Precise Pose Measurement With Single Camera Calibration for Planar Parallel Manipulators

Pose measurement is an important tool for robot calibration. This paper describes the development and implementation of a technique of camera-aided pose measurement, tested on the Planar Double Triangular Parallel Manipulator (PDTPM). A stationary camera is used to take photos of the End Effector (EE) where a certain array of Light Emitting Diodes (LED) is mounted on. Using various image processing techniques, the coordinates of the LEDs are registered in the image in order to derive the projection matrix that maps any point of the image plane to world coordinates in the EE moving plane. This homography is computed with a method where the vanishing line is treated as the principal component. This estimate is more robust and faster than the Direct Linear Transformation (DLT) method. It was shown that poses could be quickly registered at submillimetre precision notwithstanding inexpensive, relatively low resolution optics. The measurement system is easy to setup, portable, accurate, low cost and it is believed to be a valuable tool.