In the application of industrial robots, it is crucial to accurately set and maintain the tool coordinate system, which is not only related to the accuracy of operation, but also directly affects production efficiency and quality. The tool coordinate system measurement principle of KUKA robots covers a variety of measurement methods, each of which has its own specific application scenarios and operation steps. Understanding and correctly applying these methods is crucial to ensuring the accuracy and efficiency of robot operation.
The KUKA robot tool coordinate system measurement is based on the principle of kinematics. By moving the tool along a known path and recording the positions of the robot’s joints, the position and orientation of the tool relative to the robot base can be calculated. It often uses methods like the TCP (Tool Center Point) calibration to precisely define the tool’s coordinate system, enabling accurate operations like picking and placing.
1 Definition of tool coordinate system measurement
(1) Definition of tool coordinate system
The tool coordinate system is a coordinate system created with a reference point. This reference point is called TCP, and this coordinate system is called the tool coordinate system. It is a rectangular coordinate system (Cartesian coordinate system), and its X-axis is consistent with the working direction of the tool and moves with the movement of the tool.
(2) Definition of tool coordinate system measurement
The measurement of the tool coordinate system refers to the robot control system identifying where the tool tip (TCP) is relative to the flange center point and its direction through the measurement tool (tool coordinate system).
2 Tool coordinate system measurement methods
The tool coordinate system measurement is mainly divided into two aspects
①Determine the origin of the tool coordinate system
②Determine the posture of the tool coordinate system
Or you can directly input the distance value and rotation angle to the flange center point, but this method needs to be obtained through measurement and calculation, which is difficult, so it is not often used.
The origin can be determined by the XYZ 4-point method and the XYZ reference method, and the posture can be determined by the ABC world coordinate method and the ABC 2-point method. Next, we will focus on these measurement methods. (1) The origin measurement principle of the XYZ 4-point method
The XYZ 4-point method measurement is to move the TCP of the tool from 4 different directions to a reference point (generally select the tip point or a point with obvious features), and the robot control system calculates the TCP from different flange position values.
One thing to note is that the flange positions in 4 different directions cannot be in the same plane, and the distance must be far enough.
(2) The origin measurement principle of the XYZ reference method
The XYZ reference method compares a new tool with a tool that has been measured, and then the robot control system compares the flange positions of the two tools to calculate the TCP of the new tool. This method makes it easier for operators to master multiple similar tools with similar geometry in the control system. Compared with the previous method, the number of measurement strokes can be reduced to 2.
(3) The posture measurement principle of the ABC world coordinate system method
The ABC world coordinate system method adjusts the axes of the tool coordinate system to be parallel to the axes of the world coordinate system, so that the robot control system can know the posture orientation of the tool coordinate system.
The ABC world coordinate system method can determine the posture in two ways: ①5D method: only inform the robot control system of the working direction of the tool. The working direction defaults to the X axis, and the directions of other axes are determined by the system. That is, +X tool coordinates ∥ -Z world coordinates
②6D method: inform the robot control system of the directions of all three axes. The robot tool coordinates need to be adjusted to +X tool coordinates ∥ -Z world coordinates +Y tool coordinates ∥ +Y world coordinates +Z tool coordinates ∥ +X world coordinates
(4) Principle of posture measurement of ABC 2-point method
ABC 2-point method can obtain the axis data of the tool coordinate system by moving to a point on the X-axis and a point on the XY plane. This measurement method has high accuracy and is generally suitable for occasions where the axis direction must be particularly accurate.
3 Impact of tool load data on tool coordinate system measurement
Tool load data refers to all loads mounted on the robot flange. It is the mass that is additionally mounted on the robot and moves with the robot. The values that need to be entered are mass, center of gravity position, mass moment of inertia, and the main inertia axis to which it belongs. To ensure the accuracy of the robot, the load data must be entered into the robot control system and assigned to the correct tool.
It can affect many control processes, including control algorithms, speed and acceleration monitoring, torque monitoring, collision monitoring, energy monitoring, etc., so when the correct input load data is executed to execute its movement, it can benefit from its high accuracy, so that the movement process has the best cycle time and the robot reaches a longer service life.
4 The significance of tool measurement
1) Change the posture around the TCP of the tool: that is, move the tool TCP close to a fixed point, and operate the robot manually or with a 6D mouse when the base coordinate is the tool coordinate system. No matter what posture it is, it is always in contact with the fixed point.
2) Move along the tool operation direction: that is, when the robot is moved, the tool TCP always moves along the tool operation direction.
3) Maintain the programmed running speed along the trajectory on the TCP: that is, when the program is run, the speed of the tool TCP always remains at the set speed.
4) Maintain the defined posture and run along the trajectory: that is, define the robot posture at a certain point on the trajectory, move the robot in the global coordinate system, and its tool can always maintain the starting posture and run along the trajectory.