Error Mapping Considerations

Error mapping is the process of programming an X-Y stage or system to discrete points within its travel range and determining the degree with which the achieved positions correspond the commanded positions. Once the discrepancies are ascertained, future motion to the same point can be program-offset to achieve the proper position results. A table of correction factors is typically accessed automatically during any move within the X-Y plane of motion. Interpolation software algorithms are employed to correct for target locations which do not “exactly” fall on the pre-calibrated points.

A calibrated “glass master” is typically used as a reference during the mapping routine. After the glass has been properly set, the X and Y axes are commanded in “absolute mode” to the many reference fiducials on the glass. Any discrepancies of attaining a “centered image” are corrected by commanding the faulty axis (X, Y, or both) until the image is centered. This offset value is recorded for later use.

After calibration, the controller automatically calculates correction offsets for each location programmed. Targets not falling exactly on pre-calibrated points have their correction offsets determined by interpolating the values of the four closest precalibrated points. It should be noted that this feature can only correct motion to a degree that approaches the repeatability capabilities of a given system.

Additionally, plane-mirror interferometers are often used to calibrate an X-Y motion platform. In this case, two orthogonal mirrors are secured to the moving part of an X-Y stage while the laser source is secured to the common support base. Precise incremental X & Y motion is sensed by the laser system and the delta from commanded is stored for future error correctionrecall by the error mapping software algorithm.