The Dirty Truth About Magnetic Encoders

A. John Santos, Chief Engineer for Sensors


Let’s say your motor needs a rotary encoder to provide feedback to a controller. Which would you choose: a magnetic or an optical encoder?

In the past, an optical encoder might have been your only choice. Today, you have a second option. Magnetic encoders – once reserved for high-end process-industry applications – are now cost effective for virtually all encoder applications.

Magnetic encoders have benefited from overall advancements in integrated circuit technology. There is a wide range of Hall effect-based high-resolution encoder chips, modular encoders and kit encoders on the market.

Magnetic Encoder Advancements Outpace Optical Technology

The decision to use optical or magnetic technology for your next sensor will likely depend on the environmental and mechanical characteristics of your application. Magnetic encoder technology advancement has been outpacing optical technology for years. This has led to significant improvements in performance and pricing. While optics used to be the only choice for resolutions greater than 1,000 PPR, there are now 1.5 in. modular magnetic encoders and encoder kits available with up to 8,192 PPR.

Optical Encoders: How They Work

Traditional optical encoders have a light source, two or three light sensors, and a glass, metal or plastic code wheel placed between the light source and the sensors. These are the only functional components of an optical encoder. The other components you might find are used to keep contamination like dirt, dust and condensation out of the optical path or used to precisely position the optical disk in the working range between the light source and light sensors. A typical recommended tolerance range for an optical disk is +/- .003 in.

Traditional optical encoders have the optical components packaged in a sealed can with the code wheel precisely positioned using two ball bearings.  The packaged encoder is mounted using a shaft coupling or, in the case of a hollow shaft encoder, using a flexible tether. The housing, bearings and couplings are often the largest and most expensive parts of the assembly.


Magnetic Encoders: How They Work

Modular magnetic encoders can offer the same standard resolutions as optical encoders with greatly improved resistance to environmental conditions such as dirt, dust and humidity. Magnetic encoders consist of a system-on-a-chip encoder chip on a PCB and magnetic target wheel that is set to run with an air gap of between .020 in. and .060 in. above the PCB.

Magnetic encoders typically have much more lax assembly and shaft end play requirements. The ability of magnetic encoders to perform well over a wide range of assembly tolerances have eliminated the need for bearings in most magnetic encoders. This in turn has resulted in eliminating failure modes, lowering component cost and reduced overall axial length may applications.

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