DE202014100836U1 - Opto-electronic sensor for object detection in a surveillance area - Google Patents

Abstract

An optoelectronic sensor (10) comprising a light receiver (30) having a plurality of avalanche photodiode elements for detecting received light (20) from a monitor area (18), each biased with a bias above a breakdown voltage and thus operated in a Geiger mode and a receiving optical system (22) arranged in front of the light receiver (30), which has a focusing element (24) and a diaphragm (26), characterized in that the diaphragm (26) is arranged in the far field focal plane of the focusing element (24) and in that the receiving optics (22) furthermore have an optical funnel element (28) which is arranged between the diaphragm (26) and the light receiver (30).

Description

The invention relates to an optoelectronic sensor for object detection in an object area according to the preamble of claim 1.

Optoelectronic sensors are available in a broad spectrum, ranging from one-dimensional photoelectric sensors and light sensors to laser scanners and cameras. In addition to pure object detection, a distance to the object is also determined in distance-measuring systems. Distance sensors based on the time-of-flight principle measure the propagation time of a light signal that corresponds to the distance over the speed of light. Conventionally, a distinction is made between pulse-based and phase-based measurements. In a pulse transit time method, a short light pulse is emitted and the time to receive a remission or reflection of the light pulse is measured. Alternatively, in a phase method, transmitted light is amplitude modulated and a phase shift between transmitted and received light is determined, wherein the phase shift is also a measure of the light transit time. 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 most cases, and especially in distance measurement, the sensor must be able to differentiate between useful light, for example a dedicated or dedicated light emitter, and ambient light or interference from other light sources. Depending on the application, for example in particularly bright environments, poorly remitting target objects or large measuring distances, this can be a very demanding task with extremely low useful light levels.

In order to be able to detect even low reception intensities, avalanche photodiodes are conventionally used in some optoelectronic sensors (APD, avalanche photo diode). 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. The avalanche photodiode thus counts as the eponymous Geiger counter individual events.

Violin photodiodes in Geiger mode are not only highly sensitive, but also relatively inexpensive. In addition, they can be integrated with little effort on a printed circuit board. However, they have a relatively large receiving surface, which leads to an increased external light load on the imaging receiving optics. However, conventionally used receiving optics have the disadvantage that the external light coupling depends on the detector size. If the detector surface were simply reduced by a detector-side aperture, the far-field signal is also strongly suppressed with the extraneous light. This in turn limits the range of the sensor significantly. In addition, inhomogeneous illumination of the detector can cause difficult to control measurement errors.

Sensors with a wide variety of receiving optics are known from the prior art. For example, the shows DE 44 30 778 A1 a tube with scattering ribs that suppress stray light through absorption and multiple reflection. As a result, however, the dependence on the detector surface and external light exposure is not canceled, and the light-absorbing tube is not suitable to direct useful light. In the EP 1 715 313 A1 a light scanner uses a tube with a diaphragm and a lens. However, this optics serves as a transmission optics and has the function of mixing the light colors of several transmitters. Avalanche photodiodes in Geiger mode are not addressed in these documents and therefore the optics are not designed for their requirements.

It is therefore an object of the invention to improve the detection in an optoelectronic sensor with avalanche photodiodes in Geiger mode.

This object is achieved by an optoelectronic sensor for object detection in an object area according to claim 1. The invention is based on the basic idea of suppressing at least the portion of extraneous light in the receiving optics which reaches the receiving optics from the near field or medium distances. For this purpose, a diaphragm is arranged at least approximately in the far field focal plane of a focusing element of the receiving optics. The light passing through the aperture then passes into one aperture of an optical funnel element and is directed therein to its other aperture on the opposite side where the light receiver is located. As a result, the light from the far field is almost completely detected and directed to the light receiver. The optical funnel element preferably also acts simultaneously as a homogenizer.

The invention has the advantage that the far field signal is maintained despite the large detector surface and a drastic reduction of the external light coupling. The receiving optics also offer advantages in short and medium distances, because there the signal dynamics can be kept at least reasonably constant, instead of overdriving according to the simple quadratic law of distance with increasing proximity.

The receiver optics still takes up relatively little space. The detector surface is homogeneously illuminated over the entire distance range and also from far field signals, ie from large distances, without loss of signal. This is especially important for statistical signal evaluation methods, and avalanche photodiodes in Geiger mode almost inevitably have to be evaluated as individual events could also be due to interfering light photons. This is also the reason why the light receiver does not have a single but a plurality of avalanche photodiodes.

The avalanche photodiode elements preferably have a breakdown voltage of at most 70V, in particular of at most 50V, 30V or 15V. A high voltage power supply as in avalanche photodiodes conventionally used can thus be dispensed with, and significantly lower production costs are possible.

The avalanche photodiode elements are preferably produced in a CMOS process, in particular as a matrix structure on a common substrate. The light receiver is thus available at low cost. Among other things, a CMOS device enables small structures and thus also a significantly reduced breakdown voltage compared to a conventional avalanche photodiode.

The light receiver preferably has local evaluation structures for the avalanche photodiode elements on a substrate of the light receiver. The local evaluation structures can be supplemented with an evaluation unit outside the light receiver and thus sub-functions distributed to local and external evaluations. Due to the local processing many individual data are prefiltered, so that only summary measurement information has to be read out and the accumulated data volume is considerably reduced. The intrinsic amplification of an avalanche photodiode in Geiger mode allows a direct digital implementation of the evaluation circuits.

The focusing element preferably has an aspherical lens or a lens system. This achieves a particularly effective focusing from the far field on the diaphragm. Alternatively, a simple spherical lens is used.

The optical funnel element preferably has a reflective coating on its inner surfaces. As a result, the light in the funnel element is guided and homogenized even better.

The optical funnel element is preferably designed in the shape of a truncated pyramid. It can be manufactured so easily and has defined mirror surfaces with specific light guiding properties. Although the base is preferably a square, optionally a triangle, hexagon or other base may be provided.

The optical funnel element preferably has at least one jump-like taper of the cross section. Thus, the walls of the optical funnel element are not smooth sloping walls with continuous taper, but form a staircase with gradual taper. As a result, flatter mirror angles are made possible with simultaneous greater taper in a shorter funnel. Alternatively, a continuous taper, tubular course with the same cross-section or a discrete or continuously increasing cross-sectional area are conceivable.

The optical funnel element is preferably arranged immediately after the diaphragm and / or directly in front of the light receiver. There are therefore virtually no gaps between the aperture, optical funnel element and light receiver, the optical funnel element fills the entire area between the aperture and light receiver. Thus, the length of the receiving optics is reduced, and it is ruled out that outside light is captured within the receiving optics.

Between the diaphragm and the optical funnel element, a diffuser element is preferably arranged. This makes it possible to shorten the optical funnel element, since the light path from the aperture to the funnel wall is shortened by the larger deflection angle after the diffuser. In addition, the homogenization is significantly improved. The diffuser element is preferably used without gaps between the aperture and the optical funnel element.

The light receiver preferably has an area of a few square millimeters. An area on the order of a square millimeter is much compared to a simple photodiode, but is needed for the multitude of avalanche photodiode elements in Geiger mode in a photoreceiver used in the present invention. Therefore, with a conventional receiving optics especially for long distance detections Useful signal disturbed by a lot of extraneous light, which is suppressed by the invention.

The area ratio of aperture to light receiver is preferably very small and is for example at most 1:10, 1: 100 or 1: 1000. Namely, this area ratio determines to what extent extraneous light in the far field can be suppressed.

The sensor is preferably a distance-measuring sensor having a light transmitter for emitting light pulses and a light transit time measuring unit for measuring a light transit time between the emission time of a transmitted light pulse and a time of reception of the light pulse received from an avalanche photodiode element from the surveillance area. This sensor thus receives the reflected or remitted in the surveillance area own transmitted light and achieved by the receiving optical system according to the invention a particularly large range. In an advantageous embodiment of a laser scanner, the distance is measured not only in one direction, but the light pulses scan using a movable deflection, such as a rotating mirror, or by moving or rotating a measuring head from a plane or even a three-dimensional space area.

The invention will be explained below with regard to further advantages and features with reference to the accompanying drawings with reference to embodiments. The figures of the drawing show in:

1 a schematic sectional view of an optoelectronic sensor;

2 an exemplary simplified equivalent circuit diagram of a in the light receiver of the sensor according to 1 avalanche photodiode used in Geiger mode;

3a a schematic representation of the structure and beam path of the receiving optics of the sensor according to 1 ;

3b a section enlargement of the 3a ;

3c an exemplary light distribution of the receiving optics according to 3a guided reception light on the light receiver; and

4 an exemplary representation of the distance-dependent signal dynamics of a receiving optical system according to the invention.

1 shows a block diagram of an optoelectronic sensor 10 , which is exemplified as a light scanner. The sensor 10 has a light transmitter 12 on, for example, a laser diode whose transmitted light 14 in a transmission optics 16 collimated and then into a surveillance area 18 is distracted. This is the case of objects in the surveillance area 18 Remitted light is used as receiving light 20 by a receiving optics 22 that a lens 24 , a panel 26 and an optical funnel element 28 has, on a light receiver 30 having a plurality of light receiving elements in the form of avalanche photodiodes arranged in pixels.

In the sensor 10 is still a control and evaluation unit 32 provided with the light transmitter 12 and the light receiver 30 connected is. The control and evaluation unit 32 detected by the received signal of the light receiver 30 Objects in the surveillance area 18 , In this case, in one embodiment of the sensor 10 as a distance-measuring light scanner by emitting light pulses and determining the light transit time to their reception and the distance of the detected objects measured. About an exit 34 can be the evaluation unit 32 outputting processed or raw sensor measuring data or, conversely, accepting control and parameterizing instructions.

The light receiving elements of the light receiver 30 are avalanche photodiodes, which are used for the highly sensitive detection of received light 20 be operated in the violin mode. For explanation shows 2 an exemplary simplified equivalent circuit diagram of such avalanche photodiode. In practice, it is a semiconductor device whose unillustrated structure is assumed to be known here. The avalanche photodiode 100 shows on the one hand the behavior of a diode 102 , It has a capacity through a parallel-connected capacitor 104 is represented. The possible avalanche breakdown generates charge carriers whose origin in the equivalent circuit as a current source 106 is pictured. The avalanche breakdown is caused by an incident photon 108 triggered, this process as a switch 110 acts. Outward, the avalanche photodiode is a resistor 112 with a power source 114 connected. Between this power source and another resistor 116 can at one point 118 the output signal will be considered.

In standby mode is above the diode 102 a voltage above the breakdown voltage. Then creates an incident photon 108 a pair of charge carriers, this closes the switch as it were 110 so that the avalanche photodiode via the power source 106 flooded with charge carriers. But new charge carriers only arise as long as the electric field remains strong enough. Is caused by the power source 106 the capacitor 104 discharged so far that the breakdown voltage is exceeded, the avalanche comes to a standstill by itself to a standstill ("passive quenching"). After that, the capacitor becomes 104 from the external power source 114 about the resistance 112 recharged until again a voltage above the breakdown voltage at the diode 102 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").

During the avalanche, the output signal rises at the point 118 rapid and independent of the intensity of the triggering light, here a photon 108 , to a maximum value, and then drops off after the avalanche has been cleared. The pulse shape is at the point 118 indicated. The time constant of the waste, which is a dead time of the avalanche photodiode 100 is determined by the capacitance of the capacitor 104 and the resistance 112 and is typically in the range of several to several tens of nanoseconds. The dead time is not an absolute dead time, because as soon as the bias voltage is large enough to support an avalanche, the output signal can also increase again, but not to the same extent as from the standby state. The gain factor is typically on the order of 10 ^ 5 to 10 ^ 7, and is essentially the maximum number of charge carriers that avalanches in the avalanche photodiode 100 can be recruited.

If the event to be measured falls into the dead time, then the avalanche photodiode is 100 largely blind. The avalanche photodiode 100 also responds to each single event with the same signal, waiting for a sufficient dead time and triggering a pair of charge carriers. Alone is an avalanche photodiode 100 therefore largely unsuitable for evaluating reception intensity. In addition, a single photon of ambient light or a single pair of carriers may be sufficient due to dark noise to trigger an avalanche unrelated to the event being measured. On the light receiver 28 is therefore a plurality of individual avalanche photodiode elements 100 provided, the signals from the control and evaluation unit 32 be evaluated together. A corresponding component can be produced, for example, in a CMOS process. The breakdown voltage of avalanche photodiode elements 100 is significantly lower than in conventional avalanche photodiodes and is for example at most 70 V or even only 15 V-30 V.

The basis of the 1 described embodiment of the sensor 10 is only an example. Light receivers based on avalanche photodiodes in Geiger mode can be used in a large family of optoelectronic sensors, such as light barriers, range finders or laser scanners. These sensors 10 can significantly in their construction of 1 deviate, for example, lead transmit and receive channel on a common divider mirror or work passively and thus renounce entirely on a transmission channel. The invention relates to the receiving optics 24 , which will be further explained below, and less the other structure of the sensor 10 ,

3 shows the structure of the receiving optics 22 and the beam path of the receiving light 20 in the receiving optics 22 , It is 3a an overall view of the receiving optics 22 and 3b a section enlargement in the area of the aperture 26 and the optical funnel element 28 , while 3c an exemplary resulting light distribution on the light receiver shown here in plan view 30 illustrated.

The Lens 24 is a condensing lens that can be spherical, aspherical or freeform. Deviating from the figures is also conceivable as a focusing element instead of the lens 24 to use reflective or catadioptric elements. The aperture 26 becomes at least near the far field focal plane of the lens 24 arranged. The optical funnel element 28 is for example a hollow truncated pyramid with mirrored inner surfaces and square cross-section. Alternative geometries are based on a hexagonal or any other polygonal base. The funnel cross section may differ from the aperture 26 to the light receiver 30 rejuvenate continuously or discreetly. cover 26 and optical funnel element 28 can also be designed asymmetrically, for example for biaxial systems.

The optical funnel element 28 has the function, the receiving light 20 behind the focal plane of the lens 24 on the light receiver 30 To steer and to ensure its homogeneous illumination, without losing light signal. 3c shows the result of a simulation, according to which the homogeneity on the surface of the light receiver 30 is achieved very well. The geometry of the optical funnel element 28 and in particular its length should be designed so that at least one reflection of the received light 20 can take place on the inner surfaces. On the other hand, at least when used in a distance-measuring system according to the time of flight principle, the length and slope of the inner surfaces of the optical funnel element should ensure homogenization, but limit the number of reflections to such an extent that both the reflection losses and the extension of the light paths by the reflections are not becomes significant. If the number of reflections exceeds a certain level, otherwise the reflection-related optical path extension could falsify the measured distance values.

To the homogenizing effect of the optical funnel element 28 to reinforce or shorten its length, can between aperture 26 and optical funnel element 28 an additional, not shown diffuser element can be arranged.

For the efficiency of the external light suppression in the receiving optics 22 is the area ratio of aperture 26a the aperture 26 and light receiver 30 crucial. Surely the aperture should be 26a be smaller than the light receiver 30 , so that the external light coupling through the smaller aperture 26 and not by the larger area of the light receiver 30 is limited. The external light suppression is proportional to the area ratio of the aperture 26a and light receiver 30 , For example, the aperture has a size of 0.1 × 0.1 mm ² and the light receiver 30 an area of 1 × 1 mm ² , this area ratio is 1: 100, and accordingly, the extraneous light may be a factor 100 be suppressed. At the same time, care should be taken that the far field image of the transmitted light 14 in the surveillance area 18 generated light spot on the panel 26 smaller than the aperture 26a so that the signal level of the desired signal is not affected. Optimal focusing can be achieved by proper choice or appropriate design of the lens 24 can be achieved as an aspherical lens.

4 shows an exemplary representation of the distance-dependent signal dynamics of the receiving optics 22 , So here the double logarithmic applied distance dependence of the amplitude of the received signal. With dashed line the purely distance-related signal dynamics was plotted for comparison. Visible calls the optical system according to the invention in the far field no signal loss. For medium and short distances the signal dynamics are considerably reduced. However, this is an advantage in practice, because in such distances far below the specified range of the sensor 10 a sufficient signal is almost always given and on the contrary overloads are problematic, due to the flat profile of the receiving optical system according to the invention 22 be avoided.

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

• DE 4430778 A1 [0007]
• EP 1715313 A1 [0007]

Claims (9)

Optoelectronic sensor ( 10 ) with a light receiver ( 30 ) comprising a plurality of avalanche photodiode elements for detecting received light ( 20 ) from a surveillance area ( 18 ), each biased with a bias voltage above a breakdown voltage and thus operated in a Geiger mode and with a the light receiver ( 30 ) upstream receiving optics ( 22 ), which is a focusing element ( 24 ) and an aperture ( 26 ), characterized in that the diaphragm ( 26 ) in the far field focal plane of the focusing element ( 24 ) and that the receiving optics ( 22 ) further comprises an optical funnel element ( 28 ) between the diaphragm ( 26 ) and the light receiver ( 30 ) is arranged. Sensor ( 10 ) according to claim 1, wherein the focusing element ( 24 ) comprises an aspherical lens or a lens system. Sensor ( 10 ) according to claim 1 or 2, wherein the optical funnel element ( 28 ) has a silver coating on its inner surfaces. Sensor ( 10 ) according to one of the preceding claims, wherein the optical funnel element ( 28 ) is formed in the shape of a truncated pyramid. Sensor ( 10 ) according to one of the preceding claims, wherein the optical funnel element ( 28 ) has at least one jump-like taper of the cross section. Sensor ( 10 ) according to one of the preceding claims, wherein the optical funnel element ( 28 ) immediately after the aperture ( 26 ) and / or immediately before the light receiver ( 30 ) is arranged. Sensor ( 10 ) according to one of the preceding claims, wherein between the diaphragm ( 26 ) and the optical funnel element ( 28 ) A diffuser element is arranged. Sensor ( 10 ) according to one of the preceding claims, wherein the light receiver ( 30 ) has an area of a few square millimeters and / or the area ratio of aperture ( 26 ) to light receiver ( 30 ) is at most 1:10, 1: 100 or 1: 1000. Sensor ( 10 ) according to one of the preceding claims, comprising a distance-measuring sensor, in particular a laser scanner, with a light transmitter ( 12 ) for emitting light pulses and a light transit time measuring unit ( 32 ) is a light propagation time between the transmission time of an emitted light pulse and a time of reception of an avalanche photodiode element from the monitoring area ( 18 ) received light pulse to measure.

Images