Automatic Re-Calibration in the Eye-to-Hand Setup (Six-Axis Robots)

This how-to guide introduces how to complete the automatic re-calibration for six-axis robots in the eye-to-hand (ETH) setup.

Preparation before Calibration

Prepare Calibration Board

Before hand-eye re-calibration, you need to finish the following preparations:

  • Locate the calibration board provided with the project. Ensure that the circles of the calibration board are clearly visible and without obvious scratches, and the board does not suffer from deformations.

  • Mount the robot-specific bracket for calibration board onto the robot flange, and then mount the calibration board onto the bracket. Make sure that the calibration board is attached securely, and that the board is parallel to the XY plane of the robot flange frame.

    If an undetachable gripper is connected to the robot flange, you can attach the calibration board directly to the gripper.

  • Clear the workstation of any materials to ensure that the calibration area is free of obstructions.

  • Manually control the robot to move the calibration board to the starting point. For the automatic calibration in the ETH setup, the starting point is the bottom center of the camera’s field of view (the robot will move the calibration board upwards during calibration).

Adjust the Effect of Calibration Board’s Point Cloud

  1. Open Mech-Eye Viewer to adjust camera parameters.

  2. Adjust the 2D parameters to make sure that the 2D image of the calibration board is clear and neither overexposed nor underexposed.

  3. Adjust 3D parameters to make sure that the point clouds of the circles on the calibration board are complete and have clear contours. It is recommended to set Surface Smoothing and Outlier Removal to Normal in the Point Cloud Processing section to reduce the point cloud fluctuation range.

  4. Make sure that the images and point cloud of the calibration board are up to standard after completing the preceding steps.

    Normal Overexposed Underexposed

    2D image

    normal-2d

    overexposure-2d

    underexposure-2d

    Point cloud

    normal-3d

    overexposure-3d

    underexposure-3d

Pre-calibration Configuration

  1. Open Mech-Vision, select a project in the project list, and then click the Camera Calibration (Standard) button in the toolbar. The Configuration before Calibration window will be prompted.

  2. In the Select how to calibrate window, select the Load calibration parameters radio button, select the parameters that were calibrated for the project, and then click the Next button.

  3. In the Select parameter usage window, select Continue working on the calibrated camera radio button, and then click the Next button.

    • Camera status should be displayed as connected.

    • If the program uses Standard Interface communication mode, please select and run the automatic calibration program on the robot teach pendant.

    • After completing the above settings,Robot status should display as Connected.

  4. In the Select calibration task window, select Recalibrate from the start radio button, and then click the Calibration button. The Calibration (Eye to Hand) window will be prompted.

Till now, you have completed the pre-calibration configuration and can start the calibration procedure.

Re-Calibration Procedure

Connect to Camera

By default, the camera used in the project is set to be connected. Please click the Next button to continue the calibration procedure.

If the camera is not connected, please complete the camera connection by referring to the following instructions:

  1. In the Connect to camera step, select the camera to connect in the Camera ID list, and then click the connect camera icon button or double-click the camera entry to connect to it.

    eth calib auto connect camera
  2. After the camera is connected, click the Capture live or Capture once button.

    eth calib auto image capture
  3. In the Image viewer panel, ensure that the camera can capture images normally and click the Next button on the bottom bar.

Mount Calibration Board & Check Intrinsic Parameters

  1. In the Mount calibration board & check intrinsic parameters step, select the calibration board model for the Standard calibration board model parameter according to its model nameplate in the 1 Select calibration board area.

    eth calib auto select calib board
  2. In the 2 Check calibration board position and point cloud quality area, read carefully the requirements on the calibration board position and point cloud quality, and then click the Capture live button. The Capture live button will turn into Stop capturing and detect position.

  3. Control the robot to move the calibration board to the proper position (in the red rectangle), and ensure that the 2D image and depth map of the calibration board meet the requirements, and then click the Stop capturing and detect position button.

    eth calib auto install calib board

    If the captured images do not meet the requirements, click Open Mech-Eye Viewer button to open the Mech-Eye Viewer software, adjust the 2D and 3D exposure parameters and re-capture images. Please note that you need to change the Parameter group parameter to “calib” first.

  4. In the 3 Check intrinsic parameters area, click the Check intrinsic parameters button.

  5. Confirm the results of the camera intrinsic parameter check.

    • If the camera intrinsic parameter check passes, click the OK button in the prompted window, and then click the Next button on the bottom bar.

      eth calib auto check intri pass
    • If the camera intrinsic parameter check fails, adjust detection parameters by drawing aid circles or manually adjusting the calibration circle detection parameters, and then click the Recheck intrinsic parameters button. If the intrinsic parameter check still fails, please contact Technical Support.

Draw Aid Circle

  1. To draw an aid circle, click the Draw an aid circle button.

  2. In the right Image viewer panel, right-click the calibration board image, clear the Fit to window checkbox, press the Ctrl key and drag the roller to adjust the image to a suitable size.

  3. Move the mouse pointer to the cross center point of the calibration circle, press the left mouse button and make the aid circle completely include the calibration circle and then release it.

    eth calib auto adjust blob
  4. Click the Recheck intrinsic parameters button, and confirm that the camera intrinsic parameter check passes.

Manually Adjust Detection Parameters

To manually adjust detection parameters, click Calibration circle detection parameters (advanced), adjust parameter settings and click the Recheck intrinsic parameters button to confirm whether the camera intrinsic parameter check passes.

Set Motion Path

  1. In the Set motion path step, keep the Height span parameter in the 1 Set height span of calibration poses area unchanged.

    eth calib auto set robot path

    The Height span parameter should be set according to the recommended working distance range of the camera and the size of the robot’s working space.

  2. Click the Generate path parameters button, and then click the Generate motion path based on path parameters button.

  3. When the Generate motion path based on path parameters turns into Completed, click the Next button on the bottom bar.

Collect Images and Poses

  1. Click the Auto move robot along path and capture images button, read the safety window carefully, and then click the OK button.

    eth calib auto safety tips
  2. Wait until the robot finishes moving along the preset path and the camera finishes capturing images on all waypoints. In the right Image viewer panel, all the captured images are displayed.

    • Please stay away from the robot working area to keep safe.

    • Clicking the Stop robot button can exit the calibration process. But the robot will not stop until it finishes the current waypoint. In case of emergency, please tap the emergency stop button on the robot teach pendant to stop the robot immediately (the robot needs to be reconnected after tapping the emergency stop button).

  3. After all images are captured, click the OK button in the prompted window.

    eth calib auto capture finished
  4. Confirm that the collected calibration data meets the data requirements, and then click the Next button on the bottom bar.

    eth calib auto data requirements

Calculate Extrinsic Parameters

  1. In the Calculate extrinsic parameters step, click the Calculate extrinsic parameters button.

    eth calib auto calculate extri
  2. In the prompted window indicating calibration success, click the OK button.

    eth calib auto confirm calib result
  3. Click the Save button on the bottom bar. In the prompted Save Calibration Files dialog box, click the OK button. The camera calibration result will be automatically saved in the “calibration” directory of the project.

Till now, the calibration process is completed.

Verify Calibration Results

After calculating the extrinsic parameters, you can use any of the following methods to verify if the calibration results are valid.

View Extrinsic Parameter Calibration Report

After calculating the extrinsic parameters, you can click View extrinsic parameter calibration report button. The extrinsic parameter calibration report will be automatically displayed after it is generated.

If the calibration results of the Euler angle convention check, camera accuracy check, and robot absolute accuracy check are all “Pass” (or there is no significant error), then the extrinsic parameters are calibrated successfully and can be used.

If any of the checks fails, please follow the recommendations provided in the report and re-calibrate the extrinsic parameters until the check passes.

Roughly Check Coincidence Degree between the Point Cloud of the Robot and the Robot Model in Scene Viewer

  1. After calibration, move the robot arms into the field of view of the camera.

  2. In the Calculate extrinsic parameters step, click the Calculate extrinsic parameters button. This operation will trigger the camera to capture images and generate the point clouds of the robot.

  3. Click Scene viewer to check the coincidence degree between the point cloud of the robot and the robot model.

    calibration reference coincidence degree

If the point cloud of the robot roughly coincides with the robot model, the calibration is successful.

Change the Extrinsic Parameter File

After hand-eye calibration, it is necessary to change the extrinsic parameter file used in the current Mech-Vision project to a new one.

  1. Select the Capture Images from Camera Step.

    change calibration parameter group
  2. In the Step Parameters panel, click change calibration parameter group icon of the Camera Calibration Parameters parameter, and select the newly calibrated extrinsic parameter file.

Calibration-Related Status Codes

If the robot uses the Standard Interface to communicate with the Vision System during hand-eye calibration, the Vision System will return status codes in response data to indicate the execution status of the commands. The status codes indicate both the normal execution results and execution failures. You can perform further troubleshooting according to the status codes.

Calibration Error Codes

Error code Description

7001

Calibration: Parameter error

7002

Calibration: Calibration flange pose not provided by Mech-Vision

7003

Calibration: Calibration joint positions not provided by Mech-Vision

7004

Calibration: Robot failed to reach calibration point

Calibration Normal Status Codes

Normal status code Description

7100

Calibration: Robot moved to the calibration point successfully

7101

Calibration: Pose received from Mech-Vision successfully

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