Geometry Estimation of Twin Robot X-Ray Computed Tomography Systems

This thesis will focus on high accuracy control of collaborating robot systems, aiming to provide accurate positioning and orienting ability for a flexible robotic X-ray CT system. Industrial X-ray ct system has gained more and more attention because of the outstanding ability to perform quality control on both internal and external structure. However, current CT systems can only perform on fixed circular trajectories, which restrict its application in inline environment or on large targets. This thesis will develop a high accurate robot system which can be used to perform flexible and accurate movement of CT system. To improve the accuracy of robot manipulators, five aspects should be investigated: * optimal motion planning method * absolute positioning method based on laser tracker * relative positioning method based on radiograph. optimal motion planning method This task aims to develop optimal path planning method. For a given scanning geometry sequence，optimal path planner will provide feasible and optimal solution for the collaborating robots to move from the initial configuration to each of the goal state defined by the scanning geometry sequence. Objective: * execution uncertainty * execution time (path length) * path safety Different path characteristics (such as geometry, velocity, acceleration, joint type and the number of joint involved) can signification affect the execution effect, such as time and uncertainty. In order to achieve an optimal trajectory, we need to cope with following sub-tasks: * For a given scanning geometry sequence, which can be seen as Tool Center Point (TCP) sequence, inverse kinematic equations need to be solved to get the configurations of the whole robot system. * For a given start TCP/configuration and a goal TCP/configuration, how many joints should be occupied at least? * Which one pose more impact on the path uncertainty, the number or the type of joints. * How to adopt the answer of questions above to guide the selection of multi solution of inverse kinematic equations. * Combining concerns about velocity, acceleration, geometry and information above, to generate an optimal feasible path. absolute positioning method based on laser tracker The accuracy can also be improved by incorporating extra sensors. Generally, the actual robot cannot fit in its system model ideally, because of the deviation in assembling and manufacturing. This sub-task will first develop an automatic method to calibrate the robot system model (for example D-H model) by generating and tracking a series of specific movements. Then, the system will perform absolute static calibration of both TCP start positions and orientations. Dynamic calibration will also be investigated to track TCP positions and orientations, as well as improve the control accuracy. Since line-of-sight limitations are associated with external sensors, investigation on off-line calculation of trajectory pre-compensation of the effects of the nonlinear dynamics (e.g. robot joints compliances) is also important. The final accuracy of both TCP configurations should be ca. 1mm or better. relative positioning method based on radiograph. The challenge with collaborating robots and robotized X-ray CT lies in the importance of both source and detector position with respect to each other. In fact, X-ray CT systems can also be seen as an extra sensor with superior accuracy ( up to a few µm), which can be used to further calibrate the relative position accuracy between X-ray source and detector. To this aim, calibrated reference objects in between source and detector will be introduced. The principle will first be evaluated using both simulations and two complementary validation platforms: two collaborative robots with visual light and a XCT machine with additional rotation axis.