Motion system applications: encoders in aerospace and manufacturing

THERE ARE AS MANY ENCODER CHOICES as there are design objectives. When encoders provide position feedback, it’s common to specify encoder resolution exceeding what the application requires as a way to address the partial count uncertainty of digital measurements. Heavy industrial equipment for metal stamping, wind turbines, printing, and steel processing often uses high-speed rotary encoders that can reliably track speed and position even to 40,000 rpm. Other applications benefit from reflective encoders with interpolation for high-resolution measurement on small motors (usually closed-loop steppers) that drive axes on consumer products such as printers.

Other machines need rugged and rated encoders with oversized bearings, specialty seals, and reinforced housings. Industrial-rated encoders work in interior elevators, automated process machinery, and robot joints. In contrast, applications that expose encoders to liquid submersion, washdown, contaminants, shock and vibration, or EMI noise necessitate more rugged versions. These applications include those for off-road equipment, cranes, wind turbines, food-and-beverage machinery, and medical devices. Usually, it’s either the failure of the encoder’s bearing (in the case of rotary setups) or contamination and signal failure that takes the encoder out of commission. For example, wood processing tends to be dusty, while food applications and their cleaning cycles tend to be steamy and force humidity inside encoder housings.

In applications where the design must track position along a linear axis, linear encoders excel. Machine tools make copious use of linear encoders, especially for the manufacture of aerospace parts needing milling and finishing.

Bringing production lines up to speed
One industry that undergoes frequent changes is the automotive industry, as carmakers must respond to evolving customer preferences and regulatory requirements. That means regular upgrades to their production facilities as well.

One U.S. automaker recently decided to retool a Michigan engine plant for a new production program and update an automation system controlling equipment on the shop floor. The automaker hired Canadian integrator DataRealm Inc. to design and implement a new solution. The project included new IT interfaces, new PLCs, updated human-machine interfaces (HMI), new data communications and upgrades to sensors and actuators. The integrator was also responsible for programming and traceability.

The plant was once state-of-the-art with PLCs close to machinery they controlled and some units protected with IP67-rated housings. Communication between controllers and sensors and actuators was through a proprietary networking system. The integrators decided more powerful PLCs and new networking were needed.

The plant upgrade first saw IP67-rated PLCs from Phoenix Contact and an INTERBUS fieldbus network. Current upgrades include the installation of a PROFINET data communications network. PROFINET, a form of industrial Ethernet, will enhance system support of an unlimited smart devices (sensors and actuators) and communication with higher-level manufacturing management systems.

New encoders for the system had to deliver accuracy, reliability, and compatibility with the upgraded communications networks, plus compactness and IP67 ratings or better for installation on existing machinery. POSITAL IXARC absolute rotary encoders met the requirements.

“POSITAL had products that met our specs and supplied adaptor plates that simplified swapping out the old sensors,” said Dave Fortin, President of DataRealm. The availability of POSITAL encoders with both INTERBUS and PROFINET communication interfaces was another advantage.

The plant is successfully up and running with the new production program and the supporting automation system. In fact, it has emerged as the most productive plant of its type in the company.

Encoders help align telescopes doing dark energy research
Another application relies on encoders for rotary and linear position feedback to precisely position a telescope’s optical system. HEIDENHAIN encoders work in the Hobby Eberly Telescope (HET) to help researchers study dark energy. In place at the University of Texas-Austin at the McDonald Observatory, this telescope will fill in the gap of reaching longer distances in the sky than currently possible.

Dark energy is currently understood to be a mysterious force pushing the universe apart. Called cosmic acceleration, the gravitational dark energy force is pushing instead of pulling. The HET (operational since the 1990s) began upgrades in 2013 and should soon offer more insight into dark energy. Nearly-completed equipment at the top of the HET consists of a new optical system and spectrographs. Three HEIDENHAIN linear scales are part of the new optical system’s tracker, and six of the company’s rotary encoders are affixed to an adjoining six-leg hexapod on top of the HET.

“HEIDENHAIN encoders give accurate position measurement on the HET,” said Herman Kriel, HET Project Manager. “This telescope is different from conventional ones, as it does not move. Instead a tracker atop the telescope moves and aligns with a mirror, so must be extremely accurate.”

The new optical system has six axes of freedom and moves some of the optics up and down to focus the telescope and in tip and tilt to keep it aligned with the primary mirror. The rotation stage on top of the hexapod compensates for image rotation during tracks. Using HEIDENHAIN’s three LB 382 linear scales on the X and Y stages and eight EQN 437 rotary encoders on the hexapod and rotation stage, the HET tracker will move ±1.8 m in X and Y directions relative to the mirror, for a total travel of 3.6 m, and ±9.5 ˚ in tip and tilt and ±21.5 ˚ in rotation.