Patent Application
20220018764
The present invention relates to a method for determining a distance of an object surface using a laser source that emits light having a power, and using a detector that detects the light reflected or backscattered from the object surface and having an irradiance and depending thereon outputs a time-dependent voltage signal. The present invention furthermore relates to the determination of the reflectivity of the object surface. Moreover, it relates to a device that carries out the method, in particular LiDAR systems.
Such methods, which in particular are also known by the abbreviation LiDAR (Light Detection And Ranging), are based on an optical distance measurement using laser scanners. The technology has been known since the early 1970s at the latest, since LiDAR was used for measuring the topography of the surface of the moon in the orbiter modules in the context of the Apollo 15, 16 and 17 missions. The basic principle is that a laser beam is emitted in the direction of an object surface, the distance to which is to be determined, then a detector detects the reflected or backscattered light and the light time of flight (time of flight—ToF) is measured, from which in turn, given the known speed of light, the doubled path of the distance is determinable (outgoing path and return path). By means of repeated measurements, it is thereby possible to ascertain a change in distance as well, this being increasingly used in speed checks, for example.
In recent years, in particular also as a result of advances in sensor technologies, but particularly in the case of micro-opto-electromechanical components (MEMS/MOEMS) and also in the case of processor technologies, there have been strong pushes into new industrial sectors and fields of application. Mention may be made here of the traffic sector, in particular, where efforts are currently being made to enable autonomous driving, and intelligent driver assistance systems have already largely become established. LiDAR systems make it possible here to scan surroundings of the vehicles in which they are implemented, and to determine in each case distances up to moderate ranges. The results can be used to construct, in a processor-aided manner, three-dimensional images of the surroundings in which the vehicle is moving. Over and above the actual LiDAR technology, the aim here may also be to determine the reflectivity (the so-called albedo) for the object surfaces respectively measured, in order, on the basis of known values for specific materials, to obtain information about the structure and construction of the objects affected, for example the question of whether there is a tree, a road sign or an automobile etc. in the field of view.
The range is limited by the restricted sensitivity of the sensor or detector used and the power of the laser source. In order to extend the range of distance determination, the laser power could be increased, but that is at odds with stipulated safety standards for endangerment of the eye by laser beams, which standards have to be complied with; in this respect, cf. e.g. “safety of laser products—Part 1: Equipment classification and requirements”, in Technical Reports IEC 60825-1:2014 (2014). In the field of driver assistance systems and autonomous driving, moreover, the frequency range of the laser light is also kept in the near infrared (NIR) wavelength range of e.g. 840 or 900 nm to 1550 nm, and so here the human eye is unprotected owing to lack of sensitivity. The wavelength range of 840 nm to 950 nm is suitable for silicon-based applications. The range of 1100 nm to 1550 nm is suitable for III-V compound semiconductors. In the case of silicon, at the short wavelengths here the advantage of increased quantum efficiency is afforded, while the restrictions arising from the requirement of the eye safety standards in turn prove to be stricter here, however. The NIR range extends overall from 800 nm to 2500 nm. In this respect, the focus of further development is on increasing the sensitivity of the sensors, increasing the corresponding gain and improving the signal-to-noise ratio (SNR), likewise on the part of the detector.
In LiDAR applications, sensors based on avalanche photodiodes (APD) have largely become established since the latter are designed particularly for receiving and evaluating laser pulses.
This type of photodiodes represents inherently highly sensitive sensor elements which operate at high speed and which may also be regarded as a semiconductor equivalent to conventional photomultipliers. They are based on PIN diodes, but have in addition to the intrinsic i- or n-absorption layer a thin and highly doped p- or n-type layer, which generates a high electric field strength in the case of an applied reverse voltage below the breakdown voltage vis-à-vis the adjacent n+- or p+-type layer, as a result of which electric field strength the electron-hole pairs formed in the absorption layer upon absorption of a photon form charge carriers that are greatly accelerated and form further electron-hole pairs as a result of impact ionization, thus giving rise to an avalanche effect.
Multiplication factors or gain factors of from 100 to 500 can be achieved in this mode, referred to as “radiation-proportional operation”. However, this gain falls far short of that needed to detect every single photon.
The sensitivity is given by the ratio of the number of electron-hole pairs generated by absorption to the number of incident photons. It is also referred to as quantum efficiency (QE) in the case of avalanche diodes.
A very particular advantage is that there is a proportional relationship between the number of incident photons and the sensor response, i.e. the output voltage is proportional to the corresponding radiation power. This makes it possible, in the case where APDs are used, on the basis of the distance known from the time of flight determination (i.e. with local irradiance being known), for the reflectivity of the respectively relevant object surface to be deduced directly proceeding from the voltage signal output by the sensor.
While APDs thus offer a high sensitivity and the advantage of proportional behavior of the output voltage vis-à-vis the radiation power with a fast response, that is offset by only inadequate gain and not inconsiderable thermal noise and shot noise.
Specially configured avalanche photodiodes can expediently also be operated above the breakdown voltage. This operation is also referred to as the Geiger mode and the relevant photodiodes are called single-photon avalanche diodes (single-photon avalanche diode, for short: SPAD). On account of the then very high field strengths in the multiplication zone, great accelerations are achieved and as a result 106 to 108 electron-hole pairs are generated on the basis of just one photon, i.e. the gain can be more than 106, and it becomes possible to detect single photons. In order to prevent a situation in which, after an avalanche has been generated, the photodiode remains conductive on account of the high currents and is thus no longer available at all for further detection of photons, the SPAD diode can be provided with a series resistance and a suitably interconnected capacitance. After the breakdown of a charge carrier avalanche, a partial voltage is dropped across the series resistance, such that the reverse voltage across the diode falls below the breakdown voltage. This process is referred to as quenching. In the meantime, the voltage across the diode becomes charged again, and so after a dead time it is available again for a further avalanche in a cyclic manner. On account of said dead time, however, the single SPAD diode is unsuitable for use as a LiDAR detector, since once again not all of the photons can be detected.
This can be achieved, however, by a combination of large numbers of SPAD diodes respectively configured in microcells to form a so-called silicon photomultiplier (SiPM), wherein the SPAD diodes, each of which is operated in the Geiger mode, including their series resistances are interconnected in parallel with one another. Consequently, the photons impinging on the individual microcells each bring about avalanche-like output pulses, which in their entirety are statistically superposed to form an n-fold stronger voltage signal output by the SiPM sensor, wherein the number n corresponds to the number of microcells in the SiPM array and, given cell sizes of e.g. 10 μm to 100 μm and total dimensions of the SiPM sensor of 1×1 mm2, can comprise up to 10 000 microcells.
In this respect, SiPM-based detectors afford the advantage of a sufficiently high gain and moreover also comparatively low noise or a satisfactory signal-to-noise ratio for measured voltage signals.
Unfortunately, however, that is in turn offset by lower sensitivity and a dynamic region restricted by a nonlinear saturation region of the output voltage for high radiation powers in the case where SiPM sensors are used. For SiPM sensors the sensitivity is defined by the photon detection efficiency (PDE), which is a product of the quantum efficiency, an avalanche initiation probability and the fill factor. The fill factor indicates that proportion of the total area of the microcell which is constituted by the active area respectively available for photon detection. The more cells are included, i.e. the smaller the cell size for a given total area of the SiPM sensor, the lower the fill factor (e.g. more peripheral area) and hence the sensitivity. On the other hand, increasing the number n of cells results in an expansion of the dynamic region, i.e. that voltage interval of the output voltage which is available for a use and ideally yields the proportionality between radiation power and output voltage.
If the radiation power is excessively high, the relationship between a respective voltage amplitude as pulse response to the laser pulse and the pulse power in the case of SiPM sensors transitions to a nonlinear saturation region in which increasingly all the microcells are in a state of immediate photon detection after resetting by means of quenching and the dead time possibly following on cyclically. Consequently, it is always necessary to find a difficult compromise between the expansion of the dynamic region by the use of sensors having more cells and the improved sensitivity by the use of fewer but in return larger cells (given a fixed total area). This is because in the case of more cells given a predefined total area, they have an ever-decreasing cell size, such that design limits are rapidly encountered and at the same time the photon detection efficiency (PDE) decreases rapidly.
In any case the limitation of the dynamic region at a detector directly also restricts the distance range within which light signals can still be reliably detected for the distance determination and can also be evaluated with regard to ascertaining the reflectivity.
It is therefore an object of the invention to provide a method of the generic type for determining a distance of an object surface and a corresponding device in which a distance range within which light signals can still be reliably detected for the distance determination and can also be evaluated with regard to ascertaining the reflectivity is extended further.
The object is achieved by means of a method for determining a distance of an object surface having the features of patent claim 1 and by means of a corresponding device having the features of claim 15. The dependent claims relate to advantageous developments of the method according to the invention.
A method having substantially two stages is proposed here. The starting point is a method for determining a distance of an object surface, wherein use is made of a laser source that emits light having a power and a detector that detects the light reflected or backscattered from the object surface, said light arriving in the detector with an irradiance, and depending thereon outputs a time-dependent voltage signal. The detector can preferably be an SiPM sensor, or a sensor having similar properties.
Firstly, for each individual distance determination (that is to say repeatedly at high frequency for the individual pixels in the case where the surroundings are scanned by laser scanning), one or both of the two following steps is or are carried out: the laser source is set such that the latter emits light having a predetermined first value of the power in at least one pulse, and/or the detector is set, such that the latter outputs a first voltage signal having a predetermined second value for a gain factor or the gain depending on the irradiance of the reflected or backscattered light detected. The gain factor or the gain is usually set by way of the overvoltage at the detector. Here there is a generally linear relationship between the stated variables. The overvoltage is equal to the difference between the (set) reverse voltage and the respectively usable and moreover temperature-dependent breakdown voltage. The setting of the gain factor or gain is thus synonymous with the setting of the overvoltage or reverse voltage. Likewise, the setting of the power or radiation power of the laser source usually corresponds to the setting of a driver voltage (also called: drive voltage).
In a further step, a first absolute value for the distance of the object surface is determined from a measured light time of flight (ToF) assigned to the first voltage signal. In this case, it is regularly assumed that laser source and detector are substantially almost positionally identical, i.e. are at a negligible distance from one another in comparison with the distance to be measured. This holds true in particular for a possible mutual offset in the direction of the object surface. If such an offset is nevertheless present, it can correspondingly be taken into account as well in the detection and calculation of the distance from the light time of flight (ToF).
In a further step, the first value of the power of the laser source and/or the second value of the gain factor or gain of the detector are/is adapted depending on the first absolute value for the distance determined from the time of flight measurement. In this case, the adaptation can be effected in particular such that the irradiance in the detector falls within the dynamic region thereof, that is to say that, firstly, a voltage signal having a usable amplitude is actually generated in the first place by means of the adaptation and, secondly, the amplitude falls within a voltage range in which—with the distance known—the information linked with the amplitude can be evaluated further, in particular for the calculation of the reflectivity of the affected object surface. In the dynamic region there is, in a targeted manner, a substantially linear one-to-one relationship between the amplitude of the voltage signal and the irradiance (which for a given distance correlates with the radiation power of the laser source).
In contrast to the dynamic region there is a nonlinear transition region toward a saturated region in which, if the radiation power or irradiance is too high or the gain factor set is too great or the overvoltage set is too great, the amplitude response of the detector asymptotically approaches a maximum value of the voltage, i.e. the amplitude then no longer scales with the irradiance and it would then be virtually impossible to calculate for example the reflectivity, etc.
It should be noted that an amplitude-dependent time offset (or time shift or time walk) occurs in the dynamic region of the detector in the case of SiPM sensors: the smaller the amplitude, the later amplitude response is output. This phenomenon does not occur in the case of APD sensors. The result would be inherently a systematic error in the distance determination toward smaller amplitudes or irradiances. According to one aspect of the invention, this effect can be taken into account by a calibration of the detector.
In a subsequent step, on the basis of the newly adapted first and/or second values of the radiation power and/or the gain factor, once again light is emitted by the laser source in a pulsed manner and the reflected or backscattered light is detected by the detector. Accordingly, a second voltage signal is output by the detector.
Optionally, the light time of flight can be repeatedly ascertained from said second voltage signal and a second absolute value for the distance of the object surface can in turn be determined from said light time of flight. This optional second absolute value or else already the first absolute value is finally output as the measured distance. A further iteration is then regularly no longer necessary. The first absolute value may possibly already have been determined sufficiently accurately or near to the actual value. What is important is that the voltage signal for a subsequent albedo determination is present with sufficient quality, i.e. with an amplitude in the dynamic region, to be described below, which enables a corresponding evaluation.
The adaptation—proposed according to the invention—of the radiation power and/or of the gain factor for example such that the detector can detect the incident radiation as much as possible in the dynamic region enables the distance range that is to be measured, namely including an albedo determination, to be extended both toward shorter distances and toward greater distances. In the case of shorter distances, a reduction of the radiation power can draw the irradiance of the material-dependently reflected or backscattered light from the saturated region of the detector into the dynamic region.
In the case of greater distances, particularly if the radiation power of the laser source is already at the upper limit defined by safety standards, the gain or the overvoltage of the detector can also be increased if an e.g. SiPM sensor is used for this purpose. However, in this case there is also an increase in effects of so-called afterpulsing (within microcells) and of optical crosstalk (between adjacent microcells) with this type of sensors (decreasing signal-to-noise ratio), such that in this case the dynamic region becomes somewhat narrower if the gain is chosen to be excessively high, such that the distance range cannot be extended arbitrarily. Precisely in the field of autonomous driving and driver assistance systems, however, the invention enables an extension of the distance range while maintaining albedo measurements of up to 300 m or more.
According to one development of the method according to the invention, the detector includes a silicon photomultiplier, i.e. an SiPM sensor. A laser that operates in the near infrared spectral range, preferably in the range of wavelengths of 900 nm to 1550 nm, is suitable as laser source. Other wavelength ranges are not ruled out, however, particularly in the visual range of 350 nm to 900 nm. This applies to the laser source and also to the SiPM sensor, which of course have to be coordinated with one another.
According to a further development of the method according to the invention, the steps of the method are carried out repeatedly for individual pixels in the context of a LiDAR application in the field of driver assistance systems or systems for autonomous driving for scanning surroundings of a vehicle for the computer-aided construction of a three-dimensional image of the surroundings. The effects achieved by the invention have a particularly advantageous outcome in this field of application.
According to a further development of the method according to the invention, a first, upper voltage limit value for a voltage is predefined, wherein for voltages below the limit value for the detector there is a substantially linear relationship between the irradiance of the incident light and a voltage output as a consequence thereof, and above said limit value the relationship is nonlinear and/or saturated. This voltage limit value thus defines as it were the upper limit of the dynamic region.
Furthermore, an amplitude of the first voltage signal is determined and is compared with the voltage limit value, that is to say that the fact of whether or not the specific first voltage signal lies in the dynamic region is determined. In the subsequent step of adapting the first value of the power of the laser source and/or the second value of the gain factor of the detector, the extent of this adaptation is then carried out depending on the result of the comparison.
According to a refinement of this aspect, the adaptation includes in particular a decrease of the first and/or second value if the amplitude exceeds the voltage limit value, such that in the subsequent step the irradiance of the incident light is reduced in the detector and as a consequence thereof an amplitude of the second voltage signal falls below the predefined first voltage limit value. Advantageously, the dynamic region of the detector is employed again under these circumstances.
According to a further refinement of this aspect, the decrease includes a reduction of the first and/or second value by 40% or more, preferably 50% or more, and/or else by 60% or less. This decrease by e.g. 40-60% ensures that the amplitude response of the second voltage signal that is obtained in the second pass falls approximately in the center of the dynamic region.
According to a further development of the method according to the invention, a second, lower voltage limit value is predefined for a voltage, which value ensures a predefined signal-to-noise ratio, for example 2 dB or more, preferably approximately at least 10 dB, for the detector. Said second, lower voltage limit value defines the lower limit of the dynamic region. In further steps (in a manner similar to that above) an amplitude of the first output signal is determined and is compared with the second voltage limit value. In this case, the step of adapting the first value of the power of the laser source and/or the second value of the gain factor of the detector includes an increase of the first and/or second value, such that in the subsequent step the irradiance of the incident light is reduced in the detector and as a consequence thereof an amplitude of the second voltage signal lies above the predefined second voltage limit value. Analogously to the procedure described above, the increase can be effected here e.g. such that 40 to 60% of the saturation value (known in advance) of the output voltage of the detector is obtained after the adaptation, that is to say that the amplitude response in the case of the second voltage signal subsequently in the second pass lies in the center of the dynamic region here, too.
The following aspects are directed in particular to an albedo determination carried out after a distance value (first or second value for the distance) has been obtained, i.e. the determination of the reflectivity of the object surface respectively scanned.
According to a further development of the method according to the invention, provision is made of a function between the power of the laser and the distance of the object surface for a fixedly selected irradiance of the detector in relation to the reflected and/or backscattered light. The first value of the power predetermined for the adaptation and/or the predetermined second value for the gain factor are/is ascertained with the argument of the first absolute value for the distance determined from the first voltage signal by way of this function and the adaptation is carried out according to this function. The fixedly selected irradiance advantageously lies e.g. in the dynamic, i.e. substantially linear, region of the detector, preferably in the center thereof (e.g. 40-60% of the value of the output voltage at which the latter is saturated). The laser power and the distance are then assigned to one another one-to-one in order that the condition of a constant irradiance is met. The function provided forms as it were a guide for the adaptation in the second pass (i.e. adaptation of the parameters power and/or gain and generation of the second voltage signal) and ensures that the dynamic region of the detector is complied with, such that the albedo determination subsequently becomes possible.
According to a further development of the preceding aspect, before the step of the first setting of the power of the laser and/or the gain factor of the detector, a start value for the absolute value of the distance is predefined. Then in a subsequent step, the power and/or the gain factor are/is ascertained from the predefined function, on the basis of which the laser source and/or the detector can subsequently be set. By virtue of this step, in the method sequence from the outset it is possible to employ the predefined function which relates the parameter space of the settable values (power, gain) and the result (distances) obtained therefrom while complying with a condition (irradiance in the dynamic region or amplitude response in the output signal of the detector) and thus allows the cyclic passes of the method steps.
According to a further development of the preceding aspects, a lower power limit and an upper power limit are defined for the predefined function between the power of the laser and the distance of the object surface, wherein for all distances below the distance assigned to the lower power limit, only the value of the lower power limit is returned and used, and for all distances above the distance assigned to the upper power limit, only the value of the upper power limit is returned and used. This ensures that operation is effected only in the permissible power range of the laser source.
According to a further development of the preceding aspects, for example, the lower power limit is set in accordance with a minimum output power of the laser source. Likewise, the upper power limit can be set in accordance with a safety standard of the laser source.
According to a further development of the method according to the invention, then after the step of determining the second absolute value for the distance of the object surface from a measured light time of flight assigned to the second voltage signal, a further step of determining the reflectivity of the object surface on the basis of the second voltage signal and the determined second value for the distance can be carried out. This corresponds e.g. to the albedo determination itself. It is alternatively also possible to use the second voltage signal and alternatively already the first absolute value for the distance in this albedo determination. As mentioned above, by virtue of this step, the preparatory features making this possible manifest the full advantageous effect. According to an advantageous modification or supplementation of this aspect, provision is made for providing the albedo determination in each pass, i.e. also already after determining the first value for the distance out of the first voltage signal.
According to a further development of the preceding aspect, a second function is provided, which indicates a linearized response yact to an amplitude of the second voltage signal, having the form:
y
act
=x=−log(1−amp/c1)·c1/c2, (1)
wherein x corresponds to the radiation power of the laser source or a driver voltage thereof and amp corresponds to the amplitude of in each case the first or second voltage signal, and c1, c2 are coefficients determined from measurements by means of a mathematical fit. In this case, the coefficients are specific to the detector used and may differ significantly from detector to detector. However, it is applicable to SiPM sensors, in particular, and takes account of a saturation region present. The variable yact corresponds to a voltage (e.g. measured in volts) or a power (e.g. measured in watts).
Furthermore, a third function is provided, which indicates a linearized reference variable yref as a function of a distance of the object surface and a power of the laser source, having the form:
y
ref=α(d)·x, (2)
wherein x corresponds to the power of the laser source or a driver voltage thereof and a is a linear gradient factor with which a reference power as linearized reference variable and the radiation power are linked with one another and which depends on the respective distance d of the object surface. For a given distance and a given detector and optical parameters (laser and optical system), α(d) is a fixedly predefined value.
In this case, the linearized response yact is calculated from the amplitude of the second voltage signal determined by measurement. The linearized reference variable yref can be calculated from the ascertained second value for the distance and the power of the laser source. The reflectivity is finally calculated from a quotient of the linearized response yact and the linearized reference variable yref, in particular e.g. from a square root of the quotient.
According to one particular embodiment, the linearized reference response yref is calculated from:
y
refexp(k1·log(d)2+k2·log(d)+k3)·x (3)
wherein x corresponds to the power of the laser source and d corresponds to the distance of the object surface, and k1, k2 and k3 are coefficients determined from measurements by means of a mathematical fit.
A device according to the invention for determining a distance of an object surface comprises for example a laser source that emits light having a power, a detector that detects the light reflected or backscattered from the object surface and having an irradiance and depending thereon outputs a time-dependent voltage signal, and a control device. The latter is configured to carry out the method having the steps in accordance with the explanations above. The same advantages as mentioned above are afforded here.
Further advantages, features and details of the invention are evident from the claims, the following description of preferred embodiments, and with reference to the drawings. In the figures, identical reference signs designate identical features and functions.
In the figures:
In the exemplary embodiment of a device 1 as shown in
The device 1′ concerns a LiDAR system for use in vehicles in order to support an ADAS system (advanced driver-assistance system), i.e. a driver assistance system. Here it is necessary not just simply to carry out a distance determination, but to generate a three-dimensional image of the complete or partial surroundings of the vehicle (not shown) in which the device is fitted, for example in order to evaluate obstacles or signs fitted in a stationary fashion, etc. The laser source 10 here comprises a laser diode that emits light 12 in a beam (as described above) in the near infrared (NIR) wavelength range (900 nm to 1550 nm). In order to scan the surroundings, a microelectromechanical component 28 (MEMS) having one or more micromirrors 30 adjustable at high frequency is provided, which micromirrors can deflect the light beam in a manner rotating or oscillating at high frequency about an individual axis under the control of the control device 20. The deflected laser beam (light 12) is guided through a diffuser 34, which expands the beam in a vertical direction (in the schematic illustration in
Part of the backscattered or reflected light 18 passes through a lens optical unit 26, which focusses the light onto a photodiode array 24, which comprises detectors 16 embodied as SiPM sensors in this exemplary embodiment, too, which are arranged vertically in series. The number of detectors 16 is chosen in accordance with the expansion of the beam (light 12). The detectors detect the light 18 assigned to them via the optical unit, from which, with application of the method, the control device 20 ascertains for each pixel a distance d and a value for the albedo (reflectivity). The pixels are defined in a vertical direction by the detectors 16 arranged in series in the photodiode array 24, and in a horizontal direction by discrete angular positions of the micromirror(s) for the relevant pulses. The image finally constituted can have a resolution of e.g. 256×84 pixels, or 0.25°×0.3° given an image field of 60° horizontally and 20° vertically. The ranges are more than 200 m for detecting pedestrians or more than 300 m for detecting other vehicles. The values indicated are merely by way of example and on no account restrict the scope of protection defined by the claims.
In the case of photon capture, in the equivalent circuit diagram the switch S closes and so the current pulse caused by the charge carrier avalanche generated results in a discharge of the diode capacitance CD via the series resistance RS, with the consequence that the voltage proceeding from VBIAS falls back to the breakdown voltage VBD. This is referred to as discharge mode (discharge phase). As described in the introduction, the quench resistance RQ then becomes apparent by virtue of the voltage across the diode being quenched, as a result of which the switch S opens again.
In the phase that follows, referred to as recharge mode (recovery phase), the diode capacitance CD is recharged via the quench resistance RQ, and so a new cycle begins. The sequence is illustrated schematically in
In order then in view of the substantive matter shown in
In the case of short distances d (not discernible from
In accordance with the exemplary embodiment, therefore, on the basis of a distance that has already been determined, depending on the latter, an increase of the value of the laser power set can be carried out if the distance is large, or can be decreased if the distance is small. Developments of this exemplary embodiment provide for performing a dynamic real-time adaptation depending on the distance respectively determined. A corresponding exemplary embodiment is explained further below.
In the case of large distances d, in accordance with this exemplary embodiment, provision is made for increasing the gain factor (gain) in order to improve the signal-to-noise ratio and in particular also in order to keep the SiPM sensor in its dynamic region (see below for more details in this respect). By contrast, in the case of small distances, provision is made for decreasing the gain factor—likewise in order to keep the SiPM sensor in the dynamic region.
In a subsequent step 110, the laser source 10 and the detector 16 are set accordingly and a pulse is generated in the laser source 10, in the case of which pulse light having the predetermined first value of the power is emitted, wherein the detector 16, depending on the irradiance of the reflected or backscattered light detected, outputs a first voltage signal using the predetermined second value for the gain factor.
In a subsequent step 120, a (then first) distance determination is carried out, that is to say that a check is made to establish whether a distance determination is possible at all, and if so (Y in step 120), a first absolute value for the distance d of the object surface 14 is ascertained from a measured light time of flight ToF assigned to the first voltage signal.
If the distance determination is not possible (N in step 120), because the voltage signal assumes a signal-to-noise ratio SNR below a predefined minimum value, the parameters: power of the laser source 10 and/or the gain factor of the detector 16 are adapted, i.e. here increased, in a step 130.
By contrast, if the distance determination was possible and yields the first absolute value for the distance (Y in step 120), a further step 140 involves checking whether the first voltage signal output is in the saturation region 38, i.e. not in the dynamic region 39 (see
If the voltage limit value is exceeded (Y in step 140), then the method continues to step 150. In step 150, the parameters: power of the laser source 10 and/or the gain factor of the detector 16 are adapted, i.e. here: decreased.
In both cases, step 130, in which the distance d is too large to yield a usable voltage signal with a sufficient signal-to-noise ratio SNR, and steps 140, 150, in which the distance d is so small or the power of the radiation source is so high that the SiPM sensor operates in the saturation region 38, one or both parameters is or are adapted dynamically in order to start a second pass.
This is effected recursively returning to step 110, in which the laser source 10 and the detector 16 are again set accordingly or then adapted. That is to say that a pulse is generated again in the laser source 10, in the case of which pulse light having the now possibly adapted first value of the power is emitted, wherein the detector 16, depending on the irradiance of the reflected or backscattered light detected, then outputs a second voltage signal, which is ideally different than the first voltage signal, using the possibly adapted second value for the gain factor, which is then usable and is not in the saturation region 38.
Overall, therefore, in steps 130, 110 and 150, 110, respectively, the first value of the power of the laser source and/or the second value of the gain factor of the detector 16 are/is adapted (decreased or increased in accordance with the above explanations with reference to
After once again emitting light by means of the laser source 10 and detecting the reflected or backscattered light by means of the detector 16 and outputting a corresponding second voltage signal using the adapted first and/or second value, it is possible, finally, after repeating steps 110, 120, 140, to determine a second absolute value for the distance of the object surface from a measured light time of flight ToF assigned to the second voltage signal.
Said second absolute value should undergo the corresponding checks in steps 120, 140 in each case with a positive result (Y), after which the method in accordance with this first exemplary embodiment advances to step 160. Here the reflectivity of the relevant object surface 14 is calculated from the second voltage signal. Since the case of the dynamic region 39 is present here, with the indications of the distance d, the first value of the radiation power of the laser source 10, the gain factor (gain) of the detector 16 and the amplitude of the second voltage signal—optionally with suitable calibration—this value for the albedo can be calculated in a processor-aided manner by means of the central control unit 20 in step 160.
In order to determine the distance and reflectivity of a next object surface, the method returns to step 100. In this way, the surroundings of the device can be scanned step by step and a three-dimensional image can be generated as a result. This image can be evaluated by means of object-detecting software in order for example to recognize specific objects, persons or traffic signs etc. and, if appropriate, to take measures.
A second exemplary embodiment is shown in
The relationship corresponds to a function
amp=c1·(1−exp(−(c2·x)/c1)), (1)
which yields a very good fit, wherein the amplitude amp of the voltage signal is yielded by the SiPM sensor and x is related to the driver voltage V1 of the laser source 10 by:
x=(V1−2.5)/0.5. (2)
The coefficients c1 and c2 are determined by the fit. In the very specific exemplary embodiment, the coefficients are c1=0.3015 and c2=0.004296. The fit is indicated by a solid line in
This second exemplary embodiment then provides for performing a linearization of the curve shown. For this purpose, equation (1) for the nonlinear amplitude response amp is transformed according to the linearized amplitude response yact:
y
act
=x=−log(1−amp/c1)·c1/c2, (3)
The measured amplitudes of the measurements for various (known) distances with differing laser power (in accordance with x) can then be inserted in equation (3) and yield in each case straight lines having a gradient a dependent on the distance d:
y
ref=α(d)·x (4)
In order to determine α(d), it is possible once again to use a fit, for example, wherein a transformation into the logarithmic scale was carried out here as well:
log(α)=k1·log(d)2+k2·log(d)+k3 (5)
The exemplary fit is only up to the 2nd order, but without limitation could also be of a higher order.
The following arises for the thus empirical, linearized reference amplitude:
y
refexp(k1·log(d)2+k2·log(d)+k3)·x. (6)
In the second exemplary embodiment, therefore, the linearized reference amplitude yref can be immediately calculated as a reference value from equation (6), given a distance d determined by light time of flight measurement. On the other hand, the actual, linearized amplitude response yact can be directly measured or determined anew. Since the distance d is the same in both cases (after all, the distance is obtained from the same voltage signal), a difference between the two variables yact and yref is based exclusively on a difference in the underlying reflectivity or albedo. The albedo can be calculated from the quotient of yact and yref:
albendo=albedoref·(yact/yref)1/2, (7)
wherein albedoref is the albedo of a reference material used to carry out the fit. Ideally the reference material is a material with a particularly high albedo, for example aluminum with albedoref=0.88. Conversely, however, it is also possible to use other materials with a lower albedo as reference, such as e.g. steel with albedoref=0.68 or titanium with albedoref=0.34, etc. These method steps allow a particularly efficient and fast calculation of the reflectivity, which is necessary to achieve rapid updating of the detected surroundings.
The steps of the second exemplary embodiment can be carried out in the context of step 160 of the first exemplary embodiment, or else in the context of step 290 of the third exemplary embodiment described below:
In this regard, reference is made to
In this example, too, for this purpose a function V1 (d) simplifying the calculation is again predefined, this function being shown in
A fit becomes necessary since although the power of the laser source 10 is proportional to the square of the distance d, and although the driver voltage is also related linearly to the power, a laser power is brought about only starting from a certain start value. A 3rd order polynomial fit has proved itself here:
V(P(d))=e·P(d)3+f·P(d)2+g·P(d)+h. (8)
In the specific example, the coefficients were determined as follows: e=0.000793; f=−0.005521; g=2.276; h=0.8674. The curve, which is substantially parabolic nevertheless, is shown in
The laser source 10 is triggered in step 230 and, as a consequence thereof, a light pulse is generated in step 240. The light 18 reflected or backscattered from the object surface is received or detected by the detector 16 in step 250. In step 260. the first value for the distance, designated here as D, can be determined from the (first) voltage signal obtained. Step 270 involves checking whether d=D, i.e. whether the first value for the distance is equal to the distance predefined as start value. If this is not the case (N in step 270), the program sequence branches back to step 220. A new driver voltage V1 is sought here in accordance with the function in
As is shown in
On the other hand, correspondingly below d0min likewise only constant values for V1 (d) are returned. The latter only up to approximately 2 m result in irradiances that lie below the upper limit SiPM-MAX of the dynamic region.
The discrepancy between d and D in step 270 thus arises if the actual distance in
Then as described above in the second exemplary embodiment, the amplitude amp is determined in step 280 (Y in step 270) and the albedo value is calculated in step 290. Step 300 involves checking whether further pixels are to be detected and, if that is applicable (Y in step 300), the program branches back to step 210. Otherwise, the program sequence ends (step 310).
It should be noted that in this third exemplary embodiment the laser power is not adapted only when the upper voltage limit or upper limit SiPM-MAX of the dynamic region 39 is exceeded, but rather is already adapted if any change at all vis-à-vis the preset distance is established.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2018 220 932.4 | Dec 2018 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2019/081885 | 11/20/2019 | WO | 00 |
