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Understanding Precision In Linear Motion Applications

Every linear application, from simple pick-and-place to circuit board printing, requires some level of precision. This concept, one of the most vital in the world of linear motion, is best understood when broken into two separate terms: repeatability and accuracy.

Accuracy vs Repeatability BullseyesFor example, applications requiring point-to-point positioning desire a linear actuator with a high degree of repeatability for a consistent separation between points. Applications that need end-to-end positioning must have enough accuracy to reach the end of the travel length. And the repeatability might vary at the end when only one motion is necessary.

The bottom line is that precision demands further examination in linear motion applications. So let’s take a closer look at repeatability and accuracy to better understand the differences.

Repeatability. Repeatability refers to the idea of reproducing the same result within a spread of locations. We’ve specified the repeatability of our belt, ball screws and rack-and-pinion drive linear actuators, with values ranging from 50 to 100 microns for the most basic actuators. Because of their rolled thread configuration, ball screw actuators offer better repeatability when compared to belt drive units. This results in a decreased spread in their positioning.

Accuracy. On the other hand, accuracy refers to the bias in the positioning itself towards the prime target. Even though a linear actuator may not be repeatable each time to the same point, the positioning retains its bias towards the target. Further, an actuator may be repeatable to the same location with minimal spread, but completely miss the prime target every time. 

When determining the accuracy of a system, take into account everything from the elasticity of the belt or groundness on the ball screw thread, to the accuracy of the linear rail, the backlash on the gearbox and variations on the motor.