Factors Which Affect Accuracy & Repeatability

A common misconception in reviewing accuracy and repeatability in linear motion systems is that they refer to the same thing, however they do not.

The following are the accurate descriptions of each:

Accuracy

  • The ability to achieve a commanded position.
  • The difference between the actual position desired and the position attained.

Repeatability

  • The ability to go to any given spot in a consistent grouping.
  • The range of positions attained when the system is repeatedly commanded to one location under identical conditions.
Accuracy Figure 1
  • Low Accuracy
  • Low Repeatability
Accuracy Figure 2
  • Low Accuracy
  • High Repeatability
Accuracy Figure 3
  • High Accuracy
  • High Repeatability

What factors contribute to accuracy and repeatability positioning errors?

Errors in accuracy and/or repeatability may be derived from the “six degrees of freedom,” which are as follows:

  1. Z-Axis: Up/down motion
  2. Y-Axis: Side-to-side motion
  3. X-Axis: End-to-end motion
  4. Z-Axis Rotation
  5. Y-Axis Rotation
  6. X-Axis Rotation

Knowing whether one, or all, of these six degrees of freedom are required involves a thorough understanding of the application at hand. Within these six degrees of freedom, there are many different elements that can cause variances in accuracy. Some of the elements that may contribute to error in a basic actuator include, but are not limited to:

  • The base framework of the unit: aluminum, steel, granite, etc.
  • Bearing type: plain bearings, v-wheel roller bearings, ball bearings, etc.
  • Driving components: lead screws, ball screws, belts, etc.
  • Load-carrying components: bearing rails, bearings, etc.

When looking at things such as the base unit of an actuator, the key variances to keep in mind are thickness or height (Z-axis), twist (X-axis), and bow/arch (Y-axis). What cannot be forgotten is that elements of other components can cause inaccuracies, so it is imperative that each be evaluated individually. Something such as a lead screw in combination with a nut, which can provide backlash tolerance, is a great example of other contributing factors to consider. Other elements to evaluate include, but are not limited to:

  • Lead screw accuracy
  • Tooling
  • Orientation of the actuator
  • Couplings

If you require assistance in building a linear motion system specific to your application needs, please contact a PBC Linear application engineer.