Compact encoders for motion control

Aerotech’s stages power medical component laser machines, micro-machining centers, industrial robots, fiber optics and photonics systems, vision systems, machine tools, assembly machines, semiconductor equipment, and electronic manufacturing equipment.

The company’s long-time relationship with Renishaw led Aerotech to a compact, precise encoder that enhances measurement simply by being smaller. The Renishaw RLE laser interferometer encoder narrows the distance to the point of measurement, minimizing a key contributor to measurement uncertainty on precision machines.

“Our clients require high precision and give us some pretty demanding process specifications,” says William Land, business development manager at Aerotech. “We certainly have industry-specific requirements, but quite often we get customer-specific requirements as well. When supplying today’s markets, meeting those individual demands is crucial.”

Aerotech produces many custom-designed motion systems that require high precision and high throughput. In one example, the company’s stages measure components that ultimately serve as part masters for a high-value production process. Several issues can stand in the way of precision measurement, including environmental changes, thermal expansion, optics quality, and noise in the feedback loop. However, one major concern and a big influence on measurement uncertainty – Abbe offset – can be countered with the right encoder.

The Abbe offset is determined by the distance between the measuring feedback device (in this case, the encoder) and the point of measurement. Measurement uncertainty increases the further the sensor is from the point of interest. Some stage manufacturers bury the encoders in the stage due to size and design constraints, increasing the distance from the point of measurement to the point of interest. Buried encoders do not detect tip, tilt, roll, pitch, or yaw of the other devices on the stage, causing measurement uncertainty. A more compact encoder can be placed closer to the point of measurement, but measurement accuracy is also a concern.

“To avoid this altogether, we use the RLE20 laser interferometer encoders. They consistently perform well, and with them, the offset is minimized, allowing the measurement point to get as close to the tool point or point of interest as possible,” Land says. “The Renishaw encoders’ measurement axes nominally intersect with the camera axis at the point of measurement. There is practically zero offset between where the lasers are measuring and where the measurement on the part is being taken. The RLE compact laser heads give us good design flexibility while maintaining good stability.”

Small, simple, accurate

The Renishaw encoders allow direct integration into assemblies. The RLE20-DX-DG laser interferometer encoders used by Aerotech have a high digital resolution and are a double-pass homodyne with a maximum resolution setting of 10nm. With laser interferometer technology, the wavelength is the basic unit that represents the measuring device. Therefore, the wavelength stability is directly correlated to the measurement repeatability.

Land explains that Aerotech technicians take these encoders with them into the field.

“With the ease of setup and use, we take these devices out to recalibrate machines. Because of the size of the read-head and the size of the overall system, our guys put them in a case and take them to the factory to calibrate in-house,” Land says.

Customers in many markets require accurate motion control to position and test sensors in equipment such as interferometers, autocollimators, vision systems, and antennas. These high-precision applications are continually pushing the limits of precision measurement.

In turn, Aerotech works with partners who can provide process knowledge to customers, complement its capabilities, and deliver high-performance automation control solutions.

“We consistently exceed the specifications of customers with Renishaw’s encoders,” Land says. “Our philosophy of building superior motion control systems starts with designing, manufacturing, and purchasing superior components for use in our technology. We try to offer a best-in-class experience to our customers and attribute part of our continuing success to the technology advancements that companies like Renishaw offer.”

Aerotech Inc.
www.aerotech.com

Renishaw Inc.
www.renishaw.com