Abstract: This paper addresses the shortcomings of traditional tower crane control systems controlled by relays and contactors, such as poor reliability, complex operation, high failure rate, large energy waste, and low efficiency. It proposes applying programmable logic controllers (PLCs) and frequency converters to the control system. A closed-loop system consisting of a rotary encoder and a PG digital-to-analog converter is added to the tower crane's lifting mechanism. Results show that this system is easy to use, has good dynamic adjustment performance, and greatly improves system stability and reliability. Keywords: Programmable logic controller; Tower crane; Stability 1. Current Status of Traditional Tower Crane Control Tower cranes are key equipment in our construction machinery, playing a vital role in construction. We have completed in just fifty years the development path of tower cranes that took developed countries over a century, and now we have reached the level of developed countries and entered the contemporary international market. With the development of high-rise buildings, new requirements have been placed on construction machinery. Therefore, 160TM attached tower cranes, 45TM internal climbing tower cranes, and 120TM self-erecting tower cranes are all designed and manufactured in China. In the 1980s, with the rapid development of national construction, the largest tower crane for construction, the 250TM tower crane, also emerged. Entering the 1990s, the modernization process accelerated, and the domestic and international markets... The requirements for tower cranes are getting higher and higher. With the increasing number of large-scale buildings, water conservancy, power, and bridges in many cities, the market demand has accelerated the development of new products. 400TM, 900TM horizontal jib and 300TM luffing jib tower cranes have been developed one after another [1, 2]. The technical performance of tower crane products developed and produced in the 1990s has been significantly improved. The hoisting mechanism adopts a three-speed motor drive, eddy current brake and electric shift gearbox. The luffing and slewing adopts a two-speed motor hydraulic coupling drive or frequency conversion speed regulation, with multiple speeds. The operation is stable and the production efficiency is high. The safety devices are complete, the action is sensitive and reliable, and the devices are equipped to prevent misoperation and rough operation, which can eliminate safety accidents [2]. With the development of power electronics technology, as early as the late 1960s, foreign countries began to focus on the development and research of thyristor stator voltage regulation and speed regulation technology. At present, this technology has entered a mature and stable development and application stage. After the introduction of programmable logic controller (PLC) into AC electrical drive system [3, 4], the performance of the drive system has undergone a qualitative change. In tower cranes, automatic control, fault diagnosis, detection and display of grab buckets have been realized, reaching a new technical height. The AC speed regulation system composed of frequency converters can replace the DC speed regulation system. This is an inevitable result of the continuous development of computer technology, especially large-scale integrated circuit manufacturing technology. It is in line with the development trend of cranes and is suitable for developing cranes with large lifting capacities. 2. Principle of PLC control system for tower cranes This system changes the tower crane control system from relay control to PLC control. The speed regulation of the four major mechanisms all adopt frequency conversion speed regulation. The overall block diagram of the tower crane control system is shown in Figure 1 [5, 8, 9]. The hoisting, luffing, slewing and running motors of the tower crane need to operate independently. The entire system is driven by 6 motors and 4 frequency converters, and controlled by one PLC. [align=center] Figure 1 System Block Diagram[/align] The starting time of the running mechanism should meet actual needs as much as possible, and the starting should be quick and smooth; the electrical braking method of the mechanism must be given special consideration. For different working conditions, free braking and forced braking methods can be selected. When the running mechanism stops normally, the free stop method can be selected, and its stopping time can be set according to the actual operating conditions in production, so as to meet the needs of the operator of the tower crane as much as possible. In order to ensure that the hoisting mechanism has a large enough starting torque when it starts, the relationship between the opening time of the mechanical brake, the minimum operating frequency of the frequency converter, and the operating current can be set to meet the requirements of the load characteristics of the mechanism. The setting of the internal parameters of the frequency converter can ensure that the mechanism has good speed regulation accuracy and starting and braking performance. Since the hoisting mechanism motor needs to use a pulse encoder as a speed feedback device, the speed of the motor is controlled by measuring the number of pulses of the pulse encoder and using the difference between the two. Therefore, the selection and installation of the pulse encoder should be carefully considered [6, 7, 10]. 3. System Hardware Design The electrical control system schematic diagram mainly includes the main circuit and the PLC peripheral wiring diagram. 1. The main circuit has six motors and a fan cooling device. 2. The I/O wiring signals of the PLC peripheral wiring circuit correspond to the I/O names in Table 1. Table 1 S7-200 I/O Allocation Table 4. System Software Design Based on the working principle of the tower crane control circuit, the software flowchart is shown in Figure 2. [align=center] Figure 2 System Software Flowchart[/align] In this system, the main task of the PLC program design is to accept external switch signals (buttons, linkage control console relays) input, determine the current system status, and output signals to control contactors and other devices to complete the corresponding control tasks. The ladder diagram of the system is shown in Figure 3. [align=center] Figure 3 Ladder Diagram[/align] 5. Conclusion The tower crane PLC control system designed in this paper has been successfully applied to a large crane company in Changsha after laboratory debugging. After six months of continuous operation, the system has never experienced a failure. Compared with the traditional tower crane control system, it has the following advantages: 1. Easy to use; 2. Good dynamic adjustment performance; 3. Greatly improves the stability and reliability of the system; 4. Saves about 10,000 yuan in maintenance costs per year (according to the rough statistics of the company using the tower crane, the economic benefits can be increased by more than 500,000 yuan per year compared with the previous one), and the operating efficiency has been greatly improved. Practice has proven that the design of this system is effective and has good application value. The innovation of this paper is to modify the traditional relay-contactor controlled tower crane and design a tower crane control system based on PLC, which has been put into use. Practice has proven that the system is easy to use, has good dynamic adjustment performance, and greatly improves the stability and reliability of the system. References: [1] [1] China Machinery Network. Historical Development and Existing Problems of Tower Cranes [EB/OL]. [2] Liu Peiheng. Development History of Tower Crane Industry in my country. Beijing: China Academic Journals (CD-ROM Edition) Electronic Magazine, 2003 (10)-2004 (1). [3] Zhang Lihua. Application of PLC in Electrical Control of Tower Cranes [J]. Journal of Nanjing Vocational Institute of Technology. 2004.4 (3) 4-6. [4] Zhao Shan. Application of SIEMENS SIMATIC S7_300 PLC in Boiler Computer Monitoring System [J]. Microcomputer Information. 2003, 19 (8), 24-25. [5] Zhang Wanzhong. Introduction to Programmable Controllers and Application Examples [M]. Beijing: China Electric Power Press, 2005. [6] Li Taijiong, Tian Zhongping, Xie Qisheng. Improvement of Tower Crane Control System [J]. VIP Information, 2006 (2) 62-63. [7] Li Xiuzhong. Application of PLC in the control circuit of gantry crane [A]. Coal Mine Machinery, 2004(1): 97-99. [8] Wu Zhongjun, Huang Yonghong. Principles and applications of programmable controllers [M]. Beijing: Machinery Industry Press, 2004. [9] Lu Yuandong. Application design technology of PLC electromechanical control system [J]. Beijing: Electronic Industry Press, 2006: 63-130. [10] GB/T13752-1992 Design specification for tower cranes [S]. Beijing: China Standards Press, 1996.