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)