IN THE LOOP: INSPECTION SYSTEMS: ACCURACY IN MOTION.
IN THE LOOP: INSPECTION SYSTEMS: ACCURACY IN MOTION
By Kyndall Brown
WHEREVER THERE IS MOTION, ENCODERS ARE NOT FAR BEHIND. BUT WHEN IT COMES TO ACHIEVING ACCURACY IN MOTION, ENCODERS ARE A MUST.
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[TM] 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 [+ or -]1 ?m over full travel of 8 in and rotary accuracy of [+ or -]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 [+ or -]1 ?m, [+ or -]30 nm cyclic error, and resolution down to 5 nm/0.005 arc sec. These 20 ?m 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.
