Automatic door systems are a crucial component of urban rail transit vehicles, directly impacting passenger safety. Currently, most high-speed trains in China use single-wing sliding doors, while subways use double-wing swing doors. The most commonly used automatic doors in China are pneumatic doors, such as single-wing pneumatic sliding doors and double-wing pneumatic concealed swing doors. However, electric doors, as a newer type of automatic door system, are gradually being promoted and applied. 1. Working Principle of Electric Automatic Doors The working principle of an automatic door is that the door panel is supported on a guide rail by a bracket. The guide rail is connected to a drive device, which drives the door panel to slide through the guide rail. Each door has a locking mechanism that mechanically locks the door when it is fully closed. The doors have zero-speed protection and a safety interlock circuit, and there is an alarm device for opening and closing. The drive device for electric doors is a set of motor components, and each car has a main controller to control the doors of that car. The main controller is the command center of the automatic door. Through its internal instruction program, it issues corresponding commands to control the motor or electric lock system. Users can also adjust parameters such as the door's opening speed and opening radius via the main controller. External signals are provided by the sensor; when a moving object enters its working range, it sends a pulse signal to the main controller. The motor provides the main power for opening and closing the door, controlling the acceleration and deceleration of the automatic door. The workflow for one opening and closing cycle of the automatic door is as follows: The sensor transmits a detection signal to the main controller, which then controls the motor to run, monitoring the motor's speed and current to adjust the motor's power, slow operation, and reverse direction at appropriate times. Once the motor receives a certain operating current, it runs in the forward direction, transmitting power through the transmission mechanism to open the automatic door. After the door opens, the controller determines whether to reverse the motor to close the door. 2. Main Controller and Actuators of the Control System The main controller of the automatic door monitoring and control system uses a TI DSP, model TMS320C2812, an advanced 32-bit fixed-point DSP chip. It not only boasts high operating speed and powerful processing capabilities but also features abundant on-chip peripherals, facilitating interface and modular design. It is particularly suitable for monitoring and control involving large volumes of data. Utilizing DSP technology, an effective signal processing model is established to suppress interference noise and achieve maximum signal-to-noise ratio. This ensures the real-time performance of the DC brushless motor in this system. The DC brushless motor used is an ALC-TEL BG65PI motor, characterized by high efficiency, energy saving, low noise, high speed, high torque, and no overheating during continuous use, significantly surpassing traditional AC servo motors. Combined with a T-type rack and pinion synchronous belt, it provides exceptional quietness throughout the door's operation, from low to high speeds, achieving ultra-quiet operation. The control system is designed for intelligent control, allowing for arbitrary setting of the door's operating speed and opening degree; it can self-correct to maintain smooth door movement; it can automatically detect the door width to maintain optimal operation; and it has an anti-pinch function, meaning the door automatically reverses and exits when encountering obstacles or people, with a smooth transition from high to low speed during operation. 3. Sensors in the Control System For an automatic door control system, sensors detect the presence of someone outside, triggering the door opening action and then closing it. The controller not only controls the doors individually but also handles vehicle-wide communication and other interactions. Therefore, the effectiveness of door opening and timely closing, and the maintenance of a suitable environment inside the vehicle, all depend on the controller's design. Infrared pyroelectric sensors are controlled by detecting infrared radiation of 8-13 μm emitted by the human body. The radiation is emitted by a microwave device, reflected by the human body, detected and amplified by the device, and then used to control subsequent circuits. Its characteristic is that it will react regardless of whether a person is moving, as long as they are within the sensor's scanning range, preventing the door from closing. This is detrimental to minimizing changes in the environment inside the carriage, making it unsuitable for inner doors connecting rail vehicles. Therefore, we choose microwave sensors, also known as microwave radar, which react to the movement of objects. Its reaction speed is faster than that of infrared sensors, making it very suitable for use in rail passenger car doors. Its characteristic is that if a person near the door remains stationary and does not wish to exit, the radar will stop reacting, and the automatic door will close. Because this design system has an anti-pinch function, it can solve the potential problem of people being trapped. 4. Control Method and Program Design of the Control System 4.1 Control Method Employed The control method adopted is direct digital control, a relatively good online real-time control method. The PID control algorithm for the output control quantity y(t) and the input position quantity x(t) is generally as follows: Where Kp is the proportional gain, Ti is the integral time constant, and Td is the derivative time constant. To reduce the computational load, a summation and differential calculation are used instead. Therefore, when the sampling period is T, we have: During the opening and closing process of the automatic door, a switching signal is set as a position signal ε. When the calculated input position quantity x(t) is outside this position signal, i.e., x(t) > ε, the output control quantity y(t) has a faster response speed, and the automatic door opens and closes quickly; otherwise, it is slower but has a certain accuracy and achieves some preset functions such as anti-pinch. The calculation formula is: Where when x(t) > ε, K1 = 1; when x(t) ≤ ε, K1 = 0. This eliminates the integral action during the initial opening and closing of the automatic door, keeping the cumulative effect of the integral within a very small range, avoiding system oscillations, and ensuring accuracy. For a perfect measurement and control system, it must be able to accurately reproduce the waveform of the measured signal, analyze and calculate it, and provide an accurate output control signal without any time delay. In a practical system, the relationship between the input x(t) and the output y(t) at time t is: y(t) = Kx(t-t0), where K and t0 are constants. In this closed-loop measurement and control system, the lag of the system's output relative to the input will disrupt its stability. Therefore, the condition to maximize the system's accuracy or prevent distortion is t0 = 0. However, in actual feedback control, t0 cannot be 0, inevitably resulting in dead time in measurement and control. Dead time can be defined as the delay time between "the instant the measuring sensor detects the variable starting to change" and "the instant the controller begins to apply correct and effective intervention to the production process." During the experiment, the sensor was installed in the middle of the automatic door, and the shortest distance for signal detection was adjusted to reduce transmission delay and eliminate some dead time. Additionally, the controller's deviation tolerance was adjusted, i.e., the controller's tuning parameters were reduced, to slow down the system's response speed and, when elimination was not possible, reduce dead time. In the error elimination process, to ensure reliability and avoid increased costs, the previously mentioned model was selected after thorough testing to ensure the reliable use of the upper and lower components. Electromagnetic compatibility (EMC) was used in the electrical system design to overcome signal interference problems. 4.2 The programming software was written in C28x assembly language to achieve real-time signal processing. With the development of DSPs, their main work has shifted to software development, which will account for approximately 80% of the workload. Algorithms have become the core of DSPs, and the variable speed drive of the motor is also ultimately implemented in software. 5. Conclusion Through simulated testing on rail vehicles, the automatic door's operating speed is adjustable from 0 to 500 mm/s, its opening time is adjustable from 0.1 to 10 seconds, and its detection angle is greater than 150°. Technical indicators such as detection error angle and obstacle dead zone time error rate meet the relevant contract standards. The reliability and accuracy of the entire system are guaranteed even under prolonged exposure to bumps, strong vibrations, and impacts on electrical and mechanical components.