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Accuracy and Repeatability in a Motion System

March 1, 2017

Accuracy is described as how well a positioning system makes a true relative move in reference to an absolute 3D location. Repeatability is a drive mechanism’s ability to return to the same position multiple times under identical conditions.

Accuracy & Repeatability

In essence, if we lived in a perfect world, when a positioning system makes a 1.0 inch (25.4mm) move, it truly moves 1.0 inches (25.4mm). However, there are mechanical errors associated with every positioning system which may have to do with the linear bearings, structure, drive mechanism or other components. These errors will cause the positioning system to be less than perfect when making moves. This will result in move distances that will be something different than what is truly desired.

The linear bearing and structure inaccuracies can cause a positioning system to move something other than what is desired. Therefore, Lintech provides these errors in the overall “position accuracy” catalog values for a given table. Lintech also provides flatness and straightness specifications for each table series. These values can be used as a general gauge to the overall linear bearing and structural quality of a positioning table. The better these values are, the better the accuracy and repeatability of the positioning table.

The difference between accuracy and repeatability is widely misunderstood. Accuracy in a positioning system is much harder to achieve than repeatability, and it is also more expensive to accomplish. Applications like part insertion pick & place, and simple inspection stations require good repeatability, not necessarily accuracy. Why? Because once you have programmed the electronic motion controller and mechanical positioning table to position a load to a given location, you are typically more concerned as to how well the system can return to that location. Repeatability of the overall system becomes important for the application. The inaccuracies in the above systems can be offset by instructing the electronic motion controller to move a little bit more, or a little bit less. On the other hand, microscope X-Y inspection stands, semiconductor wafer probing machines, and coordinate measuring machines require both good repeatability and high accuracy. Why? Because for these applications to be successful, they need to be very accurate (in the truest sense) when making a move from one location to another. Many positioning system applications in the real world require high repeatability, not necessarily high accuracy.

Contact Lintech for guidance on selecting components for targeting accuracy and/or repeatability goals.

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