Polarization diversity detection for fmcw lidar

Abstract

The invention relates to a system and method for classifying an object. The system comprises a polarization beam splitter, a first detector, a second detector, and a processor. The polarization beam splitter generates a first source signal having a first polarization direction and a second source signal having a second polarization direction. The first detector transmits the first source signal at the object and receives a first reflection signal generated at the object in response to the first source signal. The second detector transmits the second source signal at the object and receives a second reflection signal generated at the object in response to the second source signal. The processor is configured to compare the first reflection signal and the second reflection signal to classify the object.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to sensor systems, and in particular to systems and methods for classifying objects based on polarization of signals reflected from the objects. BACKGROUND

[0002] Autonomous vehicles can employ sensors in order to localize objects in their environment. For example, light-based sensors on an autonomous vehicle, such as lidar, can be used to transmit light from the vehicle. The light reflects off of an object and is received back at the sensor, where parameters of the reflected light are measured and compared to similar parameters of the transmitted light. Such detection can yield a location and / or range of the object, but not what type of object it is. Thus, it is desirable to provide a method for operating a light sensor to determine a classification of an object as well as a range and location. SUMMARY

[0003] In an example embodiment, a method of classifying an object is disclosed. A first source signal having a first polarization direction and a second source signal having a second polarization direction are transmitted at the object. A first reflected signal generated in response to the first source signal at the object is received. A second reflected signal generated in response to the second source signal at the object is received. The object is classified by comparing the first reflected signal and the second reflected signal.

[0004] In addition to one or more features described herein, the method includes classifying the object by comparing a second reflected intensity of the second reflected signal to a first reflected intensity of the first reflected signal. The method further includes generating the first source signal and the second source signal from an elliptically polarized light beam. In an embodiment, the elliptically polarized light beam is a circularly polarized light beam. The method further includes determining a transmission ratio of the first source signal to the second source signal, determining a polarization signal ratio of the first reflected signal to the second reflected signal, and classifying the object based on a difference between the transmission ratio and the polarization signal ratio. In an embodiment, the first polarization direction is a P-polarization direction and the second polarization direction is an S-polarization direction. The method further includes adjusting a ratio of the first source signal to the second source signal to select the object for observation based on the object classification.

[0005] In another example embodiment, a system for classifying an object is disclosed. The system includes a polarization beam splitter, a first detector, a second detector, and a processor. The polarization beam splitter generates a first source signal having a first polarization direction and a second source signal having a second polarization direction. The first detector transmits the first source signal at the object and receives a first reflected signal generated in response to the first source signal at the object. The second detector transmits the second source signal at the object and receives a second reflected signal generated in response to the second source signal at the object. The processor is configured to compare the first reflected signal and the second reflected signal to classify the object.

[0006] In addition to one or more of the features described herein, the processor is further configured to classify the object by comparing a second reflection intensity of the second reflection signal to a first reflection intensity of the first reflection signal. The polarizing beam splitter generates the first source signal and the second source signal from an elliptically polarized light beam. In an embodiment, the elliptically polarized light beam is a circularly polarized light beam. The processor is further configured to determine a transmission ratio of the first source signal to the second source signal, determine a polarization signal ratio of the first reflection signal to the second reflection signal, and classify the object based on a difference between the transmission ratio and the polarization signal ratio. In an embodiment, the first polarization direction is a P-polarization direction and the second polarization direction is an S-polarization direction. The processor is further configured to adjust a ratio of the first source signal to the second source signal to select the object for observation based on the object classification.

[0007] In yet another example embodiment, a vehicle is disclosed. The vehicle includes a polarizing beam splitter, a first detector, a second detector, and a processor. The polarizing beam splitter generates a first source signal having a first polarization direction and a second source signal having a second polarization direction. The first detector transmits the first source signal at an object and receives a first reflection signal generated at the object in response to the first source signal. The second detector transmits the second source signal at the object and receives a second reflection signal generated at the object in response to the second source signal. The processor is configured to compare the first reflection signal and the second reflection signal to classify the object.

[0008] In addition to one or more of the features described herein, the processor is further configured to classify the object by comparing a second reflection intensity of the second reflection signal to a first reflection intensity of the first reflection signal. The polarizing beam splitter generates the first source signal and the second source signal from an elliptically polarized light beam. The processor is further configured to determine a transmission ratio of the first source signal to the second source signal, determine a polarization signal ratio of the first reflection signal to the second reflection signal, and classify the object based on a difference between the transmission ratio and the polarization signal ratio. In an embodiment, the first polarization direction is a P-polarization direction and the second polarization direction is an S-polarization direction. The processor is further configured to adjust a ratio of the first source signal to the second source signal to select the object for observation based on the object classification.

[0009] The above features and advantages and other features and advantages of the present disclosure are readily apparent from the following detailed description when taken in connection with the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0010] Other features, aspects, and details of the present disclosure can be described in more detail below with reference to the accompanying drawings.

[0011] Figure 1 An autonomous vehicle according to an example embodiment is shown;

[0012] Figure 2 A schematic diagram showing an optical architecture representative of a frequency modulated continuous wave sensor system in an embodiment is shown; and

[0013] Figure 3 A flowchart representative of a method for classifying objects using the methods disclosed herein is shown. DETAILED DESCRIPTION

[0014] The following description is merely exemplary in nature and is not intended to limit the present disclosure, its application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.

[0015] According to exemplary embodiments, Figure 1 An autonomous vehicle 10 is shown. In exemplary embodiments, the autonomous vehicle 10 is a so-called Level 4 or Level 5 automated system. Level 4 systems represent “high automation” and refer to driving mode-specific performance by an automated driving system of all aspects of the dynamic driving task under both internal and external conditions that can be managed by the human driver without the need for appropriate intervention. Level 5 systems represent “full automation” and refer to full-time performance by an automated driving system of all aspects of the dynamic driving task under all road and environmental conditions that can be managed by a human driver. It should be understood that the systems and methods disclosed herein can also be used with autonomous vehicles operating at any of Levels 1 through 5.

[0016] The autonomous vehicle 10 generally includes at least a navigation system 20, a propulsion system 22, a drivetrain system 24, a steering system 26, a braking system 28, a sensor system 30, an actuator system 32, and a controller 34. The navigation system 20 determines a road-level route plan for autonomous driving of the autonomous vehicle 10. The propulsion system 22 provides power for generating motion force for the autonomous vehicle 10, and in various embodiments can include an internal combustion engine, an electric machine such as a traction electric motor, and / or a fuel cell propulsion system. The drivetrain system 24 is configured to transfer power from the propulsion system 22 to two or more wheels 16 of the autonomous vehicle 10 according to selectable speed ratios. The steering system 26 affects the position of the two or more wheels 16. Although shown as including a steering wheel 27 for illustrative purposes, the steering system 26 can not include a steering wheel 27 in some embodiments contemplated within the scope of the present disclosure. The braking system 28 is configured to provide braking torque to the two or more wheels 16.

[0017] In various embodiments, the sensor system 30 includes a frequency modulated continuous wave (FMCW) system. The FMCW system includes a lidar system for generating a source signal and receiving a reflection of the source signal from an object or reflector 50 in the environment. The sensor system 30 measures the reflected signal to determine parameters such as the position and / or velocity of the reflector 50 relative to the autonomous vehicle 10. The FMCW system can also be used to classify the reflector 50 using the methods discussed herein. The parameters and classification of the reflector 50 can be provided to the controller 34 for use in navigation of the autonomous vehicle 10.

[0018] The controller 34 establishes a trajectory for the autonomous vehicle 10 based on the output of the sensor system 30. The controller 34 can provide the trajectory to the actuator system 32 to control the propulsion system 22, the transmission system 24, the steering system 26, and / or the braking system 28 in order to navigate the autonomous vehicle 10 relative to the reflector 50.

[0019] The controller 34 includes a processor 36 and a computer readable storage device or storage medium 38. The computer readable storage medium includes programs or instructions 39 that, when executed by the processor 36, operate the autonomous vehicle based on the sensor system output. The storage medium 38 can further include programs or instructions 39 that, when executed by the processor 36, determine the state of the reflector 50 to allow the autonomous vehicle to travel relative to the reflector. The processor 36 can be used to process signals from the FMCW system in order to determine range, Doppler measurements, and characterize the reflector 50 based on the polarization state of the reflected signal.

[0020] Figure 2 A diagram illustrating an optical architecture representing the FMCW system 200 is shown in an embodiment. The FMCW system 200 includes a light source 202 such as a laser, a phase plate 204, a polarization beam splitter 206, a first detector 208, and a second detector 210. The light source 202 generates a light beam 220 in a linear polarization state. In an embodiment, the light source 202 can be modulated to generate the light beam 220 in the form of a chirp signal. The light beam 220 passes through the phase plate 204, which converts the light beam 220 to an elliptically polarized light beam 222. In various embodiments, the elliptical polarization state can be a circular polarization state.

[0021] The phase plate 204 can be electronically controlled to control or adjust the ellipticity of the elliptically polarized light beam 222. The elliptically polarized light beam 222 enters the polarization beam splitter 206, which separates the elliptically polarized light beam 222 into a first linearly polarized light beam 224 and a second linearly polarized light beam 226. The first linearly polarized light beam 224 is in a first linear polarization state along a first polarization direction (PD1), and the second linearly polarized light beam 226 is in a second linear polarization state along a second polarization direction (PD2). In various embodiments, the first polarization direction (PD1) is perpendicular to the second polarization direction (PD2). In various embodiments, the first linearly polarized light beam 224 is p-polarized, and the second linearly polarized light beam 226 is s-polarized. Alternatively, the first linearly polarized light beam 224 is s-polarized, and the second linearly polarized light beam 226 is p-polarized. Those skilled in the art will recognize that controlling the ellipticity of the elliptically polarized light beam 222 controls the relative ratio of the intensity of the first linearly polarized light beam 224 to the intensity of the second linearly polarized light beam 226. The first linearly polarized light beam 224 is sent to the first detector 208, and the second linearly polarized light beam 226 is sent to the second detector 210.

[0022] The first detector 208 includes a first beam splitter 212 and a first optical mixer 214. The first beam splitter 212 separates the first linearly polarized light beam 224 into a first source signal 228 and a first reference light beam 230. The splitting ratio of the first beam splitter 212 is such that the first source signal 228 receives most of the first linearly polarized light beam 224, and the intensity of the first reference light beam 230 is a small fraction of the intensity of the first linearly polarized light beam 224. The first source signal 228 is transmitted into the environment when the first reference light beam 230 is sent directly to the first optical mixer 214. The first source signal 228 reflects off the reflector 50, as shown, to produce a first reflected signal 232. The first reflected signal 232 is mixed with the first reference light beam 230 at the first optical mixer 214 to form a first interference signal. The first interference signal can be sent to a processor, such as the processor 36, for determining the range and / or velocity of the reflector 50. Figure 1

[0023] ​The second detector 210 includes a second beamsplitter 216 and a second optical mixer 218. The second beamsplitter 216 splits the second linearly polarized light beam 226 into a second source signal 234 and a second reference light beam 236. The beamsplitting ratio of the second beamsplitter 216 is such that the second source signal 234 receives most of the second linearly polarized light beam 226, and the second reference light beam 236 is a small fraction of the intensity of the second linearly polarized light beam 226. The second source signal 234 is transmitted into the environment, while the second reference light beam 236 is sent to the second optical mixer 218. The second source signal 234 reflects off the object 50 to produce a second reflected signal 238. The second reflected signal 238 is mixed with the second reference light beam 236 at the second optical mixer 218 to form a second interference signal. The second interference signal can be sent to a processor, such as the processor 36, for determining the range and / or velocity of the object.

[0024] The first detector 208 transmits a first source signal 228 polarized along a first polarization direction (PD1) and is configured to receive signals along the first polarization direction. Similarly, the second detector 210 transmits a second source signal 234 polarized along a second polarization direction (PD2) and is configured to receive signals along the second polarization direction.

[0025] The first reflected intensity of the first reflected signal and the second reflected intensity of the second reflected signal can be compared to each other to classify the object. For example, an object made of metal can reflect s-polarized light and p-polarized light with unequal reflected intensities. For example, at a given angle, a reflection is detected for one polarization state, but not for the other polarization state, which is given by the Fresnel reflection coefficients. On the other hand, human skin can have a moderate reflectivity in both polarization directions. These intensities along the different polarization directions can be recorded and used to classify the object, for example, whether the object is a human or a metal object.

[0026] For the first source signal 228 and the second source signal 234 emanating from the elliptically polarized light beam 222, the intensity of the first source signal 228 is different from the intensity of the second source signal 234. The first source signal 228 intensity is compared to the second source signal 234 intensity to calculate a first ratio or transmission ratio. In an embodiment where the first source signal 228 is a P-polarized source signal and the second source signal 234 is an S-polarized source signal, the transmission ratio is given by equation (1):

[0027]

[0028] The first reflected intensity of the first reflected signal 232 (P-polarized reflected signal) is compared to the second reflected intensity of the second reflected signal 238 (S-polarized reflected signal) to calculate a second ratio or polarization signal ratio, as given by equation (2):

[0029]

[0030] When the reflector 50 reflects equally along both the P polarization direction and the S polarization direction, the first ratio R0and the second ratio R d are equal to each other, and the difference between them is equal to zero (R0- R d = 0). When the reflector 50 is more reflective along the P polarization direction than along the S polarization direction, the difference is less than zero (R0- R d < 0). When the reflector 50 reflects less along the P polarization direction than along the S polarization direction, the difference is greater than zero (R0- R d > 0). Thus, the difference between the ratios can be used to determine the preferred direction of polarization of the reflected signal as the polarization angle. Such a determination can be used to classify the reflector 50. In the special case where the elliptically polarized beam 222 is circularly or linearly polarized, these relationships still hold. The difference in reflection coefficients can measure the attenuation of the reflecting object.

[0031] In one embodiment, the operator can use the phase plate 204 to adjust the ratio of the first source signal and the second source signal in order to selectively observe reflectors. For example, the second source signal 234 can be removed or the intensity of the second source signal can be reduced in order to observe all reflectors except those that reflect along the polarization direction of the second source signal.

[0032] Figure 3 A flowchart 300 representing a method of classifying objects using the methods disclosed herein is shown. At block 302, an initial polarization ratio is selected for the elliptically polarized beam 222, as Figure 2 shown, using a control voltage applied to the phase plate 204. The initial polarization ratio controls the ellipticity of the elliptically polarized beam 222. At block 304, the transmission ratio between the intensity of the first source signal I(S0) and the intensity of the second source signal I(P0) is measured or established, which is R0= I(S0) / I(P0). At block 306, the beam is split into two linear states (i.e., into a P polarized beam for the first source signal and an S polarized beam for the second source signal). At block 308, each of the two linear states is further split into a small portion of the reference beam that remains in the local oscillator state and a source signal that is projected into the environment. At block 310, the reflected signal (resulting from the reflection of the source signal) is combined with the relevant reference beam. At block 312, the range and Doppler signals are determined for each linear state. At block 314, the polarization signal ratio R d = I(S d ) / I(P d ) of the reflected signal intensity is measured. The difference between the ratios (R0- R d) to identify the attenuating effect or difference in reflected intensity along the relative polarization directions. Changes in the relative intensity along the P-polarization direction and the S-polarization direction are used to identify the material of the reflector and the polarization sensitivity of the reflector.

[0033] Although the foregoing disclosure has been described in reference to the exemplary embodiments, workers skilled in the art will recognize that changes can be made in form and detail without departing from the scope of the disclosure. Also, the order of steps can be modified without departing from the scope of the disclosure. Further, the disclosure can be implemented using any number of techniques, and the specific techniques chosen to implement the disclosure should not limit the scope of the disclosure, which is to be given the broadest interpretation of the claims. Accordingly, the disclosure is not intended to be limited to the particular embodiments described for the purposes of clarity and illustration, but is capable of being practiced with other examples that are within the scope of the disclosure.

Claims

1. A method of classifying an object, comprising: transmitting a first source signal having a first polarization direction and a second source signal having a second polarization direction at an object; receiving a first reflected signal generated at the object in response to the first source signal; receiving a second reflected signal generated at the object in response to the second source signal; and classifying the object by comparing the first reflected signal and the second reflected signal; determining a transmission ratio of the first source signal to the second source signal; determining a polarization signal ratio of the first reflected signal to the second reflected signal; and classifying the object based on a difference between the transmission ratio and the polarization signal ratio.

2. The method of claim 1, further comprising classifying the object by comparing a second reflected intensity of the second reflected signal to a first reflected intensity of the first reflected signal.

3. The method of claim 1, further comprising generating the first source signal and the second source signal from an elliptically polarized light beam. adjusting the ratio of the first source signal to the second source signal to select the object for viewing based on the object classification.

4. The method of claim 1, further comprising:

5. A system for classifying an object, comprising: a polarization beam splitter for generating a first source signal having a first polarization direction and a second source signal having a second polarization direction; a first detector for transmitting the first source signal at an object and receiving a first reflected signal generated at the object in response to the first source signal; a second detector for transmitting the second source signal at the object and receiving a second reflected signal generated at the object in response to the second source signal; and a processor configured to compare the first reflected signal and the second reflected signal to classify the object; wherein the processor is further configured to: determine a transmission ratio of the first source signal to the second source signal; determine a polarization signal ratio of the first reflected signal to the second reflected signal; and classify the object based on a difference between the transmission ratio and the polarization signal ratio. The processor is further configured to classify the object by comparing a second reflected intensity of the second reflected signal to a first reflected intensity of the first reflected signal.

6. The system of claim 5, wherein, The polarization beam splitter generates the first source signal and the second source signal from an elliptically polarized light beam.

7. The system of claim 5, wherein, The processor is further configured to adjust the ratio of the first source signal to the second source signal to select the object for viewing based on the object classification.

8. The system of claim 5, wherein, ​

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