DE10103861A1 - Object space scanning and recording method based on optical pulse transit time, employs pulse burst packets for each raster element and computer to calculate range - Google Patents

Abstract

Pulses transmitted are assembled into packets or bursts and a burst is allocated to each raster element. In an electronic computer employing defined algorithms, pulses of each burst received, are related to their associated start pulses. Time of transit is determined to calculate the range of each raster element.

Description

The invention relates to a method for recording an object space with a opto-electronic rangefinder using the pulse transit time method with a sen deeinrichtung for emitting optical, esp. Laser pulses and an emp capture device for receiving optical signals, esp. Laser radiation, the is reflected by objects located in the target area, furthermore with a scanning device for especially continuous, deflection of the optical axes of transmit and emp catching device in preferably two orthogonal directions through which the object dream can be scanned like a grid, furthermore with an evaluation device which is based on the running time of the emitted optical pulses distance values determined, where appropriate, an image output system, e.g. B. a monitor or the like is provided, on which picture elements again are given, the image content of which is preferably a function of the respective grid element assigned distance value and their screen coordinates the Correspond to the coordinates of the raster elements in the object space.

Since in known methods of the type mentioned above, or in devices for implementation of the method described, a distance measurement value for each raster element Is available compared to modern laser rangefinders (range finders) the ranges, sensitivities and accuracy that can be achieved with these systems limited. It is an object of the invention to achieve ranges even with laser scanner systems, To achieve sensitivities and accuracies with those of modern Rangefin which are comparable.

This goal is achieved according to the invention in that the emitted pulses in are known to be combined into pulse packets, so-called "bursts", wherein at least one burst is assigned to each raster element and in an electronic one Computer using defined algorithms from the received signals of the pulses each Bursts and from the runtime of these receive pulses in relation to an associated star timpuls a distance value assigned to a raster element is calculated, whereby before preferably the clock rate of the emitted optical pulses within a burst min least 50 kHz, advantageously at least 100 kHz.  

It is advantageous for everyone to have a multi-channel or multi-target capable time interval measurement individual pulse of a burst a transit time interval is determined, which in its entirety define a table of runtime intervals, an electronic computer using de algorithms from these runtime intervals of the pulses of each burst with respect to an associated start pulse at least one Ent assigned to a raster element distance value determined.

An alternative method is based on digital signal processing, in which in one electronic computers from time-discrete, digitized and added reception signals of the pulses of each burst a receive pulse assigned to a raster element determined and from the runtime of this reconstructed receive pulse in relation to a a distance value is calculated for the associated start pulse.

A burst advantageously consists of 2-50, preferably 10, pulses. In an advantageous way The invention is at least partially designed during the pulse gaps between two Bursts in the electronic computer using defined algorithms from the Emp start signals of the pulses of the previous burst assigned to a raster element ter received pulse determined and based on the runtime of this received pulse a distance value is calculated for the associated start pulse and stored together in a memory men with the coordinates of the raster or image element.

On the one hand, the range is sensitive by a large number of pulses per burst speed and also accuracy increased especially when the amplitudes of the Emp Catch signals are small and the signal / noise ratio is unfavorable, on the other ren page is by a large number of pulses per burst the time to sample one given object space extended accordingly. In an advantageous development of the Invention is the pulse by an adaptive determination of the number of pulses per burst number and thus the measuring time the respective conditions optimal during a measurement customized.

In an advantageous embodiment of the invention, the Emp signals of the bursts of a scan of the object space, the signal amplitudes or the Si Signal / noise ratios of the measured values assigned to each raster element are determined and and the number of pulses per burst in the subsequent scans as a function of these Values changed in the sense that with large signal amplitudes or signal / noise ratio nissen of a raster element the number of pulses per burst and thus the duration of one  Measuring cycle reduced and with small signal amplitudes or signal / noise ratios The number of pulses per burst is increased, the scan rate of the scanning device in Ab dependence on the number of pulses per burst is variable. This optimization of the number of impulses per burst and thus hand in hand the change in the scan rate can be customized for the one Individual grid elements are made, but it is also possible to adjust this for one size re number of raster elements, e.g. B. for entire lines or for the entire scanning field or for the entire scan cycle.

In the adaptive choice of the number of transmission pulses per burst, it is expedient that the Bursts of a first scan cycle have an above-average number of pulses, so that even under unfavorable conditions a distance measurement for each individual Ra ster element is determinable in order to derive the Pa for the following scanning processes to be able to determine the parameters of the measurement in a suitable manner.

When using digital signal processing methods, it can be advantageous for each the raster element window of memory addresses which correspond to distance values, to define within which samples of the received signals for evaluation be used.

There are advantages in digital signal processing even if there is a relative phase shift of between the transmit pulse repetition frequency and the sample frequency

d = L / n

in which formula n is an integer, preferably 5, d is the phase ver shift and L mean the period of the sample frequency, so that samples of the digitized received signals are intermeshed against each other with a period of n and the effective sample frequency appears to be increased by a factor of n, in absolute terms Values, the transmission pulses are preferably shifted in their phase position.

Further features of the invention result from the following description of some Exemplary embodiments and with reference to the drawing:

Fig. 1 shows schematically a scanning device according to the invention; the

Figures 2 and 3 illustrate details of this device;

Fig. 4 shows the block diagram of a variant of the invention with digital signal processing; the

Fig. 5 and 6 show diagrams of the associated transmit and receive pulses; and the

, FIG. 7 is a block diagram illustrating an alternative embodiment with analog signal processing.

The device according to FIG. 1 comprises an active optical channel 1 , which essentially consists of a laser rangefinder, as described, for example, in Austrian Patent No. 405 105. The active channel has a transmit diode 2 and a receive diode 3 . The transmission diode 2 emits sequences of extremely short laser pulses that are reflected on objects in the recording space. The radiation reflected in the device's direction is picked up by the receiving diode 3 . In the laser rangefinder or in the active channel 1 of the device, the distance to the respective object is determined from the transit time of the laser pulses. The sensor de diode 2 and the receiving diode 3 are each preceded by an optical scanning system, with the aid of which the object space is scanned. In the interest of an overview that is as clear as possible, only the scanning device for the receiving diode 3 is shown: the beam 4 impinging on the diode 3 is deflected, for example, by an oscillating mirror 5 which is driven by an actuator 6 . A connected to the actuator 6 Winkelge 7 gives a signal α, which depends on the position of the mirror 4 . The beam 8 deflected by the oscillating mirror 4 falls on the mirror surface of a mirror prism 9 which is driven by a motor 10 at high speed. The respective Winkelpositi on of the mirror prism 9 is measured by a sensor 11 ; the corresponding signals ϕ are fed to the signal processing stage 12 . By moving the mirror 4 and the prism 9 , the object space is scanned in a line shape by the beam 13 .

The scanning device for the transmitting diode is constructed analogously and scans the object space in both directions synchronously and with the same phase position, so that the beam 13 and the corresponding beam of the transmitting device run essentially parallel. The movable optical elements 4 and 9 advantageously have the same drive elements 6 and 10 for the transmit and receive channel. It may be appropriate to axially extend the mirror 4 and the prism 9 so that the transmit and receive channels can use the same deflecting lines.

In another embodiment of the invention, the laser rangefinder is included a transmitting and receiving optics and a rotating mirror prism to a rigid Unit summarized, which ver in its entirety for scanning an object space pivots.

The object channel is scanned by the active channel 1 (laser range finder) in combination with the scanning device, a distance value being determined for each spatial direction defined by the values of the angle transmitters 7 and 11 . The corresponding data set consisting of the image coordinates and the associated distance value is fed to the signal processing stage 12 .

A beam splitting prism 14 is arranged in the beam path of the beam 4 between the oscillating mirror 5 and the receiving diode 3 , which prism feeds part of the incident radiation to a second photo diode 15 . Advantageously, the photo diode 15 has a different spectral sensitivity compared to the diode 3 , which is adapted in its spectral sensitivity of the transmit diode 2 . It may be useful to use a diode for longer-wave infrared to achieve better penetration in fog. For other applications, a photo diode for the visible range of the spectrum can be used. Instead of a single photodiode, a photo-diode triple with sensitivities corresponding to the three primary colors can also be used. The spectral sensitivities can be adjusted in a known manner by using color filters or dichroic mirrors. Advantageously, the beam splitter 15 has a dichroic mirror surface 16 through which the radiation from the laser diode 2 can pass unhindered in wesent union, while radiation for which the diode 15 has a maximum sensitivity is optimally reflected.

The signals derived from the photodiode 15 or the respective photoreceiver, in their entirety, write a passive image of the object space, which is therefore illumination-independent of the laser light from the transmitter diode.

The signals are processed in the stage 17 labeled "Passive Channel" and fed to the signal processing stage 12 , in which now for each point defined by the signals from the sensors 7 and 11 not only a distance but also a brightness and possibly a Color information is available. In stage 12 , the data are processed further.

Fig. 2 shows the invention in detail as according to the object field scanned or is scanned. The head 20 of the laser scanner is arranged on a tripod 21 . In contrast to the version shown in FIG. 1, the scanner head 20 comprises a rotating mirror prism for scanning the schematically indicated object field 22 in the direction of a line 23 . The line-by-line scanning of the object field is carried out by pivoting the movable part 20 a of the scanner head relative to the stationary part 20 b fastened to the stand 21 .

The individual grid elements 24 are essentially square with the dimensions of 3 mrad × 3 mrad. In contrast to the prior art, this raster element is scanned not by a single laser pulse but by a large number of pulses, in the present example a total of six pulses, which together result in a so-called burst. The laser diode 2 has a rectangular field which emits the laser radiation. The emitted laser beam therefore does not have a circular cross section, but has a fan-like configuration. In Fig. 3, a screen element 24 is shown, in which 25 the beam-shaped, so-called "foot prints" of the individual laser pulses are shown. In the present example, these have the dimensions 3 mrad × 0.5 mrad. In the simplest case, a received signal for this raster element is determined by averaging the reflected signals of these pulses, and a distance value is then calculated from this and the associated start pulse, which stores the values with the deflection angles α and ϕ or the coordinates of the raster element in a memory becomes.

By replacing a single pulse with a large number of pulses, a so-called ten burst, the signal-to-noise ratio can be improved and the sensitivity and accuracy measurement can be increased. Another advantage of this new process is the we Significantly increased measurement certainty: Suitable algorithms can be used for signal processing so-called "outliers" - that is, measured values that deviate significantly from the mean - hidden or suppressed when calculating the measured value. With the new The procedure is also an increase in range due to a higher "false alarm rate" possible.

In the example above, the object field is scanned at a rate of 10 lines per mi grooves scanned. Due to the relatively large aperture of the measuring beam and the limited Dimension of the mirror prism, can only around 33% of a turn of the mirror prism  mas can be used to deflect the measuring beam. With a line length of 1.4 rad (80 °) this results in a measuring beam speed of 42 rad / sec. With grid elements of 3 mrad × 3 mrad and 6 pulses per raster element or burst, the individual Pulse frequency (within one burst) is therefore 84 kHz.

FIG. 4 shows, in the form of a block diagram, the structure of a laser scanner according to the invention, in which only the systems connected to the deflection units are shown in this diagram. 30 designates a laser transmitter which controls a laser diode 2 , which is preceded by an optics 31 which preferably images the emitter zone of the laser to infinity. In addition to the optical transmission system 31 , a receiver optical system 32 is provided, the optical axis of which is aligned parallel to that of the optical transmission system 31 . A beam splitting prism 33 is provided in the beam path of the receiver optics 32 . The receiver optics 32 , on the one hand, concentrate the radiation, which is generally diffusely reflected by an object or target located in the beam path of the transmission optics, on a receiving diode 3 . An avalanche diode is advantageously used as the receiving diode. Before preferably transmit and receive diodes are matched to one another in their spectral characteristics, the receive diode having its maximum spectral sensitivity in the range in which the transmit diode emits maximally. Since the receiving diode 3 but in addition to the radiation emitted by the transmitting diode and reflected by the target receives a lot of interference radiation in the form of daylight or light from a wide variety of artificial light sources, it can be advantageous to provide the receiving diode with the narrowest possible optical filter, which has its maximum transmission in the spectral band in which the laser emits.

Part of the light emitted from the object space is concentrated on a receiving diode 15 by the beam splitter prism 33 . Preferably, the radiation prism 33 has a dichroic mirror coating, which allows radiation of the wavelength of the laser diode 2 to pass essentially undisturbed, while shorter-wave radiation, in particular visible light, is predominantly reflected on the diode 15 . Instead of a single receiving diode 15 , a diode triple can also be provided, the diodes of which are matched in their spectral sensitivity to the three primary colors. With such a variant, a black and white or color image of the object field can therefore be recorded parallel to the distance image (active channel) via the second, passive channel.

The laser transmitter 30 comprises a pulse generator which drives the laser diode 2 . The laser transmitter emits a sequence of laser pulses, so-called bursts, when it is controlled accordingly by the processor 34 . Such a burst can comprise 5-50 pulses depending on the command given by the processor. As a rule, such a burst will have about 10 pulses. The phase pulses of the individual pulses of a burst can be changed by means of a delay generator 35 which can be controlled by the processor 34 , the phase shift taking place periodically. In a preferred embodiment of the invention, for example, the phase relationships are repeated with a period of 5 pulses.

The signals received by the diode 3 are amplified and processed in an amplifier and analog signal processor stage 36 . The signals processed in this way are digitized in an analog / digital converter 37 with a sample frequency of, for example, 60 MHz. These sampled echo signals are stored in a memory 38 . Due to the phase shift of the transmit pulses of a burst compared to the sample frequency, the sampled echo signals are stored in different memory cells. If, as stated above, the phase shift has a periodicity of 5 pulses, the sampled pulse is added to the previous pulse after 5 pulses. Includes a burst e.g. B. 50 pulses and the digitized pulses with the period 5 "combed", 10 digital values are added to each memory cell and the sample frequency of 60 MHz appears increased by the number of periods of combing, in the present example to 300 MHz , so that the sampling theorem can be adhered to in relation to the reconstructed receive pulse.

The entire device is clocked by a clock generator 52 . The processor 34 and the data memory 38 are connected to one another by a data bus, which is indicated schematically and is designated by 39 . A program memory 40 for the processor 34 is also connected to this data bus 39 , and a data buffer 41 , in which, after a first evaluation by the processor 34, raw data are stored which are read out at the end of the measuring cycle. A distance value for each individual raster element is determined from this raw data using algorithms stored in the program memory.

The signal supplied by the diode (or the diode triple) 15 is amplified in a video processor level 42 and processed further. This video processor is connected via the bus 39 to the processor 34 and the other blocks of the system, in particular to a digital image memory 43 and a video interface 44 . The image coordinates belonging to the individual raster elements are fed into the system by the two deflection electronics units 45 and 46 via the data bus 39 .

The processor 34 determines the amplitude values, the signal / noise ratio etc. from the received signals, determines the number of pulses per burst on the basis of these values and controls the laser transmitter 30 accordingly. At the same time, a corresponding command is sent to the deflection electronics 45 and 46 via the data bus 39 . In unfavorable conditions (low amplitude of the received pulses, low signal / noise ratio, large measurement value dispersion), the number of pulses per burst is increased and the deflection speed of the scanning device is reduced accordingly. This adjustment can be made individually for each individual raster element, for a row of raster elements or for the entire scan cycle.

The operation of the new device or the new measuring method is explained in more detail with reference to FIGS . 5 and 6: The processor 34 is given the command to initiate a measuring cycle via a trigger device, not shown. About the delay generator 35 , the pulse generator of the laser transmitter 30 is controlled so that it with a pulse repetition frequency of z. B. 84 kHz sends out a first burst of, for example, 20 pulses. A pulse gap follows, the duration of which corresponds at least to that of the arithmetic operations to be carried out therein, whereupon the next burst is transmitted. Since the arithmetic operations are generally carried out in parallel with the emission of pulses, the pulse gap can be reduced to 1-3 pulse widths.

FIG. 6 illustrates the individual laser transmission pulses 48 (start pulses) which are emitted by the laser transmitter 30 . The impulses are reflected by various objects located in the beam path of the transmitting optics. A more or less noisy received signal from the amplifier and signal processor stage 36 is fed through the receive diode 3 . In these received signals, the echoes of the sensor deimpulse appear at different times in accordance with the target distance and thus the pulse transit time, generally strongly attenuated and often also distorted in the shape of the curve. The curve shape change can be caused by the geometry of the target, so a gradation or a corresponding inclination of the reflecting surface of the target to the axis of the transmission beam can lead to a pulse broadening. The received signals 49 are sampled in the analog digital converter 37 with a sample frequency of approximately 60 MHz. There is a corresponding phase shift between the pulse repetition frequency of the transmit pulses 48 and the frequency of the sample pulses

d = L / n

in which
L the period of the sample frequency and
n is an integer.

If, for example, n is assumed to be 5 and a stationary target is assumed, the reception signal is sampled by 5 successive transmission pulses so that they are intermeshed with one another. This process is repeated with a period of 5 pulses. The effective sample frequency is thus increased by a factor of n, in the example above by a factor of 5. The sampled and intermeshed echo signals are stored in the cells of the data memory 38 . For a burst of 20 transmission pulses and a combing corresponding to a factor of 5, 4 sample values are added up in each cell. Since the noise signals occur statistically, who suppresses them in a known manner when adding up the digital signals compared to the systematic table useful signals, so that there is a significant improvement in the signal / noise ratio.

Already during the transmission of the individual pulses of a burst and then in the pulse gaps between two bursts, the processor 34 uses the digital values stored in the memory 38 by means of a further algorithm stored in the program memory 40 , for example by calculating the maximum value of the received pulse and determining the transit time in A distance value is determined with reference to the time at which the transmission pulse 48 is transmitted. At the same time, further signal properties such as signal amplitude, signal / noise ratio and / or pulse width of the echo pulse can be calculated. This data is stored in the raw data memory 41 together with the coordinates of the raster element corresponding to the burst. The data memory 38 can then be deleted or overwritten with the data after the next burst.

A variant of the embodiment described above is shown in FIG. 7, also in the form of a block diagram. In contrast to the device shown in FIG. 4, the solution according to FIG. 7 does not have digital but analog signal processing. In Fig. 7, the same reference numerals are used for positions which correspond to those in Fig. 4.

The laser transmitter electronics 30 is controlled by the processor 34 . According to the pulse number and clock frequency specified by the processor, the laser transmitter electronics 30 sends a sequence of pulses to the laser diode 2 , which emits a corresponding sequence of laser light pulses. The emitter zone of the laser diode 2 is imaged essentially by the optics 31 to infinity. The laser beam is deflected in two orthogonal directions by line-by-line scanning of the object field by means of a deflection unit which is arranged in front of the optics and is not shown in this figure. The radiation reflected from a target is thrown by an analog constructed and synchronous and in-phase working deflection unit onto the receiving optics 32 , which directs the radiation onto a beam splitter prism 33 . The beam splitter prism 33 has a frequency-selective effect, dichroic mirroring 33 a, which allows the reflected infrared laser light to pass essentially undamped. The laser radiation is therefore concentrated on the receiving diode 3 , which is adapted in its spectral sensitivity to the transmitter diode 2 .

The output signal of the receiving diode 3 is processed in the amplifier and signal processor 36 and fed to a stage 50 , in which a distance value is determined by a microprocessor from the time of flight (TOF) in relation to a start pulse derived from a transmission pulse. From the distance values corresponding to the different pulses of a burst, the processor 34 calculates a single distance value assigned to a raster element according to predetermined algorithms. This distance value is linked by the processor 34 to the coordinates of the raster element, which are derived from the electronic units of the deflection systems 45 and 46 .

The data record assigned to a raster element is then stored in a memory 41 . The shorter-wave visible light is deflected by the dichroic mirror 33 a on the diode 15 . Instead of a single diode 15 , a triple diode can also be provided, the individual diodes of which have spectral sensitivities corresponding to the three primary colors. With such an arrangement can therefore parallel to a so-called. Distance image (active channel) a black and white or a color TV image of the object field can be recorded (passive channel). The video signal derived from the diode 15 is amplified and digitized in the stage 42 and stored together with the coordinates of the corresponding raster element in the digital image memory 43 . Since the distance images generated via the active channel and the TV images generated via the passive channel are recorded via the identical deflection unit, they are absolutely congruent and can be combined with one another in any subsequent processing. For this purpose, the processor 34 can access, via the data bus 39, both the memory 41 , in which the distance values associated with each raster element, and the memory 43 with the video information, and link them to one another in accordance with a predetermined program. The overall video information generated in this way can be output via the video interface and displayed, for example, on a monitor.

The processor 34 determines from the various measured values associated with the individual pulses of a burst, amplitude values, signal / noise ratios, mean values and scatterings etc. If the measured values have only low amplitude values or signal / noise ratios, the processor 34 gives the laser transmitter 30 the Command to increase the number of pulses per burst. At the same time, the control electronics units 45 and 46 of the scanner are controlled via bus 39 in the sense of a reduction in the scan rate.

The procedure is analogous in the case of a large scatter of the measured values compared to the mean value. If only a few results from the results of a burst have a large deviation compared to the mean, it can be assumed that such results are due to incorrect measurements conditions or malfunctions. These results are therefore eliminated from the memory and not used for further processing.

The invention is not limited to the examples described above.

Claims (10)

1. A method for recording an object space with an opto-electronic distance meter according to the pulse transit time method with a transmitter for sending optical, esp. Laser pulses and a receiving device for receiving optical signals, esp. Laser radiation, from objects located in the target area is reflected, furthermore with a scanning device for, in particular, continuous deflection of the optical axes of the transmitting and receiving device in preferably two orthogonal directions, through which the object space can be scanned in a grid-like manner, furthermore with an evaluation device which Distance values are determined from the transit time of the emitted optical pulses, with an image output system, e.g. B. a monitor or the like is provided, on which picture elements are reproduced, the picture content of which preferably represents a function of the distance value assigned to the respective raster element and whose screen coordinates correspond to the coordinates of the raster elements in the object space, characterized in that the emitted Pulses are combined in a known manner to form pulse packs, so-called “bursts”, with each raster element being assigned at least one burst and in an electronic computer using defined algorithms from the received signals of the pulses of each burst and from the transit time of these received pulses in relation a distance value assigned to a raster element is calculated on an associated start pulse. 2. The method according to claim 1, characterized in that the clock rate of the outside optical pulses within a burst at least 50 kHz, advantageously at least is at least 100 kHz. 3. The method according to claim 1 or 2, characterized in that by a Mehrka Time interval measurement for each individual pulse of a burst a runtime interval is determined, which in its entirety is a table of runtime Define intervals, whereby an electronic computer uses defined algorithms these transit time intervals of the pulses of each burst with respect to an associated one Start pulse at least one distance value assigned to a raster element determined.   4. The method according to any one of the preceding claims, characterized in that in an electronic computer from the time-discrete, digitized and added Receive signals of the pulses of each burst associated with a raster element Received pulse determined and from the runtime of this reconstructed received pulse a distance value is calculated in relation to an associated start pulse. 5. The method according to any one of the preceding claims, characterized in that a burst consists of 2-50, preferably 10, pulses. 6. The method according to any one of the preceding claims, characterized in that at least partially during the pulse gaps between two bursts in the electronics computer using defined algorithms from the received signals of the Pulses of the previous burst an emp assigned to a raster element catch pulse determined and from the runtime of this reconstructed receive pulse in Based on the associated start pulse, a distance value is calculated and in one Memory stored together with the coordinates of the raster or image element becomes. 7. The method according to any one of the preceding claims, characterized in that the signal ampli from the received signals of the bursts of a scan of the object space or the signal / noise ratios of each grid element Measured values are determined and saved and the number of pulses per burst in the fol scans can be changed as a function of these values, in the sense that with large the signal amplitudes or signal / noise ratios of a raster element the pulses per burst and thus the duration of a measuring cycle is reduced, with small NEN signal amplitudes or signal / noise ratios the number of pulses per burst is increased and the scan rate of the scanning device depending on the number of pulses is variable per burst. 8. The method according to claim 7, characterized in that the adaptive choice of Number of transmit pulses per burst over the bursts of a first scan cycle have an average large number of pulses, so that even under unfavorable conditions a measured distance value can be determined in order to be derived for the following Scanning processes to determine the parameters of the measurement.   9. The method according to any one of the preceding claims, characterized in that for each raster element windows of memory addresses, which distance values correspond, definable, within which samples of the received signals for Evaluation can be used. 10. The method according to any one of the preceding claims, characterized in that a relative phase shift of between the transmit pulse repetition frequency and the sample frequency
d = L / n
in which formula is set
n is an integer, preferably 5,
d the phase shift, and
L is the period of the sample frequency,
so that samples of the digitized received signals are intermeshed with one another with a period of n, whereby the effective sample frequency appears to be increased by a factor of n, absolute values of the transmit pulses preferably being shifted in their phase position.