Introduced today is an extremely common sensor - ultrasonic radar. If you think the ultrasonic radar is a bit strange, then it has a more popular name - parking sensor.
In the process of reversing into the warehouse and slowly moving the car, the sound of "drops" can be heard in the cab. These sounds are feedback information to the driver based on the detection distance of the ultrasonic radar.
The onboard ultrasonic radar is usually mounted above the bumper of the car and hidden in a certain position on the bumper. The appearance of the car is shown by the dot at the yellow arrow as shown below.
Type of ultrasonic radar
There are two common types of ultrasonic radar. The first type is installed on the front and rear bumpers of the car, that is, the reversing radar used to measure the obstacles in front and rear of the car. This type of radar is called UPA in the industry; the second is installed on the side of the car to measure side obstacles. The ultrasonic radar of the object distance is called APA in the industry.
The detection range and detection area of UPA and APA are too similar, as shown in the following figure. The car in the picture is equipped with a total of 8 UPAs in the front and rear, and a total of 4 APAs on the left and right sides.
The detection range of UPA ultrasonic radar is generally between 15 and 250 cm, which is mainly used to measure obstacles in front and rear of the car.
As shown in the figure, it is a schematic diagram of the detection range of a single UPA.
The detection range of APA ultrasonic radar is generally between 30~500cm. APA's detection range is farther, so it is more expensive and more powerful than UPA.
The figure shows the detection range of a single APA.
Mathematical model of ultrasonic radar
Although UPA and APA are very different in terms of detection distance and detection shape, they can still be described by the same mathematical model. Describe the state of an ultrasonic radar requires the following four parameters. The schematic diagram of the mathematical model is as follows.
α is the detection angle of the ultrasonic radar. Generally, the detection angle of the UPA is about 120°, and the detection angle of the APA is smaller than the UPA, which is about 80°.
β is one of the influencing factors of the ultrasonic radar detection width range, which is generally small. The β angle of UPA is about 20°, and the β angle of APA is special, which is 0°.
Parameter 3: R
R is also one of the influencing factors of the ultrasonic radar detection width range. The R values of UPA and APA are not much different, and are all around 0.6m.
Parameter 4: D
D is the maximum range of the ultrasonic radar. The maximum range of the UPA is 2 meters to 2.5 meters, and the maximum range of the APA is at least 5 meters. Currently, the APA radar with more than 7 meters is used in the industry.
Feature 1: Temperature sensitive
The principle of ranging of ultrasonic radar is similar to that of the previously described laser radar and millimeter wave radar. Distance = propagation speed * propagation time / 2. The difference is that the speed of the laser radar and the millimeter wave radar are both the speed of light, and the wave speed of the ultrasonic radar is related to the temperature. The approximate relationship is as follows:
C = C0 + 0.607 ∗ T, the acoustic velocity is 332m/s when C0 is zero, and T is the temperature (unit: °C).
For example, when the temperature is 0 ° C, the propagation speed of the ultrasonic wave is 332 m / s; when the temperature is 30 ° C, the propagation speed of the ultrasonic wave is 350 m / s. Obstacles of the same relative position have different measured distances at different temperatures.
For autopilot systems where sensor accuracy is extremely high, either the ultrasonic radar's ranging is conservatively calculated or the temperature information is introduced into the autopilot system to improve measurement accuracy.
Feature 2: Impossible description of obstacle location
The ultrasonic radar returns a value of the detection distance when it is working, as shown in the figure. Obstacles at A and at B return the same detection distance d. Therefore, in the case where only the detection distance d is known, it is impossible to determine whether the obstacle is at A or B through the information of a single radar.
Application of ultrasonic radar
The title of this article mentions that ultrasonic radar is an underestimated sensor because it can do a lot of things besides detecting obstacles.
The automatic parking function needs to go through two stages: 1. Identify the location; 2. Reversing the storage
The function of identifying the location is to rely on the APA installed on the side of the vehicle, as in the following scenario.
When the car slowly passes through the storage position, the relationship between the detection distance returned by the APA sensor on the right front of the car and time is as follows:
Similarly, the rear APA will generate a similar signal curve for secondary verification of the location.
With the location detection function, it is not difficult to develop the autonomous parking function.
Application 2, High speed lateral assistance
The Tesla Model S implements the cruising function of the expressway in the era of AutoPilot 1.0. In order to increase the safety and comfort of the high-speed cruising function, Tesla will use the APA ultrasonic radar for parking, which is also used for high-speed cruise. .
First look at the video of the Model A application APA. The image in the lower left corner of the video is a camera facing the back of the car. The image on the right is the angle of view towards the direction of travel.
As can be seen in the video, when the car passing by on the left is closer to the car, the Model S autonomously fine-tunes to the right while ensuring sufficient space on the right side, reducing the risk of collision with the left vehicle.