Terminology

Precision: Precision is how close the measured positions are to each other.  this is caused by a combination of:

  • the measuring error (of the encoder)
  • the positioning error (of the stage)


Accuracy: Accuracy is how close a measured position is to the actual (true) value. Stage accuracy is influenced by the feedback mechanism (e.g. encoder), drive mechanism, and trueness of bearing ways.In other words, this is also caused by a combination of:

  • the measuring error (of the encoder)
  • the positioning error (of the stage)

Resolution: (=minimal step size) The smallest possible movement of a system. Also known as step size, resolution is determined by the feedback device and capabilities of the motion system. Theoretical resolution may exceed practical resolution. For example, in a ball-screw-based positioning system, a theoretical resolution of 10 nm can be obtained by combining a 10 mm/rev screw, 1000-line encoder, and an x1000 multiplier. The actual motion system will never be able to make a single 10 nm step due to friction, windup, and mechanical compliance. Therefore, the practical resolution is actually less.

Repeatability: Repeatability is defined as the range of positions attained when the system is repeatedly commanded to one location under identical conditions.
Unidirectional repeatability – The ability of the stage to return to a given point, always coming from the same previously defined point. The value specified is the standard deviation of many moves to the same point.
Bi-directional repeatability – The ability of the stage to return to a given point coming from a random previous point. The value specified is the standard deviation of many moves from random directions to the same point.

Tilt and wobble: Angular measures of the off-axis motion, and these are specified by the three orthogonal components: roll, pitch and yaw. Look at the specific page in the technology section to get more information on these topics.

  • Roll: rotation of the stage around the axial movement direction
  • Pitch: rotation of the stage around the axis, perpendicular on the axial movement, in the same plane as the stage base. Also known as nose dive.
  • Yaw: rotation of the stage around the axis, perpendicular on the axial movement axis, typically the Z-axis.

Abbe error: A consequence of tilt and wobble. It is produced by amplifying any translator angular error by the distance between the plane of travel and point of measurement.

Play: a consequence of looseness of badly made or worn translator parts.

Backlash: Systematic error created by lost motion in the drive mechanism that appears when changing direction.

Image

The error motion of a rotary stage is usually characterised with a displacement indicator (capacitive sensor) targeting a lapped artefact which is mounted to the rotor of the stage. The error motion of a stage is fully characterised by five error motion components, namely:

Radial error motion (x(θ), y(θ)): defined as the error motion perpendicular to the axis of rotation measured at an arbitrary axial location ‘a’. The axial location must always be specified since tilt of the rotor influences the radial error motion.

Axial error motion (z(θ)): defined as the error motion collinear with the axis of rotation.

Tilt error motion (α(θ), β(θ)): defined as the rotation of the rotor about the x- and y-axis. This error motion can be derived from two radial error motion measurements made at different axial locations ‘a’.