OPTOELECTRONIC SENSOR AND METHOD FOR DETECTING OBJECTS

Abstract

An optoelectronic sensor (10) is provided for the detection of objects in a monitored zone (20) that has a light transmitter (12) for transmitting transmitted light (16), a laser scanner (26) for generating a received signal from received light (22) from the monitored zone (20), a movable deflection unit (18) for the periodic deflection of the transmitted light (16) and of the received light (22), a control and evaluation unit (32) for the detection of information on objects in the monitored zone (20) using the received signal, and an optical deflection element (18, 40), in the optical path of the received light (22), In this respect, the deflection element (18, 40) has temperature dependent beam shaping properties.

Claims

1. An optoelectronic sensor for the detection of objects in a monitored zone, the optoelectronic sensor comprising: a light transmitter for transmitting transmitted light, a laser scanner for generating a received signal from received light from the monitored zone, a movable deflection unit for the periodic deflection of the transmitted light and of the received light, a control and evaluation unit for the detection of information on objects in the monitored zone using the received signal and an optical deflection element in the optical path of the received light, wherein the deflection element has temperature dependent beam shaping properties.

2. The sensor in accordance with claim 1, wherein the control and evaluation unit is configured for a distance measurement using a time of flight measurement.

3. The sensor in accordance with claim 1, wherein the optical deflection element is a mirror element.

4. The sensor in accordance with claim 1, wherein the deflection element has a temperature dependent curvature.

5. The sensor in accordance with claim 1, that has a reception optics for bundling the received light on the light receiver.

6. The sensor in accordance with claim 5, wherein the reception optics comprises a reception lens for bundling the received light on the light receiver.

7. The sensor in accordance with claim 1, wherein the temperature dependent beam shaping properties of the deflection element counteract a temperature dependent change of the beam shaping properties of the reception optics in a compensatory manner.

8. The sensor in accordance with claim 7, wherein the deflection element and the reception optics undergo a mutually opposite focal length change on a temperature change.

9. The sensor in accordance with claim 1, wherein a diaphragm is arranged upstream of the light receiver.

10. The sensor in accordance with claim 9, wherein the diaphragm is arranged upstream of the light receiver at a distance corresponding to the focal length of the reception optics.

11. The sensor in accordance with claim 1, wherein the deflection element is flat at a desired temperature and has a convex or concave curvature on a deviation from the desired temperature depending on the sign of the deviation.

12. The sensor in accordance with claim 1, wherein the desired temperature is room temperature.

13. The sensor in accordance with claim 1, wherein the deflection element has only a convex curvature or only a concave curvature over a temperature range specified for the sensor.

14. The sensor in accordance with claim 13, wherein the temperature range specified for the sensor includes the boundary case of a flat deflection element at a margin of the temperature range.

15. The sensor in accordance with claim 1, wherein the deflection element has at least two materials having different temperature extents.

16. The sensor in accordance with claim 15, wherein the deflection element has at least two layers of the materials.

17. The sensor in accordance with claim 15, wherein the deflection element has a core composed of the one material that is surrounded by the other material.

18. The sensor in accordance with claim 17, wherein the core is a metal core having surrounding plastic.

19. The sensor in accordance with claim 17, wherein the core is annular.

20. The sensor in accordance with claim 1, wherein an actuator element is associated with the deflection element for its deformation and the actuator element is controlled to set temperature dependent beam shaping properties.

21. The sensor in accordance with claim 1, that has a temperature sensor and/or a light sensitive measurement element for determining a beam cross-section of the received light.

22. The sensor in accordance with claim 1, wherein the deflection element is arranged co-moving with the deflection unit.

23. The sensor in accordance with claim 1, wherein the deflection element is arranged co-moving with the deflection unit and forms the deflection unit as a rotating mirror.

24. The sensor in accordance with claim 1, wherein the deflection element is configured as a folding mirror arranged downstream of the reception optics in the optical reception path of the received light.

25. A method of detecting objects in a monitored zone in which transmitted light is transmitted, is received again as received light after remission at the object, and is converted into a received signal by a light receiver to generate a piece of object information from the received signal, wherein transmitted light and received light are periodically deflected with the aid of a movable deflection unit and the received light is deflected by a deflection element, wherein beam shaping properties of the deflection element change with the temperature.

Description

[0041] The invention will be explained in more detail in the following also with respect to further features and advantages by way of example with reference to embodiments and to the enclosed drawing. The Figures of the drawing show in:

[0042] FIG. 1 a schematic representation of a laser scanner;

[0043] FIG. 2 a schematic representation of a laser scanner with a folding mirror;

[0044] FIG. 3 a representation of the optical reception path in the laser scanner in accordance with FIG. 2 after a reception optics;

[0045] FIG. 4 a sectional representation of the optical reception path in accordance with FIG. 3 on a focal length change of the reception optics due to different temperatures;

[0046] FIG. 5 a schematic representation of different shapes of a deflection element at different temperatures;

[0047] FIG. 6 a three-dimensional representation of a deflection element in a flat state,

[0048] FIG. 7 a three-dimensional representation of a deflection element in a concavely curved state; and

[0049] FIG. 8 a representation of different curvatures of a deflection element at different temperatures.

[0050] FIG. 1 shows a schematic sectional representation through an optoelectronic sensor in an embodiment as a laser scanner 10. A light transmitter 12, for example having a laser light source, generates a transmitted light beam 16 with the aid of a transmission optics 14. The transmitted light beam 16 is transmitted into a monitored zone 20 by means of a deflection unit 18. To avoid optical cross-talk, the transmitted light beam 16 can be at least partly surrounded by a transmission tube, not shown.

[0051] The transmitted light beam 16 is remitted by an object that may be present in the monitored zone 20. The corresponding received light 22 again arrives back at the laser scanner 10 and is detected by a light receiver 26 via the deflection unit 18 by means of an optical reception optics 24. The reception optics 24 is preferably a single converging lens, but further lenses and other optical elements can be added. The light receiver 26, for example, has at least one photodiode or, for higher sensitivity, an avalanche photodiode (APD) or an arrangement having at least one single photon avalanche diode (SPAD, SiPM).

[0052] The deflection unit 18 is set into a continuous rotational movement having a scan frequency by a motor 28. The transmitted light beam 16 thereby scans one plane during each scan period, that is on a complete revolution at the scanning frequency. An angle measurement unit 30 is arranged at the outer periphery of the detection unit 18 to detect the respective angular position of the detection unit 18. The angle measurement unit 30 is here formed by way of example by an encoder wheel as an angular standard and a forked light barrier as a scanning device.

[0053] A control and evaluation unit 32 is connected to the light transmitter 12, to the light receiver 26, to the motor 28, and to the angle measurement unit 30. A conclusion is drawn on the distance of a scanned object from the laser scanner 10 using the speed of light by determining the time of flight between the transmission of the transmitted light beam 16 and the reception of remitted received light 22. The respective angular position at which the transmitted light beam 16 was transmitted is known to the evaluation unit from the angular measurement unit 30.

[0054] Two-dimensional polar coordinates of the object points in the monitored zone 20 are thus available via the angle and the distance after every scan period and corresponding measured data can be transmitted via an interface 34. The interface 34 can conversely be used for a parameterization or other data exchange between the laser scanner 10 and the outside world. The interface 34 can be designed for communication in one or more conventional protocols such as IO link, Ethernet, Profibus, USB3, Bluetooth, wireless LAN, LTE, 5G and many others. In applications in safety engineering, the interface 34 can be configured as safe and can in particular be a safe output (OSSD, output signal switching device) for a safety-relevant shut-down signal on recognition of a protective field infringement. The laser scanner 10 is accommodated in a housing 36 that has a peripheral front screen 38.

[0055] In the laser scanner 10 shown, the light transmitter 12 and its transmission optics 14 are located in a central opening of the reception optics 24. This is only an exemplary possibility of the arrangement. The invention additionally comprises alternative coaxial solutions, for instance having their own mirror region for the transmitted light beam 16 or having beam splitters, and also biaxial arrangements.

[0056] The deflection unit 18 appears as a flat rotating mirror in FIG. 1. In accordance with the invention, however, the rotating mirror is curved differently in dependence on the temperature, with, in dependence on the embodiment, one curvature being present for all temperatures or with the flat form being adopted at a specific temperature. The reason for the temperature dependent deformation, for the design of the deformation, and for possible measures to achieve them will be explained below with reference to FIGS. 3 to 8.

[0057] FIG. 2 shows a further embodiment of a laser scanner 10 in which, instead of the rotating mirror of the deflection unit 18, an additional folding mirror 40 is curved differently in dependence on the temperature. Here the same features are provided with the same reference numerals and will not be explained again. The optical transmission path is screened in FIG. 2 by an optional one-part or two-part transmission tube 42a-b already addressed in FIG. 1. At least the second part 42b of the transmission tube is moved along with the deflection unit 18 into the monitored zone 20 by the deflection unit 18.

[0058] Unlike the laser scanner 10 in accordance with FIG. 1, the received light 22 is additionally defected in the laser scanner 10 shown in FIG. 2 and is respectively given a different reference numeral for better distinguishability in the sequential parts of the reception light path. The received light 22a deflected by the deflection unit 18 is incident on the reception optics 24. The received light 22b beam shaped or bundled there is incident on the folding mirror 40. The received light 22c reflected back by the folding mirror 40 is then incident on the light receiver 26 through a diaphragm 44 and after passing through an optical filter 46 that is coordinated to the wavelength of the light transmitter 12. The order of the diaphragm 44 and the optical filter 46 can be reversed.

[0059] The light transmitter 12 and the light receiver 26 are arranged on a common circuit board 48, that has a cutout 50 for the passage of the received light 22a, in the embodiment shown in FIG. 2. Alternatively, respective separate circuit boards are conceivable. The reception optics 24 has a central opening 52 in which the diaphragm 44, the optical filter 46, and the light receiver 26 are accommodated. In alternative embodiments, the light receiver 26 can be arranged beneath the reception optics 24 that then does not necessarily still have a central opening 52. Instead of the central opening 52, the reception optics 24 can have a further beam shaping element at its center. The reception optics 24 then in particular forms an outer zone for the received light 22a on the forward run and the further beam shaping element forms an inner zone for the received light 22c on the return path after reflection at the folding mirror 40.

[0060] The folding mirror 40 can be equipped with spectrally filtering properties in an adaptation to a wavelength of the light transmitter 12, either by coatings, structures, or a filter element, and replaces or complements the optical filter 46 in this manner. The optical filter 46 has the advantage that the cross-section of the received light 22c and the angular range of the beams incident there are is tightly bounded there. A small optical filter 46 having a narrow bandwidth is therefore possible by which extraneous light outside the wavelength of the transmitted or useful light is particularly inexpensively and effectively filtered.

[0061] Just as in FIG. 1, the rotating mirror of the deflection unit 18 is shown as flat in simplified form; FIG. 2 shows a flat folding mirror 40 in simplified form. In the following, a temperature dependent deformation of the rotating mirror or folding mirror in accordance with the invention is presented by which a temperature range in the optical reception path is compensated, in particular the reception optics 24. Depending on the temperature and on the embodiment, the rotating mirror and/or folding mirror 40 adopt different shapes or curvatures here, with again, in dependence on the embodiment, a flat state being conceivable at a specific temperature or one curvature remaining present over all temperatures. Furthermore, complementary to the embodiments in accordance with FIG. 1 or FIG. 2, it is conceivable that there are further deflection elements that can each contribute to the compensation, or not, by temperature dependent deformation. Instead of a laser scanner 10 having a deflection unit 18 designed as a rotating mirror, a laser scanner is conceivable having a rotating measuring head in which the light transmitter and/or light receiver and at least one deflection element that deforms in dependence on the temperature are moved along in the optical reception path.

[0062] FIG. 3 again shows the folded optical reception path 22a-c in the laser scanner 10 in accordance with FIG. 2 in enlarged form and in more detail. Thanks to the folding mirror 40, the construction space between the reception optics 24 and the folding mirror 40 is used twice so that the optical reception path is also accommodated with a longer focal length of the reception optics 24. The folded received light 22c has a smaller angle fan that permits the design of an optical filter 46 having small dimensions and a narrow bandpass. The diaphragm 44 can be arranged in a focal location 54 and can thus effectively suppress extraneous light that is incident at a shallower angle.

[0063] FIG. 4 illustrates in a sectional enlargement of the optical reception path a temperature effect of the beam shaping or focusing. The favorable situation of FIG. 3 is only achieved at a specific temperature, for example room temperature of 20° C. At a changed temperature, for example a higher temperature of 70° C., the focal length of the reception optics 24 adjusts itself. A displaced focal location 54′ and thus an enlarged cross-section of the received light 22c′ in the diaphragm plane results for the received light 22c′ at the changed temperature. A diaphragm 44 arranged at the original focal location 54 then, depending on the design of the diaphragm aperture that allows tolerances or not, either blocks useful light or allows additional extraneous light to pass through. This temperature effect is particularly pronounced with a reception optics 24 of plastic, in particular a plastic lens. Plastic, however, has advantages in manufacture, design, and price with respect to the less temperature sensitive glass.

[0064] FIG. 5 illustrates a compensating temperature dependent deformation of the folding mirror 40 or analogously the rotating mirror of the deflection unit 18 in an embodiment of a laser scanner 10 in accordance with FIG. 1. The folding mirror or rotating mirror now called a deflection element in an overarching manner adopts a flat shape 56 as a nominal mirror at a desired temperature, for example room temperature. An elevated temperature effects an increasingly concavely curved shape 58; accordingly a reduced temperature effects an increasingly convexly curved shape 60. A temperature dependent focal length extent of the deflection element thus result that is opposite to that of the reception optics 24, and indeed, where possible, also quantitatively to the same degree. The resulting focal length in the optical reception path is thereby constant in the ideal case and in any case fluctuates considerably less due to the compensating deformation of the deflection element.

[0065] A deformation or change of the radius of curvature of the deflection element over temperature can be achieved by a skillful pairing of different materials. A layer structure of at least two materials is preferably selected. The basic idea is similar to a bimetallic strip, but with a considerably more precise deformation being achieved, and preferably not only metals, but, for example, the combination of a metal and a plastic or another material combination of plastics and/or metals, being used. The layers are configured in their thicknesses and materials with a respective thermal coefficient of expansion just so that a deformation of the deflection element acting opposite to the temperature behavior of the reception optics 24 is achieved in a very targeted manner. In this respect, a specified temperature range around the room temperature of 20° is considered, for example over a temperature interval of a total of 100° C. that corresponds to a permitted working environment of the laser scanner 10.

[0066] FIGS. 6 and 7 show, complementary to the schematic representation of FIG. 5, an embodiment of the deflection element having a mirror coated topmost layer in a three-dimensional representation. FIG. 6 here shows the flat shape 56, i.e. a non-deformed mirror surface at environmental temperature; and FIG. 7 shows an exemplary deformation to a concavely curved form 58 at 50° C. excess temperature.

[0067] FIG. 8 shows an exemplary temperature range of the deflection element. The effective thickness or extent in the Z direction is applied perpendicular to the mirror surface against the radius r of the deflection element. A circular shape of the deflection element is here only assumed in a simplifying manner; in practice, the deflection element can also have a different geometry. Each curve stands for the curvature at a specific temperature; from top to bottom at a maximum temperature having the largest concave curvature over less pronounced concave curvature to a flat shape at room temperature as the convex curvature increases up to a maximum convex curvature at minimal temperatures. Changes in the radius of curvature up to +/−1 mm can be achieved for an exemplary pairing of Corning glass 9740 having a thickness of 1 mm and aluminum having a thickness of 0.5 mm.

[0068] In the embodiments shown, the deflection element adopts a flat shape at room temperature and the temperature range having a convex and concave deformation is so-to-day centered around it. In other embodiments, the flat shape is adopted at a higher or lower temperature up to the marginal case of a flat shape at a maximum temperature or minimum temperature. The shape then varies between slightly convex and highly convex or slightly concave and highly convex or between flat and convex or flat and concave. In again different embodiments, the deflection element is anyway no longer flat at all in the specified temperature range, i.e. the temperature range varies from slightly convex to highly convex or slightly concave to highly concave. The direction of the curvature change in dependence on the temperature is predefined by the desired compensating effect opposite to the focal length change of the reception optics 24.

[0069] Alternatively to a deflection element of two layers, it is conceivable to surround a core of at least one material with at least one further material. An example for this is a ring of metal that is surrounded by plastic. With a larger coefficient of expansion of the plastic with respect to the metal, the plastic body escapes laterally as the temperature increases. The deflection element is dimensioned for low temperatures such that the desired opposite curvature is produced in that, for example a predefined curvature is applied to the non-effective side of the mirror.

[0070] An again alternative embodiment uses an actuator system; it, for example, replaces the passive non-reflective layer of the deflection element with an actively controllable piezoceramic material. The radius of curvature of the laminar structure can be controlled by it The shape of the deflection element is, for example, controlled or regulated using a temperature determined by a temperature sensor. It is furthermore conceivable to measure the beam cross-section of the received light 22c, for instance using a photodiode at the margin of the diaphragm aperture, to determine the matching control.