1. Introduction Currently, China's machinery manufacturing industry has multiple technical routes to choose from when developing modern mechanical automation technology for a large number of general-purpose equipment. One approach is to apply microelectronics technology to upgrade these existing general-purpose equipment, such as using digital displays and CNC devices to improve the automation level of single machines; another is to use programmable logic controllers (PLCs) to upgrade general-purpose machine tools, special-purpose machine tools, combination machine tools, and production lines composed of automatic and semi-automatic equipment. This combines the advantages of computers—complete functionality, flexible programming, and strong adaptability—with the advantages of relay control—simple control, strong anti-interference ability, and low price. This is a new path for the development and application of mechanical automation technology that is low-cost, high-efficiency, and suited to China's national conditions. 2. Automatic Shearing Machine Process Introduction Shearing machines are used in many metal processing and thin plate cutting operations. Before designing a shearing machine, several factors must be considered, including the shearing capacity, output enhancement options, and safety. The type of shearing machine is determined by many factors, such as the length, thickness, and type of materials it can handle. Shearing machines can be classified according to their shearing method and drive system. Two structural types are commonly used in electric gantry shears: guillotine (also called slide-type) and swing-arm shear. Guillotine shears utilize a drive system to move the moving blade downwards to a certain position, keeping it almost parallel to the stationary blade throughout its stroke. To maintain a proper orientation of the blade holder beams during their relative movement, guillotine shears require a slide guide system. Swing-arm shears, on the other hand, have a drive system that manipulates the moving blade, causing it to rotate downwards via roller bearings. This structure eliminates the need for U-shaped guide rails or slides to maintain the blade's proper posture during shearing. 3. System Analysis 3.1 Control Requirements After power-on, the system detects the status of each working mechanism and controls each mechanism to its initial position; feeding: the control system controls the feeding mechanism to automatically convey the sheet metal to be sheared into place; setting the shearing size: a servo motor controls the position of the stopper to ensure accurate shearing size, which can be a fixed value or set as a cyclically variable value; clamping and shearing: after the length of the sheet metal to be sheared reaches the set value, the main motor drives the clamping device and shearing cutter to first clamp the sheet metal and then shear it; operation of the feeding cart, including automatic return after unloading; setting the size of the sheared sheet metal, automatic counting, and presetting the number of sheets per cart; having power failure protection and power restoration functions; enabling automatic control of the processing process, display of processing parameters, and system detection. 9. Ensure sheet metal processing accuracy, processing efficiency, and safety and reliability. 10. Have a user-friendly human-machine interface. 3.2 Structure and Principle of the Shearing Machine An automatic shearing machine is an automated processing equipment that precisely controls the processing dimensions of sheet metal, automatically and cyclically shears large pieces of metal sheet, and transports them to the next process via a feeding trolley. Its structure and principle are shown in Figure 1. [align=center][img=397,241]http://www.ca800.com/uploadfile/maga/plc2008-6/myh-1.jpg[/img] Figure 1 Schematic diagram of an automatic shearing machine[/align] 3.3 Structure of the Control System The system is equipped with seven limit switches, which are used to detect the working status of each part. Among them, SQ1 detects whether the sheet metal to be sheared has been conveyed to the correct position. SQ2 and SQ3 detect the state of the pressure block B, respectively, to detect whether the pressure block has pressed the sheet metal that has been placed in place; SQ4 detects the state of the shearing blade A; SQ7 is a photoelectric proximity switch, which detects whether the sheet metal has been sheared and fallen into the trolley; SQ5 is used to detect whether the trolley has arrived in place; SQ6 is used to determine whether the trolley is unloaded. The feeding mechanism e, pressure block b, shearing blade a, and feeding trolley are each driven by four motors. When the system is not in operation, the limit switches sq2, sq3, and sq4 for the pressure block and shearing blade are all open, as are sq1 and sq7. 3.4 Working Principle of Automatic Shearing Machine When the system is started, input the parameters such as the sheet metal processing dimensions and quantity, press the automatic switch, and the system will run automatically. First, check the status of limit switch sq6. If the trolley is unloaded, the system starts working and starts the feeding trolley. When the trolley reaches its position, limit switch sq5 closes, and the trolley stops; the feeding mechanism e starts, driving the sheet metal c to move to the right. When the sheet metal touches the limit switch sq1, feeding stops, the brake is released, the electromagnetic clutch engages, and the main motor works through the transmission mechanism; the pressure block motor starts, causing the pressure block b to press down, and the upper limit switch sq2 for the pressure block closes. When the pressure block is in place and the sheet metal is clamped, the lower limit switch SQ3 of the pressure block closes; the shearing blade motor starts, controlling the shears to fall. At this time, SQ4 closes until the sheet metal is cut and falls into the trolley; when there are enough sheets on the trolley, the trolley control motor starts, driving the trolley to the right, sending the cut sheet metal to the next process; after unloading, the trolley starts to the left again, returning to the shearing machine to start the next work cycle. The length l of the sheet metal can be adjusted as needed, and the number of sheets in each trolley can be preset. 4 PLC Control System Design 4.1 Automatic Shearing Machine System Design To achieve automation, the design must be based on the working characteristics and action requirements of automatic and precise shearing of sheet metal. Therefore, this scheme uses a programmable controller to control the automatic shearing machine, and the design scheme is shown in Figure 2. [align=center][img=397,254]http://www.ca800.com/uploadfile/maga/plc2008-6/myh-2.jpg[/img] Figure 2 Simplified diagram of overall design scheme[/align] 4.2 System main circuit design (1) Design of feeding mechanism e: Use an AC motor to drive the feeding belt. The conveyor belt feeds in only one direction. It is only required that the motor rotates in one direction. The light-load, low-power motor can be started directly. Use fuses and thermal relays for short circuit and overload protection. So that the plate to be sheared is automatically, quickly and stably transported to the shearing position. (2) Design of pressing mechanism b: The function of pressing block b is to press the plate to facilitate the shearing blade to cut the plate. Pressing block b has two movements: rising and falling. It is required that the motor driving the pressing block has forward and reverse rotation. The control circuit has interlock protection, fuses and thermal relays for short circuit and overload protection. (3) Shearing blade: The shearing blade has two movements: downward cutting the sheet material and then upward resetting. The motor driving the shearing blade mechanism should also have forward and reverse rotation. Short circuit and overload protection are provided by fuses and thermal relays. According to the motor control requirements, the flowchart of the motor forward and reverse rotation program is shown in Figure 3. [align=center][img=397,635]http://www.ca800.com/uploadfile/maga/plc2008-6/myh-4.jpg[/img] Figure 3 Flowchart of the motor forward and reverse rotation program[/align] 4.3 Discussion on the design of motor forward and reverse rotation control PLC (1) Electric drive refers to the control of the motor. Relay-contactor control is used, and each has its own basic control circuit. PLC control should also have its own basic control links. Therefore, this paper proposes a discussion on the design of motor forward and reverse rotation control PLC to find a better control scheme. (2) Relay-contactor electrical control principle: The relay-contactor electrical control principle is shown in Figure 4. Comparing the two diagrams, the number of electrical components, contacts, and connecting wires are the same, namely 5, 9, and 13 respectively. The button wiring is also 6 wires, but the button and contactor wiring in the upper diagram is more concentrated, which is superior to the lower diagram from a maintenance perspective. (3) Meaning of component codes: sb0 - emergency stop button with grinding head. 5sb1, 5sb2 - forward and reverse start/stop buttons. 5km1 - forward contactor. 5km2 - reverse contactor. Among them, 5km1 and 5km2 require the use of AC contactors with mechanical interlock. (4) Working principle: forward start - press 5sb1, 5km1 is energized and operates, one of the contacts is self-locked, and the other contact is used for interlocking, that is, disconnecting the coil circuit of 5km2. Reverse start—Press 5SB2. First, 5km1 is de-energized, and its interlocking contact returns to closed before 5km2 is energized. One contact is self-locking, and the other is used for interlocking, i.e., disconnecting the coil circuit of 5km1. Forward start again—Press 5SB1. First, 5km2 is de-energized, and its interlocking contact returns to closed before 5km1 is energized. One contact is self-locking, and the other is used for interlocking, i.e., disconnecting the coil circuit of 5km2. 5 PLC Control System Design 5.1 Panasonic FP1-C24 Series PLC [align=center][img=397,377]http://www.ca800.com/uploadfile/maga/plc2008-6/myh-5.jpg[/img] Figure 4 Relay-Contactor Electrical Control Principle[/align] The design uses the FP1-C24 series PLC system manufactured by Panasonic Corporation of Japan as the main unit. Panasonic Electric Works' FP series PLCs are rising stars in the programmable controller market. They have a rich instruction system, with even small models having nearly 200 instructions. The CPU has a fast processing speed of 1.6μs/step. The program capacity is as high as 2700 steps/500 steps, while small models generally reach about 3,000 steps, with a maximum of 5,000 steps, and large models only up to 60,000 steps. The editing tools are powerful, whether it is a handheld programmer or a programming tool software, the programming monitoring capability is very strong. The powerful and unified programming tools are the best choice for designers. The network communication function is powerful. All models in this series provide communication functions. Panasonic Electric Works provides up to 6 types of PLC network products, integrating several communication methods in the same subnet, which can be selected according to needs. 5.2 I/O terminal allocation (1) Input devices: Limit switches: SQ1, SQ2, SQ3, SQ4, SQ5, SQ6, SQ7; System stop and start buttons: SB1, SB2. (2) Output device: When a sheet is cut and falls into the trolley, the photoelectric detection switch sq7 closes once, and the counter decrements by 1. In this design, it is assumed that the trolley can carry a maximum of 40 sheets. km1 controls the contactor of the feeding mechanism motor; km2 and km2ˊ control the contactors of the pressing motor, driving the motor to rotate forward and backward, and controlling the pressing and releasing of the sheet; km3 and km3ˊ control the contactors of the shearing motor, driving the motor to rotate forward and backward, and controlling the shearing blade to move up and down; km4 and km4ˊ control the contactors of the feeding trolley motor, driving the motor to rotate forward and backward, thereby controlling the left and right movement of the trolley. hl1 is the trolley no-load indicator. Other signal indicators can be added as needed. The input and output terminal assignments are shown in Figure 5. [align=center][img=397,274]http://www.ca800.com/uploadfile/maga/plc2008-6/myh-8.jpg[/img] Figure 5 I/O Terminal Assignment Diagram[/align] 5.3 Software Analysis of the System It can be seen that this system is a multi-step sequential control system. It utilizes a modular program structure to implement the system, offering advantages such as simple structure and convenient programming. The shift function of the PLC shift register can be used to achieve step sequence control, ensuring that each step operates strictly in sequence. A counter counts the number of plates per batch, and its value is set by the user as needed. The flow chart of the control system is shown in Figure 6. The ladder diagram of the system is shown in Figure 7. [align=center][img=397,706]http://www.ca800.com/uploadfile/maga/plc2008-6/myh-9.jpg[/img] Figure 6 Control System Flowchart[/align] 6 Conclusion An automatic shearing machine is an automated processing equipment that precisely controls the processing dimensions of sheet metal, automatically and cyclically shears large pieces of metal sheet, and transports them to the next process by a feeding trolley. Its entire process conforms well to the requirements of sequential control. Therefore, in the control process, a programmable logic controller (PLC) is used to control the automatic shearing machine. This effectively solves the problem of complex control systems using relay-contactor control, where numerous wirings reduce system reliability and indirectly decrease equipment efficiency. Therefore, applying a PLC to this control system offers advantages such as simple operation, reliable operation, strong anti-interference ability, simple programming, and high control precision. During the control process, the number of sheets sheared by the shearing machine can be easily modified according to process parameters, and the photoelectric proximity switch is used to accurately detect the sheet metal status.