DE102008025825B4 - Method for highly accurate distance measurement - Google Patents

Abstract

Method for measuring a distance to an object comprising:
Providing a pulse train of individual pulses, wherein the pulse train is provided by a radiation source,
Aligning the pulse sequence with the object whose distance is to be measured,
Detecting the radiation reflected by the object with recording means in successive exposure time windows,
wherein the distance to the object is measured by determining the transit time of the radiation between the radiation source and the recording means,
characterized in that the individual pulses are provided as a wavelength and time-coded pulse sequence, wherein the radiation reflected from the object is detected by the recording means wavelength and time discrete, and an assignment of individual pulses of the pulse sequence to the respective exposure time windows of the recording means.

Description

The present invention relates to a method for measuring a distance to an object according to the preamble of patent claim 1, as it is known from DE 37 32 347 C1 is known.

The measurement of distances between a measuring arrangement and an object is based on the principle of measuring the transit time of radiation reflected by the measuring object. The radiation may comprise an electromagnetic wave having a wavelength which is in the visible range. Also known are radiation sources, which are designed as laser beam sources and emit wavelengths that are in the near or direct infrared range. Applications find such transit time measurements in laser rangefinders, in LIDAR systems, a radar very related method for distance and speed measurement, with laser pistols of the traffic police are based on the principle of transit time measurement. The resulting accuracy of such measuring systems is limited by the limits of the technical feasibility of very short time measurements. In particular, the detection of the reflected radiation in the form of individual pulses has a time-finite resolution, so that a pulse-to-pulse measurement can not take place in arbitrarily short time intervals. In order to increase the accuracy beyond that, it is known to gain additional information in addition to the pure running time in order to further increase the accuracy of the determination of the transit time and, as a result, the determination of the distance. This happens, for example, via the determination of interferences or the measurement of phase angles of the emitted radiation. However, such a modulation of the individual pulses are also limited, so that it is desirable to further increase the accuracy of the measurement of distances.

US 4,518,256 A discloses, for example, a distance measurement based on a coincidence measurement with a multi-frequency laser. US 7,068,424 B1 discloses a device for generating a pulse train. DE 3732347 C1 discloses a method of creating a relief-like image of an object.

It is therefore the object of the present invention to provide a method for measuring distances to an object, which allows a further increase in the measurement accuracy for determining the distance to an object.

This object is achieved on the basis of a method for measuring a distance to an object according to the preamble of claim 1 in conjunction with the characterizing features. Advantageous developments of the invention are specified in the dependent claims.

The invention includes the technical teaching that means are provided with which the individual pulses are provided as wavelength and time-coded pulse sequence and sent to the object, wherein the reflected radiation from the object are detected by the recording means wavelength and time discrete.

The wavelength coding according to the invention is transmitted instead of phase or interference information with the individual pulses emitted by the radiation source. First, individual pulses are emitted by the radiation source, which are converted by the means according to the invention into a series of pulses which differ from each other both in terms of their wavelength and in their time sequence. The time coding of the individual pulses causes a splitting into a plurality of partial pulses, which together form a pulse sequence. For example, the pulse train may consist of three pulses, each having different colors. The means for transferring the individual pulses into a pulse sequence are designed in such a way that the distance of the color pulses forming the pulse sequence is known. Furthermore, the wavelength of the color pulses is known, so that the emission of the individual color pulses are known in time and resumed by the recording means, wherein the time resolution of the recording means can differentiate the color pulses of different wavelengths.

An advantageous embodiment of the pulse train includes a series of color pulses with a respective same time interval, wherein the individual pulse duration corresponds to the duration of the pulse train and is determined by the number of color pulses of the pulse train. The pulse duration .DELTA.T describes the sum of the respective time intervals .DELTA.t, which lie between the color pulses. The number of color pulses is marked with K, so that the following applies: ΔT = K · Δt.

A further embodiment of the method according to the invention has a recording means, which is formed from at least one camera, which specifies a sequence of exposure times. The exposure times can be described as T _N-1 , T _N , T _{N + 1} , etc., so that individual exposure time windows are formed. If the camera detects a pulse sequence, then the color pulses of the pulse sequence can be classified into the individual exposure time windows T _N-1 , T _N and T _{N + 1} , etc., wherein the different color assignment of the color pulses is described in more detail below.

The increase in the measurement accuracy is based on the principle of assigning the pulse sequences the exposure time windows, the assignment is wavelength dependent. Thus, individual color pulses can be assigned to respective exposure time windows. The shutter speed windows may also be referred to as time slots or so-called gates. The recording of the reflected light from the object is usually time-integrated in the gates, wherein the choice of the time length of such a gate affects the accuracy of the transit time measurement. Upon receipt of a reflected light pulse within a gate, the signal can be assigned only a single time T _LAUF . Any reduction of the exposure time to reduce the inaccuracy of the transit time measurement can not be done here, since at the same time the number of integrated photons is reduced, the number of photons integrated within the camera also limits the accuracy of the transit time measurement. If the time duration of an exposure time window was shorter, the number of photons integrated within the exposure time window is not large enough or the technical implementation of even shorter exposure time windows is not feasible. Thus, while shorter pulse train durations ΔT can be provided by the radiation source, the position of the trapped single pulse within the gate can not be determined. Consequently, the time constant of the transit time measurement is limited by the duration of the exposure time window. However, according to the invention, if the individual pulse is converted into wavelength- and time-coded color pulses and these are directed onto the object as a pulse sequence, the sequence of color pulses on the object reflects and can be recorded by the camera. If all the individual color pulses arrive within a gate, then the statement can be made that the single pulse lies centrally within the gate.

If a color is detected in an upstream gate T _N-1 and two remaining colors in the gate T _{N ,} it can be said that the single pulse within the gate N is adjacent to the gate T _N-1 . It is also possible that a color pulse in the gate T _{N + 1 is} detected, and the remaining color pulses in the gate T _N. Consequently, the single pulse in the gate N is adjacent to the gate T _{N + 1} .

As a result, a correction term δ can be formed, which is determined from the position of the pulse sequence within the exposure time windows (T _N-1 , T _N , T _{N + 1} ) by the transit time T _{LAUF of} the individual pulse first of a transit time for an exposure time window T _N is assigned, and following the correction over the correction term δ is made, so that: T _LAUF = T _N + δ.

The correction term δ can assume the value -Δt if one of the color pulses is detected in an exposure time window which is located in front of the exposure time window. By contrast, the correction term δ can assume the value + Δt if one of the color pulses is detected in an exposure time window located after the exposure time window. Only if all color pulses are detected in the same exposure time window, the correction term δ assumes the value 0.

Advantageously, the pulse train consists of three individual, successive color pulses, the wavelengths of the color pulses corresponding to the colors blue, green and red. A further increase in the accuracy of the measurement of the distance to an object can be made possible with a pulse sequence which is formed by more than three color pulses. As a result, the correction term δ assumes more than three different values when one, two or more color pulses are detected in an upstream or downstream gate.

The invention also relates to the use of a device for measuring the distance to an object with a radiation source for providing a sequence of individual pulses for irradiating the object. In this case, recording means are provided for detecting the individual pulses reflected by the object, wherein the measurement of the distance to the object takes place via the determination of the transit time of the individual pulses between the radiation source and the recording means. In this case, means are provided with which the individual pulses as wavelength and time-coded pulse sequence can be transferred and wherein the recording means detects the reflected radiation from the object wavelength and time discrete.

The means may be formed as a beam splitter which divides the individual pulses into a plurality of individual beams which pass through different path lengths.

Due to the different path lengths, the individual pulses can be converted into a pulse sequence, which have the same distances from each other by a suitable choice of the extension of the path lengths. The individual pulses emitted by the radiation source can be in the picosecond range or in the femtosecond range, so that the required difference in the path length of the individual pulses within the pulse sequence can be realized technically in a limited installation space.

Within the different path lengths, optically non-linear materials are arranged, through which the individual beams pass, in order to achieve a wavelength transformation of the individual beams to different wavelengths and to provide a pulse sequence of color and time-discrete color pulses. The optically non-linear materials can cause a wavelength transformation into the colors blue, green or red, wherein one of the colors of the base color of the individual pulse can correspond to the radiation source.

Further, measures improving the invention will be described in more detail below together with the description of a preferred embodiment of the invention with reference to FIGS.

It shows:

1 a schematic view of a transit time measurement using single pulses according to the prior art, wherein a sequence of multiple exposure time windows is shown on a timeline;

2 an embodiment of a transit time measurement using color-coded pulse series according to the present invention and

3 the schematic view of a transit time measurement using three individual pulses of a pulse series of different color, wherein three cases of detection of the color pulses are shown within adjacent gates.

In 1 a transit time measurement is shown using single pulses E, which corresponds to the prior art. At time T = 0, the single pulse E is emitted by the radiation source. The individual pulse formed as a light pulse reflects on the object or is backscattered on it. The backscattered light is captured with a camera. The recording of the backscattered light is time-integrated in exposure time windows, which are represented by T _N-1 , T _N and T _{N + 1} . As shown, the single pulse E is detected within the exposure time window T _N. It is not possible to give a statement about the position of the individual pulse within the duration of the exposure time window T _N. The following is in the 2 and 3 an embodiment of the present invention is given, with which the determination of the position of the individual pulse E within the exposure time window T _{N is} possible.

2 shows an example of a transit time measurement using color pulses, wherein three color pulses are provided, which represent a pulse train and are formed from the single pulse. The first color pulse corresponds to a red color and is marked with R, the central, central color pulse G having the color green and the third color pulse B having a wavelength corresponding to the color blue. The color pulses R, G and B are emitted at the time T = 0 from the radiation source. The exact time T = 0 corresponds to the green color pulse, so that the red color pulse is emitted at time T-Δt, and the blue color pulse is emitted at time T + Δt. Thus, the number K of color pulses corresponds to three color pulses. The transit time t is in turn divided into exposure time windows T _N-1 , T _N and T _{N + 1} , wherein the blue color pulse B and the green color pulse G in the exposure time window T _N and the red color pulse R in the exposure time window T _{N + 1 is} detected. Thus, a determination of the position of the average, green color pulse G within the exposure time window T _{N can be} determined, since the time interval .DELTA.t between the green color pulse G and the red color pulse R is known. Since the red color pulse R is detected in the exposure time window T _{N + 1} , the statement can be made that the green color pulse G must have a distance from the boundary to the subsequent exposure time window T _{N + 1} which corresponds at most to the time interval Δt. Consequently, the accuracy of the transit time measurement with the time interval .DELTA.t of the individual color pulses R, G and B can be specified.

3 again shows the transit time measurement using the color pulses R, G and B. Here, three cases are listed, which are described below.

Consider first the average exposure time window T _N. The duration of the exposure time window corresponds to the pulse duration .DELTA.T and is calculated to .DELTA.T = K · .DELTA.t, where .DELTA.t represents the time interval of the color pulses to each other.

In the first case I, the camera detects the blue color pulse in the exposure time window T _N-1 . The second, central green color pulse G is shown in bold and is located in the exposure time window T _N , wherein the third, red color pulse R is also detected in the exposure time window T _N. Due to the known time interval .DELTA.t between the individual color pulses R, G and B, the green color pulse G is adjacent to the edge of the exposure time window T _N , which merges into the exposure time window T _N-1 .

In the second case II, all three color pulses R, G and B are detected within the exposure time window T _N. In this case too, the temporal position of the mean, green color pulse G can be specified with the accuracy of the time interval Δt, since both the upstream, blue color pulse B and the downstream, red color pulse R within the middle exposure time window T _{N are} detected.

In the third case III, only the first blue color pulse B and the central, green color pulse G are detected within the exposure time window T _N. The red color pulse R is detected in the exposure time window T _{N + 1} , so that the statement can be made that the central, green color pulse G is adjacent to the edge of the exposure time window T _N , which changes to the exposure time window T _{N + 1} .

As a result, the accuracy of the transit time measurement between the emission of a single pulse by the radiation source and the detection of the radiation by the camera after reflection on the object with the time interval At is possible. In the present example, the number K of the color pulses corresponds to three color pulses, with a further increase in the number of color pulses whose total time of the pulse duration .DELTA.T corresponds to the accuracies, further increased.

LIST OF REFERENCE NUMBERS

• B

  blue color pulse

  G

  green color pulse

  R

  red color pulse

  T _N-1

  upstream first exposure time window

  T _N

  middle second exposure time window

  T _{N + 1}

  downstream third exposure time window

  e

  Single pulse

  t

  running time

  K

  Number of pulses in the pulse sequence

  .DELTA.T

  Pulse repetition period

  .delta.t

  temporal distance

  I

  first case

  II

  second case

  III

  third case

Claims (10)

Method for measuring a distance to an object, comprising: providing a pulse sequence of individual pulses, the pulse sequence being provided by a radiation source, aligning the pulse sequence with the object whose distance is to be measured, detecting the radiation reflected by the object with acquisition means in successive exposure time windows, wherein the measurement of the distance to the object via the determination of the transit time of the radiation between the radiation source and recording means, characterized in that the individual pulses are provided as a wavelength and time-coded pulse sequence, wherein the radiation reflected from the object is detected by the recording means wavelength and time discrete , and an assignment of individual pulses of the pulse sequence to the respective exposure time windows of the recording means takes place. A method according to claim 1, characterized in that the wavelength- and time-coded pulse sequence comprises a series of color pulses with a respective same time interval (Δt), wherein the pulse duration duration (.DELTA.T) corresponds to the duration of the pulse train and by the number (K) of the color pulses the pulse sequence is determined to ΔT = K · Δt. A method according to claim 1 or 2, characterized in that the recording means is formed from at least one camera having a sequence of exposure time windows (T _N-1 , T _N , T _{N + 1} ), wherein the detected pulse sequences individual exposure time windows (T _{N -1} , T _N , T _{N + 1} ). A method according to claim 3, characterized in that the assignment of the pulse sequences to the exposure time windows (T _N-1 , T _N , T _{N + 1} ) is wavelength-dependent, so that individual color pulses respective exposure time windows (T _N-1 , T _N , T _{N + 1} ). Method according to one of Claims 2 to 4, characterized in that the pulse sequence is formed from three color pulses (B, G, R), the wavelengths of the color pulses corresponding to the colors blue (B), green (G) and red (R) , Method according to one of claims 3 to 5, characterized in that a correction term (δ) is provided which is determined from the position of the pulse train within the exposure time window (T _N-1 , T _N , T _{N + 1} ) by the transit time T _{LAUF of} the individual pulse is first assigned a running time for an exposure time window (T _N ), and following the correction is made via the correction term (δ), where: T _LAUF = T _N + δ. A method according to claim 6, characterized in that the correction term (δ) assumes the value -n · Δt when the number n of one of the color pulses is detected in an exposure time window (T _N-1 ) preceding the exposure time window (T _N ), wherein the correction term (δ) assumes the value + n · at, when the number n of the color pulses in a the exposure time window (T _N) downstream exposure time window (T _{N + 1)} is detected and wherein the correction term (δ) assumes the value 0 when all Color pulses are detected in the exposure time window (T _N ) itself. Use of a device for performing a method for measuring the distance to an object according to one of the preceding claims, wherein the device comprises a radiation source for providing a sequence of individual pulses for irradiating the object, wherein a recording means for detecting the individual pulses reflected by the object is provided, and the distance to the object is measured by determining the propagation time of the individual pulses between the radiation source and the recording means, wherein means are provided with which the individual pulses can be converted as a wavelength and time-coded pulse sequence and wherein Recording means detects the reflected radiation from the object wavelength and time discrete. Use of a device according to claim 8, wherein the means comprise beam splitters which divide the individual pulses into a plurality of individual beams which travel through different path lengths. Use of a device according to claim 8 or 9, wherein optically non-linear materials are provided, through which the individual beams pass, in order to achieve a wavelength transformation of the individual beams to different wavelengths and to provide a pulse sequence of color and time-discrete color pulses.

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