WO2019149725A1

WO2019149725A1 - Device and method for determining a distance to a moving object

Info

Publication number: WO2019149725A1
Application number: PCT/EP2019/052187
Authority: WO
Inventors: Michael Patra; Jan Horn; Andy ZOTT; Vladimir Davydenko
Original assignee: Carl Zeiss Smt Gmbh
Priority date: 2018-02-05
Filing date: 2019-01-30
Publication date: 2019-08-08
Also published as: DE102018201735A1

Abstract

The invention relates to a device and method for determining a distance to a moving object. A device according to the invention comprises a transmitter (110) for broadcasting an optical transmitter signal (111), wherein the transmitter signal has a temporally varying frequency (ω_s(t)), a detector (150) for generating a detector signal from a superimposition of a first part signal (121) and a second part signal (122), wherein the first part signal (121) and the second part signal (122) arise from the transmitter signal (111) being broken down, wherein the first part signal (121) arrives at the detector (150) without previous reflection on the object (105) and wherein the second part signal (122) arrives at the detector (150) after reflection on the object (105), wherein the detector signal is characteristic for the differential frequency (Δω) between the frequency of the second part signal (122) and the frequency of the first part signal (121), and an evaluation device (160) which is designed to determine self-consistently the development over time of the distance d(t) and speed v(t) of the object from the development over time of the differential frequency (Δω(t)) and the development over time of the frequency (ω_s(t)) of the transmitter signal.

Description

Apparatus and method for determining a distance of a moving object

The present application claims the priority of German Patent Application DE 10 2018 201 735.2, filed on 05 February 2018. The content of this DE application is incorporated by reference into the present application text by "incorporation by reference".

BACKGROUND OF THE INVENTION

Field of the invention

The invention relates to a device and a method for determining a distance of a moving object.

State of the art

For optical distance measurement of objects u.a. a measurement principle also known as LIDAR, in which an optical signal, which is temporally changed in its frequency, is emitted toward the relevant object and evaluated after back reflection on the object.

5 shows only a schematic representation of a known basic structure of a measuring device 40 in which an optical transmitter signal emitted by a light source or a transmitter 10 is split over a partially transmissive mirror 11 into two partial signals with a time-linearly changed frequency. These two sub-signals are superimposed on one another at a detector 50, wherein the first sub-signal 21 without reflection at the (in Fig. 5 denoted by "5") object to the detector 50 passes. The second partial signal 22, on the other hand, is conducted to the object 5 via an optical circulator 30, and is reflected back to the detector 50 with a time delay and a correspondingly changed frequency compared to the first partial signal 21. The evaluation device 60 receives the signal from the detector 50 delivered measurement signal for determining the distance of the object 5 (in relation to the measuring device) evaluated. The superposition of the two partial signals 21 and 22 at the detector 50 leads in a known manner to a total signal, which can be represented as the product of two oscillations, one of which the average frequency of the partial signals 21, 22 and the other the difference frequency the frequencies of the partial signals 21, 22 possesses, whereby with a real detector 50 only the difference frequency is temporally resolvable.

If now the object 5 to be measured with respect to its distance is in motion, the difference frequency of the frequencies of the partial signals 21, 22 is not constant in time and only dependent on the time delay, as shown in FIG. 6a, but results from the Doppler effect a speed-dependent shift of the frequency of the second partial signal 22 reflected at the moving object 5 and thus also a temporal variation of the finally measured difference frequency, so that erroneous values are obtained in the distance determination without appropriate consideration.

Figs. 6b-6c are diagrams for explaining a conventional approach for overcoming this problem. In this case, the influence of the Doppler effect on the obtained measurement result according to FIG. 6b is regarded as a constant offset and the measurement process is divided into two measurement sections, in which according to FIG. 6c the time derivative of the frequency generated by the transmitter 10 is opposite to each other. This inversion of the slope of the temporal frequency dependence generated by the transmitter 10 is intended to ensure that the portion of the differential frequency dependent only on the distance of the object 5 is of opposite sign in both measuring sections, However, the Doppler shift of the measured frequency has the same sign in both measuring sections. The object movement thus has an apparent increase in the object distance in one of the two measurement sections, while an apparent reduction in the object distance results in the other measurement section, so that the "true" distance is set as the mean value of the measurement results obtained in both measurement sections.

However, the approach described above with reference to FIG. 6c proves to be insufficient in practice in scenarios in which a highly accurate distance measurement to a few micrometers (pm) is to be precisely realized for moving objects with distances of the order of several meters. This is due, on the one hand, to the fact that, as a result of the temporal change in the frequency of the signal emitted by the transmitter 10, the intensity of the Doppler effect also changes over time, and consequently the difference frequency is time-variable, as shown in FIG. 6d, the treatment of the Doppler effect is more constant Offset is just not justified. In addition, the distance determination from two successive partial measurements described with reference to FIG. 6c disregards the fact that the object 5 to be measured generally continues to move during the two partial measurements, with the consequence that the compensation of the effects of the Doppler effect does not occur more completely given. Last but not least, the movement of the object 5 during a single measuring section already leads to a change of the difference frequency and thus to a reduction in the accuracy achieved during the distance determination in the distance determination based on a single difference frequency on the basis of FIG. 6c ,

SUMMARY OF THE INVENTION

Against the above background, it is an object of the present invention to provide an apparatus and a method for determining a distance of a wegten object provide, even with moving objects with distances in the order of several meters, a highly accurate distance measurement on a few microns (gm) can be realized.

This object is achieved by the device according to the independent patent claim 1 and the method according to the independent claim 12.

An apparatus according to the invention for determining a distance of a moving object has:

a transmitter for transmitting an optical transmitter signal, the transmitter signal having a time-varying frequency;

a detector for generating a detector signal from a superimposition of a first partial signal and a second partial signal, wherein the first partial signal and the second partial signal have arisen by decomposing the transmitter signal, wherein the first partial signal reaches the detector without prior reflection at the object and wherein the second sub-signal reaches the detector after reflection from the object, the difference frequency signal detection signal being characteristic of the frequency of the second sub-signal and the frequency of the first sub-signal; and

- An evaluation device which is designed to, from the time course of the difference frequency (Aco (t)) and the time course of the frequency (co _s (t)) of the transmitter signal self-consistent the time course of distance d (t) and speed v ( t) of the object.

The invention is based in particular on the concept of not determining, for the distance measurement of a moving object based on the conventional concept described with reference to FIG. 5, not just a single difference frequency and based on the distance calculation, but rather the search the desired distance of the object on the basis of a time profile of this difference frequency. The present invention is intended to include both embodiments in which the relevant time profile of the differential frequency over a predetermined time interval is determined, as well as embodiments in which, as explained in more detail below one or more of the Characteristic parameters are determined over the course of the difference frequency.

The determination of the distance on the basis of the time profile of the difference frequency has the advantage that the effects of the movement of the object to be measured in terms of its distance, in particular due to the Doppler effect, are not taken into account - in an incorrectly simplified way - as a constant offset, but rather with a higher one Accuracy (in a sense, taking into account higher orders of the Doppler effect) are recorded varying over time and, as a result, highly accurate distance determinations with accuracies in the micrometer range can be realized even with moving objects in several meters distance.

Furthermore, the concept according to the invention has the advantage that according to the invention a linearization of the temporal frequency characteristic of the transmitter emitting the optical transmitter signal can be dispensed with, with the result that the apparatus cost associated with such a linearization can also be dispensed with.

This advantageous further effect is based on the fact that with the determination according to the invention of the time profile of the differential frequency, which in turn comprises the measurement of "instantaneous" or instantaneous frequencies, the requirement of a linear frequency existing in the prior art as a result of a constant difference frequency - no amendment. In other words, according to the invention, there is no specific chronological frequency profile with regard to the transmitter signal emitted by the transmitter (in particular, no signal that changes necessarily linearly with time). de frequency of the transmitter signal), but only a precise knowledge of the temporal frequency response of the transmitter signal is required.

This exact knowledge of the temporal frequency response of the transmitter signal can be achieved according to the invention in that either the said temporal frequency characteristic of the transmitter or the transmitter signal emitted by it is known from the outset due to sufficient knowledge of the characteristics of the transmitter, or that as described below the frequency of from the transmitter emitted optical transmitter signal via a suitable frequency measuring device, for is measured in the form of an interferometer.

According to one embodiment, the evaluation device is designed to determine the time profile of distance d (t) and speed v (t) by determining the parameters of a model for distance d (t) and speed v (t).

According to one embodiment, at least one of the parameters distance and speed is not constant in this model.

According to one embodiment, this model includes either an acceleration of the object or a temporal change of the distance due to a finite speed of the object.

According to one embodiment, the evaluation device is configured to determine at least one parameter characteristic of the time profile of the difference frequency.

According to one embodiment, the evaluation device is configured to determine the time profile of the difference frequency over a predetermined time interval. According to one embodiment, this time interval comprises two sub-intervals with mutually opposite slope of the temporal frequency response.

According to one embodiment, the device further comprises a frequency measuring device for measuring the frequency of the signal emitted by the transmitter.

According to one embodiment, this frequency measuring device has an interferometer.

According to one embodiment, the time dependence of the frequency of the transmitter signal over a predetermined time interval with monotonous time dependency of this frequency deviates from a time-linear course by at least 0.1%, in particular by at least 0.5%, more particularly by at least 1%.

This criterion is to be understood as meaning that the time derivative of the function describing the time dependency of the frequency of the transmitter signal in the predetermined time interval from the mean time derivative over this time interval by at least 0.1%, in particular by at least 0.5%, more particularly by at least 1% deviates; this applies for at least 10% of this time interval.

With this criterion, the invention differs in particular further from the prior art cited at the outset with reference to FIG. 5 and FIGS. 6a-6d, since in this prior art deviations of the temporal frequency characteristic from linearity are already in the same range Deviations of the time dependency of the frequency of the transmitter signal from a time-linear course with an aspired high accuracy of the distance measurement would not be acceptable in the state of the art on the order of 0.1%. According to one embodiment, the time dependency of the frequency of the transmitter signal deviates over a predetermined time interval with monotonous time dependency of this frequency from a time-linear course by a maximum of 25%, in particular a maximum of 20%, further in particular a maximum of 10%. If the time derivative of this frequency becomes very large, the difference frequency to be measured at a given distance from the object may become so great that it can no longer be resolved by the detector. If the time derivative becomes very small, the difference frequency to be measured decreases for a given distance to the object, which can reduce the resolution of the measurement. For each special application there is therefore an optimum value (best compromise) for the time derivation of the frequency. An approximate (but not necessarily exact) linear progression of the frequency allows a derivative of the frequency, which is approximately equal to this ideal value, to be obtained at all times.

The invention further relates to a method for determining a distance of a moving object, the method comprising the following steps:

- emitting an optical transmitter signal, the transmitter signal having a time-varying frequency;

Detecting a superimposed partial signal generated by decomposing this transmitter signal with a detector, wherein a first partial signal of these partial signals impinges on the detector without prior reflection on the object and wherein a second partial signal of these partial signals after Reflection on the object on the Detector hits; and

- Self-consistent determination of the time course of distance d (t) and speed v (t) of the object from the time course of the difference frequency (Aco (t)) between the frequency of the second partial signal and the frequency of the first partial signal and the time profile of the frequency (co _s (t)) of the transmitter signal.

For objects measured in the context of the invention with regard to the distance, it can only be described by way of example (and without the invention being would be limited) to robot components such as robot arms or also in the traffic or automotive sector relevant objects (eg foreign vehicles) act.

Further embodiments of the invention are described in the description and the dependent claims.

The invention will be explained in more detail with reference to embodiments shown in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Show it:

Figure 1 is a schematic representation for explaining the possible

Structure of a device according to the invention;

FIG. 2 shows a diagram for explaining the mode of operation of a device according to the invention in an exemplary embodiment;

FIG. 3-4 show schematic representations of different exemplary embodiments for determining an instantaneous frequency used for distance determination in the present invention;

Figure 5 is a schematic representation for explaining the structure of a conventional device for distance measurement; and

6a-d schematic representation for explaining further aspects of the conventional device of Figure 5. DETAILED DESCRIPTION PREFERRED

EMBODIMENTS

In the following, the structure and mode of operation of a device according to the invention for determining a distance of a moving object in an exemplary embodiment will be described with reference to the schematic illustrations in FIG. 1 and FIGS. 2a-2b.

According to FIG. 1, a measuring device 100 according to the invention has a transmitter 110 for emitting an optical transmitter signal 111 with a time-varying frequency, insofar as it is initially analogous to the conventional concept already described with reference to FIG. 5. Exemplary optical transmitters which can serve as transmitters 110 in the measuring device are MEMS-VCSEL, DBR (Distributed Bragg Reflector) laser, SGDBR (Segmented Grating Distributed Bragg Reflector) and WGMR (SGDBR). stabilized / WGMR tunable lasers (WGMR = Whispering Gallery Mode Resonator).

The optical transmitter signal 111 may be e.g. Have wavelengths in the visible range or in the infrared range. According to FIG. 1, this transmitter signal 111 is likewise split analogously to the structure of FIG. 5 via a partially transmissive mirror 115, with the result that two partial signals 121, 122 resulting from decomposition of the transmitter signal 111 arrive at a detector 150. Of these partial signals 121, 122, the first partial signal 121 reaches the detector 150 without prior reflection at an object 105 to be measured with respect to its distance, whereas the second partial signal 122, after reflection at the object 105, reaches the detector 150 via an optical circulator 130.

In the exemplary embodiment, the detector 150 has a suitable sensor, for example in the form of a photodiode, and converts the optical heterodyne signal generated by superposing the partial signals 121, 122 into an electrical one Signal around.

According to FIG. 1, another semitransparent mirror 112 is used to decouple a part of the first sub-signal 121, this part being guided via an interferometer 140 for the purpose of measuring the time-frequency curve of the transmitter signal 111.

The device according to the invention differs from the conventional concept according to FIG. 5, in particular with regard to the mode of operation of an evaluation device 160 present for evaluating the detector signal generated by the detector 150:

In contrast to the conventional concept according to FIG. 5, in which the electrical signal of the detector 150 (generated by conversion of the optical signal generated by superposition of the partial signals 121, 122) is generated

Superposition signal in an electrical signal) determines a single difference frequency and is used for distance measurement, according to the invention from the time course l (t) of the electrical signal, a determination of the time course of the so-called instantaneous frequency. To various approaches and algorithms for the determination of the so-called instantaneous frequency is on the publication Boualem Boashash: "Estimating and interpreting the instantaneous frequency of a signal" I - Fundamentals. II - Algorithms and Applications, Proceedings of the IEEE 80, 520-538, 540-568 (1992).

The detector signal generated by the detector 150 finally gives the time profile of the difference frequency Aco (t) between the received frequency co _e (t) of the second partial signal 122 and the frequency co _s (t) of the first partial signal 121 and the transmitter signal 111, the latter due to the measurement with the interferometer 140 (or already due to accurate knowledge of the transmitter 110) is known.

Aw (ί) = w _e (ί) - wcί) (1) Sensible maximum difference frequencies can range from 100 MHz to 1 GHz, for less demanding applications including.

From equation (1), the time profile d (t) of the searched distance of the object 105 can be determined by parameterizing d (t) suitably eg according to the following equation (2):

In the following, a possible embodiment for determining the distance progression d (t) from the time profile of the difference frequency Dw is described. For easier understanding, the paths within the measuring device 100 (for example, from the beam splitter 115 via the beam splitter 112 to the detector 150 or from the circulator 130 to the detector 150) are ignored. Since these are constant offsets, they can easily be taken into account when realizing the method described here.

The distance d (t) of the object 105 at a certain point in time t results from the fact that a signal 122a was sent from the measuring device 100 at an earlier time ti and reaches the object 105 at the time t. The reflected signal then reaches 122b at a later time t ₂ again, the measuring device 100. The relationship of the various times is given by the speed of light c of the medium between the measuring device 100 and object 105:

Due to the reflection on the moving object 105, the frequency Wi22 _a (t) of the signal 122a differs from the frequency (0i _22b (t) of the signal 122b.) This effect is called the Doppler effect eg in the publication Aleksandar Gjurchinovski: "Reflection from a moving mirror - simple derivation using the photon mode! of light ", European Journal of Physics 34, L1 -4. The result is:

The velocity v (t) in equation (4) is only the component of the velocity of the object 105 along the direction between the measuring device 100 and the object 105. This definition is advantageous in that the distance d (t) to be determined is along is measured in this direction and then v (t) is the time derivative of d (t).

The frequency m (ti) of the transmitted signal is known, either because of a measurement using the interferometer 140 or because the characteristics of the transmitter 110 are sufficiently well known. This frequency is identical to the frequency wi ₂₂₃ (ίi).

The signal 122 enters the detector 150 at the time t ₂ . This has the frequency c0i _22b (t ₂ ), which depends in particular on the distance d (t), velocity v (t) and emitted frequency m (ti) according to the formulas given above. In the formulas, all three points in time h, t and t ₂ occur, which is why a determination of c0i _22b (t ₂ ) in the general case is not possible in a closed form, but only iteratively.

On the other hand, the signal 121 enters the detector at time t ₂ . This has the frequency u) m (t ₂ ). Flieraus results directly in the difference frequency Dw (ί ₂ ) = (0i _22b (t ₂ ) - UJ _I 1 1 (t ₂ ) .This system of equations makes it possible to derive Dw (ί ₂ ) from the measured time course and the knowledge of Frequency course m (ti) self-consistent to determine the time course of distance d (t) and speed v (t) of the object 105. The determination of d (t) and v (t) and the further processing can be simplified by d (t) and v (t) are parameterized appropriately, eg as a polynomial. According to FIG. 2 a, the measurement according to the invention of the time profile of the difference frequency can take place over predetermined time intervals or in a sequence of sections of signals, wherein in each section the temporal progression of the transmitter frequency is monotonous. Typical measuring period durations, that is to say time ranges with a monotonous change in the wavelength of the emitted radiation, can be in the range from 1 microsecond (ps) to 10 milliseconds (ms), for example. In this case, the corresponding time profile of the distance d ^{k \ t) can in principle be determined for every k th section from the difference frequency measured during this section. Thus, according to the invention, it is taken into account that the difference frequency can already change over time within a measuring section.

FIG. 2b shows a diagram for explaining a further possible embodiment, in which each time interval comprises two subintervals with mutually opposite slope of the temporal frequency profile. As a result, the sensitivity of the distance determination according to the invention with respect to measurement errors can be reduced.

The time derivation of the frequency of the transmitter signal 111 emitted by the transmitter 110 may be within the scope of the invention, for example in the range of 10 ¹⁵ Flz / s to 10 ¹⁸ Hz / s.

In the following, further exemplary embodiments for the at least approximate determination of the instantaneous frequency used in the present invention for determining the distance will be described with reference to the schematic illustrations in FIGS. 3 and 4.

The exemplary embodiment according to FIG. 3 is based on the consideration that an instantaneous frequency can be approximated by determining in each case a mean frequency over a multiplicity of (overlapping or non-overlapping) time windows. One possible approach for determining the respective mean frequencies over a respective time window can be based on the implementation of fast Fourier transforms (FFT). FIG. 3 shows the calculation both for the time profile of the differential frequency and for the frequency of the transmitter signal.

In FIG. 3, "311" denotes the intensity of the detector signal generated by the detector 150. For this purpose, in a function block 310, the energy content is determined over different frequency intervals, and in a function block 320 of these frequency intervals, the frequency interval with the highest energy content is searched out. Then again the energy as a function of the frequency is determined only for frequencies within the frequency interval in a function block 330. This process can be repeated until the width of an interval is smaller than the necessary measurement accuracy of the frequency.

3, the calculation for the temporal progression of the frequency of the transmitter signal, in which case the detector signal measured at the interferometer 140 is used as the input signal 351 and wherein the functional blocks corresponding to the function blocks 310, 320 and 330 with 350, 360 or 370 are designated.

Another method known per se for calculating instantaneous frequency is based, as explained below with reference to FIG. 4, on a determination of the instantaneous frequency from the time derivative of the phase. Also in Fig. 4, the calculation is shown both for the time course of the difference frequency and for the frequency of the transmitter signal.

In this case, 411 or 451 is advantageously used instead of the real signal 411 or 451 with a complex signal 415 or 455, which consists only of the positive frequency component. This signal can be determined in a manner known per se via a Flickertransformation (function block 410 or 450) from the real signal 411 or 451. For conversion into hardware, it may be helpful to attribute the Hilbert transform according to function block 410 or 450 to fast Fourier transforms (FFT). The Hilbert transform provides a complex signal (with phase and amplitude) as a function of time.

In the functional block 420 or 460, the complex signal 415 or 425 is decomposed into phases 425 or 465 and amplitudes, wherein the amplitudes are not used in the further. The phase derivative of the phase calculated in function blocks 430 and 470 is respectively smoothed with respect to possible measurement errors or existing signal noise via a low-pass filter 440 or 480 and then supplies the desired output signal 445 or 485 for the time profile of the instantaneous frequency or the frequency the transmitter signal.

Achievable accuracies for distances measured within the scope of the invention are better than 10 micrometers. This also applies in particular if the object in question moves at speeds of a few 100 m / s and accelerations of a few multiples of the gravitational acceleration. Sensible maximum measuring distances can be in the range of (10-50) m, for example.

While the invention has been described in terms of specific embodiments, numerous variations and alternative embodiments, e.g. by combination and / or exchange of features of individual embodiments. Accordingly, it will be understood by those skilled in the art that such variations and alternative embodiments are intended to be embraced by the present invention and that the scope of the invention is limited only in terms of the appended claims and their equivalents.

Claims

claims

1. A device for determining a distance of a moving object, with

A transmitter (110) for transmitting an optical transmitter signal (111), the transmitter signal having a time-varying frequency (co _s (t));

A detector (150) for generating a detector signal from a superimposition of a first partial signal (121) and a second partial signal (122), the first partial signal (121) and the second partial signal (122) being broken down by the transmitter signal (111) the first partial signal (121) passes without prior reflection at the object (105) to the detector (150) and wherein the second partial signal (122) reaches the detector (150) after reflection at the object (105), the detector signal for the difference frequency (Dw) between the frequency of the second partial signal (122) and the frequency of the first partial signal (121) is characteristic; and

• an evaluation device (160), which is designed to be consistent with the time course of the difference frequency (Aco (t)) and the time course of the frequency (co _s (t)) of the transmitter signal consistently the time course of distance d (t ) and velocity v (t) of the object.

2. Device according to claim 1, characterized in that the evaluation device (160) is designed to determine the time profile of distance d (t) and speed v (t) by the parameters of a model for distance d (t ) and velocity v (t).

3. A device according to claim 2, characterized in that at least one of the sizes distance and speed is not constant in this model.

4. Apparatus according to claim 2 or 3, characterized in that this model includes either an acceleration of the object or a temporal change of the distance due to a finite speed of the object.

5. Device according to one of the preceding claims, character- ized in that the evaluation device (160) is configured to determine at least one characteristic of the time course of the difference frequency (Aco (t)) parameter.

6. Device according to one of the preceding claims, character- ized in that the evaluation device is configured to determine the time course of the difference frequency (Aco (t)) over a predetermined time interval.

7. The device according to claim 6, characterized in that this time interval comprises two sub-intervals with mutually opposite slope of the temporal frequency response.

8. Device according to one of the preceding claims, character- ized in that it further comprises a frequency measuring device for measuring the frequency of the transmitter signal (111).

9. Apparatus according to claim 8, characterized in that said frequency measuring device comprises an interferometer (140).

10. Device according to one of the preceding claims, character- ized in that the time dependence of the frequency of the transmitter signal (111) over a predetermined time interval with monotonous time dependence of this frequency of a time linear course by at least 0.1%, in particular by at least 0.5%, on in particular by at least 1%, deviates.

11. Device according to one of the preceding claims, character- ized in that the time dependence of the frequency of the transmitter signal (111) over a predetermined time interval with monotonous time dependence of this frequency of a time-linear course by a maximum of 25%, in particular a maximum of 20%, more particularly by a maximum of 10%, deviates.

12. A method of determining a distance of a moving object, the method comprising the steps of:

a) emitting an optical transmitter signal (111), wherein the transmitter signal (111) has a time-varying frequency;

b) detecting overlapping sub-signals generated by decomposing this transmitter signal (111) with a detector (150), a first sub-signal (121) of these sub-signals impinging on the detector (150) without prior reflection at the object (105) and wherein a second sub-signal (122) of these sub-signals impinges upon the detector (150) upon reflection at the object (105); and

c) self-consistent determination of the time course of distance d (t) and speed v (t) of the object from the time course of the difference frequency (Aco (t)) between the frequency of the second partial signal (122) and the frequency of first sub-signal (121) and the time course of the frequency (co _s (t)) of the transmitter signal.