DE102008013164A1 - Material measure for optical position measuring unit, which operates with light of central wavelength, comprises substrate with two areas, which code position information, which is selected with light of wavelength - Google Patents

Abstract

The material measure (4,5) comprises a substrate (9) with two areas. The two areas code a position information, which is selected with the light of a wavelength. The area of a lattice structure (7) comprises a period, which affects the optical characteristics of the material measure. The period of the lattice structure is smaller than the wavelength. An independent claim is included for an optical position measuring unit has a light source for linear polarized light of a central wavelength.

Description

The The invention relates to a material measure for an optical position measuring device. Such a material measure has structures that are read out by optical means can and contain position information. Further the invention relates to an optical position measuring device, that of the material measure according to the invention is adjusted. Such position measuring devices are used to determine the Position of moving elements in machines of various kinds Art.

One simple scale in an optical position measuring device wears light and dark areas, their superimposition with light and dark areas on one opposite the Scale movable scanning can be detected. For this Light from a light source falls on a sensor when it is on his way to the sensor each bright areas of scale and Scanning plate happens. However, if it falls on dark areas, so the light does not reach the sensor. By a relative movement between the scanning plate and the scale arise on Sensor modulated signals. The sensor can be a photodiode, for example be.

scale and scanning are also used as a measuring standard designated. Distinguish in a length measuring device They usually only by their extent in the direction of measurement. If one uses a structured photodetector, so can on a Scanning plate are omitted, since the detector then like itself a scanning plate acts. The light and dark areas become then imaged directly on its detector fields.

at the patterns of light and dark areas may be one Incremental track that leads to periodic sensor signals, from which a relative movement between material measure and scanning can be calculated. The light and dark areas but can also encode absolute positions. A reference mark generates a reference pulse at the sensor in a certain position. A code track with a pseudo random code (PRC) supplies against it in each relative position information about the absolute position, if each a certain number of bits are evaluated. An absolute Position can also consist of several adjacent incremental tracks different graduation period are calculated.

material measures for linear encoders are also called Scales and measuring standards for Angle encoders also known as part discs.

In the EP 1081457 A2 a position measuring device is described, which is based on a material measure with an incremental track and a reference mark. In the 1C This document illustrates the light and dark areas of the scale formed here as a reflection scale with reflective (light) and non-reflective (dark) areas. Also common are transmitted light scales, where transparent (light) areas alternate with opaque (dark) areas.

material measures with light and dark areas, for example, by coating a glass substrate with chromium or another metal becomes. This layer is then partially removed, reducing the glass scale becomes transparent at these points. The remaining metal acts reflective (ie bright) in reflected light mode, or the light path blocking (ie dark) in transmitted light mode.

Other approaches to making light and dark areas on gauge scales are also known. For example, plastic part disks can be provided with oblique, totally reflecting structures (eg V structures) which block incident light and thus act as dark areas in transmitted-light mode. The US 4820918 shows a position measuring device, in which both the scale incremental track and reference mark and a suitable scanning are realized with such structures. Such material measures can be produced cheaply.

Another method for producing light and dark areas on a plastic disk shows the EP 1801546 A1 from which this application originates. Because of a full-surface metallic coating, the indexing disk is initially opaque. In some areas, however, the disc is provided with periodic structures that are tuned to the wavelength of the incident light to excite plasmon modes in the metal layer. As a result, the dividing disk becomes transparent in these areas. The disadvantage is that for such part discs a metal coating is absolutely necessary, which significantly increases the production cost.

task The invention is an economically producible material measure as well as based on this material measure specify optical position measuring device.

These Task is solved by a scale according to claim 1 or by an optical position measuring device according to claim 12. Advantageous embodiments will be apparent from the Features listed in the dependent claims.

It becomes a material measure for an optical Position measuring device as well as an optical position measuring device described with light of a central wavelength works, with the material measure a for Light of this wavelength transparent substrate with first and second areas, the arrangement of the first and second Areas of position information are coded to match the light is readable and at least the first areas a grid structure having a period that satisfies the optical properties of Measuring standard influenced at this point. The period of the lattice structure is smaller than the central one Wavelength and dimensioned so that in the direction of Grid lines linear (TE) polarized light of central wavelength from the structured first areas almost completely reflected in 0. diffraction order and thus extinguished in transmission becomes.

Of the The basic idea of the invention is therefore that of the optical Properties of an actually transparent substrate or a on a transparent, thin layer applied through a Structuring the surface with periodic lattice structures, whose period is smaller than the wavelength of the irradiated Light, so change that linearly polarized Light in such structured areas almost completely is reflected. The structured areas behave in this case almost like a mirror, diffraction orders do not occur on. In transmitted light mode, such areas are dark, in incident light mode bright. But since this applies only to linearly polarized light, must have a position measuring device with such a material measure have a light source for polarized light. there the E vector of the linearly polarized light must be parallel to the periodic lattice structures.

polarized Light is obtained when z. B. a conventional lighting unit, consisting of an infrared LED and a collimating lens around a simple polarizing film is added. A polarization ratio of about 1:50 is sufficient in most cases to achieve a good degree of modulation in the scanning signals. Alternatively, a light source may be used that is already polarized Emits light, such as B. a laser.

The Period of the grating structures is approximately between half the wavelength of light and the light wavelength itself, preferably just below the Light wavelength, between 90% and 95% of the light wavelength.

For the structured areas of a transmitted light system become a transmission aimed at less than 5% of the incident light. A transmission but up to 10% may still be acceptable depending on the application.

The Lattice structure can either be incorporated in the substrate itself, or in a thin layer deposited on the substrate.

When Substrate or layer are various materials that preferably transparent to the light of the light source are. Here are substrates or layers as possible high refractive indices of advantage. The periodic lattice structures, with which the substrate or the layer must be provided to To achieve optimal results (eg in the sense of a possible low residual transmission or the highest possible reflection), strongly depend on the refractive index of the structured material from. An optimization of the grating structures with regard to the refractive index, the wavelength of light, the grating period, the step height of the Grid and the web-gap ratio of the grid can only by means of exact lattice theory, as in some software packages for grid simulation (such as PC-ridges, GSolver, DiffractMOD) is implemented, because the limits the scalar lattice theory are at structure sizes in the range of 5 times the wavelength of light. Meaningful starting points and useful hints for such simulations are given below.

A the first category of measuring standards has one provided with grid structures substrate without additional Layers on. The substrate must always have a refractive index of more than 1.6, that is n> 1.6. Plastic substrates with something higher refractive index of about n = 1.8 For example, by using conventional plastic nanoparticles be mixed. To the transmission in first areas of the substrate to suppress or to maximize the reflection must such a substrate can be provided with a phase grating in which in general my case graduation period, step height and web width at concrete refractive index to be optimized. Such phase gratings on plastic substrates can by embossing or Injection molding process produced inexpensively become.

A second category of measuring scales comprises a substrate with an additional thin layer of high refractive index applied by a thin-layer method such as vapor deposition or sputtering. This layer is etched back to grid structures. Suitable materials for this thin layer are, for example, Ta ₂ O ₅ (n = 2.12), TiO ₂ (n = 2.4), Fe ₂ O ₃ (n = 3) or silicon (n = 3.8). A low absorption of these materials at the wavelength of light used is of minor importance for the transmission, since the layer thicknesses are at most a few tenths of a micrometer. The refractive index of the substrate (eg made of glass or quartz) is of secondary importance in this case, it merely has to be transparent be.

Of the Advantage of this arrangement is that such layer materials depending on the application, very different refractive indices a simple glass substrate can be realized.

in the general case must also be dependent here from the actual refractive index, the lattice parameter period, Step height and step width are optimized to extinguish the to reach transmitted light.

In special cases where, for example, the underlying scanning method operates at specific scanning distances (the so-called Talbot levels), a phase height of 90 ° or 180 ° is advantageous. These phase heights correspond to step heights of a light wavelength L h (90 °) = L / (4 * (n-1)) respectively. h (180 °) = L / (2 * (n-1)).

manufactures one the subwavelength grating structures in this way, so can be on the material measure also completely ordinary phase gratings in the same production step and produce with uniform etch depth. Such ordinary Phase grating with a period significantly larger as the wavelength of light, for example, serve as an incremental track to accurate measurement of a positional shift. One for determination the absolute position additionally required PRC track encodes an absolute position with its light and dark areas. The dark areas are not now as in the prior art with additional generated by absorbing layers, but by having sub-wavelength grating structures provided areas.

The desired optical effect (ie the extinction of the transmitted light) is achieved at a phase height of 180 ° only with refractive indices of n> 2 and at a phase height of 90 ° for n> 2.4.

Structured Substrates or layers with particularly high refractive indices are less sensitive to soiling, as typical soiling have significantly lower refractive index. Pollution with Water (n = 1.3) or oil (n = 1.6) the optical effect and thus the position measuring device less, because the refractive index difference is still sufficient to to ensure the functional safety. There are such Structures especially for external, environmental influences exposed areas of Maßverkörperungen suitable.

With The described measures are obtained by structuring a transparent substrate or a layer deposited on the substrate Areas in which the material measure a transmission almost zero and almost complete reflection equal to a mirror. The deposition and structuring of additional absorbent layers is not more necessary. Incremental tracks, reference marks and absolute tracks for detecting an absolute position immediately at power-up of the position measuring device can be generated in the same way be like labels or apertures that act as apertures.

Also the production of a phase grating for the reflected light mode is possible with an actually transparent substrate, if the lattice structure of the phase grating (with a grating period greater than the wavelength of light) becomes (with a grating period less than the light wavelength). This makes the phase grating reflective in all areas, the phase height is above the step height h set.

Mixed amplitude and phase gratings (MAP grids), as in the above-mentioned EP 1081457 A2 are explained in detail, can also be produced particularly cost-effective, since both phase and amplitude structures can be produced with a single manufacturing step.

differs one with the lattice structures whose period is smaller than that Wavelength of the incident light, from the ideal Conditions such as the orientation of the grating structures relative to the E-vector of irradiated, linearly polarized light or the grating period and changes them in the direction of measurement locally, this allows different transmission or Set reflection levels. This allows continuous transitions between the areas and thus also any intensity values to adjust.

Farther are also material measures possible which allow position detection in two directions, such as Cross lattice or checkered structures.

Further Advantages and details of the present invention will become apparent from the following description of preferred embodiments based on the figures. It shows

1 an optical position measuring device,

2 a section of a material measure,

3 an embodiment of a scale,

4 another embodiment of a scale,

5 another embodiment of a scale,

6 another embodiment of a scale,

7 a periodic sampling signal,

8th another embodiment of a scale,

9 another embodiment of a scale.

1 shows an optical position measuring device with a light source 1 , As a light source 1 For example, an LED with a central wavelength L of 855 nm is suitable. The light of this light source 1 is from a collimator lens 2 collimated, so that parallel light is irradiated to the subsequent optical arrangement. A polarizer 3 lets go of the light of the light source 1 just pass linearly polarized light. To the polarizer 3 no high demands are made, a simple, inexpensive polarizing film can suffice.

The light of the light source 1 falls perpendicular to a first measuring scale in the form of a scanning 4 and perpendicular to a second measuring scale in the form of a scale 5 , The order of scanning plate 4 and scale 5 can be reversed. The length of the scale 5 in measuring direction X determines the measuring length. The two measuring standards 4 . 5 are movable relative to each other in the measuring direction X.

Finally, the light falls from the light source 1 on a photodetector 6 which converts the incident light into an electrical signal from which position information can be derived in a known manner. In this embodiment, the light source 1 , the collimator lens 2 , the polarizer 3 , the scanning plate 4 and the photodetector 6 arranged firmly to one another in a scanning head, not shown. This scanning head as a whole is opposite to the scale 5 movably mounted in measuring direction X.

For the light of the light source 1 actually transparent material measures have first areas with lattice structures 7 whose period P is smaller than the wavelength L of the light used and which influence the optical properties of the material measure at this point in such a way that the light polarized in parallel to the grating lines is almost completely extinguished in these regions in transmission. Second areas without lattice structure, however, are transparent.

Only when light from the light source 1 neither on the scanning plate 4 still on the scale 5 on areas with lattice structures 7 it hits the photodetector 6 , Is the scale moving? 5 relative to the scanning plate 4 , so the amount of light on the photodetector 6 and thus modulate the detector signal.

In an alternative embodiment, a structured photodetector is used whose arrangement of detector fields in approximately the arrangement of the first and second regions of the scanning 4 equivalent. On the scanning plate 4 can then be dispensed with, since the detector then acts as a scanning itself. The light and dark areas are imaged directly on its detector fields.

2 shows a section of a material measure 4 . 5 with a grid structure 7 , For maximum reflection or minimal transmission of the structured areas 7 the electric field vector E of the incident light must be parallel to the grating lines of the grating structures 7 be aligned. This illumination case is called TE polarization.

3 shows a section of a scale 5 in cross section. The scanning plate 4 can here and in all other embodiments just like the scale 5 be constructed. The scale 5 consists of a substrate 9 that is for the light of the light source 1 is permeable. First ranges of the scale 5 are with the grid structures 7 provided whose period P is smaller than the light wavelength L. A substrate 9 made of plastic is very simple z. B. structurable with an injection molding or embossing process. The height h of the lattice structures 7 for the casting mold or for the die, it must be optimized by simulations or tests, yes depending on the application, to the lowest possible residual transmission or the highest possible reflection. A simple position measuring device in reflected light mode can already be used with a scale 5 built in the first areas with lattice structures 7 a reflection for example 70% and in the areas without lattice structure 7 has a reflection of less than 10%.

In this and the following embodiments, the grating structures exist 7 of webs of width b or trenches of width P-b with rectangular cross-section. But there are also other structures such. B. sine grid with wavy cross section or grid with V-shaped cross-section possible. The advantage is that the ratio of b to P can be near 0.5. This is a relatively favorable value for manufacturing processes, since the dimensions of the structural elements (lines, gaps) do not differ greatly from one another.

As a concrete embodiment in connection with 3 (corresponding to the first category mentioned above) is a substrate 9 of nanoparticle-enriched plastic with a refractive index of n = 1.8. When illuminated with polarized light of wavelength L = 855 nm, the optimization of the lattice parameters gives a grating period P of 820 nm, a ridge width b = 330 nm and a ridge height h = 415 nm. The theoretical transmission is then almost zero.

According to another, in the 4 (according to the above-mentioned second category) embodiment is a thin layer 8th on a substrate 9 separated from glass. This embodiment is particularly suitable for higher refractive index materials having a refractive index n above two (n> 2), since glass substrates with such high refractive indices are not considered for cost reasons.

As a concrete embodiment according to 4 be a glass substrate 9 indicated with a Ta ₂ O ₅ layer of thickness 382 nm with a refractive index of n = 2.12. The grating period is at P = 800 nm and the ridge width at b = 400 nm. At a central light wavelength L of 855 nm, the layer thickness or the height h = 382 nm corresponds to a phase step of 180 ° in a transmitted light system.

Another numerical example is the 4 a sub-wavelength grating 7 indicated in which on a glass substrate 9 the layer 8th made of silicon with n = 3.8 with a thickness of 76 nm. This corresponds to a phase grating with 90 ° phase height in a transmitted light system which operates with light of wavelength L = 855 nm. With a grating period P of 495 nm and ridge widths b between 130 nm and 260 nm, a transmission of almost zero can be achieved.

The particular advantage of the last two embodiments according to 4 is that both conventional 180 ° or 90 ° phase gratings for an incremental scan as well as subwavelength gratings for generating light / dark structures z. B. for PRC tracks for absolute value or other apertures can be produced in a single manufacturing step, since a uniform etch depth can be used.

For a transmitted light system, the substrate must be 9 or the layer 8th the embodiments according to the 3 respectively. 4 for light of the light source 1 be transparent. The reflective areas with a grid structure 7 are then for the photodetector 6 dark. For a reflected light system, the substrate 9 also opaque or slightly translucent. The reflective areas with a grid structure 7 are then for the photodetector 6 bright. The photodetector is in a reflected light system on the same side of the scale 5 arranged like the light source 1 ,

5 shows an embodiment in which cost on a substrate 9 a reflective phase grating with phase steps 10 is made. As a reflective layer is not used as usual a metal layer. Rather, the phase grating is on an actually transparent substrate 9 produced by embossing or similar inexpensive methods. The reflection properties only result from the whole-surface under-structuring of the substrate 9 or an applied layer 8th with the grid structures 7 with a period P smaller than the wavelength of light L. The period Λ of the reflective phase grating depends on the measuring principle used and is in most applications between 2 and 100 micrometers.

In 6 Finally, an example of a MAP grid already mentioned above is shown, whose phase and amplitude structures can be generated with a single etching step. The grid structures 7 do not have like in the already quoted EP 1081457 A2 a distracting effect (as the lattice structures of 4a , Federation 5a , b of this document), but in fact a reflective effect. Thus, the light transmission in these areas is reduced to almost zero, which is for the function of this MAP grid is of crucial importance and so far only with additional coating and structuring steps of layers z. B. could be achieved from chrome. The grid structures 7 thus act as dark areas, the unstructured areas form a phase grating, so that a mixed amplitude and phase grating arises.

Also Here comes the particular advantage of the fact that both a conventional phase grating as well as a sub-wavelength grating for generating light blocking areas in a single Manufacturing step can be generated.

7 shows by way of example the course of a detector signal I, as it is with a position measuring device according to the 1 can be obtained. By counting the periods of this detector signal and dividing (interpolating) such a period, the displacement between the scanning head and the scale 5 of such a position measuring device can be determined.

The 8th and 9 show further embodiments of the invention measure scales or scales 5 , These are designed so that it is possible to dispense with a scanning plate. Nevertheless, a scanning plate can be used for structured illumination of the scale 5 be used. One makes use of the fact that transmission and reflection of linearly polarized light of a specific wavelength L and polarization direction E can be controlled by the variation of parameters of the grating structures 7 on the scale 5 ,

So is according to 8th the full scale with lattice structures 7 whose period P fluctuates periodically. Thus, the period P is tuned to the incident light only in some areas so that it is almost completely reflected and virtually no light is reflected by the photodetector 6 reached. This is due to a minimum in the detector signal I in the overlying 7 indicated. On both sides of such a region, the period P of the lattice structure 7 larger (possibly even larger than the light wavelength L) or smaller, the scale 5 becomes transparent at these points and the detector signal correspondingly higher, up to complete transmission in the region of the maxima of the detector signal I.

The same effect can be achieved, if not as in 8th the period P, but as in 9 the orientation of the lattice structures 7 is varied continuously or discreetly. Only if the grating lines are parallel to the polarization direction E, incident light is the scale 5 strongly reflected and the detector signal in the transmitted light case minimal. Lying the lattice structures 7 perpendicular to the polarization direction E incident light passes almost completely to the photodetector 6 , the detector signal I is maximum.

With such variations of parameters of the grating structures can be not just two states (light / dark), but several, up to continuously varying detector signals, which are directly proportional to a position. So there are others To realize dimensional standards as usual Incremental tracks or binary coded absolute tracks.

For all embodiments described here is that the polarized light perpendicular or nearly perpendicular to the measuring scale must come up. It depends on the specific circumstances (especially light wavelength L and period P) from how big the Deviation from the vertical can be to get a sufficient Extinguishing the transmission or sufficient reflection to get in 0. diffraction order. The deviation from the vertical can be quite in the order of 10 °.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - EP 1081457 A2 [0006, 0029, 0058]
• - US 4820918 [0008]
• EP 1801546 A1 [0009]

Claims (12)

Measuring standard for an optical position measuring device that works with light of a central wavelength (L), wherein - the material measure ( 4 . 5 ) a substrate ( 9 ) having first and second regions, - the arrangement of the first and second regions encodes position information such that it can be read out with the light of the wavelength (L), - at least the first regions have a lattice structure ( 7 ) having a period (P) which determines the optical properties of the material measure ( 4 . 5 ), characterized in that - the period (P) of the lattice structure ( 7 ) is smaller than the wavelength (L), - in the direction (E) of the lattice structure ( 7 ) linearly polarized light of wavelength (L) is almost completely reflected by the structured first regions in the order of diffraction. Measuring standard according to claim 1, in which the second areas are not structured and thus incidental Light mostly transmitted or partially is absorbed. Measuring standard according to claim 1 or 2, characterized in that the substrate ( 9 ) has on its structured surface the first and second regions. Measurement standard according to one of the preceding claims, characterized in that the refractive index (n) of the substrate ( 9 ) is greater than 1.6. Measurement standard according to claim 1 or 2, characterized in that a structured thin layer ( 8th ) on the substrate ( 9 ) having the first and second regions. Measurement standard according to one of claims 1, 2 or 5, characterized in that the refractive index (n) of the layer ( 8th ) is greater than two. Measuring standard according to claim 5 or 6, characterized in that the lattice structure ( 7 ) is formed as a phase grating having a step height (h) of h (180 °) = L / (2 * (n-1)). Measurement standard according to claim 5 or 6, characterized in that the refractive index (n) of the with the lattice structure ( 7 ) layer ( 8th ) is greater than 2.4, and that the grid structure ( 7 ) is formed as a phase grating having a step height (h) of h (180 °) = L / (2 * (n-1)) or h (90 °) = L / (4 * (n-1)). Measuring standard according to one of the preceding claims, characterized in that on the material measure ( 4 . 5 ) both a phase grating with a step height (h) and a grating period (Λ) greater than the wavelength of light (L) is arranged, as well as an amplitude grating whose light or dark areas through the grating structures ( 7 ) having the step height (h) and the period (P) smaller than the wavelength (L) are formed. Measuring standard according to claim 9, characterized in that the material measure ( 4 . 5 ) carries a MAP grid. Measuring standard according to claim 9, characterized in that the material measure ( 4 . 5 ) carries the phase grating with the grating period (Λ) greater than the wavelength of light (L) as an incremental track for measuring positional shifts, and that the material measure ( 4 . 5 ) the lattice structures ( 7 ) having the period (P) smaller than the wavelength (L) as a reference mark or absolute track for detecting an absolute position. Optical position measuring device with a light source ( 1 . 2 . 3 ) for linearly polarized light of a central wavelength (L), a material measure ( 4 . 5 ) and a photodetector ( 6 ), characterized in that the linearly polarized light with a polarization direction (E) parallel to grating structures ( 7 ) of the material measure ( 4 . 5 ), whereby the material measure ( 4 . 5 ) is a material measure according to one of the preceding claims.