1. Introduction Currently, the vast majority of mine hoists in China (over 70%) use traditional AC hoist electrical control systems (represented by TKD-A). The TKD control system is a contact-based control system composed of relay logic circuits, large air contactors, and tachogenerators. While the TKD-A series hoist electrical control system has developed its own characteristics over many years, its shortcomings are also obvious. Its electrical circuits are overly complex, with numerous intermediate relays, electrical contacts, and electrical connections, leading to frequent hoist shutdowns due to electrical faults. New electrical control systems using PLC technology have been successfully applied in mine hoisting practices and have achieved good operational experience, overcoming the shortcomings of traditional electrical control systems and representing the trend in AC mine hoist electrical control technology development. 2. Overall Design Scheme The control circuit structure of the mine AC hoist electrical control system based on PLC technology is shown in Figure 1. It consists of the following five parts: high-voltage main circuit (including high-voltage commutator, motor, starter cabinet, and power brake power supply), main control PLC circuit, hoisting stroke detection and display circuit, hoisting speed detection, and hoisting signal circuit. The high-voltage main circuit still uses the traditional relay control circuit. [align=center] Figure 1 Block diagram of mine AC hoist electrical control system[/align] Working process: When the start-up signal is sent from the wellhead or bottom through the signal communication circuit, the start-up conditions are met. The driver pushes the brake handle forward away from the tight brake position, and the main motor releases the brake. The driver pushes the operating handle of the master controller to the positive (or negative) extreme position. The main control PLC controls the high-voltage commutator to be energized first through the program, so that the high-voltage signal is sent to the stator winding of the main motor. The main motor starts by connecting all rotor resistances, and then disconnects 8 sections of resistance in sequence to achieve automatic acceleration. Finally, it runs on the natural mechanical characteristics. When the AC hoist is running, the rotary encoder rotates with the main motor, outputting two columns of A/B phase pulses, which are respectively connected to the A/B phase pulse input terminals of the high-speed counter HSC0 of the main control PLC. The main control PLC automatically determines the increment/decrement counting mode of HSC0 based on the phase relationship of the A/B pulses. Based on the count value of HSC0, the hoisting stroke can be calculated and displayed. At the same time, the main control PLC increments the count based only on the A phase pulses output by the rotary encoder. Based on the count value of HSC1 within a constant interval, the hoisting speed can be calculated. 3 Hardware Design 3.1 Design of the Main Circuit of the Hoist The main circuit is used to supply power to the hoisting motor, realize undervoltage and overcurrent protection, control the direction of the motor and adjust the speed. The main circuit consists of a high-voltage switch cabinet, normally open contacts of the high-voltage commutator, normally open main contacts of the power brake contactor, power brake power supply device, hoisting motor, motor rotor resistance, normally open main contacts of the acceleration contactor (1JC~8JC), and indicating ammeters and voltmeters mounted on the driver's console. The system schematic diagram is shown in Figure 2. [align=center]Figure 2 Schematic diagram of the main circuit system of the hoist[/align] Main drive motor selection: Although squirrel-cage asynchronous motors are simple in structure, inexpensive, and easy to maintain, they are difficult to meet the requirements of hoist starting and speed regulation performance. Therefore, wound-rotor asynchronous motors are selected as the main drive motors for the AC drive system of mine hoists. The rotor resistance of the wound-rotor asynchronous motor can limit the starting current and increase the starting torque, and can regulate the speed within a certain range. Two 6kV power supplies from the ground substation, one for operation and one for standby, pass through the isolating switch GLK1, oil switch gyd, main contacts of the high-voltage commutator line contactor XLC, and forward (or reverse) contactor ZC (or FC) of the TGG-6 type high-voltage switchgear to the stator of the main motor. The high-voltage switchgear also contains a voltage transformer YH, an undervoltage trip coil SYQ, a current transformer LH, and an overcurrent trip coil GLQ for undervoltage or overcurrent protection. Emergency stop switch JTK1 and commutator chamber gate interlock switch LSK are also connected in series in the SYQ coil circuit. 3.2 Braking Circuit Design Most mine hoists are driven by wound-rotor asynchronous motors, and in most cases, speed regulation is achieved by step-switching rotor circuit resistance. Their braking systems mostly use thyristor-controlled power braking and adjustable brake braking systems. The former is an electrical brake, and the latter is a mechanical brake. When the hoist is running in the deceleration section, when the speed is within the range of 0-5%, the electrical brake is active, and the adjustable brake is inactive; when the overspeed is within the range of 5%-10%, the electrical brake limits the speed and maintains the maximum braking power, while the adjustable brake is active, increasing the total braking torque; when the overspeed is 10%, the overspeed relay GSJ1 acts on the safety circuit, and the adjustable brake stops the hoist drum. The thyristor power supply device mainly consists of two parts: a main circuit and a trigger circuit. In this design, a KZG type three-phase thyristor-controlled power braking system is used. This system is a single closed-loop dynamic braking system. The system block diagram is shown in Figure 3. As can be seen from the figure, speed deviation control and foot pedal control have an "OR" relationship; whichever signal is larger allows the signal to pass, meaning the corresponding control method takes effect. Therefore, in single closed-loop control, the driver can use the foot pedal brake for control. However, if the hoist overspeeds during foot pedal control, the closed-loop system can also provide monitoring and protection. [align=center] Figure 3 Block Diagram of Single Closed-Loop Dynamic Braking System[/align] 3.3 Speed ​​Setting Loop The speed setting method generates the speed setting signal according to the stroke principle. In the electrical control system of mine hoists, a cam plate setting method is usually used, where the cam plate controls the output voltage of the synchro. Because the synchro has no sliding contacts, the voltage change is relatively stable, the operation is more reliable, and the maintenance is less. The schematic diagram is shown in Figure 4. [align=center] [/align]Figure 4 Speed ​​Setting Circuit When the synchro is used as a setting device, the excitation winding is supplied with a single-phase 110V AC current, and the output of any two phases from the three-phase synchronous winding is used as the output of the setting voltage. Its output voltage is AC; if DC is required, it should be output through a bridge rectifier. 3.4 Power Braking Circuit The thyristor rectifier and its triggering device are installed as a set in the power cabinet. The magnitude of the output voltage of the power braking power supply device is related to the level of the control signal voltage input to the triggering device. [align=center] [/align]Figure 5 Power Braking Voltage Formation Circuit The control signal voltage consists of two loops forming an OR gate circuit, as shown in Figure 5. As long as one of them meets the triggering requirements, the thyristor can be triggered to perform the braking function. One of these two loops is the speed deviation value formed by the actual speed and the given speed, which automatically controls the output of the CF3 magnetic amplifier and the power braking output. The other loop is controlled by the driver to manually adjust the output of the synchro CD2. When the power braking system is manually controlled, the driver controls the foot pedal to drive the synchro CD2 to generate the control voltage. During adjustment, it should be matched with the output of the magnetic amplifier CF3. When the heel is just pressed down and the toes have not yet pressed down, it is equivalent to the control switch being closed, causing the DZC to be energized and engaged, and the thyristor power brake to be activated. However, at this time, the output of the synchro CD2 is very small, and the power brake current is minimal. When the driver's toes are pressed down, the output of the synchro CD2 is at its maximum. In the foot pedal power brake and CF3 output circuits, two diodes, Z1 and Z2, respectively form an OR gate circuit. These two control signals are connected in parallel and do not affect each other. 3.5 Stroke Detection and Display The running position of the hoist is converted into pulses using a rotary encoder. The PLC counts these pulses at high speed and automatically generates relevant data of the hoist position through corresponding calculations, which is then transmitted to the storage unit of the high-speed counter inside the PLC. In order to improve the pulse accuracy of the counter, the E6C-CWSC type reversible rotary encoder from Omron Corporation of Japan is selected. Its pulse accuracy is high and it will not lose pulses at low speeds. To facilitate operation by the hoist operator, the hoist's electrical control system needs a reliable travel display device (also known as a depth indicator) to show the position of the hoisting container in the shaft. This paper designs a hoist position display using three LED seven-segment displays based on the operating distance (0-570m) measured by the encoder. [align=center] Figure 6 PLC Digital Display Circuit[/align] In the circuit shown in Figure 6, a CD4513 chip with latching, decoding, and driving functions drives the common cathode LED seven-segment displays. The data input terminals a-d of the three CD4513 chips share the four output terminals of the programmable controller, where a is the least significant bit and d is the most significant bit. LE is the latch enable input terminal. On the rising edge of the LE signal, the BCD number input from the data input terminal is latched into the on-chip register, and the number is then decoded and displayed. If the input is not a decimal number, the display is off. When LE is high, the displayed number is not affected by the data input signal. Obviously, the number of output points occupied by n displays is 4+n. 3.6 Auxiliary Circuit Design The auxiliary circuit is used to power and control auxiliary equipment. The power supply voltage of the auxiliary circuit is AC 380V, with two circuits providing power. The loads connected to the auxiliary circuit include: thyristor-driven power brake device, brake oil pump motor, lubricating oil pump motor, etc. 4 Calculation of Rotor Resistance of Hoist Main Motor The calculation of the motor rotor resistance plays an important role in the normal operation of the hoisting equipment. When calculating the starting resistance, the number of preparatory stage stages and acceleration stage stages should be determined first. Because the selected number of stages directly affects the increase or decrease of the maximum switching torque and the increase or decrease of the average starting acceleration, and may even require increasing the motor capacity due to insufficient overload capacity, a comprehensive consideration should be given to select an economical and reasonable number of stages. Generally, the selection of the preparatory stage stage and acceleration stage stages is shown in the attached table. [align=center] [/align] There are many methods for calculating the three-phase balanced starting resistance, but they can be basically divided into two types: one is to calculate the starting resistance based on a given acceleration, and the other is to calculate it based on fully utilizing the overload capacity of the motor. Because the first method is simple and accurate in calculation, it is adopted in this paper. 5 PLC Control System Design 5.1 Main Control PLC Control Circuit Design Based on the operating mode of the hoist and the inherent characteristics of coal mining enterprises, the application of PLCs in the electrical control system of domestic mine hoists has developed rapidly. However, from the perspective of field use, currently, in the control system of domestic coal mine hoists, PLCs are mainly used to process switching quantities to replace the numerous relays, contactors, complex wiring, and signal display systems in the old hoist control system. The analog quantities and automatic adjustment processes in the braking system, which are crucial for the safe operation of the hoist, are mostly handled by ordinary electronic modes using adjustable brakes and thyristor-controlled braking with semiconductor devices and operational amplifiers. During use, zero-point drift and electronic component damage frequently occur, and there are disadvantages such as difficulty in maintenance and re-adjustment, and poor reliability, thus reducing the reliability of the hoist electrical control system. To address these problems, in-depth research on using PLCs to control the coal mine hoist control system is essential. The main control unit programmable logic controller (PLC) design in this paper consists of a CPU226 main unit and two I/O expansion modules EM223 and EM222. The design contains 40 input points and 40 output points. The specific I/O wiring is shown in Figure 7. [align=center] Figure 7 Main control PLC circuit and expansion I/O wiring[/align] 5.2 PLC control software design [align=center] Figure 8 Main program control flowchart[/align] The main program flowchart of the PLC control software is shown in Figure 8. (1) The initialization subroutine is used to perform the following operations on the high-speed counters HSC0 and HSC1: write control word, define working mode, clear, write set value, set timer interrupt, connect interrupt, start counting. (2) The control of brake oil pump, lubricating oil pump, power brake power supply, five-way valve solenoid, four-way valve solenoid and safety valve solenoid, etc., belongs to the control of auxiliary equipment required for the safe operation of AC hoist. (3) The display and control of brake oil overpressure signal, brake oil overheat signal and lubricating oil overpressure signal are used to display and control the working status of AC hoist. (4) The rope adjustment interlocking circuit plays a safety protection role in the rope adjustment process. When the double drum hoist changes to horizontal rope adjustment, the rope adjustment conversion switch 1hk-3 is disconnected, so that the rope adjustment interlocking link is connected in series with the safety circuit. During normal operation, 1hk-3 is connected, and the rope adjustment interlocking does not work. (5) The hoisting signal circuit is used to prepare for the start or deceleration of the AC hoisting motor. (6) The position measurement subroutine is used to measure the position of the hoist in the mine. (7) The stroke display subroutine displays the current stroke position according to the number of pulses of the rotary encoder. (8) The deceleration signal circuit and deceleration signal bell are used for deceleration control and to issue a bell prompt signal. (9) The automatic reversing working circuit and the manual forward and reverse working circuit are used to control the forward and reverse rotation of the AC hoisting motor in automatic and manual modes, respectively. (10) The safety circuit is used to prevent and avoid accidents of AC hoist. (11) The timer control circuit and rotor resistance switching control are used for rotor resistance switching control when the AC hoisting motor starts or decelerates. (12) The power braking circuit is used for power braking power supply input and output control. (13) The foot brake interlock and working brake relay are used for AC hoisting motor braking control. 6 Conclusion The hoisting machine control system adopts a combination of PLC control and TKD-A control system, which has the advantages of reliability, safety and convenient implementation. The main control logic of the hoisting machine is implemented by PLC, which increases the control function and realizes efficient automated production. The key is to give full play to the advantages of PLC, use its comprehensive measurement and control mechanism, solve the problems of speed measurement and protection, realize good connection with the original system, improve the comprehensive performance of the system, and achieve low input and high output. From the application of the system, there are still some issues that need to be further improved, such as network communication function and advanced control technology and strategies such as intelligent control. Further functional expansion on the basis of existing PLC technology will further improve the modernization level of mine hoisting power control system in China. Author Profile Xu Chengyi, male, graduated from Liaoning Institute of Technology with a major in Electrical Engineering and Automation. He is currently employed in the Technical Research and Development Department of Dalian Sanyo Refrigeration Co., Ltd. References [1] Lu Yan. Electric Drive and Control of Mine Hoists. Beijing: Metallurgical Industry Press, 2001 [2] Wang Yonghua, Chen Yuguo. Modern Electrical Control and PLC Application Technology. Beijing: Aerospace University Press, 2003 [3] Yu Fazhi. Current Status and Development of Mine Hoists in China. Mine Electromechanical, No. 3, 1995 [4] Ye Yuguang. Mine Hoist Electrical Control System Based on PLC Technology. Mechatronics, No. 6, 2004 [5] Jiang Hongmin. Research on the Application of PLC Technology in AC Hoisting System of Mine in China. Metallurgical Mine Design and Construction, No. 4, 1998 [6] Zhang Hongyan. Application of Programmable Controller in AC Hoist Electrical Control. Zhongzhou Coal, No. 4, 2005