Accuracy and Repeatability

Accuracy and Repeatability

Don't pay for what you don't need.

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 terms.

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.



In the above diagrams, the "spread" of the measurements indicates how repeatabile the positioning is - the more repeatable the smaller the spread. The mean value of the position inicates how accurate the system is - the closer the mean is to the center of the target, the more accurate it is.

To make good parts, it is often sufficient to be very repeatable with moderate accuracy.
To make precision parts it is often necessary to be very repeatable and also have high accuracy.

IMPORTANT TO NOTE:

In many applications where REPEATABILITY is relevant and accuracy is not critical, asking for a high accuracy system adds unnecessary cost. Applications that warrant high accuracy are precision machined parts but applications such as video inspection or dispensing, often only need REPEATABILITY. Even applications such as wafer handling usually only need high repeatability and not accuracy.