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Technical Information techquestions@techno-isel.com 7 2)    Terms and Definitions for Linear Motion Systems There 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, others may not. This section is provided so that such people can use these terms in proper context, to relieve the possibility of design criteria and specifications being too tightly constrained due to lack of effective communication. Having the following terms explained and understood allows for a more efficient design process, and a more cost-effective solution. Point to point accuracy or accuracy is defined as the difference between the statistical mean of a series of measurements and the theoretically correct position.  Another way of stating this is to say that point to point accuracy is the ability to travel to a desired point or series of points with respect to some known reference. Straight line accuracy is the ability of a machine to accurately travel in a straight line with respect to a known reference plane, and the specification refers to the maximum possible deviation from the desired straight line path. Accuracy affects how closely parts are made to specifications.  There are many factors that contribute to the accuracy of a system, but the most significant ones  are the accuracy of the drive mechanism, the accuracy of the motor, and the presence of play, or backlash.  Accuracy may also be referred to as “system error”. Repeatability is defined as the degree to which repetitive measurements on a single system are in agreement. Another way of stating this definition is to say that repeatability is how close a system returns to a desired location or locations time after time under repeated cycling.  Major contributing factors to repeatability 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 essential that the terms are understood.  If an application involves a motion stopped by an operator, a position sensor, or a mechanical stop, then the application requires only repeatability.  Similarly, if an application requires that the same location be found time after time as with assembly applications, then only repeatability is needed.  If an application involves cycled point to point motion or exact length motion as with high-precision parts machining, then both accuracy and repeatability are required.  The following diagrams provide a good visual representation of these two terms. Mean Position ¯¯ X ––– Error ––– Absolute Position Case 1:  Accurate and Repeatable (Mean = Absolute Position, s small) Absolute Position= 12  — 10  — 8  — 6  — 4  — 2  — 0  — Mean Position ¯¯ X 3 12  — 10  — 8  — 6  — 4  — 2  — 0  — Case 2:  Not Accurate but Repeatable (Mean = Absolute Position, s small)