AppNote - Rotary Table Accuracy

A common misconception is that the resolution of the rotary table must also be its accuracy. For example, if a digital readout displays to four decimal places (0.0001), then it must also be accurate to that same value. That is usually not the case.  Although high resolution is a prerequisite for high accuracy, it does not guarantee it. Consider the two graduated scales:

Both scales have 15 graduations over equal arcs; therefore, both have identical resolutions of 1/15th arc. For arc A the resolution increments are equal; however, for arc B the resolution increments are obviously not the same.  That difference, scale accuracy, is a component of position accuracy, and while both examples have the same resolution, each will provide very different results. 

Accuracy is the difference between the actual position and the position measured by a reference measurement device. Rotary table accuracy is influenced by the feedback mechanism (rotary encoder, drive mechanism (ball screw, lead screw, linear/torque motor), and roundness/parallelism/run-out of the bearing guide-way. IntelLiDrives uses laser interferometers (for linear axes) and autocollimators (for rotary tables) as a reference measurement tools.

Repeatability is defined as the range of positions attained when the rotary table is repeatedly commanded to one location under identical conditions. Uni-directional repeatability is measured by approaching the point from one direction, and ignores the effects of hysteresis within the system. Bi-directional repeatability measures the ability to return to the point from both directions.

Resolution - The smallest possible movement of a rotary table. Also known as step size, resolution is determined by the feedback device and capabilities of the motion system.

Low Accuracy High Repeatability	Low Accuracy Low Repeatability	High Accuracy High Repeatability