Abstract: This paper reconstructed the electrical control system and DC drive system of a large-scale B2025 gantry planer using PLC control technology and DC speed governing technology. The paper discusses the machine tool's control and drive requirements, completes the hardware and software design of the entire system, and provides detailed speed control circuits and control programs for crossbeam lifting and tool post feeding. The reconstruction improved the machine tool's reliability and stability, and has practical significance for the reconstruction of similar machine tools. Keywords: Gantry planer; Programmable logic controller; DC drive Introduction The B2025 gantry planer of a certain factory was manufactured by Wuhan Heavy Machine Tool Factory in the early 1970s. Its control system is a traditional relay-contactor control system. Important positions such as table forward, backward, acceleration, deceleration, and overtravel protection use contact limit switches. These switches operate frequently, and the contacts often stick together or fail to close properly, leading to table malfunctions, a large maintenance workload, and production disruptions. Meanwhile, the machine tool's main drive system uses a DC generator-DC motor drive system, and the speed control system uses discrete semiconductor components and a connector structure. This not only results in high noise and energy waste, but also, after nearly thirty years of use, the entire system is severely aged. It can no longer meet machining requirements at low speeds, sometimes even exhibiting crawling behavior, and its speed control performance is extremely unstable. To fully utilize the machine tool's still good mechanical precision and intact high-power DC motor, it is necessary to technically upgrade the machine tool's control and drive systems using modern control and power electronics technologies. 2 System Requirements Analysis When designing the overall machine tool upgrade, the coordination between the control and drive systems needs to be considered. The motion control of this machine tool is mostly logic control, therefore a programmable logic controller (PLC) is selected to complete the motion control. Since the original machine tool's DC motor has a power of 60KW, which is costly and still in good working order, the worktable will continue to be driven by the original DC motor. However, the DC drive involves speed adjustment. On the one hand, the forward and reverse speeds of the worktable need to be automatically adjusted according to the worktable's position during normal machining; on the other hand, the cutting speed of the worktable needs to be adjusted according to the cutting parameters. Considering the actual situation of the factory, this machine tool is only used for rough machining. Therefore, the different speed requirements are mainly caused by the reciprocating motion of the worktable, and the accuracy requirements for specific speed values ​​are not high. Therefore, the Eurotherm 590 series fully digital DC driver 594 is selected for the drive system. 3 Hardware Composition 3.1 Hardware System Scheme Based on the system requirements analysis, the hardware scheme of the entire control and speed regulation system is determined as shown in Figure 1. The overhead operator station mainly provides signals for worktable forward start, worktable backward start, worktable step forward, worktable step backward, crossbeam rise, crossbeam fall, vertical tool post rapid traverse, left tool post rapid traverse, right tool post rapid traverse, and emergency stop. The control cabinet buttons mainly provide signals for working mode selection, tool post selection, 594 enable, 594 enable stop, program stop, stop, and emergency stop. Various limit switches include deceleration and reversing proximity switches for table forward and backward movement, tool post lifting/lowering limit switches, tool post feed limit switches, and crossbeam release limit switches. Various protection signals include thermal protection for tool post lifting/lowering motors, thermal protection for feed motors, overcurrent protection for crossbeam tensioning motors, thermal protection for ventilation motors, position protection for left and right vertical tool posts, protection for crossbeam up/down position, and position protection for left and right side tool posts. The speed setting circuit uses a potentiometer to adjust the output voltage. The PLC selects the output voltage of the speed setting circuit and transmits this voltage signal to the 594, thereby controlling the speed and direction of the table's DC motor to achieve the table's speed regulation and reversing requirements. [align=center]Figure 1 Hardware Composition Scheme of Control System[/align] 3.2 Hardware Design All inputs and outputs of this system are digital quantities. There are a total of 66 digital input points and 37 digital output points. Therefore, a Siemens PLC CPU224 (AC/DC/relay) is selected as the main unit, and five digital expansion modules are added: one EM223 (16DI/16DO, relay output), one EM223 (16DI/16DO transistor output), and three EM221 (8DI). The speed control circuit is shown in Figure 2. B3 of the DC driver 594 is the +10V reference voltage terminal, B4 is the -10V reference voltage terminal, A1 is the 0V reference voltage terminal, A4 is the speed setpoint voltage terminal, and R1 to R8 are adjustable potentiometers. There are four circuits between B3 and the 0V reference voltage terminal: from B3 through R5, R6, R7, and R8, then to A1. Each time one of these four circuits is connected, the speed setting voltage terminal A4 obtains the corresponding voltage from the corresponding potentiometer, thus setting a specific speed. The voltage obtained by A4 from these four circuits is positive, causing the motor to rotate forward and drive the worktable forward. Similarly, A4 obtains four negative voltages from the circuit formed between B4 and A1, thus reversing the motor and driving the worktable backward. [align=center] Figure 2 Speed ​​Setting Circuit[/align] 4 Software Design According to the machine tool's working requirements, and considering the need to conform to the operating habits of the operators before the modification, an automatic working mode and a manual working mode were set up. In manual working mode, the following actions can be completed: vertical tool post rapid feed, left tool post rapid feed, right tool post rapid feed, crossbeam lifting control, worktable step forward, and worktable step backward. 4.1 Beam Lifting Control The prerequisite for beam lifting and lowering is that the beam is in a released state. After the beam descends to the designated position, it is necessary to ensure that the beam remains horizontal and to brake the beam's descent as quickly as possible. The design incorporates a brief upward movement of the beam to achieve this. After the beam has finished rising or falling, it is also necessary to clamp it. Figure 3 shows the PLC ladder diagram for beam lifting control. [align=center] Figure 3 Beam Lifting Control Program Segment[/align] In Figure 3, the Network 4 program segment completes the beam descent control. When the beam descent button (I0.1) is pressed, the beam begins to relax (Q0.2). After relaxation, the limit switches (I0.2, I0.3) are activated, and the beam begins to descend (Q0.1). When the beam has descended to the desired position, the descent button (I0.1) is released, and its normally closed contact is activated. The rising edge of the contact activates the beam rising (Q0.0) circuit and holds. Simultaneously, timer T37 starts timing. When the timing expires, the beam rise ends. Other inputs and outputs include the beam rise button I0.0, the beam clamping current relay I0.4, the beam rise limit switch I0.5, the left tool post and beam collision limit switch I0.6, the right tool post and beam collision limit switch I0.7, and the beam clamping contactor Q0.3. 4.2 Tool Post Feed and Braking Control In automatic operation mode, tool post feed and braking control is required. To detect the feed motor's rotation speed, the feed motor's rotation signal is transmitted to the limit switch via a cam mechanism. The feed amount is controlled by controlling the feed motor's rotation speed; the feed amount represented by each rotation of the feed motor is achieved by adjusting the transmission ratio of the feed box. Figure 5 shows the ladder diagram for the automatic feed control of the right tool post in automatic operation mode. [align=center]Figure 5 Right Tool Post Feed Program Segment[/align] When the right tool post is selected (I3.4), if the feed motor overload protection relay (I3.5) does not activate and the automatic feed selection switch (I3.2) is on, and the right tool post is in its normal position (i.e., the right tool post and crossbeam mutual contact limit switch (I3.1) and the right tool post lower limit switch (I3.0) are not pressed), then the feed action (Q2.4) is performed. When the feed motor rotates, the right tool post feed detection limit switch (I3.3) goes from on to off to on again, and its rising edge is used to count using the counter (C3). When the counter counts to the preset value, the feed is completed, and the feed motor stops. On the other hand, the rising edge of the counter is used to start the capacitor braking circuit (Q2.5) and start the timer (T44). When the timer expires, the electromagnetic braking circuit is disconnected, thus completing the feed process. The feed and braking of the left tool post and the vertical tool post are similar. The feed motor speed (limited to 1-4 revolutions) can be set via a DIP switch and stored in VW4, thus adjusting the feed rate. 5. Conclusion Over a year of operation after the modification of this machine tool has proven that the modification using a PLC and Eurotherm 594 system did not change the operator's operating habits, enhanced the feed control function, and improved the reliability and stability of the control and drive systems. The economic benefits generated by this project amounted to 1.5 million yuan. The author's innovation lies in designing a convenient and practical speed setting circuit using a PLC. Compared with setting speed via the operator panel or communication function, this method is less expensive and suitable for ordinary machine tools with low speed accuracy requirements. References: 1. Luo Bing, Yang Chongchang, Development of Automatic Sample Loom Control System Based on PLC [J], Microcomputer Information, 2006, 4: 57-59 2. Chen Boshi, Automatic Control System of Electric Drive, Beijing: Machinery Industry Press, 1995 3. Liao Changchu, PLC Programming and Application, Beijing: Machinery Industry Press, 2003