FIELD OF THE INVENTION
The present invention relates to a hydrophilic material and coating for automotive LiDAR sensor covers.
BACKGROUND OF THE INVENTION
Vehicles with advanced driver-assist, semi-autonomous or fully autonomous systems heavily rely on the input of various sensors (Optical, Ultrasonic, Radar, LiDAR, IR, etc.) to capture environmental and traffic data. Clear sensor vision under all vehicle operating conditions has to be ensured to guarantee safe and uninterrupted operation. Different sensor types require different boundary conditions regarding applicability and performance (e.g., camera and LiDAR need unobstructed optical view, while a radio frequency radar might be covered by certain materials, which may affect its performance and range). Taking measures to optimize boundary conditions is absolutely critical to ensure robust and reliable sensor performance for safety. Soiling during adverse weather conditions is hazardous for autonomous driving because droplets and particles can cause obstructions and degradation of sensor signals. It also reduces the ability of an autonomous vehicle to navigate safely. It is desirable to develop coatings that will improve the LiDAR sensor performance in adverse weather conditions by creating a homogenous film of water on the LiDAR cover.
SUMMARY OF THE INVENTION
A weather resistant autonomous driving sensor unit having one or more light detection and ranging sensors connected to an autonomous vehicle driving system. The sensor unit further includes a cover having an inside surface facing the one or more light detection and ranging sensors, and an external surface facing an external environment of a vehicle. The cover is formed of molded polycarbonate and also forms a vehicle component selected from the group consisting of a vehicle grille, bumper and front end module. A hydrophilic coating applied to the external surface of the coating. The hydrophilic coating can be made of several different compounds that are applied to the external surface using spraying, dipping or vapor deposition. The hydrophilic coating selected must provide a droplet thickness to diameter ratio of less than 0.3 when the water contact angle on the external surface of the cover is less than 40 degrees or between about 25 degrees to about 40 degrees.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:
FIG. 1a is a schematic view of droplet thickness to contact diameter ratio.
FIG. 1b is a schematic view of a droplet having a T/D ratio of ˜1.25.
FIG. 1c is a schematic view of a droplet having a T/D ratio of about 0.5.
FIG. 1d is a schematic view of a droplet having a T/D ratio ˜0.25.
FIG. 1e is a schematic view of a water contact angle measurement on a droplet having a T/D ratio of less than 0.25.
FIG. 2 is a graph showing the relationship between droplet T/D ratio and water contact angle on a cover surface.
FIG. 3 is a schematic diagram showing how WCA is measured using the sessile drop technique.
FIG. 4 is a schematic diagram showing a LiDAR housing and cover.
FIG. 5 is a schematic diagram showing a cross section of a portion of the cover and the various optional material layers.
FIG. 6 is a graph showing the results of tests demonstrating LiDAR visibility for coatings with different T/D ratios when driving at 50, 75, and 100 km/hr. in moderate rain conditions.
FIG. 7 is a perspective front view of a vehicle having a LiDAR housing and cover connected.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.
Referring now to the figures, this invention focuses on the findings made on passive soiling mitigation through coatings of different material properties. The effectiveness of the coating is quantified by the wettability and the respective LiDAR sensor visibility % in adverse weather conditions such as driving-in-rain.
Method
Coating properties are first identified and are classified according to their static water contact angle, which is linearly correlated to the droplet thickness to contact diameter ratio (T/D). FIG. 2 contains a graph demonstrating the linear relationship between T/D and WCA. FIG. 1a is a schematic diagram showing a droplet 10a on a surface 12 and how the thickness T is measured relative to the diameter D of the droplet 10a. This is how the T/D ratio is determined for other droplet types described herein. FIG. 1b is a schematic view of a droplet 10b located on a surface 12b that is more hydrophilic. The droplet 10b has a T/D ratio of ˜1.25. FIG. 1c is a schematic view of a droplet 10c on a surface 12c that is a less hydrophilic (i.e., more hydrophobic) than surface 12b. The droplet 10c has a T/D ratio of about 0.5. FIG. 1d is a schematic view of a droplet 10d on a surface 12d that is hydrophobic. The droplet 10d has a T/D ratio of ˜0.25. FIG. 1e is a schematic view demonstrating how a water contact angle is measured on a droplet 16 on a surface 18, which is described in greater detail with respect to FIG. 3 below. The droplet 10b is thicker than droplet 10c and droplet 10d, while the droplet 10d is more spread out with a larger diameter and covers a larger surface area than droplet and droplet 10c.
A wide range of coatings of different water contact angles (WCA) in the range of ˜25°-150° is evaluated under controlled conditions in various driving-in-rain scenarios in a wind tunnel. LiDAR visibility is extracted based on the percent of point-cloud present. WCA measurements are defined herein to be a measure of surface wettability, where its value is commonly used to classify the material type and determine predicted droplet behaviors (e.g., shape and motion) that are related to the surface energy of adhesion. Droplet thickness and the droplet contact diameter are two critical parameters that affect optical behaviors. Referring to FIGS. 1a and 3, WCA is measured using the sessile drop technique 14 where a known volume of the water droplet 16 dispensed onto the surface 18, which is a flat and leveled material surface. A 2-dimensional projected image is then immediately captured by an imaging device 20, such as camera and microscope and analyzed using a computer 22. A line 24 tangent to the droplet profile that intersect with a base contact line 26 is drawn, as shown schematically in FIG. 1e. The T/D is measured from the height and width of the droplet.
A conventional low-cost approach to mitigate soiling on vehicle surfaces is to employ hydrophobic coatings; a lot of the products currently in the automotive market are hydrophobic in nature with WCA ˜90-100°. However, our studies found that hydrophobic coatings are detrimental to LiDAR sensor signals due to the shape of the droplet adhering to the cover being hemispherical with T/D ˜0.35-0.5 (FIG. 6). The mildly hydrophilic or hydrophobic coatings that lie in the median spectrum of the wettability class result in poor LiDAR visibility due to a large number of laser deflection events. Although enhanced hydrophobic or superhydrophobic coatings with WCA >140° (or T/D ˜1.2) have demonstrated the capability of maintaining LiDAR visibility in high rain intensity conditions, there are several disadvantages to consider, such as poor durability, fabrication difficulty, and high maintenance cost. Also, it is risky to apply superhydrophobic coatings that may lose its superior hydrophobicity and impair LiDAR visibility completely without warning signs or a mild degradation curve for failure prediction.
On the other hand, hydrophilic coatings, which are typically not the general approach, are found to result in outstanding LiDAR visibility. When raindrops impact a hydrophilic surface, due to the higher surface adsorption energy than the molecular interactions within the droplet, a thin water film is formed.
Referring now to FIG. 6 are the results of an analysis of coating of various LiDAR visibility with different T/D ratios when driving at 50, 75 and 100 kilometers per hour (km/h) in moderate rain conditions. Line 30 are the results for speeds of 50 km/h and demonstrate that the film is stable at lower rain intensity conditions (i.e., 0.25 T/D) to maintain high LiDAR signal transmittance. When rain intensity increases as a result of driving speed increasing to 75 km/h as shown by line 32 or 100 km/h as shown by line 34, the density of droplet impact also increases, which creates a disruption to the water film and results in partial point-cloud obstruction. Under extreme rain intensity conditions, splashes occur in front of the surface, causing more vision loss as shown by line 34. Nonetheless, the LiDAR performance is predictable with respect to rain intensity. Based on the results of these studies, we propose the application of hydrophilic coatings for automotive LiDAR sensor cover in adverse weather conditions. In particular, the use of highly transparent and hydrophilic materials by nature such as glass for automotive LiDAR sensor applications is implemented. The benefit of glass includes its readiness in accepting different kinds of coatings (anti-reflective, anti-abrasion, etc.), chemical resistant, and durable, which have been proven by existing automotive applications such as windshields and headlights.
FIG. 1a is a schematic depiction of the droplet 10a and how it is measured for determining thickness and diameter on the surface 12a, which can be any suitable surface for a particular application. The surface 12a can be hydrophobic, hydrophilic or a combination thereof. Glass and coated polycarbonate are two hydrophilic materials that offer similar soiling mitigation capabilities. The results from using a glass cover also fall within the trend of LiDAR visibility with respect to T/D measurements. The droplet is a single rain droplet on the surface 12a. The droplet 10a has a droplet thickness (T) to surface contact diameter (D) is measured, with only considering one single droplet on the surface. For hydrophilic surfaces, droplets are widespread with large diameter and small thickness for a given droplet volume. Therefore, T/D ratio should be less than 0.5, while lower ratios are achieved when the surface is more hydrophilic. Water film was observed for T/D less than 0.25 and rain situation where there are sufficient number of droplets to spread into each other. (FIG. 1d). When a water film is established, LiDAR visibility is not severely affected compared to mildly hydrophobic surfaces in most rain driving conditions.
For mildly hydrophobic surfaces, the water contact angle is approximately 90 degrees, forming a semi-spherical shape, thus resulting in T/D of approximately 0.5. (FIG. 1c). As hydrophobicity increases, T/D also increases due to low adhesion on the surface, contact diameter decreases. (FIG. 1b). The rounder the droplet, the higher the T/D ratio. For example, a superhydrophobic surface would result in T/D of approximately 1.25.
Referring now to FIGS. 4 and 7, there is a schematic diagram showing a LiDAR housing and cover arrangement 36. The LiDAR housing and cover arrangement 36 as shown in FIG. 7 is integrated into a vehicle 100 as part of the bumper fascia. As shown, there is a LiDAR unit 37 mounted on an internal surface 38. The LiDAR unit 37 includes laser emitting sources 33 and one or more sensors 35 for sensing reflected laser beams. The one or more sensors 35 are light detection and ranging sensors that are connected to an autonomous vehicle driving system. A cover 40 according to the present invention separates the LiDAR unit 37 from an external side of the cover 40. The cover has an inside surface that faces the one or more sensors of the LiDAR unit 37 and an external surface facing an external environment of a vehicle. While FIG. 7 shows the cover 40 to be part of the bumper fascia of the vehicle 100 it is within the scope of this invention for the cover arrangement 36 to be integrated into other components such as a vehicle grille or front end module, which helps to provide desired aesthetics of the cover arrangement 36. Also, it is within the scope of this invention for the cover arrangement 36 to be part of a separate LiDAR dome mounted to the vehicle and not necessarily be integrated into the different vehicle components. The LiDAR unit 37 is operably positioned relative to the cover 40 so that it can send and receive laser beam data through the cover 40.
FIG. 5 is a schematic diagram showing a cross section of a portion of the cover 40 and the various optional material layers. The cover 40 includes the inside surface 38 and external surface 42. Depending on the make-up of the cover there will be different measurable levels of LiDAR visibility. FIG. 5 shows a total of five layers, which can be used in various combinations. There is a core layer 44 that can be glass, polycarbonate or some other suitable transparent material. As shown there the cover has an external side and an internal side. Applied to the external side of the core layer 44 is an anti-abrasion coating layer 46 that protects the core layer from abrasion. An anti-abrasion coating layer 48 can also be applied to the inside of the core layer 44. The anti-abrasion coating layer 46 and anti-abrasion coating layer 48 are any suitable materials and can be sacrificial in nature, such that reapplication might be necessary. Examples of suitable materials for the anti-abrasion coating layer 46 and anti-abrasion coating layer 48 include, but are not limited to epoxy coatings, polyacrylic coating, ceramic coatings, polyurethane coatings, fluoropolymer coatings, etc. A hydrophilic coating layer 50 is applied over the anti-abrasion coating layer 46 and serves as the outermost layer external to the cover 40. The hydrophilic coating layer 50 is any suitable material and can include, but is not limited to, glass, ceramic, polycarbonate, etc. An anti-reflective coating 52 is applied over the anti-abrasion coating layer 48. The anti-reflective coating layer 52 is a suitable material, but includes silicon dioxide, titanium dioxide or a polarizing coating material, which can be a film or a layer of cured liquid. While layers there are five layer described above it is possible to have different applications where not all of them are used in a particular cover. It has been determined that in dry conditions the LiDAR visibility can vary depending on the layers combination. Set out in Table 1 below are various layers combinations with the determined LiDAR visibility for the different combinations. The layers combination column on Table 1 correspond to the layers shown FIG. 5. It is within the scope of this invention for a cover to have all of the layers mentioned above, or to have just the core layer 44, the core layer 44 plus the anti-abrasion coating layer 46 and anti-abrasion coating layer 48. It is further within the scope of the invention for a cover to be provided with the core layer 44 plus the anti-abrasion coating layer 46 and anti-abrasion coating layer 48 and the anti-reflective coating layer 52.
TABLE 1
|
|
Layers Combination (Corresponds to
LiDAR visibility in dry
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reference numbers in FIG. 5)
condition
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None
100%
|
44 only
97%
|
46 + 44 + 48
101%
|
46(already hydrophilic) + 44 + 48 + 52
102%
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|
The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.
Patent Application
20240027583
