Terms and Definitions for Linear Motion Systems
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 dispensing 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
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.