OPTOELECTRONIC SENSOR AND METHOD FOR MEASURING THE CONTAMINATION OF A WINDSCREEN

Description

[0001] The invention relates to an optoelectronic sensor, in particular a laser scanner, for detecting at least one object in a monitoring area and to a method for measuring the contamination of a front window of an optoelectronic sensor according to the preamble of claim 1 or 13.

[0002] In a laser scanner, a light beam generated by a laser periodically sweeps across a surveillance area with the aid of a deflection unit. The light is remitted by objects in the surveillance area and evaluated in the scanner. The angular position of the deflection unit is used to determine the angular position of the object, and the distance of the object from the laser scanner is also determined from the light travel time, using the speed of light in a phase or pulse method. To increase the range with limited light pulse energy, some conventional laser scanners emit multiple light pulses for each distance value and calculate the results of these individual measurements to produce a common measured value. A laser scanner of this type with a pulse averaging method is known, for example, from DE 10 2010 061 382 A1. Using the angle and distance information, the location of an object in the surveillance area is recorded in two-dimensional polar coordinates.This allows the position of objects to be determined or their contours to be determined. In most laser scanners, the scanning movement is achieved by a rotating mirror or a polygon mirror wheel. Instead, in some laser scanners, such as the one described in DE 197 57 849 B4, the entire measuring head, including light transmitters and light receivers, rotates. While most known laser scanners operate with a single scanning beam and therefore only cover a central scanning plane, there are also efforts to create a multi-plane scanner using a multitude of scanning beams. This can be combined with a pulse averaging method.

[0003] Laser scanners are also used in safety technology to monitor a hazard source, such as a dangerous machine. Such a safety laser scanner is known from DE 43 40 756 A1. It monitors a protective field that operating personnel are prohibited from entering while the machine is in operation. If the laser scanner detects an unauthorized intrusion into the protective field, such as an operator's leg, it triggers an emergency stop of the machine. Sensors used in safety technology must be particularly reliable and therefore meet stringent safety requirements, such as the EN 13849 standard for machinery safety and the EN 61496 device standard for electro-sensitive protective devices (ESPE).

[0004] A laser scanner is often used in harsh environments where it comes into contact with dust, dirt, water, and similar contaminants. This impairs the transmittance of the laser scanner's front screen and increases scattering. The laser scanner is generally designed to tolerate this to a certain extent. However, beyond this point, a loss of range or incorrect measurements with potentially serious consequences occurs. Therefore, monitoring the transmittance of the front screen, also known as contamination measurement, is known. This enables the laser scanner to issue warnings or error states in the event of impairments. For use in security technology, front screen monitoring is even essential.

[0005] A conventional solution, described, for example, in DE 43 45 446 C2, provides for a multitude of independent optical test channels consisting of respective transmitter-receiver pairs, which are distributed across the entire angular range of the windscreen. This principle is still used in many laser scanners. Since each test channel can only perform a point measurement of the windscreen, a correspondingly large number of components is required. In addition, one partner of the transmitter-receiver pair or, alternatively, at least one reflector in a reflective arrangement must be mounted outside the windscreen. However, it would be desirable for the windscreen to enclose the device, especially since external components themselves are exposed to contamination.

[0006] To avoid the multitude of test channels, a so-called rotating contamination measurement is possible. In this case, a transmitter-receiver pair moves with the deflection unit or the rotating mirror for the scanning movement, thus successively testing the transmittance of the windshield. However, this again requires a reflector outside the windshield. Optical crosstalk to the actual measuring system can also be problematic. EP 2 237 065 A1 discloses such a laser scanner in which the entire measuring unit, including light source and detector, rotates. A test light source and a test detector are also housed on the corresponding rotor, while a reflector element is arranged outside the housing. In this way, the test light source and test detector scan the windshield with the help of the reflector element as they rotate. A similar window pane monitoring system is also known from EP 2 388 619 A1.In EP 4 086 661 A1, a test light transmitter and a test light receiver are arranged near the outer circumference of a rotor of the rotating measuring head in order to irradiate the front window from below, with a reflector on the top completing the test light path.

[0007] Another conventional approach is to derive the contamination measurement from the actual distance measurement. The escaping measuring light causes an internal windscreen reflection, which is usually considered an interference effect and deflected, for example, into an optical trap. However, it essentially contains information about the optical properties of the windscreen. One difficulty is that this only checks the exit area of ​​the windscreen, not a different entry area. Furthermore, the measurement function can be overloaded and impaired, for example, because part of the measurement time is consumed.

[0008] In EP 3 078 985 A1, a reflector rotates relative to stationary pairs of test light transmitter and test light receiver. This allows a somewhat wider area of ​​the windshield to be tested per test channel, but the need for multiple test channels is not eliminated. EP 2 508 914 A1 uses a similar concept, except that the reflector is attached to a transmitter tube and not directly to the rotating mirror. US 2008 / 0158555 A1 shows another windshield monitoring system using a co-rotating reflector.

[0009] DE 100 25 511 C1 directs measuring radiation for contamination monitoring of a laser scanner window via its rotating mirror to the underside of the window. There, the measuring radiation is redirected by a mirror surface extending the length of the window to the top of the window, where a row of measuring receivers is arranged. Due to the curved contour of the window, the measuring radiation is guided through the window several times on its way from the underside to the top. This is a type of hybrid between rotating contamination measurement and a multitude of test channels, but it still requires numerous components. DE 197 06 612 A1 has a slightly different design but follows the same principle. EP 3 511 739 A1 expands on the idea by feeding the test light for checking the windscreen back into a receiver arranged on the axis of rotation, effectively implementing a rotating contamination measurement.

[0010] DE 10 2016 111 615 B3 discloses a laser scanner whose angle measurement unit is based on a co-rotating image sensor based on the principle of a computer mouse chip. The sensor is directed at a stationary part, in particular the windshield, and tracks the rotation via the optical flow. As a further function, the data from the image sensor can be evaluated again, for example, with regard to amplitude and contrast, to monitor the windshield for contamination.

[0011] EP 3 392 679 A1 describes a laser scanner that uses a test unit to test the transmittance of a windshield. The test unit has two test devices, the first of which has a rotating test detector and test transmitters distributed in the circumferential direction, and the second of which has a rotating vertical arrangement of several light sensors.

[0012] It is therefore an object of the invention to further improve windscreen monitoring or contamination measurement.

[0013] This object is achieved by an optoelectronic sensor, in particular a laser scanner, for detecting at least one object in a monitored area, as well as a method for measuring the contamination of a front screen of an optoelectronic sensor according to claim 1 or 13. The sensor comprises a measuring unit with at least one measuring light transmitter and at least one measuring light receiver for emitting measuring light into the monitored area and receiving it from there in order to generate a measurement reception signal. A front screen is provided in a housing of the sensor to allow the measuring light to pass through. A movable deflection unit periodically deflects the measuring light, thus ensuring repeated scanning of a partial area of ​​the monitored area, for example a scanning plane. The movable deflection unit is preferably designed as a movable measuring head with the measuring unit; alternatively, however, a rotating mirror or the like can also be used.

[0014] A contamination inspection unit moves along with the deflection unit to check the windshield for contamination. The contamination inspection unit has at least two contamination inspection light transmitters and at least two contamination inspection light receivers. This generates a contamination inspection reception signal from the contamination inspection light reflected from the windshield. Thus, a windshield reflection is measured; the evaluated portion of the contamination inspection light does not penetrate the windshield or only penetrates to a contamination on the outside. Measuring unit and contamination inspection unit are initially merely collective names for the corresponding functional units, but they can also be designed as structural modules. The corresponding prefixes are intended merely to distinguish the affiliation, for example, of the respective light transmitters and light receivers.

[0015] In a control and evaluation unit, the received measurement signal is evaluated to detect the object, for example, to determine whether an object is located in the respective radiation direction and, preferably, to measure its distance using a time-of-flight method. The received contamination detection signal is also evaluated to assess the contamination of the windshield and thus, at least indirectly, its transmittance.

[0016] The invention is based on the fundamental concept of multi-channel contamination measurement. Multi-channel means that, depending on the position of the deflection unit, multiple contamination detection reception signals are acquired and evaluated. For this purpose, the contamination detection unit has at least two contamination detection light transmitters and / or at least two contamination detection light receivers. Each contamination detection channel comprises a contamination detection light transmitter and a contamination detection light receiver. By selecting such pairs, various combinations and thus light paths and points of incidence of the contamination detection light on the windshield are created. A contamination detection light transmitter or a contamination detection light receiver can be used multiple times in different contamination detection channels. The contamination detection channels preferably operate with a time offset. The sequence can be changed very quickly due to the extremely short light paths.The multiple contamination test reception signals are then evaluated together to arrive at an overall assessment of the contamination of the windscreen.

[0017] The invention has the advantage of enabling continuous monitoring of the front screen in all positions of the movable deflection unit. This allows the entire passage of both the outgoing and incoming measuring light to be inspected. The sections or segments of the front screen to be inspected can be configured virtually arbitrarily in the direction of movement of the deflection unit. Only a few components are required, keeping hardware and cost requirements low. The front screen can represent the outermost edge of the device; external components for contamination measurement are not required. The light paths of the measuring light and the contamination inspection light can be decoupled, so that the contamination measurement does not influence the actual measurement.The evaluation according to the invention also enables a distinction to be made between nearby objects in the monitoring area and contamination of the windscreen, so that false triggering of the contamination test can be intercepted.

[0018] If the sensor has a measuring unit that moves with the deflection unit, data and energy transmission to the moving part of the sensor is already required, which can also be used by the contamination measurement. In this case, the additional space and hardware requirements for the contamination measurement are particularly minimal. The sensor can be designed with multiple beams, particularly as a multi-layer scanner, to cover a larger portion of the monitoring area. For this purpose, several scanning beams are generated in the measuring unit on the transmitting and / or receiving sides by a plurality of measuring light transmitters and / or beam outputs or a plurality of measuring light receivers.

[0019] The radiation direction of the contamination detection light is preferably transverse, especially perpendicular, to the radiation direction of the measurement light. By different orientations of the actual measurement and the contamination measurement, optical crosstalk is greatly reduced or completely eliminated. Even without temporal decoupling, the contamination measurement does not interfere with the actual measurement.

[0020] The control and evaluation unit is preferably designed to compare a respective contamination test reception signal with a reference value for evaluating the transmittance of the windshield. This comparison is preferably performed for each contamination test reception signal or contamination test channel. The reference value forms the basis for evaluating, for example, a new or clean windshield.

[0021] The control and evaluation unit is preferably configured for a teach-in mode in which contamination test reception signals are stored as reference values. This preferably occurs for each contamination test reception signal or contamination test channel. This calibrates the sensor to a target state, either ex works or, for example, with a new windscreen or one that has been cleaned manually. The teach-in mode can be called up for recalibration, for example, at the push of a button or by selecting a corresponding function in configuration software, such as after the windscreen has been replaced or cleaned.

[0022] The control and evaluation unit is preferably designed to measure a contamination test received signal when the contamination test light transmitter is inactive in order to compensate it as a background signal of a contamination test received signal detected when the contamination test light transmitter is active. This implements background suppression, which reduces or eliminates the influence of ambient light on the contamination measurement. Background suppression is preferably performed for each contamination test channel, whereby it is sufficient to measure contamination test signals only once per contamination test light receiver when the contamination test light transmitter is inactive. Background suppression is preferably performed for the contamination test received signals of a contamination measurement during operation and / or for the acquisition of reference values.To accommodate the dynamic behavior of the sensor with its moving deflection unit, the two measurements are preferably taken very close together, with the contamination detection light transmitter inactive and active, and thus in close temporal and spatial proximity. Very close together means that the deflection unit is still in practically the same angular position; however, mathematical precision is not required here; an identical angular segment to be evaluated is sufficient.

[0023] The sensor preferably has a temperature sensor, wherein the control and evaluation unit is configured to adapt a contamination check reception signal or a reference value based on a temperature measured by the temperature sensor. This provides temperature compensation for the contamination measurement. It is possible to adapt the contamination check reception signals and / or the reference values ​​to the measured temperature. The temperature adaptation can be performed using a calculation rule, in particular a simple linear temperature factor, or, for example, using a lookup table (LUT).

[0024] The sensor preferably has a temperature reference channel with a temperature reference light transmitter, a temperature reference light receiver, and an internal temperature reference target arranged within the housing for adjusting a contamination check reception signal or a reference value based on a temperature reference signal from the temperature reference light receiver. This transfers the temperature behavior from the temperature reference channel to the contamination measurement. Temperature measurement is then no longer required, but can still be redundantly performed.

[0025] According to the invention, the contamination inspection unit comprises at least two contamination inspection light transmitters and at least two contamination inspection light receivers in an alternating linear arrangement. The minimal configuration of one contamination inspection light transmitter and two contamination inspection light receivers is thus expanded to create additional contamination inspection channels. The alternating arrangement can be interpreted in a strictly alternating sense ... ESESE... (S for contamination inspection light transmitter, E for contamination inspection light receiver), but also in a more irregular sense such as EESEESEE, ESEESSEESE. A non-linear, i.e., two-dimensional arrangement is conceivable as an alternative.

[0026] The arrangement is preferably arranged at an angle or parallel to a rotational axis of the movable deflection unit. Illustratively, the contamination detection light transmitters and contamination detection light receivers are arranged vertically within the device to inspect overlapping sections of the windshield. A certain inclination is permissible if, for example, it offers structural advantages.

[0027] The contamination inspection channels preferably have at least one direct channel formed by a contamination inspection light transmitter and an immediately adjacent contamination inspection light receiver. A direct channel is thus spanned by a pair of adjacent contamination inspection light transmitters and contamination inspection light receivers. This enables direct inspection of the corresponding point of impact on the windshield with the shortest light paths and steepest angles.

[0028] The contamination detection channels preferably have at least one indirect channel, formed by a contamination detection light transmitter and a contamination detection light receiver not directly adjacent. An indirect channel is thus a counterpart to a direct channel, with somewhat longer light paths and flatter angles. A combination of at least one direct channel with at least one indirect channel is particularly advantageous. Indirect channels are particularly suitable for distinguishing between contamination and a nearby object in the monitoring area, since the latter generates a significantly stronger contamination detection reception signal in an indirect channel.

[0029] The control and evaluation unit is preferably designed to evaluate the transmittance of the windscreen in several angular segments according to the positions of the movable deflection unit, and in particular, to not evaluate the transmittance for at least one configurable angular segment. The contamination measurement is thus related to sections of the windscreen. This enables a differentiated diagnosis and, for example, an indication to the user where the windscreen needs to be cleaned. Furthermore, angular segments can be treated differently; for example, contaminated angular segments can be tolerated if no measurement is required there or if the corresponding section is already installed and blocked from the measuring light.

[0030] The control and evaluation unit is preferably designed to compare measurement results from the measuring unit and the contamination testing unit, in particular to not evaluate the transmittance of the windscreen when an object is detected by the measuring unit in this position of the deflection unit and / or to treat a rapid increase in contamination as a detected object. Such cross-comparisons between the actual measurement and the contamination measurement can lead to more robust results and prevent false alarms from the contamination measurement. Conversely, the contamination measurement can possibly support the actual measurement at extremely close range. This is because contamination is a slow process, so that a rapid change can be attributed to an object, for example, within one or a few periods of movement of the deflection unit.It can happen that, for example, a drop of rain or a splash of mud on the windshield is treated as a close object instead of as dirt, which however leads to the correct result of assuming a close object as a precaution.

[0031] The method according to the invention can be further developed in a similar manner and thereby exhibits similar advantages. Such advantageous features are described by way of example, but not exhaustively, in the subclaims following the independent claims.

[0032] The invention will be explained in more detail below with regard to further features and advantages, using exemplary embodiments and with reference to the accompanying drawings. The figures of the drawing show: Fig. 1 a schematic sectional view of a laser scanner with rotating contamination measurement; Fig. 2 a simplified schematic sectional view of another laser scanner with a rotating mirror instead of a rotating measuring head; Fig. 3 a further simplified schematic view of a laser scanner with rotating contamination measurement from above; Fig. 4 a view of a contamination testing unit with its direct channels; Fig. 5 a view of the contamination testing unit according to Figure 4 now with their indirect channels; Fig. 6 shows a comparative representation of the signals of a contamination detection unit in the presence of contamination and in the presence of a nearby object in the monitoring area, depending on the distance between the contamination detection light transmitter and the contamination detection light receiver; and Fig. 7 shows an exemplary flow diagram of a contamination measurement.

[0033] Figure 1shows a schematic sectional view of an optoelectronic sensor 10 in an embodiment as a laser scanner. The sensor 10 roughly comprises a movable deflection unit 12 and a base unit 14, which can also be considered a rotor and stator. The deflection unit 12 carries the optical measuring head, while the base unit 14 houses additional elements such as a power supply, evaluation electronics, connections, and the like. During operation, the deflection unit 12 is set in motion about a rotational axis 18 with the aid of a drive 16 of the base unit 14 in order to periodically scan a monitoring area 20.

[0034] A measuring unit 22 is moved along with the deflection unit 12. In this measuring unit, a light transmitter 24 with multiple light sources 24a, for example, LEDs or lasers in the form of edge emitters or VCSELs, generates, with the aid of a common transmission optics 26, multiple transmitted light beams 28 that are emitted into the monitored area 20. In the example shown, there are four transmitted light beams 28 for four scanning planes; there can be more, even significantly more, or just as few transmitted light beams 28. Instead of a common transmission optics 26, individual optics are possible. The multiple transmitted light beams 28 can also be created by splitting the light from one or more light sources using a beam splitter element, a diffractive optical element, or the like.

[0035] If the transmitted light beams 28 strike an object in the monitored area 20, corresponding remitted light beams 30 return to the sensor 10. The remitted light beams 30 are guided by a common receiving optics 32 to a light receiver 34 with several light receiving elements 34a, each of which generates an electrical received signal. The light receiving elements 34a can be separate components or pixels of an integrated matrix arrangement, for example, photodiodes, APDs (avalanche diodes), or SPADs (single-photon avalanche diodes). The comments regarding the transmitting side also apply here. In particular, several individual optics can be provided, and several remitted light beams 30 can be detected on a common light receiving element.

[0036] Scanning with multiple light beams 28, 30 creates a larger detection area in elevation, particularly a multi-layer scanner. Alternatively, only one light source 24a or only one light receiving element 34a can be provided to implement a single-beam sensor, particularly a laser scanner with only one scanning plane. The basic optical design with a biaxially adjacent light transmitter 24 and light receiver 34 is also not mandatory and can be replaced by any design known per se from single-beam optoelectronic sensors or laser scanners. One example of this is a coaxial arrangement with or without a beam splitter.

[0037] A contactless supply and data interface 36 connects the movable deflection unit 12 to the stationary base unit 14. A control and evaluation unit 38 with at least one computing unit is located there. Examples of these are digital computing components such as a microprocessor or a CPU (Central Processing Unit), an FPGA (Field Programmable Gate Array), a DSP (Digital Signal Processor), an ASIC (Application-Specific Integrated Circuit), a KI processor, an NPU (Neural Processing Unit), a GPU (Graphics Processing Unit), or the like. Thanks to the contactless data interface 36, the control and evaluation unit 38 can also be provided partially or completely in the movable deflection unit 12, deviating from the illustration.

[0038] The control and evaluation unit 38 controls the light transmitter 24 and receives the received signals from the light receiver 34 for further evaluation. It also controls the drive 16 and receives the signal from an angle measuring unit (not shown) that is commonly used in laser scanners and determines the respective angular position of the deflection unit 12. For evaluation purposes, the distance to a scanned object is measured. Together with the information about the angular position from the angle measuring unit, two-dimensional polar coordinates of all object points in a scanning plane are available after each scan period, including the angle and distance. The respective scanning plane, if there are multiple scanning planes, is also known from the identity of the respective light beam 28, 30, so that a total three-dimensional spatial region is scanned.

[0039] The object positions or object contours are thus known and can be output via a sensor interface 40. Conversely, the sensor interface 40 or another connection (not shown) serves as a parameterization interface. The sensor 10 can be designed as a safety laser scanner, in which case the control and evaluation unit 38 is particularly designed to compare the position of detected objects with protective fields and, upon detection of an impermissible intrusion into the protective field, to trigger a safe output with a shutdown signal.

[0040] All of the aforementioned functional components are arranged in a housing 42, which has a circumferential front panel 44 in the area of ​​the light exit and entry. The front panel 44 is often, but not necessarily, designed as a rotating body and in both cases does not necessarily have to extend over 360°, so that a certain angular range remains as a dead zone. The front panel 44 shown is frustoconical and is therefore inclined to the optical axis of the measuring unit 22. Alternative shapes of the front panel 44, for example, with a spherical or cup-shaped contour or curvature, are also conceivable.

[0041] A contamination checking unit 46 is provided in the sensor 10, which is moved along with the deflection unit 12 and which makes it possible to detect contamination of the windscreen 44 and which will be described later with reference to the Figures 4 to 7will be explained in more detail. The contamination measurement is based on the backscattering of contamination inspection light 48 from the front screen 44 and enables a large-area vertical scan of the front screen 44 and continuous monitoring of the contamination in the direction of movement of the deflection unit 12. The contamination inspection unit 46 can be arranged and aligned as shown so that it does not influence the measuring unit 22 optically, mechanically, electrically, or digitally. The contamination inspection unit 46 is connected via the contactless supply and data interface 36, so that the control and evaluation unit 38 is also responsible for its control and evaluation.

[0042] Figure 2shows a simplified schematic sectional view of another embodiment of the sensor 10. The measuring unit 22 is located in the base unit 14 and therefore does not follow the movement of the deflection unit 12. Instead, the periodic scanning is implemented with the aid of a rotating mirror 50, which rotates with the deflection unit 12. The contamination testing unit 46 is still arranged to move with the deflection unit 12 and is therefore connected to the contactless supply and data interface 36. The beam direction of the contamination testing light 48 is different here than in Figure 1, which is merely intended to illustrate that variations are possible in this regard in all embodiments of the sensor 10. It is important that the contamination detection light 48 actually tests an area of ​​the windshield 44 that is also used by the light beams 28, 30 of the measuring unit 22, whereby an angular and resulting time offset is permissible in the direction of movement of the deflection unit 12.

[0043] Figure 3 shows a further simplified schematic representation of the sensor 10 from above to further illustrate the basic arrangement. Figure 3 can be seen as a plan view of both the sensor 10 according to Figure 1 as well as the sensor 10 according to Figure 2As the deflection unit 12 moves, the contamination testing unit 46 or its contamination testing light 48 reaches every angular position, enabling continuous monitoring of the front screen 44. For the evaluation of the contamination measurement, the front screen 44 can be divided into any number of angular ranges or segments. The only limits here are the required measuring and processing times, which do not permit any arbitrarily fine-grained subdivision. A division into segments makes it possible, for example, to exclude selected segments from monitoring if the sensor 10 is installed in such a way that only part of the front screen 44 is still accessible. Since contamination is generally a slow process, a measurement can be extended to several periods of movement of the deflection unit 12 or accumulated over them, for example, averaged.This makes it possible to further increase the number of segments.

[0044] Figure 4 shows a representation of the contamination inspection unit 46. It has light transmitters 52 and light receivers 54, which are sometimes referred to as contamination light transmitters and contamination light receivers to distinguish them from the light transmitter 24 and light receiver 34 of the measuring unit 22, which for the same reason can be called measuring light transmitters and measuring light receivers. The light transmitters 52 and light receivers 54 can form various pairs and generate a respective contamination inspection reception signal with the contamination inspection light 48 that is respectively emitted and received after reflection at the front screen 44. The respective pairs are called channels. Figure 4 Pairs of directly adjacent light transmitters 52 and light receivers 54 are formed. This is referred to as a direct channel.

[0045] In order to form multiple channels, according to the invention, at least more than one light transmitter 52 and more than one light receiver 54 must be provided. An embodiment with two light transmitters 52 (S) and three light receivers 54 (E) in alternating sequence ESESE is shown as an example. This makes it possible to form the four direct channels 56 d1 - 56 d4 shown. Other numbers and other sequences are conceivable, for example ESEESE and many more. Instead of an arrangement vertically one above the other, an inclined arrangement would also be conceivable if this promises advantages, for example, with regard to the use of installation space or heat dissipation or for a specific windshield geometry, such as a curved freeform surface. However, care should preferably be taken to ensure that the distance between the light transmitters 52 or light receivers 54 and the windshield 44 is as small as possible and remains constant during the movement of the deflection unit 12.

[0046] The light transmitters 52 preferably have a wide beam angle and a non-transparent base or a lateral shield such as a light-tight ring to prevent lateral scattered light, especially directly into the light receivers 54, as this would generate an interfering signal component. A large reception angle is advantageous for the light receivers 54 in order to be able to detect as much backscattered contamination detection light 48 as possible.

[0047] Figure 5 shows a representation similar Figure 4but now with indirect channels 56 i1 - 56 i2 , each of which combines light transmitters 52 and light receivers 54 that are not directly adjacent. The component spacing between the light transmitter 52 and the light receiver 54 in an indirect channel is therefore greater than in a direct channel and also varies between the indirect channels depending on the arrangement. This allows additional information to be obtained, which enables an improved assessment of the contamination status of the windscreen 44. In particular, the indirect channels enable a distinction to be made between contamination and a nearby object in the monitoring area 20.

[0048] This shows Figure 6shows a comparative representation of the signals of a contamination detection unit in the presence of contamination and in the presence of a nearby object in the monitoring area 20 as a function of the distance between the light transmitter 52 and the light receiver 54 of a channel. The dark spots represent the signal from the nearby object, the bright spots represent the signal from a dirty windscreen 44. For small distances between the light transmitter 52 and the light receiver 54, there is no noticeable difference. For larger distances, in Figure 6 Above approximately 20 mm, for example, the values ​​differ significantly. A nearby object produces a significantly higher signal than contamination, and thus, a distinction can be made between these two cases.

[0049] Direct channels have a smaller distance between light transmitter 52 and light receiver 54, indirect channels a larger distance corresponding to the right part of the Figure 6. Thus, if the front screen 44 is dirty, a strong signal is produced in the direct channels, while the indirect channels show only a very weak signal due to the flat angle of incidence on the front screen 44. However, if the object is close, all channels show strong signals. The indirect channels therefore make the difference in the right part of the Figure 6 accessible and allow a distinction between a nearby object and pollution.

[0050] Figure 7 shows an example flow chart of a contamination measurement. In step S1, reference values ​​are learned. These are the signal levels of the channels to be used later in operation in a specified state of the windscreen 44, for example, at the factory or when the windscreen 44 has been cleaned or replaced. The reference values ​​can therefore be considered target values ​​for a sufficiently clean windscreen 44.

[0051] When determining reference values, as well as later during operation when measuring contamination test signals, background suppression is preferably performed. For this purpose, for example, a measurement is taken in the respective channel once with the light transmitter 52 active and once with the light transmitter 52 inactive, and the difference is calculated. Digital background suppression with a background measurement with the light transmitter 52 inactive can be supplemented by an electrical high-pass filter.

[0052] In step S2, temperature compensation is performed. This at least partially compensates for the temperature dependence of the behavior of the light transmitters 52 and light receivers 54. According to the sequence of Figure 7The reference values ​​are adjusted to a currently measured temperature. Instead, the measured contamination test reception signals could be adjusted. The temperature adjustment can be simple, for example, multiplying an empirical temperature constant by the temperature difference between the current temperature and a reference temperature and adding this temperature component to the reference values. Alternatively, more complex calculations or a lookup table are conceivable.

[0053] In step S3, a contamination check reception signal is measured for each participating channel. Here, too, the aforementioned background compensation is preferably performed with a further measurement while the light transmitter 52 is inactive. Optionally, channels in which the background light alone generates a high signal level are excluded from further analysis, because in this case, it is not expected that a relevant contamination check reception signal can be generated even in the event of contamination. The respective, preferably background-compensated, contamination check reception signal is compared with the corresponding reference value of the channel, for example, a quotient is calculated.

[0054] The measurements in the channels are preferably performed with a time delay to avoid crosstalk or because the light transmitter 52 and light receiver 54 are involved in multiple channels. However, a single measurement, in which, for example, the light transmitter 52 of a channel is pulsed and the corresponding light receiver 54 is read out, is very short, so no excessive demands need to be placed on the timing. In particular, the temporal sequence of the channels is arbitrary. All channels should only measure in the same angular segment, which means that the smallest granularity of the angular segments has a certain lower limit.

[0055] In step S4, the results from the individual channels are summarized to obtain a common value for the respective angular segment. For example, the highest signal level among the direct channels is used as the value to preventively overestimate the contamination, or another calculation such as an average, a quantile, or the like is used. Optionally, some exceptions are caught. For example, if the signal level in the indirect channels is too high, a nearby object is suspected, and therefore no contamination measurement is possible for this angular segment at that moment. It is also possible that too many channels are invalid due to too much background light; for example, fewer than two direct channels have even contributed a meaningful contamination test received signal. Even then, no contamination measurement is possible for this angular segment at that moment.

[0056] In step S5, the value is translated into a contamination level. For this purpose, a threshold can be specified, for example, for a binary decision between still sufficiently transmittable and contaminated, or multiple thresholds for a more differentiated contamination level, such as no relevant contamination, still functional but imminent failure, and measurement capability no longer guaranteed.

[0057] In step S6, the degree of soiling is output or displayed on sensor 10 or a connected device. This can be information as described in the previous paragraph, a descriptive output such as "windshield lightly / moderately / heavily soiled," or a warning. Which soiling levels are relevant and how they are to be responded to can vary from application to application, and therefore a configuration that can include step S5 is also conceivable.

[0058] In an advantageous development of the invention, it is conceivable to include measurements from measuring unit 22 in the contamination measurement or vice versa. For example, a contamination measurement for an angular segment can be skipped if measuring unit 22 detects or has recently detected a nearby object there. Conversely, a rapid change in the degree of contamination can be an indication for the actual measurement with measuring unit 22 that a nearby object must be present, since contamination generally accumulates slowly and does not appear suddenly. Cases of rapid contamination, such as from a splash of mud or a raindrop, cannot be clearly assigned here, but also cannot actually be clearly classified as either a nearby object or contamination. A time factor could be included here.A persistent nearby object ultimately has the same detrimental effect on further measurements as contamination. In any case, sensor 10 has all the necessary knowledge that a measurement at greater distances in the affected angular segment is currently not possible, so it can respond appropriately.

[0059] In some applications, it may be useful to recalibrate the contamination measurement, particularly to relearn the reference values ​​according to step S1. Examples include cleaning the front screen or replacing the front screen 44 with a spare part. Recalibration can be triggered by an actuating element of the sensor 10 or by configuration software, and during the learning process, care should be taken to ensure that no objects are in the vicinity of the sensor 10.

Claims

1. An optoelectronic sensor (10), in particular a laser scanner, for the detection of at least one object in a monitored zone (20), comprising a housing (42) having a front screen (44); a measuring unit (22) having at least one measurement light transmitter (24) for transmitting measurement light (28) through the front screen (44) and at least one measurement light receiver (34) for generating a received measurement signal from measurement light (30) remitted by the object; a movable deflection unit (12, 50) for the periodic deflection of the measurement light (28, 30); a contamination test unit (46) moved along with the deflection unit (12) and having at least two contamination test light transmitters (52) for transmitting contamination test light (48) onto the front screen (44) and at least two contamination test light receivers (54) for generating a received contamination test signal from contamination test light (48) remitted at the front screen (44); wherein the contamination test light transmitters (52) and the contamination test light receivers (54) form a plurality of contamination test channels (56); and a control and evaluation unit (38) that is configured to detect the object by evaluating the received measurement signal and to evaluate the contamination of the front screen (44) by evaluating the received contamination test signals of the plurality of contamination test channels (56); characterized in that the contamination test light transmitters (52) and the contamination test light receivers (54) form an alternating linear arrangement.

2. A sensor (10) in accordance with claim 1, wherein a direction of radiation of the contamination test light (48) is transverse, in particular perpendicular, to a direction of radiation of the measurement light (28, 30).

3. A sensor (10) in accordance with claim 1 or claim 2, wherein the control and evaluation (38) is configured to compare a respective received contamination test signal with a reference value for the evaluation of the contamination of the front screen (44).

4. A sensor (10) in accordance with claim 3, wherein the control and evaluation unit (38) is configured for a teaching mode in which received contamination test signals are stored as reference values.

5. A sensor (10) in accordance with any one of the preceding claims, wherein the control and evaluation unit (38) is configured to measure a received contamination test signal with an inactive contamination test light transmitter (52) to compensate it as a background signal of a received contamination test signal detected with an active contamination test light transmitter (52).

6. A sensor (10) in accordance with any one of the preceding claims, that has a temperature sensor, and wherein the control and evaluation (38) is configured to adapt a received contamination test signal or a reference value using a temperature measured by the temperature sensor.

7. A sensor (10) in accordance with any one of the preceding claims, that has a temperature reference channel having a temperature reference light transmitter, a temperature reference light receiver, and an internal temperature reference target arranged within the housing (52) to adapt a received contamination test signal or a reference value using a temperature reference signal of the temperature reference light receiver.

8. A sensor (10) in accordance with any one of the preceding claims, wherein the linear arrangement is arranged slanted to or in parallel with an axis of rotation (18) of the movable deflection unit (12, 50).

9. A sensor (10) in accordance with any one of the preceding claims wherein the contamination test channels (56) have at least one direct channel (56 d1 - 56d4) that is formed by a contamination test light transmitter (52) and a directly adjacent contamination test light receiver (54).

10. A sensor (10) in accordance with any one of the preceding claims wherein the contamination test channels (56) have at least one indirect channel (56i1 - 56i2) that is formed by a contamination test light transmitter (52) and a contamination test light receiver (54) not directly adjacent.

11. A sensor (10) in accordance with any one of the preceding claims wherein the control and evaluation unit (38) is configured to evaluate the contamination of the front screen (44) in a plurality of angular segments corresponding to positions of the movable deflection unit (12) and in particular not to evaluate the contamination for at least one configurable angular segment.

12. A sensor (10) in accordance with any one of the preceding claims wherein the control and evaluation unit (38) is configured to compare measured results of the measuring unit (22) and of the contamination test unit (46) with one another, in particular not to evaluate the contamination of the front screen (44) and / or to treat a fast increase in the contamination of the front screen (44) as a detected object on a detection of an object by the measuring unit (22) in this position of the deflection unit (12).

13. A method of measuring the contamination of a front screen (44) of an optoelectronic sensor (10) that transmits measurement light (28) through the front screen (44) by at least two measurement light transmitters (24) and that generates a received measurement signal from measurement light (30) remitted by the object by at least one measurement light receiver (34); that periodically deflects the measurement light (28, 30) by a movable deflection unit (11); that transmits contamination test light (48) onto the front screen (44) by at least two contamination test light transmitters (52) moved along with the deflection unit (12) and that generates a received contamination test light signal from contamination test light (48) reflected at the front screen (44) by at least two contamination test light receivers (54); wherein the contamination test light transmitters (52) and the contamination test light receivers (54) for a plurality of contamination test channels (56), the object is detected by evaluating the received measurement signal; and the contamination of the front screen (44) is evaluated by evaluating the received contamination test signals of the plurality of contamination test channels (56), characterized in that the contamination test light transmitters (52) and the contamination test light receivers (54) form an alternating linear arrangement.