The term accuracy is probably on of the most misunderstood specifications in the positioning field. Accuracy is always understood as absolute accuracy in terms of theoretically exact distance relative to a machine coordinate system. No positioning system can truly create an absolute accurate position due to error of various magnitude.
Typical specifications would read "Accuracy ± 5 µm/300 mm" the interpretation of this specification is that the maximum error of position this stage will have is ± 5 µm over 300 mm travel range.
Repeatability is a term used to describe the ability of a positioning machine to repeat a target position within a certain range of error. There are two principal ways of describing repeatability;
Uni-Directional Repeatability: in this scenario, the targeted position is approached from one direction only. The deviation of true position on a series of repetitive moves to that targeted position, is expressed as uni-directional repeatability.
Bi-directional Repeatability: In this scenario the targeted position in approached from both directions and the recorded deviation or error from true position is expressed as bi-direction repeatability.
The values for bi-directional repeatability are always larger then the uni-directional repeatability because they include the composite hysteresis of the electronic, position feedback ,electro-mechanical drive and mechanical hysteresis. For the end user, the bi-directional repeatability is usually more relevant and meaningful unless the motion patter for which the stage is used is a single direction step and repeat mode requiring an optimized repeatability capability.
The smallest achievable positioning increment that can be achieved by a stage or system. Resolution is determined by the stages mechanical capability and its feedback device.