Encoders Ensure Accurate Measurement of Conformal Optics

When a leading manufacturer of CNC optical manufacturing technology needed accurate and stable positioning for a new measurement system for conformal optics, it turned to ABTech Manufacturing Inc., specialists at custom-made precise air bearings and motion systems. The engineers, in turn, chose new high-resolution linear and rotary optical encoders from Renishaw to meet the positioning resolution requirements. The finished product is a five-axis optical measurement platform with three ABTech linear air bearings, two ABTech rotary air bearings, and Renishaw’s SiGNUM™ series linear and rotary encoders.

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 measured surface point. The platform achieves exact repositioning by coordinated motion in up to five axes. The inspection system requires position 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.

Five-axis system — three linear, two rotary — provides ultra-accurate positioning for high-level automated measurement and process feedback in production of critical lens optics.

Overall volumetric system accuracy is mapped and corrected by the customer, so position repeatability and thermal stability were the most important demands for the application. The  requirements for individual axis position accuracy were:

• Linear — ±1 µm over full travel of 8 in.

• Rotary — ±1 arcsec total error over a 360 degree move

Tim Goggin of Renishaw recommended the SiGNUM rotary and linear system – a dual-readhead DSi rotary encoder and RELM high accuracy linear encoder. These encoders are accurate to better than±1 µm, ±30 nm cyclic error, and resolution down to 5 nm/0.005 arcsec. Through dynamic signal control, the 20 µm scale position encoders offer “fine pitch” performance without the fragility and optical cleanliness constraints of glass encoders. The encoders are dependable in manufacturing environments with high tolerance of shock, vibration and temperature (to 85°C).

The RELM linear encoder system has a scale made of stabilized Invar. This nickel/iron alloy offers low coefficient of expansion (˜0.6 µm/m/°C, 0 °C to 30 °C), which proved crucial to thermal
stability for the metrology system. The 20 µm Invar scale has a small cross section of 1.5 mm x 15 mm (0.059 in. x 0.591 in.).

For rotary position accuracy, ABTech selected the DSi (Dual SiGNUM™ interface) and REXM
rotary encoder system from Renishaw. Capable of total installed accuracy of better than ±1 arc second, the DSi configuration combines two error-correcting SiGNUM™ SR read heads with the high-accuracy REXM ring/scale, and provides a customer-selectable propoZ™ reference (index) position that is unaffected by bearing wander or power cycling. Located in 180° opposition, the two read heads cancel out odd error harmonics, including eccentricity, and compensate for the effect of bearing wander.

SiGNUM DSi/REXM rotary ring encoder from Renishaw enables resolution to 0.018 arcsec/count for B-axis rotary table. Dual readheads cancel out eccentricities, while direct mounting of the ring on the table rotor eliminates reversal errors, coupling losses, and other hysteresis errors.

Once the interface has eliminated eccentricity effects, the only significant errors remaining are minor even-harmonic distortions in installation, graduation and cyclic error (sub-divisional error – SDE). These are exceedingly small, as low as ±0.5 arc second and ±0.03 arc second respectively.

The integral ring/scale locks directly to the rotor, eliminating reversal errors, coupling losses, oscillation, shaft torsion and other hysteresis errors.

To create the desired propoZ™ reference position, the user drives the axis to the chosen angle and simply presses a button. The selected angle is then stored in the DSi’s memory, where its output position is automatically compensated for error by algorithms, ensuring perfect angular repeatability, even if the center of rotation of the axis moves while the DSi is switched off.

While the five-axis platform is substantial at 53 in. wide, 38 in. deep and 66 in. tall, the individual axes are packed full of motors, encoders and air bearing surfaces, Abbott stressed. “The small size of the Renishaw package lets us maximize the size of our air bearing pads, which is critical to achieving high stiffness and accuracy.”

The linear and rotary encoders share the same miniature read heads, just 14.8 mm x 36 mm x 16.5 mm (HxLxW) (0.583” x 1.417” x 0.650”), while the RELM scale is 15 mm wide (0.591”) wide and the REXM scale only 10 mm (0.394”).

PC-based SiGNUM™ software handles calibration and set-up of real-time diagnostics.

Renishaw Inc.
www.renishaw.com/encoders