Light detection and ranging system

<u>Background</u>

The present invention relates to a method for determining a range to a target. It finds particular application in conjunction with a light detection and ranging (LIDAR) system and will be described with particular reference thereto. It will be appreciated, however, that the invention is also amenable to other applications.

A light detection and ranging (LIDAR) system transmits a pulse of light (e.g., atom a Isser), which is reflected from a target. An optical receiver detects the reflected light, and the range to the target is computed &amp;om the delay time between tho transmission of the light pulse and the detection of the reflected light. The receiver field-of-view and the transmitted light beam are usually matched and co-aligned to ensure maximum light collection efficiency. If the LIDAR contains a fast optical scanner (such as a rapidly moving mirror), it is possible for the field-of-view of the receiver to lose alignment with respect to the projected light beam. Such alignment loss in caused by a change in the pointing direction that occurs during the time required for the light pulse to travel to the target, reflect, and then travel back to the receiver. The extent of this misalignment is a "lag angle," which depends on the speed of the scanner and the range of me target. For a scanning LIDAR that is "diffraction-limited" (i.e., the light beam divergence is limited only by the wavelength and the diameter of the beam at the exit aperture of the LIDAR), the scanner angular speed ω_κ (measured in radiana/ second) for which the lag angle is half of the transmitted light beam divergence is: <maths id="math0001" num=""><math display="block"><msub><mi>ω</mi><mrow><mn>1</mn><mo>/</mo><mn>2</mn></mrow></msub><mo>=</mo><mover><mi>θ</mi><mo>˙</mo></mover><mo>=</mo><mfrac><mrow><mn>0.5</mn><mo>⋅</mo><mi>λ</mi><mo>⋅</mo><mi>c</mi></mrow><mrow><mn>2</mn><mo>⋅</mo><mi>D</mi><mo>⋅</mo><mi>R</mi></mrow></mfrac><mo>,</mo></math><img file="EP2187232B1_D0001.tif" /></maths> where λ is the wavelength of the transmitted light (in meters), c is the speed of light (- 3 x 10⁸ meters/second). <i>D</i> is the transmitted light beam diameter at the exit aperture of the LIDAR (in meters), and <i>R</i> is the range to the target (in meters).

If the receiver field-of-view is initially aligned to a transmitted light cone of illumination, the received signal is reduced by the lag angle. The effect becomes worse at longer ranges and as the scanner speed increases.

If the LIDAR system is not diffraction-limited and the transmitted beam divergence is instead Δ, then the formula above becomes: <maths id="math0002" num=""><math display="block"><msub><mi>ω</mi><mi>½</mi></msub><mo>=</mo><mover><mi>θ</mi><mo>˙</mo></mover><mo>=</mo><mfrac><mrow><mn>0.5</mn><mo>⋅</mo><mi mathvariant="normal">Δ</mi><mo>⋅</mo><mi>c</mi></mrow><mrow><mn>2</mn><mo>⋅</mo><mi>R</mi></mrow></mfrac><mn>.</mn></math><img file="EP2187232B1_D0002.tif" /></maths> For example, if the divergence of the transmitted light beam is 2 milliradians (mrad) and the range of a target is 3 km, the angular speed at which the lag angle is half of the transmitter beam width is 50 radians/second, or 480 revolutions/minute (rpm). In this case if the receiver field-of-view is matched to the transmitted light beam divergence, the lag angle is still sufficiently small for the optical receiver to detect some reduced amount of scattered light from the target, but for scanner speeds greater than 960 rpm, the lag angle effect causes the receiver field-of-view to completely miss or obscure the signal from 3 km and beyond. The condition for this complete obscuration is: <maths id="math0003" num=""><math display="block"><msub><mi>ω</mi><mi mathvariant="italic">obscure</mi></msub><mo>=</mo><mover><mi>θ</mi><mo>˙</mo></mover><mo>=</mo><mfrac><mrow><mi mathvariant="normal">Δ</mi><mo>⋅</mo><mi>c</mi></mrow><mrow><mn>2</mn><mo>⋅</mo><mi>R</mi></mrow></mfrac><mn>.</mn></math><img file="EP2187232B1_D0003.tif" /></maths>

All LIDAR systems, whether they are scanning or staring, must often contend with another issue-large signal dynamic range. Signal dynamic range is the ratio of the maximum detectable light signal intensity (i.e, detector saturation) to the minimum detectable light signal intensity. The detected signal decreases rapidly with increasing target distance. Therefore, the received signals from targets at closer ranges may over-saturate the detector, while those from targets at longer ranges may be barely detectable. A design technique known as "geometric compression" can reduce the signal dynamic range by controlling the fixed overlap of the transmitter and receiver optical fields-of-view, the separation of the receiver and transmitter optics, and the shadowing of the receiver by the transmitter optics to attenuate the close-range signal. The time delay of the received light signal with respect to the transmitted light pulse does not enter into this compression calculation since these design parameters are static. Geometric compression can benefit both scanning and staring LIDAR systems. <patcit id="pcit0001" dnum="US4311385A"><text>US 4311385</text></patcit> discloses a detection scanning laser system having optics that compensate for angular deviation of a received signal. A control component rotates the lag angle compensating optics to optically align a reference signal and a received signal to be parallel to each other along an optical path.

The present invention provides a new and improved apparatus and method which addresses the above-referenced problems.

Summary

In one embodiment, a light detection and ranging system includes a mirror unit rotating around a scan axis. The mirror unit includes a receiving portion and a transmitting portion offset by an angle about the scan axis relative to a surface plane of the receiving portion. Respective centroids of the receiving and transmitting portions are positioned at a common point on the scan axis while the receiving and transmitting portions rotate around the scan axis. A transmitter transmits a light pulse toward the mirror unit. The transmitting portion is positioned to reflect the light pulse toward a target. A receiver is positioned to reflect the light pulse reflected from the target toward the receiver. The angle offset compensates for a change between a cone of illumination of the transmitting portion and a field-of-vlew of the receiving portion resulting from the rotation of the mirror unit.

Brief Description of the Drawings

In the accompanying drawings which are incorporated in and constitute a part of the specification, embodiments of the invention are illustrated, which, together with a general description of the invention given above, and the detailed description given below, serve to exemplify the embodiments of this invention.

<figref idref="f0001"><b>FIGURE 1</b></figref> illustrates a side view of a scanning system in accordance with one embodiment of an apparatus illustrating principles of the present invention;

<figref idref="f0002"><b>FIGURE 2</b></figref> illustrates a top view of the scanning system of <figref idref="f0001"><b>FIGURE 1</b></figref>;

<figref idref="f0003"><b>FIGURE 3</b></figref> illustrates a schematic representation of uncompensated transmitter and receiver fields-of-view;

<figref idref="f0004"><b>FIGURE 4</b></figref> illustrates a graph of a received signal vs. range for the uncompensated transmitter cone of illumination and the receiver field-of view shown in <figref idref="f0003"><b>FIGURE 3</b></figref>;

<figref idref="f0003"><b>FIGURE 5</b></figref> illustrates a schematic representation of a compensated transmitter cone of illumination and the receiver field-of view, in accordance with one embodiment of an apparatus illustrating principles of the present invention; and

<figref idref="f0005"><b>FIGURE 6</b></figref> illustrates a graph of a received signal vs. range for the compensated transmitter cone of illumination and the receiver field-of-view shown in <figref idref="f0003"><b>FIGURE 5</b></figref>.

Detailed Description of Illustrated Embodiment

With reference to <figref idref="f0001"><b>FIGURE 1</b></figref><b>,</b> an optical scanning LIDAR system <b>10</b> includes a transmitter <b>12</b> and an optical receiver <b>14.</b> A mirror unit <b>16</b> includes a transmitting portion <b>20</b> and a receiving portion <b>22.</b> In one embodiment, both the transmitter <b>12</b> and optical receiver <b>14</b> rotate (scan) together at a constant rotation (speed) in one direction. More specifically, rotating elements, specifically the mirror unit <b>16,</b> rotate (as illustrated by <b>24</b>) at a predetermined speed about a scan axis <b>26.</b> The transmitting and receiving portions <b>20, 22,</b> respectively, are positioned concentrically to have a common centroid <b>30,</b> The centroids <b>30</b> are positioned at a common point on the scan axis <b>26.</b>

A light pulse <b>32</b> leaving the transmitter <b>12</b> passes through at least one lens <b>34</b> for collimating the light pulse <b>32.</b> The light pulse <b>32</b> is then reflected off a first mirror <b>36</b> (e.g., a flat mirror) toward the transmitting portion <b>20</b> of the mirror unit <b>16,</b> where the light pulse <b>32</b> is reflected toward a target <b>40.</b> A transmitter cone of illumination <b>42</b> (see <figref idref="f0003"><b>FIGURES 3</b> and <b>5</b></figref>) of the light pulse <b>32</b> transmitted from the transmitting portion <b>20</b> to the target <b>40</b> is defined by a divergence determining the spot size of the light pulse <b>32</b> on the target <b>40.</b> The light pulse <b>32</b> reflects off the target <b>40</b> back toward the receiving portion <b>22</b> of the mirror unit <b>16.</b> A receiver field-of-view <b>44</b> (see <figref idref="f0003"><b>FIGURES 3 and 5</b></figref>) of the light pulse <b>32</b> reflected from the target <b>40</b> to the receiving portion <b>22</b> is defined by the full range of incidence angles that could detect a light pulse <b>32</b> on the receiving portion <b>22</b> and is normally matched in divergence to the transmitted cone of illumination. The reflected light pulse <b>32</b> is reflected off the receiving portion <b>22</b> of the mirror unit <b>16</b> toward a circular (e.g., spherical) mirror <b>46,</b> which reflects the light pulse <b>32</b> toward a second mirror <b>50</b> (e.g., a flat mirror). The light pulse <b>32</b> is reflected off the second mirror <b>50,</b> passes through at least one lens <b>52,</b> and is detected by the optical receiver <b>14.</b>

Electronics <b>54</b> communicating with the optical receiver <b>14</b> determine a range to the target <b>40</b> as a function of the time the light pulse is received at the optical receiver <b>14.</b> In one embodiment, the range of the target <b>40</b> is determined measuring a delay time of the light received at the optical receiver <b>14</b> with respect to the transmission of the original light pulse.

The period of time between which the light pulse <b>32</b> leaves the transmitter <b>12</b> and is received at the optical receiver <b>14</b> is referred to as the delay time. The rotational motion of the rotating elements during the delay time displaces the receiver field-of-view <b>44</b> (see <figref idref="f0003"><b>FIGURES</b> 3 <b>and 5</b></figref><b>)</b> with respect to the transmitter cone of illumination <b>42</b> (see <figref idref="f0003"><b>FIGURES 3 and 5</b></figref>)<b>.</b> This displacement between the cone of illumination and receiver field-of-view <b>42, 44</b> results in a lag angle <b>56</b> (see <figref idref="f0002"><b>FIGURE 2</b></figref>) between the transmitter cone of illumination and receiver field-of-view <b>42, 44.</b> The lag angle <b>56</b> increases as a function of the delay time and scan rate. Therefore, the lag angle <b>56</b> increases as a function of a distance between the transmitting portion <b>20</b> of the mirror unit <b>16,</b> the target <b>40,</b> and the optical receiver mirror portion <b>22.</b> The distance between the mirror unit <b>16</b> (which includes mirrors <b>20</b> and <b>22)</b> and the target <b>40</b> is referred to as the target range. If the transmitter cone of illumination <b>42</b> and receiver field-of-view <b>44</b> are co-aligned for targets at relatively closer ranges (as is typically the case with LIDAR systems), the lag angle increases with the target range.

As illustrated in <figref idref="f0002"><b>FIGURE 2</b></figref><b>,</b> an angular offset (adjustment) is made between the transmitting and receiving portions <b>20, 22</b> to compensate for this lag angle <b>56.</b> Therefore, the angular adjustment is also referred to as lag angle compensation. More specifically, the transmitting portion <b>20</b> is rotated slightly about the axis <b>26</b> in the same direction <b>24</b> as the scan. Hence, the transmitter cone of illumination <b>42</b> always leads the receiver field-of-view <b>44,</b> out to the maximum range. The angular adjustment is equal to the delay time of the maximum working target range multiplied by a rotational speed of the rotating elements. Therefore, for this adjustment, an overlap of the transmitter cone of illumination <b>42</b> and receiver field-of-view <b>44</b> and the receiver collection efficiency, is greatest at the maximum working target range.

<figref idref="f0003"><b>FIGURE 3</b></figref> illustrates an overlap of the transmitter cone of illumination <b>42</b> and receiver field-of-view <b>44</b> in a system without the lag angle compensation. <figref idref="f0004"><b>FIGURE 4</b></figref> illustrates a graph <b>60</b> of the logarithm of the intensity at the receiver vs. the target range for the configuration in <figref idref="f0003"><b>FIGURE 3</b></figref><b>.</b> With reference to <figref idref="f0003"><b>FIGURES 3</b></figref><b>and</b><figref idref="f0004"><b>4</b></figref><b>,</b> the intensity of light received at the optical receiver <b>14</b> (see <figref idref="f0001"><b>FIGURE 1</b></figref>) is determined as a function of the target range. For example, as the target range increases, for a constant target reflectivity, the light intensity received at the optical receiver <b>14</b> (see <figref idref="f0001"><b>FIGURE 1</b></figref>) decreases (e.g., decreases rapidly). Consequently, for a given target, the intensity of light at the optical receiver <b>14</b> (see <figref idref="f0001"><b>FIGURE 1</b></figref>) may vary significantly over the working range. The intensity of light detected at the optical receiver <b>14</b> (see <figref idref="f0001"><b>FIGURE 1</b></figref>) at the range <b>62</b> corresponds to the receiver threshold. Because of the delay time, a minimum overlap <b>64</b> occurs at the receiver threshold range <b>62,</b> which contributes to the weak intensity at the optical receiver <b>14</b> (see <figref idref="f0001"><b>FIGURE 1</b></figref>). In addition, the graph <b>60</b> illustrates that a maximum overlap <b>66</b> occurs at a receiver saturation range <b>70</b> (closer to the optical receiver <b>14</b> (see <figref idref="f0001"><b>FIGURE 1</b></figref>)), which contributes to saturating the receiver <b>14</b> (see <figref idref="f0001"><b>FIGURE 1</b></figref>). A signal drop-off reduction at very short ranges occurs due to the transmit mirror obscuration of the receiver field-of-view <b>44,</b> a form of geometrical compression. Although no signal reduction at short range is evident in <figref idref="f0004"><b>FIGURE 4</b></figref> (graph <b>60),</b> a signal reduction is illustrated in <figref idref="f0005"><b>FIGURE 6</b></figref> (graph <b>72</b>).

<figref idref="f0003"><b>FIGURE 5</b></figref> illustrates an overlap of the transmitter cone of illumination <b>42</b> and receiver field-of-view <b>44</b> in a system with the lag angle compensation. <figref idref="f0005"><b>FIGURE 6</b></figref> illustrates a graph <b>72</b> of the logarithm of the intensity at the receiver vs. the target range. With reference to <figref idref="f0003"><b>FIGURES 5</b></figref><b>and</b><figref idref="f0005"><b>6</b></figref><b>,</b> the intensity of light received at the optical receiver <b>14</b> (see <figref idref="f0001"><b>FIGURE 1</b></figref>) is determined as a function of the target range. For example, as the target range increases, the light intensity received at the optical receiver <b>14</b> (see <figref idref="f0001"><b>FIGURE 1</b></figref>) decreases (except at very short ranges). However, unlike the embodiment illustrated in <figref idref="f0003"><b>FIGURES 3</b></figref><b>and</b><figref idref="f0004"><b>4</b></figref><b>,</b> the graph <b>72</b> shows the intensity of light detected by the optical receiver <b>14</b> (see <figref idref="f0001"><b>FIGURE 1</b></figref>) is between the detector threshold <b>74</b> and detector saturation limit <b>76</b> over the entire working target range (e.g., about 2 m to about 3,000 meters). In addition, because of the lag angle compensation, the maximum overlap <b>82</b> between the transmitter cone of illumination <b>42</b> and receiver field-of-view <b>44</b> occurs at approximately the maximum target range (e.g., about 3,000 meters), while the minimum overlap <b>84</b> between the transmitter cone of illumination <b>42</b> and receiver field-of-view <b>44</b> occurs at approximately the minimum target range. The lag angle compensation increases the light signal detected by the optical receiver <b>14</b> (see <figref idref="f0001"><b>FIGURE 1</b></figref>) from a target located at the maximum working range, and reduces the signal received at close range, and thus the dynamic range of the detected light intensity is compressed over the entire working range.

The decreased dynamic range of the light signals at the optical receiver <b>14</b> (see <figref idref="f0001"><b>FIGURE 1</b></figref><b>)</b> reduces demands on the receiver sensor and the associated electronics. Highspeed electronics with high dynamic range are often expensive and difficult to fabricate.

In one embodiment, the lag angle compensation is made smaller than the transmitter cone of illumination and/or receiver field-of-view at all working ranges of the system. Similar results are obtained if the receiver field-of-view and the transmitter divergence are different and the compensation lag angle is the same as me maximum expected lag angle. The amount of lag relative to the receiver field of view and/or transmitter cone of illumination may be tailored to minimize the dynamic range of light intensity received by the receiver while maximizing the light intensity received at the receiver for long-range targets.

It is contemplated that the LIDAR system scans quickly (eg., time of flight of the light pulse is not negligible short relative to the associated scan movement in percent of spot size) and also unidirectionally, where the lag angle is always in the same direction and the magnitude of the lag angle is linearly related to the range. For a slowly scanned system (e.g., the time-of-flight of the light pulse is negligibly short relative to the associated scan movement in percent of spot size), signal compression by optimization of lag angle compensation (SCOLAC) is unnecessary and cannot significantly compress the intensity range of received light. For fast scanning systems where the scan direction reverses or moves in different directions (e.g., a Palmer mirror scan), SCOLAC might still be used, but may be more difficult to implement if, for example, the reversing motion is not constant in velocity (e.g., as in a sinusoidal scan).