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.
“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.
For rotary position accuracy, ABTech selected the DSi (Dual SiGNUM(TM) interface) and REXM rotary encoder system, Renishaw’s highest accuracy SiGNUM(TM) rotary encoders. Capable of total installed accuracy of better than ±1 arcsec, the DSi configuration combines two error-correcting SiGNUM(TM) SR readheads with an ultra-high-accuracy REXM ring/scale and provides a customer- selectable propoZ(TM) reference (index) position, which is completely unaffected by bearing wander or power cycling.
“Located in 180° opposition, the two readheads cancel out odd error harmonics, including eccentricity, and compensate for the effect of bearing wander,” explained Goggin. “By combining the incremental signals from the two SiGNUM(TM) readheads and using Renishaw’s patented reference mark processing, the DSi appears to the controller as a single, very high accuracy encoder.”
The other element in the accuracy equation, the REXM ring, features a thick cross-section to minimize all installation errors except eccentricity, which is corrected by the DSi. Once the interface has eliminated the effects of eccentricity, the only significant errors remaining are minor even-harmonic distortions in installation, graduation and cyclic error (subdivisional error – SDE). These are exceedingly small, according to Goggin, as low as ±0.5 arc second and ±0.03 arc second respectively. “When the REXM ring is used with the DSi, it is possible to realize a total installed accuracy of better than ±1 arc second,” he says. “In fact, DSi/REXM rotary encoders have achieved total installed accuracies below ±0.25 arc seconds, depending on ring size.”
The one-piece REXM stainless steel ring is available with diameters from 52 mm to 417 mm (2.047 in. to 16.417 in.) and has scale graduations marked directly onto the outer periphery. The integral ring/scale locks directly to the rotor, eliminating reversal errors, coupling losses, oscillation, shaft torsion and other hysteresis errors that plague enclosed encoders.
To create the desired reference position, the user drives the axis to the chosen angle and simply presses a button. In this way, Renishaw’s propoZ(TM) technology makes aligning the reference process far easier, quicker, and accurate. The selected angle is then stored in the DSi’s memory, where its powerful algorithms automatically compensate for output position errors, thus ensuring perfect angular repeatability even if the center of rotation of the axis moves while the DSi is switched off.
The linear and rotary encoders share SiGNUM(TM) encoder design features, including rugged IP64 sealed readheads, dynamic signal processing for excellent reliability, and ultra-low cyclic error (<±30 nm). Patented Renishaw optics scan and average the contributions from many scale periods and effectively filter out non-periodic features such as dust, dirt and other contamination. The non-contact readheads ride above the scale, thus eliminating friction, hysteresis and wear.
Renishaw’s compact readhead design met ABTech’s tight size constraints. While the five-axis platform is substantial at 53in. wide, 38in. deep and 66in. tall, the individual axes are packed full of motors, encoders and air bearing surfaces, Abbott stressed. “The small size of the Renishaw package allows us to 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 miniaturized readheads, just 14.8mmH × 36mmL × 16.5mmW (0.583 in. × 1.417 in. × 0.650 in.), while the RELM scale is just 15mm wide (0.591in.) wide and the REXM scale only 10mm (0.394in.).
Powerful PC-based SiGNUM(TM) software provides comprehensive calibration and set-up optimization with real-time diagnostics. The SiGNUM(TM) Si interface connects to the PC via a USB connector. Analysis of the SiGNUM(TM) Si interface is available to users at all times, providing information on system configuration such as resolution, clock frequency, error and warning outputs.
The software simplifies encoder installation and provides an innovative indicator that enables fine adjustment of the readhead pitch angle, which is useful for readhead set-up on small diameter rings. Integral LEDs on the readhead and interface provide quick, visual feedback to help optimize set-up and real-time system diagnosis.