Distance measuring device and distance measuring method

A distance measuring device includes a light emission portion for emitting light; a light receiving portion for receiving measurement light that is emitted by the light emission portion and reflected by a measurement object, the light receiving portion comprising a plurality of pixels, each pixel having at least one light receiving portion and outputting a light reception signal that depends on the measurement light incident on the pixel; a discrimination portion for discriminating whether the pixel receives measurement light; a pixel output control portion for selectively outputting the light reception signal of each pixel individually, depending on the determination result of the discrimination portion; and an evaluation portion for receiving the light reception signals output by the pixel output control portion and outputting a distance signal that is indicative of a distance between the measuring device and the measurement object based on these light reception signals.

FIELD OF THE INVENTION

The present invention relates to a distance measuring device, and more specifically to a distance measuring device comprising a light receiving portion having a plurality of pixels, as well as to a measurement method using the distance measuring device.

TECHNICAL BACKGROUND

A variety of distance measuring devices are known, in which measurement light is incident on a light reception area constituted by an array of light reception elements or pixels. Such distance measuring devices are used for example as distance sensors that evaluate the time of flight of a light beam that is emitted from the distance measuring device, reflected by the measurement object and received with the light reception area of the distance measuring device.

If the pixels receive not only the measurement light but also ambient or stray light, then this may falsify or corrupt the determination of the distance to the measurement object performed by the distance measuring device.

JP 2014-77658 A discloses an optical distance measuring device comprising a light source that projects irradiation light, light reception means that receive the reflection light from the object and is provided with a plurality of photodiodes in which each photodiode can be independently turned off, and addition means that adds an output of the plurality of photodiodes.

It is one object of the present invention to mitigate the influence of ambient light on the measurement result and to improve the accuracy of the distance measuring device.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a distance measuring device comprises:a light emission portion configured to emit light;a light receiving portion configured to receive measurement light that is emitted by the light emission portion and reflected by a measurement object, the light receiving portion comprising a plurality of pixels, each pixel having at least one light receiving portion and being configured to output a light reception signal that depends on the measurement light incident on the pixel;a discrimination portion configured to discriminate whether the pixel receives measurement light;a pixel output control portion configured to selectively output (forward) the light reception signal of each pixel individually, depending on the determination result of the discrimination portion; andan evaluation portion configured to receive the light reception signals output (forwarded) by the pixel output control portion and to output a distance signal that is indicative of a distance between the measuring device and the measurement object based on these light reception signals.

The light emission portion may emit the light as pulses. In that case, the measurement light may also take the form of pulses. In particular, the pulses of the received measurement light are delayed in time with respect to the pulses of the emitted light. A time of flight of the measurement light, which corresponds to the time it takes for the light to travel from the distance measuring device to the measurement object and back, can be determined from the time delay of the pulses. The distance to the measurement object may be calculated as a characteristic of the measurement object based on the time of flight. In addition to the distance to the measurement object, other properties or characteristics of the measurement object, such as its shape, color, reflectivity and/or luminescence, can be determined by the distance measuring device.

The light that is received by the light receiving portion may include not only light that is reflected once by the measurement object, but also light that is reflected multiple times, diffused light, scattered light, background light from the environment, light that is transmitted through the measurement object and then reflected, or otherwise returned to the light receiving portion.

Here, “distance signal that is indicative of a distance” may mean any signal that contains information that allows the specification of the distance. Examples of such signals are digital signals representing the measured distance or the time of flight of the returned light beam (“time of flight”) and analog signals whose amplitude is proportional to the measured distance or the time of flight.

The light reception signal can be indicative of a light amount incident on the pixel and/or of a light intensity of the light incident on the pixel. The pixel output control portion may be configured to forward the light reception signal of those pixels for which the discrimination portion determines that measurement light is received. In particular, the pixel output control portion may be configured to not forward the light reception signal of all other pixels of the light receiving portion (e.g., those receiving only ambient light or background noise, or just a lower signal intensity). This increases the signal-to-noise ratio and thus improves the measurement accuracy. In particular, the light reception signals that are not forwarded are not taken into account for the measurement result.

The distance measuring device may further comprise an evaluation portion configured to receive the light reception signals forwarded by the pixel output control portion and to determine a distance between the measuring device and the measurement object based on these light reception signals. The evaluation portion may be configured to output a signal that is indicative of that distance.

The evaluation portion may determine the distance between the measuring device and the measurement object based on only the light reception signals forwarded by the pixel output control portion. In particular, only the light reception signals from the pixels that do receive measurement light are taken into account when determining the distance to the measurement object. That way, the measurement can be performed with higher accuracy, because signals resulting from ambient light are disregarded.

The discrimination portion may comprise a detection portion for detecting whether light is received at a pixel connected to the discrimination portion (also referred to as “associated pixel” below) during a plurality of first predetermined time intervals during a predetermined time period and during a plurality of second predetermined time intervals during the predetermined time period; and, for example, a counting portion for counting the number of times that the detection portion detects reception of light during the first predetermined time intervals and during the second predetermined time intervals during the predetermined time period; wherein the discrimination portion is configured to determine whether each pixel receives measurement light based on a counting result of the counting portion. The detection portion may detect that a light signal is received during the first or second predetermined time interval when at least one photon or at least a predetermined number of photons is detected at the associated pixel during the first or second predetermined time interval. The ambient light arriving on a pixel can take the form of randomly scattered photons. At a pixel receiving only ambient light, the probability of receiving a photon at any time during the predetermined time period is substantially constant. That is to say, the ambient light may fluctuate over time or vary spatially, but can be regarded as constant for the purposes of distance detection, as long as such variations do not substantially affect the distance measurement. In other words, the distribution of received ambient light photons is spatially and/or temporally different from the distribution of the measurement light.

In particular, for constant, uniform ambient light, the probability of receiving a photon at any time during the predetermined time period is constant while measurement light is received only during predetermined time intervals, so that the probability of receiving a photon during those time intervals is not constant. In other words, the distribution of received measurement light photons is not constant. Counting or discriminating how often a light signal is received during the first or second predetermined time interval of the predetermined time period is indicative of the type of light that is received at a given pixel, i.e. whether measurement light or only stray light is received.

The discrimination portion may be configured to determine that a pixel receives measurement light if the number (or distribution) of times that the detection portion detects a reception of light during the first predetermined time intervals during the predetermined time period differs from the number (or distribution) of times that the detection portion detects reception of light during the second predetermined time intervals during the predetermined time period by at least a predetermined comparison threshold. If a pixel receives measurement light, the distribution of the received photons is in particular not constant (and differs from the case of receiving only ambient light). The number (or distribution) of times at which a light signal is received during the first predetermined time interval is thus expected to be significantly different from the number (or distribution) of times at which a light signal is received during the second predetermined time interval. The comparison of the number (or distribution) of times at which a light signal is received during the first predetermined time interval and the number of times at which a light signal is received during the second predetermined time interval can hence provide an indication on whether measurement light or only ambient light is received at the pixel.

In some embodiments, the discrimination portion is configured to determine that a pixel receives measurement light if a count of the number of times that the detection portion detects reception of light during the first predetermined time intervals during the predetermined time period is smaller than an absolute or self-adjusting threshold and if the count of the number of times that the detection portion detects reception of light during the second predetermined time intervals during the predetermined time period is larger than the absolute or self-adjusting threshold, or vice versa.

The discrimination portion may be configured to determine that a pixel does not receive measurement light if the count of the number of times that the detection portion detects reception of light during the first predetermined time intervals during the predetermined time period differs from the count of the number of times that the detection portion detects reception of light during the second predetermined time intervals during the predetermined time period by less than the predetermined comparison threshold.

The first predetermined time intervals and/or the second predetermined time intervals may be periodic. Thereby, the precision of the measurement may be improved. Moreover, the length of the first predetermined time interval may be the same as a duration of the second predetermined time interval. However, different timings or durations can be also implemented and are covered by another embodiment. For example, the first predetermined time intervals and/or the second predetermined time intervals can be pseudo-random intervals.

The distance measuring device may further comprise a weighting portion for assigning a first weight to the counts of each first predetermined time interval at which the detection portion detects reception of light and/or for assigning a second weight to the counts of each second predetermined time interval at which the detection portion detects reception of light during the second predetermined time intervals. For example, the weighting portion may associate the number “+1” to every first predetermined time interval during which a light signal is detected and the number “−1” to every second predetermined time interval during which a light signal is detected. The number “0” may be associated to first and second predetermined time intervals during which no light signal is detected at all. The difference between the number of times that the light signal is detected during the first predetermined time interval and the number of times that the light signal is detected during the second predetermined time interval can then be easily determined by simply adding up these weighted counts.

The counting portion may further be configured to sum up the weighted counts of the first predetermined time intervals and to sum up the weighted counts of the second predetermined time intervals. The result of addition of the weights of the first predetermined time intervals and of the weights of the second predetermined time intervals allows to easily compare the number of times that the detection portion detects reception of light during the first predetermined time intervals to the number of times that the detection portion detects reception of light during the second predetermined time intervals and to determine whether measurement light is received therefrom.

A plurality of pixel output control portions may be connected to a single evaluation portion. Furthermore, a discrimination portion and a pixel output control portion may be provided for each pixel. The light reception signals forwarded by several pixel output control portions may be fed to a single evaluation portion. It is possible that all pixel output control portions are connected to a single evaluation portion. Moreover, the plurality of pixel output control portions may be connected to a single discrimination portion.

The discrimination portion may receive the light reception signal of each of a plurality of pixels. More specifically, a plurality of pixels may be connected to the same discrimination portion such as to provide the discrimination portion with their light reception signals. The discrimination portion receiving light reception signal of each of a plurality of pixels may also be connected to a single output control portion connected to a single evaluation portion. The light receiving portions constituting the pixels may be single photon avalanche diodes.

The plurality of pixels connected to the same discrimination portion may be non-adjacent to each other. Non-adjacent pixels can be non-neighboring pixels having at least one other pixel arranged between them. In particular, there is no direct path between non-adjacent pixels that does not cross any other pixel provided between the non-adjacent pixels.

A measurement method for performing a measurement using the distance measuring device in accordance with one aspect of the invention includes:emitting light with the light emission portion;receiving measurement light that is emitted by the light emission portion and reflected by the measurement object by the light receiving portion, the light receiving portion comprising a plurality of pixels, each pixel having at least one light receiving portion and being configured to output a light reception signal that depends on the measurement light incident on the pixel;discriminating, in particular for each pixel individually, whether the pixel receives measurement light;selectively outputting (forwarding) the light reception signal of each pixel individually, depending on a determination result of the step of determining for each pixel individually whether the pixel receives measurement light; andoutputting a distance signal that is indicative of a distance between the measuring device and the measurement object based on the received light reception signals.

The evaluation portion may be configured to detect a plurality of light reception signals from a plurality of objects, to determine a shape according to which the objects are arranged, and/or to determine distances to the respective objects.

Further possible implementations or alternative solutions of the invention also encompass combinations—that are not explicitly mentioned herein—of features described above or below with regard to the embodiments. The person skilled in the art may also add individual or isolated aspects and features to the most basic form of the invention.

Further embodiments, features and advantages of the present invention will become apparent from the subsequent description and dependent claims, taken in conjunction with the accompanying drawings, in which:

EMBODIMENTS OF THE INVENTION

In the figures, like reference numerals designate like or functionally equivalent elements, unless otherwise indicated.

FIG.1shows an example of an optical system20including a distance measuring device1according to a first embodiment and a measurement object2. The distance measuring device1of this embodiment is configured as a photoelectric sensor and is a distance determination device for determining the distance to the measurement object2, which is also called “object2” in the following. The distance measuring device1comprises a light emission portion3, a light receiving portion4, a collimator6and a converging lens7.

The light emission portion3is a laser source emitting pulsed light at a predetermined frequency and at a predetermined intensity. Alternatively, the pulses may be emitted in a non-periodic manner and/or the pulses may not have constant amplitudes. For example, the amplitudes and frequencies of the pulses can be self-determined based on a signal-to-noise ratio.

The light emitted by the light emission portion3passes the collimator6when exiting the distance measuring device1. The collimator6forms the light emitted by the light emission portion3into a substantially parallel light beam, referred to as emitted light8below.

When the emitted light8reaches the measurement object2, it is reflected back towards the distance measuring device1by the measurement object2. The emitted light that is reflected by the measurement object2forms measurement light5. In other words, the measurement light5is obtained by reflection of the emitted light8at the measurement object2. The measurement light5is converged onto a spot19of the light receiving portion4by the converging lens7located at the entrance of the distance measuring device1. The spot19is the surface of the light receiving portion4that receives the incident measurement light5. Although the spot19is shown to be circular, it should be clear to the skilled person that it may also have a different shape, for example elliptical.

Depending on how far the object2is from the distance measuring device1, the size of the spot19may vary. This is illustrated inFIG.2, which shows an example of a light receiving portion4. As shown inFIG.2, the light receiving portion4forms a detection region with a plurality of pixels10arranged in an array. The array has seven columns and seven rows, as an example. The light receiving portion4can be used to detect incident light, in particular incident measurement light5. In the present example, each pixel10has a square light receiving surface9. It should be noted that, for illustrative reasons, the pixels10are shown to be contiguous to each other inFIG.2, but they will ordinarily be not contiguous to each other, but spaced slightly apart, so that signal lines or the like may be arranged between adjacent pixels. Further, inFIG.2, the pixels10are arranged in a pattern with several rows and columns. However, they could also be arranged in a different geometry, for example in a circle, in radial arrangements, in asymmetric arrangements and the like. The same is also true for the drawings discussed below.

FIG.2shows four light spots19a-19das examples for the light spot19shown inFIG.1. The substantially round light spots19a-19ddiffer from one another with regard to their diameters. Depending on the diameter of the light spot19a-19d, a different number of pixels10are illuminated by the measurement light5. The diameter of each light spot19a-19dvaries, e.g. depending on how far the object2from which the measurement light5was received is located. The smaller the diameter of the light spot19a-19d, the further away (or the closer, depending on the optical system) the object2is located from the distance measuring device1. In the example ofFIG.2, the light spot19dis hence obtained from an object2that is closer than an object2that creates the light spot19cand so on. As shown inFIG.2, there may also be a shift in the center of the light spots19a-19ddepending on the distance of the object2due to the parallax effect.

The distance to the object2can be determined by analyzing a time of flight of the measurement light5. The time of flight of the measurement light5corresponds to the time it takes for the measurement light5to travel from the distance measuring device1to the object2and back to the distance measuring device1. By measuring a difference in time between the time at which a certain light pulse was emitted by the light emitting portion3and the time at which this light pulse is received by the light receiving portion4, the time of flight of the measurement light5can be detected. These time of flight measurements will be described in greater detail with reference toFIGS.7A-7Ebelow.

In addition to receiving the measurement light5that is emitted by the light emission portion3and reflected back by the object2, the light receiving portion4may also receive ambient (stray) light. Such ambient light may be caused by other light sources or may be the result of multiple reflections (echoes) of the emitted light beam at other reflection surfaces. The reception of such ambient light falsifies the determination of the distance to the object2performed by the distance measuring device1and is therefore undesirable. The distance measuring device1is capable of discriminating between pixels that receive measurement light5and pixels that do not receive measurement light5but only the ambient light, as will be described below.

FIG.3shows a view of a part of the distance measuring device1. In particular,FIG.3shows one pixel10of the light receiving portion4, which is connected to a discrimination portion11, a pixel output control portion12and an evaluation portion13. The discrimination portion11successively receives light reception signals RS1from the pixel10and determines whether the pixel10receives measurement light5based on the light reception signals RS1. In accordance with the light reception signal RS1, the discrimination portion11outputs a discrimination signal DR (discrimination result) indicating whether the light received by the pixel10includes measurement light5or only comprises ambient light. The discrimination signal DR may be a binary signal varying between a low and a high level. For example, the high level may indicate that the light received by the pixel10includes measurement light5and the low level may indicate that the light received by the pixel does not include measurement light5but includes only ambient light.

This discrimination signal DR is sent to the pixel output control portion12, which enables or disables the output of the pixel10depending on the received discrimination signal DR. That is, if the discrimination signal DR indicates that the pixel10receives measurement light5, the pixel output control portion12enables the output of the pixel10and forwards the light reception signal RS1as a light reception signal RS2to the evaluation portion13. In other words, if the pixel10receives measurement light5, the pixel output control portion sends the light reception signal RS2to the evaluation portion, with the light reception signal RS2corresponding to the light reception signal RS1from the pixel10.

By contrast, if the discrimination signal DR indicates that the pixel10only receives ambient light, the pixel output control portion12disables the output of the pixel10and forwards a light reception signal RS2which is not equal to the light reception signal RS1to the evaluation portion13. In this case, the light reception signal RS2may be, for example, a constant low-level signal.

Thus, the light reception signal RS2transferred from the pixel output control portion12to the evaluation portion13corresponds to the light reception signal RS1from the pixel10if the pixel10receives measurement light5. On the other hand, the light reception signal RS2is a constant low-level or the like if the pixel10does not receive any measurement light5, as indicated by the discrimination result DR.

In order to enable and disable the output of the pixel10, the pixel output control portion12may be implemented as a switch. For example, it may be a transistor, such as a FET (field effect transistor), where the discrimination signal DR is applied to the gate of the FET. The pixel output control portion12may also comprise one or more logic gates. It should be noted that the pixel output control portion12is located between the pixel10and the evaluation portion13. Also, in the present example, the pixel output control portion12is connected directly to the pixel10. In other words, no further circuit elements are provided between the pixel10and the pixel output control portion12.

The evaluation portion13generates a signal that contains information about the time of flight of a light beam emitted by the distance measuring device1and received by the pixel10associated with that evaluation portion13.

In the present embodiment, the light reception signal RS is evaluated by the evaluation portion13only when the pixel10receives measurement light5. If it only receives ambient light, the light reception signal RS is discarded and not considered for evaluating the distance to the object2.

Discrimination portions11and pixel output control portions12having the functionality described above may be provided for each of the pixels10of the light receiving portion4. In other words, there may be a one-to-one-to-one relationship between the pixels10, the discrimination portions11and the pixel output control portions12.FIG.4shows a distance measuring device100with such a configuration. In this distance measuring device100, a discrimination portion11a-11cand a pixel output control portion12a-12cis associated with each of the pixels10a-10cof the light receiving portion4. AlthoughFIG.4shows only three pixels10a-10cwith corresponding discrimination portions11a-11cand pixel output control portions12a-12c, it should be understood that the light receiving portion4can comprise more pixels10a-10calso each having their own discrimination portion11a-11cand pixel output control portion12a-12c. In one preferable embodiment, the distance measuring device100comprises 400 pixels arranged in an array of 50 rows of 8 pixels or 10 rows of 40 pixels each.

The functions of the discrimination portions11a-11cand pixel output control portions12a-12cassociated with each pixel are similar to the functions of the discrimination portion11and pixel output control portion12described in view ofFIG.3and will hence be explained only briefly in the following. The discrimination portions11a-11ccan also be regarded collectively constituting a single discrimination portion11. The pixel output control portions12a-12ccan also be regarded as collectively constituting a single pixel output control portion12.

The discrimination portion11adetermines, based on a signal RS1areceived by the pixel10a, if the pixel10areceives measurement light5or not, and accordingly sends a discrimination signal DRa to the pixel output control portion12a. The pixel output control portion12aenables or disables the output of the pixel10adepending on the discrimination signal DRa, i.e. depending on whether the pixel10areceives measurement light5or only ambient light. The light reception signal RS1ais only forwarded to an evaluation portion13aas the light reception signal RS2awhen the output of the pixel10ais activated by the pixel output control portion12a. Otherwise, if the pixel10adoes not receive the measurement light5, the light reception signal RS1ais not transmitted to the evaluation portion13a.

Similarly, the discrimination portion11bdetermines, based on a signal RS1breceived by the pixel10b, if the pixel10breceives measurement light5or not, and accordingly sends a discrimination signal DRb to the pixel output control portion12b. The pixel output control portion12benables or disables the output of the pixel10bdepending on the discrimination signal DRb, i.e. depending on whether the pixel10breceives measurement light5or only ambient light. The light reception signal RS1bis only forwarded to the evaluation portion13bas the light reception signal RS2bwhen the output of the pixel10bis activated by the pixel output control portion12b. Otherwise, if the pixel10bdoes not receive the measurement light5, the light reception signal RS1bis not transmitted to the evaluation portion13b.

Similarly, the discrimination portion11cdetermines, based on a signal RS1creceived by the pixel10c, if the pixel10creceives measurement light5or not, and accordingly sends a discrimination signal DRc to the pixel output control portion12c. The pixel output control portion12cenables or disables the output of the pixel10cdepending on the discrimination signal DRc, i.e. depending on whether the pixel10creceives measurement light5or only ambient light. The light reception signal RS1cis only forwarded to the evaluation portion13cas the light reception signal RS2cwhen the output of the pixel10cis activated by the pixel output control portion12c. Otherwise, if the pixel10cdoes not receive the measurement light5, the light reception signal RS1cis not transmitted to the evaluation portion13c.

Based on the light reception signals RS2a, RS2b, RS2c, the evaluation portion13a,13band13crespectively output a distance signal that is indicative of the distance to the object2. For example, the evaluation portion13a,13b,13cmay calculate an average value of the time of flight values determined from the light reception signals RS2a, RS2b, RS2c. Alternatively, the evaluation portions13a,13b,13cmay determine the distance to the object2using histograms, as explained further below. The evaluation portions13a,13b,13cmay be hard-wired on the same semiconductor chip as the pixels10, but it is also possible to realize the functionality of the evaluation portions13a,13b,13cwith a CPU that performs the necessary calculations.

The result of the evaluation, i.e. the distance to the object2, may be output on a display (not shown) with which the distance measuring device100is provided. Alternatively, it is also possible that the distance to the object2is output to another processing portion, such as a controller, a CPU, a computer, another electronic circuit or the like, or used to control another process.

The distance to the object2is determined based only on the signals from the pixels10a-10cthat receive measurement light5. The signals from pixels10a-10con which only ambient light is incident are not taken into account when determining the distance to the object2. The distance to the object2is thus determined with a higher accuracy.

In the distance measuring device100according to this embodiment, each pixel10a-10chas its own evaluation portion13a,13b,13cassociated thereto. The distance measuring device100does not comprise any combiners or multiplexers, thereby simplifying the structure of the distance measuring device100. In alternative embodiments, the signals from the respective pixels10a-10cwhich are forwarded by the respective pixel output control portions12a-12cmay also be combined using a combiner or a multiplexer (not shown). The use of a combiner or a multiplexer can be advantageous because light reception signals RS1from several pixels10can be forwarded to a single discrimination portion11via the combiner or multiplexer. A size of the distance measuring device100can thereby be reduced.

FIG.5is a flowchart illustrating a measurement method for determining a distance to a measurement object2using the distance measuring device1or100.

In a step S1, the distance measuring device1,100emits light8using the light emission portion3. In a step S2, measurement light5reflected from the object2is received by the light receiving portion4of the distance measuring device1,100.

In a step S3, the discrimination portion11,11a-11cdetermines for each pixel10,10a-10cwhether the received light is measurement light5and accordingly generates a discrimination signal DR, DRa-DRc. If it is determined in step S3that the light received by a pixel10,10a-10cis measurement light5, the pixel output control portion12enables the output of the light reception signals RS1of the pixel10,10a-10cas the light reception signal RS2and forwards the output to the evaluation portion13in step S4. Then, in a step S5, the evaluation portion13,13a-13cdetermines the distance to the object2based on the light reception signals RS2.

Alternatively, if it is determined in step S3that the light received by a pixel10,10a-10cis not measurement light5(i.e. when it is ambient light or noise), the pixel output control portion12disables the output of the pixel10,10a-10cand does not forward the light reception signal RS1as the light reception signal RS2in step S6. It should be noted that “enabling the output” and “disabling the output” may correspond to turning a switch of the pixel output control portion12on or off, respectively.

FIG.6shows a distance measuring device101according to a second embodiment. In the distance measuring device101of this embodiment, the discrimination portion11includes a detection portion14and a counting portion15. The detection portion14and the counting portion15contribute to determining whether the light received by the associated pixel10is measurement light5or ambient light. The functions of the detection portion14and the counting portion15will be explained in greater detail below in view ofFIGS.7A-7E and8.

FIG.7Ashows an example of the intensity of the light emitted by the light emission portion3over time. As shown inFIG.7A, the light emission portion3emits light8at regular intervals towards the object2. That is, a light pulse17of light is emitted at times tA1, tA2, tA3, tA4, tA5and tA6, which are spaced from one another by a time interval Δt, during a predetermined time period ΔT. The time interval Δt may be fixed, but there is no limitation thereto.

FIG.7Bshows the light intensity of the light received by a pixel10of the light receiving portion4that receives measurement light5as a function of time. The light received by the pixel10has two components: an ambient light component18that basically corresponds to a background noise and measurement light5superimposed on the ambient light18. The intensity of the measurement light5is much larger than that of the ambient light18.

The ambient light18is a random distribution of photons that reach the pixel10. The measurement light5is received as pulses27separated by the same fixed time interval Δt that spaces the emitted pulses. The pulses27of measurement light5are received at times tB1, tB2, tB3, tB4, tB5and tB6, which are respectively shifted by a time shift Δd as compared to the times tA1, tA2, tA3, tA4, tA5and tA6. The time shift Δd results from the time it takes to the light8emitted by the light emission portion3to travel to the object2and back to the distance measuring device101, plus any other time delays due to, for instance, the electronics, wiring or the like. The time shift Δd varies as a function of the distance to the object2(time of flight) and can be evaluated to determine the distance to the object2. InFIG.7B, the time shift Δd is not necessarily depicted in the same scale as the time interval Δt.

FIG.7Cshows the light intensity of the light received by a pixel10of the light receiving portion4that does not receive measurement light5. The received light signal of such a pixel10only comprises ambient light18and noise, characterized by the random distribution of photons reaching the pixel10.

FIG.7Dshows an example of a counting process performed using the counting portion15. The detection portion14and the counting portion15contribute to determining whether the light received by the associated pixel10is measurement light5or ambient light18. To this end, the detection portion14determines whether light is received at the corresponding pixel10during a plurality of first predetermined time intervals TM1-TM6and a plurality of second predetermined time intervals TP1-TP6.

The first predetermined time intervals TM1-TM6are set such that they include the times tB1, tB2. . . , at which the pulsed measurement light5is incident on the distance measuring device101and the second predetermined time intervals TP1-TP6are set such that they do not include these times tB1, tB2. . . . For example, the first predetermined time intervals TM1-TM6may be set to start at the time when the pulsed light8is emitted by the distance measuring device101and to stop at a time that is sufficiently long such that the measurement light5reflected from the object2is included in the time interval. The length of the first predetermined time intervals TM1-TM6is equal to the length of the second predetermined time intervals TP1-TP6and may be several nanoseconds, for example 5 to 500 nanoseconds, depending on the range of the distance measuring device101. Furthermore, this length may be variable. For example, it is possible that the length of this period can be adjusted manually by the user, or that it is adjusted automatically depending on the brightness of the ambient light or the desired distance range to measure. Thus, it is possible to adjust the sensing conditions to the ambient light conditions, thereby improving the accuracy even further.

In this example, each pixel10is constituted by one single photon detector, for example a single photon avalanche diode (SPAR), which allows the detection of single photons incident on the pixel10. That is, every time a photon is incident on a pixel10, the pixel generates a SPAD current that is detected by the detection portion14.

Photons are incident on the pixel10in discrete events that are subject to a certain statistical distribution. Accordingly, during a given time interval T, there may be zero, one or a plurality of photons incident on the pixel10. The detection portion14is configured such that it determines that the pixel10has received light during a given time interval T if a minimum number n of photons are incident on the pixel during that interval T. This minimum number n may be 1, for example, but it may also be a greater number.

FIG.7Eshows a table indicating the detection results of the detection portion14. That is, the table shows for each of the first and second predetermined time intervals TM1-TM6and TP1-TP6whether light is received or not, i.e. whether the minimum number n of photons has been received. InFIG.7E, “+1” indicates the first predetermined time intervals TM1-TM6at which light is received at the pixel10, “−1” indicates the second predetermined time intervals TP1-TP6at which light is received at the pixel10and “0” indicates that no signal is received at the pixel10.

Assigning the number “+1” to first predetermined time intervals TM1-TM6during which a signal is received at the pixel10corresponds to assigning a first weight (namely “+1”) to said first predetermined time intervals TM1-TM6. Assigning the number “−1” to second predetermined time intervals TP1-TP6during which a signal is received at the pixel10corresponds to assigning a second weight (namely “−1”) to said second predetermined time intervals TP1-TP6. This weighting can be performed by a weighting portion (not shown). If no photon is received by the pixel10during the interval T, then it is assigned a weight of “0”.

InFIG.7E, the first row (“inside spot”) comprises an example of results from the detection portion14when the corresponding pixel10is inside the light spot19and irradiated by light comprising measurement light5(as shown inFIG.7B), while the second row (“outside spot”) comprises an example of results from the detection portion14when the corresponding pixel10is outside the light spot19and is irradiated by light that only comprises ambient light18(as shown inFIG.7C).

A pixel10receiving only ambient light18is irradiated randomly by photons. Thus, when the detection portion14measures the presence of light at the first and second predetermined time intervals TM1-TM6and TP1-TP6and over a long enough time interval ΔT, statistically, the number of times that the detection portion14detects light at the first predetermined time intervals TM1-TM6should be more or less equal to the number of times that it detects light at the second predetermined time intervals TP1-TP6.

By contrast, a pixel10receiving measurement light5reliably receives the measurement signal5so that overall, the number of times that the discrimination portion11detects light at the first predetermined time intervals TM1-TM6is greater than the number of times that it detects light at the second predetermined time intervals TP1-TP6. It should be noted that since the incidence of light as photons on the pixels10is a stochastic process, light may not necessarily be detected during all time intervals TM1-TM6, even if the pixels10are within the spot19and thus subject to measurement light5. An example of this is given for the period TM3, during which no light is detected inside the spot of measurement light5. However, generally speaking, the probability that light is detected inside the spot of measurement light5during the time intervals TM1-TM6is much greater than during the time intervals TP1-TP6

The counting portion15counts the number of times that the detection portion14detects a signal at the first predetermined time intervals TM1-TM6and the number of times that the detection portion14detects a signal at the second predetermined time interval TP1-TP6and compares these. When the result of this comparison is equal to or greater than a predetermined comparison threshold, for example greater than or equal to +2, the detection portion14determines that the signal received by the pixel10comprises measurement light5. Otherwise, if the result of the comparison is smaller than the predetermined comparison threshold, the detection portion14determines that the signal received by the pixel10comprises only ambient light18. The pixel output control portion12accordingly enables or disables the output of the pixel10, as described above. This calculation corresponds to summing up (integrating) the weighted counts in each row of the table ofFIG.7E. For example, for the row marked “inside spot”, the sum is +2, which is equal to the comparison threshold, so that the discrimination portion11regards the pixel10as being inside the spot and sends a corresponding discrimination result signal DR to the pixel output control portion12.

An advantage of the present embodiment is that the discrimination between pixels10that receive measurement light5and pixels10that do not receive measurement light can be continued during the actual measurement. That is to say, the discrimination portion11can be configured to constantly monitor the pixel10, so that only the signals from those pixel that actually receive measurement light5are taken into account for the calculation of the measurement result. In the present embodiment, the reception of six light pulses is monitored, but needless to say, the number of light pulses whose reception is monitored may also be larger or smaller than that. In one possible embodiment, the above-described discrimination is performed always on the basis of the last m (e.g. 6) light pulses emitted during a predetermined time period ΔT (e.g. 1 microsecond). Such a dynamic forwarding of the output of the pixels10is advantageous over an arrangement in which it is first determined which pixels are inside the spot, and then the pixels that are outside the spot are turned off, e.g. by interrupting the voltage supply to those pixels.

FIG.8is a flowchart illustrating a measurement method for determining a distance to a measurement object according to a second embodiment. The method ofFIG.8can be implemented by the distance measuring device101according to the third embodiment.

In the following, only the differences between the method according to the first embodiment (FIG.5) and the second embodiment will be described. In the second embodiment, step S3comprises the steps S7, S8and S9. In step S7, the detection portion11detects the signals at the pixel10at the first and second predetermined time intervals TM1-TM6and TP1-TP6. In step S8, the counting portion15counts the number of times that the detection portion14detects a signal at the first predetermined time intervals TM1-TM6and the number of times that the detection portion14detects a signal at the second predetermined time intervals TP1-TP6during the time ΔT.

In step S9, the number of times that the detection portion14detects a signal at the first predetermined time intervals TM1-TM6and the number of times that the detection portion14detects a signal at the second predetermined time intervals TP1-TP6during the time ΔT are compared and the result of this comparison is compared to the comparison threshold. If the comparison result is greater than or equal to the comparison threshold, the output of the pixel10is enabled in step S4. Otherwise, the output is disabled at step S6, and the steps S1, S2, S7, S8and S9are repeated for the entire time ΔT.

After the output of the pixel10has been evaluated by the evaluation portion13in step S5or after the output of the pixel10has been disabled in step S6, the process returns to step S1.

FIG.9shows a histogram generated at step S5by the evaluation portion13and allowing to determine a travel time tdof the measurement light5. The histogram is generated by counting the number of pixels for which a certain time of flight is measured and corresponds to a distribution of the determined times of flight. The mean value of the times of flight of the histogram is determined as the travel time tdand the distance to the object2is derived therefrom. It is also possible to estimate the distance without using a histogram, for example by using the averaging described above.

FIG.10A to10Gshow alternative layouts for the distance measuring device1,100,101. The layout ofFIG.10Acorresponds to the layout shown inFIG.4, in which each pixel10a-10chas its own discrimination portion11a-11c, its own pixel output control portion12a-12cand its own evaluation portion13a-13cassociated therewith.

The layout inFIG.10Bdiffers from that inFIG.10Ain that the three pixels10a-10cshare a single discrimination portion11. Using a single discrimination portion11can be advantageous in that less components are needed, leading to a less space-consuming and power-consuming distance measuring device1,100,101. As illustrated inFIG.10B, the pixels10a-10cmay be non-adjacent.

The layout inFIG.10Cdiffers from that inFIG.10Ain that the three pixels10a-10cshare a pixel output control portion12. Using a single pixel output control portion12can be advantageous in that less components are needed, leading to a less space-consuming and power-consuming distance measuring device1,100,101.

The layout inFIG.10Ddiffers from that inFIG.10Ain that the three pixels10a-10cshare a single evaluation portion13. Using a single evaluation portion13can be advantageous in that less components are needed, leading to a less space-consuming and power-consuming distance measuring device1,100,101.

The layout inFIG.10Ecorresponds to a mixture of the layouts ofFIGS.10B and10D, in which each pixel10a-10cis connected to own pixel output control portion12a-12c, but only a single discrimination portion11and a single evaluation portion13are provided for a plurality of (or all) pixels10a-10c.

The layout inFIG.10Fcorresponds to a mixture of the layouts ofFIGS.10C and10D, in which each pixel10a-10chas its own discrimination portion11a-11c, but in which the pixels10a-10cshare a single pixel output control portion12and a single evaluation portion13.

In the layout ofFIG.10G, the pixels10a-10cshare a single discrimination portion11, a single pixel output control portion12and a single evaluation portion13.

Although the present invention has been described in accordance with preferred embodiments, it is obvious for the person skilled in the art that modifications are possible in all embodiments. For example, the number of pixels10of the light receiving element4can be increased or reduced. Instead of determining a comparison result, the counting portion can integrate the received light signals over time, for example in case of analog photosensors, providing an electric signal proportional to the incoming detected light (e.g. APD, CCD, . . . ) and not just digital pulses as in case of SPADs and related sensing circuitry.

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