A. John Santos, Chief Engineer
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
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
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
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
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