Blog: Definions in Precision in Motion Control - Repeatability
Effectively automating any industrial operation, test procedure, or prototype process involving motion requires determining the movement precision needed to yield acceptable results.
A motion system ideally positions to a given target point with some small amount of tolerance or uncertainty. However, motion system performance is specified by many factors affecting performance.
Repeatability, positioning accuracy and resolution primarily determine precision. Understanding these parameters, their causes, and their influence on system performance is essential in determining the capability needed in a given application and specifying the appropriate motion components.
Definition: The tolerance within which a positioner returns to a particular location. Unidirectional repeatability defines performance for approaches from only one side of the target. Bidirectional Repeatability defines the performance for approaches from either side of the target.
Contributing factors: System friction, Torsional stiffness, Load, Acceleration, Backlash, Motion performance.
Repeatability, the most fundamental standard of system performance, defines the variation in results for a series of moves or, more analytically, the width of the dispersion about the mean for a significant number of positioning trials. Repeatability, a statistical quality, is commonly defined for a normal distribution by the dispersion width corresponding to a number of standard deviations.
Usually, three-standard deviation repeatability (3 Sigma) is specified. Consider, for example, a positioning system with a repeatability specification of 0.0001". For 3 Sigma specification, any series of identical movements will fall within a dispersion width of 0.0001" with 99.74 confidence. By way of comparison, 2 Sigma equates to a 95.44 confidence, while 6 Sigma corresponds to a 99.9997 confidence interval. Frequently, motion systems are only required to demonstrate consistency or minimal variability to provide uniformity of the resulting product. In such cases, repeatability is the only attribute necessary to satisfy the precision requirement.
Unidirectional repeatability involves approaching the target position from only one side. It is affected by non-constant static friction, termed" stiction," and the degree of torsion stiffness in the drive train. Stiction gives rise to movement characterized by a breakaway jump as force is applied to initiate motion, while inadequate torsion stiffness causes windup, which is motion input without a corresponding output displacement.
A high level of unidirectional repeatability is relatively easy to achieve since backlash, the motion lost on reversal that contributes to bi-directional repeatability, does not affect unidirectional movement. Of course, approaching targets from a single direction sacrifices throughput times. Bidirectional repeatability is more demanding.
A high degree of bidirectional repeatability presupposes high-level unidirectional repeatability. Tolerances between drive train elements (lead screws/nuts, meshed gears, multi-piece couplings, etc.) must be very closely controlled and preloads must be adjusted in order to limit backlash, which may be considered as a mechanical dead band in the motion system. In programmable motion systems,
it is possible to remove backlash by means of a small incremental move before making normal moves in a given direction. Minimizing either the number of interacting drive train components (gear trains, belts/pulleys, etc.) or the play or looseness between components (which develops as components wear) will also reduce backlash.
In rolled ball screw systems, a backlash is typically less than 0.001", while high-precision ground ball screw drives exhibit backlash of less than 0.0001".
When high performance and maximum production throughput are required, bidirectional repeatability will usually be required as well.
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