DE102018108925B4 - Lighting circuit and method for a time-of-flight camera - Google Patents

Abstract

Lighting (10) for a time-of-flight camera system,
with at least two light sources (12),
in which the light sources (12) are electrically connected in parallel and each lie in their own current path,
the light sources (12) each having a first connection side connected to a single, common switch (S) and having a second connection side connected to a supply voltage (UB, UV),
wherein the parallel connection of the light sources (12) is designed in such a way that the signal propagation times (t1, t2, t3) from the first connection side to the switch (S) or a common node for all light sources (12) are of the same length,
and in which each light source (12) has its own controllable voltage source (130) and a current control (SR) in the current path of its second connection side.

Description

The invention relates to an illumination circuit for a time-of-flight camera or time-of-flight camera system according to the species of the independent claim.

Systems for three-dimensional image acquisition that work with the aid of active lighting are known from the prior art. These include so-called time-of-flight (TOF) or transit time measurement systems. These use amplitude-modulated or pulsed lighting to illuminate the three-dimensional scenery to be captured.

The invention also relates to a time-of-flight camera system with a time-of-flight sensor of the type mentioned above. Such time-of-flight camera systems relate in particular to all light-of-flight or 3D-TOF camera systems that obtain transit-time information from the phase shift of emitted and received radiation. PMD cameras with photomixing detectors (PMD) are particularly suitable as time-of-flight or 3D-TOF cameras, as they are used, for example, in DE 197 04 496 C2 described and can be obtained from 'ifm electronic GmbH' or 'PMD Technologies GmbH' as a frame grabber O3D or as a CamCube. In particular, the PMD camera allows a flexible arrangement of the light source and the detector, which can be arranged either in one housing or separately. Of course, the term camera or camera system should also include cameras or devices with at least one receiving pixel, such as the applicant's distance measuring device O1D.

From the DE 197 04 496 A1 the determination of a distance or a corresponding phase shift of the light reflected by an object is also known. In particular, it is disclosed that the transmitter modulation can be shifted in a targeted manner by 0°, 90°, 180° or 270° in order to determine a phase shift and thus a distance from these four phase measurements using an arctan function.

From the GB 2492833A a lighting circuit for a TOF system with two lighting strands is known. To modulate the light sources, each lighting strand has a switch on the low side, which is then followed by current measurement and regulation.

The object of the invention is to improve the distance measurement of a time-of-flight camera system.

The object is advantageously achieved by the illumination according to the invention for a time-of-flight camera system and the method according to the species of the independent claims.

Lighting for a time-of-flight camera system is advantageously provided, with at least two light sources, in which the light sources are connected electrically in parallel and are connected to a first connection side to a common switch and to a second connection side to a supply voltage, the parallel connection of the light sources being configured in such a way that that the signal propagation times from the first connection side to the switch are the same for all light sources.

This procedure has the advantage that there is no phase shift of an object when a light source is partially shaded and there is no asymmetry with regard to the viewing direction if all light sources are switched on or off at exactly the same time.

In a further embodiment, each current path of the light sources has its own controllable voltage source.

As a result, the currents for the light sources can be optimally set individually.

It is particularly advantageous if each current path has its own current control. This means that the most suitable current can flow to each light source, even if the light source is aging or due to thermal changes.

In a particularly preferred embodiment, a common controller is provided for all current paths of the light sources, with the controller detecting a current in each current path and adjusting the supply voltage by adjusting the controllable voltage sources in such a way that the current is regulated to an actual current specified for each current path .

By using a common controller for all current paths, the current regulation of the light sources connected in parallel can be implemented particularly cost-effectively.

• - bei dem die Lichtquelle in einer Modulationsphase mit mehreren Pulsen moduliert wird,
• - bei dem der Controller in den Modulationsphasen ausgehend von den Strommessungen den zu den jeweiligen Lichtquellen fließenden mittleren Strom durch Verändern der Versorgungsspannung auf einen mittleren Sollstrom einstellt,
• - bei dem die Stromregelung in den Modulationsphasen mit einer Start-Stellgröße beginnt, die als Spannungsendwert am Ende der Stromregelung der vorhergehenden Modulationsphase anlag.

A method for operating the aforementioned lighting for a time-of-flight camera is also advantageously provided, in which the lighting has a controller for current and voltage regulation and in each current path a modulatable light source, a controllable voltage source, a voltage measurement and a current measurement.

• - in which the light source is modulated in a modulation phase with several pulses,
• - in which the controller sets the mean current flowing to the respective light sources in the modulation phases based on the current measurements by changing the supply voltage to a mean target current,
• - In which the current control in the modulation phases begins with a start manipulated variable that was applied as a final voltage value at the end of the current control of the previous modulation phase.

This procedure has the advantage that electrical and thermal changes in the lighting in particular can be dynamically compensated.

The invention is explained in more detail below using exemplary embodiments with reference to the drawings.

• 1 schematisch ein Lichtlaufzeitkamerasystem,
• 2 eine modulierte Integration erzeugter Ladungsträger,
• 3 eine aus dem Stand der Technik bekannte Serienschaltung von Lichtquellen,
• 4 eine erfindungsgemäße Beleuchtung mit parallelgeschalteten Lichtquellen,
• 5 eine Anordnung gemäß 4 mit Stromregelungen in jedem Strompfad,
• 6 eine Strom- und Spannungsregelung für eine Beleuchtung,
• 7 eine Zeitdiagramm des Reglers gemäß 6,
• 8 einen erfindungsgemäßen Aufbau mit einem Regler gemäß 6.

Show it:

• 1 schematic of a time-of-flight camera system,
• 2 a modulated integration of generated charge carriers,
• 3 a series connection of light sources known from the prior art,
• 4 lighting according to the invention with light sources connected in parallel,
• 5 an arrangement according to 4 with current controls in each current path,
• 6 a current and voltage control for a lighting,
• 7 a timing diagram of the regulator according to 6 ,
• 8th a structure according to the invention with a controller 6 .

In the following description of the preferred embodiments, the same reference symbols designate the same or comparable components.

1 shows a measurement situation for an optical distance measurement with a time-of-flight camera, such as that from DE 197 04 496 A1 is known.

The time-of-flight camera system 1 comprises a transmission unit or an illumination module 10 with an illumination 12 and associated beam-shaping optics 15 and a receiving unit or time-of-flight camera 20 with a receiving optics 25 and a time-of-flight sensor 22.

The time-of-flight sensor 22 has at least one transit-time pixel, preferably also a pixel array, and is designed in particular as a PMD sensor. The receiving optics 25 typically consist of several optical elements to improve the imaging properties. The beam-shaping optics 15 of the transmission unit 10 can be designed, for example, as a reflector or lens optics. In a very simple embodiment, it is also possible to dispense with optical elements both on the receiving and on the transmitting side.

The measuring principle of this arrangement is essentially based on the fact that the propagation time and thus the distance covered by the received light can be determined based on 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 by a modulator 30 together with a specific modulation signal M _o with a basic phase angle φ0. In the example shown, a phase shifter 35 is also provided between the modulator 30 and the light source 12, with which the base phase φ0 of the modulation signal Mo of the light source 12 can be shifted by defined phase positions _φvar . For typical phase measurements, phase angles of φ _var =0°, 90°, 180°, 270° are preferably used.

According to the set modulation signal, the light source 12 emits an intensity-modulated signal S _p1 with the first phase position p1 or p1=φ ₀ +φ _var . In the illustrated case, this signal S _p1 or the electromagnetic radiation is reflected by an object 40 and, due to the distance covered, arrives with a phase shift Δφ(t _L ) with a second phase position p2=φ0+ _{φ var} +Δφ(t _L ) as the received signal S _p2 to the time-of-flight sensor 22. In the time-of-flight sensor 22, the modulation signal M _o is mixed with the received signal S _p2 , the phase shift or the object distance d being determined from the resulting signal.

The system also has a modulation control unit 27 which changes the phase position φ _var of the modulation signal Mo depending on the measurement task at hand and/or sets the modulation frequency via a frequency oscillator 38 .

Infrared light-emitting diodes are preferably suitable as the illumination source or light source 12 . Other radiation sources in other frequency ranges are of course also conceivable, in particular light sources in the visible frequency range are also possible.

The basic principle of phase measurement is shown schematically in 2 shown. The upper curve shows the time profile of the modulation signal M ₀ with which the lighting 12 and the time-of-flight sensor 22 are controlled. The light reflected from the object 40 hits the time-of-flight sensor 22 as a received signal S _p2 phase-shifted Δφ(t _L ) according to its time-of-flight t _L . The time-of-flight sensor 22 collects the photonically generated charges q over several modulation periods in the phase position of the modulation signal Mo in a first accumulation gate Ga and in a phase shifted by 180° M ₀ +180° in a second accumulation gate Gb. The phase shift Δφ(t _L ) and thus a distance d of the object can be determined from the ratio of the charges qa, qb collected in the first and second gates Ga, Gb.

3 shows one from the DE 10 2011 006 613 A1 known lighting circuit with three light-emitting diodes D1, D2, D3 and designed as a MOSFET switch Q1. The three light sources, in particular light-emitting diodes VCSEL, are connected in series, with the cathode of the third light-emitting diode D3 being switchably connected to the reference potential GND via switch Q1. The anode of the first light-emitting diode D1 is connected to the supply voltage U_IN via a limiting resistor R1. A parallel capacitor C1 is arranged in parallel with the limiting resistor R1. The gate side of the switch Q1 designed as a MOSFET is connected to a switching potential UT.

The switch Q1 is opened and closed in time with the desired modulation frequency and accordingly modulates the flow of current through the light-emitting diodes, which then naturally emit a correspondingly modulated light signal.

The disadvantage of such a linear arrangement of the light sources is that the supply voltage increases with the number of light sources. Also, different efficiencies of the lighting elements cannot be taken into account and, in particular, cannot be calibrated.

According to the invention, therefore, a lighting circuit according to 4 proposed in which the light sources 12 are arranged in parallel and switched via a common switch S. The operating point of the light sources 12 or the current flow I _B can be set separately for each light source 12 via a series resistor R.

In order to ensure that each light source 12 is switched on at the same time, provision is made for the light sources 12 to be electrically at the same distance from the switch S or from a common nodal point. The lines between switch S and the light sources 12 must have exactly the same transit time t1(ℓ ₁ ) = t2(ℓ ₂ ) = t3(ℓ ₃ ). The running time can be adjusted by the cable length ℓ, for example.

In a preferred embodiment according to 5 it is provided that each light source 12 has its own current control SR. To control the current I _B , a voltage drop U _I (I _B ) can be tapped off as the actual current variable I _{B_actual} via the series resistor R, for example, and the supply voltage Uv for the light source 12 can be adjusted so that the desired target current I _{B_soll} flows.

This procedure ensures that each light source 12 can be operated at its optimum operating point.

The path to the common node is entered here as an example of the signal path, after which the path to the switch S is the same for all light sources 12 .

6 shows a structure for carrying out a current regulation and an additional voltage regulation, for example in the exemplary embodiment according to FIG 5 could be used. A controllable voltage source 130 is provided for supplying current and voltage to the light source 12 and is controlled by a controller 140 on the basis of current and voltage values determined. A switch 120 is provided on the cathode side of the light source 12 and is controlled on the basis of the modulation signal of the modulation signal source or of the modulator 30 or phase shifter 35 . The modulation signal source 30, 35 also provides an envelope signal pulse _H of the modulation for the controller 140.

The average current flowing to the light source 12 I _B is determined via a voltage drop in a measuring resistor R and fed to the controller 140 as a current signal U _I via a differential amplifier 110 . Depending on the size of the resistor R and the amplification of the differential amplifier 110, an actual value with the unit ampere per volt [A/V] results.

The controller 140 also detects the supply voltage Uv present at the output of the controllable voltage source 130 . The voltage U _V can be measured either before or after the resistor R. The time window for current regulation is determined by the Pulse _H envelope. For the measurement, the current I _B flowing through the light source 12 is averaged or smoothed by the capacitor C, so that an average electricity I _B of the current I _B flowing through the light source 12 can be tapped.

Starting from the envelope signal Pulse _H , it is provided to switch between current and voltage regulation.

In the current control mode, it is intended to use the average current flow I _B to regulate the light source 12. The control variable I _B is via the resistor R and differential amplifier 110 to the controller 140 as the actual current value I _{Are you} or current signal actual value U _I ( I _{Are you} ) [A/V] provided. The controller 140 serves as a regulator and generates from the actual current value I _B _is and a reference variable or setpoint value that is preferably stored in the controller 140 I _{B_set} a control variable (U _PWM ) for the voltage regulator 130 to the mean current I _B to be controlled via the supply voltage Uv as a manipulated variable.

In the voltage regulation mode, the supply voltage Uv applied to the light source 12 is kept constant. For regulation, the controller 140 detects the supply voltage Uv present at the voltage source 130 or alternatively the supply voltage Uv present at the light source 12 as a controlled variable or actual voltage value U _{V_actual} . According to the invention, the supply voltage U _{V_end,n} that is present at the end of the preceding current regulation is used as the command variable or setpoint value. The controller 140 generates a control variable (U _PWM ) for the voltage regulator 130 from the present desired and actual values in order to regulate the supply voltage Uv to the desired value.

In 7 the aforementioned regulations are described in detail in three time diagrams. As shown in the first diagram, a modulation envelope curve pulse _H is made available to the controller 140 based on signals from the modulation signal source 30 , 35 . The beginning of the modulation phase Mein and the end of the modulation phase Maus can be recognized, for example, by the rising and falling flanks of the Pulse _H modulation envelope.

In the second graph is a time course of the mean current I _B to illumination source 12 is shown. As described, the mean current I _B to its setpoint during the modulation phases I _{B _should be} kept constant. The switch 120 is open between the modulation phases n and no current flows. In principle, the beginning Mein and the end Maus of a modulation phase n could also be determined at the flanks of the mean current signal.

The third diagram shows the supply voltage Uv as a function of time. When the modulation is initially started, a voltage U _{V_init} stored in the system is preferably first applied to the light source 12 .

In the first modulation phase n-1, the current control uses the applied supply voltage U _{V_init} as the first manipulated variable or as the voltage start value U _{V_start} .

The mean current is regulated as described I _B by changing the supply voltage Uv. The changes in the manipulated variable due to the control interventions are shown schematically.

According to the invention, the supply voltage U V_end,n-1 present at the end of the current control is used as a reference variable U _{V_soll} = U _{V_end, n-1} for the subsequent voltage control and as a starting manipulated variable U _{V_start,n} = U _{V_end} for the subsequent current control _,n-1 to use.

This means that after the end of the mouse modulation phase, the supply voltage Uv is kept constant, as shown, at the supply voltage U _{V_end,n-1} last applied in the current control. The current regulation of the subsequent modulation phase n then also begins with this voltage at the point in time Mein.

The current and voltage controls thus always take place as a function of the final voltage value U _{V_end,n-1} which was present in the last or previous current control.

This procedure has the advantage that the operation of the lighting 12 can be dynamically adapted to thermal and/or electrical changes in the lighting 12 .

8th shows a structure according to the invention with the aforementioned current and voltage regulation. Each lighting current path has at least one light source 12, a series resistor R and an adjustable voltage source 130. The voltage Uv present in each current path and the current I _B or U _I (I _B ) flowing is detected by a controller 140 which all current paths use jointly. To regulate the current I _B and the supply voltage Uv, the controller 140 generates a separate control variable U _PWM for each voltage regulator 130 in order to regulate the current I _B and the voltage Uv to their actual values.

Reference List

1

time-of-flight camera system

10

Lighting, lighting module

12

light source

15

beam shaping optics

20

Receiver, time-of-flight camera

22

time-of-flight sensor

25

receiving optics

27

evaluation unit

30

modulator

35

Phase shifter, lighting phase shifter

38

modulation controller

40

object

110

Amplifier, differential amplifier

120

Switch

130

adjustable voltage source

140

controllers

φ, Δφ(tL)

runtime-related phase shift

φvar

phasing

φ0

base phase

R

Resistance

C

capacitor

Mon

modulation signal

H

envelope

p1

first phase

p2

second phase

Sp1

First phase transmit signal

Sp2

Receive signal with second phase

Δq

charge difference

i.e

object distance

Claims (8)

Lighting (10) for a time-of-flight camera system, with at least two light sources (12), in which the light sources (12) are electrically connected in parallel and each lie in their own current path, the light sources (12) each having a first connection side connected to a single, common switch (S) and having a second connection side connected to a supply voltage (UB, UV), wherein the parallel connection of the light sources (12) is designed in such a way that the signal propagation times (t1, t2, t3) from the first connection side to the switch (S) or a common node for all light sources (12) are of the same length, and in which each light source (12) has its own controllable voltage source (130) and a current control (SR) in the current path of its second connection side. lighting after claim 1 , in which the lighting has a common controller (140) for all current paths of the light sources (12), the controller (140) detecting a current (I _B ) in each current path and by adjusting the controllable voltage sources (130) the supply voltage (Uv ) is adjusted in such a way that the current (I _B ) is regulated to an actual current (I _{B_ist} ) specified for each current path. Method for operating lighting (10) according to one of the preceding claims, in which the lighting (10) has a controller (140) and in each current path a modulatable light source (12), a controllable voltage source (130), a voltage measurement and a current measurement ( 110), - in which the light sources (12) are modulated in a modulation phase (n) via a common switch (S) with a plurality of pulses, - in which the controller (140) in the modulation phases (n) based on the current measurements ( 110) the average current ( I _B ) by changing the supply voltage (U _Vn ) to an average target current ( I _{B_set} ) is set, - in which the current control in the modulation phases (n) begins with a start manipulated variable (U _{V_start,n} ) which, as a final voltage value (U _{V_end,n-1} ) at the end of the current control of the preceding modulation phase (n-1) system procedure after claim 3 , in which, after the end of the modulation phase (n), the voltage is regulated to the final voltage value (U _{V_end,n} ), which was present at the end of the modulation phase (n). procedure after claim 3 or 4 , which switches between current and voltage control depending on an on/off signal (my, mouse). Procedure according to one of claims 3 until 5 , in which the switch-on and switch-off signals (Mein, Maus) are determined based on a rising and falling switching edge of the envelope signal (pulse _H ). Lighting required to carry out any of the procedures referred to claim 3 until 6 is trained. Time-of-flight camera system with lighting according to one of Claims 1 until 2 and 7 .

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