Eye to hand Calibration for Four-Axis Robots

About 4308 wordsAbout 14 min

This article describes how to perform eye-hand calibration for four-axis Robots in an Eye to hand setup.

1. Random Pose Sampling Calibration

1.1 Preparation Before Calibration

Before starting eye-hand calibration, complete the following preparations:

（1）Complete the 3D vision guidance kit hardware setup

Please refer to 3D Vision Guidance Kit Hardware Setup to complete the installation and connection of the Robot, Camera, and industrial PC

（2）Create a new Project and a new task

Please refer to Project Operation Guide and task Operation Guide to create a new Project and task

（3）Complete Camera connection and camera imaging quality adjustment, then configure them in the task information

Please refer to Camera Connection and Parameter Adjustment Guide to connect the Camera, perform camera imaging quality adjustment, verify Camera accuracy, and configure it in the task information

（4）Complete Robot communication configuration

Please refer to Robot Configuration and Communication Operation Guide to establish the robot communication connection between the Robot and PickWiz.

（5）Prepare the materials required for calibration

Ensure that the Calibration Board is flat and clear, with no obvious scratches or dirt, and no bending or deformation
If the calibration method is Random Pose Sampling Calibration, ensure that the Calibration Board is placed on the work platform and is within the Camera field of view.

1.2 Calibration Information Configuration

Go to the eye-hand calibration interface and click Add Eye-Hand Calibration to open the calibration information configuration interface

Calibration Name can be used to define the name of the current calibration configuration
Calibration ID is used by the Robot to switch calibration configurations
Camera is the IP address of the currently connected Camera
Camera Brand is the brand of the currently connected Camera
Robot Type must be consistent with the Robot type in Robot Configuration
For Camera Installation Method, select Eye-to-Hand
For Calibration Board Type, select the current Calibration Board type

The appropriate Calibration Board is mainly determined by the Robot type and the Camera mounting height. Select a suitable Calibration Board for the actual application scenario by referring to the table below.

Robot Type	Camera Mounting Height	Select Calibration Board
Six-axis Robot	Below 0.5 m	A6 multi-concentric-circle Calibration Board
	Below 1.5 m	A5 multi-concentric-circle Calibration Board
	1.5 m -- 2.5 m	A4 multi-concentric-circle Calibration Board
	Above 2.5 m	A3 multi-concentric-circle Calibration Board
Three-axis/Four-axis Robot	Below 0.5 m	A6 multi-concentric-circle Calibration Board
	Below 1.5 m	A5 multi-concentric-circle Calibration Board
	1.5 m -- 2.5 m	A4 multi-concentric-circle Calibration Board
	Above 2.5 m	A3 multi-concentric-circle Calibration Board

For Calibration Method, select Random Pose Sampling Calibration
If the Robot has a corresponding calibration program and must run it to send the pose to PickWiz while sampling the Calibration Board, then for Get Coordinates Online** select Yes; if the Robot does not have a corresponding calibration program and the pose of the Robot end Tool must be entered manually while sampling the Calibration Board, then for Get Coordinates Online **select No.

1.3 Checks Before Calibration

Ensure that the Robot base is securely installed

Before eye-hand calibration, carefully check the installation of the Robot base. If the Robot base is not securely installed, the Robot may shake noticeably during motion, affecting Robot accuracy and therefore the calibration result.

Check whether the Robot base installation meets the requirements as follows:

First, the surface on which the Robot base is installed must be flat and clean;

Second, operate the Robot at 100% speed with large translation or rotation movements and observe whether there are signs of shaking. If shaking occurs, readjust and secure the Robot base to ensure that no displacement or tilting occurs during Robot motion;

Third, check whether the Robot body and base are tightly connected, and tighten the screws to prevent loosening.

Ensure that the Camera and its bracket are securely installed

Before eye-hand calibration, carefully check the installation of the Camera and its bracket. If the Camera or bracket is not securely installed, Camera imaging quality may be affected, which in turn affects the calibration result.

Check whether the Camera and its bracket installation meets the requirements as follows:

First, check whether the Camera bracket is a machined part. Aluminum profile material should be avoided for the bracket;

Second, shake the bracket and observe whether there is any obvious wobble. If so, readjust and secure the bracket;

Third, shake the Camera and observe whether there is any obvious wobble to ensure that the Camera is securely installed.

Ensure that the Calibration Board is securely mounted within the Camera field of view

In an Eye to hand setup, mount the Calibration Board on the end Tool and ensure that it is within the Camera field of view.

Ensure that Robot accuracy meets the Project requirements

If Robot accuracy does not meet the Project requirements, please refer to Calibration Verification to correct Robot accuracy.

1.4 Collect Samples

1.4.1 Collect Multiple Sets of Calibration Board and Robot Coordinates

（1）Control the Robot carrying the Calibration Board to move so that the Calibration Board is positioned at the center of the Camera field of view and is parallel to the Camera imaging plane.

（2）Click Start Sampling. The Start Sampling button changes to Stop Sampling, and the Inspection Results below displays Camera and Robot checks. If all Camera and Robot checks show green icons, the checks pass. If any red icon appears, the check has failed and should be investigated.

（3）The Camera checks consist of 4 aspects:

（4）The Robot checks consist of 3 aspects:

（5）When coordinate axes appear on the Calibration Board in the visualization window and the center of each concentric circle is green, the current Calibration Pose meets the requirements.

（6）Click New Sample. If the sample is valid, a Sample Validity Check Passed pop-up dialog appears. After clicking OK, Collected Samples displays the Sample 1 label. Click Sample 1 to view the saved sample, or click the close button after Sample 1 to delete it.

If the sample is close to an existing sample, it is invalid. A Sample Check Failed pop-up dialog appears with the message Close to Existing Sample. Click Back, then control the Robot carrying the Calibration Board to move and collect the sample again.

（7）Control the Robot carrying the Calibration Board to move so that the Calibration Board is positioned at the four corners of the Camera field of view, and the rotation angles around the X, Y, and Z axes are all greater than 10°. Repeat the above sample collection steps until at least 5 samples are saved.

（8）After saving at least 5 valid samples, click Finish Collection to enter the Calculation Results interface.

1.4.2 Identify the Calibration Board to be touched

（1）Place the Calibration Board within the Camera field of view and ensure that the Robot with the installed needle tip can touch the Calibration Board.

（2）Click Start Sampling until coordinate axes and feature points appear in the visualization window and all points are green, indicating that the current Calibration Pose meets the requirements

（3）Click Add Sample. If the sample is valid, Saved Successfully is displayed; if the sample is close to an existing sample, the sample is invalid and saving fails

（4）Click Next

1.4.3 Touch the Calibration Board

（1）Click Start Sampling, control the Robot to move, and touch the center of each corresponding concentric circle with the needle tip in the numerical order shown in the visualization window.

（2）Click New Sample to collect the Robot coordinates at the moment of contact.

（3）Repeat the above steps to collect at least 3~6 concentric-circle point-touch samples.

Click Next

1.4.4 Calculate the Calibration Result

Click 计算结果. PickWiz calculates the calibration result based on the collected samples. If the calibration error does not meet the requirements, perform Calibration Result Inspection and Analysis.

1.5 Calibration Result Inspection and Analysis

If the Calibration Error exceeds the normal range, the cause of the error must be identified. Click Calibration Diagnosis and refer to Calibration Result Inspection and Analysis to analyze the calibration result, resolve the issue, and check again until the calibration accuracy meets the application requirements.

1.6 Select the eye-hand calibration configuration

After the calibration process is completed, return to the Run Configuration interface, click Task Information, click the Eye-Hand Calibration drop-down list, and select the corresponding eye-hand calibration configuration.

2. Needle-Tip Tool Point-Touch Calibration

2.1 Preparation Before Calibration

Before starting eye-hand calibration, complete the following preparations:

（1）Complete the 3D vision guidance kit hardware setup

Please refer to 3D Vision Guidance Kit Hardware Setup to complete the installation and connection of the Robot, Camera, and industrial PC

（2）Create a new Project and a new task

Please refer to Project Operation Guide and task Operation Guide to create a new Project and task

（3）Complete Camera connection and camera imaging quality adjustment, then configure them in the task information

Please refer to Camera Connection and Parameter Adjustment Guide to connect the Camera, perform camera imaging quality adjustment, verify Camera accuracy, and configure it in the task information

（4）Complete Robot communication configuration

Please refer to Robot Configuration and Communication Operation Guide to establish the robot communication connection between the Robot and PickWiz.

（5）Prepare the materials required for calibration

Ensure that the Calibration Board is flat and clear, with no obvious scratches or dirt, and no bending or deformation
Install the needle-tip device on the Robot end, create the corresponding Tool coordinate system, and verify that the Tool coordinate system is accurate
- After TCP calibration is completed, try moving the Robot under this Tool coordinate system. For a four-axis Robot, the positive Z-axis direction is usually downward along the Robot end
- You can also inspect the calibrated Tool coordinate system values. If the needle-tip device is installed as shown below and extends along the center of the flange, the TCP x and y should be 0
For needle-tip tool point-touch calibration, ensure that the Calibration Board is placed horizontally on the work platform and is within the Camera field of view, as shown below.

2.2 Calibration Information Configuration

Go to the eye-hand calibration interface and click Add Eye-Hand Calibration to open the calibration information configuration interface

Calibration Name can be used to define the name of the current calibration configuration
Calibration ID is used by the Robot to switch calibration configurations
Camera is the IP address of the currently connected Camera
Camera Brand is the brand of the currently connected Camera
Robot Type must be consistent with the Robot type in Robot Configuration
- If a six-axis Robot is displayed here, go to Robot Configuration and change it to a four-axis Robot. Save the change, then return here and create a new calibration for the change to take effect
For Camera Installation Method, select Eye-to-Hand
For Calibration Board Type, select the current Calibration Board type

Robot Type	Camera Mounting Height	Select Calibration Board
Six-axis Robot	Below 0.5 m	A6 multi-concentric-circle Calibration Board
	Below 1.5 m	A5 multi-concentric-circle Calibration Board
	1.5 m -- 2.5 m	A4 multi-concentric-circle Calibration Board
	Above 2.5 m	A3 multi-concentric-circle Calibration Board
Three-axis/Four-axis Robot	Below 0.5 m	A6 multi-concentric-circle Calibration Board
	Below 1.5 m	A5 multi-concentric-circle Calibration Board
	1.5 m -- 2.5 m	A4 multi-concentric-circle Calibration Board
	Above 2.5 m	A3 multi-concentric-circle Calibration Board

For Calibration Method, select Needle-tip Tool Touch Calibration
If the Robot has a corresponding calibration program and must run it to send the pose to PickWiz while sampling the Calibration Board, then for Get Coordinates Online** select Yes; if the Robot does not have a corresponding calibration program and the pose of the Robot end Tool must be entered manually while sampling the Calibration Board, then for Get Coordinates Online **select No.

2.3 Checks Before Calibration

Ensure that the Robot base is securely installed
- Before eye-hand calibration, carefully check the installation of the Robot base. If the Robot base is not securely installed, the Robot may shake noticeably during motion, affecting Robot accuracy and therefore the calibration result.
- Check whether the Robot base installation meets the requirements as follows:
  1. First, the surface on which the Robot base is installed must be flat and clean;
  2. Second, operate the Robot at 100% speed with large translation or rotation movements and observe whether there are signs of shaking. If shaking occurs, readjust and secure the Robot base to ensure that no displacement or tilting occurs during Robot motion;
  3. Third, check whether the Robot body and base are tightly connected, and tighten the screws to prevent loosening.
Ensure that the Camera and its bracket are securely installed
- Before eye-hand calibration, carefully check the installation of the Camera and its bracket. If the Camera or bracket is not securely installed, Camera imaging quality may be affected, which in turn affects the calibration result.
- Check whether the Camera and its bracket installation meets the requirements as follows:
  1. First, check whether the Camera bracket is a machined part. Aluminum profile material should be avoided for the bracket;
  2. Second, shake the bracket and observe whether there is any obvious wobble. If so, readjust and secure the bracket;
  3. Third, shake the Camera and observe whether there is any obvious wobble to ensure that the Camera is securely installed.
Ensure that the Calibration Board is stably placed within the Camera field of view
- In an Eye to hand setup, place the Calibration Board at a potential incoming-material position and ensure that it is within the Camera field of view.
Ensure that Robot accuracy meets the Project requirements
- If Robot accuracy does not meet the Project requirements, please refer to Calibration Verification to correct Robot accuracy.
Ensure that the point-touch device Tool coordinate system meets the accuracy requirements
- You can verify TCP accuracy by performing needle-tip-to-needle-tip rotation under the Tool coordinate system

2.4 Collect Samples

2.4.1 Collect Calibration Board and Robot Coordinates

（1）Adjust the position of the Calibration Board, place it within the workpiece placement area, and click Start Sampling.

（2）Click Add Sample. If the sample is valid, Saved Successfully is displayed. If the sample is close to an existing sample, the sample is invalid and saving fails.

（3）Check the Vision Result. If all inspection items display green icons, coordinate axes and feature points appear in the visualization window, and all circle centers are green, the current Calibration Pose meets the requirements.

The inspection consists of 4 aspects:

2.4.2 Touch the Calibration Board with the Needle Tip

（1）Click Start Sampling, control the Robot to move, and touch the center of each corresponding concentric circle with the needle tip in the numerical order shown in the visualization window.