Machine Positioning Uncertainty with Laser Interferometer Feedback

The purpose of this discussion is to explain the major contributors to machine positioning uncertainty in systems with laser interferometer feedback near the work point. We will use an example to quantify these uncertainties in a real implementation of a laser-feedback-driven machine.

Laser interferometers are used as a measurement reference for machine correction and accuracy validation in the production of many high precision motion systems. Under controlled environmental conditions, laser interferometer measurement can provide low measurement uncertainty relative to the achievable accuracy of most commonly used motion control devices. As such, when processes require the utmost precision, laser interferometer measurement near the machine’s work point is frequently used as the feedback mechanism for machine control. In these instances, the use of laser interferometry to characterize the machine’s motion is unjustified because the measurement uncertainty of the metrology system is equivalent or higher than the motion error. The accuracy of these machines‘ motion must be equated to an uncertainty in the feedback system’s measurement of the defined work point’s motion.

The purpose of this discussion is to explain the major contributors to machine positioning uncertainty in systems with laser interferometer feedback near the work point. We will use an example to quantify these uncertainties in a real implementation of a laser-feedback-driven machine. This is not meant to be an introduction to measurement uncertainty, laser interferometer feedback, error/uncertainty budgeting, or rigid body error motions. It is meant to be an overview of the level of work point position measurement uncertainty that can be obtained in a specific machine design.