Understanding Resolution in optical and magnetic Encoders

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The EMS22A from Bourns for digital output

Rotary encoders: devices that convert rotary movement or angular position into analogue or digital signals for use within measurement or control systems.
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For absolute encoding with a digital output, Bourns offers the EMS22A, which is also a non-contacting magnetic encoder. This has 1024-position angular resolution with the data output serially as a 10-bit code.

It is also suited to harsh environments and, because it uses a ball bearing, it can operate up to 10,000 rpm. Factory ordering options allow for 3.3 or 5V supply, clockwise or anti-clockwise coding, different bushing types and various shaft style/length and terminal configurations. Its target applications are industrial automation, robotics and automotive.

Bourns also manufactures optical encoders for incremental applications where a simple 2-channel quadrature output, for interfacing directly to microcontrollers without A/D conversion, is sufficient to measure rotational position changes or speed. The EN series includes a ball-bearing bush option, supporting shaft speeds to 3,000 rpm and a rotational life of 200 million revolutions or a regular spring-loaded bush (for optimum feel) that is good for 300 rpm and 10 million revolutions.

Different ordering codes allow a choice of resolution from 25 to 256 positions (i.e. 8-bit) as well as various mechanical and electrical termination options. Bourns’ EM14 optical encoder, with push switch, is clearly designed for less demanding applications, with an operating temperature range from –40 to 70 °C, a maximum speed of 120 rpm, a life of 2 million revolutions (at 30 rpm) and a maximum positional resolution of just 64 pulses/revolution.

These optical encoders are primarily targeted at HMI applications like panel-mount controls in test & measurement equipment and professional & consumer audio products.

Vishay favours magnetic encoders

Vishay Intertechnology is another leading manufacturer of magnetic rotary encoders. All provide either a linear analogue or PWM output from a single signal wire, like the Bourns AMS22S.

The Vishay 981HE is targeted at harsh environments and intended for throttle position sensing so has the widest operating temperature range (–40 to 125 °C) and enhanced performance in terms of vibration, shock and immunity to electromagnetic interference. It is rated for 120 rpm and a life of >10 million cycles (for the D-shaft model).

The Vishay 34THE is a ten-turn servo encoder, which means it can measure angles up to 3600 degrees with 12-bit resolution. It is targeted at “off-road” steering and actuator applications. The servo model has ball bearings to provide long life (50 million cycles). Next is Vishay’s 50SHE single-turn, 120 rpm, servo-mount model. This is their smallest unit, with a body measuring just 12.7mm in diameter and 19.5mm in length. Like the 981HE it is targeted at harsh environments but ball bearings provide a longer 50 million-cycle life.

Finally, there is the 351HE 120 rpm, single-turn, bush-mount encoder. This has a more conventional 22mm diameter body and generally a lower specification compared to the 50SHE.

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