An asynchronous modulation/demodulation technique for robust identification of a target for 3-D pose determination

Engineers at Oak Ridge National Laboratory have been investigating the feasibility of computer-controlled docking in resupply missions, sponsored by the US Army. The goal of this program is to autonomously dock an articulating robotic boom with a special receiving port. A video camera mounted on the boom provides video images of the docking port to an image processing computer that calculates the position and orientation (pose) of the target relative to the camera. The control system can then move the boom into docking position. This paper describes a method of uniquely identifying and segmenting the receiving port from its background in a sequence of video images. An array of light-emitting diodes was installed to mark the vertices of the port. The markers have a fixed geometric pattern and are modulated at a fixed frequency. An asynchronous demodulation technique to segment flashing markers from an image of the port was developed and tested under laboratory conditions. The technique acquires a sequence of images and digitally processes them in the time domain to suppress all image features except the flashing markers. Pixels that vary at frequencies within the filter bandwidth are passed unattenuated, while variations outside the passband are suppressed. The image coordinates of the segmented markers are computed and then used to calculate the pose of the receiving port. The technique has been robust and reliable in a laboratory demonstration of autodocking.