# Part-based Cross Calibration

This mode uses a part as the calibration target. This technique is used in applications where the gripper can transfer only the run-time part and cannot be adapted to transfer a checkerboard.

Part-based cross calibration has the following advantages

- A special fixture to hold the calibration target is not necessary. This is because the alignment system is designed to hold the part
- Special lighting is not necessary to illuminate the part. The illumination that is used during run-time can be used during calibration.

The disadvantages to using a part for hand-eye calibration are as follows

- Unlike the checkboard-based cross calibration, the user will have to add feature finders to locate the features that are used to compute the calibration results
- It is difficult to use this mode of calibration to compute lens and perspective distortion as it is not practical to add many feature finders that would be needed for computation.
- Each camera in SubStation1 should see at least three features which have already been located in SubStation0 for successful calibration, this may be hard to achieve when there are very few features on run-time part.

Cam2 in SubStation1 is cross-calibrated to SubStation0 that has been hand-eye calibrated. Note that even though Cam2 locates some features from Cam0 and other features from Cam1, the calibration would still succeed because the locations of all the features are known in Home2D in both the substations.

## Sequence of Commands

The following are the sequence of commands that must be sent from the PLC to AlignPlus to perform cross-station calibration for the above system

1. The gripper should present the part to the substation that has been hand-eye calibrated, SubStation0 in the figure above, and should send the calibration command along with gripper pose at this station.

2. The gripper should present the part to the substation that must be cross-calibrated, SubStation1 in the figure above, and should send the calibration command along with gripper pose at this station. Care should be taken to ensure that relationship between the part and the gripper does not change during the entire calibration process.

If feature finders needed to cross-station calibrate the cameras have not been added, they should be done. Feature finders in each station should be given unique names. Feature finders that find the same features across the stations (the hand-eye calibrated station and the station to be cross-station calibrated) should be given the same names in order to establish correspondence. If the feature finders have already been added, before the start of this step, the calibration process is complete. Otherwise Steps 1-4 should be repeated.