1. Overview The ship unloader at a certain power plant is a bridge crane designed and manufactured by a renowned international crane manufacturer. Its operator's cab is suspended on the main beam/cantilever rails, 25m above the ground, with a travel distance of 42m. The coal unloading operation of the ship unloader is a high-altitude operation, and the operator's cab is the core mechanism for human-machine communication. However, the cab design and the surrounding environment significantly limit the operator's field of vision, with an approximately 240° blind spot. Furthermore, the cab lacks protective devices when the trolley mechanism is moving. Since the plant began operation, the ship unloader's operator's cab has experienced several serious collisions with coal ships. Therefore, installing an anti-collision system in the cab is directly related to the operator's safety and equipment safety. To prevent personal injury and equipment damage accidents caused by collisions in the operator's cab during ship unloader operation and to reduce potential safety hazards, we conducted a safety analysis and technical modifications. 2. Analysis and Design of the Cab Collision Avoidance System 2.1 On-site Working Environment of the Ship Unloader Cab 2.1.1 Due to the influence of astronomical tides, and the changes in the load and position of the moored ships during coal unloading, the moored ships and the ship unloader may experience relative motion exceeding safe limits. 2.1.2 The structures of incoming ships vary greatly; some have masts and cranes, which are uncontrollable and may move at any time, causing collisions. 2.1.3 The operator's field of vision in the ship unloader cab is 160°, with an effective field of vision of 120°. The operator's blind spot is large, which is very detrimental to coal unloading operations. 2.2 Based on practical experience in ship unloader operations, we require the following: 2.2.1 The detection sensors must be installed at the bottom rear of the cab, with an effective detection distance of at least 6m and adjustable sensitivity. The display should be installed inside the cab, showing the distance to obstacles (accurate to millimeters), with audible and visual alarms, and the ability to set the unloader's deceleration and stop points. During the movement of the unloader's trolley mechanism, an alarm should sound and the trolley mechanism should decelerate to 10% when the cab is less than 3.00m from an obstacle, and stop normally when it is 1.50m away. 2.2.2 The cab anti-collision system is activated when the cab moves towards the sea, but not towards the land. 2.2.3 The detection sensors must have at least one analog output (4-20mA) and two digital outputs. 2.2.4 The detection sensors must be able to distinguish objects such as coal ship antennas and lightning rods, and be unaffected by external environmental factors (wind, temperature changes, dust, etc.). 2.3 Determination of Anti-collision System Scheme Ultrasonic waves have strong directivity, slow energy consumption, and long propagation distance in a medium. Therefore, ultrasonic waves are often used for distance measurement, such as in rangefinders and level gauges. However, the effective dimensions of the unloader's cab in our factory are 4578×4216×2250mm (length×width×height), meaning that the rear side needs an anti-collision range of approximately 5m. The performance parameters (divergence angle and reflection characteristic curve) of the ultrasonic sensor do not meet the standard of covering a 5m width over a short distance. Therefore, a radar sensor with a larger coverage area is added to protect the cab. 2.3.1 Design of Control System Block Diagram and Selection of Detection Sensors Based on the analysis and requirements of the cab anti-collision system, it was decided to use the Banner QT50R radar sensor (two-way switching output) to realize the alarm stop signal output; and the QT50U ultrasonic sensor (analog output) to realize the distance display between the cab and obstacles. Three sets of combined sensor devices with slightly lower parameters are installed at equal intervals behind the cab. This meets the system's technical requirements and also reduces costs. As shown in Figure 1, the main control system completes the human-machine interface functions of the QT50 module, including window settings, analog signal acquisition, parameter calculation, result display, and control. Figure 1 shows the main control system block diagram. The detection range of the combined sensor is shown in Figure 2, demonstrating that it can cover the entire cab with a small blind spot, and its performance meets the requirements for on-site operation. [align=center] Figure 2 Schematic diagram of Banner sensor detection[/align] 3. Program structure and design of cab anti-collision system Add the following program segment to the PLC program (PB stands for Program Block, I refers to input point, Q refers to output point, and F refers to intermediate flag word): 3.1 In segment PB41/2, add the following logical relationship: ______ I104.4 · (I113.1+I113.3+ I113.5)＝F41 Where: I113.1 1.5m switch input of radar sensor 1 I113.3 1.5m switch input of radar sensor 2 I113.5 1.5m switch input of radar sensor 3 I104.4 Input of cab landside deceleration limit switch F41.0 Normal stop intermediate flag word for trolley mechanism direction 1 (i.e., backward movement) The input of the three radar sensors is required to be high level under normal conditions and low level when there is a fault or an obstacle is detected. The function of this relationship is to enable the landside deceleration limit switch I104.4=1 when the cab leaves the landside for normal coal unloading operations. When any radar sensor detects an obstacle within 1.5m behind the cab, it will interlock the trolley mechanism to stop it normally, but will not affect its forward movement, thus avoiding collisions with the cab. At the same time, F41 can be used as an alarm input point to realize the alarm function. 3.2 In segment PB41/9, add the following logical relationship: ______ I104.4 · (I113.0+I113.2+ I113.4)＝F44 Where: I113.0 3.0m switch input of radar sensor 1 I113.2 3.0m switch input of radar sensor 2 I113.4 3.0m switch input of radar sensor 3 I104.4 Input of the cab landside deceleration limit switch F44.0 Predetermined speed 10% for the trolley mechanism in direction 1 (i.e., backward movement) The input of the three radar sensors is required to be high level under normal conditions and low level when there is a fault or an obstacle is detected. The function of this relation is to interlock the trolley mechanism when any radar sensor detects an obstacle within 3.0m behind the cab, even when the cab is not in a landside parking space, causing it to slow down and move backward at 10% speed. This does not affect forward movement. Simultaneously, F41 can serve as an alarm input point. 3.3 In section PB41, a standard analog-to-digital conversion control block is added to convert the analog signals of the three ultrasonic waves to display the distance between the cab and the obstacle, which is then transmitted to the driver via a display. 3.4 In section PB41, the following logical relationship is added to implement an alarm when the cab is at a dangerous distance from an obstacle. The logical relationship is as follows: F41 + F44 = Q40 Where: F41 is the intermediate stop sign for the trolley mechanism when the distance between the cab and the obstacle is 1.5m; F44 is the intermediate deceleration sign for the trolley mechanism when the distance between the cab and the obstacle is 3.0m (radar sensor); Q40 is the alarm output when the cab and the obstacle are at a dangerous distance. 4. Conclusion: In the implementation of the anti-collision system for the ship unloader cab, we innovatively combined ultrasonic and radar sensors, fully utilizing their working characteristics, increasing their sensitivity and operational reliability, and realizing the detection and anti-collision protection of the cab and obstacles, greatly improving safety and preventing accidents. We utilized the advanced Siemens S5-155U program function and ET200 spare I/O input/output points of the ship unloader, employing advanced industrial programming language combined with electrical control technology to make various interlocks more reliable and easier to maintain. It is understood that the cabs of similar ship unloaders generally lack this type of interlock protection, making it worthwhile to promote its application in similar equipment to increase the safety of ship unloaders.