THE CONTROL OF A ROBOT END-EFFECTOR USING PHOTOGRAMMETRY

摘要

Most robots rely upon their rigidity to perform tasks. In the automotive industry the accuracy requirement for operations such as spot welding will be of the order of a millimetre. The aerospace industry provides a challenging environment to apply robotics in that the accuracy requirements are at least a factor of ten to twenty higher. Conventional robots are not capable of achieving this accuracy. However, combining a measurement system and a robot makes use of the ability of the robot to move precisely and overcomes the accuracy deficiencies. Photogrammetry provides a suitable method to measure the six degrees of freedom of many objects simultaneously. This paper describes work conducted at the Optical Metrology Centre, City University and Sowerby Research Centre, BAe Systems. 1. INTRODUCTION Photogrammetry provides the unsual ability to simultaneously measure the six degrees of freedom of multiple objects at instant in time. This characteristic can be used in many ways, this paper considers how robotic systems might benefit. There are two alternative ways in which photogrammetric systems might be used: with the cameras viewing the robot’s entire working volume (Beyer, 1999) or the only viewing the volume close to an end-effector (Clarke, 1999). Both schemes have advantages and disadvantages. In the former case a high relative accuracy is required but the robot position can be computed in a large area, while in the second case a lower relative accuracy may be acceptable but targets must be placed in known positions close to the task. This paper describes the development of an end-effector based system that has been integrated with an industrial robot. The development of the physical and software components are considered. The physical aspects include: the image processing hardware, configuration of the cameras, lighting, coded targets, and location of the targets with respect to the CAD of the components. The software aspects concern the methods by which targets are identified, the 3-D estimation of the target locations, matching the targets to the CAD information of the components, and estimation of the robot tool centre point. The functionality of the system has been demonstrated for drilling and assembly operations showing that the scheme is feasible, the current status of the work is designed to assess the capability of such a system to operate within realistic tolerances.