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Though frequently taken for granted, the use of encoders to convert the rotary or linear motion of speed, rate, velocity, distance, position, or direction into information and feed it back into the controller of a motion control system is a centerpiece of advanced inspection systems.

For example, ABTech Manufacturing Inc.(Swanzey, NH), a manufacturer of ultra-precision air bearings and motion systems, had a customer that needed ultra-accurate and stable positioning capabilities for a new system that measured conformal optics. Their solution was to develop a five-axis nanometer/sub-arc second processing platform around advanced air bearing and encoder technologies.

"This was a joint development project with our customer," says ABTech president Ken Abbott. "They developed the concept, controller and software while we designed and built the mechanical system, the multi-axis air bearing platform."

This five-axis optical measurement platform is 53 in wide, 38 in deep and 66 in tall and uses three linear air bearings, two rotary air bearings, and SiGNUM™ ultra high-resolution linear and rotary optical encoders from Renishaw Inc. (Hoffman Estates, IL).

This system requires the confocal probe to be positioned normal (perpendicular) to each surface point being measured. The platform accomplishes its exacting repositioning by coordinated motion in up to five axes.

The next-generation CNC optics system requires position resolution of 5 nm for the three linear axes. Resolution for rotary motion is 0.009 arc sec/count for the C-axis and 0.018 arc sec/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 position repeatability and thermal stability are the most important demands on us for this application," explains Abbott, who had to meet an individual axis position linear accuracy of ±1 μm over full travel of 8 in and rotary accuracy of ±1 arc sec total error over a 360 deg move.

To do this, Abbott called Tim Goggin of Renishaw, who recommended a dual-readhead DSi rotary encoder system and a RELM high accuracy linear encoder system. These encoders provide accuracy better than ±1 μm, ±30 nm cyclic error, and resolution down to 5 nm/0.005 arc sec. These 20 μm scale position encoders use dynamic signal control for ‘fine pitch' performance without the fragility and optical cleanliness constraints of glass encoders. They are dependable in manufacturing environments, with a high tolerance of shock, vibration and temperature to 85 deg C.

The linear encoder system uses a scale of stabilized Invar, a nickel/iron alloy with an exceptionally low coefficient of expansion of ≈ 0.6 μm/m/deg C, 0 deg C to 30 deg C. "We were most concerned about thermal stability," states Abbott. "Most of the machine is fabricated from stainless steel, granite and ceramic to minimize thermal growth. The Invar scale was perfect for this application. The encoder had to be small, and this scale met those needs with a smaller cross section of just 0.059 x 0.591 in."

The scale incorporates an IN-TRAC™ optical reference mark for a bi-directional repeatable datum point across the entire speed and temperature range without increasing the overall system's width. Dual optical limits are position markers that indicate end of travel.

For rotary position accuracy, the DSi (Dual SiGNUM™ interface) and REXM rotary encoder are capable of total installed accuracy of better than ±1 arc sec. The DSi configuration combines two error-correcting SR readheads with an ultra-high accuracy REXM ring/scale and provides a propoZ™ reference (index) position that is completely unaffected by bearing wander or power cycling.

"Located in 180 deg opposition, the two readheads cancel out odd error harmonics, including eccentricity, and compensate for the effect of bearing wander," notes Goggin. "By combining the incremental signals from the two readheads and using this reference mark processing, the DSi appears to the controller as a single, very high accuracy encoder."

The integral ring/scale locks directly to the rotor to eliminate reversal errors, coupling losses, oscillation, shaft torsion and other hysteresis errors. This one-piece stainless steel ring has scale graduations marked directly onto the outer periphery of a thick cross-section that minimize installation errors except eccentricity, which is corrected by the DSi.

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