I. Introduction

In the efficient operation system of modern airports, the smooth flow of baggage and cargo is crucial. Empty containers, as key carriers of baggage and cargo transportation, require precise monitoring of their status. Laser rangefinders, with their unique advantages, have emerged as a leading technology in airport empty container measurement, providing strong support for ensuring transportation efficiency, improving safety, and optimizing resource allocation.

II. Analysis of the Working Principle of Laser Rangefinder Sensors

(I) Time of Flight (ToF) Principle

Time-of-flight (ToF) is one of the core principles of laser ranging sensors used in airport applications. Taking a common pulsed ToF sensor as an example, its operation resembles a precisely timed "race against time." The sensor's internal laser emitting module emits an extremely fine laser pulse towards the empty container. This laser beam travels at approximately 3 × 10⁸ m/s. When the laser pulse contacts the container's surface, it scatters in all directions according to the law of reflection. Some of the reflected light is fortunate enough to "return along the same path" and be captured by the sensor's receiving module. At this point, the high-precision timer inside the sensor plays a crucial role, accurately recording the time difference Δt between the laser emission and reception. Using the simple yet elegant physical formula: distance d = c × Δt/2 (where c is the speed of light), the distance between the sensor and the empty container can be calculated quickly and accurately. For example, if the measured time difference is 2 × 10⁻⁸ s, substituting into the formula, the distance d = 3 × 10⁸ × 2 × 10⁻⁸ ÷ 2 = 3 m. This principle endows the sensor with powerful long-distance measurement capabilities, easily handling empty container storage and transportation scenarios at airports where the distance is often several meters or even tens of meters, and the measurement accuracy can reach the millimeter level, meeting the stringent requirements of airports for high-precision empty container measurement.

(II) Phase ranging principle

Phase ranging is another effective way to achieve accurate measurement, and its working mechanism is more like a precise "phase dance." The sensor emits a modulated continuous laser beam, the intensity of which changes periodically according to a specific pattern (such as a sine wave). When this modulated laser beam strikes the surface of an empty box and reflects back, a phase difference is generated between the reflected and emitted light due to the propagation distance. The sensor uses complex and precise circuits and algorithms to accurately measure this phase difference Δφ. Given the laser modulation frequency f, the distance to the empty box can be accurately calculated using the formula d = c × Δφ/(4πf). For example, if the modulation frequency is 10MHz, the measured phase difference is π/2, and the speed of light c = 3 × 10⁸ m/s, the distance d = 3 × 10⁸ × (π/2) ÷ (4π × 10 × 10⁶) = 3.75m. The advantage of phase ranging is its extremely high measurement accuracy, making it particularly suitable for scenarios with extremely stringent requirements for the size accuracy of empty containers, such as determining whether an empty container meets the cargo hold loading standards of a specific aircraft model. It can accurately identify minute size deviations, ensuring size control for safe and efficient air transport.

III. Key Application Scenarios in Airport Empty Container Measurement

(1) Accurate verification of empty box dimensions

In airport cargo and baggage handling processes, empty containers must strictly adhere to standardized size specifications. Taking a common air cargo container as an example, such as the 1AAA type, its standard dimensions are 3184×2438×2591mm. If an empty container is deformed or damaged due to long-term use, exceeding the allowable tolerance range (e.g., ±5mm), it will severely impact subsequent aircraft compatibility, potentially leading to loading difficulties or affecting aircraft flight balance. Using laser rangefinders, the length, width, and height of an empty container can be accurately measured from all angles before it enters the transportation process. At the entrance of the empty container conveyor line, a cleverly designed array of laser rangefinders is used: three sets of sensors are installed vertically along the container's direction of travel. One set at the top measures the container's height vertically downwards, while the other two sets at 45° angles illuminate the sides of the container. The length of the container is precisely calculated using triangulation combined with geometric relationships. As the container slowly passes through the measurement area, the sensors rapidly collect data, which is then analyzed and processed by a backend algorithm and compared in real-time with standard size data. Once a size anomaly is detected, the system immediately triggers an alarm and diverts the problematic empty container to the repair or scrapping channel, effectively preventing unqualified empty containers from entering the transportation process and ensuring the safety and efficiency of air transport.

(2) Intelligent monitoring and space optimization of stacking height

In airport empty container storage areas, rational stacking planning is crucial for improving storage space utilization and ensuring operational safety. Laser rangefinders play a vital role as "intelligent guardians" in this scenario. Carefully installed ToF laser rangefinders, positioned vertically above the stacking aisles, constantly monitor the distance between the top of the stack and the sensor. By pre-setting the height of a single empty container (e.g., 2.6m for a common plastic empty container) and the maximum allowed stacking height (assuming 6 layers, totaling approximately 15.6m), the system can quickly calculate the stacking height and number of layers based on real-time sensor measurements. If the stacking height approaches or exceeds the safety threshold, the system immediately issues an alarm, reminding staff to adjust the stacking method or stop stacking operations to prevent stack collapse and accidents. Simultaneously, in conjunction with side-mounted LiDAR or multi-ToF sensor arrays, the system performs a comprehensive scan of the stack outline, generating precise point cloud data. Advanced algorithms analyze the space occupied by the stack, planning the optimal loading and unloading path for automated guided vehicles (AGVs), significantly improving storage space utilization—for example, increasing storage density by over 20% and reducing airport operating costs.

(3) Real-time identification of empty container position and attitude

In automated loading and unloading processes, precise positioning and attitude recognition of empty containers on conveyor lines and loading/unloading platforms are crucial for ensuring accurate grasping and placement by robotic arms, AGVs, and other automated equipment. In these key operational areas, two sets of ToF laser rangefinders (1m apart) are cleverly installed along the container conveying direction, vertically illuminating the two corners of the top of the empty container. The sensors continuously measure the distance to the top of the container, and by measuring the distance difference, the trigonometric formula is used to accurately calculate the center coordinates of the empty container, guiding the AGV to precise alignment. The grasping error can be controlled within ±5mm. Simultaneously, if the distance difference measured by the two sensors exceeds ±10mm, the system can sensitively detect that the empty container is tilted, promptly triggering the robotic arm to adjust the grasping angle, avoiding grasping failures or collisions that could damage the equipment and the empty container due to abnormal container posture. This application effectively improves the efficiency and safety of automated loading and unloading, significantly reduces human intervention, and makes airport baggage and cargo handling processes smoother and more efficient.

IV. Key Points for Sensor Selection and System Setup

(I) Key Considerations for Sensor Selection

1. Range Adaptation : The sensor range is appropriately selected based on the actual needs of empty container measurement at airports. For typical small and medium-sized airports with limited dimensions and stacking heights of empty luggage containers, laser rangefinders with a range of 0.5-10m, such as the Keyence LR-TW5000, covering a range of 0.1-5m, are suitable, sufficient for measuring common empty container sizes and monitoring lower stacking heights. Large hub airports, facing larger cargo containers and higher stacking and storage requirements, need to select industrial-grade sensors with ranges of 15-30m or even higher, such as the Pepperl+Fuchs VDM28-8-133-476, with a range of 0.05-30m, easily handling complex and diverse measurement tasks. 

2. Accuracy Guarantee : High accuracy is a core requirement for airport empty container measurement. The sensor resolution should be no less than 0.1mm, with an accuracy of ±1mm - ±5mm. For example, the SICK TIM571 series boasts an accuracy of ±1mm, precisely identifying minute deformations, dimensional deviations, and attitude changes in empty containers, providing reliable data support for accurate assessment of their status. 

3. Anti-interference performance : Airport environments are complex, with drastic changes in lighting (natural light in the terminal, strong light in the loading and unloading area), and pervasive dust and moisture (baggage handling area). Sensors must possess excellent anti-interference capabilities. Using sensors employing infrared lasers (such as 905nm wavelength) in conjunction with narrowband filters effectively filters ambient light interference; IP67 or higher protection ratings and integrated anti-scattering algorithms resist dust and moisture corrosion, ensuring stable operation in harsh environments. For example, some SICK laser rangefinders maintain measurement accuracy and stability under various complex conditions.

4. Response speed and data output : Airport empty container transportation is fast-paced, requiring sensor measurement frequencies ≥5kHz. For instance, the SICK TIM310 has a measurement frequency as high as 10kHz, enabling rapid capture of dynamic information about empty containers. Meanwhile, sensors with 4-20mA analog input or digital interfaces such as EtherNet/IP and RS485 are preferred to facilitate seamless integration with the airport's existing PLC and SCADA systems, enabling high-speed data transmission and real-time processing.

(II) System Setup and Integration

1. Installation Layout Optimization : The sensor installation position and angle directly affect measurement accuracy and reliability. In empty container size measurement scenarios, the sensor needs to be installed directly above or to the side of the conveyor line to ensure that the laser beam accurately illuminates the key measurement points of the empty container vertically or at a specific angle. The horizontal/vertical accuracy of the mounting bracket should be ≤0.5°, and it should be strictly calibrated using a laser level. In stacking height monitoring scenarios, the sensor should be installed directly above the stacking aisle, avoiding proximity to the stacking edge or obstacles to prevent reflection interference. In empty container position and attitude recognition scenarios, the sensor should be installed at specific locations on the loading/unloading platform or AGV operating area to ensure that the laser beam effectively covers the top monitoring area of the empty container. 2. Data Processing and Control System Construction : Build a data processing and control system based on an industrial computer or high-performance PLC. The raw distance data collected by the sensors is first pre-processed by signal conditioning circuitry, including amplification and filtering, to improve signal quality. It is then transmitted to the data processing unit, where algorithms such as median filtering and sliding window filtering are used to eliminate measurement noise and interference, ensuring stable and reliable data. Next, through preset measurement models and judgment logic, such as dimension calculations based on geometric relationships and state judgments based on threshold comparisons, the size, position, and attitude of the empty container are precisely analyzed. Finally, based on the analysis results, control commands are output, such as triggering alarms, controlling the start and stop of conveyor lines, and guiding AGV movements, to achieve intelligent control and optimized management of the airport's empty container transportation and storage process. Simultaneously, the system should also have data storage and traceability functions to facilitate subsequent querying and analysis, providing data support for airport operation management.