Universal Magnetic Rotary Encoders, Universal Magnetic Position Encoder Includes Linear Hall Sensors
Rotary encoders convert rotary movement or angular position into analogue or digital signals for use in 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 to recognize 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 the system powers down. All other applications can use an incremental encoder. Encoders also can 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, including consumer, automotive, industrial, and medical. Encoders essentially provide one of two functions, enabling a human machine interface (HMI) or a machine-to-machine interface (MMI).
In HMI applications, encoders often are 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.
In MMI applications, encoders invariably are used as part of a feedback control system, where they count spindle revolutions or monitor speed. Many systems feature encoders that close the loop on an HMI input. So in automotive throttle controls, a second encoder is likely to measure engine speed (rpm) to control fuel injection and engine timing to achieve the desired acceleration. Or, it could measure the position of a rudder on a boat to ensure it corresponds to the setting at the helm.
How Do They Work?
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 the sensor detects changes in the 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° 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 (Fig. 1). 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. For example, 1024 positions (or 10-bit resolution) can be achieved with four sensors and 128 poles.
1. A magnetic rotary encoder comprises two poles and two sensors. The second sensor makes it possible to not only detect the direction of rotation but also to interpolate the absolute position of the shaft.
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 photodetector. 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 also can be applied to linear encoders.
Choosing An Encoder
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 part of an 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 rpm, and angular resolution, either quoted as the number of positions per revolution or the equivalent number of bits. Magnetic Rotary Encoders
Of course, if the intended application is going to subject the encoder to extended use at high speeds, 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 types of applications they target.
Bourns makes both magnetic and optical encoders for heavy-duty applications in tough environments such as industrial, medical, and military. The AMS22S single-turn, magnetic rotary encoder is highly resistant to vibration or shock and the ingress of fluid or dust. The robust design’s sturdy mechanical construction resists side loads and axial forces acting on the shaft.
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