Abstract: Escalators are widely used in public places such as shopping malls, hotels, subways, and airports. As a type of electromechanical equipment that operates year-round, escalators are mostly lightly loaded or unloaded, thus offering significant energy-saving potential. This paper proposes a simple and easy-to-implement energy-saving method for escalators based on a PLC controller and a frequency converter. Keywords: Escalator ; PLC; Frequency converter; Energy saving 1. Introduction Escalators are widely used in hotels, shopping malls, subways, train stations, office buildings, airports, and other places, playing a significant role in facilitating customers and improving service quality. However, due to the special nature of their use, some escalators are frequently under light load or empty, which inevitably wastes a lot of electrical energy and causes wear and tear on the escalators. According to current technical standards, escalators can be divided into two types based on their workload: ordinary and public transportation type. The most typical and common public transportation type escalator is the exit escalator located in subway stations. Our field tests on this type of escalator show that after a subway arrives, the surge of people on the escalator lasts for about 45 seconds. If we calculate based on a subway train every 3 minutes, the escalator's "load duration" is only about 25%, with the escalator operating under empty or very low load for the rest of the time. If this is the case for public transportation escalators, the situation for other ordinary escalators is even more severe. Therefore, it is essential to take energy-saving measures for escalators. 2. Common Energy-Saving Methods and Control Systems Common energy-saving methods for escalators include the "automatic restart" energy-saving method and the "Δ-Y switching" energy-saving method. The first method involves stopping the escalator when no one is using it and starting it again when someone is using it, thus saving electricity. Because it involves a complete power outage, the energy savings are considerable, typically reaching 50%. However, this method has some drawbacks. First, when no one is using it, the escalator is stopped, so most passengers will assume it's out of service or under maintenance and go up the nearby stairs, especially when the escalator and stairs run parallel, thus negating the escalator's purpose. The latter method uses the classic star-delta connection switching for starting. However, this compromise also has some drawbacks. The star connection has only 1/3 the torque of the delta connection, resulting in a slower speed. Furthermore, there is a 2-3 second switching time between the two. If the photoelectric sensor is too close to the escalator entrance, the switching between the star and delta connections after passengers board will cause a shock, worsening the escalator's comfort and potentially leading to accidents. If the photoelectric sensor is too far from the escalator entrance, the shock can be avoided, but it is prone to malfunctions. Frequent starting of the escalator due to continuous passenger flow will also have adverse effects. There are three types of escalator control systems: relay control systems, PLC control systems, and microcontroller control systems. Currently, some escalators on the market still use relay control. However, due to the high noise, inconvenient maintenance, and outdated control methods of relay control systems, they are no longer suitable for modern needs and are gradually being phased out. Although microcontroller control systems have made great progress in intelligence, their application is limited by the long development time, difficulty in program modification, and anti-interference issues. PLC control systems, on the other hand, have significant advantages such as high reliability, good stability, and convenient maintenance. In addition, with the increasing popularity of computerized electrical equipment, PLC-controlled escalators perfectly meet people's expectations and are closely linked to market demand. Their development prospects are highly promising. 3. Basic Principles of PLC and Frequency Converter Control Due to the various defects of the above methods, an energy-saving control method based on PLC and frequency converter is proposed here. At the same time, using PLC as the main controller has the following advantages: (1) High reliability and good stability. The input signal threshold allowed by a general PLC is much larger than that of a typical microcontroller, and it is opto-isolated from external circuits, resulting in strong anti-interference ability. For example, a general PLC can withstand the peak interference of a rectangular pulse with a peak value of 1000V and a pulse width of 1s, and has multiple protection functions. Once a fault occurs, it can quickly stop the escalator. (2) Simple programming and convenient use. At present, PLCs generally adopt the "ladder diagram" programming method of relay control, which is easily accepted by electrical and automatic control technicians. (3) Convenient maintenance and repair. PLC products have complete monitoring and diagnostic functions, such as prominent internal working status, communication status, I/O status and abnormal status display. Each control link of the escalator can be represented by fault codes. This can greatly reduce the average repair time of faults. After some PLCs adopt intelligent I/O modules, they can also separate the external fault judgment and detection functions from the CPU, thereby improving the external fault detection function. Generally speaking, the idle time of the escalator is often greater than the passenger carrying time. After the PLC controller automatically detects that the escalator has been running idle for a certain period of time, it issues an instruction to control the frequency converter to reduce its speed. When a passenger steps onto the escalator bed cover, the escalator can automatically sense the arrival of the passenger through the photoelectric sensor set at the entrance of the bed cover and start to accelerate. Here, the main function of the frequency converter is to adjust the escalator speed, start and run smoothly. When no one is around, the PLC, after a delay instruction, automatically switches the system to creep mode to achieve energy saving. A photoelectric switch is installed at the escalator entrance. When a passenger is present, the switch sends a signal, and the escalator is regulated to its rated speed via frequency converter. After all passengers have disembarked, the escalator automatically enters a low-speed waiting state after a short delay. This saves energy, reduces mechanical wear, and extends the escalator's lifespan. Figure 1 shows the principle diagram of this energy-saving method. After the escalator starts, the PLC continuously monitors the electrical pulse signals from the photoelectric sensors to determine if anyone needs to ride the escalator, thus controlling the frequency converter to operate at normal or low speed. During normal speed operation, if no photoelectric pulse signal is detected, the system automatically switches to low-speed operation after a delay of t seconds (passengers moving from one end to the other). Simultaneously, the PLC continuously samples the frequency converter overload alarm signal OLW, the frequency converter abnormal alarm signal FB, and the motor flywheel speed signal after system power-on, and uses this information in conjunction with the program design to protect the system. Meanwhile, the system can also switch between energy saving and the original operating mode via a manual switch, allowing the system to operate in frequency conversion mode. When the frequency converter fails, it can switch back to the original operating mode, improving system reliability and enhancing redundancy. 4. System Overall Design: The PLC main controller uses the CPU 226 control unit from Siemens' S7-200 series. The S7-200 series PLC is suitable for automation of detection, monitoring, and control in various industries and occasions. It can achieve complex control functions whether operating independently or connected in a network. The CPU 226 integrates 24 inputs/16 outputs, totaling 40 digital I/O points. It can connect to 7 expansion modules, expanding to a maximum of 248 digital I/O points or 35 analog I/O points. It has 13KB of program and data storage space, 6 independent 30kHz high-speed counters, 2 independent 20kHz high-speed pulse outputs, and a PID controller. It features two RS485 communication/programming ports with PPI, MPI, and free-mode communication capabilities. The I/O terminal block can be easily disassembled as a whole. Designed for demanding control systems, it offers more input/output points, enhanced module expansion capabilities, faster operating speeds, and more powerful internal integrated special functions. Its input circuit employs opto-isolation, relay-type output, and a 24V DC power supply to provide a reliable operating voltage for the sensor. Its stability and ease of use have long been favored by electrical technicians. The sensor used is the BANNER Q23SN6LP reflective photoelectric sensor, with a maximum detection distance of 2m, and optimal performance at a distance of 0.5-1.5m. Its operating voltage is 10-30VDC. It should be installed on the upper or lower platform of the escalator, 1.0-1.5m from the entrance (allowing passengers to prepare for the escalator's acceleration). The inverter selected is the SAMCO-i high-performance silent I-IF type. Its two-speed selection terminal 2DF and second acceleration/deceleration terminal AD2 are used to achieve both normal and low speeds, as well as acceleration suitable for passenger comfort. The system hardware block diagram is shown in Figure 2. When the inverter reduces the speed to 20% under no-load conditions, for motors below 11kW, the escalator's energy consumption is only 3% to 5% of the motor's rated power, approximately 300 to 400 watts. Whether going up or down, an energy saving of 80% can be achieved at a 20% creeping speed. Using an inverter allows for extremely low frequency and voltage power supply to the motor when the escalator is unloaded, enabling the escalator to run at a slow speed. This provides excellent energy saving when the escalator is unloaded, without causing the escalator to stop running and avoiding the illusion of escalator damage or inoperability for passengers. When the photoelectric sensor detects someone entering the escalator, the escalator begins to accelerate. The time required for the escalator to accelerate from slow speed to rated speed can be set by the PLC, and the set time can be considered to be 2 to 4 seconds. When a person steps onto the escalator step, the escalator has approached or reached the rated speed. This avoids the acceleration process of people on the escalator and ensures the safety of people. When the PLC detects that the escalator is unloaded for t seconds, it sends a command to the frequency converter to change the rated speed back to the creeping speed. The entire control flow of the energy-saving operation mode of the automatic escalator is shown in Figure 3. The advantages of adopting the above energy-saving method are: (1) When no one is riding the escalator, it ensures that the escalator automatically and smoothly transitions to energy-saving operation and runs at 1/5 of the rated speed, saving energy. (2) When someone is riding the escalator, it ensures that the escalator automatically and smoothly transitions to the rated speed at the energy-saving speed, ensuring normal use. (3) Since the energy-saving speed is very slow when no one is riding the escalator, the wear of the mechanical parts is greatly reduced, which relatively extends the service life of the escalator. (4) The technology is mature and easy to implement and promote. The PLC adopts a relay type, which is simple to program and easy for engineers to modify according to actual needs. The frequency converter used is fully functional and easy to operate. Moreover, the device is small in size and easy to install, and can be directly installed in the upper and lower platforms of the escalator. (5) The use of frequency conversion technology greatly reduces the impact on the power grid when the escalator starts. The use of frequency converter can effectively improve the power factor of the power grid and reduce reactive power loss. 5. Conclusion This article mainly introduces an energy-saving method for automatic escalators based on PLC and frequency converter control. The PLC, as the main controller of the system, has stable and reliable performance, powerful functions, and is easy to maintain. At the same time, it also has a certain upgrade space. The use of frequency converter also enables the energy-saving effect of automatic escalators to be brought into play. Under the premise of safe and comfortable operation, it saves valuable energy and has good social promotion value and market prospects. The innovation of the author of this article: Unlike the traditional energy-saving method, the method proposed in this article not only ensures the normal operation of the elevator and makes passengers comfortable, but also saves energy, reduces unnecessary waste, and helps to extend the service life of the escalator. It can achieve significant results, especially in public places such as airports and hotels. References: [1] Nie Hui. Application of frequency converter in escalators [J]. Frequency Converter World, October 2005 [2] Zhu Ruihua. A simple energy-saving method for escalators [J]. Design Research, No. 9, 2000 [3] Hao Xiaohong, Zhang Ping. Application of VS-616G5 frequency converter in elevator speed control system [J]. Microcomputer Information, No. 1-1, 2006: P36-38