Design of dynamic vehicle intelligent width and height detection system based on two-dimensional laser pulse ranging sensor

The intelligent trend of modern instruments and meters has led to the increasing application of various sensors. As lasers have many advantages, the laser detection and control systems developed using these characteristics have advanced technical performance, convenient use performance and simple system structure. Laser sensors are generally composed of laser generators, optical parts and optoelectronic devices. They can convert the measured physical quantity (such as distance, flow, speed, etc.) into optical signals, and then use photoelectric converters to convert optical signals into electrical signals. The output signal is obtained through filtering, amplification and rectification of the corresponding circuits, thereby calculating the measured quantity. With the advantages of lasers (such as good directionality, high brightness, good monochromaticity, good coherence, etc.), laser sensors usually have the advantages of simple and reliable structure, strong anti-interference ability, non-mechanical contact, high resolution, high accuracy, small indication error, good stability, and suitable for rapid measurement.

With the continuous development of science and technology, the country's requirements for overload control are also constantly increasing. Whether in terms of efficiency or accuracy, intelligent overload detection tools will be widely used. In view of the above situation, a high-efficiency and high-precision intelligent vehicle over-width and over-height detection system based on laser scanning sensor technology is designed.

1 Measurement principle of laser ranging sensor

Laser ranging is an active optical detection method. The detection mechanism of active optical detection is: the detection system emits a beam to the target (in optical detection, it is generally infrared or visible light), and the beam is reflected by the target surface to generate an echo signal. The echo signal directly or indirectly contains the information to be measured. The receiving and signal processing system obtains the measured value by receiving and analyzing the echo signal. Laser has the advantages of strong coherence, high brightness, and good directionality. Therefore, after the appearance of laser, it immediately became the preferred light source for most active optical detection systems.

At present, pulse laser ranging has been widely used, such as topographic measurement, measurement of the distance from the earth to the moon, etc. Figure 1 is a schematic diagram of the pulse laser ranging system. Its working principle is as follows: the human-machine operator issues a ranging command, triggering the laser to emit a laser pulse. A small part of the energy passes through the beam splitter and is directly sent to the pulse acquisition system as a reference pulse. As the starting point of timing, the digital ranging timer starts timing; the other part is reflected by the refracting prism and shot to the target. Generally, the emission front end has a telescopic optical system in order to reduce the divergence angle of the outgoing light beam, so as to increase the surface density of light energy, increase the working distance, and reduce the interference of background and surrounding non-target objects. Part of the laser beam reaching the target is diffusely reflected back to the rangefinder by the surface; it reaches the detector APD through the receiving objective lens and optical filter. The main function of the narrowband optical filter is to make full use of the excellent monochromaticity of the laser and improve the signal-to-noise ratio of the system; the optical detector APD converts the optical signal into an electrical signal, and then amplifies and filters the electrical signal. The shaped echo signal turns off the time interval processing module to stop timing. In this way, the distance L of the target to be measured can be calculated according to the result t of time interval processing:

L=ct／2 (1)

In formula (1), c is the speed of light. In Figure 1, the filter and aperture can reduce the influence of background and stray flashes, and reduce the background noise in the detector output signal. According to formula (1), the pulse ranging accuracy △L can be expressed as:

△L=c△t／2 (2)

It can be seen from formula (2) that the time interval accuracy △t of the system processing directly determines the ranging accuracy △L of the pulse laser ranging system.

2 System composition and data processing

This system uses the LMS series outdoor non-contact laser sensor of the German SICK company as the data acquisition device and the industrial computer as the data processing device. The system composition is shown in Figure 2. The LMS series sensor is an outdoor non-contact high-precision, high-resolution external sensor. Its working principle is based on the measurement of the flight time of the laser beam. It emits laser pulses at a defined time interval and calculates the time interval between the emitted pulse and the received pulse through a timer to obtain the distance to the measured object. The pulsed laser beam is reflected by a rotating reflector inside the ranging sensor to form a fan-shaped scan of the surrounding environment. The contour curve of the target object is determined by the series of pulse sequences received. The scanning frequency of the LMS sensor is 25Hz/50Hz, the angle frequency is (0.25°/0.5°, the scanning angle range is 0°~270°, the maximum scanning distance is 20 m, and the standard measurement accuracy is ±0.30 m, the safety protection level is IP67, which is safe for human eyes. Harsh environmental factors have no effect on the measurement range, and it can be used in an outdoor temperature environment of -30 to +50°C. The LMS series laser sensor uses the laser pulse ranging method to calculate the distance from several points on the scanning contour line of the measured object to the sensor, and uploads it to the industrial computer in real time through the high-speed network interface in the form of polar coordinates for post-processing. Since the data transmission speed of the network port can reach 100Mb/s, there will be no data loss problem, ensuring that the measured data can be uploaded to the industrial computer in real time for data processing. The industrial computer directly exchanges data with the LMS series sensors through the network communication kernel Winsock of the visual programming language VB. When using Wins When you are using a ock control, you should first consider whether to use the TCP or UDP communication protocol. The TCP communication protocol control is a communication protocol that requires a connection, similar to a telephone system. Before starting data transmission, the user must first establish a connection, which also has an error checking mechanism to prevent data from being transmitted in a scattered manner. The transmission process is slower and has fewer errors. If the data is more important, this method is better. The UDP communication protocol is a communication protocol that does not require a connection. The transmission between two computers is similar to sending mail: information is sent from one computer to another, but there is no clear connection between the two. Compared with the TCP method, its error checking is simpler, so it is faster. When speed is required, this method is more appropriate.

This system has high requirements for data real-time performance and relatively low requirements for data accuracy. Therefore, before data exchange, the IP addresses of the LMS series sensors and the industrial computer must be set to a network segment, and then the UDP communication protocol must be used to exchange data. After the industrial computer receives the polar coordinate information uploaded by the LMS series sensors, it processes all the data through a data processing program designed based on the VB kernel. The data processing process is as follows.

First, the vehicle’s entry and exit are determined through data uploaded by the sensor.

In the first step, the sensor uploads the polar radius (ρ1, ρ2, ρ3, ..., ρn) and the corresponding polar angle (θ1, θ2, θ3, ..., θn) of the polar coordinates of different points of the measured vehicle from the sensor to the measured vehicle;

in the second step, the polar coordinates of the measured point are converted into plane rectangular coordinates through the transformation of the coordinate system, that is,
(x1=ρ1·cosθ1, y1=ρ1t·sinθ1)(x2=ρ2·cosθ2, y2=ρ2·simθ2)(x3=ρ3·c∞θ3, y3=ρ3·sinθ3)……(xn=ρn·cosθn,yn=ρn·sinθn);

Step 3, compare y1, y2, y3,……,yn respectively, and take the smallest y value ymin; then compare x1, x2, x3,……,xn respectively, and take the leftmost x value xmin and the rightmost x value xmax of the scanned vehicle; calculate the maximum height Height=H-ymin and the maximum width Width=xmax-xmin of the vehicle scanned in a single scan (H is the height of the sensor from the ground). Compare the width and height obtained from a single measurement, and judge the entry and exit

of the vehicle based on the change curve of the data. Then, by comparing the width and height information of each single measurement of the vehicle, compare them one by one, and calculate the maximum height Height and maximum width Width of the vehicle. Finally, the maximum width and height of the passing vehicle are compared with the width and height limits of the vehicle stipulated by the state to determine whether the passing vehicle is over-width or over-height. The comparison results are displayed on the monitor of the industrial computer, and the measurement results are saved in the SQL database. If the vehicle is over-width or over-height, the industrial computer will also generate an audible and visual alarm to remind the over-limit control personnel and the driver of the over-limit vehicle that there is an over-limit problem.

3 Experimental results and analysis

The laser sensor in this system is installed on a gantry with a height of 6 m at the application site. The scanning and measuring sector area of ​​the laser sensor is perpendicular to the lane. When a vehicle passes through the sector scanning area, the detection system automatically calculates the width and height of the vehicle and displays it on the monitor of the industrial computer for staff monitoring.

During the field experiment, 30 vehicles of different styles were randomly measured for width and height by manual and automatic methods (hereinafter referred to as actual value and measured value, respectively). The statistical analysis results show that the error of the automatic measurement data of this system is within 0.10 m for 28 vehicles. There are 2 vehicles within 0.15 m. That is, 93% of the vehicle detection errors are less than 0.10 m, which meets the user's requirement of less than 0.15 m error. The data analysis table is shown in Table 1.

4 Conclusions

The intelligent vehicle over-width and over-height detection system designed with LMS series two-dimensional laser pulse scanning sensors and the network kernel of the visual programming language VB has high measurement accuracy and good real-time performance. The width and height information of the vehicle under the sensor is immediately displayed on the monitor. The application of this system realizes the automation of over-width and over-height detection in vehicle over-limit detection, avoids the randomness of manual measurement errors, can measure the width and height of vehicles that cannot be directly measured manually, and the measurement data is automatically saved in the database and can be queried at any time. It can also greatly improve the work efficiency and detection accuracy of over-limit detection. Therefore, this system has a good promotion prospect.