DE102015202501B4 - time-of-flight sensor - Google Patents

Abstract

Time-of-flight sensor (22) with a plurality of light-time-of-flight pixels (23), which are arranged in a pixel array, the light-time-of-flight pixels (23) in a light-sensitive area having modulation gates (yarn, Gbm) for electrical modulation of this area, characterized in that within the pixel array a plurality of modulation drivers (38) for providing modulation signals (ModA, ModB) for the modulation gates (Garn, Gbm) are arranged, that clock and potential lines are provided, a clock signal (CLK) and two modulation DC voltages (VmodH, VmodL). introduce the modulation driver (38) in the pixel array.

Description

The invention relates to a time-of-flight sensor and a time-of-flight camera according to the species of the independent claims.

The time-of-flight sensor or time-of-flight camera is intended to encompass systems that obtain distances from the phase shift of an emitted and received radiation. PMD cameras with photomixing detectors (PMD) are particularly suitable as time-of-flight cameras or 3D-TOF cameras, such as those in the applications EP 1 777 747 A1 , U.S. 6,587,186 B2 and also DE 197 04 496 A1 described and can be obtained, for example, 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.

From the DE 10 2012 204 512 A1 a time-of-flight sensor for phase measurement is also known, in which the light-time-of-flight pixels are designed as photomixing detectors and are connected to modulation drivers. The modulation drivers are connected to at least one phase shifter and allow time-of-flight pixels in different areas on the time-of-flight sensor to be operated with different phase angles.

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

A time-of-flight sensor with a plurality of light-time-of-flight pixels (23) arranged in a pixel array is advantageously provided,
in which the light propagation time pixels in a light-sensitive area have modulation gates for electrical modulation of this area, a large number of modulation drivers for providing modulation signals for the modulation gates being arranged within the pixel array. The input signals required for generating the modulation signals, namely a clock signal (CLK) and two modulation DC voltages (VmodH, VmodL) are fed to the modulation driver (38) in the pixel array via clock and potential lines.

This procedure has the advantage that the capacitive load which has to drive the signal lines according to the invention is significantly lower than the load of conventional modulation signal lines. This allows a novel design of the time-of-flight sensor with, for example, narrower and/or narrower signal lines. On the one hand, this reduces the space requirement and, on the other hand, also the intrinsic capacitance of these lines and thus also capacitive influences on the signal to be transmitted at high frequency.

Such a procedure also has the advantage that the structure and construction of the signal lines can be adapted or scaled to pixel matrices of different sizes without major adaptations. Conventional modulation data lines cannot be scaled in this simple way, since the signal changes greatly over a column or line length, particularly in the case of very large pixels, due to line losses caused by load, and counteracting precautions must be taken.

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

• 1 ein Lichtlaufzeitkamerasystem,
• 2 eine modulierte Integration erzeugter Ladungsträger,
• 3 eine aus dem Stand der Technik bekannte Anordnung eines Lichtlaufzeitpixels,
• 4 ein erfindungsgemäßes Lichtlaufzeitpixel,
• 5 unterschiedliche Spannungsversorgungen der Lichtlaufzeitpixel,
• 6 eine einfache Treiberschaltung,
• 7 eine Treiberschaltung mit Buffer,
• 8 eine Treiberschaltung mit einer Frequenzverdopplung,
• 9 eine Treiberschaltung mit einem Takteingang.

They show schematically:

• 1 a time-of-flight camera system,
• 2 a modulated integration of generated charge carriers,
• 3 an arrangement of a time-of-flight pixel known from the prior art,
• 4 a time-of-flight pixel according to the invention,
• 5 different voltage supplies for the light runtime pixels,
• 6 a simple driver circuit,
• 7 a driver circuit with buffer,
• 8th a driver circuit with a frequency doubling,
• 9 a driver circuit with a clock input.

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 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 an array of multiple light-time-of-flight pixels 23 and is designed in particular as a PMD sensor. The Emp Capture 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 supplied via a modulator 30 together with a specific modulation signal M o} with a basic phase position φ ₀ . 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 φ _{0 of} the modulation signal M _{0 of} the light source 12 can be shifted by defined phase positions φ _{var .} 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 . This signal S _p1 or the electromagnetic radiation is reflected by an object 40 in the illustrated case and, due to the distance covered, arrives as a received signal _{with a phase shift Δφ(t L} ) with a second phase position p2=φ ₀ +φ _var +Δφ(t _{L ).} S _p2 to the time-of-flight sensor 22. In the time-of-flight sensor 22, the modulation signal Mo is _{mixed with the received signal S p2} , the phase shift or the object distance d being determined from the resulting signal.

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 can also be considered.

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 by the object 40 hits _{the time-of-flight sensor 22 as a received signal S p2} with a phase shift 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 M ₀ in a first integration node Ga and in a phase shifted by 180° Mo + 180° in a second integration node 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.

At least two modulation gates Gam, Gbm, which are operated in push-pull, are arranged in the light-sensitive area of the light transit time pixel for the phase-synchronous distribution of the charges to the first and second integration nodes Ga, Gb or in the A and B channels. The first modulation gate Yarn is connected, for example, to a modulation signal ModA of a so-called A-channel and the second modulation gate Gbm to a modulation signal ModB of a B-channel running in push-pull, ie shifted by 180°.

3 1 shows an arrangement known from the prior art for controlling time-of-flight pixels 23 of a time-of-flight sensor 22. A modulation signal Mo or a corresponding clock signal CLK is generated via the modulator 30 and made available to a modulation driver 38. The modulation driver 38 generates an A- and B-channel modulation signal ModA, ModB for each pixel column of the time-of-flight sensor 22, based on the clock signal CLK that is present and the high and low DC voltage that is present.

According to the invention, as in 4 shown, provided that the A and B modulation signals ModA, ModB are not generated globally for each column, but in each light transit time pixel 23 individually. For this purpose, a driver circuit 38 is arranged in each pixel, which generates the respective modulation signals ModA, ModB from the applied high and low DC voltages and the clock signal CLK. In order to keep the circuit complexity low, the pixel-specific driver 38 can also be supplied with an inverse clock signal CLKN. the inside 4 The area marked PMD is intended to represent the area of the pixel in which the modulation gates Gam, Gbm and the integration nodes Ga, Gb of the light transit time pixel 23 are arranged.

The procedure according to the invention has the advantage, among other things, that the signal can be fed in in a more space-saving manner. According to the usual design 3 is it like in 5 a) shown necessary to arrange the signal lines carrying the modulation signal ModA, ModB at the greatest possible distance in order to keep parasitic capacitances and the resulting crosstalk between the lines as low as possible.

In the embodiment according to the invention, however, it is not the driving modulation signal ModA, ModB that is fed to the pixels 23, but rather the high and low direct voltages VmodL, VmodH. Since these lines do not carry AC voltage, the arrangement is not critical. It is even advantageous to run these lines as close together as possible in order to form as high a capacitance as possible between the lines. This intrinsic capacitance is particularly useful as a blocking capacitance that stabilizes the high and low DC voltages.

5 b) shows a line routing with two adjacent high/low DC voltage lines. In 5c) a variant is shown in which the high/low DC voltage lines H, L are arranged in four quadrants and are electrically isolated from one another _{via an insulator SiO 2 .} Since the load is essentially borne by the DC voltage lines as a result of this procedure, there is the possibility of making the clock signal lines, which now essentially only carry a high-frequency signal and no load, narrower and/or also to arrange them more closely.

6 shows a very simple structure of a possible pixel-specific driver circuit 38, in which two inverting OPs are driven by a clock and inverse clock signal CLK, CLKN and provide modulation signals ModA, ModB for the A and B channels.

7 shows a driver circuit with buffers, which are each made up of two inverters. The transistor geometries can be matched to one another in a particularly advantageous manner in order, for example, to minimize variations in the clock signal, in particular the duty cycle, as a result of temperature or process fluctuations.

8th shows a further embodiment in which the modulation signals ModA, ModB are implemented via an XOR/XNOR driver circuit. By shifting the second clock signal CLK90 by 90° instead of by 180°, the frequency of the incoming clock signal can be doubled in a simple manner.

9 FIG. 1 shows a further arrangement in which only one clock signal CLK has to be fed to the driver circuit 38 in order to provide the modulation signals ModA, ModB. The driver circuit 38 consists of an XOR and XNOR gate, with the clock signal CLK being present at one input and a constant reference potential being present at the other.

It is also conceivable that the driver circuits according to 8th and 9 to combine so that an input of the logic gate can be switched to the reference potential and the clock signal CLK90 shifted by 90°. It is thus possible in a simple manner to switch between two frequencies. In particular, it is conceivable that the 90° clock signal CLK90 is switched over to a reference potential not in the pixel itself, but already globally outside the pixel.

Furthermore, it is also possible to share the driving circuit in the pixel array with multiple pixels. In particular, it is advantageous to share driver circuitry with neighboring pixels. For example, the driver circuit can supply two to four pixels in the immediate vicinity with a modulation signal.

Reference List

1

time-of-flight camera system

10

lighting module

12

lighting

20

Receiver, time-of-flight camera

22

time-of-flight sensor

23

time-of-flight pixels

30

modulator

35

Phase shifter, lighting phase shifter

φ, Δφ(tL)

runtime-related phase shift

φvar

phasing

φ0

base phase

M0

modulation signal

p1

first phase

p2

second phase

Sp1

Transmission signal with first phase

Sp2

Receive signal with second phase

i.e

object distance

VmodH

High DC voltage

VmodL

low DC voltage

ga, gb

modulation gates

ModA

Modulation signal A channel

ModB

Modulation signal B-channel

CLK

clock signal

CLKN

inverse clock signal

Claims (9)

Time-of-flight sensor (22) with a plurality of light-time-of-flight pixels (23) which are arranged in a pixel array, the light-time-of-flight pixels (23) having modulation gates (yarn, Gbm) in a light-sensitive area for electrical modulation of this area, characterized in that within the pixel array a large number of modulation drivers (38) for providing modulation signals (ModA, ModB) for the modulation gates (Garn, Gbm) are arranged, that clock and potential lines are provided that a clock signal (CLK) and two modulation DC voltages (VmodH, VmodL). introduce the modulation driver (38) in the pixel array. Time of flight sensor (22) after claim 1 In which the modulation drivers (38) are assigned to groups of light transit time pixels (23) and the modulation drivers (38) supply this group together with modulation signals (ModA, ModB). Time-of-flight sensor (22) according to one of the preceding claims, in which the modulation drivers (38) supply one to four light-time-of-flight pixels (23) with modulation signals (ModA, ModB). Time-of-flight sensor (22) according to one of the preceding claims, in which the modulation driver (38) for doubling the frequency of the incoming clock signal (CLK) has an XNOR and XOR gate, with a clock signal (CLK) at one input of the gate and a clock signal at the other A clock signal (CLKN) shifted by 90° is present at the input. Time-of-flight sensor (22) according to one of the preceding claims, in which the modulation driver (38) has an XNOR and XOR gate, a clock signal (CLK) being present at one input of each gate and a constant reference potential being present at the other input. Time of flight sensor (22) after claim 4 and 5 Is formed with at least one switching element which is connected to a switching signal line, and for switching the signal line to the constant reference potential or the clock signal (CLKN) shifted by 90°. Time-of-flight sensor (22) according to one of the preceding claims, in which the potential lines for the modulation DC voltage (VmodH, VmodL) are arranged spatially close together or one below the other and have a spacing of less than 1 µm. Time-of-flight sensor (22) according to one of the preceding claims, in which the driver circuit (38) has a buffer circuit which is optimized with regard to the temperature and process variation-dependent duty cycle variation. Time-of-flight camera (20) with a time-of-flight sensor (22) according to one of the preceding claims, with a modulator (30) for generating a clock signal (CLK) or inverse clock signal (CLKN).

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