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Over the past two decades, machine vision has consolidated its early promise and has become a vital component in the design of advanced manufacturing systems. On the one hand, it provides a means of maintaining control of quality during manufacture, and on the other, it is able to feed the assembly robot with the right sort of information to construct complex products from sets of basic components. Automated assembly helps to make flexible manufacture a reality, so that costs due to underuse of expensive production lines are virtually eliminated.
These two major applications of vision - automated visual inspection and automated assembly - have many commonalities and can on the whole be performed by similar hardware vision systems employing closely related algorithms. Perhaps the most obvious use of visual inspection is to check products for quality so that defective ones may be rejected. This is easy to visualize in the case of a line making biscuits or washers at rates of the order of one million per day.
Another area of application of inspection is to measure a specific parameter for each product and to feed its value back to an earlier stage in the plant in order to "close the loop" of the manufacturing process. A typical application is to adjust the temperature of jam or chocolate in a biscuit factory when the coating is found not to be spreading correctly. A third use of inspection is simply to gather statistics on the efficiency of the manufacturing process, finding, for example, how product diameters vary, in order to provide information that will help management with advance planning.
Another aspect of inspection is what can be learned via other modalities such as X-rays, even if the acquired images often ressemble visible light images. Similarly, it is relevant to ask what additional information color can provide that is useful for inspection. Sections 22.10 and 22.11 aim to give answers to these questions.
In automated assembly, vision can provide feedback to control the robot arm and wrist. For this purpose it needs to provide detailed information on the positions and orientations of objects within the field of view. It also needs to be able to distinguish individual components within the field of view. In addition, a well-designed vision system will be able to check components before assembly, for example, so as to prevent the robot from trying to fit a screw into a nonexistent hole.
Inspection and assembly require virtually identical vision systems, the most notable difference often being that a linescan camera is used for inspecting components on a conveyor, whereas an area (whole picture) camera is required for assembly operations on a worktable. The following discussion centers on inspection, although, because of the similarity of the two types of application, many of the concepts that are developed are also useful for automated assembly tasks.