Diffraction Encoded Position Measuring Apparatus

The present invention is generally concerned with position detection and more specifically, with position detection using light-beams such as produced by lasers in conjunction with a grating whose movement or position is being detected. Even more specifically, the invention is related to the effective change in phase of a laser signal after passing through a grating and producing diffracted orders higher than zero, and using detection apparatus to measure the difference between the zero order beam and one of the higher order beams for use in accurately measuring displacement of the grating.

Prior art position detector apparatus have used gratings as an encoder for linear or angular measurements and comprises a common technique used in many commercial devices. One such prior art device is illustrated in a paper authored by myself and S. Holly in the proceedings of SPIE--The International Society For Optical Engineering, Volume 887, as presented Jan. 14, 1988, in Los Angeles, Calif., in the Acquisition, Tracking and Pointing group. The title of the article was, "Development of An Interferometric Encoder For High Resolution Angular Measurements". The prior art technology of this paper as concerns heterodyne detection and other well-known prior art techniques is incorporated by reference.

The grating for many prior art devices is produced holographically by two beams which interfere at an angle of incidence theta (.theta.), on a substrate which retains the image of the laser pattern. The spacing of the grating will be an amount G which equals lambda (.lambda.) divided by (2 times sine of the angle theta) where lambda is the wavelength of the beam. Prior art techniques have compared the position of a reference grating with one being produced at the time of measurement by the laser involved. Data from the two gratings are then compared in a phase measurement type test to provide an indication of position. A device such as a computer can keep track of the phase changes for total number of cycles changed to get absolute position relative to a reference.

While the use of gratings for an encoder for linear or angular measurements has become a common technique used in many commercial devices, all prior devices known to applicant have involved the use of a grating whose spacing is considerably larger than the wavelength of the lightwave comprising the laser beam signal. The prior art approach has used increasingly accurate and accordingly expensive phase detectors to get the position detection accuracies required for some applications. The approach discussed here would use a small wavelength laser to achieve a small grating space thereby relaxing the requirement on phase detection.

Since the present invention involves the discovery that the grating can have a spacing comparable to or less than the wavelength, one aspect of the present invention allows the gratings to be produced by a short wavelength, (more expensive) laser from the fringe effects produced by the interference of two such lasers and then this grating be used with a longer wavelength but less expensive laser in combination with a low cost phase detector to produce the measurement desired.

A further realization in the present invention is that if a diffracted beam is redirected through the grating N times before being detected, the change in phase will be N times the change in phase of a single pass system.

A multipass system using only the multipass concept portion of the present invention is illustrated by an article in Motion magazine, July/August, 1986, authored by Nishimura and Ishizuka.

It is also recognized in the present invention that while there is less power in a diffracted beam of greater than first order, the higher order diffracted beams have greater phase changes than do the lesser order beams and thus, this factor can be used to increase the phase change in the output signal relative the movement of the grating.

All of the above features presented in this concept when combined lessen the requirement for both the cost of the laser (i.e., a low cost laser may be used) and the cost of the phase detector since the phase detector can be of a much lower accuracy than was required in the prior art.

It is thus an object of the present invention to provide an improved motion detector or diffraction decoder apparatus involving high accuracy motion detection without the necessity of high cost components.

Other objects and advantages of the present invention will be apparent from a reading of the specification and appended claims in conjunction with the drawings wherein;

FIG. 1 is a representation of a two pass encoder used to easily describe the inventive concept;

FIG. 2 is a four pass beam reflected encoder system using the principles of FIG. 1; and

FIG. 3 is a further single pass encoder using a minimum of parts while still providing acceptable accuracy for some applications.

In FIG. 1 a laser source supplies a light beam signal on a path or beam labeled 10. This beam is passed through a block 12 labeled AOM for acouso-optic modulator which superimposes a modulated signal on the laser beam at a frequency that can be easily detected. In one embodiment of the invention, the laser beam had a wavelength of 633 nanometers which is approximately equivalent to 4.7.times.10.sup.12 megahertz. On the other hand, the modulating frequency from the AOM 12 may be a much lower value such as 40 megahertz. A second laser beam 14 is shown passing through a second AOM 16 and producing an output beam 18 modulated at 41 megahertz. The source 14 may be the same as source 10 and redirected using mirrors or may be a separate source if it is in phase with 10. The light beam 18 after passing through a grating 20 continues as a zero order beam 22 and a portion of the original beam 18 is diffracted as an N order beam such as first order beam 24. The output of AOM 12 is supplied as a zero order beam 26 to the grating 20 and after passage through the grating 20, continues as a zero order beam 28 and a diffracted beam 30. The phase of the diffracted beams 24 and 30 change phase in opposite directions for a given direction of movement of grating 20. Again, beam 30 may be any of N orders but for the purposes of explanation will be a first order beam. The beam 30 is deflected from a mirror 32 to a beam splitter 34. The beam 24 is reflected off a further mirror 36 and is also supplied to beam splitter 34. Within the beam splitter 34, the signals are combined to produce a fringe pattern which is detected with a detecting device of the square law type such as a photo detector. Detector 38 is labeled as the detector of the fringe pattern. A portion of the beam reflected from beam 32 is labeled as beam 40 and is the remaining portion of beam 30 which is still in existence after the combining or mixing process. Beam 40 would be supplied to some type of beam dump 42 and would be unused. The output of detector 38 would be an electrical signal indicative of the difference frequency between the modulated signal on beams 24 and 30. This output is labeled 44. The beams 22 and 28 are combined in a beam splitter generally designated as 46 and a fringe pattern resulting from the combination of 22 and 28 is detected by a detector 48 which operates in a manner similar to that of 38. A reference signal output is obtained from the detector 48 and is supplied on a lead 50. The extension of light beam 28 that is the unused portion, is also supplied to a beam dump 52.

In FIG. 2, a laser 75 is shown supplying a light beam to a beam splitter generally designated as 77 and the beam of light is output as two separate beams 79 and 81 from beam splitter 77. Beam 79 goes through a modulating device such as an AOM 83 and then passes through a grate 85 where it continues as a beam 87 of zero order and a first order beam 89. The beam 89 is reflected by a pair of mirrors generally designated as 91 and returned to the grate 85 where again a zero order beam continues, and is dissipated in a bean dump and a further deflected beam of the first order 93 is returned to beam splitter 77. The beam 81 is passed through a further modulator (AOM) 95 which modulates the signal at a slightly different frequency than does the modulator 83 and provides an output 97. The output 97 passes through grating 85 and produces a zero order beam 99 and a first order diffracted beam 101. Beam 101 is reflected from a pair of mirrors generally designated as 103 and is returned as a beam 105 which is again passed through grating 85. A first order component of beam 105 is designated as 107 and it is returned to beam splitter 77. The beams 93 and 107 produce a fringe effect which is detected by a detector 109. Detector 109 provides an electrical signal on a lead 111 to a phase detector 113. The beam 87 is reflected by a mirror 115 and supplied to a beam splitter generally designated as 117. The beam 99 is reflected by a mirror 119 and also supplied to beam splitter 117. The two beams received by beam splitter 117 interfere to produce a fringe effect and the fringe effect is detected by detector 121 and an output signal is produced on lead 123 and supplied to phase detector 113. The output of phase detector 113 is a voltage proportional to the phase difference between signals from 121 and 111, and this provides a total movement indication of the grating 85. This signal can be converted to a voltage by block 125 and stored in a computer or viewed on any voltage measuring device such as an oscilloscope 127. It will be noted that the beam starting out as 79 passes through the grating 85 on two separate passes each of which has the same relative angle for a given direction of movement of the grating 85. The two beams which pass through grating 85 as a result of initial beam 81 are aligned such that they have a component which is opposite the previously referenced beam. Movement of the grating 85 produces an effect similar to a Doppler effect in phase shifting the modulating frequency of the light beam passing through. This phenomena is well-known in the art and may be verified in many reference books such as on Page 864 of a book entitled, "Applied Optics--A Guide To Optical System Design/Volume 2", by Leo Levi, copyrighted in 1980 and published by John Wiley and Sons.

In FIG. 3, a laser 150 supplies a laser beam through an acousto-optic modulator 152 to provide an output zero order beam 154 and a first order beam 156. The beam 154 goes directly to a beam splitter 158 while the first order beam 156 is supplied to a grating 160 where it is output as a beam 162 which continues to a beam dump 164 and a diffracted first order beam 166 is passed to the beam splitter 158. The two beams 154 and 166 interfere or combine in the splitter 158 and produce a fringe effect output which is detected by a detector 170. The continuation of beam 154 is passed to and terminated by a beam dump 172. An electrical output of detector 170 is supplied to a dash line block generally designated as 174 which contains an RF amplifier 176. An output of the amplifier 176 is supplied to a mixer 178 whose output is supplied to a phase detector 180. A 40 megahertz signal is supplied both to the AOM 152 to provide modulation for it and to a mixer 182 within block 174. A 41 megahertz signal is supplied on a lead 184 to the two mixers 178 and 182. An output of phase detector 180 is supplied on a lead 186 which is indicative of the phase difference between the reference beam 154 which does not pass through grating 160 and the beam 156 which has a diffracted output 166 after passing through grating 160.

The grating 20 can be produced holographically by two beams which interfere at an angle of incidence on a substrate which retains the image of the interference pattern. The spacing is a function of the wavelength of the beams and the angle of incidence according to well-known formulas. An ultraviolet laser having a wavelength of 300 nanometers could be used to produce a spacing on a grating at 300 nanometers where the angle of incidence is 30 degrees. A cheaper laser such as a helium neon laser with a wavelength of 633 nanometers could be used to provide the laser beams 10 and 14. It may be assumed that each of the acoustic modulators 12 and 16 modulate the main laser beams at slightly different values which, for purposes of discussion, might be 40 and 41 megahertz. Thus, the beams 18 and 26 will each have a basic wavelength of 633 nanometers which is approximately equivalent to 4.7 times 10.sup.12 megahertz with a superimposed ripple of about 40 megahertz on this lightwave. As the beam 18 passes through the grating 20, a zero order beam 22 appears at the output side in line with the beam 18. A higher order beam such as a first order beam 24 is produced from diffraction. As is known in the art, for equally spaced gratings of the "Ronchi" type, there is the most intensity of the first order beam and the higher order beams have relatively less intensity and thus are more difficult to detect. However, the first order beam has the least phase shift in the diffraction and thus, if the intensity of an even higher order beam can be detected (as is the case for phase gratings or specially designed sinusoidal gratings), more phase shift is involved and thus, from the electrical phase detection standpoint, it is easier to detect a small movement of the grating 20. The beam 22, which is the zero order extension of beam 18, is combined in the beam splitter 46 with the zero order extension 28 of the beam 26. The interference or combination of the two beams 22 and 28 within the beam splitter 46 produces a fringe pattern which is detected by a square law detector such as a photodiode within detector 48. An electrical output signal is produced on lead 50 indicative of the difference between base frequency 40 and 41 megahertz signals modulated onto the original light beams by the AOM's 12 and 16. The diffracted beam 24 is reflected by mirror 36 to a beam splitter 34 where it combines with or interferes with a similar beam diffracted from beam 26 which is shown as beam 30 after being reflected from mirror 32. An interference or fringe pattern is produced by beam splitter 34 and the combination of the two incoming light beams, and this fringe pattern is detected by detector 38 and output on lead 44 as a signal which is very close to the one megahertz difference signal output by detector 48 but is phase shifted in accordance with the position of the grate 20. The difference in phase between the signals 44 and 50 can never exceed 2 pi, unless a phase counter is added which can count two pi changes, as I have used in my heterodyne systems.

Since the approach of FIG. 1 is a two pass system, the phase change at output 44 is twice as much as would be indicated by movement of grating 20. In other words, if the movement of grating 20 was 75 nanometers or 1/4 the spacing, then the phase change at output 44 would be pi or 180 degrees of phase difference with respect to the signal on lead 50.

As the number of passes through the grating is increased, the phase change per amount of movement of the grating is changed by a directly proportional amount. With the approach shown in FIG. 2, wherein a four pass system is shown, the same amount of movement of grating 85 will produce twice the phase change at output 123 as was produced by the same movement of grating 20 in FIG. 1 at output 44. Likewise, the single pass system of FIG. 3 would have an output from detector 170 which would only be half the phase change of that of FIG. 1 for a like movement of grating 160 as compared to grating 20 if the two gratings had the same spacings between grid lines.

The operation of FIG. 2 is relatively straightforward once FIG. 1 is understood in that the laser beam from laser 75 is split into two separate components 79 and 81 modulated at two different frequencies such as 40 and 41 megahertz by the acoustic modulators 83 and 95. Each of these modulated light beams is then sent through the grating, reflected and returned through the grating to be combined within splitter 77. The fringe effect that is produced upon the combining is detected by detector 109 to produce a movement output signal on lead 111. The original modulated beams as appear at the output of modulators 83 and 95 have a component which is reflected by mirrors 115 and 119 wherein they are combined in splitter 117 and applied to detector 121. Detector 121 has an output which is primarily equivalent to the difference between the modulating signals applied by the two modulators 83 and 95 or, in other words, a one megahertz signal which is always within one cycle or one complete phase change of the signal appearing at the output of detector 109. The phase detector 113 detects the phase difference at any point in time between the reference detector 121 and the test detector 109. Thus, an accumulated summary of the incremental phase changes is then stored or sent to 127 indicative of the exact position of grating 85 with respect to an initial reference.

As will be realized by those skilled in the art, if the grating comprises part of a disk which is being measured for radial movement, or in other words, to measure angular motion, the only requirement is that the plane of incidence and diffraction be perpendicular to the disk upon which a grating has been produced.

The single pass version of FIG. 3 operates in a manner extremely similar to that described previously for FIGS. 1 and 2. The laser beam from laser 150 is passed through the acoustic modulator 152 and provides as an output a zero order beam represented by indicator 154 which is passed directly to the beam splitter 158 and an N order beam such as first order which is represented by 156. As shown, light beam 154 being the zero order beam is not modulated by AOM, while beam 156 carries the AOM modulation frequency of 40 megahertz. However, after beam 156 is passed through grating 160, the diffracted portion of the beam represented as 166 will be of a phase relative to the beam 154 in accordance with the position of the grating 160. The incident extension of beam 156 which is labeled 162 is merely terminated in a beam dump 164. The beam splitter 158 combines the two beams 154 and 166 and through the fringe effect and modulating difference, provides an output which is detected as moving fringes by detector 170. The output of detector 170 is indicative of the phase difference between the two beams 154 and 166 and this difference as previously indicated, is as a result of the absolute position of the grating 160. The two signals which have a base of 40 megahertz as illustrated in this drawing, are then mixed in respective mixers with a 41 megahertz signal 184. Basically, the signal source 184 is providing a constant reference for the two mixers. The output signals of the two mixers are phase detected by phase detector 180 which then provides an output on 186 to be supplied to some type of accumulator such as 125 in FIG. 2.

It may be noted that alignment of the apparatus of the present invention is trivial. If a diffracted order is produced and interferes with other diffracted orders, the system works. Alignment of prior art systems has not been as easy. Also, as has been implied previously, higher diffracted orders will produce more grating movement phase detection sensitivity. Diffracted orders of order M will produce 2 times M times pi phase changes for a grating movement equal to G for a single pass. Both higher diffracted orders and multiple passes may be used in combination to even further reduce the phase detection requirements of the phase detector or, in the alternative, to greatly increase the accuracy of motion or movement detection of a grating being monitored.

In summary, the present invention is directed to a concept which either greatly increases the accuracy of motion detection over prior art, or greatly reduces the complexity of the electronic phase detecting apparatus for a given accuracy with respect to the prior art or some intermediate combination of both. This feat is accomplished by utilizing multiple passes of a laser beam through a grating, or using higher order diffracted beams than the first order or both. A further aspect of the invention is the realization that the spacing of the grating can be equal to or less than the wavelength of the laser thereby further decreasing the detector phase resolution requirements of the electronic portion of the circuit for a given frequency laser beam generating device.