Generally, when the robot leaves the factory, the zero point has been calibrated by the manufacturer. However, during transportation, the zero point may be offset due to road bumps or other reasons, or the robot may need to be recalibrated due to maintenance after use. Therefore, it is necessary to understand the method of robot zero point calibration and the robot load offset learning, and understand the necessity of robot zero point calibration and how to perform zero point calibration.
Zero point setting for KUKA robots is crucial. It defines the reference position for accurate movements. First, ensure the robot is in a safe state. Then, use the appropriate tools and procedures in the robot’s operating system. Some models may require moving axes to specific marks. This process calibrates the position sensors, enabling the robot to perform tasks with precision. Regularly checking and resetting the zero point can maintain high – quality performance.
- Why is zero point calibration necessary?
The zero point is the reference of the robot coordinate system. Only when the industrial robot is fully and correctly calibrated at the zero point, its use effect will be the best. Because only in this way can the robot achieve its highest point accuracy and trajectory accuracy or be able to run in the programmed action.
If the robot axis is not zero-point calibrated, the robot’s function will be severely limited, resulting in the following consequences:
1) Unable to program operation
2) Unable to perform Cartesian manual operation
3) Software limit switch is closed
In principle, the robot must always be in the state of zero point calibration. Normally, zero calibration is necessary in the following situations:
1) After maintenance measures have been taken on components involved in the sensing of positioning values (such as motors with resolvers or RDCs)
2) When the robot axis is moved without the control system (e.g. with the aid of a free-wheeling device)
3) After mechanical repairs (such as replacing a drive motor) or after replacing a gearbox, after hitting an end stop at a speed higher than 250mm/s, or after a collision, the zero point must be recalibrated
The zero point calibration position of all robots is similar, but not exactly the same. The exact position may also vary between different robots of the same robot model. The zero point position of different robots of the same model is slightly different. - How to perform zero point calibration?
Zero point calibration can be performed by determining the mechanical zero point of the axis. This process can be performed by using the technical auxiliary tool EMD (Electronic Mastering Device) to specify a reference value (e.g. 0°) for any axis at the mechanical zero point position, so that the mechanical position and electrical position of the axis are consistent, so each axis has a unique angle value. Small robots use MEMD, and large robots use SEMD.
When calibrating the robot, each axis should be placed in a defined mechanical position, the so-called mechanical zero point. This mechanical zero point position indicates the corresponding relationship between the coaxial drive angles, usually represented by a measuring groove or line, that is, the pre-zero point calibration mark.
In the process of accurately determining the mechanical zero point position of a robot axis using EMD, the axis will continue to run until the probe reaches the deepest point of the measuring groove and stops.
①-EMD (electronic controller) ②-measuring cylinder ③-probe ④-measuring groove ⑤-pre-zero point calibration mark
Note: 1) The mechanical zero point must be found in the direction of the arrow shown in the figure from “+” to “-“. If the direction must be changed, it must first turn past the mark of the pre-calibration position and then return to this mark. In this way, the transmission direction gap can be eliminated.
2) The calibration process is always carried out under the same temperature conditions to avoid errors caused by thermal expansion.
- What are the methods of zero point calibration?
There are two ways to calibrate a robot: 1) Standard zero point calibration – suitable for lower accuracy and lower load specifications; 2) Zero point calibration with load correction, suitable for high precision requirements or multiple load specifications.
(1) Standard zero point calibration
When the robot is put into operation for the first time, it needs to calibrate the zero point. The first zero point calibration can only be performed when the robot has no load, no installation tools or additional loads.
The steps for the first zero point calibration of the robot are as follows:
1) Move the robot to the pre-zero point calibration position
2) In the main menu, select “Start-up” → “Adjustment” → “EMD” → “Load correction” → “First adjustment” in sequence. A window will automatically open and all the axes to be zero-point calibrated will be displayed, among which the axis with the smallest number has been selected.
3) Remove the protective cover of the measuring cylinder from the axis selected in the window. The turned-over EMD can be used as a screwdriver to screw the EMD onto the measuring cylinder.
4) Then connect the measuring wire to the EMD and connect it to the interface X32 of the robot junction box.
Note: Always screw the EMD without measuring wires onto the measuring cylinder before connecting the measuring wires to the EMD, otherwise the measuring wires will be damaged. Similarly, when removing the EMD, the measuring wires of the EMD must be removed first, and then the EMD is removed from the measuring cylinder. After adjustment, remove the measuring wires from the interface X32, otherwise interference signals will appear or damage will occur.
5) Click “First adjustment”.
6) Press the confirmation switch to the middle position, then press and hold the start key. If the EMD passes the lowest point of the measuring cutout, it has reached the zero point calibration position, the robot automatically stops running, the value is saved, and the axis disappears from the window.
7) Remove the measuring wires from the EMD, then remove the EMD from the measuring cylinder and reinstall the protective cover.
8) Repeat steps 2 to 5 for all axes that need to be zero-calibrated.
9) After calibration, close the window.
10) Remove the measuring wires from the interface X32.
Note: Do not forget to replace the protective cover of the measuring head after the zero point calibration of an axis. If foreign matter is mixed in, the sensitive measuring device will be damaged and expensive repair costs will be incurred.
(2) Offset learning with load
If the robot is equipped with a heavy tool or carries a heavy workpiece after the first calibration, the increased load may cause positioning deviation. In order for the robot to compensate for this deviation, the corresponding tool weight must be “learned”, otherwise the robot’s accuracy will be affected. “Offset learning” is carried out with load, and the difference between it and the first calibration (without load) is stored. Only robots that are frequently calibrated with load and zero point have the required high accuracy. Therefore, offset learning must be performed for each load condition! The premise is that the geometric measurement of the tool has been completed and the tool number has been assigned.
The operating steps for the robot to perform offset learning with load are as follows:
1) Move the robot to the pre-zero point calibration position.
2) In the main menu, select “Start operation” → “Adjustment” → “EMD” → “Calibration with load” → “Offset learning”.
3) Enter the tool number and confirm. At this point, a window automatically opens and all axes of the unlearned tool are displayed. The default option is the axis with the smallest number.
4) Remove the protective cover of the measuring cylinder from the axis selected in the window, screw the EMD onto the measuring cylinder, and then connect the measuring wire to the EMD and connect it to the interface X32 of the base terminal box.
5) Click the “Learn” button and hold down the confirmation key and the start key. When the EMD recognizes the lowest point of the measuring cutout, it means that the zero point calibration position has been reached, the robot stops running, and a window automatically opens. The deviation from the first zero point calibration on the axis is expressed in the form of increments and angles.
6) Click the “Confirm” button, and the axis disappears from the window, indicating that the offset learning setting is complete.
7) Remove the measuring wire from the EMD, then remove the EMD from the measuring cylinder and reinstall the protective cover.
8) Then, repeat steps 3 to 6 for all axes that need to be zero-calibrated.
9) Remove the measuring wire from the X32 interface and click the “Close” button to close the window.
After the KUKA robot has learned the offset with load, it can perform a load zero calibration check with offset. This function can check and, if necessary, rebuild the robot’s old first calibration parameters without removing the tool. The robot is calibrated with the tool. If the tool has been learned, the parameters of the first calibration will be recalculated based on the learned offset and overwritten with the user’s consent. This function is only valid in T1 operation mode.
The operating steps for the robot to perform a load zero calibration check with offset are as follows:
1) Move the robot to the pre-zero calibration position.
2) In the main menu, select “Start” → “Adjustment” → “EMD” → “Calibration with load” → “Load calibration” → “With offset”.
3) In the pop-up window, enter the tool number and confirm. A window will pop up automatically, showing all axes that have not been checked for load zero calibration with offset. The default option is the axis with the smallest number.
4) Press the axis selected in the window, remove the protective cover of the measuring cylinder, screw the EMD onto the measuring cylinder, and then connect the measuring wire to the EMD and connect it to the interface X32 of the base terminal box. During this process, align the red dot of the plug with the notch in the EMD. 5) Click the “Check” button.
6) Press and hold the confirmation key and press the start key. When the EMD recognizes the lowest point of the measuring cutout, the zero point calibration position has been reached and the robot automatically stops running. A pop-up window will be opened to record the data.
7) If necessary, use “Backup” to store these values. The old zero point calibration values are deleted. If you want to restore the lost first zero point calibration, you must save these data.
8) Remove the measuring wire from the EMD, then remove the EMD from the measuring cylinder and reinstall the protective cover.
9) Repeat steps 4 to 7 for all axes that need to be zero-calibrated.
10) Click the “Close” button to close the window and remove the measuring wire from the interface X32.