・A higher degree of precision in an automation device is considered as making the positioning more precise.Degradations in the precision of the positioning, however, are caused by a variety of factors, and need to be recognized as one of the most difficult production technologies.

・The positioning precision obtained by the multiple times (n times) of positioning operations will have the number of variations equal to that of attempts (n) of positioning.Typical variations in positioning in this case are plotted in Fig.1.

・Fig.1 shows the positioning precision consists of two errors, namely "bias error" for target value for positioning, and "variation error" for limits of positioning precision (positioning tolerance).

・Performance with low "bias error" is expressed as "high accuracy", whereas performance with low "variation error" is expressed as "high precision".Low "variation error" is also expressed as positioning with high reproducibility.

・While "bias error" can be corrected toward the target value using positional measurement and control, "variation error" requires some countermeasures for the main cause of the variation error, when a strict tolerance is applied to positioning, due to positioning errors because of various factors.

・Typical factors for errors for "variation error" are as follows:
(1)Factor specific to positioning system: Coordinates precisions of position sensor and mechanical components, backlash in drive mechanism, etc.
(2)Factor specific to workpiece for positioning: Error of shape, error in reference plane, etc.
(3)Accidental variations caused by dirt, foreign particles, etc.