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Motion control systems have grown in sophistication and precision. However the precision of an entire machine process, for example an assembly operation, involves factors beyond the movement of the controlled mechanism. The initial location of parts prior to handling is subject to a tolerance often much greater than the placement tolerance. The correct position of a part is relative to the final assembly, not the part handling mechanism.

When precision beyond the motor shaft is required an "outer loop" or "dual loop" approach is often taken to improve machine performance. Measuring and controlling the resultant position of a handling mechanism is more precise than just controlling the motor which drives it. Even better than measuring the resultant position of the mechanism is measuring the position of the part being handled itself. This ultimate outer loop control can be implemented with machine vision to monitor part position and target placement positions resulting in improved placement performance.

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