Compact encoder boosts resolution, cuts costCompact encoder boosts resolution, cuts cost

Croglio, Switzerland--Encoders are a great way to improve servo-system performance. Until now, however, having an encoder capable of resolutions in the 500-cpr (counts per revolution) range required relatively expensive optical encoders. These devices use a light source, a lens or grating, and an optical sensor, and come in housed or modular designs.

Engineers typically choose between coupling a housed encoder to the motor shaft or assembling modular designs to cut costs and save space. Either choice requires costly assembly time, and results in a relatively fragile feedback device that is sensitive to harsh environments. That's why some engineers prefer magnetic encoders. Magnetic feedback offers immunity to external influences, does not depend on an unreliable light source that decays over time, and is much more rugged.

The trade-off is that until now, commercially available magnetic encoders were only capable of resolutions in the 10 to 32 cpr range--not enough for even moderate servo-system performance. All that has changed since MicroMo engineers developed a proprietary reluctor-wheel design and a custom ASIC (application-specific integrated circuit) that provide a low-cost, compact-magnetic encoder that achieves up to 512 cpr.

Magnetic encoders use a wheel or reluctor with magnetic domains on its edge surface. As the wheel rotates with the motor shaft, Hall elements pick up the change in flux density as each domain passes by. After electronics process and condition the encoder signals, the resulting position feedback is transmitted to the controller.

The design challenges were two-fold, according to MicroMo's VP of Advanced Research and Planning Stephen O'Neil. "The first was understanding how magnetics work in encoder-based systems, and the second was the custom ASIC development.

"Overcoming the limited number of magnetic domains that can be packed on a wheel was key to the project's success," says O'Neil. "And one custom ASIC offers engineers the choice of 16, 64, 128, 256, or 512 cpr."

Called the IE series, these dual-channel encoders with digital outputs are available as integral parts of the 15-, 22-, and 23-mm rare-earth motor series. No assembly, soldering, cable making, or termination additions are required. The signals are CMOS and TTL compatible.

The integrated plastic design has all miniaturized electronics on board, and adds a premium to the base-motor price of about $20 and up, depending on the model. This is typically below the unassembled unit cost of larger, less reliable encoders, according to O'Neil.