introduction
Shearing machines are indispensable equipment in continuous steel plate production lines. Their purpose is to cut steel plates to specific dimensions, trim edges, cut samples, and remove localized defects. Currently, the functional requirements for shearing machines are constantly expanding, while higher demands are being placed on their production efficiency and processing precision. By applying PLC control technology to shearing machines, the electrical performance of the equipment has been greatly improved, the level of automation has been increased, continuous production has been achieved, production efficiency has been significantly improved, and the labor intensity of workers has been reduced.
1. Structural Design of Automatic Shearing Machine
Automated shearing machines should be able to automatically adjust the shearing stroke and blade gap according to the material, thickness and shearing length of the sheet metal being sheared. They can be equipped with a front feeding system or a rear material support device, integrating feeding and unloading into one unit, effectively improving the automation level of the equipment, and can perform single-step execution or continuous cycle operation as needed.
Figure 1 Schematic diagram of automatic shearing machine system
Accordingly, the designed automatic shearing machine consists of five parts: a material handling module, a leveling module, a length-fixing module, and a shearing module. These modules work together to achieve automatic shearing and meet accuracy requirements. Rollers are used in the material handling, leveling, and length-fixing modules, with an electric motor as the power source. For the shearing module, due to significant vibration and its linear reciprocating motion, a pneumatic transmission method is considered. During operation, attention must be paid to the synchronization of the electric motor and roller drive system. Figure 1 shows a schematic diagram of the automatic shearing machine system.
The material handling module consists of an uncoiling module and a clamping module. In the uncoiling module, the raw material clamping section of the feeding mechanism can move left and right to adjust the maximum width of the output material. The clamping module primarily conveys the steel plate to be sheared forward. This module has two rotating shafts: the upper one is the driven shaft, and the lower one is the driving shaft. The driving shaft is directly driven by a motor, while the driven shaft can move up and down to accommodate clamping and conveying steel plates of different thicknesses. Because the object being sheared is steel plate, supports must be installed at the steel plate output section of the clamping mechanism.
The leveling device uses upper and lower pressure rollers to press the product to be processed, bringing it to the desired level. The positions of the pressure roller shafts in this module must be rationally arranged, and the upper and lower shafts must rotate in opposite directions to achieve the desired transmission effect. A gear transmission system and a parallel roller row mechanism with a single adjustable upper roller are used, and pressure gauges installed on the leveling device control the specific leveling requirements.
The fixed-length module mainly consists of a bracket, upper and lower rollers, support rods, and a slider. Automatic shearing machines are required to shear steel plates of different widths and thicknesses; therefore, the distance between the upper and lower rollers should be adjustable. For the rollers of the fixed-length module, to ensure high transmission accuracy, the upper and lower rollers should rotate synchronously. Therefore, a synchronous toothed belt is selected for the transmission, offering advantages such as high transmission accuracy, high transmission efficiency, and balanced operation, further improving the equipment's performance.
The automatic shearing machine uses a parallel blade shearing structure for its shearing module. Due to the significant vibration during operation and the linear reciprocating motion, a fluid drive system is considered for the shearing module. Pneumatic drive is generally preferred due to its cleanliness, safety, ease of implementation, and lower cost compared to hydraulic systems. Based on the significant vibration during operation and calculations of the shearing force workload, a pneumatic drive system was chosen for steel plate shearing, proving to be effective in practice.
2. Design of Automatic Shearing Machine Control System
2.1 Control System Hardware Design
Based on the working requirements of the shearing machine, its control system uses a PLC as the main controller. The motors in the material handling module, leveling module, and length fixing module are speed-regulated by frequency converters to meet the speed requirements during processing. To ensure the synchronous operation of the three motors, speed regulation by frequency converters alone is insufficient, as it can lead to error accumulation. Excessive error accumulation can cause material buildup, damaging the steel plate, or cause deformation due to excessive tensile stress. Therefore, a synchronization controller is used in the control system to achieve synchronous control of the motors. To meet the high-precision requirements of the shearing machine for the shearing length, a rotary encoder is used on the roller of the length fixing module to count the linear speed and rotational length of the roller, achieving precise control of the steel plate length. Figure 2 shows the block diagram of the automatic shearing machine control system.
Figure 2. Block diagram of the automatic shearing machine control system
2.1.1 PLC Selection and Application
The characteristics of the process flow and application requirements are the main basis for selection. The control system requires 13 input ports and 8 output ports. Six of the outputs are dedicated to three frequency converters to control the forward and reverse rotation of the motors. To ensure that the COM terminals of the outputs do not cross-connect, a 40-point I/O configuration is chosen (originally, a 24-point PLC would have been sufficient based on the I/O point count). The PLC outputs must drive not only the frequency converters but also the solenoid valves, handling both AC and DC loads; therefore, relay outputs are required. After comprehensive consideration, a Mitsubishi PLC, model FX1N-40MR, is selected for control. Tables 1 and 2 show the PLC input/output allocation in the control system.
Table 1 PLC Input Allocation Table
Table 2 PLC Output Allocation Table
2.1.2 Inverter Selection and Application
Since the PLC is FX1N-40MR, a Mitsubishi FR-E500 inverter was also selected for better performance. The FR-E500 inverter is a small, high-performance, general-purpose inverter that uses flux vector control to achieve 150% torque output at 1Hz operation. It has a built-in RS-485 communication interface and flexible PWM for low-noise operation.
There are two ways to set the operating frequency of a frequency converter. The first is to fix the frequency of the converter ports, which means using the converter to output different frequencies by selecting different ports. The advantage of this method is simple wiring and no need for complex software, but the disadvantage is that the settings cannot be changed arbitrarily, resulting in poor flexibility. The second method is to use the communication module of the FR-E500 that supports the USS protocol (Universal Serial Interface Protocol). Using the USS protocol, the output frequency of the frequency converter can be set in the host computer via software, and the settings can be updated online in real time to change the output frequency. The corresponding disadvantage is that it makes the PLC program very complex, and the speed of setting the frequency via the USS protocol is slower than the first method (the first method takes less than 10ms, while the second method takes 20-30ms). Considering that the PLC output points and the number of connections in the system designed in this paper are relatively small, from the perspective of system stability and real-time performance, the first method is more reasonable.
According to the technical specifications of the frequency converter (FR-E500), taking the fixed-length module as an example, its port allocation is shown in Table 3.
Table 3. Port Allocation Table for Frequency Converters Used in Fixed-Length Modules
2.1.3 Selection and Application of Synchronous Controllers
There are relatively few manufacturers supplying synchronous controllers on the market, among which Delta Electronics (Taiwan) offers the best price-performance ratio. Therefore, Delta's SCD-B series synchronous controller, model SCD0882lA, was selected for this equipment. Each synchronous controller can control the speed chain of 8 units. Each speed chain consists of a feedback signal and a unit output. The feedback signal should be input to the synchronous controller in analog form, based on the sensor's measurement of the steel plate tension. The unit output port is connected to the frequency setter port of the frequency converter.
When the synchronous controller is working, it uses the speed of one module motor as a reference (master motor) and matches the speeds of the other motors to it (slave motors). In this shearing machine, the motor of the fixed-length module is used as the reference, so the feedback signal should be connected to Unit 2 and Unit 3 respectively at the feedback signal input terminal. The frequency setting ports of the three frequency converters should all be connected to the unit output ports of the synchronous controller to achieve the purpose of synchronous control through the internal calculation of the controller. According to the technical specifications of the synchronous controller (SCD08821A), its port allocation is designed as shown in Table 4.
Table 4 Synchronization Controller Port Allocation Table
2.1.4 Wiring diagram of automatic shearing machine control system
Figure 3. Wiring diagram of PLC control system
Figure 3 shows the wiring diagram of the PLC control circuit for the automatic shearing machine. Since the PLC can generate 24V DC power internally, it can directly power the external encoder and displacement sensors. The encoder is an OMRON product, model E6A2-CWZ5B1000P/ R0.5M . The encoder's outputs A and B phases are connected to the X0 and X1 terminals of the PLC (the PLC's internal high-speed counter terminals), respectively, and are counted through an internal 2-phase input (AB phase) counter. The input terminals of the synchronization controller are two non-contact displacement sensors connected to Unit 2 (leveling module) and Unit 3 (feeding module), which are GEFRAN IK1A model products (analog output). These are signals input by the synchronization controller to detect the tightness of the steel plates.
2.2 Control System Software Design
For typical shearing processes, corresponding PLC control programs were developed, which have functions such as manual operation and automatic operation.
2.2.1 Initialize the ladder diagram
Program initialization refers to setting the initialization parameters of the control program. The initialization program is executed once at the beginning, and its results are stored in the component image register. The states of these components mostly remain unchanged during program execution. Figure 4 shows the ladder diagram program for the initialization of the control system. M8044 serves as the origin condition, and the initial state detects the origin condition as the start condition for automatic operation. M8000 is used for operation monitoring; M8000 is turned on when the PLC is running. After the RUN monitors the M8000 drive state initialization instruction IST is executed, the following components are automatically switched and controlled: S0, the initial state for manual operation; S1, the initial state for origin reset; S2, the initial state for automatic operation.
Figure 4 Initialization Ladder Diagram
2.2.2 Manual Operation of SFC Diagram
Figure 5 shows the manual operation SFC diagram. The most significant feature of the state transition diagram is that after transitioning from one state to the next, the previous state is automatically reset. By selecting the two-position rotary switch to the manual operation function position (X20), the shearing machine can be manually operated, enabling manual operations such as jogging the motor (forward and reverse rotation) and jogging the gate (jogging up and jogging down).
Figure 5. Manual operation of SFC diagram
2.2.3 Automatically run SFC diagram
Figure 6. Automatic SFC operation diagram
Figure 6 shows the SFC (Self-Controlled Flow Control) diagram for automatic operation. The automatic operation program can execute cyclic and single-cycle commands to achieve different control requirements. Automatic mode is further divided into cyclic and single-cycle modes. Cyclic mode is the most commonly used mode in production; after pressing the automatic start button, the shearing machine should be able to achieve continuous automatic feeding and shearing. Single-cycle mode involves the shearing machine automatically feeding and shearing, but only one cycle is performed at a time. This mode is less efficient than the automatic cyclic mode and is mainly used for debugging, allowing the operator to easily adjust to achieve satisfactory shearing results.
