1. Introduction Traditional lifting platforms generally use AC wound-rotor asynchronous motors with rotor resistance for speed regulation. The switching of the resistor is controlled by a relay-contaminator. This control method has obvious drawbacks: large mechanical shock during braking and speed shifting, poor speed regulation performance, high energy consumption from the external resistor, complex wiring, frequent malfunctions, and poor safety. By using a simple and inexpensive squirrel-cage motor and modifying the lifting platform's control system with a PLC and frequency converter, soft starting and soft braking of the lifting motor can be achieved. This means slow acceleration during startup and slow stopping during braking. It also allows for multi-speed program control, accelerating the lifting process and ensuring fast, smooth, and safe transport of goods. 2. Basic Structure of a Small Cargo Lift The lifting process of a small cargo lift is achieved by the forward and reverse rotation of the motor winding a steel wire rope, which drives the cage up and down. A small cargo lift generally consists of a motor, pulleys, steel wire rope, cage, and various control electrical components. Its basic structure is shown in Figure 1. SQ1 to SQ4 can be limit switches or proximity switches, used for position detection and acting as limit switches. Figure 1 shows the structure of the hoist. 1. Hoist cage 2. Pulley 3. Drum 4. Motor 5. Limit switches SQ1 to SQ4 3. Speed ​​control system controlled by PLC and frequency converter 3.1 Multi-speed control According to the requirement that the hoist cage has a process of slow to fast and then fast to slow during the hoisting process, that is, it slowly increases speed at the start, runs quickly after reaching a certain speed, and decelerates and then slowly stops when approaching the end point, the hoisting process in Figure 1 is divided into three travel intervals, and the hoisting speed of each interval is shown in Figure 2. Pressing the lifting start button SB2 (or the lowering button SB3) will cause the hoist cage to start smoothly at a lower first speed. When it reaches the predetermined position, it will run quickly at a second speed, and when it reaches the predetermined position again, it will stop smoothly at the first speed. Figure 2. Lifting speed of the elevator. Figure 3.2. Hardware composition of the system. The automatic control system of the elevator mainly consists of a Mitsubishi FX2N-32MR programmable controller, a Sanken SAMCO-i frequency converter, and a three-phase squirrel-cage asynchronous motor. The hardware wiring of the system is shown in Figure 3. [align=center] Figure 3. Hardware wiring of the system[/align] The PLC control replaces the relay circuit. On the other hand, for the lifting and lowering required by the system, and the position information of the cage obtained by the limit switches, the PLC processes the information through its internal program and outputs corresponding "0" and "1" signals at the Y0~Y2 terminals to control the state of the frequency converter input terminals 2DF, FR, and RR, so that the frequency converter outputs the corresponding frequency as shown in Figure 2 in a timely manner, thereby controlling the operating characteristics of the elevator. The speed range is selected by 2DF. The magnitude of each speed range is determined by preset function settings of the frequency converter and then controlled by the PLC program to switch frequencies. When the PLC output terminals Y0Y1Y2 are in the state of "010", the frequency converter outputs the first-speed frequency, and the elevator rises at the speed corresponding to 10Hz. When the state is "110", the frequency converter outputs the second-speed frequency, and the elevator rises at the speed corresponding to 30Hz. Correspondingly, when the states of Y0Y1Y2 are "001" and "101", the elevator descends at the speeds corresponding to 10Hz and 30Hz, respectively. QF in the diagram is a circuit breaker, providing isolation, overcurrent, and undervoltage protection. Emergency stop button SB1, up button SB2, and down button SB3 can be installed at the bottom and top, or both, depending on ease of operation. During operation, simply pressing SB2 or SB3 will automatically initiate program control. 3.3 SAMCO-i Inverter Main Function Command Setting Cd000=1; Select inverter monitor display frequency (HZ) Cd001=1; Select external terminal signal as inverter operation command Cd002=1; Select inverter 1st speed frequency set by operation panel Cd007=30; Inverter upper limit frequency is 30HZ Cd029=10; Inverter first speed frequency is 10HZ Cd030=30; Inverter second speed frequency is 30HZ Cd049=5; Use braking resistor Cd050=1; Motor can rotate forward and reverse 3.4 PLC Ladder Diagram When the cage is at the bottom position and the normally open contact of SQ1 is closed, press SB2, and the motor will slowly rise at first speed. When it reaches the SQ2 and SQ3 positions, it will rise quickly and slowly in turn. The descent is similar. In case of emergency, press SB1 and the elevator will stop at any position. 4 Conclusion The speed regulation method controlled by PLC and frequency converter replaces the original rotor series resistance speed regulation method, which has smooth acceleration and deceleration, reliable operation, and greatly improves the automation level of the system. This system can be widely used in the vertical transfer system of goods in construction, warehouses, restaurants and catering industries. References : [1] Wang Tingcai, Wang Wei. Principles and Applications of Frequency Converters [M]. Machinery Industry Press, 2005. [2] Xiong Yongchao, Tao Yong. Application of PLC in Control System of Small Cargo Lifting Platform [J]. Coal Mine Machinery, 2006, 4: 681～683 [3] Mitsubishi FX2N Programmable Controller Manual [Z]. [4] SAMCO-i Frequency Converter User Manual [Z].