Abstract: This paper analyzes and compares the existing automatic train operation systems of Beijing, Shanghai, and Guangzhou metro systems in China, and points out the design ideas for domestically produced automatic train operation systems. Keywords: Metro, Automatic Train Control System, Automatic Train Operation System, Domestic Production For the high efficiency and high density requirements of urban rail transit systems, Automatic Train Control (ATC) is indispensable. One important subsystem, Automatic Train Operation (ATO), can simulate an experienced driver to complete the task of driving the train. The ATO subsystem uses ground information to control the traction and braking of the train, so that the train is always in the optimal operating state, improving passenger comfort, improving train punctuality, and saving energy. Many countries are researching ATO systems and have achieved certain results. China is still blank in this technology. This paper will compare and analyze the technical characteristics of three ATO systems. 1 Introduction to ATC and ATO ATC is an automatic control device for adjusting train intervals with the aim of safety and efficiency. It completes train operation control through a control system composed of onboard equipment, ground equipment, stations, and control centers. The ATC system comprises three subsystems: Automatic Train Monitoring (ATS), Automatic Train Protection (ATP), and Automatic Train Operation (ATO). The ATS subsystem monitors and guides trains to operate according to a predetermined timetable, ensuring the stability of the metro system. It establishes departure routes by switching tracks and provides trains with monitoring commands from the control center. The ATP subsystem has functions such as overspeed protection, zero-speed detection, and door restriction. ATP provides speed limit information to maintain safe intervals between trains, ensuring trains operate within the speed limits. Before opening doors, the ATP checks various conditions that allow door opening; only after passing the checks is the door allowed to open. The ATO subsystem automatically adjusts train speed and performs station-level stops, ensuring the train smoothly stops at the correct position within the station. The ATO receives train operation commands from the ATS. This information is transmitted to the train via track circuits or trackside communicators. After processing, the information is transmitted to the ATO, and relevant information is displayed. Upon obtaining useful information, the ATO calculates the operating speed based on track conditions, derives control parameters, executes control commands, and displays relevant information. Upon arrival at the station, and once conditions permit, the ATO opens the doors. During the stop, the train transmits train information to the ground communicator via the train-to-ground communication system, and then to the ATS. The ATS, based on the train information, transmits the operational information to the onboard ATO. The working principle diagram of the ATO is shown in Figure 1. [align=center] Figure 1 ATO Working Principle Diagram[/align] 2. Comparison of ATO System Technical Characteristics In the early 1990s, some trains on Beijing Metro Line 1 were equipped with ATO equipment from Westinghouse (UK) (not yet in use); the ATO equipment on Shanghai Metro Line 1 was imported from GRS (USA) and began trial use on the entire line in November 1996. Guangzhou Metro Line 1 introduced ATO equipment from Siemens (Germany), which officially began operation in June 1999. Because their ATO system designs are not entirely the same, it is necessary to compare the differences (mainly ATO equipment, ATO demand data and transmission channels, and control strategies), and then analyze the characteristics of various designs to facilitate the localization of ATO equipment. 2.1 Beijing Subway Line 1 ATO System 1. ATO Equipment Onboard Equipment: Consists of an ATO controller located in the driver's cab at each end of the train, and two ATO receiving antennas and two ATO transmitting antennas installed under the train body at each end of the driver's cab. Ground Equipment: Platform ATO Communicators (PACs) are installed in the equipment rooms of each station. The PAC stores track information to the next two stations and receives control commands from the ATS subsystem through an interface with the LPU or RTU. Xd or X2 loops and Rd loops are installed on the up and down platforms and the turnaround tracks for ATO turnarounds at each station. During train stops at stations, data is transmitted to the outdoor loops under the control of interlocking circuits and track circuits. 2. ATO Request Data and Transmission Channel During ATO data acquisition, the onboard ATP receives safety information. Safety information is transmitted by the AF900 track circuit of the current train operating section, using low-frequency pulse amplitude modulation (LPAM). There are 8 different modulation frequencies: 6 for ATP speed commands and 2 for gate control commands. In addition, the onboard TWC system receives ground TWC information. This information is generally non-safety control function data, such as operating class, train number, destination, and stop information. This information is transmitted to the train via ground TWC equipment using FSK modulation. Finally, the onboard ATO receives information from the onboard ATP, TWC, and marker coils. 3. Control Strategy Speed ​​Regulation: The ATO calculates the train's operating speed curve based on the MSS and TS obtained from the ATP. This curve is relatively simple, mainly calculating the points where acceleration transitions to constant speed and constant speed transitions to braking, to ensure that the train does not exceed the MSS and that the speed does not exceed the target speed at the target distance in each track circuit section. The controller automatically controls the train's traction and braking output according to the track conditions, trying to make the train run according to the operating speed curve. When the train speed exceeds the target speed, the ATP equipment alarms; when the maximum permissible speed is exceeded, the ATP implements emergency braking. Station Stopping: Station-based stopping is achieved through the X2 and Xd loops. After the train enters the X2 loop of the station, it obtains the distance to the stopping point through the induction between the ground and the train, performs the first position adjustment, and makes the speed as close as possible to the preset stopping speed curve. At the Xd loop, the second and final position adjustment is performed. If the running time needs to be adjusted, the ATS will give control commands, such as coasting control, stopping the train, passing the next station, etc., which are executed by the ATO. 2.2 Shanghai Metro Line 1 ATO System [3] 1. ATO Equipment Onboard Equipment: mainly includes the ATO main controller, as well as the ATP/TWC receiving coil, TWC transmitting antenna (TWC is the train-to-ground communication subsystem), alignment antenna, and marker coil on the undercarriage. Ground Equipment: includes the station parking module in the ATC equipment room of each station and a set of ground marker coils arranged along each platform. 2. ATO Demand Data and Transmission Channel During the ATO data acquisition process, the onboard ATP receives safety information. Safety information is transmitted via the AF900 track circuit in the current train operating section, using low-frequency pulse amplitude modulation (LPAM) with eight different modulation frequencies: six for ATP speed commands and two for gating commands. Additionally, the onboard TWC system receives ground TWC information. This information is generally non-safety control function data, such as operating class, train number, destination, and stop/skip information. This information uses FSK modulation and is transmitted to the train via ground TWC equipment. Finally, the onboard ATO receives information from the onboard ATP, TWC, and marker coils. 3. Control Strategy: Speed ​​Adjustment: The ATO works in conjunction with the ATP to adjust speed. The ATP has six speed commands: 20, 30, 45, 55, 65, and 80 km/h. The ATC system has four ATS operating classes, each with a corresponding corrected speed for each ATP speed command. The reference speed is the minimum of the received ATP speed command, the corrected speed of the ATS operating class, and the stop/skip speed curve. The ATO (Automatic Train Operation) system obtains operating speed information based on the operating class information received from the trackside and adjusts the train speed, acceleration, and programmed deceleration to conform to the received operating class. If a speed limit decrease is detected and the train speed exceeds the new speed command under normal braking conditions, service braking is initiated with a braking deceleration of 0.97 m/s². The ATO subsystem utilizes closed-loop feedback technology for speed regulation, using the difference between the actual speed and the reference speed as the error control quantity. A certain traction or braking force is applied to the train via the traction/braking lines to make the error control quantity zero. Station Stopping: The onboard ATO system corrects the programmed stopping curve to conform to the accepted operating class. Precise station stopping is achieved by applying track circuit IDs and boundary transitions, as well as the station loop. The track circuit ID is used to determine the starting point of the correct stopping curve. The station stop curve is initiated when the train passes the first pair of ground markers 350m outside the station. The station stop curve is based on a fixed deceleration rate. When the ATS (Automatic Train Service) speed matches the station stop curve speed, the train enters station stop control mode. When the train passes the ground markers at 150m and 25m, the distance information from the last stopping point is constantly updated. When the train passes above the active ground marker at 8m and receives the signal sent by the marker, the train immediately switches to the positioning and stopping mode, implements full service braking, and stops the train. The vehicle positioning antenna is aligned with the ground positioning antenna. Adjustment of running time: mainly achieved by selecting different running levels. The coasting mode is already included in the running level. Change of running mode: the logic requirement of the ATC system is that the conversion can only be carried out when the train stops, otherwise it will result in an emergency braking. 2.3 Guangzhou Metro Line 1 ATO system [4] 1. ATO equipment Onboard equipment: mainly includes ATC equipment rack, speedometer, control console, ATP receiving antenna, PTI transmitting antenna. Ground equipment: includes station cross loop and PTI loop. 2. ATO demand data and transmission channel Since Guangzhou Metro adopts FTGS digital frequency track circuit, it can transmit message information. All data transmitted from the ground to the train is received by the onboard ATP through the track circuit. The information required by the ATO is mainly obtained through the onboard ATP (Automatic Train Protection). This includes information processed by the ATP (actual speed, direction of travel, actual position, train length, speed limit command, braking deceleration, and additional information: fine-tuning of the next section, stopping position, and station stopping), as well as information transmitted from the ATS to the ATO via the ATP (gating, time to the next station, station number, train number, destination number, and track circuit number). Messages consist of all types of codes in a specific order and are cyclically sent by the track circuit. 3. Control Strategy: Speed ​​Regulation: The ATO receives speed command information from the ATP with four markers (including maximum speed, first speed limit, second speed limit, and the starting point, ending point, and speed value of the entry speed) and calculates the required train speed. The ATO provides three mode curves according to the timetable and operational needs: the maximum permissible curve, the normal speed curve (10% lower than the maximum speed curve), and the energy-saving speed curve (20% lower than the maximum speed curve); then, based on various line conditions and vehicle information, it calculates the required traction or braking force to bring the train to the required speed. The train is set with a maximum acceleration rate to ensure smooth operation. The control algorithm includes a warning curve, which is always slightly lower than the maximum permissible speed curve of the ATP. An alarm is triggered when the warning curve is exceeded. Station Stopping: Position adjustment points within the station are provided by multiple intersecting loops, as shown in Figure 2. The beginning and end of the loop are called loop boundaries. Correspondingly, the loop intersections in the middle of the station are used to determine distances, typically 6 sleeper spacings. Additionally, some coarse adjustment points are defined, with distances reduced to 3 sleeper spacings, grouped in sets of four. [align=center]Figure 2 Fixed-point stopping intersecting loops[/align] The ATP onboard equipment receives these intersections and transmits the processing signal of each intersection to the ATO. The ATO calculates the distance between each intersection. Coarse adjustment points can only be identified within the desired position window. If a coarse adjustment point is identified, the next intersection can be used for position synchronization. The positions of these intersections are preset in the ATO. Cruise/coasting is an auxiliary function of the ATO. If time permits, cruise/coasting can be used to adjust running time and save energy. Changing the operating mode on the main line: At any moment during train operation, the driver can move the control lever to move it out of the zero position, thus enabling manual driving. At any time and at any stage of driving, the ATO will display that ATO driving is possible. The driver moves the control lever to the zero position and presses the ATO start button, changing the train's operating mode to ATO mode. 2.4 System Analysis and Comparison: Among the three systems above, the Guangzhou Metro Line 1 ATO system has the best operating effect, followed by the Shanghai Metro Line 1 ATO system. The above analysis and comparison revealed that: From the perspective of information acquisition, Beijing uses station ATO communicators, and the ATO only obtains information within the station, resulting in poor real-time information; Shanghai Metro Line 1 uses track circuits and trackside TWC, while Guangzhou Metro Line 1 uses track circuits, both enabling the ATO to receive the latest information during operation. From the perspective of ATP speed limiting mode, Beijing Metro Line 1 and Shanghai Metro Line 1 use a graded speed control mode; Guangzhou Metro Line 1 uses a mode curve speed control mode. The mode curve ATP speed limiting mode makes ATO train control more efficient and smoother. In terms of parking methods, Beijing Metro Line 1 and Shanghai Metro Line 1 use a point-based system with corresponding coils at fixed locations; Guangzhou Metro Line 1 uses a continuous system, laying continuous intersecting loops within stations. In addition to adjusting distance at fixed points, it can also track train positions via pulse signals from the intersecting loops. Regarding operating time adjustments, Beijing Metro Line 1's ATO (Automatic Train Operation) adjusts based on coasting commands given by the ATS (Automatic Train Service) at the station; the ATO equipment itself simply executes operations based on various speed commands. Shanghai Metro Line 1's ATO adjusts operating time by receiving operating level commands from trackside equipment via the ATS, operating at the corresponding speed. Guangzhou Metro Line 1's ATO can calculate the desired operating level to select different traction percentages for control, thus adjusting operating time. Guangzhou Metro Line 1's ATO can also calculate the traction cut-off point in coasting mode to achieve on-time operation. Relatively speaking, Guangzhou Metro Line 1's ATO is more flexible in achieving on-time performance and adjusting operating time. 3. Research on the Localization of ATO System Onboard Equipment Through analysis and comparison, the design requirements for the localized ATO are as follows: information can be transmitted in the form of messages via track circuits; speed limiting modes can adopt a mode curve approach; parking equipment can adopt the laying of continuous intersecting loops; time adjustment requirements can be calculated in real time. 3.1 Working Principle Based on the ATO system of Guangzhou Metro Line 1, and combined with actual conditions, the onboard equipment of the ATO system was developed. The ATO receives the train operation task command from the ATS. This information, together with ground track information, forms a message, which is transmitted through track circuits and uniformly received by the onboard ATP. The ATP transmits the processed information useful to the ATO to the ATO, displays relevant information, and continuously monitors the operation of the ATO. After obtaining useful information, the ATO calculates the operating speed based on the actual operating speed and the maximum permissible speed of the ATP, derives the control quantity, and executes the control command. The cruise/coasting module is assisted by an independent controller. After arriving at the station, the ATO transmits train information to the ground via PTI (vehicle-to-ground communication transmitting antenna) and transmits it to the ATS to identify the train's position. After determining the train's new task based on this train information, ATS transmits it to ATO again via track circuit. During operation within a section, ATO receives new ground information each time it enters a new track section to adjust speed. When ATO conditions are met during operation, a flexible switch to ATO mode is allowed. 3.2 Design of ATO Onboard Equipment The ATO onboard equipment is the core of the ATO system and a challenging aspect of its design. The following analyzes the interfaces of the ATO onboard equipment. The ATO onboard equipment interface is shown in Figure 3. The CCU is the central control unit, controlling data communication between ATO, ATP, and the display via a bus; L1 and L2 are indicator lights that interface with ATO; multiple signal lines directly connect ATO and ATP, including system excitation lines, ATO enable lines, etc.; E1 to E10 are switches, buttons, or indicator lights that interface with ATP, including driver key, ATO enable lines, etc. All ground information is received by the ATP receiving antenna; PTI is the vehicle-to-ground communication transmitting antenna. [align=center] Figure 3 ATP Onboard Equipment Interface[/align] The above analysis and comparison of the existing automatic train operation systems of metro trains in China, and a brief introduction to the localization of the design of onboard equipment for automatic train operation systems. 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