Understanding Resolution in optical and magnetic Encoders

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Choosing the right encoder for the application

From a user’s perspective, the technology behind an encoder (magnetic or optical) is only relevant if it determines the performance that can be achieved. Particularly when employed as MMI, the encoder undergoes mechanical stress, and has to function also while on fast rotation.

Therefore, having determined whether incremental or absolute encoding is required, the key selection criteria are rotational speed, measured in revolutions per minute (rpm), and angular resolution, either quoted as the number of positions per revolution or the equivalent number of bits.

Of course, if the intended application is going to subject the encoder to extended use at high speeds then some consideration of life expectancy is appropriate i.e. choosing an encoder with suitable high-performance ball bearings. Beyond this, normal considerations of quality, reliability and price will apply.

Rather than considering these factors from an abstract position, it is easier to look at a number of representative products from various manufacturers to understand their advantages in the type of applications they are intended for.

Products from various manufacturers and their advantages

Bourns manufactures a wide range of encoders, featuring both magnetic and optical types. It places considerable emphasis on quality and reliability, especially for heavy-duty applications in tough environments such as industrial, medical and military.

The AMS22S product is a single-turn, magnetic rotary encoder that is highly resistant to vibration or shock and the ingress of fluid or dust. The robust design, shown in an exploded view in the picture on the left page, features a sturdy mechanical construction to resist side loads and also axial forces acting on the shaft.

The encoder uses a programmable quad Hall Effect IC to measure rotational angle, which can output either as a linear analogue voltage or a PWM waveform, in both cases with 12-bit resolution. These are factory pre-set options, as is the output voltage range, which can be programmed between 1 to 99% of V_DD and the effective electrical angle, which can be defined between 10 and 360 degrees in 10-degree increments.

From an environmental and mechanical perspective, the AMS22S is specified for an operating temperature from –40 to 125 °C with a rotational life of 50 million revolutions at speeds up to 120 rpm. Target applications are controls (valves, pedals, steering), geared DC servo motor systems and camera pan/tilt positioning.

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