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XY Grid Encoders Offer Higher Precision and Lower Cost for Position Control, Channel, SEMI, January 7, 1999, Vol. 12, No. 1 (click to go)

High Accuracy 360° Rotation Stage and Metrology Solution for NIST

A research and integration program was awarded to develop, test and deliver a high accuracy rotation stage for X-ray diffractometry applications to the National Institute of Standards and Technology (NIST). The task was accomplished with a design that adapted several standard components to accurately align the independent X-ray sample and detector axes to a fixed inertial reference and developed a metrology system that measures rotation angle with a resolution of better than 0.027 arc-sec over one revolution.

Innovative Design

The completed system integrated two independently controllable 300-mm diameter rotary tables from Anorad (PLR-300) in a stacked configuration for the X-ray sample and detector axes. Each axis uses the sum of two diametrically opposed linear encoder sensors from OPTRA (NanoScale Model B) whose signal is obtained from a metal tape encoder scale wrapped around the circumference of each table. These encoders provide positioning information in an A-quad-B format from a newly designed signal processing board to a closed-loop servo controller (an Anorad IDAC-3U). The use of two encoders to measure rotation angle for each table serves the two-fold purpose of providing continuity at the abutting of the encoder scale as well as completely canceling table errors to the first order. Each axis also has an OPTRA home optical sensor to provide a stable reference position as well as the appropriate mounting hardware and brackets for an X-ray sample and detector.

Results

Measured system performance is:

The results of this work achieved a high accuracy rotation system suitable for accurate X-ray diffractometry measurements. The developed metrology systems are applicable to other high-accuracy, closed-loop rotational measurements and stage control.

Precision Diamond Turning Lab Includes Submersed NanoScales

The Stanford Quiet Hydraulics Lab utilizes hydraulic means to deal with vibration and temperature problems in precision machining. Hydraulic bearing and actuators without seals can totally eliminate backlash and/or stiction. With temperature controlled oil, uniform temperature distribution throughout the machine can be achieved. Submersing the NanoScale sensors in the temperature controlled oil was achieved with a protective housing. This special effort assisted in reducing thermal measurement error to a negligible level.

High Accuracy XY Metrology in a Manufacturing Environment

Although originally designed to operate in a “clean” environment, OPTRA’s customer desired to use the NanoGrid system in a manufacturing environment. The difference in environments made it more difficult to control and eliminate some contaminants that resulted from the manufacturing process. The customer also had a need to synchronize our high-speed measurements with their built-in clock.

Working closely to customer specifications, OPTRA made custom modifications to both the NanoGrid encoder and hi resolution electronics to solve these problems. The resulting customized NanoGrid system now works to within an accuracy of +/- 30 nanometers with contaminants of up to 50 microns falling on the optical grid. The NanoGrid is integrated into the customers’ system which sells for in excess of $1 million.