2009

Renishaw Encoder Improves Positional Accuracy for CNC Optical Maker

When a leading manufacturer of CNC optical manufacturing technology needed ultra-accurate and stable positioning for a new system to measure conformal optics, it turned to ABTech Manufacturing Inc., specialists of custom- made ultra-precision air bearings and motion systems. In order to meet tighter customer specifications for positioning resolution, ABTech found the solution in high-resolution linear and rotary optical encoders from Renishaw. ABTech created a five-axis optical measurement platform featuring three of its linear air bearings, two of its rotary air bearings, and Renishaw’s ultra-high accuracy SiGNUM(TM) series linear and rotary encoders.

alt“This was a joint development project with our customer,” said Ken Abbott, ABTech’s president. “They developed the concept, controller and software, while ABTech designed and built the mechanical system — the multi-axis air bearing platform.”

In operation, a lens is placed on a high-precision B-axis rotary table for measurement by a non-contact confocal imaging probe mounted horizontally on a rotary C-axis air bearing. Confocal imaging performs micro-topographic mapping of the lens geometry and surface.

The system requires that the confocal probe be positioned normal (perpendicular) to each surface point to be measured. The platform created by ABTech accomplishes the exacting repositioning by coordinated motion in up to five axes. The next-generation CNC optics inspection system requires positional resolution of 5 nm for the three linear axes. Resolution for rotary motion is 0.009 arcsec/count for the C-axis and 0.018 arcsec/count for the B-axis. Fully programmable 32-bit Windows-based measuring software drives the non-contact probe to automatically collect micro-topographic data.

“Overall volumetric system accuracy is mapped and corrected by the customer, so positional repeatability and thermal stability was the most important demands on us for this application,” said Abbott. ABTech’s requirements for individual axis position accuracy were:

  • Linear — ±1 μm over full travel of 8 in.
  • Rotary — ±1 arc-sec total error over a 360 degree move

To achieve that accuracy, Abbott turned to Renishaw’s Tim Goggin, who. recommended an advanced SiGNUM rotary and linear system – a high performance dual-readhead DSi rotary encoder and the RELM high accuracy linear encoder. These SiGNUM(TM) encoders provide accuracy better than ±1 μm, ±30nm cyclic error, and resolution down to 5nm/0.005 arcsec. Dynamic signal control enables the 20μm scale position encoders to offer ‘fine pitch’ performance without the fragility and optical cleanliness constraints of glass encoders, says Goggin. The encoders provide outstanding dependability in manufacturing environments with high tolerance of shock, vibration and temperature (to 85°C).

The RELM linear encoder system features a scale of stabilized Invar. This nickel/iron alloy offers exceptionally low coefficient of expansion (≈0.6 μm/m/°C, 0 °C to 30°C). “This being a metrology system, we were most concerned about thermal stability,” said Abbott. “Most of the machine is fabricated from stainless steel, granite and ceramic to minimize thermal growth. The Invar scales were a perfect fit for this application.”

Besides accuracy and reliability, he said, price, ease of use and physical size were key factors for ABTech in selecting position encoders, “The encoder needs to be small,” Abbott emphasized. The 20μm Invar scale met those needs with a smaller cross section than glass scales of just 1.5 mm × 15mm (0.059in. × 0.591in.), along with easier handling and installation without risk of breakage.

The scale incorporates Renishaw’s INTRAC( TM) optical reference mark providing a bi-directionally repeatable datum point across the entire speed and temperature range, without increasing overall system width. Dual optical limits are also available as position markers to indicate end of travel.

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