

Further features.of the present invention are described in the-folowing detailed description of the embodiments thereof. Besides these embodiments,.various apparatuses can be easily made on the basis of the idea of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates a conventional interference signal and the state of a binarized'signal formed from the interference signal.

Figure 2-schematically shows the construction of a linear encoder according to an embodiment of the present invention.

Figure 3 illustrates the states of an interference signalf a reference signal and a binarized signal obtained by the linear encoder shown in Figure 2.

Figure.4 is a block diagram showing an example of an electrical circuit for forming a signal of constant amplitude from the interference s17n-,1'hy the use.of tbe. reference signal.

i 5? Q Figure 5 schematically shows a rotary encoder according to another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 2 is a schematic view of an optical system according to an embodiment of the present-invention as it is applied to a linear encoder. In Figure 2, the reference numeral 1 designates a monochromatic light source which emits a coherent light beam, such as a semiconductor laser, and the reference numeral 2 denotes a diffraction grating formed on or connected to an object to be measured, not shown, which is moving in the direction of arrow 21. The diffraction grating 2 is moved with the movement of the object to be measured.

The reference numerals 31 and 32 designate corner cubes, the reference numerals 41 and 42 denote quarter wave- length plates,- the reference numerals 51 and 52 designate non-polarizing beam splitters (half-mirrors), the reference numerals 61 and 62 denote polarizing plates, and-the reference numerals 71, 72 and 73 designate photoreceptors. Particularly, the photo receptor 73 is disposed to monitor the intensity of diffracted light.

The light beam from the light source 1 enters the diffraction grating 2 perpendicularly thereto, and is diffracted in various directions by the diffraction grating 2. At this time, diffracted lights of particular 1 positive and negative orders, e.g., 1st-order diffracted lights, are reflected by the corner cubes 31 and 32, respectively, and again enter the diffraction grating 2 through the quarter wavelength plates 41 and 42. The positive and negative diffracted lights again diffracted thereby are superposed one upon the other and enter the beam splitter 51P and are divided into two reflected and transmitted light beams. Of these, the transmitted light beam does not interfer with the reflected light beam for the reason set forth later and is received simply as intensity by the photoreceptor 73, which thus outputs a reference signal. On the other hand, the reflected light is again divided into two reflected and transmitted light beams by the beam splitter 52, and these light beams become coherent light beams through the polarizing plates 61 and 62, respectively, and enter the photoreceptors 71 and 72, respectively. At this time, the light beams received by the photoreceptors 71 and 72 correspond to the intensit ies of the light and shade of interference fringes which interfere with each other, and the photoreceptors 71 and 72 output interference signals.

That is, if the pitch of the-diffraction grating 2 is P and the order of the positive and negative diffracted lights is-m, the photoreceptors 71 and 72 output a.signal of sine waveform-for each arount.nf mo,.,,ement,:f the - - d..

i 1 1 Q 1 - 9 1 In the present embodiment, by adjusting the combination of the rectilinearly polarized light of the light source 1 and the polarizing state of the quarter wavelength plates 41 and 42 and the polarizing plates 61 and 62, a phase difference of 90 is provided between the output signals of the photoreceptors 71 and 72 to thereby discriminate the direction of movement of the diffraction grating 2.

That is, in the linear encoder shown in Figure 2, the laser light emitted from the light source 1 is a rectilinearly polarized light polarized in a predetermined direction, and each of a plurality of diffracted lights of predetermined orders produced by being first diffracted by the diffraction grating 2 is also said rectilinearly polarized light. The quarter wavelength plates 41 and 42 provided in the optical path on the light emission side of the corner cubes 31 and 32 are set so that the.directions of the optic axes thereof form an angle of 450 with respect to said predeermined direction and moreover, the directions of the optic axes of the quarter wavelength plates 41 and 42 form an angle of 900 with respect to each other. Accordingly, two diffracted lights reflected by the corner cubes 31 and 32 and passed through the quarter wavelength plates 41 and 42 are converted to circularly polarized lights_ opposite in direction to each other. In this regard,if the diffracted light reflected by the corner cube 31 1 is a righthanded circularly polarized light and the diffracted light reflected by the corner cube 32 is a lefthanded circularly polarized light, these diffracted lights are re-diffracted by the diffraction grating 2 and emerge in the same direction to overlap each other, whereby the light beams having thus overlapped each other become a rectilinearly polarized light.

This rectilinearly polarized light has its direction of polarization fixed when the diffraction grating_2 is not displaced, but has its direction of polarization changed when the diffraction grating 2 is displaced in the direction of arrow 21. The intensity of the rectilinearly polarized light does not change in conformity with the displacement of the diffraction grating-2. Accordingly, the photoreceptor 73 receives part of the diffracted light which has been made into said rectilinearly polarized light by the non-polarizing beam splitter, but the intensity of the diffracted light can be stably monitored independently of the displacement of the diffraction grating 2.

On the other handr the light-beam consisting of two diffracted lights overlapping each other and comprising rectilinearly polarized lights which has been reflected by the non-polarizing beam splitter 51 is divided into two light beams by the non-polarizing beam splitter 52. The light.beam reflected by the nonpolarizing beam splitter 52 is received by tche j; i 1 1 S; I photoreceptor 71 through the polarizing plate 61, and the light beam transmitted through the non-polarizing beam splitter 52 is received by the photoreceptor 72 through the polarizing plate 62.

The polarizing Plates 61 and 62 are disposed so that at this timet their polarization axes are fixed and the directions of their polarization axes form an angle of 450 with respect to each other. Accordingly, as previously described, with the displacement of the diffraction grating 2, the directions of rectilinear polarization of the light beams entering the polarizing plates 61 and 62 rotate and the intensities of the light beams reaching the photoreceptors 71 and 72 are varied. That is, a change in light and shade occurs on the light- receiving surfaces of the photoreceptors 71 And 72. The photoreceptors 71 and 72 output interference signals corresponding to this change in light and shade, but a phase difference of 90 is formed between the interference signals from the respective photoreceptors 71 and 72 because the polarization axes of the polarizing plates 61 and 62 form an angle of 45 with respect to each other. In the- embodiment shown in Figure 2, the disposition of the photoreceptor 73 and the disposition of the photoreceptors 71 and 72 may be replaced with each other. 25 Description will now be made of a method.of processing the output signals from the photoreceptors 71, 72_and 73 in the present embodiment.

1 1 - 12 1 Expressing the rectilinear polarization of the light source 1 as a sin wt, 0 (a: amplitude, w: frequen- cy), positive and negative diffracted lights m.G) and m 9 diffracted by the diffraction grating 2, passed through the quarter wavelength plates 41 and 42 and again diffracted by the diffraction,grating 2 can be expressed as follows:

m ag/ Y,'2 (sin (wt + 4 5 cos (wt + 4 5 m ag/yr2-(sin(wt + 459 + 6), cos(wt + 4511 + 6)) where 6 is the phase difference between the positive and negative diffracted.lights brought about by the movement of the diffraction grating 2, and g is the diffraction efficiency of the mth-order diffracted lights produced by the diffraction grating 2. Accordingly, the light beam comprising the positive and negative diffracted lights superposed one upon the other.is expressed as follows:

m @) + m v/2--ag sin (wt + 4 5 0 + 612) x (cos 612, sin 6/2) The intensity 13 of the light beam passed through the non-polarizing beam splitter 51 and entering the photoreceptor 73 is 13 = m @ + m(912 = 2a 2g2s in 2(wt + 450 + 612) X' (cos 2 612 + sin 2 6/2) 2a 2 g 2 sin 2 (wt + 45 + 6/2) and the output FR'Af the reference signal obtained from the photoreceDjx--or 77.3' ALs 1 1 E3 = 2a 2 g 2. (1) If the polarization azimuth angle of the polarizing plate 61 is o51, theincident light beam entering the photoreceptor 71 is expressed as v/-2ag sin(wt + 45 + 612)(cosoS 1 - 612) x (costbl, sinOl) and the intensity Il thereof is expressed as I1 = 2a 2 g 2 cos 2 612) sin 2 (wt + 45 + 612).

Thus, the output El of the interference signal obtained by the photoreceptor 71 is E1 = 2a 2 g 2 cos 2 (l. 6/2) = a 2 g 2 {1 + cos (26 1 - M}. (2) Likewise, the output E2 of the interference signal obtained by the photoreceptor 72 is 2 2 E2 =-.a g {1 + cos(26 2 - 6)}.

(3) If the azimuth angles of the polarizing plates 6,1 and 62 are made to form 45 with respect to each other, it follows from equations (2) and (3) that a phase difference of 90' is obtained between the outputs ofthe photoreceptors 71 and 72 since 95 1 - 56 2 = 4511.

Also, from equation (1), the output E3 of the photoreceptor 73 is a signal component of the output S El and E2 shown in equations (2) and (3) which is equal to the term of amplitude and which will not change even if the diffraction grating 2 is displaced, and is of a value dependent on the output variation a 2 of the light source.1, the diffraction efficiency fluctuation g 2 0.

i the diffraction grating 2. etc.

Let it be assumed that the output value of the light source 1 has fluctuated or the diffraction efficiency of the diffraction.grating 2 has been varied by the manufacturing error or the like of the diffraction grating 2 and the amplitude of the output signal El from the photoreceptor 71 has beenfluctuated due to these factorsas-shown in Figure 3A. At such time, the output signal from the photoreceptor 73 becomes such as shown in Figure 3B. In the present embodiment,-the output signal El is corrected by being divided by the then obtained output signal E3 by the use of an operator, and this value fl = El/E3 is found as.the output signal from the photoreceptor 71. Thereby, an output signal El of constant amplitude as shown in Figure 3C is obtained. The output signal E1 is processed by a predetermined slice levelt whereby there is obtained a binarized signal comprising a rectangular wave signal which is stable in "H" level and "L" level as shown in Figure 3D.

Thus, in the present embodiment, the output signal E3 from the photoreceptor 73 is utilized, whereby there are always obtained stable output signals of constant amplitude as shown in Figures 3C and 3D even if the output signal El from.the photoreceptor 71 fluctuates due to the, output fluctuation of the light source 1 and the manufacturing error or the like of the diffraction f i i 1 grating 2 and a signal having an error is f ormed.

Obtainment of such stable signals makes thesignal processing such as electrical division in the subsequent processing circuit easy to accomplish and enables measurement of high accuracy and high resolution to be accomplished.

For the output E2 from the photoreceptor 72,, a stable output signal is obtained entirely in the same manner as the output El.

Figure 4 is a block diagram of an electrical circuit for obtaining t he output signals shown in Figures -3C and 3D. In Figure 4, the output signals El and E2 from the photoreceptors 71 and 72 are divided by the output signal E3 from the photoreceptor 73 by the use of dividers 74 and 75# respectively, whereby the output signal E1 = El/E3 and the output U2 = E2/E3 are found. These output signals fl and g2 are input to the subsequent counter circuit 76, whereby the-signal as-shown in Figure 3D is obtained. 20 The linear encoder shown in Figure 2 causes the two diffracted lights produced by the diffraction grating to enter the diffraction grating again, and causes the re- diffracted lights-produced there to be superposed one upon the other. Howevery according to the present invention, the diffracted light used to form interference fringes on the light-receiving surface can be sufficiently used 1 1 1 if it is a light produced by being at least once diffracted by the diffraction grating. Also, even if the diffracted lights are not always superposed one upon the other, interference fringes can be formed by superposing a predetermined diffracted light and a reference light one upon the other as shown also in the aforementioned U.S. Application Serial No. 880,207.

Also, thediffracted light usedto form the reference signal must be at least one of the diffracted lights produced from the diffraction grating. Accordingly, when interference fringes are formed by a diffracted light and a reference light as shown in U.S. Application Serial No. 880,207, it is necessary to adjust the intensities of the remaining diffracted.light and the reference light so as to be finally equal to each other even if part of the diffracted light is taken out for the formation of the reference signal. Also when a plurality of diffracted lights are superposed one upon anothe r, adjustment similar to that described just above is necessary if-part of-one diffracted light is used for the formation of the reference signal.

If, as shown in Figure 2, the two diffracted lights produced by thediffraction grating 2 differ in direction of polarization from each other and the intensity of the light beams having overlapped each other-does not change even if the.diffraction grating 2 is displaced, -art of the respective diffracted lights J i C.

fr 1 A can be received in that state and the intensity of-the diffracted lights can be detected. Accordingly, when it is desired that part of the plurality of diffracted lights produced by the diffraction grating which contribute to the formation of interference fringes be received to thereby form a reference signal, contrivance is made so.as to change-the direction of polarization so that-the intensity of the light beam does not vary even if these partial diffracted lights interfere with each other and a change in phase occurs. For example, the respective diffracted lights are made into predeter mined linearly polarized lights so that the directions of polarization of the two diffracted lights are orthogonal to each other, or as'in the above-described embodiment, the two diffracted lights are made into circularly polarized light opposite in direction to each other, and are received-by,a predetermined photo receptor.

of course, when an interference signal is to be detected, it is necessary that these overlapping diffracted lights be caused to enter a predetermined polarizing plate and as shown in Figure 2, the'rotation of the plane of polarization of a rectilinearly polarized light comprising the overlapping diffracted lights be converted to a change in light and shade on the lightreceiving surface of a photoreceptor, or that lights of the same polarized components be extracted from two I diffracted lights whose directions of polarization are orthogonal to each other and a change in the phases of the respective diffracted lights be converted to a change in light and shade on the light-receiving surface 5 of the photoreceptor.

Figure 5 is a schematic view of an optical system according to an embodiment of the present invention as it is applied to a rotary encoder. In Figure 5,, members-functionally similar to those in Figure 2 are given similar reference numerals. In Figure 5, the reference numeral 10 designates a collimator lens, and the reference numeral 11-denotes a parallel glass plate inclined by 45 with respect to the optic axis. The reference numeral 12 designates a polarizing prism, the reference numeral 13 denotes a radiation grating formed on the circumferential portion of a rotary scale along the direction of rotation thereof, the reference numeral 14 designates a photoreceptor, the reference numeral 15 denotes a rotary shaft to be connected to a rotating object to be examined, the reference numeral 16 designates a non-polarizing prism, the reference numeral 17 denotes a cylindrical lens, the reference numerals 81 and 82 designate reflecting means, the reference numerals 91 and 92 denote reflecting prisms, and the reference numerals 41, 42 and 43 designate cruarter wavelength plates. In Figure 5, a light beam emitted froma light source 1 such as a laser is made Z1 7 v 1 1 1 into a substantially parallel light beam by the collimator lens 10. This light beam enters the polarizing prism 12 through the parallel glass plate 11. The light beam having entered the polarizing prism 12 is reflected therewithin, is directed to a polarizing beam splitter formed on the cemented surface of the polarizing prism 12 and is divided thereby into a reflected light beam and a transmitted light beam.

Of the two light beams divided by the polarizing J0 beam splitter, the reflected light beam (the S-polarized light) is repetitively reflected by the internal surfaces of the polarizing prism 12 and emerges from the polarizing prism 12 in a direction parallel to the direction of incidence. This reflected light beam is then reflected by the reflecting prism 92 and enters the position M1 on the radiation grating 13 at a predetermined angle. Of the transmitted diffracted lights having entered the radiation grating 13 and diffracted thereby, a diffracted light of particular order is reflected by the reflecting means 82 through the quarter wavelength plate 42 and is caused to retrogress along the same optical path and again enter the same position M1 on the radiation grating 13 through the quarter wavelength plate 42. Accordingly, the diffracted light of particular order again diffracted by the radiation grating 13 is caused to reciprocate in the quarter wavelength plate 42, whereby it is made into a rectilinearly polarized light k i 1 (P-polarized light) differing by 90 in the direction of polarization from the incident light and is directed to the reflecting prism 92. The diffracted light reflected by the reflecting prism 92 retrogresses along the original optical path and again enters the polarizing prism 12 and arrives at the polarizing beam splitter.

In the present embodiment, the optical paths from the polarizing beam splitter of the polarizing prism 12 to the reflecting means 82 along which the diffracted light of particular order reciprocates are identical.

The reflecting means 82 and 81 may be ordinary plane mirrors or precision optical elements such as corner cubes. Alternatively, a reflecting mirror may be disposed substantially on the focal plane of a condensing lens so that only a diffracted light of particular order which has entered the condensing lens in parallelism thereto-may be caused to pass through an opening in a mask and may be reflected by the reflecting mirror, whereafter it may retrogress along the original optical path, while diffracted lights of the other orders may be intercepted by the mask. Besides these, the reflecting means may be of any construction such as, for example, a cat's eye optical system. If such an optical system is employed, the diffracted lights can be returned to the diffraction grating along substantially the same optical path even if the oscillation wavelength of the 1 11 1 laser, for example, is varied and the angles of diffraction of diffracted lights are more or less varied.

Turning back to Figure 5, of the two light beams divided by the polarizing beam splitter, the transmitted light beam (the P-polarized light) is repetitively reflected by the internal surfaces of the polarizing prism 12,, whereafter it emerges from the polarizing prism-12 and is caused to enter a position M2 substantially point-symmetrical with the position M1 on the radiation grating 13 with respect to the rotary shaft 15, through the reflecting prism 91. Of the transmitted diffracted lights having entered the radiation grating 13 and diffracted thereby,.a diffracted ' light of particular order is caused to retrogress along the same optical path by the reflecting.means 81 which is similar to the aforementioned reflecting.means 82, and is again diffracted to the same position M2 on the radiation.grating 13 through the quarter wavelength plate 41. Accordingly, the diffracted light of particular order again diffracted from the radiation grating 13 is caused to again enter the reflecting prism 91 so as to become a rectilinearly polarized light (S- polarizedlight) differing by 900 in the direction of polarization from the incident light.

The diffracted light reflected by the reflecting prism 91 retrogresses along the original optical path 1 and again enters the polarizing prism 12, and then arrives at the polarizing beam splitter.

At this time, for the transmitted light beam, as for the afored.escribed reflected light beam, the optical paths from the polarizing beam splitter to the reflecting means 81 along which the diffracted light of particular order reciprocates are identical. This diffracted light is caused to overlap the diffra:cted light having entered through the reflecting means 82, whereafter-it is caused to emerge from the polarizing prism 12, and the P- polarized light and the S-polarized light are made into circularly polarized lights opposite in direction to-each other through the quarter wavelength plate 43, and then are caused to enter the non-polarizing prism 16.

Part of the light beam having entered the nonpolarizing prism 16 is transmitted through non-polarizing beam splitters to be described andenters the photoreceptor 73, and is photoelectrically converted thereby in a state in which two diffracted lights do not interfere with each other, whereby a reference signal is obtained. On the non-polarizing prism 16, non-polarizing beam splitters 16a and'116b are provided at-a predetermined interval in the optical path of the two overlapping diffracted lights, and light beams successively reflected thereby,enter the photgreceptors 71 and 72 through the polarizing plates 61 and 62 whose directions of 1 14 1 9 polarization deviate from each other by 45 and-are photoelectrically converted thereby, whereby an interference signal is obtained. As in the aforedescribed embodiment of Figure 2, an electrical circuit as shown in Figure 4 is provided at the stage subsequent to the photoreceptors 71, 72, 73, whereby a stable signal of substantially constant amplitude can be obtained.

In the rotary encoder shown in Figure 5, some light-beam reflected.by plane parallel glass 14 irradiates a zero-point detecting mark formed at a predetermined location on the rotary scale. The light from this mark is transmitted through the plane parallel glass 14 and received by the photoreceptor 14, whereby detection of the mark formed at said predetermined location is 1 effected. The detection of this mark is effected each time the rotary scale makes one full rotation and the reference signal when the rotational state of the rotary scale is measured is formed.

If use is made - of the encoder of the embodiment shown in Figures 2 to 5, the amount of displacementlof the object to be'measured can of course be detected accurately. The displacement measuring apparatus of the present invention can be applied not only to an encoder, but also to a speed meter.

A.

1 f Z I Also, in the encoder of the embodiment shown in Figures 2 to 5, the linear scale and rotary scale on which diffraction gratings are formed may take various forms. For example, the diffraction gratings formed on these scales may be amplitude type or phase type gratings and further, these two types of diffraction gratings can be constructed as transmission type or reflection type gratings.. According to the present invention, as described above, the intensity of the diffracted light from the diffraction grating is detected to form a reference signal, whereby even if there is a fluctuation of the output of the-light source or a fluctuation of the diffraction efficiency of the diffraction grating, the signal by which the light and shade of interference - - 1 1 1 -1 i 1 v IL 4 1 fringes is counted does not become unstable but can be made constant and thus. a displacement measuring apparatus which ensures highly accurate measurement to be accomplished can be achieved.

1

Claims

CLAIMS:

1. A displacement measuring apparatus including: means for directing a light to a diffraction grating formed on an object to be measured; optical means for forming interference fringes from a diffracted light produced by said diffraction grating; means for photoelectrically converting said interference fringes and detecting an interference signal; means for detecting the intensity of said diffracted light and forming a reference signal; and means for using said reference signal to convert said interference signal to a signal of constant amplitude, the displacement of the object to be measured being measured by said signal.

2. A-displacement measuring apparatus according to Claim 1, wherein said directing means has a semicon- ductor laser, and the coherent light beam from said laser is directed to said diffraction grating.

3. A displacement measuring apparatus according to Claim 1, wherein said optical means causes two diffracted lights of particular orders produced by said diffraction grating to be superposed one upon the other and forms interference fringes.

1 11 V, 4. A displacement measuring apparatus according to Claim 3, wherein said. two diffracted lights comprise circularly polarized light beams opposite in direction to each other, and said optical means directs said two diffracted lights to said detecting means through a polarizing plate while kdepin.g said two diffracted lights superposed one upon the other..

5. A displacement measuring apparatus according to Claim 4, wherein said reference signal forming means directly receives and photoelectrically converts said, two diffracted lights to thereby detect the intensities thereof.

f 6. A displacement measuring apparatus according to Claim 1, wherein said converting means divides said interference signal by said reference signal to thereby form a signal of.constant amplitude.

7. A displacement measuring.apparatus including:

means for directing a coherent light beam to a movable diffraction grating and producing a plurality of diffracted lights by said diffraction grating; means for superposing said plurality of diffracted lights one upon another to thereby form interference fringes and outputting an interference signal-corresponding to the light and shade of the f v - 28 1 interference fringes; means for receiving part of said plurality of diffracted lights and forming a reference signal corresponding to.the intensity of said diffracted lights and means for converting said interference signal to a signal of constant amplitude by said reference signal, the displacement of said diffraction grating being measured by said signal.

1 8. A displacement measuring.apparatus according to Claim 7, further including means for binarizing said signal of constant amplitude, and means for counting the binary signal output from said means, the amount of displacement of said diffraction grating being measured by said counter means.

9. A displacement measuring.apparatus according to Claim 7, wherein said producing means includes a semiconductor laser.

10. A displacement measuring apparatus according to Claim 7, wherein said producing means produces mth-order diffracted lights differing in direction of polarization from each other.

11. A displacement measuring apparatus according k d 1 1 r 1 % 1 v pc to Claim 10. wherein said mth-order diffracted lights comprise circularly polarized lights opposite in direction to each other.

12. A displacement measuring apparatus according to Claim 10, wherein said mth-order diffracted lights comprise rectilinearly polarized lights differeing in direction of polarization from each other.

13. A displacement measuring apparatus according to Claim 7, wherein said converting means includes a.' division circuit for dividing said-interference signal by said reference signal.

-14. A displacement measuring apparatus according to Claim-10, wherein said forming means has a photoreceptor and receives and photoelectrically converts part of said mth-order diffracted lights, whereby said reference signal is formed.

15. A displacement measuring apparatus according to Claim 14, wherein said output means receives the remainder of said mth- order.diffracted lights through a polarizing plate and-forms interference fringes on a light-receiving surface, and photoelectrically converts said interference fringes to thereby output-the interference signal.

i 1 16, A displacement measuring apparatus including:

means for directing a coherent light beam to a movable diffraction grating and causing a diffracted light to be produced from said diffraction grating; means using-said diffracted light to form interference fringes-and photoelectrically converting said interference. fringes to thereby obtain an interference signal; and means for receiving part of said diffracted light and monitoring the intensity of said diffracted light, the displacement of said diffraction grating being measured by the utilization of said interference signal and the output signal from said monitor means.

17. A displacement measuring apparatus according to Claim 16, wherein said monitor means receives part of the diffracted light forming---said interference-fringes.

18. A displacement measuring apparatus according to Claim 17,.wherein said interference fringes are formed by causing two diffracted lights produced by said diffraction grating to interfere with each other, and said monitor means receives part of each of said two diffracted lights.

t R 1 k 11 1 k 19. A displacement measuring method in which interference fringes of varying amplitude are detected, and in which compensation is made for the variations in amplitude of the detected fringes.

20. A displacement measuring apparatus or method substantially.as described in the description with reference to Figures 2 to 5 of the drawings.

V, t Published 1988 at The Patent Office, State HouBe, 66/71 High Holborn, London WCIR 4TP. Further copies may be obtained from The Patent office, Sales Branch, St Mary Oray, Orpington, Kent BR5 3P.D. Printed by Multiplex techniques ltd, St Maa7 Cray, Kent Con. V87.