EP3894886A1

EP3894886A1 - Flight time sensor and surveillance system comprising such a sensor

Info

Publication number: EP3894886A1
Application number: EP19809075.5A
Authority: EP
Inventors: Sylvain BEAUDOIN; Sebastien TRAPANI; Frédéric AUTRAN
Original assignee: Valeo Comfort and Driving Assistance SAS
Current assignee: Valeo Comfort and Driving Assistance SAS
Priority date: 2018-12-14
Filing date: 2019-11-26
Publication date: 2021-10-20
Also published as: US20220050202A1; FR3090124B1; CN113167874A; FR3090124A1; WO2020120128A1

Abstract

The invention relates to a flight time sensor (10) comprising: - a lighting device (11) comprising a light source (12) which emits a source beam (13) in the direction of a scene (3) comprising an object (4) which is capable of reflecting the source beam; - a detector (15) comprising a matrix (16) of photo-sensitive pixels (16A) which receive a portion (18) of the source beam reflected by the object; and - an electronic unit (19) which is configured: - to generate a modulation signal and to control the device by means of this signal so that the source beam has a source light power which is modulated temporally; - to process electric signals which are supplied as a function of time by the detector, each electric signal representing a fraction of the source light power reflected in the direction of a pixel; and - to deduce from the electric signals a characteristic distance (D_obj) between the object and the device. According to the invention, the unit is configured, when the object is detected as being at a characteristic distance smaller than a predetermined threshold distance, to control the device in order to reduce the average source light power.

Description

Title of the invention: Time of flight sensor and monitoring system comprising such a sensor

TECHNICAL FIELD TO WHICH THE INVENTION RELATES

The present invention relates generally to the field of vision and object detection by means of an optical detection system.

It relates more particularly to a time-of-flight sensor making it possible to reconstruct the three-dimensional mapping of a scene comprising one or more objects.

It also relates to a monitoring system comprising such a sensor.

TECHNOLOGICAL BACKGROUND

Time of flight sensors are known comprising:

an illumination device comprising a light source emitting a source beam, towards a scene comprising an object capable of reflecting this source beam; and

a detector comprising a matrix of photosensitive pixels and receiving part of the source beam reflected by this reflecting object.

Generally, the light source of the illumination device is either a light-emitting diode (DE) or a laser diode (for example of the VCSEL type). This source generally emits in a narrow band of the infrared domain, around 900 nanometers for example. This means that the source beam is not visible to users present near the sensor (the human eye is only sensitive up to around 800 nm).

The detector, which is of the "matrix" type, is for example formed by a CMOS type camera, the photosensitive pixels of which are sensitive in IR, at least in an absorption band including the narrow emission band.

As described in the document “Time-of-Flight Camera - An Introduction” (Texas Instruments Technical White Paper SLOA190B, May 2014), the sensor measures the time, therefore called “flight time”, taken by the source beam to go from the source to the object, then by the "reflected" beam) to go from the object to the detector. For this, the source beam must be modulated temporally.

For this purpose, there is generally provided an electronic unit configured to generate a modulation signal and to control the illumination device by means of this modulation signal so that the emitted source beam has a temporally modulated source light power.

Generally, this electronic unit is also configured to: process electrical signals delivered as a function of time by the detector, each electrical signal being representative of a fraction of the source light power reflected by the object towards an associated photosensitive pixel ; and

deduce from the processed electrical signals a characteristic distance between the object and the illumination device.

Thus, a time-of-flight sensor makes it possible to reconstruct a three-dimensional cartography of the scene it observes and to detect shapes there, or even recognize objects (a human body or part of a human body, the eyes of an individual, etc ...).

To “see” far (long range) and well (good signal-to-noise ratio), the electronic time sensor controls the illumination device so that the average light power emitted from the source beam is high enough.

OBJECT OF THE INVENTION

In order to remedy the aforementioned drawback of the state of the art, the present invention provides a time-of-flight sensor having increased security and avoiding the risks of damage by the source beam, in particular the risks of damage to persons.

More particularly, a time of flight sensor as defined in the introduction is proposed according to the invention, in which said electronic unit is also configured for, when said object is detected as being at a characteristic distance less than a predetermined threshold distance:

control said illumination device so as to reduce said source light power on average below a predefined maximum value.

Thus, thanks to the time-of-flight sensor of the invention, the level of infrared radiation undergone by the objects present in the scene and liable to reflect this radiation is limited. In a preferred embodiment, said electronic unit is also configured to generate a modified modulation signal for controlling said illumination device by means of this modified modulation signal.

In this case, when the time-of-flight sensor detects that an object is at a characteristic distance too small from the light source, then the electronic unit modifies the modulation signal almost instantaneously so that the source beam presents limited light output.

The predefined maximum value can for example be a danger threshold for the skin or the eyes of an individual. For example, when the illumination device comprises an LED type source, this maximum value is defined in the international standard IEC 62471 ("Photobiological safety of lamps and devices using lamps").

The predetermined threshold distance may in particular depend on this predefined maximum value.

The characteristic distance of the object detected by the flight time sensor of the invention can be, for example, the average distance or else a weighted distance.

Preferably, said characteristic distance is equal to the minimum distance between said illumination device and a particular point of the object reflecting said source beam towards a photosensitive pixel of said detector. In this way, we are sure that as soon as the object (or the individual) enters a particular danger zone, then the average light power is reduced so that no point of said object is illuminated with a source beam. too intense.

Advantageously, in cases where said predetermined threshold distance is less than 30 centimeters, preferably less than 20 centimeters, the modified modulation signal controls the extinction of said light source.

Other non-limiting and advantageous characteristics of the time-of-flight sensor according to the invention are as follows:

said modulation signal being such that said modulated source light power comprises a periodic succession of light pulses, said modified modulation signal is adjusted so that the source light power comprises a reduced number of light pulses and / or pulses brighter with narrower width and / or lower intensity; and

said electronic unit is further configured to, when said object is then detected as being at another characteristic distance greater than said predetermined threshold distance, generating another modified modulation signal and controlling said illumination device by means of this modulation signal modified so as to increase said source light power beyond a predefined minimum value.

The time of flight sensor described above advantageously enters into the embodiment of a monitoring system intended to monitor the interior of a passenger compartment of a motor vehicle.

The invention therefore also provides a monitoring system comprising a time of flight sensor as defined above.

In a preferred embodiment, the reduction of the source light power by the time-of-flight sensor is also conditioned by the recognition of said object as being the head of an occupant (driver, passenger, etc.) of said motor vehicle. .

DETAILED DESCRIPTION OF AN EXAMPLE OF IMPLEMENTATION

The description which follows with reference to the appended drawings, given by way of nonlimiting examples, will make it clear what the invention consists of and how it can be carried out.

In the accompanying drawings:

[Fig. 1] is a schematic view of a vehicle and its passenger compartment which includes a time of flight sensor integrated into a passenger compartment monitoring system;

[Fig. 2] is a schematic view of the time-of-flight sensor of FIG. 1 showing the operating principle of the time-of-flight measurement;

[Fig. 3] shows a human head detected in the passenger compartment of the vehicle in FIG. 1; and

[Fig. 4] and

[Fig. 5] are graphs showing examples of modulation of the source power emitted by the time-of-flight sensor of FIG. 3.

In Figure 1, there is shown a motor vehicle 1 and its passenger compartment 2, with the front and rear seats.

A monitoring system 20 is on board inside the passenger compartment 2 of the motor vehicle 1 to allow the acquisition of the cabin environment and its occupants (driver, front and / or rear passenger (s)).

This type of monitoring system is called in English "Interior Monitoring System" or IMS. It comprises a time-of-flight sensor 10 (hereinafter referred to as “sensor 10”) which is generally positioned in the roof modules of the motor vehicle 1 but can also be in front of the driver, near the upright or in the center console.

The sensor 10 is directly oriented towards the occupants of the car and has a field of vision 17 (see FIG. 1), here so as to cover all the possible positions of the occupants of the motor vehicle 1.

As shown schematically in FIG. 2, the sensor 10 comprises three elements which we will describe in more detail below: an illumination device 11, a detector 15 and an electronic unit 19.

In the general case, the sensor 10 faces a scene 3 comprising different “objects” (represented here by geometric shapes) capable of reflecting light, in particular infrared (IR) light. These objects may for example be the head of the driver or the passenger. For the rest, we will consider as an example for the explanation the particular case of cube 4 as a reflecting object.

The illumination device 1 1, which can be controlled by the electronic unit 19 (we will see how in the following description), comprises a light source 12 whose light power can be modulated, for example via a current control.

Thus controlled, the illumination device 1 1 emits a source beam 13 (see fig. 2) in the direction of scene 3 comprising the object 4 which will reflect the source beam 13.

The light source 12 is preferably a source emitting electromagnetic radiation at a wavelength little or not visible to the human eye. Advantageously, this light source 12 emits in the infrared range.

Here, the light source 12 is a light-emitting diode (LED) emitting in the IR, at a wavelength of 940 nm, with an emission width of 60 nm (+/- 30 nm around the central wavelength). As a variant, the diode could emit at a wavelength of 850 nm, or else at any other wavelength in the near infrared range between 800 nm and 1100 nm, or even possibly at a wavelength in the visible spectrum of deep reds, between 700 nm and 800 nm.

In other embodiments, the light source of the device can be a laser diode, in particular of the VCSEL (“Vertical-cavity surface-emitting laser”) type, for example a GaAs / AIGaAs laser diode emitting between 700 nm and 1100 nm.

In particularly advantageous embodiments, the illumination device 11 may comprise an optical system placed downstream from the light source 12 for shaping the source beam 13 (see FIG. 2) emitted by the light source 12. This optical system can be simply formed of a single lens, or of a doublet. Alternatively, a complex optical system can be provided to give particular properties to the source beam 13 (digital aperture, polarization, optical quality, etc.).

The electronic unit 19 is configured to generate a modulation signal, for example a modulated current signal, intended for the illumination device 11 (see arrow between the unit 19 and the device 11 in FIG. 2).

Controlled by means of this modulation signal, the light source 12 emits a source light beam 13 which has a source light power Ps which is temporally modulated.

This can be seen in FIG. 4 where the variations in the source light power Ps (in Watts W) emitted by the illumination device 11 have been shown on the ordinate as a function of time t (in seconds s).

The modulation signal is here such that during an activation period Ton (also called “integration period”) typically between 1 ps and 10 ms, the illumination device emits a periodic succession of pulses 21 square of power peak Po and of width At between 5 nanoseconds and 500 nanoseconds, at a frequency between 1 and 100 megahertz (MHz).

During another period known as Tott inactivation, the modulation signal is such that the illumination device 1 1 does not emit a source beam (the current signal received at the input of the illumination device - and therefore from the source luminous - being for example zero current signal).

The set formed by the Ton activation period and the period Tott inactivation period constitutes a Tacq vesting period.

In other embodiments, the modulation signal could be a sinusoidal signal so that the source light power is modulated sinusoidally as a function of time.

As shown in FIG. 2, part 14 of the source beam 13 emitted by the illumination device 11 is reflected by the object 4 present in the scene 3. To be detected, the object 4 must be in the field of view 17 of the sensor 10 so that the reflected beam 18 is intercepted by the detector 15.

This detector 15 is a matrix detector (for example “focal plane array”) and comprises a matrix 16 of photosensitive pixels (“pixel array”), in particular at the emission wavelength of the light source 12, c that is to say here in the infrared.

Advantageously, the detector 15 can also include collection optics, for example a simple lens or a more complex optical system.

The detector 15 receives part 18 of the source beam 13 (reflected beam 18) reflected by the object 4 in the direction of the detector 15.

The source beam 13 being temporally modulated, the reflected beam 18 is also temporally modulated (the reflection on the object 4 does not modify this property).

Because of the propagation of light from the light source 12 to the object 4, then from the object 4 to the detector 15, there is a time shift between the moment when the source beam 14 is emitted and the moment when the reflected beam 18 is collected by the matrix 16 of the detector 15. By determining this time offset, it is possible to go back to the time of flight of the light between the illumination device 1 1 and the detector 15 (see Texas Instruments document cited above).

This determination of the time of flight can be made for each pixel of the matrix 16 so that it is then possible to reconstruct a real three-dimensional map of the scene 3 located opposite the sensor 10. Indeed, each photosensitive pixel of the matrix 16 delivers an electrical signal as a function of time which is proportional to the amount of light received by the pixel.

For example, for the particular pixel 16A in FIG. 2, the electrical signal is representative of the fraction (reflected beam 18) of the light power source P _s reflected by the object 4 in the direction of said photosensitive pixel 16A.

The electronic unit 19 of the sensor 10 is precisely configured to: process these electrical signals delivered as a function of time by the detector 15; and

deduce from the processed electrical signals a characteristic distance Dobj between the object 4 of scene 3 and the illumination device 1 1.

According to the invention, the electronic unit 19 is further configured for, when the object 4 is detected as being at a characteristic distance Dobj less than a predetermined threshold distance Dmin:

generating a modified modulation signal; and

control the lighting device 1 1 by means of this modified modulation signal so as to reduce on average the source light power P _s below a predefined maximum value Pmax.

In other words, if the sensor 10 detects that the object 4 is too close then it decreases the average power of the light source 12 so as to limit the infrared radiation received by the object 4 with the source beam 13.

In a preferred embodiment of the monitoring system 20 of FIG. 1, the reduction of the source light power is further conditioned by the recognition of said object as being the head 5 of an occupant of the motor vehicle.

In fact, in an automotive context, this can be useful when the object 4 detected is the head of an individual (driver or passenger) since this avoids irradiating the skin and / or the eyes of the individual, which could be problematic in certain situations.

The recognition of the object as being the head of an occupant of the vehicle can for example be carried out by determining that the apparent surface or the volume of said object corresponds to that of a human head.

In another example, the monitoring system includes software processing capable of distinguishing on a three-dimensional representation of the observed scene that the volume detected is a human head.

FIG. 5 shows an example of the source light power Ps modified by the control of the illumination device 11 with the modified modulation signal generated by the electronic unit 19 after the object 4 has been detected as being at a characteristic distance Dobj less than the distance threshold Dmin.

As can be seen in this FIG. 5, this modified light source power P _s is such that only one square pulse 21 out of two has been retained (the even order pulses 22 in dotted lines have been deleted), while keeping its level peak Po and its activation period Ton. Thus, on average, the source light power P _s is here halved.

In an alternative embodiment, one could keep all the pulses of the modulation of the source beam 13 and reduce the peak power of the pulses, for example to a value Po / 2 (to have an average light power of half). This can easily be done by dividing the modulation current sent to the light source 12 by two.

Yet in another alternative embodiment, one could keep all the pulses of the Ton activation period but reduce the width of the pulses (for example by dividing it by two).

Still in another embodiment, one could keep the pulses with their original width and power but reduce the duration of the Ton activation period. For example, by dividing this duration by 2, the average lighting power is also divided by 2.

In a particularly safe embodiment, the characteristic distance Dobj of the object 4 can be taken equal to the minimum distance between the illumination device 1 1 and a particular point of the object 4 reflecting the source beam 14 in the direction d a photosensitive pixel 16A of the detector 15.

Thus, in this embodiment, it is sufficient that a single particular point of the object 4 (for example the head 5 of the conductor, see FIG. 3) is at a distance less than the threshold distance Dmin for the electronic unit 1 9 modifies the modulation signal in order to limit the light source power Ps.

In the general case, the threshold distance Dmin depends on the average light power Ps.

In practice, the average light source power Ps is determined so as to be less than the danger levels defined by standard IEC 62471 for light-emitting diodes and by standard IEC 60825-1 for laser diodes.

In another even safer embodiment, the predetermined threshold distance Dmin being less than 30 centimeters, or even less than 20 cm, the modified modulation signal generated by the electronic unit 19 controls the extinction of the light source 12 of the illumination device 1 1. In other words, in this particular embodiment, the light source is completely cut off, by example by canceling the source drive current.

Advantageously, the electronic unit 19 is further configured for, when the object 4 previously detected as too close to the sensor 10 is then detected as being at another characteristic distance Dobj greater than the predetermined threshold distance Dmin:

generating another modified modulation signal; and

- controlling said illumination device by means of this modified modulation signal so as to increase said source light power beyond a predefined minimum value.

In this way, it is then possible to detect the object previously too close when it has moved away. Indeed, the range of the sensor 10 increases with the average light power of the source beam 13. By increasing the source light power, it is ensured that the object 4 can be detected again when it approaches.

In practice, the other modified modulation signal is preferably identical to the modulation signal generated before the object is detected as being too close. The profile of the source light power P _s is then that shown in FIG. 4.

Claims

1. Time of flight sensor (10) comprising:

• an illumination device (1 1) comprising a light source (12) and adapted to emit a source beam (13) towards a scene (3) comprising an object (4) capable of reflecting said source beam ( 13);

• a detector (15) comprising a matrix (16) of photosensitive pixels (16A) and adapted to receive a part (18) of the source beam (13) reflected by said object (4) of the scene (3); and

• an electronic unit (19) configured for:

• generate a modulation signal and control said illumination device (1 1) by means of this modulation signal so that the source beam (13) emitted has a source light power (P _s ) modulated in time;

• process electrical signals delivered as a function of time by said detector (15), each electrical signal being representative of a fraction of the source light power (P _s ) reflected by the object (4) in the direction of a pixel ( 16A) associated photosensitive; and

• deduce from said processed electrical signals, a characteristic distance (Dobj) between said object (4) and said illumination device (1 1), said time of flight sensor (10) being characterized in that said electronic unit (19) is further configured for, when said object (4) is detected as being at a characteristic distance (Dobj) less than a predetermined threshold distance (Dmin):

control said illumination device (1 1) so as to reduce on average said source light power (Ps) below a predefined maximum value (Pmax).

2. time-of-flight sensor (10) according to claim 1, wherein said electronic unit (19) is further configured to generate a modified modulation signal to control said illumination device (1 1) by means of this signal modified modulation.

3. Time-of-flight sensor (10) according to claim 1 or 2, wherein said characteristic distance (Dobj) is equal to the minimum distance between said illumination device (1 1) and a particular point of the object (4) reflecting said source beam (13) towards a photosensitive pixel (16A) of said detector (15).

4. time-of-flight sensor (10) according to claim 2 or claim 3 taken in dependence on claim 2, wherein, said predetermined threshold distance (Dmin) being less than 30 centimeters, the modified modulation signal controls l extinction of said light source (12).

5. Time-of-flight sensor (10) according to one of claims 2 to 4, in which said modulation signal being such that said modulated source light power (P _s ) comprises a periodic succession of light pulses (21) , said modified modulation signal is adjusted so that the source light power (P _s ) comprises a reduced number of light pulses (21) or light pulses of narrower width or lower intensity.

6. Flight time sensor (10) according to one of claims 2 to 5, claims 3 to 5 being taken in dependence on claim 2, wherein said electronic unit (19) is further configured for, when said object (4) is then detected as being at another characteristic distance (Dobj) greater than said predetermined threshold distance (Dmin):

generating another modified modulation signal; and

controlling said illumination device (11) by means of this modulation signal modified so as to increase said source light power (P _s ) beyond a predefined minimum value (Pmin).

7. Monitoring system (20) for monitoring the interior of a passenger compartment (2) of a motor vehicle (1) and comprising a time-of-flight sensor (10) according to one of claims 1 to 6.

8. Monitoring system according to claim 7, in which the reduction of the source light power (P _s ) is further conditioned by the recognition of said object (4) as being a head (5) of an occupant of said motor vehicle ( 1).