Autocollimator device and its use

Abstract

Autocollimator device (12) for measuring an angle Θ of the surface normal (N) of a reflecting surface (36) relative to a main optical axis (26) of the autocollimator device (12), comprising a base unit (10) which has a light source (16), a detector (18) and an optical system (20), wherein the reflecting surface (36) reflects the light emitted by the base unit (10) close to the axis in a central angular range ΔΘ of the angle Θ around the main axis (26) in such a way that the reflected light can be detected by the detector (18), characterized by a supplementary unit (14) with an optical device (40) comprising a plurality of optical elements (42, 44, 46, 48), which is designed such that the optical elements (42, 44, 46, 48) reflect the light emitted by the reflecting surface (36) in at least one of the central angular ranges ΔΘ deviating from the other angle range of the angle Θ reflects light so close to the axis,that it is detectable by the detector (18), wherein the optical device (40) is designed as a mirror arrangement with optical elements (42, 44; 46, 48) designed as mirrors, wherein the mirrors (42, 44, 46, 48) of the mirror arrangement (40) are exclusively plane mirrors.

Description

The present invention relates to an autocollimator device for measuring an angle of the surface normal of a reflecting surface with respect to an optical main axis of the autocollimator device, having a base unit which has a light source, a detector and an optical unit, wherein the reflecting surface reflects the light emitted by the base unit in the vicinity of the axis in a central angular range of the angle around the main axis in the vicinity of the axis in such a way that it is detectable by the detector.The invention further relates to a use of this autocollimator device.An autocollimator device of this type for measuring an angle of the surface normal of a reflecting surface with respect to an optical axis of the autocollimator device is known, for example, from the publication EP 2 461 132 A1. The autocollimator device described therein comprises a light source, an imaging detector and an optical system having two beam splitters, a focus lens and a converging lens on a common optical axis. The light source and the imaging detector are connected to the rest of the optics via one of the beam splitters each. An autocollimating target constitutes the reflecting surface. This reflecting surface reflects the light emitted off the base unit off-axis in a central angular range ΔΘ of the angle θ around the optical axis off-axis in such a way that it is detectable by the detector. Such an autocollimator device can, however, only be used for a limited angular range. The document further describes the use of this autocollimator device for automatically aligning a telescope of a surveying instrument defining the optical axis. The optical axis is set perpendicular to a specular surface of a coated plane mirror serving as an autocollimation target.For certain applications, however, this structure of the autocollimator device cannot be used, or can only be used to a very limited extent, since the usable angular range is not sufficiently large.EP 2 461 132 A1 discloses a telescope for use in a surveying instrument and a method for automated autocollimation of a surveying instrument in which the telescope is inserted.DE 20 2010 008 615 U1 shows an optoelectronic sensor for detecting objects having at least one glossy surface.The object of the invention is thus to specify an autocollimator device with an extended usable angular range and a use for such an autocollimator device.The object is achieved according to the invention by the features of the independent claims. Advantageous embodiments of the invention are specified in the dependent claims.In the autocollimator device according to the invention for measuring an angle θ of the surface normal of a reflecting surface with respect to an optical main axis of the autocollimator device, which has a base unit with a light source, detector and optics, it is provided that it also has a supplementary unit with an optical device comprising a plurality of optical elements, which is set up in such a way that the optical elements reject the light reflected by the reflecting surface in at least one other angle range of the angle θ deviating from the central angle range ΔΘ in a manner close to the axis in such a way that it can be detected by the detector, wherein the optical device is, however, designed as a mirror arrangement with optical elements designed as mirrors, wherein the mirrors of the mirror arrangement are exclusively planar mirrors. In this way, the supplementary unit can be realized particularly easily. Such a mirror assembly augmentation unit is similar in construction to a periscope or catadiopteric imaging system. Planar mirrors with high imaging accuracy can be produced relatively easily. Furthermore, a mirror arrangement with planar mirrors can be realized particularly easily. The mirror arrangement of the supplementary unit has in particular two mirrors per other angular range. If the surface normal of the reflecting surface thus has an angle which lies in the other angle range, the reflected light is not reflected directly back near the axis, but the corresponding light beam is deflected via two mirrors onto a path near the axis back into the base unit. The term "close to the axis" always refers to the main optical axis of the autocollimator device. If the surface normal of the reflecting surface thus has an angle which lies in the other angle range, the reflected light is not reflected directly close to the axis of the reflecting surface, but the corresponding light beam is deflected via the optical elements onto a path close to the axis. Essential degrees of freedom in the design of the optical device are:Number of optical elements, type of optical elements, arrangement of the optical elements and alignment of the individual optical elements with one another and with respect to the base unit and the main optical axis.In principle, it is of course possible for the central angular range and the at least one other angular range to overlap. According to a preferred embodiment of the invention, however, the at least one other angular range (ΔΘ 1 around the angle θ 1 ) is outside the central angular range ΔΘ. Preferably, it is provided that the at least one other angular range directly adjoins the central angular range ΔΘ. In this way, the central angle range ΔΘ is expanded.According to a further preferred embodiment of the invention, two other angular ranges ΔΘ 1 are provided, which lie on one side of the central angular range in each case. If the two other angle ranges ΔΘ are immediately adjacent to the central angle range ΔΘ, the central angle range ΔΘ is extended on both sides. In this case, according to a further preferred embodiment of the invention, the optical elements of the optical device are arranged symmetrically with respect to the main axis.In principle, the optical device of the supplementary unit can have one or more of the following optical elements: lenses, prisms, gratings, diffractive optical elements or other beam shape elements.According to a further preferred embodiment of the invention, the optics of the base unit has a beam splitter and at least one lens. A base unit with such components is known, for example, from the aforementioned publication EP 2 461 132 A1.When using the autocollimator device according to the invention, it is provided that an autocollimator device mentioned above is used for characterizing a microelectromechanical mirror device. Microelectromechanical mirror devices of this type are known as MEMS mirror devices (MEMS: microelectromechanical systems) or, for short, MEMS mirrors or MEMS (microelectromechanical mirrors). A microelectromechanical mirror device has a mirror element which forms the reflective surface and can be periodically tilted using electrical control variables. The angle of rotation or tilt of the mirror element of such MEMS mirrors is often significantly greater than the central angular range of the autocollimator device which results via the base unit. The microelectromechanical mirror device is a device capable of oscillation with the parameters usual for such devices, such as resonant frequency, quality, etc.According to a preferred embodiment of the use according to the invention, the characterization of the microelectromechanical mirror device is a temperature-dependent characterization. The characteristic parameters of a microelectromechanical mirror device change with temperature. Alternatively or additionally, the characterization of the microelectromechanical mirror device is a characterization as a function of the electrical control variable.According to a preferred embodiment of the use according to the invention, at least one look-up table is created for characterizing the microelectromechanical mirror device. This look-up table can then also be used to correct control parameters of the microelectromechanical mirror device. In this way, the temperature dependence of the characteristic parameters of the microelectromechanical mirror device can be compensated.The invention is explained in more detail below with reference to the attached drawings on the basis of preferred embodiments.The following are shown: FIG. 1 shows a base unit of an autocollimator device according to a preferred embodiment of the invention, FIG. 2 shows a supplementary unit of an autocollimator device according to a preferred embodiment of the invention, FIG. 3 shows the transfer function of a MEMS mirror at different control voltages, and FIG. 4 shows the transfer functions of a MEMS mirror at different ambient temperatures.In FIG. 1, a base unit 10 of an autocollimator device 12 is shown, which, in addition to the base unit 10, also has an extension unit 14 shown in FIG. 2. The base unit 10 has a light source 16, an imaging detector 18 and an optical unit 20, the optical elements 22, 24 of which lie on a common main optical axis 26. The optical elements are: a beam splitter 22 and a converging lens 24. the light source 16 and the imaging detector 18 are connected to the rest of the optics 20 via the beam splitter 22. The light source 16 in turn has a lighting means 28, a condenser lens 30 and a diaphragm 32. These components 28, 30, 32 result in a high-intensity light source 16 of a shape predefined by the aperture 32. At the end of the optical main axis 26, an autocollimating target 34 having a reflecting surface 36 is located in front of the base unit 10 of the autocollimator device 12, the autocollimating target 34 being mounted rotatably about a rotational axis 38. The surface normal N of the reflecting surface 36 forms an angle θ with respect to the main optical axis 26. The reflective surface 36 reflects off-axis light emitted from the base unit 10 in a central angular range ΔΘ of the angle θ about the main optical axis 26, such that this light is detectable by the detector 18.In addition to a representation of the autocollimating target 34 in an orientation in which the surface normal N of the surface of the autocollimating target 34 has an angle θ=0° with respect to the optical main axis 26, an orientation of the autocollimating target 34 is shown in which this angle θ is a few degrees. While the light reflected by the reflective surface 36 is centrally focused on the imaging detector 18 at θ=0°, an offset x of the corresponding focal point on the imaging detector 18 with respect to the center of the imaging detector 18 results at the other angle θ. Overall, the reflected light can be focused on the imaging detector 18 at each angular position in the central angular range ΔΘ.FIG. 2 now also shows the autocollimating target 34 and the optical main axis 26 of the autocollimator device 12 as well as its extension unit 14, wherein the position of the base unit 10 is indicated. The expansion unit 14 comprises an optical device 40 having a plurality of optical elements 42, 44, 46, 48. the optical device 40 shown here is designed as a mirror arrangement having two parts, each of the parts having in each case two mirrors 42, 44; 46, 48. The optical elements 42, 44; 46, 48, i.e. here the mirrors of each of the parts of the optical device 40, are arranged, configured and aligned such that the optical device 40 reflects the light reflected by the reflective surface 36 into a respective other angle range ΔΘ (around an associated angle θ 1) in the direction of the optical unit 20 close to the axis of the main optical axis 26 in such a way that it can be detected by the detector 18, i.e. can be focused on the active detector surface. The optical elements 42, 44; 46, 48 are arranged in such a way that they do not interfere with the direct reflection in the region of the axis 26. Thus, the function described in connection with the base unit 10 is fully maintained in the central angular range ΔΘ. The resulting two other angular ranges ΔΘ 1 are found on each side of the central angular range outside the central angular range ΔΘ. In FIG. 2, only one of these angle ranges is indicated, but for reasons of symmetry a completely analogous situation results on the other side of the main axis 26.Of course, a plurality of such parts of the mirror arrangement or optical device 40 can also be provided on each side. In this case, individual groups of the mirrors can then form mirror elements of polygonal mirrors.In the example shown, the optical device 40 is described as a pure mirror arrangement with mirrors. In principle, the optical device 40 of the supplementary unit 14 can have one or more of the following optical elements: lenses, prisms, gratings, diffractive optical elements or other beam shape elements.The autocollimator device 12 with base and extension unit 10, 14 can be used for characterizing a microelectromechanical mirror device 50, also referred to for short as MEMS (MicroElectroMechanischer Mirror). In this case, the autocollimating target 34 is a mirror element 52 of the microelectromechanical mirror device 50. the mirror element 52 forms the reflective surface 36 and can be periodically tilted via electrical drive parameters. The angle of rotation or tilt of the mirror element 52 is the angle θ of the surface normal N of the reflecting surface 36 with respect to the main optical axis 26 indicated here. The microelectromechanical mirror device 50 is an oscillatory device with the parameters usual for such devices, such as resonant frequency, quality, etc.MEMS mirrors 50 find wide use in the art in beam deflection for laser scanners, among others. Here, the application requires not only the excitation of the mirror element, but also the reliable determination of the applied angle of rotation or tilt.The characterization of a MEMS mirror 50 therefore comprises the determination of the resonant frequency and transfer function as a function of external interference variables, such as e.g. the ambient temperature. For this purpose, the mechanical deflection angle must be set by means of optical measurement methods as a function of the MEMS excitation signal and the MEMS sensor signal. The characterization of the microelectromechanical mirror device 50 is therefore in particular a temperature-dependent characterization. The characteristic parameters of a microelectromechanical mirror device 50 change with temperature, as is quite common in the case of systems that can oscillate. For characterizing the microelectromechanical mirror device 50, a look-up table is created, for example. This look-up table is stored in a data memory of a control device of microelectromechanical mirror device 50 and can then also be used to correct control parameters of microelectromechanical mirror device 50. In this way, the dependence of the characteristic parameters of the microelectromechanical mirror device 50 on the disturbance variable (e.g. the temperature) can be compensated.FIGS. 3 and 4 illustrate the control voltage and temperature dependence of the characteristic parameters of a microelectromechanical mirror device 50.FIG. 3 shows, by way of example, the transfer function of a MEMS mirror 50 at different control voltages U 1, U 2 and U 3. The amplitude A is plotted here against the frequency f. As can be seen from the graph, the amplitude, i.e. the mechanical deflection angle, changes the resonant frequency with increasing control voltage.Furthermore, it can be observed that the transfer function deviates greatly from its idealised Gaussian distribution, as can be observed at low control voltages, at high control voltages. This is due to increasing non-linearities in the spring constant of the MEMS mirror 50. the spring constant thus becomes a function of the instantaneous deflection angle and can be described by a third degree polynomial. Here, the cubic term k 2 basically determines the slope of the transfer function.FIG. 4 shows exemplary transfer functions of a MEMS mirror 50 as a function of the ambient temperature T.In the upper row, the amplitude A is plotted against the frequency f, in the lower row the phase P is plotted against the frequency f. All graphs show the resonant frequency f 0N at standard temperature T N. While the situation at standard temperature T=T N is shown in the middle column, the left column shows the situation at lower temperature T<T N and the right column shows the situation at higher temperature T>T N.It can be seen that there is also a dependence on the ambient temperature. The resonant frequency "migrates" with the temperature T.Furthermore, the transfer function may vary from individual MEMS mirror 50 to individual MEMS mirror 50. For reliable control of the MEMS modules, it is therefore necessary to characterize the resonant frequency and the control voltage individually as a function of the mechanical deflection angle and to store them in systemic form.Another application of microelectromechanical mirror (MEMS) devices 50 is light detection and ranging (LIDAR) systems for optical object detection. A corresponding system is known, for example, from the publication DE 10 2012 025 281 A1. The use of the measuring system described above results in a number of advantages for the production process of complex LIDAR systems. On the one hand, the position and angular offset of a rotating object outside the original working range of the measuring device can be determined. This not only allows the adjustment duration of complex LIDAR systems to be significantly reduced, but at the same time increases the adjustment accuracy, since additional possible error influences due to respanning of the workpiece are dispensed with.

Claims

Autocollimator device (12) for measuring an angle θ of the surface normal (N) of a reflecting surface (36) with respect to an optical main axis (26) of the autocollimator device (12), having a base unit (10) which has a light source (16), a detector (18) and an optical unit (20), wherein the reflecting surface (36) reflects the light emitted axially by the base unit (10) axially in a central angular range ΔΘ of the angle θ around the main axis (26) in such a way that the reflected light is detectable by the detector (18), characterized bya supplementary unit (14) having an optical device (40) which comprises a plurality of optical elements (42, 44, 46, 48) and is designed in such a way that the optical elements (42, 44, 46, 46, 48) reflects the light reflected by the reflecting surface (36) in at least one other angle range of the angle θ deviating from the central angle range ΔΘ close to the axis in such a way that it can be detected by the detector (18), wherein the optical device (40) is designed as a mirror arrangement with optical elements (42, 44; 46, 48) designed as mirrors, wherein the mirrors (42, 44, 46, 48) of the mirror arrangement (40) are exclusively planar mirrors.Autocollimator device according to claim 1, characterized in that the at least one other angular range lies outside the central angular range ΔΘ of the angle θ.Autocollimator device according to Claim 1 or 2, characterized in that two other angular ranges are provided, which lie on one side of the central angular range ΔΘ in each case.Autocollimator device according to Claim 3, characterized in that the optical elements (42, 44, 46, 48) of the optical device (40) are arranged symmetrically with respect to the main axis (26).Autocollimator device according to one of Claims 1 to 4, characterized in that the optical unit (20) of the base unit has a beam splitter (22) and at least one lens (24).Use of the autocollimator device (12) according to at least one of Claims 1 to 5 for characterizing a microelectromechanical mirror device (52).Use according to Claim 6, characterized in that the characterization of the microelectromechanical mirror device (52) is a temperature-dependent characterization.Use according to Claim 6 or 7, characterized in that at least one look-up table is created for characterizing the microelectromechanical mirror device (52).

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