Optoelectronic sensor and method for detecting an object

Abstract

The present application relates to an optoelectronic sensor and a method for detecting an object. An optoelectronic sensor (10) for detecting an object in a monitored region (20) is proposed, having a light emitter (12) for emitting an emitted light (16), a light receiver (26) for generating a reception signal from a received light (22) from the monitored region (20), a movable deflection unit (18) for periodically deflecting the emitted light (16) and the received light (22), a control and evaluation unit (32) for detecting information about an object in the monitored region (20) on the basis of the reception signal, and an optical turning element (18, 40) in the beam path of the received light (22). Here, the turning element (18, 40) has a temperature-dependent beam shaping property.

Description

TECHNICAL FIELD

[0001] The present application relates to an optoelectronic sensor, in particular a laser scanner, and a method for detecting objects in a monitored area. BACKGROUND

[0002] Optoelectronic systems, in particular laser scanners, are suitable for detection and distance measurement in which a large horizontal angular range of the measuring system is required. In a laser scanner, a light beam generated by a laser is periodically scanned over a monitored area by means of a deflection unit. The light is diffusely reflected on objects in the monitored area and evaluated in the laser scanner. From the angular position of the deflection unit, the angular position of the object is inferred, and in a phase method or a pulse method, the distance of the object from the laser scanner is also inferred from the light flight time using the speed of light. With the angular information and the distance information, the position of the object in the monitored area is recorded in two-dimensional polar coordinates. Here, the position of the object can be determined or its contour can be determined.

[0003] In addition to such measuring applications, laser scanners are also used in safety technology for monitoring dangerous sources, for example dangerous machines. Such safety laser scanners are known from DE 43 40 756 Al. Here, a protective zone in which an operating person is not allowed to step during machine operation is monitored. If the laser scanner recognizes an unpermitted protective zone intervention, for example a leg of an operating person, the laser scanner triggers an emergency stop of the machine. Sensors used in safety technology must work particularly reliably, so that higher safety requirements must be met, for example the standard EN 13849 on machine safety and the equipment standard EN 61496 on non-contact protective devices (BMS).

[0004] The detection sensitivity of a simple photodiode is not sufficient in many applications. In an avalanche photodiode (APD), the incident light triggers a controlled avalanche breakdown (Avalanche Effect). As a result, the number of charge carriers generated by the incident photons increases exponentially and generates a photo current which is proportional to the light reception intensity, but much larger than in a simple PIN diode. In the so-called Geiger mode, the avalanche photodiode is biased above the breakdown voltage so that even a single charge carrier released by a single photon can trigger an avalanche which then recruits all available charge carriers due to the high field strength. The avalanche photodiode therefore counts single events like a Geiger counter as it is named. Avalanche photodiodes in Geiger mode are also called SPADs (Single-Photon Avalanche Diodes), and the corresponding light receivers are called multi-pixel photon counters (MPCC) or silicon photomultipliers (SiPM).

[0005] In APDs, the electronic system is more temperature-dependent and therefore has to track the high voltage. There are more inherent noise sources, which are limited in their area-to-bandwidth relationship, and they are usually more expensive. All this speaks in favor of a light receiver based on SPADs or SiPMs. However, in the case of single-photon detection, it is not only the useful photons that trigger an avalanche, but also weak disturbance events due to extraneous light, optical crosstalk or dark noise. This disturbance event then contributes to the measurement result with a comparatively strong signal comparable to the received useful light and cannot be distinguished from it either. Avalanche photodiodes working in Geiger mode then remain insensitive for a dead time of approximately 5 ns to 100 ns and stop further measurements during this time. On the other hand, for APD diodes or PIN diodes, it is possible to select the electronic derivation of the cross current (Querstrom) generated by extraneous light.

[0006] For these reasons, in particular for single-photon detection, it is necessary to suppress extraneous light particularly effectively for useful light, where these measures are likewise advantageous for other technical light receivers. One strategy consists in not allowing extraneous light to penetrate to the light receiver in the first place. This is achieved by means of spectral separation by means of optical bandpass, by reducing undesired scattering paths or reflection paths in the sensor and preferably by means of a diaphragm in front of the light receiver, which reduces the acceptance angle deliberately.

[0007] The acceptance angle is proportional to the quotient of the field stop area of the receiving optics and the focal length. In turn, the focal length usually varies by a few percent within a certain temperature range of the sensor. This variation is usually added to the tolerance chain and ultimately increases the field stop diameter and thus the acceptance angle of the system. However, more extraneous light must be accepted.

[0008] DE 10 2009 055 988 B3 describes a device for optically scanning and measuring an environment, which utilizes a converging lens and multiple turning downstream of the light receiver. Here, a color camera is arranged on the optical axis of the receiving lens, which records a color image around the laser scanner.

[0009] The lidar sensor with a multi-beam light source for emitting light beams according to DE 10 2017 209 294 A1 uses a directional filter, a converging primary mirror element and a scattering secondary mirror element in the receiving beam path.

[0010] In the laser scanner according to EP 3 246 729 B1, a converging mirror is used as an optical unit, which combines the functions of a receiving optics and a beam folding. Such a receiving-side beam guidance is also proposed in the embodiment of US 7 544 945 B2.

[0011] However, these documents do not aim at any method for compensating the temperature-dependent focal length change of the receiving optics. Only EP 3 246 729 B1 focuses on this aspect and avoids the receiving lens and its temperature effects. This is a possible approach, but the receiving lens is absolutely reasonable and advantageous, so it does not solve the real problem.

[0012] EP 3 699 637 B1 relates to a laser scanner which compensates the refractive influence of a front window on the passing emitted or received light, in particular by suitably shaping the shape of a rotating mirror. This does not reduce the influence of the described temperature changes.

[0013] EP 3 699 638 B1 describes a laser scanner with a common rotating shield for emitted light. Without any further steering, the received light beam path is very traditionally guided through a flat rotating mirror and a receiving lens. SUMMARY

[0014] It is therefore the task of the present invention to enable an improved measurement using a universal sensor.

[0015] This task is solved by the optoelectronic sensor, in particular laser scanner, and the method described below for detecting an object in a monitored area. An optical emitter generates and emits emitted light into the monitored area. The emitted light is at least partially diffusely reflected by the object in the monitored area and is then received again as diffusely reflected emitted light. A received light is thus generated in which the diffusely reflected emitted light is superimposed with extraneous light. As long as there is no contact with the object, only extraneous light will form the received light. A scanning movement, preferably a rotational movement, is generated by means of a movable deflection unit, by which the emitted light and the received light are periodically emitted at different deflection angles or detected from different deflection angles.

[0016] The respective received signals are evaluated to obtain optically detectable information about the object, for example binary presence information, distance, position or color or diffuse reflectivity. Preferably, the control and evaluation unit is designed to determine the distance of the object from the light flight time between the emission of the emitted light and the reception of the received light. This will result in a distance measuring sensor and its distance determination as a measurement information about the object.

[0017] An optical turning element, in particular a mirror element, is arranged in the beam path of the received light, through which the received light is turned. Depending on the embodiment, the turning can involve a periodic deflection or be the periodic deflection itself, or the turning is an additional deflection which is not responsible for the periodic deflection and is preferably arranged downstream of the periodic deflection.

[0018] The basic idea of the present application is to use a turning element for temperature compensation. For this purpose, the turning element has a temperature-dependent beam shaping. In particular, the turning element is curved, so that the turning element can be associated with a focal length. This curvature and focal length are changed by targeted deformation in a known temperature range. Possible curvature states can include a flat, non-curved state for certain temperatures. According to the application, a targeted thermal adaptation is achieved, which leads to a desired temperature-dependent change in the beam shaping properties. This therefore does not simply mean that all objects are inevitably subject to temperature influences. For example, this would not allow a flat mirror to become a converging mirror or a diverging mirror, and in any case this change would be uncertain, it would be random, and thus would generate additional disturbances and would not be compensated.

[0019] The advantage of the present application is that, for example, within a temperature change process of 100°C, the spatial viewing angle remains small. By means of the preferred passive compensation of the turning element, or alternatively the active compensation, the thermal influences in the receiving beam path, for example the inherent change in the focal length of the receiving lens within the temperature, are reduced in whole or in part. Thereby, a small spot is achieved, so that a high spatial resolution and a low foreign light entry are achieved. There is no need to provide an enlarged acceptance angle to compensate for temperature tolerances. This improves the conditions for using highly sensitive avalanche photodiodes (SPAD, Single Photon Avalanche Diode, or SiPM, Silicon Photomultiplier) in Geiger mode, which are falsely triggered by foreign light and are then no longer available for useful light in the dead time. A powerful foreign light concept can be implemented economically and retrofitted in existing systems.

[0020] Preferably, the turning element has only a single mirror surface, wherein alternatively a plurality of individual mirror elements with a joint action of the mirrors in the same plane are envisaged, in particular mirror elements which are very close to one another. A further conceivable variant uses front and rear sides of the mirrors with a mutual spacing of, for example, 1-2 mm.

[0021] Preferably, the sensor has a receiving optics, in particular with at least one refractive element or a receiving lens, for converging the received light onto the light receiver. Preferably, the receiving optics guides the received light by converging or focusing it onto the light receiver. For reasons of simple construction, it is particularly preferred to use only a single receiving lens. Preferably, the receiving optics or the receiving lens is made of plastic. This enables low-cost production and can be arbitrarily shaped, including additional functions such as fixation, etc. Alternative glass lenses have a more powerful temperature-dependent process, but reduce the design freedom and are significantly more expensive, especially for large aspherical lenses.

[0022] Preferably, the temperature-dependent beam shaping properties of the turning element counteract the temperature-dependent changes in the beam shaping properties of the receiving optics in a compensatory manner. It is particularly preferred that the turning element and the receiving optics change their focal lengths in opposite directions when the temperature changes. In other words, the temperature-dependent process of the turning element is opposite to that of the receiving optics. The beam shaping properties, in particular the focal length, of the turning element change in the opposite direction to the receiving optics. The aim of this is to keep the focal point position at least substantially constant when the temperature changes.

[0023] Preferably, the aperture is arranged upstream of the light receiver, in particular at a distance corresponding to the focal length of the receiving optics, i.e. the receiving optics focuses the received light onto the aperture opening. The compensatory temperature-dependent process of the turning element ensures that this focal point position is maintained even if the focal length of the receiving optics changes due to the temperature. As a result, the acceptance angle or the aperture opening can be kept small without taking temperature changes or tolerances into account. The foreign light is cut off as much as possible at the outer cross section of the received light without affecting the useful light.

[0024] Preferably, the turning element is flat at the target temperature, in particular at room temperature, and has a convex or concave curvature depending on the sign of the deviation from the target temperature. Thus, if the focal length of the receiving optics changes with the temperature, this is compensated by a compensatory change in the focal length of the turning element. Depending on the direction to be compensated, the turning element either has a converging effect and thus a positive focal length of the appropriate size, or a diverging effect and thus a negative focal length of the appropriate size. The resting state at the target temperature is a flat turning element which leaves the beam shaping properties of the receiving optics optimized for this case unchanged, in particular the focal length of the receiving optics and the focal point position at the aperture opening in front of the light receiver. A flat turning element in the form of a plane glass mirror can achieve a high-quality optical design at very low cost.

[0025] Preferably, the turning element has only convex curvature or only concave curvature in the temperature range specific to the sensor, in particular including the borderline case of a flat turning element at the edge of the temperature range. In this alternative, the turning element does not deform between convex and concave around a flat initial shape, but always remains convex or concave depending on the embodiment. The degree of deformation of the convex or concave shape changes with the temperature change, the turning element assumes different radii of curvature. The borderline case of a flat turning element at the lowest or highest specified temperature can still be included, i.e. the shape changes between flat and convex or flat and concave and vice versa.

[0026] Preferably, the turning element has at least two materials with different thermal expansion. Exactly two materials are particularly preferred. This enables a passive design of the turning element which adapts itself to the temperature. No control device or circuit is required for this.

[0027] Preferably, the turning element has at least two material layers. The layer thicknesses and the materials and their thermal expansion coefficients are free parameters for achieving the turning element. Thus, the sensitivity to temperature changes can be adjusted to achieve the desired temperature-dependent beam shaping properties of the turning element for compensation. In particular, the third or further material or the third and further layers relate to the case of an additional protective layer or a layer system on a carrier layer or multiple carrier layers.

[0028] Preferably, the turning element has a core made of one material, which is surrounded by another material, in particular a metal core surrounded by plastic. When the temperature changes, the surrounding material has to give way to the core. Preferably, the core has a low thermal expansion coefficient, as in the case of metals, so the reaction to temperature changes is relatively small. The basic shape is still determined by the core, the desired different curvature is produced by the surrounding material.

[0029] Preferably, the core is ring-shaped. The surrounding material then assumes a shape symmetrical about the central axis as it expands and contracts with the temperature change. The curvature over the entire surface changes, the radial shape is comparable over the entire circumference. This exactly corresponds to the desired focal length change.

[0030] Preferably, an actuating element is associated with the turning element to deform the turning element, and the actuating element is manipulated to adjust the temperature-dependent beam shaping properties. This is an active embodiment, for example based on piezoceramics. The turning element is brought into a shape adapted to the temperature by the actuator.

[0031] Preferably, the sensor has a temperature sensor and / or a light-sensitive measuring element for determining the cross section of the light beam of the received light. From the measured temperature, in the case of a known receiving path of the sensor, the changes in the received light beam path can be predicted using a theoretical model, and using the light-sensitive measuring element, these changes in the received light beam path can be remeasured in practice. This can be used for diagnostic purposes. Thus, in combination with the actuating element, a temperature adjustment of the light beam formed by the receiving optics and the turning element can be provided. For example, the light-sensitive measuring element can be placed annularly around the diaphragm opening.

[0032] Preferably, the movable deflection unit is configured as a rotating mirror with a mirror surface. Typically, the rotating mirror is at an angle of 45°, so that the emitted light is generated along the axis of rotation or the received light is received along the axis of rotation and is scanned perpendicular to the plane of the axis of rotation by 90° turning using the rotating mirror. Preferably, the mirror surface is the only mirror surface of the rotating mirror. Thus, it is not a polygon mirror. Alternatively, a laser scanner is known, in which the entire measuring head rotates together with the light emitter and the light receiver.

[0033] Preferably, the turning element is arranged to move together with the deflection unit. In the case of the deflection unit being configured as a rotating mirror, the turning element preferably serves as the mirror surface of the rotating mirror. Thus, the temperature compensation is achieved by a special design of the rotating mirror.

[0034] Preferably, the turning element is configured as a folding mirror, which is arranged downstream of the receiving optics in the receiving beam path of the received light. In this implementation, the turning element is not part of the deflection unit and, in particular, is a separate component with respect to the rotating mirror and is arranged downstream thereof. With the folding mirror, the received light beam path is folded, given a new direction to the received light beam path and, in particular, at least partially returns to itself. Thereby, a longer optical path can be accommodated in a smaller space. Particularly preferably, the folding mirror is arranged downstream of the receiving optics. Thus, the received light, which has been beam-shaped or focused by the receiving optics, is shot onto the folding mirror, which then turns it to the light receiver. By the dual function of the folding mirror as a turning element according to the invention, the temperature compensation of the beam shaping is superimposed with the folding.

[0035] Preferably, the reflecting surface of the folding mirror is adapted to the receiving beam path, particularly in a ring-shaped manner (having a non-reflective center and an annular segment corresponding to the projected shadow in the receiving beam path). This reduces the ingress of extraneous light, thereby improving the signal-to-noise ratio. Due to this adaptation, the folding mirror does not reflect any additional extraneous light from areas in the light receiver where no useful light is incident. If the light receiver is located at the center of the receiving beam path, the annular shape is suitable. The incident received light is then blocked there, so the light reflected at the center can only be extraneous light. The corresponding projected shadow can be generated by the light emitter and the carrier element for the light receiver and / or the light emitter, and can be taken into account in the shaping of the reflecting surface.

[0036] Preferably, the folding mirror is constructed and arranged such that the received light is directly redirected towards the direction of the light receiver. This means there is only one folding mirror, and multiple folding mirrors are not required before the received light is guided to the light receiver in a new direction, and therefore, there is no need to redirect it one after another. Thus, the received light from the monitoring area is sequentially redirected by the deflection unit through the receiving optics to the folding mirror, and then by the folding mirror to the light receiver, without any further change in the direction of the received beam path. Optical elements such as filters or apertures can still be used between the folding mirror and the light receiver, but new redirecting elements or mirrors cannot be used.

[0037] Preferably, the folding mirror is oriented perpendicular to the beam path of the received light incident upon it. Preferably, the lens plane of the receiving optics, configured as a receiving lens, is parallel to the folding mirror, and more preferably, also parallel to the receiving plane of the light receiver. Therefore, the received light incident on the folding mirror is reflected back, or redirected by 180° except for tolerances, etc. Due to the converging effect of the receiving optics, the reflection angle of each beam of received light is not 180°, but can specify the common direction of the entire beam of received light, such as the average reflection angle, or alternatively, can be defined by the receiving optics whose optical axis is perpendicular to the folding mirror. Preferably, the light receiver is arranged between the deflection unit and the folding mirror. Then, the deflection of the folding mirror and other deflecting elements will not guide the received light from the light receiver into the plane on the other side of the folding mirror.

[0038] Preferably, the sensor has a transmitting tube for shielding the emitted light, which moves at least partially with the deflection unit. Essentially, the transmitting tube surrounds the path of the emitted beam and prevents the emitted light from being scattered within the sensor before entering the monitoring area. Preferably, emitting optics for beam shaping, particularly for collimating the emitted light, are arranged within the transmitting tube.

[0039] The method according to the application can be further developed in an analogous manner and at the same time exhibits analogous advantages. Such advantageous features are described below exemplarily but not exhaustively. BRIEF DESCRIPTION OF DRAWINGS

[0040] Further features and advantages of the present application will be described in more detail below exemplarily and with reference to embodiments and with reference to the drawings. In the drawings:

[0041] Figure 1 A schematic diagram of a laser scanner is shown;

[0042] Figure 2 A schematic diagram of a laser scanner with a folding mirror is shown;

[0043] Figure 3 A diagram of the receiving beam path in a laser scanner according to the application after the receiving optics is shown; Figure 2

[0044] Figure 4 A cross-sectional diagram of the receiving beam path in a laser scanner according to the application in the case of a change in the focal length of the receiving optics due to different temperatures is shown; Figure 3

[0045] Figure 5 A diagram of different shapes of the turning element at different temperatures is shown;

[0046] Figure 6 A three-dimensional diagram of the turning element in a flat state is shown;

[0047] Figure 7 A three-dimensional diagram of the turning element in a concave curved state is shown; and

[0048] Figure 8 A diagram of different curvatures of the turning element at different temperatures is shown. DETAILED DESCRIPTION

[0049] Figure 1 A schematic cross-sectional diagram of a photosensor as a laser scanner 10 in an embodiment is shown. A light emitter 12 (e.g. with a laser source) generates an emission beam 16 by means of emission optics 14. The emission beam 16 is emitted into a monitoring region 20 by means of a deflection unit 18. In order to avoid optical crosstalk, the emission beam 16 can be at least partially surrounded by an emission tube (not shown).

[0050] ​​In the monitoring region 20, the emitted light beam 16 is diffusely reflected by possibly present objects. The corresponding received light 22 is returned to the laser scanner 10 and is detected by means of a light receiver 26 by means of a receiving optics 24 by means of the deflection unit 18. Preferably, the receiving optics 24 is a single converging lens, but further lenses and other optical elements can be added. The light receiver 26 has at least one photodiode, or for higher sensitivity an avalanche photodiode (APD) or an arrangement of at least one single-photon avalanche diode (SPAD, SiPM), for example.

[0051] The deflection unit 18 is placed in a continuous rotary motion with a scan frequency by means of an electric motor 28. Thereby, the emitted light beam 16 scans a plane within each scan cycle, i.e. during one full rotation at the scan frequency. An angle measurement unit 30 is arranged on the outer circumference of the deflection unit 18 in order to detect the respective angular position of the deflection unit 18. Here, the angle measurement unit 30 is formed by a line disk (Strichscheibe) as a Winkelmaßverkörperung and a forked light barrier (Gabellichtschranke) as a scanning device, for example.

[0052] A control and evaluation unit 32 is connected with the light emitter 12, the light receiver 26, the electric motor 28 and the angle measurement unit 30. By determining the light flight time between emitting the light beam 16 and receiving the diffusely reflected light 22, the distance of the scanned object from the laser scanner 10 is inferred with the speed of light. Here, the evaluation unit learns the respective angular position of the emitted light beam 16 from the angle measurement unit 30.

[0053] Thus, after each scan cycle, the two-dimensional polar coordinates of the object points in the monitoring region 20 can be obtained by angle and distance, and the respective measurement data can be transmitted by means of an interface 34. In turn, the interface 34 can be used for 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, WLAN, LTE, 5G and many others. In the application of safety technology, the interface 34 can be designed to be safe, in particular as an Output Signal Switching Device (OSSD) for a safety-oriented shutdown signal upon recognition of a protection area violation. The laser scanner 10 is mounted in a housing 36, which has a surrounding front window (Frontscheibe) 38.

[0054] In the illustrated laser scanner 10, the light emitter 12 and its emitting optics 14 are located within the central opening of the receiving optics 24. This is merely one example of possible arrangement. The invention also includes alternative coaxial solutions, such as those with separate reflector areas for emitting the beam 16 or with beam-splitting reflectors, as well as biaxial arrangements.

[0055] Deflection unit 18 in Figure 1 The rotating mirror appears to be flat. However, according to the present invention, the rotating mirror bends differently depending on temperature, wherein, according to an embodiment, curvature exists at all temperatures, or it exhibits a flat shape at a specific temperature. The reasons for the temperature-dependent deformation, the design scheme of the deformation, and the possible measures to achieve the deformation will be referred to later. Figures 3 to 8 To elaborate.

[0056] Figure 2 Another embodiment of the laser scanner 10 is shown, wherein instead of the rotating reflector of the deflection unit 18, an additional folding reflector 40 bends differently depending on the temperature. Here, the same features are indicated by the same reference numerals and will not be described further. Figure 2 In the middle, the emission beam path utilization has been targeted Figure 1 The optional one-piece or two-piece transmitter tubes 42a-42b are used for shielding. At least the second part 42b of the transmitter tube entering the monitoring area 20 from the deflection unit 18 moves together with the deflection unit 18.

[0057] According to Figure 1 Unlike the laser scanner 10, the receiving light 22 is... Figure 2 In the laser scanner 10 shown, the light is additionally deflected, and consecutive portions of the received beam path are assigned different reference symbols for better differentiation. The received light 22a, deflected by the deflection unit 18, strikes the receiving optics 24. The received light 22b, forming a beam or bundle on the receiving optics, falls onto the folding mirror 40. The received light 22c, reflected by the folding mirror 40, then passes through the aperture 44 and, after passing through the optical filter 46 matched to the wavelength of the light emitter 12, strikes the light receiver 26. The order of the aperture 44 and the optical filter 46 can be reversed.

[0058] exist Figure 2In the illustrated embodiment, the light emitter 12 and the light receiver 26 are arranged on a common printed circuit board 48, which has an aperture 50 through which the received light 22a passes. Alternatively, separate printed circuit boards can be envisaged. The receiving optics 24 have a central opening 52 in which the diaphragm 44, the optical filter 46 and the light receiver 26 are arranged. In alternative embodiments, the light receiver 26 can be arranged below the receiving optics 24, which then do not necessarily have a central opening 52 either. Instead of the central opening 52, the receiving optics 24 can have a further beam-shaping element in the center. In particular, the illustrated receiving optics 24 then form an outer region for the received light 22a in the forward path (Hinweg) and the further beam-shaping element forms an inner region for the received light 22a in the return path (Rückweg) after reflection at the fold mirror 40.

[0059] The fold mirror 40 can be equipped with spectral filtering properties adapted to the wavelength of the light emitter 12 by means of a coating, a structuring or a filter element and in this way replace or supplement the optical filter 46. The optical filter 46 has the advantage that the cross section of the received light 22c and the angular range of the light beam incident thereon are strictly limited. As a result, a small optical filter 46 with a narrow bandwidth is possible, by means of which extraneous light outside the wavelength of the emitted or useful light is filtered out particularly cost-effectively and effectively.

[0060] As in Figure 1 for simplicity, the rotating mirror of the deflection unit 18 is shown as flat, for simplicity, Figure 2 the fold mirror 40 is shown as flat. Now, a temperature-dependent deformation of the rotating mirror or the fold mirror according to the application is introduced below, with which temperature changes in the received beam path of the receiving optics 24 are compensated, in particular. Here, depending on the temperature and the embodiment, the rotating mirror and / or the fold mirror 40 assumes different shapes or curvatures, wherein, depending on the embodiment, a flat state at a specific temperature or a curvature that is maintained over all temperatures can also be envisaged. Furthermore, in addition to the embodiments according to Figure 1 or Figure 2 additional turning elements can also be envisaged, which can contribute to or not contribute to the compensation by means of a temperature-dependent deformation. Instead of a laser scanner 10 with a deflection unit 18 designed as a rotating mirror, a laser scanner with a rotating measurement head can be envisaged, in which the light emitter and / or the light receiver and at least one temperature-dependent deformation of the turning element move together in the received beam path.

[0061] Figure 3 Again, the laser scanner 10 according toFigure 2 The laser scanner 10 features folded receiving beam paths 22a-22c. Due to the folded reflector 40, the structural space between the receiving optics 24 and the folded reflector 40 is doubled, thus accommodating the receiving beam path even if the receiving optics 24 has a long focal length. Because of the long focal length of the receiving optics 24, the folded receiving beam 22c has a smaller fan angle, allowing for the design of a small-sized and narrow-passband optical filter 46. The aperture 44 can be positioned at the focal point 54, thus effectively suppressing incoming light incident at a flatter angle.

[0062] Figure 4 The temperature effect of beam shaping or focusing is shown in a magnified view of the received beam path. Figure 3 The favorable conditions in this process are only achieved at specific temperatures (e.g., room temperature of 20°C). If the temperature changes, for example to a higher temperature of 70°C, the focal length of the receiving optics 24 changes. The received light 22c' produces a shifted focal position 54' when the temperature changes, thus magnifying the cross-section of the received light 22c' in the aperture plane. The aperture 44, then positioned at the original focal position 54, then cuts off the useful light or allows additional external light to pass through, depending on whether the aperture opening design allows for tolerance. This temperature effect is particularly pronounced in the receiving optics 24 made of plastic, especially plastic lenses. However, plastic has advantages in manufacturing and design, as well as price, compared to glass, which is less sensitive to temperature.

[0063] Figure 5 It is shown in accordance with Figure 1 In embodiments of the laser scanner 10, the folding mirror 40 or a rotating mirror similar to the deflection unit 18 exhibits compensated, temperature-dependent deformation. The folding mirror or rotating mirror, now commonly referred to as a steering element, presents a flat shape 56 at the target temperature (e.g., room temperature) as a nominal mirror (Nennspiegel). Increasing temperatures result in an increasingly concave curved shape 58, and correspondingly decreasing temperatures result in an increasingly convex curved shape 60. This results in a temperature-dependent focal length distribution of the steering element that is opposite to, and quantitatively equal to, that of the receiving optics 24. Thus, ideally, the final focal length in the received beam path is constant, and in any case, fluctuates significantly less due to the compensating deformation of the steering element.

[0064] By a clever pairing of different materials, a deformation or change in the radius of curvature of the turning element over temperature can be achieved. Preferably, a layer structure of at least two materials is chosen. The basic idea is similar to a bimetallic strip, but a significantly more precise deformation is achieved, and preferably not only metals are used, but for example a combination of metal and plastic or other material combinations of plastic and / or metal. These layers are designed in their thickness and the materials with the respective coefficients of thermal expansion in such a way that a deformation of the turning element is achieved in a completely targeted manner which counteracts the temperature characteristics of the receiving optics 24. Here, a specific temperature range of approximately 20° room temperature is considered, for example within a temperature interval of a total of 100°C, which corresponds to the permissible operating environment of the laser scanner 10.

[0065] As a supplement to the schematic illustration of the turning element 22, Figure 5 and Figure 6 and Figure 7 an embodiment of the turning element with a mirrorized uppermost layer is shown in a three-dimensional illustration. Here, Figure 6 a flat shape 56 is shown, i.e. a mirror surface which is not deformed at ambient temperature, while Figure 7 an exemplary deformation of the shape 58 into a concave curvature is shown at an overtemperature of 50°C.

[0066] Figure 8 An exemplary temperature change process of the turning element is shown. The effective thickness or range is plotted in the Z direction perpendicular to the mirror surface with respect to the radius r of the turning element. Here, for simplicity, only a circular turning element is assumed, in reality, the turning element can also have a different geometry. Each curve represents a curvature at a temperature, from top to bottom: with the greatest concave curvature at the highest temperature, through less pronounced concave curvatures to the flat shape at room temperature, with increasing convex curvature until the greatest convex curvature at the lowest temperature. With an exemplary pairing of Corning glass 9740 with a thickness of 1 mm and aluminum with a thickness of 0.5 mm, a change in the radius of curvature of up to + / - 1 m can be achieved.

[0067] In the shown embodiment, the turning element assumes a flat shape at room temperature, and the temperature change process with convex and concave deformations can be said to be centered on this flat shape. In other embodiments, it is assumed that the flat shape is at a higher or lower temperature, up to the limiting case of the flat shape at the highest or lowest temperature. Then, the shape changes between slightly convex and strongly concave or slightly concave and strongly convex, or between flat and convex or flat and concave. In yet other embodiments, the turning element is not flat at all within a specific temperature range, i.e. the temperature change process changes from slightly convex to strongly convex or from slightly concave to strongly concave. The direction of the temperature-dependent change in curvature is predetermined by the desired compensation of the effects opposite to the focal length change of the receiving optics 24.

[0068] For steering elements made of two layers, it is alternatively conceivable to surround a core made of at least one material with at least one other material. An example in this regard is a metal ring surrounded by plastic. If the expansion coefficient of the plastic is greater than that of the metal, the plastic will tend to escape laterally as the temperature rises. For low temperatures, the dimensions of the steering element are designed such that a desired curvature in the opposite direction is produced, for example by introducing a predetermined curvature on the inactive side of the mirror.

[0069] A further alternative embodiment is the use of actuators, for example active controllable piezoceramics instead of the passive non-reflective layer of the steering element. This can be used to control the radius of curvature of the layer composite. For example, the shape of the steering element is controlled or adjusted on the basis of the temperature determined using a temperature sensor. It is also conceivable to use, for example, photodiodes at the edge of the diaphragm opening, to measure the beam cross-section of the received light 22c, in order to determine the appropriate steering.

Claims

1. An optoelectronic sensor (10) for detecting objects in a monitored area (20), wherein the sensor (10) has a light emitter (12) for emitting an emission light (16), a light receiver (26) for generating a reception signal from a reception light (22) coming from the monitored area (20), a reception optics (24) for converging the reception light (22) onto the light receiver (26), a movable deflection unit (18) for periodically deflecting the emission light (16) and the reception light (22), a control and evaluation unit (32) for detecting information about objects in the monitored area (20) on the basis of the reception signal, and a turning element (18, 40) in the beam path of the reception light (22), which turning element (18, 40) is a mirror element, wherein the turning element (18, 40) has temperature-dependent beam shaping properties which counteract temperature-dependent changes in the beam shaping properties of the reception optics (24) in a compensatory manner.

2. The sensor (10) according to claim 1, wherein The sensor (10) is a laser scanner.

3. The sensor (10) according to claim 1, wherein The control and evaluation unit (32) is designed to carry out distance measurements using a light time-of-flight method.

4. The sensor (10) according to claim 1, wherein The temperature-dependent beam shaping properties of the turning element (18, 40) are temperature-dependent curvatures.

5. The sensor (10) according to any one of claims 1 to 4, the reception optics (24) is a reception lens.

6. The sensor (10) according to any one of claims 1 to 5, wherein the turning element (18, 40) and the reception optics (24) have focal length changes which are opposite to one another on temperature changes.

7. The sensor (10) according to any one of claims 1 to 6, wherein, a diaphragm (44) is arranged upstream of the light receiver (26).

8. The sensor (10) according to claim 7, wherein The diaphragm (44) is arranged at a distance which corresponds to the focal length of the reception optics (24).

9. The sensor (10) according to any one of claims 1 to 8, wherein the turning element (18, 40) is flat at a target temperature and has a convex curvature or a concave curvature depending on the sign of a deviation from the target temperature.

10. The sensor (10) according to claim 9, wherein The turning element (18, 40) is flat at room temperature.

11. The sensor (10) according to any one of claims 1 to 8, wherein the turning element (18, 40) has only a convex curvature or only a concave curvature in a temperature range which is specific to the sensor (10), including the critical case of a flat turning element (18, 40) at the edges of the temperature range.

12. The sensor (10) according to any one of claims 1 to 11, wherein the turning element (18, 40) has at least two materials which differ in their thermal expansion.

13. The sensor (10) according to claim 12, wherein, the turning element (18, 40) has at least two material layers.

14. The sensor (10) according to claim 12 or 13, wherein, The turning element (18, 40) has a core made of one material, which is surrounded by another material.

15. The sensor (10) according to claim 14, the turning element (18, 40) having a metal core surrounded by plastic.

16. The sensor (10) according to claim 14 or 15, wherein, The core is ring-shaped.

17. The sensor (10) according to any one of claims 1 to 11, wherein An actuating element is associated with the turning element (18, 40) to deform the turning element, and the actuating element is manipulated to adjust the temperature-dependent beam shaping properties.

18. The sensor (10) according to any one of claims 1 to 17, The sensor (10) has a temperature sensor and / or a light-sensitive measuring element (22) for determining a beam cross section of the received light.

19. The sensor (10) according to any one of claims 1 to 18, wherein, The turning element is arranged to move with the deflection unit (18).

20. The sensor (10) according to claim 19, the deflection unit (18) being configured as a rotating mirror.

21. The sensor (10) according to any one of claims 1 to 18, wherein The turning element is configured as a folding mirror (40), which is arranged downstream of the receiving optics (24) in the received beam path of the received light (22).

22. A method for detecting objects in a monitored area (20), wherein Emitting an emission light (16), which, after diffuse reflection at the object, is received by a receiving optics (24) onto a light receiver (26) as received light (22) and is converted by the light receiver (26) into a received signal in order to generate object information from the received signal, wherein the emission light (16) and the received light (22) are periodically deflected by means of a movable deflection unit (18) and the received light (22) is turned by means of a turning element (18, 40), wherein the turning element (18, 40) is a mirror element, wherein The beam shaping properties of the turning element (18, 40) change with temperature, the temperature-dependent beam shaping properties of the turning element (18, 40) counteracting the temperature-dependent change in the beam shaping properties of the receiving optics (24) in a compensatory manner.

23. The method of claim 22, wherein, The method is used for distance measurement with a light time-of-flight method.

24. The method of claim 22, wherein, The beam shaping properties are the curvature of the turning element.

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