In recent years, the application cases of industrial robots in China have rapidly increased, mainly focusing on welding, painting, and material handling, with limited application in the field of bending. However, workpiece bending is a widely used and somewhat hazardous task, making the market prospects for robot bending very promising, as there have been many successful experiences abroad. Currently, 40% to 50% of bending machines in sheet metal processing workshops in Europe and the United States are equipped with robot automatic bending systems, while China is just beginning to automate bending. In the next 10 years, the demand for bending robots in China is expected to increase significantly.
A CNC sheet metal bending flexible processing unit with a robot as the core executing component is a highly automated equipment combination that offers high efficiency, high quality, and high flexibility. In a bending flexible processing unit, selecting the appropriate component combination can provide better support for processing efficiency and flexibility. Bending precision depends on the bending machine's own precision, the robot's positioning precision, and the coordinated control between the robot and the bending machine. The difficulty of coordinated control lies in matching the speed of the robot and the bending machine, as well as the running trajectory of the robot supporting the workpiece. Poor tracking performance will seriously affect the bending angle forming effect and surface flatness, thereby affecting the quality of the finished product.
The composition of a bending processing unit typically includes a robot and a bending machine as the core, with auxiliary components such as grippers, loading and unloading stations, positioning worktables, flipping frames, transfer devices, and various detection sensors.
The gripper is the "hand" of the robot that replaces manual handling of the workpiece. The gripper of a bending robot usually consists of multiple suction cups mounted on a metal frame.
The loading and unloading stations usually use stacked pallets or conveyors and roller conveyors to transport raw materials and transfer finished products. Oily sheets are prone to adhesion, resulting in picking up multiple sheets at once. Various separation devices (such as magnetic separators) and detection sensors can be installed next to the loading station to ensure that the sheets are picked up individually.
The positioning worktable is a tilted platform with a stop edge, and the surface is equipped with small raised balls. The robot transfers the steel plate to the positioning table, and the plate freely slides down to the stop edge under gravity. Since the position of the positioning table and the stop edge are fixed, when the robot re-grabs the plate, the position of the plate relative to the gripper is relatively fixed, providing a reference for the next bending step.
The flipping frame is a fixed frame for holding the gripping device. When the robot needs to change its position to pick up the workpiece, it can place the workpiece on the flipping frame to secure it, and then the robot can re-grab the workpiece in the new position. In some special cases, the bending machine mold can also hold the workpiece, and the gripping position can be changed.
Workflow of the Bending Processing Unit:
The work of the bending processing unit mainly consists of six processes: loading, retrieval, centering, flipping, bending, and stacking.
Loading: Manually stack the plates to be processed on the loading station. The loading station is equipped with plate detection switches to prevent the robot from grabbing the tray after all the plates have been processed.
Retrieval: The robot moves to the position of the loading station and uses the ultrasonic sensor installed on the gripper to detect the height of the plates. Based on the detection data, it automatically moves to the appropriate position for plate retrieval. After grabbing the plates, the thickness of the plates is measured using a thickness measuring device to avoid picking up multiple sheets at once, which can cause processing failures. After thickness measurement, the system prepares for centering.
Alignment: The robot moves to the position of the positioning table and places the sheet on the positioning table for precise alignment. After alignment, the robot grabs the sheet again, preparing for bending.
Flipping: Based on process requirements, it is determined whether the flipping frame is needed. If so, the robot moves to the position of the flipping frame and places the sheet on it. The robot releases the sheet and moves aside. After the flipping is completed, the robot retrieves the sheet.
Bending: The robot moves to the position of the bending machine and lays the sheet flat on the lower die of the bending machine. Precise positioning is achieved using a backstop sensor on the bending machine. After positioning, the robot sends a bending signal to the bending machine and collaborates with it to complete the bending action. It determines whether further bending is required to decide whether to perform continuous bending. Bending is a critical step, and the technical challenge lies in the coordination between the robot and the bending machine, known as bending follow-up. When the robot grips or supports the sheet for bending, the sheet may deform, and the robot needs to follow the sheet's specific trajectory algorithm to perform arc movements while maintaining a relatively fixed position relative to the sheet.
Stacking: The robot moves to the position of the unloading station. Depending on the shape of the workpiece, there are various stacking processes, such as conventional matrix stacking, single or double-layer crossover stacking, and positive or negative interlocking stacking.
Technical Key Points:
Currently, whether it is a general-purpose six-axis robot or a bending-specialized robot with optimized arm length or shape for bending processes, they all require bending follow-up algorithms. The situation where bending is not followed is rare. Without good follow-up performance, the fixture or suction cup gripper may pull and wrinkle the workpiece due to poor follow-up trajectory, affecting the forming quality. Establishing an accurate robot bending follow-up motion model helps to develop good follow-up trajectory algorithms and achieve excellent follow-up performance.
The parameters are represented as follows: 1) Upper die arc radius: R/mm. 2) Lower die arc radius: r/mm. 3) Lower die opening: V/mm. 4) Lower die angle: ∠b/degrees. 5) Workpiece thickness: T/mm. 6) Thickness from the neutral layer to the upper surface of the workpiece: λ/mm. 7) Bending angle of the workpiece: ∠a/degrees. 8) Bending machine downward movement from the clamping point: S/mm.
Based on the motion model, the relationship between the bending angle and the downward movement of the bending machine can be calculated as follows:
S = {r × tan[(45°-∠b/4) + V/2] × sin(90°-∠a/2) - (r+R+T)}/cos(90°-∠a/2) + (r+R+T)
Based on different mechanical parameters and the formula relating bending angle to bending downward movement, the trajectory curve of the displacement change in the X and Z directions from 180° to 10° can be obtained.
Conclusion:
As the sheet metal manufacturing industry continues to develop, robot bending has increasingly broad application prospects. Developing bending follow-up model algorithms for general-purpose six-axis robots, instead of developing specialized bending robots, reduces development costs. Combined with robots from reputable brands in the industry and other auxiliary hardware, the application of robot bending can be rapidly promoted.
