1. Introduction The performance of elevators has an increasingly significant impact on people's lives. Therefore, it is essential to improve the performance of elevator systems to ensure that elevator operation is both efficient and energy-saving, as well as safe and reliable. There are three main types of elevator control systems in China: relay control systems, microcomputer control systems, and PLC control systems. Among them, PLC control systems have become the mainstream due to their significant advantages. PLC control systems mainly include dual-speed elevator systems and variable frequency speed control systems. The latter smoothly adjusts the elevator speed by changing the voltage and frequency of the motor power supply, resulting in better ride comfort. It has high leveling accuracy and significant energy-saving effects, ensuring elevator reliability and successfully solving the problem of elevator ride comfort. In the past, medium and low-speed elevators mainly used traction systems to form their traction systems, employing pole changing to achieve motor speed regulation. Because this system can only achieve stepped speed regulation and cannot accurately control the motor speed and acceleration/deceleration, the ride comfort and leveling accuracy of this method are relatively poor. Later, elevators using AC voltage regulation speed control were adopted for closed-loop speed control, which significantly improved comfort and leveling accuracy. However, it was difficult to achieve precise control, and it consumed a lot of energy with a low input power factor, affecting the overall system performance. For high-speed elevators, thyristor DC speed control systems were mainly used in the past, which had problems such as difficult maintenance and low power factor. Compared with the aforementioned methods, variable frequency speed control is one of the most efficient and performant speed control methods. 2. Variable Frequency Speed ​​Control Elevator System and its Drive Technology The schematic diagram of a variable frequency speed control elevator system controlled by a PLC is shown in Figure 1. [align=center] Figure 1 Schematic diagram of a PLC-controlled variable frequency speed control elevator system[/align] Elevator systems need to process a large number of complex signals, and the problem of fast and accurate signal processing must be carefully considered during the design. It is necessary to adopt many advanced drive and control technologies, including vector transformation control technology, high-speed CPU technology, DSP technology, and high-performance variable frequency speed controllers using new high-power devices such as IGBTs. In Figure 1, the PLC mainly processes monitoring signals, including floor counting signals, call and floor selection signals, directional signals, speed change signals, and main control signals. It also handles various control signals such as door opening/closing control, floor display, call and floor selection display, single/double control, automatic safety condition detection, automatic leveling, and fire protection. When a frequency converter is used in an elevator, it is often called a VVVF elevator. Generally, frequency converters are of two types: AC-AC and AC-DC-AC. For AC-DC-AC frequency converters, they can be classified into voltage-type and current-type frequency converters based on the characteristics of the DC link voltage and current (determined by the capacitance and inductance of the filter). Elevators generally use voltage-type frequency converters. Changing the power supply frequency of the motor stator allows for speed regulation of the asynchronous motor. However, to maintain the maximum torque of the motor during speed regulation, the magnetic flux needs to be constant, requiring the voltage and frequency to be constant; that is, the voltage should be changed in a coordinated manner during frequency conversion. Currently, although dedicated elevator frequency converters exist, they are expensive. Therefore, general-purpose frequency converters can be used, and through proper design, they can achieve the control effect of dedicated frequency converters. To meet the requirements of elevator control, the parameter settings are much more complex than for dedicated frequency converters. To reduce starting shock and increase the comfort of speed regulation, the proportional coefficient of the speed loop should be smaller (3s), while the integral time constant should be larger (5s). To improve operating efficiency, the high-speed frequency should be selected as the power frequency (50Hz), while the creeping frequency should be as low as possible (4Hz) to reduce stopping shock. The maintenance slow-speed frequency can be selected at 10Hz. To ensure leveling accuracy and operational reliability, the traction motor speed is controlled in a closed loop, and its speed is detected by a rotary encoder. To ensure the frequency converter operates at its optimal state, it needs to perform self-learning on the driven motor. The method is as follows: disengage the traction machine brake wheel from the motor shaft, leaving the motor in an unloaded state, and then start the motor. The frequency converter can then automatically identify and store the relevant motor parameters, enabling it to perform optimal control of the motor. 3. Control Technology of Variable Frequency Speed ​​Control Elevator Systems Elevator system control can be mainly divided into speed control of the drive system and logic control of the floor selection system. The VVVF elevator drive speed control system essentially uses AC asynchronous motor drive and vector transformation control technology. The characteristics of the VVVF elevator speed control system are: the elevator starts with a reduced-frequency soft start, the motor starting current is very small, not exceeding the rated current. During the braking phase of the elevator, the speed control system operates in a regenerative braking state, without needing to draw power from the power grid, thus reducing energy consumption, avoiding motor overheating, and resulting in a relatively high power factor (close to 1). Speed ​​control refers to controlling the speed change pattern of the elevator throughout the entire process from start-up to leveling, thereby reducing the discomfort (floating and sinking sensations) caused by acceleration and deceleration during start-up and braking, and ensuring accurate and reliable leveling and stopping. Compared with the logic control of the floor selection system, speed control is more complex, and its control performance largely determines the performance and quality of the elevator. Elevator operation can be divided into three stages: start-up, steady-speed operation, and braking. During steady-speed operation, considering energy saving and interference with the power grid, the system employs open-loop control. However, during start-up and braking, closed-loop control is used to ensure the operating speed follows the given ideal speed. The ideal speed, which integrates comfort (meeting human requirements for acceleration and rate of change of acceleration), operating efficiency, and motor speed regulation performance, is stored in the program memory according to positional principles. The speed control system consists of a main circuit and a control circuit. The main circuit includes the traction motor (three-phase asynchronous motor) drive and functional circuitry. The speed feedback signal comes from a pulse generator coaxial with the motor rotor, and the frequency of its output pulses corresponds to the elevator's operating speed. Speed ​​control is achieved by changing the control angle (phase shift angle) of the trigger pulse. After a corresponding phase shift delay, the system controls the main circuit's operating state based on the difference between the actual elevator speed and the given speed. Due to factors such as load changes, power grid fluctuations, wire rope slippage, and expansion/contraction, the deceleration process may not meet the leveling requirements for direct stopping. Therefore, 1-2 leveling correctors are installed at a distance of 100-200mm from the floor. When the car reaches this correction point, the actual speed is compared with the leveling speed setpoint issued by the leveling corrector at that point. If there is no difference, the car will run according to the original deceleration curve; if there is a difference, the slope of the original speed setpoint curve will be corrected using the difference to ensure accurate leveling.