Time-of-flight camera system with function monitoring

Abstract

Time-of-flight camera system (1), comprising a light source (12) for emitting modulated light, a time-of-flight sensor (22) having at least one receiving pixel for receiving and demodulating the emitted light reflected from a scene (40), a modulator (30) connected to the time-of-flight sensor (22) and the light source (12), and an evaluation unit which determines a phase shift of the received light based on the demodulated light signal, wherein the time-of-flight camera system (1) has an optical return channel (100) which couples out a portion (a') of the light (a) emitted by the light source (12) and directs it to the time-of-flight sensor (22), characterized in that the optical return channel (100) has a first switchable optical attenuation unit (200) which switchably interrupts the transmission of the coupled-out light (a') to the time-of-flight sensor (22).and that the time-of-flight camera system is designed such that in a measurement operation the optical return channel (100) is closed via the attenuation unit (200) and is open in a functional measurement phase.

Description

The invention relates to a time-of-flight camera system and a method for operating such a system according to the preamble of the independent claims.The time-of-flight camera system or time-of-flight camera is intended to include, in particular, all time-of-flight or 3D TOF camera systems which acquire time-of-flight information from the phase shift of emitted and received radiation. Suitable time of flight or 3D TOF cameras are, in particular, PMD cameras with photomix detectors (PMD), as described, inter alia, in the applications EP 1 777 747 A1, U.S. Pat. No. 6,587,186 B2 and also DE 197 04 496 A1 and available, for example, from the company "ifm electronic gmbh" as Frame-Grabber O3D. The PMD camera allows in particular a flexible arrangement of the light source and the detector, which can be arranged both in a housing and separately. Of course, the term camera or camera system is also intended to include cameras or devices having at least one reception pixel, such as, for example, the distance measuring device O 1D of the applicant.For the function monitoring of such a time-of-flight camera system, for example, applications DE 10 2010 041 390 A1, DE 10 2011 081 561 A1 or DE 10 2011 081 563 A1 disclose embodiments in which, for example, a modulation signal applied to the illumination is tapped and electrically transmitted back to the camera module 300.EP 3 064 962 A1 discloses an optoelectronic distance meter having an optical return channel by means of which it is possible to determine distances over a large distance range in a short time.It is the object of the invention to further develop a time-of-flight camera system with regard to function monitoring.This object is advantageously achieved by the time-of-flight camera system according to the invention and the method for such a system.The time-of-flight camera system has a time-of-flight sensor having at least one receiving pixel, and a modulatable light source, and also a modulator which is connected to the time-of-flight sensor and the light source, wherein the time-of-flight camera system has an optical return channel which couples out part of the light emitted by the light source and directs it onto the time-of-flight sensor, wherein the optical return channel has a first connectable optical attenuation unit which interrupts a forwarding of the coupled-out light to the time-of-flight sensor in a connectable manner, wherein the time-of-flight camera system is designed such that, in a measurement mode, the optical return channel is closed via the attenuation unit and, in a function measurement phase, is open.The optical return channel is preferably designed such that the coupled-out light impinges on the entire light transit time sensor.In a further preferred embodiment, a second, connectable, optical attenuation unit is provided in the light path of the light emitted to the outside by the light source.This has the advantage that during a function measurement from the lighting side no modulated light reaches the outside and from there can be reflected back to the time of flight sensor in an interfering manner.In the normal measurement mode, the optical attenuator is switched such that the optical return channel is closed.Only at quite specific times is the optical return channel "opened" and a function measurement carried out. The essential advantage of the invention is that no additional sensor surface is required for the function measurement, and that a function measurement is available for a large part or for all pixels.Such an arrangement has the advantage that the function of the time-of-flight sensor can be monitored, for example after a regular distance measurement (normal measurement operation), by radiation modulated via the opened optical return channel being impressed on the time-of-flight sensor and being able to be evaluated. If the measurement results of the function measurement lie within a tolerance, it is possible to infer a fault-free function of the time-of-flight sensor or of the subsequent signal chain.Likewise advantageously, a method for operating the time-of-flight camera is provided, in which the optical return channel is opened and a function measurement is carried out in a measurement pause.The function monitoring is characterized by the following method steps: a) interrupting the light path leading to the outside and / or interrupting the light path b incident from the outside) disconnecting the optical return channel c) determining the functionality of the light source and of the time of flight sensor on the basis of the light detected on the time of flight sensor. d) terminating the function measurement phaseIn a further embodiment, it is provided that the function measurement takes place at predetermined time intervals. For example, the function measurements can take place after each distance measurement, but depending on the application it is also conceivable to repeat the function measurements at relatively long time intervals or after a certain number of distance measurements.The invention is explained in more detail below on the basis of exemplary embodiments with reference to the drawings.They show schematically: FIG. 1 shows a time-of-flight camera according to the invention, FIG. 2 shows a system according to the invention with an optical return channel, FIG. 3 shows a schematic illustration of a system according to the inventionIn the following description of the preferred embodiments, like reference numerals designate like or comparable components.FIG. 1 shows a measurement situation for an optical distance measurement, as is known, for example, from DE 197 04 496. The time-of-flight camera system 1 comprises a transmitting unit or an illumination module 10 with a light source 12 and an associated beam shaping optical unit 15 and a receiving unit or TOF camera 20 with a receiving optical unit 25 and a time-of-flight sensor 22. The reception optics 25 typically consist of a plurality of optical elements to improve the imaging properties. The beam shaping optics 15 of the transmission unit 10 is preferably designed as a reflector. However, diffractive elements or combinations of reflective and diffractive elements can also be used.The measuring principle of this arrangement is essentially based on the fact that the propagation time of the emitted and received light is determined on the basis of the phase shift of the emitted and received light. For this purpose, the light source 12 and the time of flight sensor 22 are acted upon jointly by a specific modulation frequency with a first phase position a via a modulator 30. According to the modulation frequency, the light source 12 emits an amplitude-modulated signal having the phase a. This signal or the electromagnetic radiation is reflected by an object 40 in the case shown and, on account of the distance covered, impinges on the time of flight sensor 22 in a phase-shifted manner with a second phase position b. In the time of flight sensor 22, the signal of the first phase position a of the modulator 30 is mixed with the received signal, which has meanwhile assumed a second phase position b, and the phase shift or the object distance I is determined from the resulting signal.FIG. 2 schematically shows a first exemplary embodiment of a system according to the invention. The light a emitted modulated by the light source 12 is reflected by an object 40 or by a scene 40 and received as a phase-shifted light signal b by the time of flight sensor 22, from which a distance signal is determined in a usual manner on a pixel-specific basis. In addition to the embodiment according to FIG. 1, an optical return channel 100 is provided, which directs a portion a' of the emitted light a directly onto the time of flight sensor 22.In the example shown, the optical return channel 100 consists of a first and a second mirror 101, 102. The first mirror 101, which can also be designed, for example, as a cover glass of the illumination module 10, deflects a part a' of the radiation a emitted by the illumination source 22 from the main beam path, wherein the deflected light a' is projected onto the light transit time sensor 22 via the second mirror.Furthermore, to interrupt the light path, an connectable optical first and second attenuation unit 200, 201 is provided both in the optical return channel 100 and in the outwardly directed light path a. In an embodiment not shown, the second attenuation unit 201 could also be arranged in the inwardly directed light path b and interrupt the light received by the scene 40 in the direction of the time of flight sensor 22.In normal measuring operation, the second damping unit 201 is not switched on, i.e. transparent, and the first damping unit 200 is switched on, i.e. opaque. In normal operation, the objects 40 in the exterior are illuminated and the optical return channel 100 is closed. While in function measuring operation the light path a towards the outside is interrupted and the optical return channel 100 is opened, so that the light transit time sensor 22 is acted upon by modulated deflected light a' via the optical return channel 200.If the time of flight sensor 22 receives a modulated light signal in the case of an open optical return channel 100, it can be assumed that the light source 12 is functioning correctly.It can typically be assumed that the light travels only a negligible small distance via the optical return channel 100, so that the mixture of the first phase position a with the phase position of the coupled-out light a' generates a phase shift which substantially corresponds to the distance zero.However, the absolute value is not absolutely essential in this measurement, but rather the functionality of the light transit time sensor 22 can also be checked via this function measurement.For example, during the production of the time-of-flight camera system, signal values that are expected during a function measurement can be stored as reference values in an evaluation or monitoring unit for each pixel of the time-of-flight sensor 22. If these values deviate significantly from the stored reference values during a function measurement, a malfunction of the light transit time sensor 22 or individual pixels of the sensor 22 can be deduced.Of course, further evaluation possibilities are also conceivable.The damping units 200, 201 can be implemented in a wide variety of forms. In particular, the damping units 200, 201 can be constructed differently. The following technical solutions are possible by way of example in a non-exhaustive list:Magneto-optical Kerr effect (MOKE): doping a portion of each of the function measurement optical path and the optical path across the scene with polarizing material that changes polarization alternately depending on the mode upon application of a magnetic field. As a result, the transmission of an optical path in conjunction with a polarization filter can be changed or blocked.(Electro-Optic) Kerr effect: doping a section of the optical path of the function measurement and of the optical path over the scene with polarizing material, which changes in polarization alternately when an electric field is applied depending on the mode. As a result, the transmission of the light guide can be changed or blocked electrically, optionally in conjunction with a polarization filter.Electrochromic: A portion of the optical path of the function measurement and of the optical path over the scene is each made of electrochromic material which, when an electric voltage is applied, changes or blocks the light transmission alternately depending on the mode.Mechanical: A section of the optical path of the function measurement and of the optical path over the scene is each composed of a mechanical aperture which, when an electrical voltage is applied, changes or blocks the light transmission alternately depending on the mode.Switchable mirror: There is a switchable mirror in front of the illumination unit, which depending on the mode alternately switches the light of the illumination source in the direction of the scene or in the direction of the optical return channel.Switchable absorber: A section of the optical path of the function measurement and of the optical path over the scene is made up of one or together via a switchable absorber, which changes or blocks the light transmission alternately when an electrical voltage is applied depending on the mode.SPS (suspended particle devices): a section of the optical path of the function measurement and of the optical path over the scene each consists of an element in which rod-shaped nanoparticles float in a liquid. By applying a voltage, the light transmission can be changed or blocked alternately depending on the mode.Micro-lamellae (micro-blinds): A section of the optical path of the function measurement and of the optical path over the scene each consists of an element of micro-lamellae which can change their position by applying a voltage. By applying a voltage, the light transmission can be changed or blocked alternately depending on the mode.Polarization plates rotatable relative to each other: A section of the optical path of the function measurement and of the optical path over the scene is each made up of at least 2 polarization plates rotatable relative to each other. By rotating the polarization plates with respect to one another, the light transmission can be changed or blocked alternately depending on the mode.As already described, the switching possibility of the optical paths is to be used to switch on the optical return channel only when a function measurement is carried out. Otherwise, the optical return channel 10 is not transparent, so that no deflected light a' falls on the time of flight sensor 22 and falsifys the ToF measurement.Preferably, the optical return channel 100 is switched in between the active measurements (distance measurements). The surface of the light transit time sensor is illuminated, for example laterally, with the light a' of the optical return channel 100, and corresponding reference values can be determined for each amplitude value.Since it can typically be assumed that the light from the optical return channel 100 is orders of magnitude higher than the reflected light b from the actively illuminated scene, integration times which are substantially shorter than integration times for the normal distance measurement operation can preferably be used for the function measurement.FIG. 3 shows an exemplary embodiment in which the optical return channel 100 is designed as a light guide 300. As before, the modulator 30 is connected to the time of flight sensor 22 and the light source 12. Furthermore, an evaluation unit or a control unit 250 is provided, which is connected to the first attenuation unit 200 in the optical return channel and to the second attenuation unit 201 in the outward light path a. The control unit 250 is also connected to the time of flight sensor 22, so that the attenuation units 200, 201 and the time of flight sensor 22 can be controlled appropriately depending on the respectively present operating mode (normal operation, function measurement operation).List of reference characters1 Time-of-flight camera system 10 Illumination module 12 Light source 15 Beam shaping optical unit 20 Receiving unit, TOF camera 22 Time-of-flight sensor 25 Receiving optical unit 30 Modulator 40 Object 100 Optical return channel 101 First mirror 102 Second mirror 200 First attenuation unit 201 Second attenuation unit 250 Evaluation unit, control unit, 300 Light guide a Emitted light a' Deflected light b Received light

Claims

Time-of-flight camera system (1), having a light source (12) for emitting a modulated light, having a time-of-flight sensor (22) which has at least one reception pixel for receiving and demodulating the emitted light reflected by a scene (40), having a modulator (30) which is connected to the time-of-flight sensor (22) and the light source (12), and having an evaluation unit which, on the basis of the demodulated light signal, determines a phase shift of the received light, wherein the time-of-flight camera system (1) has an optical return channel (100) which couples out a part (a') of the light (a) emitted by the light source (12) and directs it onto the time-of-flight sensor (22), characterized in that the optical return channel (100) has a first connectable optical attenuation unit (200), interrupting a forwarding of the coupled-out light (a') to the time of flight sensor (22) in a connectable manner, and in that the time of flight camera system is designed in such a way that, in a measurement mode, the optical return channel (100) is closed via the attenuation unit (200) and is open in a function measurement phase.Time-of-flight camera system (1) according to Claim 1, in which the optical return channel (100) is designed in such a way that the coupled-out light (a') impinges on the entire time-of-flight sensor (22).Time-of-flight camera system (1) according to one of the preceding claims, in which a second, connectable, optical attenuation unit (201) is provided in the light path of the light (a) emitted outwards by the light source (12) and / or in the light path (b) of the light received externally.Time-of-flight camera system (1) according to Claim 3, which is designed in such a way that, in the function measurement phase, the second attenuation unit (201) interrupts the light path (a) to the outside and / or the light path (b) of the light received from the scene (40) in the direction of the time-of-flight sensor (22).Method for monitoring the function of a time-of-flight camera system according to one of the preceding claims, characterized bythe following method steps: a) interruption of the light path (a) leading to the outside and / or interruption of the light path (b) b) incident from the outside, opening of the optical return channel (100) c), determination of the functionality of the light source (12) and of the time-of-flight sensor (22) on the basis of the light detected on the time-of-flight sensor (22). d) termination of the function measurement phaseMethod according to Claim 5, in which the function measurement phases are initiated at predefined time intervals and / or after a predefined number of distance measurement phases.Method according to either of Claims 5 and 6, in which a malfunction of the time-of-flight sensor (22) and / or individual time-of-flight pixels is detected if the values of the time-of-flight sensor determined in the function monitoring deviate from stored reference values.

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