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Application of robot in bending machine

In recent years, the application cases of industrial robots in China have increased rapidly, mainly in welding, spraying, handling and other fields, and there are not many applications in bending. Workpiece bending is a kind of work which is widely used and has certain danger, so the market prospect of robot bending is very optimistic, and there are many successful experiences abroad. At present, 40% - 50% of the bending machines in sheet metal processing workshops in Europe and America are equipped with robot automatic bending system, while the bending automation in China is just in its infancy. In the next 10 years, the domestic demand of bending robot will rise in a straight line.

The CNC plate bending flexible machining unit with robot as the core executive part is a set of highly automatic equipment combination, which has the advantages of high efficiency, high quality and high flexibility. In the bending flexible machining unit, choosing the appropriate component combination can provide better support for improving the machining efficiency and flexibility. The bending accuracy depends on the accuracy of the bending machine itself, the positioning accuracy of the robot and the cooperative control between the robot and the bending machine. The difficulty of the cooperative control lies in the speed matching between the robot and the bending machine, as well as the running track of the robot supporting the workpiece. The poor follow-up effect will seriously affect the forming effect of the bending angle and the plate.

Composition of bending unit

The quasi bending processing unit (Figure 1) takes the robot and bending machine as the core, and the gripper, loading platform, blanking platform, positioning workbench, turning frame, hand changing device and various detection sensors as auxiliary components

The grab is a robot's "hand" instead of manual work. The gripper of bending robot is generally composed of several suction cups installed on a metal frame. Loading platform and unloading platform usually adopt palletizing pallet, and conveyor belt or roller table are also used to transport raw materials and finished products. Oily plates are easy to be adhered, which leads to picking up multiple plates at a time. A separation device (such as magnetic separator) and a detection sensor can be installed beside the feeding table to ensure that the plates to be grabbed are single sheets. The positioning table is an inclined platform with a baffle, on which there are micro raised balls. The robot transfers the steel plate to the positioning platform, and the plate slides freely to the retaining edge under the gravity. Because the position and the edge of the positioning table are fixed, when the robot grabs the plate again, the position of the plate and the grab is relatively accurate and fixed, which provides a reference for the next bending. The tripod is a fixed frame of the grabbing device. When the robot needs to take the workpiece at a different position, it can put the workpiece on the turnover frame to fix it, and then the robot can grasp the workpiece at a new position again. In some special occasions, it is also possible to use the bending machine mould to clamp the workpiece and change the grab position.

Work flow of bending unit

The work of bending processing unit is mainly divided into six processes: feeding, reclaiming, centering, turning, bending and stacking, as shown in Figure 2.

1. Loading. Manually place the whole stack of plates to be processed on the loading platform, and install the plate detection switch on the loading platform to prevent the robot from grabbing the tray after all the plates are processed.

2. Take out materials. The robot runs to the position of the loading platform, and detects the height of the plate through the ultrasonic sensor installed on the grab. According to the detection data, it automatically runs to the appropriate position to grab the plate. After grabbing the plate, it measures the thickness of the plate through the thickness measuring device, so as to avoid grabbing multiple plates at one time, resulting in processing failure. After passing the thickness measurement, it is ready for centering.

3, alignment. The robot moves to the position of the positioning table, places the plate on the positioning table for precise positioning (Figure 3), grabs the plate again after the positioning is completed, and prepares to bend.
 
4.Turn over. According to the process requirements, judge whether it is necessary to use the turnover frame. If necessary, run the robot to the position of the turnover frame, place the sheet on the turnover frame, release the sheet, and run to the other side of the sheet to grab the sheet.
 
5.bending. The robot runs to the position of the bending machine, flattens the sheet metal to the lower die of the bending machine, and precisely positions it through the back finger sensor of the bending machine. After the positioning is completed, the robot sends the bending signal to the bending machine, and cooperates with the bending machine to complete the bending action, and judges whether it needs to bend again to decide whether to carry out continuous bending, as shown in Figure 4. Bending is the key link, and the technical difficulty of bending lies in the cooperative action of robot and bending machine, that is, bending following. When the robot pinches or supports the plate bending, the plate deforms. The robot needs to follow the plate to make arc motion according to the specific trajectory algorithm, and keep a relatively fixed position with the plate all the time.
6.Palletizing When the robot moves to the position of the blanking table, due to the difference of workpiece forming, there are many kinds of palletizing process actions, such as conventional matrix palletizing, single and double layer cross palletizing, positive and negative buckle palletizing, etc., as shown in Figure 5.

Echnical Points

 
At present, in the market, whether it is a general standard six axis robot, or a special bending robot whose arm span or body is optimized for bending process, it needs the support of bending following algorithm, and there are few cases that do not follow bending. If there is no good follow effect, the gripper or suction cup grab will pull the workpiece due to poor follow track, forming sheet wrinkles and affecting the forming quality. It is helpful to establish a good tracking algorithm and obtain excellent tracking effect by establishing an accurate bending following motion model of robot. Fig. 6 is a schematic diagram of bending process, from which a mathematical model of bending following is obtained, as shown in Fig. 7.
The parameters in Figure 7 are as follows:
1) r arc radius of upper die: R, unit: mm;
2) r arc radius of lower die: R, unit: mm;
3) lower die opening: V, unit: mm;
4) angle of lower die: ∠ B, unit: °;
5) workpiece thickness: T, unit: mm;
6) thickness from neutral layer to upper surface of workpiece: λ, unit: mm;
7) bending angle of workpiece: ∠ a, unit: °;
8) downward movement of the bending machine slider from the clamping point: s, unit: mm. According to the mathematical model, calculate the relationship between the bending angle and the bending downward amount:  according to the mechanical parameters in Table 1, the comprehensive formula of the relationship between the bending angle and the bending downward amount can get the track curve of the displacement change of the bending angle from 180 ° to 10 °, in X direction and Z direction, as shown in Figure 8.
 

Expectation

With the continuous development of sheet metal manufacturing industry, robot bending has a more and more broad application prospect. Compared with the development of special bending robot, the development of robot bending following model algorithm suitable for general six axis robot, and applied to general robot, the development cost is lower. With most excellent brands of robots and other auxiliary hardware in the industry, it can quickly promote the application of robot bending.
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