IN THE LOOP: ACCURACY IN MOTION

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.

Both the linear and rotary encoders use rugged 0.583 in tall x 1.417 in long x 0.650 in wide IP64 sealed readheads, dynamic signal processing for excellent reliability, and ultra-low cyclic error (<±30 nm). Unique patented optics scan and average the contributions from many scale periods and effectively filter out dust, dirt and other contamination. The non-contact readheads ride above the scale to eliminate friction, hysteresis and wear.

Encoder technology also provides high levels of accuracy on CMMs for laser scanning inspection. The exact movement of a robotic arm, for instance, can be controlled by determining the position of individual pivot joints through the use of data gathered by an optical rotary encoder.

For example, consider the intrinsic accuracy of the RCA Robot CMM Arm from Metris (Rochester Hills, MI). This 7-axis robot scanner, designed for on-site 3D laser inspection, increases scanning productivity and flexibility through encoder technology and stiff carbon fiber axes that achieve absolute measurement accuracy within an inspection volume of 4.2 m diameter.

The arm can quickly produce repeatable and accurate part-to-CAD measurements wherever repetitive inspection is performed on a series of parts or part variants.

Video inspection systems also benefit from the use of encoders. For example, Starrett's Galileo AV1824 Video Measurement System uses encoder technology with a resolution of 0.5 µm (0.0000039 in) for accuracy in its multi-sensor measuring capabilities of vision, touch probe, and laser scanning.

High accuracy is ensured by a stable granite base design within 0.00060 in overall at E1=5.0+15L/1,000. The encoder in this system enables users to obtain, store, manage and distribute precision measurement data directly from the lab or the shop floor.

Without question, encoders have become ubiquitous in inspection processes throughout the metalworking industry. That's one reason we take these sophisticated devices for granted.

Whether it's the heavy-duty encoders that are not affected by the harsh conditions of a steel mill, or the absolute encoders with multi-turn capabilities used in gantry systems, or the compact motor-mounted encoders needed to monitor motor speed and motion control position in precise factory automation, one thing is certain: wherever there is motion, encoders are not far behind. But when it comes to accurate motion, encoders are a must.

– – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – –

Kyndall Brown is the assistant editor of Fabricating & Metalworking magazine. Contact her at 866-832-8476 or [email protected].

ABTech Manufacturing Inc., 126 Monadnock Highway, Swanzey, NH 03446, 603-358-6431, Fax: 603-358-0196, www.abtechmfg.com.

Renishaw Inc., 5277 Trillium Boulevard, Hoffman Estates, IL 60192, 847-286-9953, Fax: 847-286-9974, www.renishaw.com.

Metris, 1577 Star Batt Drive, Rochester Hills, MI 48309, 248-853-6907, Fax: 248-853-6945, www.metris.com.

The L.S. Starrett Company, 121 Crescent Street, Athol, MA 01331, 978-249-3551, Fax: 978-249-8495, www.starrett.com.