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Discussion on the trajectory and beat improvement of stamping robot

    This article mainly introduces the influence of the robot’s placement position, the rotation angle and running speed of each axis of the robot, and the robot auxiliary components on the stamping production cycle; and analyzes the robot’s running trajectory, waiting position, parameter setting, etc., and optimizes the robot placement The position and its trajectory improve the production beat of the press line.
    With the rapid development of industries such as automobiles, aviation and home appliances, my country’s sheet metal processing equipment has made great progress. According to incomplete statistics, more than 40% of car parts are sheet metal stamping parts. It can be seen that stamping equipment has an important position in the automotive industry, and sheet metal transportation is at the center of stamping production.
    The traditional stamping production method uses manual picking and unloading, which has high labor intensity, low production efficiency, and poor precision, which poses safety risks. The quality of stamping parts is greatly affected by factors such as the skill and quality of the operator, resulting in a reduction in production quality and efficiency. Continuous, efficient, and non-intermittent are the characteristics of stamping automation production. As a flexible, efficient, reliable and universal processing unit, stamping automation system ensures the stability and safety of stamping production. Therefore, traditional conveying methods such as manual feeding are gradually replaced by automatic feeding mechanisms.
    Figure 1 Layout of stamping production line
    At present, the domestic six-axis robot technology is becoming more and more perfect, the cost is getting lower and lower, the use is becoming more and more popular, and the high degree of flexibility is increasingly showing its superiority. In the stamping automation system, as the main part of the automatic conveying system, the robot mainly completes the work of sheet material destacking, automatic loading and unloading between the presses, and sheet material transfer and flipping to replace heavy and dangerous manual operations. The robot realizes follow-up and interlocking with the press through the control system to complete the robot’s motion control, pneumatic and vacuum system monitoring, and safety protection. In order to improve the production cycle and improve the production efficiency of the entire line, based on the premise of the balance between each process, the robot’s running trajectory, waiting position, parameter setting, etc. are analyzed, and the robot placement position and its running trajectory are optimized. The layout of the stamping production line is shown in Figure 1. To improve the tempo of the robot, the most basic thing is to increase the running speed of the robot. In addition to excluding the speed factor, the following three factors are mainly considered.
    Where to place the robot
    In the stamping production process, the main role of the robot is to transport the stamping parts from the previous process to the next process, and to transport the stamping parts from position A to position B. As shown in Figure 2, the stamping parts are transported from position A to position B. In theory, the robot can be placed in many positions, as long as the robot arm can reach point A and point B. In fact, the placement of the robot is very limited. The three main constraints are as follows: (1) The weight and size of the stamping parts; the arm span of the robot is inversely proportional to the load. The larger the arm span, the smaller the load. When the load is met, the arm span is bound to be restricted. At the same time, if the shape of the stamped part is large, the arm length of the robot must also be increased. ⑵Cost restriction, under the condition of ensuring the load capacity, the longer the arm span; the higher the price of the robot, mainly because the power of the motor and reducer used by the robot increases, and the structural strength increases. ⑶The space planning of the press line is different, and the position where the robot can be placed will also be restricted.
    In the robot position in Figure 3, ∠a is greater than ∠b. It shows that when the stamping part is transported from A to B, the smaller ∠b is, the smaller the angle of robot rotation and the less time it takes, that is, the shorter the time for the stamping part to move laterally, that is, the shorter the tact time. It can be seen from the above examples: within the arm length range of the robot, appropriately adjusting the position of the robot can improve the production cycle.
    Figure 3 Comparison of robot positions
    How the robot moves
    The fewer the number of axes of the robot, the simpler the action, and the more the number of axes, the more complex the action. Take the 6-axis robot shown in Figure 4 as an example (Note: Axis 1, 2 and 3 are the main axis; Axis 4, 5 and 6 are the wrist axis). It is still assumed that point A and point B remain unchanged, and the position of the simulated stamping part is simulated. Picking time and placing time are not counted, and the impact of robot load is not considered. In theory, there are countless kinds of trajectories for robots to meet the requirements. But there must be one that is relatively more reasonable and has the greatest beat.
    Figure 4 6-axis robot structure
    In order to maximize the performance of the robot, it is necessary to confirm the maximum operating speed and acceleration/deceleration of each axis of the robot (it must be within the range designed and verified by the robot design company and related to the load). Take axis 1 as an example. Assuming that axis 1 is to reach the target, the angle of rotation is α, α=a1×t1+v×t2+a2×t3. a1 is the maximum angular acceleration, fixed value; a2 is the maximum angular deceleration, fixed value, under normal circumstances a1=a2; v is the maximum angular velocity, fixed value; t1 is the acceleration time from 0 to the maximum angular velocity, fixed value; t2 is the maximum Angular speed operation time; t3 is the time to decelerate from the maximum angular speed to 0, which is also a fixed value. Under normal circumstances, t1=t3. Then it can be known that the shorter the time of t2, the faster the beat of the robot. The same applies to other axes.
    In order for the robot to reach the target position quickly, it is necessary to preliminarily determine the running trajectory of the robot. After the trajectory is determined, calculate the running angle of each axis, obtain the required running time of each axis, and conduct overall analysis. ⑴ In the “Minimum Running Time” column, select the maximum value, assuming it is the axis 1 time t1. ⑵Consider axis 2~6 “shortest running time”, consider whether it can run simultaneously with axis 1. Assuming that axis 2 can run simultaneously with axis 1, the total tact time is T=t1+t3+t4+t5+t6. ⑶ Re-select the running track and perform the same operation selection. Note: The shortest running time of each axis is different for different trajectories. What’s more important is that axis 2 may not be able to run at the same time as axis 1. Maybe other axes can run at the same time. ⑷Assuming that the “minimum running time t1” of axis 1 is the largest, try to consider how to change the trajectory of the robot, or even change the position of the robot, or the model specification of the robot, and reduce t1. ⑸ After t1 becomes smaller, the “minimum running time” of other axes may become larger, as long as the total tact time T becomes smaller. ⑹ If the axis 2 running time t2 is shorter than the axis 1 running time t1, and it can run simultaneously with axis 1, then the maximum running speed and acceleration/deceleration of axis 2 should be reduced, as long as the rotation angle is completed within t1.
    The trajectory can be simulated multiple times, and after all the data statistics are completed, compare and determine a compromise plan (the robot sometimes arcs or avoids, but the way is the same). The shortest time is the fastest beat.
    Robot accessories and tooling
    The robot’s accessories and tooling also affect the production cycle of the press line. The end of the robot is equipped with an end picker during stamping production. Because the plate is a thin-walled piece, it is suitable for grasping by the working principle of vacuum adsorption. The end picker is equipped with a vacuum suction cup. The suction cup is arranged on the end picker bracket made of high-strength alloy and carbon fiber material. The number and arrangement of the suction cups The method depends on the specific panel, and the efficiency of the suction cup is affected by the efficiency and power of the vacuum generator. And then affect the robot’s beat.
    In addition, when the end picker picks up the board, the board must be positioned first. If the positioning system is slower than the robot, it will affect the robot’s beat. The suction cup system is equipped with a vacuum detection sensor, and the sensitivity and accuracy of the vacuum generator will also affect the beat of the robot.
    There is also an abnormal condition that will also affect the robot’s beat. Under normal circumstances, we transport objects from point A to point B, which is one-to-one transportation, that is, an object is positioned at point A and then the robot is transported to point B; if the object is lost in the process from point A to point B, Then the robot stops and alarms, and manual intervention is required. The latest approach is: if the object is lost and does not affect the continued operation of the equipment, the robot immediately generates a new trajectory, and returns to point A to grab another object and transport it to point B. In this way, the loss of downtime can be reduced. A more advanced point is the temporary storage mechanism at point A. Regardless of whether the equipment before point A is faulty or not, the conveying work from point A to point B will not stop, so that the average beat (average beat is the working time divided by For the output, such as the weekly average beat, a total of 10,000 panels are conveyed in a week, the weekly average beat is: 8h×5d/10000 pieces = 14.4 seconds/piece).
    Concluding remarks
    In recent years, in order to overcome the shaking of the plate during the six-axis robot handling process and further improve production efficiency, the robot handling system has developed a rotating seven-axis and end picker automatic replacement technology. The rotating seven-axis technology is to install a servo-controlled rotating arm on the sixth axis of the robot to realize the translation of the workpiece in the process of conveying between the upper and lower station presses, avoiding the jitter and falling off of the previous workpiece due to 180° rotation, which is convenient Speed ​​up the robot handling process.
    The seven-axis robot is developed on the basis of the six-axis robot. The linear seven-axis is added to the end of the robot arm, which has a faster conveying speed than the six-axis robot, and can realize the same direction conveyance of parts between various processes. Like a six-axis robot, it rotates 180° back and forth in two sequences. This kind of robot does not have high requirements on the mold, but it has a little more than the six-axis robot’s requirements on the mold, that is, the clear height of the mold opening is higher to ensure that the seventh axis can be used. Enter between the upper and lower molds. This kind of automation is about 1.6 times higher than the cost of six-axis robot automation.