Optical angle detector with beam shaping element

Abstract

Position detector for determining the angle of rotation of an element rotatable about an axis of rotation (A) with: a light source (3), a detector element (18, 19, 20, 21) for detecting the light radiation emitted by the light source (3), which is arranged around the axis of rotation (A) and has at least two photosensors (19, 20) arranged in a plane (23), a beam shaping element (7, 207) arranged between the light source (3) and the detector element (18, 19, 20, 21), wherein light is directed from the light source (3) to the detector element (18, 19, 20, 21) by the beam shaping element (7, 207), a light blocking element (5) which is arranged between the beam shaping element (7, 207) and the detector element (18, 19, 20, 21) such that it can rotate together with the rotatable element about the axis of rotation (A), thereby preventing the incidence of light on different parts of the detector element (18, 19, 20, 21) depending on the angle of rotation, characterized in that the beam shaping element (7, 207) has at least one freeform surface (6, 206) which is a non-planar, non-spherical surface and on which the light from the light source (3) is reflected on its way to the detector element (18, 19, 20, 21) and the desired beam shaping is achieved by the fact that light rays entering the beam shaping element from the light source are reflected on the non-planar, non-spherical surface on their way to the photosensors and wherein the beam shaping element is designed such that the light directed to the detector element is concentrated on the detector element in an annular area.

Description

[0001] The invention relates to an optical angular position detector (also referred to as a "position detector" for short) for determining the angular position of a rotating element, such as is used, for example, in galvanometer drives. In addition to moving read heads in hard disk drives and CD players, galvanometer drives are used in particular for rotating mirrors in (laser) scanning devices. Such scanning devices are used, for example, in the field of additive manufacturing processes, where a laser beam is directed onto selected locations in a layer of a building material in order to selectively solidify the building material. For this purpose, a deflection mirror is attached to a rotatably arranged shaft, whereby the point of impact of the deflected laser beam can be influenced by rotating the mirror.It is obvious that to ensure high manufacturing accuracy, the rotational position of the mirror must be adjusted as precisely as possible and must be constantly checked.

[0002] US 2011 0 101 209 A1 describes a rotary position detector with a housing having a reflective inner surface. A light source is arranged at the bottom of the housing in line with the rotational axis of a rotary element. The light emitted upwards by the light source is reflected by the reflective inner surface toward photosensors arranged in a ring around the light source. A light-blocking element rotating with the rotary element prevents the emission of light toward some of the photosensors, depending on the rotary position. Therefore, depending on the rotary position of the light-blocking element, the angular position of the rotary element can be detected based on the signal responses of the individual photosensors.

[0003] US Patent No. 3,875,406 A describes an optical system for the automatic reading of electricity meters. The optical system detects the angular position of coding discs. For this purpose, the coding discs are arranged between a light source and a detector. To ensure that the light only penetrates the edge of a coding disc, the light is reflected by two different reflectors on its path from the light source to the detector.

[0004] US 2006 0 118 707 A1 deals with optical controls and switches that provide an output indicating the state of the switch. In particular, an optical rotary encoder is disclosed in which a Gray code disk, a light source, and three photosensors are arranged in series. On its path from the light source to the photosensors, the light passes through the transparent areas of the code disk and is then reflected by a reflector with a parabolic surface toward the photosensors.

[0005] US 2009 0 108 188 A1 describes an absolute encoder for rotary motion. Here, too, light from a series of light elements passes through an encoder disk, is reflected off a reflective surface, and then reaches a detector located on the rotary axis.

[0006] DE 10 2004 019 332 A1 describes an optical rotary encoder for detecting rotational information from a shaft. The light beams emitted by a light source arranged on the rotational axis pass through a code wheel, are reflected and parallelized by a reflector, and then pass through the code wheel again and impinge on light-receiving elements arranged around the light source.

[0007] US 2002 0 038 848 A1 describes an optical motion detector using an encoder disc. Light rays emanating from a light source arranged on a rotating axis are reflected by a parabolic surface and pass through the openings of a encoder disc to a light sensor.

[0008] US 7 113 269 B2 describes an angle measuring device in which laser light strikes a rotating mirror-prism structure, whereby it is split into a substantially parallel portion and a portion with convergent beams.

[0009] US 5 569 912 A describes an optical speed measuring device in which two encoder discs are arranged on a rotating shaft and circular light penetrates the openings of these encoder discs in order to be directed by a light deflection device to a detector element arranged on the rotation axis.

[0010] WO 2004 063 671 A2 describes an optical encoder for measuring a rotation angle. Optical guides in the encoder direct light radiation in a circular pattern onto a code disk with a mirror arranged behind it. After reflection, the same light deflection elements are used to return the radiation to a detector.

[0011] Checking the angular position is typically performed using a position detector that determines the angular position. To ensure high measurement accuracy, such a detector requires good temperature stability, a high signal-to-noise ratio, and good reproducibility of the output values. Regarding operating speed, it is important that the addition of the sensor does not significantly increase the moving mass during rotation. A small size of such a detector is also advantageous for ease of handling.

[0012] While capacitive position detectors were mainly used in the past, the recent trend has been towards optical position detectors, as these tend to be more cost-effective and smaller in size.

[0013] Optical position detectors primarily consist of those in which light from a light source passes through a disc with opaque lines on its way to a detector array. The disc rotates around the axis of rotation of the shaft whose angular position is to be determined. The angular information is encoded via the barcode on the disc. On the one hand, the angular position is determined by counting the pulses generated by a light / dark transition caused by the movement of the barcode (incremental encoder). Alternatively, the absolute angular position can be encoded by the code on the disc (absolute encoder).

[0014] In contrast, this application is directed to optical position detectors in which the angular position is determined without the aid of a code (uncoded) from the analog output signal of the detector array. The general operating principle of such an optical detector is that light is incident on a sensor, whereby different areas of the sensor surface are covered depending on the angular position of a shaft. This results in a correlation between the signal output by the light sensor area, which is proportional to the area onto which the light falls, and the angular position of the rotating shaft.

[0015] An example of such a sensor is described in European patent EP 1 071 929 B1. In the position detector according to EP 1 071 929 B1, which is described with reference to Fig. 5 and Fig. 6, a rotating shaft 114 is housed in a housing 116 and rotatably supported therein by bearings 118 and 120. A rotating mirror is attached to one end 125 of the shaft 114. The position detector 112 is located at the other end 132 of the shaft. The latter consists of a detector housing 121 in which photosensors 134, 136, 138, and 140, a light-blocking element 130, and an LED light source 146 are housed. While the LED light source 146 and the photosensors 134, 136, 138, and 140 are stationary, the light-blocking element 130 is connected to the rotating shaft 114.

[0016] As in particular the Fig. As can be seen in Figure 6, the circular ring-segment-shaped photosensors 134, 136, 138, 140 are arranged symmetrically around the rotating shaft. The light-blocking element 130 has the shape of a tie loop (commonly referred to as a "butterfly") and covers different portions of the photosensors depending on its angular position.

[0017] The measurement accuracy of such a position detector is affected by radial and axial movements of the shaft.

[0018] To achieve the highest possible signal-to-noise ratio, EP 1 071 929 B1 uses a wide-angle LED, which is arranged as close as possible to the photosensors, so that as much light as possible from the light source can fall on the photosensors without any additional optical elements in between. Such a design also has the further advantage of being very compact. For increased insensitivity to radial movements of the shaft 114, the signals from two opposing photosensors are summed during photosensor readout, and the resulting two summed signals are fed to the different inputs of a differential amplifier.

[0019] A basic requirement for the state-of-the-art position detector described above is the presence of a wide-angle LED that radiates as homogeneously as possible in all angular directions. Since such homogeneity is only achievable to a limited extent with wide-angle LEDs, there is a risk that the different photosensors will deliver different output signals due to different amounts of light, which ultimately leads to reduced angle measurement accuracy, particularly insufficient linearity.

[0020] Furthermore, the close positioning of a wide-angle LED to the photosensors also results in an increased dependence of the position detector's output signals on axial movements of the shaft. This is due to the strong divergence of the light radiation incident on the photosensors. This is the reason why, for example, in US patent US 7,688,432 B1, a lens is used to collimate the light incident on the sensors. Fig. Figure 7 shows a structure according to US Pat. No. 7,688,432 B1 with an LED 41, a lens 42, light sensors 44, a light blocking element 45, and a circuit board 46, which are arranged as a rotational position detector 40 above a motor housing 47. In addition to collimation, the use of the lens 42 also ensures that light rays emitted at a large angle by the LED 41 are not lost but are also utilized, thereby increasing the signal-to-noise ratio.

[0021] A disadvantage of the embodiment just described, however, is that in order to utilize light rays emitted by the LED 41 at a particularly large angle, the diameter of the lens 42 must be quite large. Furthermore, although parallel light reaches the sensors 44 from the lens 42, not all of the parallel light is needed, since the sensors 44 occupy only a fraction of the surface area of ​​the circuit board 46. This relativizes the improvement in the signal-to-noise ratio achieved by using the lens 42.

[0022] In view of the problems described above, it is an object of the invention to provide a position detector having a high signal-to-noise ratio.

[0023] The object is achieved by a position detector according to claim 1. Further developments of the invention are specified in the subclaims. Fig. 1 shows a schematic lateral cross-section through an angular position detector according to the invention. Fig. 2 shows a plan view of that section of the Fig. 1, in which the detector element and the light blocking element are located. Fig. 3 shows the beam path in an angular position detector according to a first embodiment. Fig. 4 shows the beam path in an angular position detector according to a second embodiment. Fig. 5 and Fig. 6 show position detectors according to the prior art according to EP 1 071 929 B1 in a side sectional view and in a plan view of the photosensors used in the position detector. Fig. 7 shows a position detector according to the prior art according to US 7 688 432 B1.

[0024] In the following, with reference to Fig. 1 to 4 describe an embodiment of the present invention. Fig. 1 shows a lateral cross-section through the position detector according to the invention, which is shown only schematically. Fig. 2 shows a top view of the detector section with the photosensors and the light blocking element and Fig. 3 and Fig. 4 show the beam paths of the light emitted by the light source when using the Fig. 1 shown structure. In Fig. 2 shows a section line S1-S1, which shows the position of the cross-sectional plane in Fig. 1 is intended to indicate.

[0025] When presented in Fig. 1 and Fig. 2, the basic structure is very similar to that in Fig. 5 and Fig. 6. As already mentioned in the introduction, the present invention is directed to a position detector in which the angular position is determined without the aid of a code (uncoded) from the analog output signal of the detector arrangement. In particular, a printed circuit board 9, photosensors 18, 19, 20 and 21, an aperture mask 10 and a light blocking element 5 are arranged one above the other. It should be noted that in Fig. 1 the photosensors 19, 20 are shown only for better understanding. Strictly speaking, along the Fig. No photo sensor is visible in the section line shown in Figure 2. Not shown in Fig. 1 is still a the overall structure in Fig. 1 surrounding housing that seals the structure off from the surrounding light.

[0026] Analogous to the structure of Fig. 5 is the Fig. 1 is connected to a rotating shaft, to which, for example, a galvanometer mirror can be attached. The rotating shaft is located below the structure in Fig. 1 is arranged such that its longitudinal axis, which is also the rotation axis A, passes vertically centrally through the fastening screw 12. By means of the fastening screw 12, the light blocking element 5 is fastened to the rotating shaft such that it can rotate with the rotating shaft, while the printed circuit board 9, the photosensors 18, 19, 20, and 21, and the aperture mask 10 remain stationary.

[0027] How to Fig. 2, four photosensors 18, 19, 20, 21 are arranged symmetrically around the longitudinal axis of the shaft as the detector element, and the light-blocking element 5, which, like the prior art, is a butterfly-shaped element, is positioned such that its axis of symmetry coincides with the longitudinal axis A of the shaft. The aperture mask 10 partially covers the photosensors 18, 19, 20, 21 and has a circular ring-shaped recess above each of the photosensors. A circular ring-shaped recess is advantageous because it results in uniform shading of the photosensors 18, 19, 20, 21 upon rotation of the butterfly-shaped element, which leads to very good linearity. In a preferred embodiment, the outer edge of the light blocking element 5 has a greater distance from the longitudinal axis A of the rotary shaft than the outer edge of a circular ring segment.It should be emphasized that, within the scope of the invention, the light-blocking element 5 can certainly have a shape other than the "butterfly" shape. Likewise, the recesses in the aperture mask 10 can also have a different shape. However, if there are sensor areas that cannot be covered by the light-blocking element 5, this results in a deteriorated signal-to-noise ratio. Furthermore, compromises in linearity may have to be accepted if the shape of the light-blocking element deviates significantly from the bow tie shape, for example, if it is rectangular.

[0028] It should be noted that the aperture mask 10 can also be omitted. In this case, a deteriorated signal-to-noise ratio must also be accepted in the event that there are sensor areas that cannot be covered by the light-blocking element 5. However, it is possible to freely select the shape of the photosensors 18, 19, 20, 21. In particular, they can each have a circular ring-segment shape.

[0029] Gap diodes can be used as photosensors 18, 19, 20, 21. However, the use of gap diodes is not absolutely necessary for the invention. Other types of photosensors can also be used. Likewise, four photosensors do not necessarily have to be used; instead, only one, two, three, five, six, seven, eight, or more photosensors can be used.

[0030] For increased insensitivity to radial movements of the Fig. 1, the signals of two opposing photosensors are summed, analogous to the procedure described in EP 1 071 929 B1 for photosensor readout of four photosensors, and the resulting two sum signals are fed to the different inputs of a differential amplifier. However, the evaluation of the photosensor signals can also be performed in a different manner as an alternative to the procedure described above.

[0031] In the present case, a single light source 3 can be used as the illumination source. This can be a commercially available LED or laser diode. LEDs or laser diodes that can be used for the present invention can already contain lenses or aperture elements. In the figures, the light source 3 is shown only schematically; the beam path is depicted as if all rays emanate from one point. Fig. 1 additionally shows a holder 11, for example a circuit board, on which the light source 3 is mounted.

[0032] The basic idea of ​​the present invention is that the signal delivered by the detector is greater, the better the utilization of the light emitted by the light source 3 is. Ideally, as much of the light emitted by the light source 3 as possible should fall on the photosensors, if one disregards coverage by the light blocking element 5. The light emitted by the light source 3 is therefore directed into an annular area on the printed circuit board 9 with the photosensors 18, 19, 20 and 21, in which the circular ring-shaped photosensors or circular ring-shaped openings in the aperture mask 10 are located. Since this objective can be achieved with a lens as in Fig. 7 cannot be achieved, it is the idea of ​​the invention to Fig. 1, a beam-shaping element designated by reference numeral 7 is used. In the following, this beam-shaping element 7 is also referred to as a free-form element, since the desired beam shaping is achieved by the beam-shaping element 7 having at least one non-planar, non-spherical surface at which light rays entering the beam-shaping element 7 from the light source are reflected on their way to the photosensors.

[0033] The beam shaping element 7 in Fig. 1 has a truncated cone shape, with the axis of symmetry of the truncated cone passing through the center of the fastening screw 12 and, when the position detector is attached to a rotating shaft, coinciding with the axis of rotation A. A truncated cone-shaped recess 4 is provided on the rear side 30 of the beam-shaping element 7 facing the light source 3. The side 32 of the beam-shaping element 7 facing the fastening screw 12 has a central region 32a, which is substantially parallel to the plane of the photosensors, and an edge region 32b. Since the central region 32a is set back more with respect to the fastening screw 12 than the edge region 32b, the surface 32b corresponds to the side surface of a truncated cone.

[0034] The side surface 6 of the beam-shaping element 7 has a convex shape. As can be seen in Fig. 1, the side surface 6 can be curved outwards with respect to the axis of symmetry and can be a free-form surface. The exact shape (curvature) of the individual surfaces of the beam-shaping element 7 can be determined by simulation calculations using commercially available software, for example ASAP. The free-form surfaces can be described, for example, by polynomials, splines or in another way. Since the side surface 6 can be curved outwards, the shape of the beam-shaping element 7 was deliberately described above as "truncated cone-shaped". A truncated cone-shaped shape, in which the side surface in the sectional view of Fig. 1 is not curved outwards, is a special case of this. Since the outer surface 4b of the recess 4 in the sectional view of Fig. 1 can have a curvature, the shape of the recess 4 was also generally characterized as “truncated cone”.

[0035] The beam-shaping element 7, which can be made of a plastic material (e.g., Plexiglas) or, for example, glass, is characterized in that the light is directed onto the photosensors not only by refraction, but also by reflection in the beam-shaping element. In the beam-shaping element 7 described here as an example, a reflection takes place at a boundary surface of the beam-shaping element 7 to the outside, namely a reflection at the side surface 6. Ideally, this is total reflection, in which more than 99% of the light is reflected in order to achieve the greatest possible light yield. However, one can also work without total reflection, e.g., by coating the side surface 6 with a metal. The mere fact of using the light reflected at the side surface 6 already leads to an improvement over the Fig. 7. Even if only more than 10%, more than 50%, more than 70%, 80%, 90%, or even 95% of the light is reflected, the invention represents an improvement over the prior art. The improvement is, of course, greatest when as little light as possible is lost.

[0036] The structure and mode of operation of the beam-shaping element are explained below using various embodiments. First embodiment

[0037] Fig. Figure 3 shows the beam path when using a beam-shaping element 7 according to a first embodiment. The arrangement of the light source and the beam-shaping element 7 are the same as the arrangement in Fig. 1. The structure below the beam shaping element 7 is in Fig. 3 identical to the structure in Fig. 1. Instead of the Fig. 1 shown structure below the beam shaping element 7 is in Fig. 3, however, only schematically shows a plane 23, which is intended to represent a plane in which the planar sensors 18, 19, 20, 21 are arranged. The position of the photosensors 19, 20 is shown only schematically.

[0038] How to Fig. 3, the light source 3 is arranged near the side of the beam-shaping element 7 facing away from the photosensors in such a way that it does not protrude into the recess 4. The rays emanating from the light source 3 are first refracted at the edge of the recess 4 in the beam-shaping element 7 and subsequently directed by total reflection at the side surface 6 of the beam-shaping element 7 in the direction of the photosensors 19 and 20, wherein a further refraction takes place at the exit side 32. In the special design according to the first embodiment, the exit-side surface 32 is not divided into two as in Fig. 1, but essentially flat.

[0039] It can be seen that in the forward direction (within a certain angular range around the axis of symmetry, assuming the direction on the axis of symmetry towards the photosensors is 0°), the light rays from the light source 3 end at the edge of the recess 4. This is because at this point the edge of the recess 4 is coated with an opaque material. This prevents light from falling onto the fastening screw 12 and being reflected by it in an uncontrolled manner. Fig. 3 the surface of the truncated cone-shaped recess 4 facing the photosensors, i.e. the bottom 4a of the recess 4, is opaque.

[0040] Opacity can be achieved by applying black paint (e.g., based on carbon black) as an opaque layer. Alternatively, an opaque disc, such as a black anodized aluminum disc, can be glued or otherwise attached to the base 4 of the recess.

[0041] Specifically, in the first embodiment, the light is focused only on a partial area of ​​the photosensors. The “light spot” has the shape of a circular ring, the width of which is significantly smaller than the width of the circular ring-shaped photosensors 18, 19, 20, 21 or circular ring-shaped openings in the aperture mask 10. The reflection of the light at the side surface 6 causes a redistribution of the light from the light source: light that is reflected at small angles in the forward direction (the direction in which Fig. 1 the axis A is assumed to be the 0° direction) reaches the outer part of the circular ring and light that is emitted by the light source 3 at smaller radiation angles reaches the inner part of the circular ring-shaped area.

[0042] Although the light incident on the photosensors is not parallel, this embodiment still achieves insensitivity to axial displacements (a change in the distance between light source 3 and photosensors 18, 19, 20, 21). Because the width of the circular ring within which light falls on the photosensors is significantly smaller than the width of the circular ring segments in the radial direction, the "light spot" remains on the photosensors even in the event of axial displacements. Thus, there is no change in the light intensity registered by the photosensors due to axial displacements. Similarly, the shape of the light also ensures insensitivity to radial displacements of the photosensors.

[0043] The beam-shaping element 7 ensures that the largest possible proportion of the light emitted by the light source 3 reaches the photosensors. Although a certain angular range of the light emitted in the forward direction by the light source 3 is not used to illuminate the photosensors due to the opaque coating on the beam-shaping element 7, very good signal-to-noise values ​​were achieved for the position detector according to the invention. This is due, among other things, to the fact that the solid angle component of the light is large at large emission angles (assuming the forward emission direction is 0°). The solid angle here refers to that portion of a spherical surface surrounding the light source 3 that is penetrated by the light rays emitted by the light source 3, whereby it is assumed that the light source is a point-shaped emitter.If the largest possible percentage of light is directed onto the photosensors at large beam angles, a high percentage of the total solid angle range into which light source 3 radiates is utilized. This leads to improved utilization of the light from light source 3 itself, taking into account the fact that the radiance of the light source is generally lower at large beam angles than at small beam angles.

[0044] The Fig. The angle 14 shown in Figure 3 at the bottom of the recess 4 is slightly greater than 90°. The angle can be between 90° and 105°, preferably between 91° and 95°. The inclination of the side walls of the recess 4 influences the refraction behavior. The closer the angle is to 90°, the further outward the rays are refracted. This results in the beam-shaping element 7 being able to be designed correspondingly shorter. For angles close to 90°, a particularly compact structure is therefore achieved. However, the angle should preferably be slightly greater than 90°, as this offers advantages in the manufacture of the beam-shaping element 7. Since the beam-shaping element 7 is preferably manufactured by injection molding, an angle of more than 90° facilitates the removal of the injection molding tool from the recess 4.

[0045] For reasons of clarity, the figures do not show how the beam-shaping element 7 is held, i.e. how it is fixed in its position between the light source 3 and the structure consisting of the fastening screw 12, the light-blocking element 5 and the photosensors 18, 19, 20, 21. There are, in principle, several options for holding the beam-shaping element 7. It is advisable to fix the beam-shaping element 7 to a housing (not shown in the figures) which surrounds the overall structure shown. Since the beam-shaping element 7 is preferably manufactured by injection molding, it is also advisable to form the holding elements in one piece with the beam-shaping element 7 by injection molding. For example, lugs, webs or snap hooks can be considered as holding elements, which ensure a positive connection with the housing.Of course, recesses or eyelets can also be provided on the side wall of the beam-shaping element 7, into which projections of the housing then engage. If the mounting elements are arranged on the side surface 6, they should preferably be positioned where there is no reflection of the light incident on the photosensors.

[0046] Due to the rotational symmetry of the overall structure, a cylindrical housing is particularly feasible. In this case, an inner cylinder surrounding the structure can be formed by injection molding, which inner cylinder is located within the cylindrical housing and is fixed to the cylindrical housing. The connection between the inner cylinder and the beam-shaping element 7 can be achieved via connecting webs formed by injection molding between the beam-shaping element 7 and the inner cylinder, so that the beam-shaping element 7, the connecting webs, and the inner cylinder are ultimately formed as a single, injection-molded part.

[0047] As an alternative to the integral construction of the beam-shaping element 7 and the support elements, the support elements can also be glued to the beam-shaping element 7. If the contact surface between the support elements and the beam-shaping element 7 is located at a point where the light incident on the photosensors is reflected, a layer is preferably applied that can reflect light toward the interior of the beam-shaping element 7. This ensures that the contact areas between the support elements and the beam-shaping element 7 can also be used as reflective surfaces. Second embodiment

[0048] Fig. Figure 4 shows the beam path when using a beam-shaping element 207 according to a second embodiment. The second embodiment differs from the first embodiment only in the modified beam-shaping element 207. All other elements of the second embodiment are identical to the first embodiment. Furthermore, all possible modifications described with regard to the first embodiment are also applicable to the second embodiment.

[0049] How to refer to Fig. 4, the surface 232 of the beam-shaping element 207 facing the photosensors 19, 20 is, in contrast to the first embodiment, not flat, but as in Fig. 1 is divided into a recessed central area 232a and an outer area 232b.

[0050] As with the first embodiment, it can be seen that the light rays emitted by the light source 3 do not penetrate the bottom of the frustoconical recess 204. This is again due to the fact that the bottom 204a of the recess 204 is coated with an opaque material. As with the first embodiment, an opaque layer could, of course, alternatively be applied to the beam-shaping element 207 in the central region 232a of the exit surface 232.

[0051] The light rays emitted by the light source 3 are first refracted at the edge (the lateral surface 204b) of the recess 204 and subsequently directed toward the photosensors 18, 19, 20, 21 by total internal reflection at the side surface 206, with another refraction occurring at the surface 232. The side surface 206 is again, as in the first embodiment, a freeform surface whose shape was determined by simulations.

[0052] In contrast to the first embodiment, in the second embodiment the light of the light source 3 is concentrated onto a circular ring whose width is greater than the width of the circular segment-shaped photosensors or the circular segment-shaped recesses in the aperture mask 10. The beam-shaping element 207 acts as a collimator, with parallel light exiting at the exit side 232 in the direction of the photosensors.

[0053] Compared to the use of a lens as a collimation element as in US patent US 7 688 432 B1, the second embodiment has the following advantage:

[0054] The light leaving the light source 3 is reshaped by the beam-shaping element 207 so that it radiates in the shape of a circular ring toward the plane 23 containing the photosensors 18, 19, 20, and 21. Specifically, the presence of the recess 204 and the arrangement of the light source 3 directly behind the side of the beam-shaping element 207 facing away from the photosensors ensure that light rays emitted at a large radiation angle relative to the forward radiation direction of the light source 3 (the 0° direction defined above) are directed toward the photosensors by reflection. Since the light is reflected toward the photosensors by the correspondingly designed side surface 206 of the beam-shaping element 207, more light from the light source 3 is directed to the photosensors than with an aperture of the same diameter.In contrast to an aperture, the light is not directed into an arbitrarily large area outside the area covered by the photosensors. Although the circular illumination area generated by the beam-shaping element 207 according to the second embodiment is, in contrast to the first embodiment, wider in the radial direction than the width of the circular ring-segment-shaped photosensors 18, 19, 20, 21 or the circular ring-segment-shaped recesses in the aperture mask 10, the width of the circular ring does not have to significantly exceed the width of the circular ring segments or circular ring-segment-shaped recesses. In contrast to an aperture, this concentrates the light more strongly onto the photosensors.

[0055] In the second embodiment, parallel light strikes the photosensors, resulting in insensitivity to axial displacements between the light source and the photosensors. Furthermore, since the width of the annular illumination region is greater than the width of the annular segments or annular segment-shaped recesses, insensitivity to radial displacements of the photosensors relative to the light source 3 is also achieved. The amount by which the width of the annular illumination region exceeds the width of the annular segments or annular segment-shaped recesses depends on the axial displacements expected. The width of the annular region can be adjusted via the shape of the side surfaces 206 and thus the reflection behavior of the side surfaces 206. Possible variations in the designs

[0056] The invention is not limited to the explicit design of the two embodiments just described. Rather, the following modifications are possible in both embodiments, among others: - Since it is also possible in the above embodiments for the light in the beam-shaping element to be reflected on a paraboloid segment-shaped surface, one can explicitly choose a surface as the free-form surface which deviates from the paraboloid segment shape in order to thereby obtain more freedom to realize a specific pattern of the light leaving the beam-shaping element. - As already mentioned in relation to Fig. As described in Figure 1, the exit surface 32 or 232 does not have to be flat. Rather, this surface can have a curvature, recesses, or steps. For example, a recess can be provided that allows the fastening screw 12 to be inserted. Since this allows a very small distance between the beam-shaping element and the photosensors 18, 19, 20, 21 including the light-blocking element 5 to be achieved, such a measure is advantageous with regard to a compact design.

[0057] In particular, the exit surface 32 or 232 can also be a freeform surface of any shape. Its exact configuration, which can be determined by simulations (e.g., using ASAP), depends on the type of "light spot" to be generated, i.e., the planar shape of the light incident on the plane 23 with the photosensors. - The bottom 4a, 204a of the recess 4 or 204 and / or their lateral surface 4b, 204b can also be free-form surfaces. Instead of applying the opaque layer to the edge or bottom of the recess 4, 204, an opaque layer can also be applied to the beam-shaping element 7 in the central region of the exit side 32, i.e., where the exit side 32 faces the fastening screw 12. The opaque layer can be implemented in the same way as described for the first embodiment.

[0058] It should be mentioned that it is also possible to completely dispense with the application of opaque layers on the beam-shaping element 7. In such a case, the beams should be directed at small beam angles, which are Fig. 3 and Fig. 4 at the bottom 4a or 204a of the recess 4 or 204 in the opaque layer, do not strike the photosensors directly. Furthermore, the head of the fastening screw 12 should preferably be blackened to prevent disruptive reflections on the screw head. If opaque layers are omitted from the beam-shaping element 7, it is also possible, in particular, to design the shape of the recess 4, 204 and / or the exit side 32, 232 such that the beams are directed specifically onto the blackened head of the fastening screw 12 at small radiation angles, e.g., for angles less than ±30° or less than ±25°. If the screw head has a recess (e.g., a hexagon socket), the beams can also be directed specifically into this recess. With this approach, you can even do without blackening the screw head, although blackening is also advantageous in this case.Ultimately, it is only important that the rays not reflected on the side surface 6, 206 do not reach the photosensors. Furthermore, the recess 4 or 204 can have inwardly curved side walls 4b that taper to a point at the bottom. For example, the area where the side walls 4b meet can resemble the area around the lowest point of a conventional or shortened cycloid. With such a configuration, better utilization of the light rays emitted by the light source 3 in the forward direction (radiation angle range close to the 0° direction) can be achieved. Ideally, all measures for blocking the rays leaving the light source 3 at small radiation angles (e.g., opaque layers) can be completely dispensed with. - Although in Fig. 3 and Fig. 4 If the light source is arranged outside the recess 4 or 204, the light source 3 can also be immersed in the recess. If the light source does not protrude into the recess 4 or 204, the distance between the light source and the beam-shaping element should be so small that as many rays of the light source as possible (including those with large beam angles) hit the beam-shaping element. - Although in Fig. 3 and Fig. 4, the beam-shaping element has a rotationally symmetrical shape, it is also possible for the surfaces 30 and 230 opposite the light source and / or the surfaces 32 and 232 opposite the photosensors to be polygons. In particular, the beam-shaping element 7, 207 and / or the recess 4, 204 can have a truncated pyramid shape. - The freeform surface in the beam-shaping element at which the light is reflected does not necessarily have to be circular-cylindrical. - Although the invention is not limited thereto, the light-blocking element in the present invention can be designed, in particular, with a diameter of less than 10 mm, preferably between 2 and 9 mm. This results in a very compact position detector with a small diameter perpendicular to the rotation axis A.

[0059] In contrast, position detectors that encode the angular position are reaching their limits with increasing demands on the accuracy of the angle measurement. This is because with encoded position detectors, which typically use an encoder disk, the angular resolution can only be achieved by increasing the disk diameter, since the barcode on the disk cannot be printed with any desired degree of detail. - Regarding the moment of inertia with respect to the rotation axis, the present invention also allows the realization of position detectors with a particularly low moment of inertia of the element rotating about the rotation axis (here, the light-blocking element). In particular, the moment of inertia of the element rotating about the rotation axis (here, the light-blocking element) can be below 0.010 g cm. 2 , preferably below 0.008 g·cm 2 and more preferably below 0.006 g·cm 2 The invention even allows the realization of position detectors in which the moment of inertia of the element rotating around the axis of rotation (here the light blocking element) is between 0.003 and 0.004 g·cm 2 lies.

[0060] As already mentioned, the (angular) position detector according to the invention can be used in a scanning device mentioned in the introduction to the description, by means of which a light beam, in particular a laser beam, is directed onto different points of a surface in order to detect properties at these points or to perform a processing operation there using the laser beam. The angular position of a rotating mirror used to deflect the light beam, which is attached to a rotation axis A connected to the light-blocking element 5, can then be determined and used to control a scanning process. In particular, the position detector according to the invention can be used in a scanning device as described in EP 1 295 090 B1.Details of the use in a scanning device (an example of the mechanical attachment of an angular position detector in a scanning device) are already described in EP 1 295 090 B1 and are therefore not repeated here.

Claims

[1] Position detector for determining the angle of rotation of an element rotatable about a rotation axis (A) with: a light source (3), a detector element (18, 19, 20, 21) for detecting the light radiation emitted by the light source (3), which is arranged around the axis of rotation (A) and has at least two photosensors (19, 20) arranged in a plane (23), a beam-shaping element (7, 207) arranged between the light source (3) and the detector element (18, 19, 20, 21), wherein the beam-shaping element (7, 207) directs light from the light source (3) to the detector element (18, 19, 20, 21), a light blocking element (5) arranged between the beam transforming element (7, 207) and the detector element (18, 19, 20, 21) such that it can rotate together with the rotatable element about the axis of rotation (A), whereby the incidence of light on different sub-areas of the detector element (18, 19, 20, 21) is prevented depending on the angle of rotation, characterized by , that the beam-shaping element (7, 207) has at least one free-form surface (6, 206) which is a non-planar, non-spherical surface and at which the light from the light source (3) is reflected on its way to the detector element (18, 19, 20, 21), and the desired beam shaping is achieved by light rays entering the beam-shaping element from the light source being reflected on the non-planar, non-spherical surface on their way to the photosensors, and wherein the beam transforming element is designed such that the light directed to the detector element is concentrated on the detector element in an annular region. [2] Position detector according to claim 1, wherein the beam transforming element (7, 207) is designed such that the light emerges from the beam transforming element (7, 207) in a circular ring shape. [3] Position detector according to claim 1 or 2, wherein the light is totally reflected at the at least one free-form surface (6, 206). [4] Position detector according to one of claims 1 to 3, wherein the surface of each photosensor onto which the light from the light source (3) can impinge has the shape of a circular ring section. [5] Position detector according to claim 4, wherein the beam transforming element (7, 207) is designed such that the light emerges from the beam transforming element (7, 207) in a circular ring shape and the width of the circular ring-shaped light distribution in the radial direction is smaller than the width of the circular ring section in the radial direction. [6] Position detector according to claim 4, wherein the beam transforming element (7, 207) is designed such that the light emerges from the beam transforming element (7, 207) in a circular ring shape and the width of the circular ring-shaped light distribution in the radial direction is greater than the width of the circular ring section in the radial direction. [7] Position detector according to one of claims 1 to 6, wherein the beam transforming element (7, 207) has a truncated cone or truncated pyramid shape, the cover surface (30, 230) facing the light source (3) and the axis of symmetry coinciding with the axis of rotation (A). [8] Position detector according to claim 7, wherein the side surface (6, 206) of the truncated pyramid or truncated cone is curved outwards with respect to its axis of symmetry and this side surface is the surface at which the light is reflected. [9] Position detector according to claim 7 or 8, wherein the beam transforming element (7, 207) has a recess (4, 204) in its surface facing the light source (3). [10] Position detector according to claim 9, wherein the recess (4, 204) has the shape of a truncated cone or truncated pyramid and the beam transforming element (7, 207) is provided with a light-tight coating on the cover surface of this truncated cone or truncated pyramid. [11] Position detector according to one of claims 1 to 10, wherein the light blocking element (5) has a diameter perpendicular to the axis of rotation (A) of less than 10 mm. [12] Position detector according to one of claims 1 to 11, wherein the light blocking element (5) has a moment of inertia with respect to the rotation axis (A) of less than 0.010 g·cm 2 has. [13] Scanning device with a position detector according to one of claims 1 to 12.

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