Basic Mechanism of Calibration under EIH

When using the multiple random board pose method, the camera is mounted on a frame which is fixed at the end of the robot. The calibration board is placed within the camera’s field of view. The pose of the calibration board in the robot base coordinate system, denoted by A, is a measurable constant. The pose of the flange in the robot base coordinate system denoted by B, is a known variable. The spatial relationship between the camera’s optical center and each circle on the calibration board, denoted by C, can be calculated by capturing images of the calibration board with the camera. The spatial relationship between the center of the flange and the camera’s optical center, denoted by X, is an unknown constant. The above constants and variables are shown in Figure 1. Thus, A, B, C, and X form a closed loop, as shown in the following equations. Since A is a constant, when the first two equations are combined, X will be the only unknown variable. As the robot moves to different poses, the camera will capture pictures from different angles and produce multiple sets of A, B, and C. Then, these values can be used to perform numerical fitting and obtain an optimal solution of X.

Figure 1. The multiple random board poses method (EIH)

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When using the TCP touch method, place the calibration board on the object plane and install a sharp-tipped tool with known dimensions on the robot flange. Then, touch the mark points on the calibration board with the center point of the tool by moving the robot. As shown in Figure 2, since the values of A, B and C are known, the value of X can also be obtained.

Figure 2. TCP Touch Method (EIH)

Calibration under EIH calibrates the transformation relationship between the coordinate systems of the camera’s optical center and the robot base. If the camera moves relative to the flange, the extrinsic parameters will be altered and must be recalibrated.