Bearing Encoder vs Modular Encoder - Making the Right Choice

March 20, 2013

The following is a comparison of Bearing Encoders vs Modular (Bearingless) Encoders for use in Brushless DC Motors.

Construction

Bearing encoders have a shaft that is centered to the body of the encoder on a bearing system.

Modular encoders have a shaft that does not use bearings so the shaft/disk is separate from the body of the encoder.

An examination of traditional bearing less encoder misconceptions:

“No bearing to wear out.”

The modular design relies upon the motor bearing and shaft tolerances to hold the necessary air gap. Motor bearings have typically lower tolerances than that of Encoder bearings. This places the responsibility of holding air gap tolerances on the motor bearings. In encoders with bearings, the encoder bearing holds the air gap. Any excessive endplay or TIR is either held in place by the encoder bearing as the entire encoder assembly moves, not just the disk. The bearing in the encoder is essential for holding the air gap between the disk and sensor.

“Unlimited RPM”

While a bearingless design would allow for unlimited RPM in theory, in practice all encoders have an upper frequency response, which ultimately limits their top speed. The higher the line count, the less RPM required before the frequency limitation is hit.

“Fewer parts means the encoder costs less.”

The initial encoder may be cheaper, but in order to hold tolerances, a higher quality bearing must be used in the motor that the encoder is being mounted on. There are also many “infant death” failures for modular encoders due to the precision that is required to set the disk to sensor air gap. This increases overall cost of using modular encoders in a design.

The Bottom Line

In the end, the decision comes down to whether or not you should take on the burden and responsibility of holding encoder air gap and alignment.

If the application is cost critical, and you feel confident you can handle the technical challenges and efficiency needed to keep from incurring labor costs that make a modular purchase prohibitive, you may be better off with a modular encoder.

If you want the speed and ease of installation, with the added benefit of having the encoder manufacturer hold air gap tolerances for you, a bearing encoder is the right choice..

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