DE202017103345U1 - Tasting light grid for detecting objects - Google Patents

Abstract

Tasting light grid (10) for detecting objects (20a-c) in a monitoring area (12) by means of a plurality of mutually parallel monitoring beams (24a-c), the at least one light transmitter (14) for emitting transmitted light (18), a plurality of Light receivers (26) for receiving the transmitted light reflected from the objects as a respective monitoring beam (24a-c) and a control and evaluation unit (28) which detects the objects (20a-c) based on respective received signals of the light receiver (26) , characterized in that more light receivers (26) are provided as light emitter (14) and that a plurality of monitoring beams (24a-c) are generated by the same light emitter (14).

Description

The invention relates to a momentary light grid for detecting objects in a surveillance area by means of a plurality of mutually parallel monitoring beams according to the preamble of claim 1.

Light grids are used in automation technology to measure objects in order to measure the position and extent of objects based on the position and number of intermittent beams. For example, the height of objects moved on a conveyor belt can be determined in this way. In safety engineering, the parallel light beams serve as a kind of virtual wall, and when interrupted by an object, for example, a source of danger is secured.

Most light curtains comprise a plurality of transmitters and associated receivers, so that a respective pair of transmitters and receivers forms a light barrier which detects whether the light beam spanned between the transmitter and the receiver is interrupted by an object or not. The transmitters and receivers are each combined in a transmitting unit and a receiving unit, which are mounted opposite each other.

Light grids can comprise several hundred light beams, which are also referred to as monitoring beams or channels. In order to keep the adjustment requirements manageable, although beam-shaping optics are used to limit the respective transmit and receive beam. However, they are designed so that at usual distances between transmitting unit and receiving unit a receiver receives transmitted light not only from the associated transmitter, but also from its neighbors. For a unique channel assignment, the sender and receiver are then activated cyclically and in pairs. Nevertheless, at least in the angular direction still a correct adjustment is required.

So-called momentary light grids are a special design in which the transmitting and receiving unit are combined. It is therefore not monitored whether a light beam between a transmitter and an opposite receiver is interrupted by an object. Instead, a light beam is emitted and received again at the same location, provided that an object is located in the light beam that reflects the light beam. In this description, a simplified distinction between directional reflection and diffuse remission is not made in this description.

A momentary light grid can also measure the distance of the touched object in a light transit time method in addition to the pure presence detection. Here, a distinction is made between pulse transit time methods in which a short light pulse is emitted and the time until reception of the reflected light pulse is measured, and phase methods in which transmitted light is amplitude modulated and a phase shift between transmitted and received light is determined, the phase shift also being a measure of the light transit time is. However, the boundary between the two methods can not always be drawn sharply, because, for example, in the case of complex pulse patterns, a pulse transit time method is more similar to a phase method than to a classical single pulse measurement.

In order to be able to detect even low reception intensities, avalanche photodiodes (APD, avalanche photo diode) can be used as the light receiver. The incident light triggers a controlled avalanche breakdown. As a result, the charge carriers generated by incident photons are multiplied, and there is a photocurrent, which is proportional to the light receiving intensity, but much larger than a simple PIN diode.

An even greater sensitivity is achieved with avalanche photodiodes, which are operated in the so-called Geiger mode (SPAD, Single Photon Avalanche Diode). Here, the avalanche photodiode is biased above the breakdown voltage, so that even a single, released by a single photon charge carrier can trigger a no longer controlled avalanche, which then recruits all available charge carriers due to the high field strength. Then the avalanche comes to a standstill (passive quenching) and is no longer available for detection for a certain dead time. Alternatively, it is also known to detect and extinguish the avalanche from the outside (active quenching).

A SPAD thus counts as a Geiger counter individual events. Not only are SPADs highly sensitive, but they are also comparatively inexpensive and efficient to integrate into silicon semiconductors. Next, they can then be integrated with little effort on a printed circuit board. A special feature is the fact that even a minimal disturbance event, such as an extraneous light photon or dark noise, generates the same maximum received signal as a useful light signal.

From the EP 3 091 272 A1 For example, a momentary light grid is known whose light receiver has a single photon avalanche diode. However, this light grid still uses one light transmitter per light receiver. In one embodiment, with an array of single photon avalanche diodes, their signals are combined but without intervening Single photon avalanche diodes with useful light and without useful light to distinguish.

In the EP 1 906 368 A1 For example, an alignment method for a light grid having a light receiving unit with a matrix array of photodiodes is described. Photodiodes are then combined in each case to form a common processing region, for example those which are hit by a light signal, as well as the other photodiodes. It is not a groping, but a classic light grid with opposing light emitters and light receivers.

The EP 1 947 481 B1 discloses a light grid which achieves a strict limitation of the surveillance beams to avoid reflections only by designing the receiving optics. This too is a classic light grid.

The EP 1 927 867 B1 deals with a multilevel sensor. In this case, a plurality of line scan cameras, each with its own illumination, are arranged one above the other at a certain grid spacing. This leads to a rather complex structure.

Also known are 3D cameras based on a light transit time method, such as the DE 197 57 595 C2 , However, 3D cameras and light grids differ considerably in structure and field of application.

It is therefore an object of the invention to improve a momentary light grid.

This object is achieved by a momentary light grid for detecting objects in a surveillance area by means of a plurality of mutually parallel monitoring beams according to claim 1. As is common in a momentary light grid, at least one light transmitter transmits transmitted light and, if it encounters an object, reflects to a light receiver which is located on the same side as the light transmitter with respect to the surveillance area. A control and evaluation unit evaluates the respective received signal of the light receiver. Depending on the embodiment, it is, for example, summarily detected whether an object is located in the monitoring area, or the position or size of the object is measured on the basis of the monitoring beams concerned.

The invention is based on the basic idea of abandoning the usual pairwise arrangement of light transmitter and light receiver for a monitoring beam. Instead, multiple monitor beams are generated by the same light emitter. In this case, a light beam or light fan is emitted, which illuminates a larger angular range. From this, as it were, several light receivers intersect their respective monitoring beams. Several light receivers generate their received signal from the reflected transmitted light of the same light transmitter. Accordingly, there are more light receivers than light emitters, in particular by an integer factor more, i. in each case one light transmitter is assigned to two, three or even more light receivers.

The invention has the advantage that the structure of the light grid is significantly simplified. It eliminates a variety of light emitters. The touch principle also requires significantly less installation space on site, as no receiver unit has to be mounted. The adjustment during installation largely eliminated, in particular a mutual adjustment of a transmitting and receiving unit. The light grid must only be mounted at the correct height and with an orientation that corresponds to the desired monitoring. Also, the adjustment of the components within the light grid can be considerably simplified in particular embodiments to be explained.

Preferably, only one light transmitter is provided. In this embodiment, especially many light transmitters are saved. The usual concept of pairs of light emitters and light receivers is completely canceled, one and the same light emitter is responsible for all light receivers. The monitoring beams are produced only at the receiving end. However, a single light emitter does not exclude multiple light sources to achieve higher optical output power, such as through an array of light sources such as a VCSEL array. Even such a light transmitter is still considered to be largely punctiform for all practical purposes of the light grid.

The light receivers are preferably spaced from each other by at least one millimeter. The distance of the light receiver and thus the monitoring beams determines the resolution of the light grid, which is demensprechend at one millimeter, in other embodiments, a value of a few millimeters or even centimeters, conversely, less than one millimeter. Due to the parallel monitoring beams, the resolution within the monitored area of the light grid is the same everywhere, unlike, for example, a camera. For light curtains used in safety technology, the distances correspond to the specification of a safety standard, for example 7-10 mm for finger protection, 10-20 mm for arm protection or 30-38 mm for leg protection.

The arrangement of the light receiver preferably has a longitudinal extent of at least half a meter, one meter or several meters. The length of a grooving light grid, unlike the typical small dimensions of a camera, is not limited in principle. Depending on the opening angle of the light emitter or the light emitter, a certain dead zone in the near zone may result. This can be absorbed by large opening angles or a recessed mounting of the light emitter in a light curtain housing, or it is accepted in measurement applications. The longitudinal extent corresponds to the extent of the monitoring area in the height direction, or the corresponding direction in the case of non-vertical mounting of the light grid. In the lateral direction of the surveillance area is not sharply defined, as even from large distances still some light components can be reflected. Here, a range is specified from which objects are still reliably detected.

The light receivers are preferably each associated with a receiving optical system with a small receiving angle. Since a light transmitter is responsible for several light receivers, the light emitter contributes little to the demarcation of surveillance beams. A strongly limiting receiving optics subdivides the transmitted light into the monitoring beams. To prevent unwanted crosstalk between channels, i. E. registering transmission light in a non-responsible light receiver, the light receivers are preferably cyclically activated one after the other despite receiving optics with a narrow reception angle.

The control and evaluation unit is preferably designed to determine a distance of detected objects from a light transit time of the transmitted light. Thus, not only the lateral position and extent of detected objects is measured on the basis of the affected monitoring beams, but also the contour in the beam direction. The emitted light signal more preferably has a light pulse. The sensor thus measures distances according to the pulse method. Even more complicated forms such as double pulses or even pulse codes are conceivable. It is also possible for a plurality of light pulses to be transmitted one after the other, received, and the respective individual results jointly evaluated statistically, for example in a pulse averaging process. Alternatively, a phase method is conceivable.

The light receivers preferably each have at least one avalanche photodiode element which is biased with a bias voltage above a breakdown voltage and thus operated in a Geiger mode. The high sensitivity of such avalanche photodiode elements in Geiger mode or SPADs is particularly advantageous for a momentum sensing grating with distance measurement.

The light receivers preferably have a multiplicity of avalanche photodiode elements, and the control and evaluation unit is designed for a learning mode in which the region of the light receiver to be activated is taught in using the position of a light spot. As a result, the requirements for a mutual alignment of light emitters and light receivers are lower or corresponding tolerances are compensated. In addition, the signal-to-noise ratio increases significantly because only avalanche photodiode elements contribute to the received signal that is actually reflected by the transmitted light. The activation may in fact relate to the physical activation, for example by supplying only active avalanche photodiode elements with a bias, in particular above the breakdown voltage. But for the improved adjustment and measurement accuracy is sufficient if avalanche photodiode elements that are not hit by the light spot, do not contribute to the measurement, for example, by their received signal is not evaluated.

The control and evaluation unit is preferably designed to activate the light receivers cyclically by biasing avalanche photodiode elements. The usual cycle, with which the individual monitoring beams are checked successively for the detection of objects, is thus controlled in a special way via the bias voltage. This reduces power consumption and prevents SPADs from being dead in their cycle even before the actual measurement, for example due to an avalanche caused by dark noise.

• 1 eine schematische Schnittdarstellung eines Lichtgitters;
• 2 ein vereinfachtes Ersatzschaltbild einer Lawinenphotodiode im Geiger-Modus; und
• 3 eine schematische Draufsicht auf einen Lichtempfänger zur Erläuterung des Einlernens eines Empfangslichtflecks.

The invention will be explained in more detail below with regard to further features and advantages by way of example with reference to embodiments and with reference to the accompanying drawings. The illustrations of the drawing show in:

• 1 a schematic sectional view of a light grid;
• 2 a simplified equivalent circuit of an avalanche photodiode in Geiger mode; and
• 3 a schematic plan view of a light receiver for explaining the learning of a received light spot.

1 shows a schematic cross-sectional view of an embodiment of a grooving light grid 10 for object detection, position detection or surveying. According to the scanning principle is the light grid 10 laterally without an opposite receiving counterpart at its surveillance area 12 arranged. A light transmitter 14 with a transmission optics 16 generates transmitted light 18 in the visible or invisible, especially infrared spectrum. In the illustrated Embodiment, the opening angle is large enough to the monitoring area after a short distance 12 illuminate. An even larger opening angle with a correspondingly reduced non-illuminated near area would be conceivable, but at the same time leads to greater light losses by illuminating regions outside the monitoring area 12 ,

Meets the transmitted light 18 in the surveillance area 18 on an object 20a-c , so a share becomes a light grid 10 reflected or remitted. Respective receiver optics 22a-c direct a section of the reflected transmitted light as a monitoring beam by means of suitable narrow receiving characteristic 24a-c to an associated light receiver 26 , The receiving optics 22a-c may include other elements such as additional lenses, apertures and the like and ensure that the monitoring beams 22a-c have a desired beam profile. There are only three receiving optics 22a-c and surveillance beams 24a-c shown, in fact, should preferably all the light receivers 26 a same receiving optics 22a-c be assigned. Also the transmission optics 16 can be more complicated than shown. In particular, cylindrical properties of the image make sense to the transmitted light 12 to focus on the surveillance area.

The light grid 10 in 1 has only a single light transmitter 14 on, for example, an LED or a laser diode. The surveillance beams 24a-c arise from it only at the receiving end. This is a preferred embodiment with very few components. To get enough light output, the light transmitter can 14 have multiple LEDs or laser diodes, such as a VCSEL array. But it is also conceivable, deviating from 1 several light transmitters 14 including transmission optics 16 provide, which then spaced from each other along the arrangement of the light receiver 26 distributed, for example, every two, three or even more light receivers 26 a light transmitter 14 , As a result, very large ranges of the light grid can be achieved 10 achieve.

Because of their high sensitivity and dynamic compression, the light receivers have 26 preferably avalanche photodiode element in Geiger mode (SPAD, Single Photon Avalanche Diode). In 1 is symbolically each a matrix as a light receiver 26 shown because in a preferred embodiment, a SPAD matrix is provided. A large number of SPADs is advantageous for statistical evaluation because of their special properties. In addition, this allows learning of the receiving light spot, which will be described later with reference to FIG 3 will be explained.

A control and evaluation unit 28 is with the light transmitter 14 and the light receivers 26 connected. This allows the transmitted light 18 be controlled, for example in the form of short pulses at certain transmission times. In addition, the control and evaluation unit 28 light receiver 26 cyclically activate, so that the monitoring beams 24a-c remain separated from each other, and on the received signals of the light receiver 26 access. At least parts of the control and evaluation unit 28 can also use the light receivers 26 integrated on a common chip whose area then share photosensitive areas of avalanche photodiode elements and individual or groups of avalanche photodiode elements associated circuits for their evaluation and control.

A very simple evaluation provides only a threshold value comparison of the reception levels to detect in binary whether an object 20a-c was recorded. But preferably also the distance of the respective object 20a-c determined by a light transit time method. Then in a situation like in 1 recognized that there are several objects 20a-c at different distances, or indefinitely measure a common object with a corresponding contour for the light grid.

For the time of flight measurement, for example, the light transmitter 14 pulsed operated and with a TDCs (Time-to-Digital Converter), the time to register the reflected transmitted light in the light receiver 26 certainly. Depending on the embodiment, the TDCs are started and stopped directly by signals at the time of transmission and reception, or they measure time intervals from a reference time point or up to a reference time point, from which the actual light transit time through subtraction also results. Are SPAD matrices as light receivers 26 provided, the light propagation time for individual SPADs, groups of SPADs or all SPADs can be determined together. However, statistical billing is conceivable not only on the level of received signals by combining SPADs, but also later on the level of light transit times. The measurement accuracy can be further improved in addition to the just described local statistical evaluation by repeated measurement and thus temporal statistical evaluation.

2 shows an exemplary simplified equivalent circuit diagram of a single avalanche photodiode in Geiger mode. In the light receiver 26 avalanche photodiodes used in Geiger mode or SPADs do not necessarily have to be constructed in this way, but at least follow a similar basic principle. Furthermore, it is in the Practice around a semiconductor device, whose construction not shown here is assumed to be known. The avalanche photodiode shows the behavior of a diode on the one hand 30 , It has a capacity through a parallel-connected capacitor 32 is represented. The possible avalanche breakdown generates charge carriers whose origin in the equivalent circuit as a current source 34 is pictured. The avalanche breakdown is caused by an incident photon 36 triggered, this process as a switch 38 acts. There are then different possibilities, the output signal 40 to look at, which will not be discussed here.

In standby mode is above the diode 30 a voltage above the breakdown voltage. Then creates an incident photon 36 a pair of charge carriers, this closes the switch as it were 38 so that the avalanche photodiode via the power source 34 flooded with charge carriers. However, new charge carriers only arise as long as the electric field remains strong enough. Is caused by the power source 34 the capacitor 32 discharged so far that the breakdown voltage is exceeded, the avalanche comes to a standstill by itself to a standstill ("passive quenching"). Then the capacitor charges 32 again, until again a voltage above the breakdown voltage at the diode 30 is applied. There are alternative embodiments in which the avalanche is detected from the outside and then a discharge below the breakdown voltage is triggered ("active quenching").

3 shows a schematic plan view of a light receiver 26 with a matrix arrangement of SPADs. In a preferred embodiment, not all SPADs are active, but only a part of them are activated, in particular those that are from a receiving light spot 42 are taken, the reflected transmitted light on the light receiver 26 generated. Thus, receive signals from SPADs, which can not hit any useful light, do not contribute to the measurement, and the signal-to-noise ratio is significantly improved. In addition, in this way, the receiving light spot can be taught in a final test in the production or even later on the site, by such a calibration production tolerances of light receiver 26 to light receiver 26 compensate.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 3091272 A1 [0010]
• EP 1906368 A1 [0011]
• EP 1947481 B1 [0012]
• EP 1927867 B1 [0013]
• DE 19757595 C2 [0014]

Claims (9)

Tasting light grid (10) for detecting objects (20a-c) in a monitoring area (12) by means of a plurality of mutually parallel monitoring beams (24a-c), the at least one light transmitter (14) for emitting transmitted light (18), a plurality of Light receivers (26) for receiving the transmitted light reflected from the objects as a respective monitoring beam (24a-c) and a control and evaluation unit (28) which detects the objects (20a-c) based on respective received signals of the light receiver (26) , characterized in that more light receivers (26) are provided as light emitter (14) and that a plurality of monitoring beams (24a-c) are generated by the same light emitter (14). Light grid (10) after Claim 1 , wherein only one light transmitter (14) is provided. Light grid (10) after Claim 1 or 2 wherein the light receivers (26) are spaced from one another by at least one millimeter. A light grid (10) according to any preceding claim, wherein the array of light receivers (26) has a longitudinal extent of at least one-half meter, one meter or several meters. Light grid (10) according to one of the preceding claims, wherein the light receivers (26) each have a receiving optics (22a-c) is associated with a small receiving angle. A light grid (10) according to any one of the preceding claims, wherein the control and evaluation unit (28) is adapted to determine a distance of detected objects (20a-c) from a light transit time of the transmitted light (18). A light grid (10) according to any one of the preceding claims, wherein the light receivers (26) each comprise at least one avalanche photodiode element biased with a bias voltage above a breakdown voltage and thus operated in a Geiger mode. The light grid (10) according to one of the preceding claims, wherein the light receivers (26) comprise a plurality of avalanche photodiode elements and the control and evaluation unit (28) is designed for a teach-in mode in which the region of the light receiver (26) to be activated is determined by the position a light spot (42) is learned. Light grid (10) after Claim 7 or 8th wherein the control and evaluation unit (28) is adapted to cyclically activate the light receivers (26) by biasing avalanche photodiode elements.

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