Encoder Understanding Resolution in optical and magnetic Encoders

Autor / Redakteur: Giovanna Monari * / Margit Kuther

How the different types of rotary encoders work and what differentiates the performance of the many types available today.

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Giovanna Monari, Avnet Abacus: „Rotary encoders can be classified in a number of ways.“
Giovanna Monari, Avnet Abacus: „Rotary encoders can be classified in a number of ways.“
(Bild: Avnet)

Rotary encoders are devices that convert rotary movement or angular position into analogue or digital signals for use within measurement or control systems. They can be classified in a number of ways; primarily by the type of output they provide, either absolute or incremental.

The incremental signal consists of two phase-shifted, square-wave signals. The phase shift is required for recognition of the direction of rotation. The absolute signal consists of discreet coded binary values and may be from 4 to 16 bits wide. In application, absolute encoders are required if a particular setting must be recognized and available after a power down of the system.

All other applications can use an incremental encoder. Encoders can also be classified by the sensor technology employed, which may use mechanical contacts or, more likely these days, contactless optical or magnetic sensors.

The applications for encoders are extremely wide-ranging, from consumer through to automotive, industrial and medical. Essentially encoders provide one of two functions, either enabling a human machine interface (HMI) or a machine-to-machine interface (MMI).

Encoders in HMI and MMI applications

In HMI applications, encoders are often encountered in panel controls, where they are commonly regarded as the “digital version of a potentiometer”. A less obvious but increasingly popular use is in control lever or pedal assemblies, replacing mechanical cables or rod linkages e.g. automotive throttle control.

The use of encoders in MMI applications is invariably as part of a feedback control system, where they are used to count spindle revolutions or monitor speed. Many systems feature encoders that close the loop on an HMI input; so in the automotive throttle control example above, a second encoder is likely to be measuring engine speed (rpm) to control fuel injection and engine timing to achieve the desired acceleration.

Another example would be measuring the position of a rudder on a boat to ensure it corresponds to the setting at the helm.

Magnetic encoders use a combination of permanent magnets and magnetic sensors to detect movement and position. A typical construction uses magnets placed around the edge of a rotor disc attached to a shaft and positioned so that the sensor detects changes in magnetic field as the alternating poles of the magnet pass over it.

The simplest configuration would have a single magnet, with its north and south poles on opposite edges of the rotor, and a single sensor. Such a device would produce a sine wave output with a frequency equal to the rotational speed of the shaft.

With a second sensor, set 90 degrees apart from the first and therefore generating a cosine output, it becomes possible to not only detect the direction of rotation but also to interpolate the absolute position of the shaft from the sine and cosine signals.

For incremental encoders, the sinusoidal outputs from the sensors are converted to square waves so the resulting quadrature waveforms can only be encoded to one of four possible angular positions. Greater resolution is achieved by increasing the number of magnetic poles around the rotor and by having more sensors e.g. 1024 positions (or 10-bit resolution) can be achieved with 4 sensors and 128 poles.

Hall effect sensor: a magnetic rotary encoder with two poles and two sensors
Hall effect sensor: a magnetic rotary encoder with two poles and two sensors
(Bild: Avnet)
Optical encoders use a rotor disc made of plastic or glass that is patterned with transparent and opaque areas that can be detected as the disc rotates between a light source and a photo detector.

Like the magnetic encoder, the simplest configuration might use just one sensor and have one half of the disc transparent and the other half opaque; but for higher resolution the disc is usually divided into many more segments (often in concentric rings) with two or more sensors. Again, an appropriate arrangement of rotor pattern and sensor position can provide the quadrature output that characterizes incremental encoders.

To minimize pin count the data from an absolute encoder is usually output serially. And, while most commercial encoders are rotary, the same sensing and coding principles can also be applied to linear encoders.

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