Techno Linear Motion Catalog7Technical Information2) Terms and Definitions for Linear Motion SystemsThere are many terms that are commonly used in industry that are not always understood by designers,end users, and other technical persons. Although some of the terms take on obvious meanings, othersmay not. This section is provided so that such people can use these terms in proper context, to relieve thepossibility of design criteria and specifications being too tightly constrained due to lack of effectivecommunication. Having the following terms explained and understood allows for a more efficient designprocess, and a more cost-effective solution.Point to point accuracy or accuracy is defined as the difference between the statistical mean of a seriesof measurements and the theoretically correct position. Another way of stating this is to say that point topoint accuracy is the ability to travel to a desired point or series of points with respect to some knownreference.Straight line accuracyis the ability of a machine to accurately travel in a straight line with respect to aknown reference plane, and the specification refers to the maximum possible deviation from the desiredstraight line path.Accuracy affects how closely parts are made to specifications. There are many factors that contribute tothe accuracy of a system, but the most significant ones are the accuracy of the drive mechanism, theaccuracy of the motor, and the presence of play, or backlash. Accuracy may also be referred to as “systemerror”.Repeatabilityis defined as the degree to which repetitive measurements on a single system are inagreement. Another way of stating this definition is to say that repeatability is how close a system returnsto a desired location or locations time after time under repeated cycling. Major contributing factors torepeatability are the precision of the bearing ways and the amount of play, or backlash in the system.Repeatability affects how identical parts may differ slightly.There is a direct relationship between system cost, accuracy and repeatability; therefore it is essentialthat the terms are understood. If an application involves a motion stopped by an operator, a positionsensor, or a mechanical stop, then the application requires only repeatability. Similarly, if an applicationrequires that the same location be found time after time as with inlay applications, then only repeatability isneeded. If an application involves cycled point to point motion or exact length motion as with high-precisionparts machining, then both accuracy and repeatability are required. The following diagrams provide a goodvisual representation of these two terms.12 —10 —8 —6 —4 —2 —0 —Case 2: Not Accurate but Repeatable(Mean ¹ Absolute Position, s small)MeanPosition¯¯X3––– Error –––3AbsolutePositionCase 1: Accurate and Repeatable(Mean = Absolute Position, s small)AbsolutePosition=12 —10 —8 —6 —4 —2 —0 —MeanPosition¯¯X
