# Hybrid Cross Calibration

When performing pure part based cross-station calibration, we assume that the lens does not introduce any non-linear distortion during imaging. The presence of lens distortion affects the accuracy of calibration adversely. The hybrid mode solves this problem. This mode of calibration uses two calibration targets. It first uses a checkerboard to train image correctors for all the cameras to be cross-station calibrated. This step models the nonlinear distortion in the imaging process. Then, it uses a part to cross-station calibrate the remote cameras.

The advantage of this technique is that during the second step of the calibration the features are found on a corrected image that is free of lens and perspective distortion.

The figure shows the cameras in SubStation1 being checkerboard calibrated. Following this calibration, all the cameras in the substation would be able to map the image features to Plate2D

The locations of the features F1, F2 and F3 are computed in Home2D in SubStation0. The corresponding feature locations are computed in SubStation1. Even though we need three features to perform cross-calibration, the three features need not be in the same camera because one can map the features in SubStation1 to Plate2D

## 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. A checkerboard calibration plate should be placed in the substation to be cross-station calibrated and the calibration command with an arbitrary gripper must be sent. The pose of the gripper is not relevant in this step.
2. The gripper should present the part to the substation that has been hand-eye calibrated, SubStation0 in the figure above, and should send another calibration command along with the gripper position at this station. At this command, the vision system will acquire images from all cameras in SubStation0.
3. The gripper should present the part to the substation that must be cross-calibrated, SubStation1 in the figure above, and should send the third command along with the gripper position at this station. At this command, the vision system will acquire images from all cameras in SubStation1 and then process cross calibration. 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 to establish correspondence. If the feature finders have already been added, before the start of this step, the calibration process is complete. Otherwise Steps 2 and 3 should be repeated.