Expansion of the dynamic range of SPAD-based detectors

Abstract

A method for operating a lidar device by means of a control device is disclosed, wherein at least one radiation pulse is emitted by a radiation source into a scanning area, and the radiation reflected and / or backscattered from the scanning area is received by a detector having a plurality of SPAD cells and converted into electrical count pulses, wherein at least one radiation pulse having an intensity profile with an extended fall is generated and the detector is read out by means of DC-coupled readout electronics. Furthermore, a control device and a lidar device are disclosed.

Description

Technical Field

[0001] The present invention relates to a method for operating a lidar device by a control device and a lidar device for scanning a scanning area. Background Art

[0002] Automatically operable vehicles and driving functions are becoming increasingly important on public roads. For the technical implementation of such vehicles and driving functions, lidar sensors are generally required. The lidar sensor generates electromagnetic radiation, such as a laser beam, here, and uses this radiation to scan the scanning area. Based on time-of-flight analysis, the distance between the lidar sensor and an object in the scanning area can be determined.

[0003] The detectors of lidar sensors with a large operating range can be configured to be particularly insensitive to the influence of temperature and aging if these detectors consist of SPAD (single photon avalanche diode) units. Each macro pixel of the detector consists of a plurality of SPA diodes or SPAD units and can receive incident radiation from a defined solid angle range.

[0004] Determined by the operating mode of the SPAD unit, the dead time (Totzeit) must be considered in the structural design of the detector. The dead time is caused by the time period required to suppress the avalanche current in the SPAD unit and the renewed elevation (Anheben) applied to the SPAD unit for further photon detection. During the dead time of the SPAD unit, photons cannot be further detected, and in addition, it cannot be determined whether only one photon or multiple photons are detected during the activation time of the SPAD unit.

[0005] Generally, the readout electronics are operated based on alternating current to suppress the avalanche current in the SPAD unit, thereby increasing the dead time and reducing the number of photons detected maximally per time (pro Zeit). In addition, the dynamic range of the hitherto known SPAD-based detectors is insufficient. Summary of the Invention

[0006] The task on which the present invention is based can be regarded as: providing a method for operating a lidar device and a lidar device with an expanded dynamic range.

[0007] This task is solved by the corresponding subject matter of the independent claims. Advantageous configurations of the present invention are the subject matter of the corresponding dependent claims.

[0008] According to one aspect of the present invention, there is provided a method for operating a lidar device by a control device. In one step, at least one radiation pulse is emitted into the scanning area by a radiation source.

[0009] The lidar device can thus operate as a pulsed lidar device. Depending on the configuration of the lidar device, the radiation angle of at least one radiation pulse can vary in the vertical direction and / or the horizontal direction and, thus, a solid angle can be scanned.

[0010] The radiation reflected and / or backscattered from the scanning area is received by a detector having a plurality of SPAD cells and converted into electrical count pulses. The detector can thus be constituted by an array of SPAD cells and has a planar extension.

[0011] According to the invention, at least one radiation pulse having a prolonged descending intensity profile is generated and the detector is read out by means of a direct current coupled readout electronics. The descending pulse edge of the radiation pulse extends over a longer time period than the ascending pulse edge of the radiation pulse.

[0012] According to another aspect of the invention, a control device is provided which is arranged to carry out the method according to the invention. The control device can be integrated in the lidar device or configured as an external control device.

[0013] The analysis and processing electronics can preferably be configured as an avalanche suppression circuit or a so-called quenching circuit. By using direct current coupled analysis and processing electronics instead of alternating current coupled analysis and processing electronics, the activation time of the SPAD cells can be precisely controlled and matched to the duration of the generated or received radiation pulse in order to measure the arrival time of the radiation pulse with high precision. In particular, the activation time can be prolonged during which the SPAD cells are light-sensitive.

[0014] Due to the targeted control of the activation time of the SPAD cells in combination with the prolonged radiation pulse, a higher average exposure time of the SPAD cells can be achieved, which results in a larger dynamic range of the detector. The dynamic range can be extended by the method according to the invention to far more than one million detectable photons during the received radiation pulse.

[0015] In particular, the dynamic range of the lidar device can be increased in a technically simple manner by matching the operation of the radiation source and by changing the analysis and processing electronics of the detector.

[0016] Furthermore, by means of the prolonged descending edge or intensity profile of the radiation pulse, multiple radiation pulses of decreasing intensity can be simulated by controlling the radiation source by means of the control device, so that the exposure of the scanning area with multiple short radiation pulses can be dispensed with and thus the use of a particularly high-frequency radiation source can be dispensed with.​

[0017] The use of SPAD cells in detecting reflected radiation pulses is particularly advantageous due to their small size, low preload (Vorspannung), and power consumption, as well as their compactness, robustness, and reliability. In addition, SPAD cells offer higher quantum efficiency and higher precision in detecting the arrival time of photons.

[0018] According to one embodiment, the radiation source is operated by the control device such that at least one radiation pulse with an exponentially decreasing, quadratically decreasing, or linearly decreasing intensity profile is generated. Based on the shape of the decreasing intensity profile of the generated radiation pulse, multiple successively emitted short radiation pulses can be targeted simulated. With the increase in the length of the decreasing intensity profile and thus the radiation power, the dynamic range of the lidar device can be increased.

[0019] According to another embodiment, the readout electronics are configured as an active or passive avalanche suppression circuit. The avalanche suppression circuit enables the activation of the SPAD cell to detect photons by applying a voltage shortly before the breakdown voltage of the SPAD cell. Accordingly, the SPAD cell triggered by photons can be reactivated by reducing the voltage applied to the SPAD cell to the breakdown voltage or lower.

[0020] By receiving photons, the breakdown voltage of the SPAD cell can be overcome and a short-duration current rise can be measured. This current rise can be measurable or persistent, for example, over a duration of 10 ns. By configuring the readout electronics as an active or passive avalanche suppression circuit, the analysis and processing electronics can be flexibly matched to the requirements of the lidar device.

[0021] According to another embodiment, the SPAD cells of the detector are controlled by the readout electronics with a variably adjustable activation time. Through the DC coupling of the analysis and processing electronics, the avalanche current of each SPAD cell of the detector is not interrupted by the compulsorily short activation time of the readout electronics, but by the slowly decreasing edge of the reflected and / or backscattered radiation, which must be below a defined value. Thus, the SPAD cells are activated throughout the entire duration of the received radiation pulse or at least during the partial duration for receiving photons. By this measure, the SPAD cells can be exposed for a longer time, thereby increasing the dynamic range of the detector.

[0022] Furthermore, the DC coupling of the analysis and processing electronics of the SPAD cells enables targeted control of the activation time and thus targeted control of the possible sensitive duration of each emitted radiation pulse. The activation time of the SPAD cells can be achieved by setting the following time period during which the SPAD cells are loaded with a predetermined operating voltage slightly below their breakdown voltage.

[0023] According to another aspect of the invention, a lidar device for scanning a scanning area is provided. The lidar device has at least one radiation source for generating electromagnetic radiation, at least one detector for receiving the radiation backscattered and / or reflected from the scanning area, and a control device connected to the readout electronics.

[0024] The detector is configured as a SPAD array and is connected to the readout electronics to operate the SPAD array, wherein the control device is arranged to analyze the output of the readout electronics and to control the at least one radiation source.

[0025] According to an advantageous embodiment of the lidar device, the analysis and processing electronics are configured as DC-coupled analysis and processing electronics. By this measure, in contrast to AC coupling, it is possible to prevent multiple photons successively received by the SPAD cells from extending the so-called dead time of the SPAD cells.

[0026] Furthermore, this enables the omission of complex control of multiple laser sources, and the analysis and processing electronics of the SPAD cells can be configured in a technically simpler manner.

[0027] According to another embodiment, the at least one radiation source is controllable by the control device such that the generated radiation can be emitted into the scanning area as a radiation pulse with an extended falling intensity profile. This enables the avoidance of using multiple radiation sources or technically complex control of the radiation sources for generating multiple rapidly successive emitted radiation pulses in order to achieve an increase in the dynamic range. Description of the Drawings

[0028] The preferred embodiments of the invention are explained in more detail below with the aid of highly simplified schematic diagrams. Shown here are:

[0029] Figure 1 A schematic diagram of a lidar device according to one embodiment;

[0030] Figure 2 A schematic curve showing the temporal voltage profile for explaining the AC-coupled analysis and processing electronics;

[0031] Figure 3Shows a schematic graph for illustrating the voltage change process over time of an analysis and processing electronic device for DC coupling;

[0032] Figure 4 Shows a schematic graph for indicating the intensity change process of a matched radiation pulse;

[0033] Figure 5 Shows a graph of photons received or counted by a detector in a time - correlated manner. Detailed implementation mode

[0034] Figure 1 Shows a schematic diagram of a lidar device 1 according to an embodiment. The lidar device 1 has a radiation source 2, which is used to generate radiation or radiation pulses 3.

[0035] The radiation source 2 is configured as a laser and can be electrically controlled by a control device 4 and be excited to generate radiation 3. The radiation source 2 can generate radiation 3 having a wavelength, for example, in the infrared, visible or ultraviolet wavelength range.

[0036] In addition, the lidar device 1 has a detector 6. The detector 6 has a plurality of SPAD units 8, and all of the plurality of SPAD units are connected to an analysis and processing electronic device 10. The analysis and processing electronic device 10 is preferably configured as a DC - coupled analysis and processing electronic device 10, wherein, for example, the DC component of the voltage U applied to the SPAD unit 8 is not filtered by a capacitor. The SPAD units 8 of the detector 6 are arranged in a planar manner and can receive or detect the radiation 12 reflected and / or backscattered from the scanning area A.

[0037] The received radiation 12 and especially the photons of the received radiation 12 are detected by the analysis and processing electronic device 10 in the form of short current pulses and converted into digital measurement data. Alternatively or additionally, this step can be carried out in combination with the control device 6.

[0038] By using a DC - coupled analysis and processing electronic device 10, not only is a so - called count triggered by the received photons as in an AC - coupled analysis and processing electronic device, and through an extended activation time Z, for example, six or more counts can be detected per activation time.

[0039] In Figure 2 exemplarily presents the voltage U applied to the SPAD unit 8 in an exemplary AC - coupled analysis and processing electronic device. The voltage U required to activate the SPAD unit 8 only exists briefly due to the oscillation of the voltage U.

[0040] As an alternative thereto, in Figure 3A schematic graph showing the voltage change process U over time of the analysis and processing electronic device 10 for explaining DC coupling is shown.

[0041] In Figure 3 two different voltage change processes U are shown, which can be applied to the SPAD unit 8 so that photons of the received radiation 12 can be detected. The difference between the voltage change processes U lies especially in the activation time Z, which can be variably set by the analysis and processing electronic device 10 with DC coupling.

[0042] In the case of the analysis and processing electronic device 10 with DC coupling, the dynamic range is expanded because the length of the voltage signal or the activation time Z provides information about how much intensity I the received radiation 12 has. In Figure 2 the shown AC coupling, this is no longer possible after a very short time.

[0043] Figure 4 A schematic graph showing the matched intensity change process I of the generated radiation pulse 3 is shown. Different from a Gaussian-shaped radiation pulse, the shown intensity change process I of the radiation pulse 3 has an extended descending intensity edge 14. In the presented embodiment, the descending intensity edge 14 is configured to be exponentially decreasing. For example, the descending intensity edge 14 can have a remaining intensity I of 1% after a time t of 30 ns.

[0044] Figure 5 A diagram showing the photons received or counted by the detector 6 in a time-correlated manner is shown. Especially shown is the change of the photon number N over time t. Here, as the time t for detecting the received radiation 12 increases, a larger photon number N can be detected.

[0045] The dynamic range can be described as the range between the minimum detectable photon number N and the maximum detectable photon number N. Here, the expansion of the dynamic range enables an improved distinguishability of the detectable photon number N. For example, a smaller detectable photon number N and an increasing detectable photon number N can be distinguished by a larger dynamic range. This relationship and distinguishability are schematically shown in Figure 5 In.

[0046] Digital data is Figure 5 exemplarily illustrated in and is only used to show the difference.

Claims

1. A method for operating a lidar device (1) by means of a control device (4), wherein, - At least one radiation pulse (3) is emitted into the scanning area (A) by means of a radiation source (2), - radiation (12) reflected and / or backscattered from the scanning area (A) is received by a detector (6) having a plurality of SPAD cells (8) and the radiation reflected and / or backscattered from the scanning area is converted into electrical count pulses (N). It is characterized in that at least one radiation pulse (3) with an extended falling intensity edge (14) is generated, such that the falling pulse edge of the radiation pulse (3) extends over a longer time period than the rising pulse edge of the radiation pulse (3), and the detector (6) is read out by means of DC-coupled readout electronics (10), wherein the radiation source (2) is operated by means of the control device (4) such that at least one radiation pulse (3) with an exponentially falling intensity edge (14) is generated.

2. The method according to claim 1, wherein, The readout electronics (10) is configured as an active or passive avalanche suppression circuit.

3. The method according to claim 1 or 2, wherein, The SPAD cells (8) of the detector (6) are controlled by means of an activation time (Z) that can be variably set by means of the readout electronics (10).

4. A control device (4), wherein, The control device (4) is provided for carrying out the method according to any one of the preceding claims.

5. A lidar device (1) for scanning a scanning area (A), the lidar device having at least one radiation source (2) for generating electromagnetic radiation (3), at least one detector (6) for receiving radiation (12) backscattered and / or reflected from the scanning area (A), and having a control device (4) connected to readout electronics (10) according to claim 4, wherein, The detector (6) is configured as a SPAD array (8) and is connected to the readout electronics (10) for operating the SPAD array (8), wherein the control device (4) is provided for analyzing the output of the readout electronics (10) and for controlling the at least one radiation source (2).

6. The lidar device according to claim 5, wherein, The analysis electronics (10) is configured as DC-coupled analysis electronics.

7. The lidar device according to claim 5 or 6, wherein, The at least one radiation source (2) is controllable by means of the control device (4) such that the generated radiation (3) can be emitted into the scanning area (A) as a radiation pulse with an extended falling intensity edge (14).

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