Different from common industrial robot automation applications, polishing and grinding automation has always been full of challenges. Based on years of understanding of the breakthrough of robotics technology and the understanding of process scenarios, flexiv provides a new idea for polishing and grinding automationWorkshop on robot grinding and polishingon topic"Advantages of using a torque-controlled robot for grinding".An in-depth sharing was conducted.
Launched on the eve of the holidayAdaptive robots are one of the representatives of force-controlled robotsTorque sensors are fitted to each joint, and additional six-dimensional force torque sensors are added to the ends of some models for better sensing resolution and control performance.
Adaptive robots have a total of seven joints, and this extra degree of freedom allows the robot to easily adapt to confined workspaces compared to six-degree-of-freedom (six-axis) robots, making it useful in specific application scenarios.
Direct Force Control
First, let's talk about direct force control.
When the traditional industrial robot is carrying out force control, the force command will be converted into speed command, and the speed command will be converted into joint torque and current command. This consists of two steps, in effect indirect force control.
Using this method, the robot will be stiff at the force level, which is not conducive to grinding and polishing applications. This is one of the reasons why most people choose to add a submissive device to the end of the robot so that it can do force control for the robot.
And for moment-controlled robots,Force commands can be directly converted into joint torque commandsThere is no need for any intermediate steps. That's why we call it direct force control. With direct force control, the robot will get very fastForce Response,And give a sense of the intuitive feeling that the robot operates very gently, and thisIt does not sacrifice the speed and stiffness of the robot at the level of motion.
In the plane grinding test, the adaptive robot operates at a speed of 1000mm s while maintaining a constant force on the plane. Not only that, but adaptive robots perform just as well on curved surfaces, and they work very similarly. Based on Feixi's high-precision force sensor, the weight and inertia of the tool at the end of the robot can be automatically measured and factored into the force control, so no matter which direction we want to polish, we can have a consistent effect.
So how precise is this force control?In the mobile phone glue line polishing application test, the robot robot performs rapid polishing with gentle force on the side of the mobile phone, running at a speed of 1000mm s and an acceleration of 8000mm s, while maintaining a small force.
Mobile phone glue line polishing test.
This diagram shows the actual force measurement: the blue line is the target value, which is 10n;The green line is the actual measurement. Of course, this is a zoom graph of the measurements, so it looks like it's going up and down. But if you look closely at the data, you will find that the average force error is only 02n, the maximum force error is less than 07n - This data is enough to reflect the accuracy of the force control of the adaptive robot.
Since the non-solar robot can achieve precise force control at high speed without the help of a compliance device, and the entire working space of the robot can be used as a floating range. Combining all of these advantages, we open up even more possibilities for design grinding and polishing applications.
When there is no need to comply with the device,The design of the end tool can have more design space. This makes the total load much lighter, allowing for the use of smaller robots, and it is much easier to mount or even flip smaller robots at different angles, such as flipping up 4 adaptive robots to clean and polish a roof rack.
The Adaptive Robot Dawn series has 7 degrees of freedom, so it has more degrees of freedom to avoid collisions, so we can fit four robots in a confined space without worrying about collisions.
Roof rack cleaning.
Another advantage is:Adaptive robots are able to resist external interference to a certain extent. For example, when the base of the robot is unstable (the human hand is shaking it vigorously), the robot can still maintain contact with its surface and make the surface well. Or when the robot is installed on the ** track, moving in the same direction of the frame, but the frame and the guide rail are not completely synchronized, there is a speed difference between them, but the robot will not be affected by these interferences, and can still maintain the contact force well and brush to the designated area.
The same line of thought canExpand to more complex scenariosFor example, when adaptive robots are mounted on a moving base or a**, they can handle uncertainty better than traditional industrial robots.
Omnidirectional compliance
All-round compliance refers to the fact that the adaptive robot's compliance is not limited to axial or radial directions, it canControls the forces in all directions in Cartesian space.
It is common to have multiple grinders for a grinding project, and it is more efficient to alternate all the grinders mounted on the robot flange as this saves man-hours. In the case of traditional industrial robots, it is likely that each mill will need to be fitted with a compliant device, which will increase the cost, project complexity and subsequent maintenance difficulties.
Using an adaptive robot with all-round force compliance, users can:Define force control in any TCP framework you want, and you can switch TCP frameworks. For example, we first set up the force control in the left mill, and then switch the force control function to the right mill, which does not have much impact on the robot.
In practice, when you want to add a contact angle between the mill and the polished surface, it is the surface normal force that really needs to be concerned, not the actual force. Therefore, if the axial force is used in the programming, the axial force value needs to be changed again every time the contact angle is modified, which is very inconvenient.
In the case of an adaptive robot, the normal force acting on a surface can be set directly, and it will not be affected by the contact angle in the slightest. In fact, the operator can also set different contact angles along the grinding path without changing any force control settings, whether on a flat or curved surface.
Based on this all-round compliance, we have developed several clever features for adaptive bots, such as the bot canDo surface following without knowing the workpiece in advance.
When we just give the robot an initial speed and let it go up, we can see that the robot will automatically turn around corners while maintaining contact with that surface. And during the whole process, the TCP of the robot does not rotate, only the direction of the force automatically rotates. In addition, it can turn quickly even when hitting sharp corners, which demonstrates the sensitivity of the surface following feature, which is a feature when deployed at the customer's siteThis will significantly reduce the time required for trajectory adjustment.
Another interesting and smart feature is called:Surface bonding. In the test, we programmed the robot to run back and forth in a streamlined pattern, without giving the robot any information about the surface (i.e., the transparent plastic workpiece in this case), but the robot could automatically turn the direction of the mill so that it was in full contact with the surface. This is achieved by adjusting the force and moment on the TCP.
When turning the surface, the robot also adapts to the change in attitude, but the robot can always adjust its orientation and adapt to the new attitude;No matter which direction the workpiece surface is turned, the robot is always quick to adapt and can even lift it up, or give it a little interference to prove that it is not doing position control.
As you can see below, the diameter of the fixture is about 120mm, which is the smallest curvature we can fit. For this curvature, two lines (at the top of the fixture in the second **) need to be given at the time of setting, represented by two red lines, forming a triangle. We pass this triangle to the robot, and the robot will do the rest, and it will follow the actual curve.
Interestingly,This feature is not limited by the millIt doesn't matter what shape the mill is, in fact even a single point of contact can still use this feature.
All-round complianceAnother typical application scenario is the use of stand-alone grinding equipment. For example, a robot can hold a workpiece and grind it on a grinding mill, and the direction of the force relative to the robot is constantly changing. With an adaptive robot, the independent mill can be set to an external TCP and the direction of the force can be defined compared to the mill, so that no matter how the robot turns, it will conform in the same direction. This means that users no longer need to customize independent grinding equipment, and can directly use any standard products on the market, and the robot can adapt.
Another easy-to-use feature is:When programming the non-sunset robot, there is no need to operate the robot in JOG mode all the time. The robot can be dragged to a point and the waypoint can be recorded directly through manual teaching.
Vibration reduction
Vibration damping is an inherent feature of torque controlled robotic arms.
The graph below shows the performance of the adaptive robot vibration test. The test was done by installing a 5-inch grinder at the end of the robot and commanding the robot against a surface and then observing the vibration data of each joint of the robot. The way to measure vibration is to use the moment derivative, the moment derivative we measure on each joint represents how much it vibrates, and the seven graphs represent the vibration of the seven joints.
The blue line on the diagram is the vibration signal of the robot when it is in position control, and the orange line is the vibration signal of the robot in torque control mode. The data comes from the same robot, but with different control modes. As you can see,Vibration can be reduced by 25 to 50 percent using torque controlThis means that the robot can work longer in harsh environments.
Example of force-controlled grinding by an adaptive robot
InAutomotive chassis solder joint grinding applicationThe vibration caused by grinding is very strong, making it difficult to hold the grinder stably even with two hands, and the operating space is small, so it is difficult to grind in a straight line. It takes about 20 seconds to manually complete a single solder joint.
The adaptive robot barely changes when it comes into contact with the grinding workpiece, and the vibrations caused by the grinding are disposed of. Robotsapplied on the weld pointAbout 4nConstant force,And with streamlined grinding, a weld point can be completed in less than three seconds. The image on the right shows the final result of the robot's grinding, which is much cleaner than the manual grinding on the left. We have several of these stations deployed at our customers' factories, working 20 hours a day and grinding more than 700 vehicles.
Another example from the automotive industry is:Welding spatter around the grinding door frame. The picture on the left is a frame that has just been welded and looks dirty. For this trajectory, we don't need to specify all the details of the door frame, we only need to program 7 to 8 waypoints to roughly describe the shape of the door frame. Because the adaptive robot has a large floating range, it can handle these small details automatically.
Over time, the diameter of such a brush wheel will shrink by about 10-20mm, but this is not a problem because the robot automatically compensates for tool loss. Therefore, using Feixi's adaptive robot, each brush wheel can polish more than 4,000 vehicles with quality and quantity.
Guitar side panelsIt needs to be finely sanded before painting. The sides of the guitar have complex curved surfaces that are difficult to program if you use a robot holding a grinder. With an adaptive robot, you can set an external TCP on a belt mill and drag the robot around in a full turn, making it easier to program.
At the same time, the operator can also set the force control relative to the belt mill. The grinding efficiency is much higher with this belt grinder compared to a hand-held belt grinder, so the total cycle time is only 10 seconds. We didn't finish the whole week in the test screen because the radius of the sander we were using was a bit too large and it didn't fit in the last corner, but we could have sanded there.
Silicone ear tip clamping line removalApplicationsIt demonstrates the micro-force processing capability of the adaptive robot. As you can see from the diagram, the clamping line of the earbuds is very tiny and difficult to see, but if you don't remove it, the product will look very cheap. Therefore, the goal of this application is to remove those clamping lines without compromising the smoothness of the silicone surface.
At the same time, the silicone earbuds are very soft, and every time you put it into the clamp, its position will change a little, so you can't use position control, you have to use force control to deal with this situation.
The force that the robot can use in this application is only 05n, otherwise the surface will be damaged. Our adaptive robot can not only control a small force, but also rotate the direction of the force so that the force can always be perpendicular to the surface. The final result can be seen in the image on the right – the clamping line is gone, and the surface of the earbuds is still very smooth.
In addition, there are mobile phone degumming apps. When assembling a phone, one step is to press the screen glass of the phone. Some of the glue may spill out of the gap when pressed, and it must be treated, otherwise the glue will remain there unsightly.
In order to do this, the robot must operate at a fairly high speed - 600 millimeters per second. But at the same time, it is necessary to maintain a very precise force so that it does not damage the phone and does not leave any scratches on the glass, so it is necessary to have precise force control.
PassedDirect force control（direct force control）、All-round compliance(omnidirectional compliance) andVibration damping(Vibraton Reduction), a robot that uses torque control can perform fine, real-time force control at high speeds, so as to flexibly handle grinding and polishing tasks for various surfaces. Therefore, as a representative of force-controlled robots, non-solar adaptive robots can have excellent performance in surface treatment applications and continue to help customers in different fields solve automation problems.