The laser cutting technology of the six-axis series robot and fiber laser can be used to trim and cut holes .
Compared with traditional open die stamping technology shortens the process flow, greatly reduces labor cost and mold cost, improves product quality and added value, and is widely used in automobile manufacturing and aerospace industries .
This post mainly analyzes the main factors affecting the cutting accuracy of robotic laser cutting machine after offline programming and provides corresponding solutions.
Robotic Cutting Machine Programming Mode
In recent years, the concept of industry 4.0 has been deeply rooted in people's hearts, and the development of laser cutting machine robots towards intelligence has become an important trend.
If we want to develop and transform, we need to understand how the robotic laser cutting machine completes the established work, and then we need to talk about its programming method.
Generally, serial robot laser cutting machine programming methods are mainly divided into teaching programming and offline programming.
Teaching programming means that the operator's “eye (observation) + brain (judgment) + hand (registration)” will manually guide the end effector along the required track and store it in the robot controller.
After the robot reads and learns, it will repeat these trajectories.
Its advantages are low threshold, easy to learn, and can correct errors caused by mechanical structure.
However, the disadvantages are also obvious, the actual operating environment is necessary.
Cut quality depends on operator experience and is not suitable for complex trajectories.
And the machine needs to stop production, which affects productivity.
Offline programming consists of reconstructing the 3D virtual environment of the entire work scene on the computer through offline simulation software.
Then, according to the size, shape and material of the parts to be processed, the software can automatically generate the robot's trajectory with some operations from the software operator, that is, the control command.
Then, the trajectory is simulated and adjusted in the software, and the robot program is generated and transmitted to the robot.
The advantage of this method is that it has trajectory simulation, trajectory optimization and collision detection functions, and does not depend on the operator's experience.
It can be far from the actual operating environment and is suitable for complex trajectories.
In addition, there is no need to stop production, improve production efficiency, and overcome many shortcomings in programming teaching.
Comparatively speaking, offline programming is more suitable for the intelligent development direction of robotic laser cutting machine.
However, many users use offline programming and generally reflect that the cutting accuracy is low.
Next, we will analyze what factors affect the cutting accuracy after robot laser cutting machine using offline programming.
Absolute positioning accuracy error of six-axis serial robot
Offline programming needs to import the simulation model into computer software.
The model represents the perfect design and there are no geometric and kinematic errors in the simulation.
However, there are tolerances in the actual processing and assembly of robot parts, and there are also deviations between the encoder feedback value and the actual movement value in the joint axis movement process, which leads to errors between the actual position of the end effector and the theoretical position of the target when the robot executes the instructions given by offline programming.
As shown in Figure 1, cutting a straight line from a to B, the offline programming software measures the theoretical length L between AB and sends the command to the robot controller, but the geometric size of the robot itself and the motion error just walk L1 in reality execution, and there is a difference △L with the theoretical length, which is the absolute positioning accuracy error of the robot.
Fig. 1 absolute positioning difference △L
The six-axis series robot generally has the characteristics of high repositioning accuracy (about 0.05mm) and extremely low absolute positioning accuracy (about 3mm difference per meter), which leads to worse dimension accuracy geometric when the robot adopts offline programming.
Solution:
(1) modify the part model
For some parts with simple contour and low precision, cut a cuboid as shown in Figure 2.
First, the workpiece coordinate system is parallel to the robot coordinate system, then a solid sample is cut or marked according to the instructions generated by the normal model of the workpiece, and the difference between the cutting dimension and the target is measured along the part coordinate. system.
Then, through 3D modeling software or offline simulation software, the part model is enlarged or reduced along the part coordinate system according to the difference, and the cutting program is regenerated to improve the dimensional accuracy.
Fig. 2 the model built by cutting a cuboid
(2) The laser tracker compensates for the absolute positioning accuracy of the robot's local area.
For parts with complex contour and high precision requirements, the laser tracker can be used to compensate for the accuracy of the robot's end effector in the cutting area, as shown in Figure 3.
Fig. 3, the laser tracker compensates for the absolute positioning accuracy of the robot's local area
Using offline programming to generate different positions of spherical points from a known spherical model. The robot reads and executes.
The laser tracker captures and maps these points and finally draws the actual spherical model by computer software.
After comparison with the known spherical model through offline programming, the absolute positioning accuracy of the robot in this area is calibrated and compensated.
TCP tool midpoint error
The robot end actuator, also known as the tool, is installed on the robot end flange.
When we let the robot approach a certain point in space manually or programmatically, its essence is to let the center point of the tool approach the point.
Therefore, it can be said that the movement of the robot trajectory is the movement of the tool center point (TCP).
When the robot leaves the factory, it only knows the coordinates of the center point of the end flange, and the coordinates of the tool center point (TCP) need to be input into the robot controller after teaching.
At present, the center point of the robotic tool is generally calculated by the four-point ball drawing method, as shown in Figure 4.
Fig. 4 Central point of the TCP calculation tool
That is, taking the center point of the tool as the center of the ball, drawing the sphere by collecting the center points of four different positions of the robot flange, and then calculating the coordinates of the center of the ball, which is the center point of the tool.
Since the position of the ball center is manually calibrated by observing the relative position of the TCP and the teaching accessory, there is an error between the tool center point calculated by this method and the actual tool center point, as shown in Figure 5, which it is usually about 0.5mm.
Fig. 5 There is an error between the calculated tool center point and the actual tool center point
In offline programming software, there is no error in the tool center.
In the actual cutting of parts, as long as the laser cutting head rotates around the center of the tool, it will produce large size and position deviations.
Use the infrared tool gauge to calibrate the center point of the tool
As shown in Figure 6, there are two orthogonal infrared transmitters and corresponding inductive switches on the circle of the tool gauge.
Figure 6 TCP tool center point correction with infrared tool gauge
When performing a specific calibration procedure, the robot makes the tool move on the calibration circle.
When the robot tool blocks the infrared ray, the corresponding induction switch sends a signal to the robot controller, and finally the controller calculates and evaluates the position of the center point of the tool.
After calibration, the TCP accuracy of the tool center point can reach 0.1mm.
Final thoughts
In addition to the above two main factors, the accuracy of the robotic laser cutting machine based on offline programming is also related to the establishment of the workpiece coordinate system, the attitude and load of the robot, and the adjustment degree of the physical model of the workpiece .
However, the cutting accuracy errors caused by these factors are acceptable to most users, and can also be compensated by personnel's operating experience to reduce these errors.
In short, in the near future, with the development and deep integration of vision technology, sensing technology, intelligent control, network and information technology, big data and other technologies, laser cutting machine offline programming technology robotics will be further developed to intelligence, such as automatic perception, identification and reconstruction of the part and processing path, realizing independent path planning, automatic correction and adaptive environment.
With less and less human intervention, the user's operation will be simpler and the cutting precision will be higher and higher.
