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Encoder sensors are a type of mechanical motion sensor that create a digital signal from a motion. It is an electro-mechanical device that provides users (commonly those in a motion control capacity) with information on position, velocity and direction. There are two main types of encoder: linear and rotary. Here, we look at encoder sensors in more depth.
Encoder sensors have become a widely used class of sensors where feedback information from a moving mechanical system is required. It is a device that can provide precise information on the speed, direction and positioning of a piece of mechanical equipment. In recent years, encoders have become a lot more sensitive and tough with higher resolutions at a lower cost, and as a result are now widely used in many industries.
From an industry perspective, encoder sensors are used across the automotive, consumer electronics, medical, military, manufacturing and scientific instrument industry sectors. In terms of specific applications, encoder sensors can be found in printers, food processing, robotics, material handling, axis controllers, medical scanners, dispensing pumps, military-grade antennas, drilling machines and telescopes, to name but a few.
The difference between the two different types of encoder is the way in which they respond to motion in a given area, and the clue is in their names. Linear encoder sensors measure a motion along a linear path, whereas a rotary encoder sensor responds to rotational motion. Both types often employ magnetic principles (a magnetic scale or magnetic field) to detect any changes. The sensor will either detect changes in the magnetic field or magnetic position and this provides an output which represents a speed, position or directional change.
Each class of sensor can be further broken down into absolute and incremental sensors. Absolute encoders use a series of pulses to measure the position and speed of the equipment, whereas an absolute encoder uses bit configurations to directly track positions. So, the type of equipment being analyzed, and how it moves, determines what type of encoder sensor is needed.
In a linear encoder a magnetic sensor passing over a magnetic scale. As the sensor moves along this scale, it detects changes in the magnetic field which are proportional to the measuring speed and the displacement of the sensor. As linear sensors only detect changes in the magnetic field, external factors such as light, debris or oil have no effect on the sensing capabilities, and as a result they are often used in harsher environments. Optical linear encoders are not as widely used, but use parallel beams of light on a glass scale to generate sinusoidal wave outputs that are detected using a photodetector.
There are many key components to linear encoder sensors, including the scanning unit, sensor unit, transducer and a transmissive/reflective scale. Overall, the linear encoder converts the motion into either a digital or analog signal and this can be used to determine the positional change over time.
Rotary encoders can be magnetic or non-magnetic in nature. In a magnetic rotary sensor, the sensor is passed over a rotating disc of alternating (north and south) magnetic regions. The sensor detects the smalls changes in the magnetic field either via the Hall effect (change in voltage compared the deflection of the electrons in the magnetic field) or the magneto resistive effect (a change in resistance caused by the magnetic field).
Electronic rotary encoder sensors are a little more complex and are typically controlled via the rotation of a shaft which is connected to the circuitry of the encoder. The shaft is connected to the encoder via a part known as the hub. When the shaft rotates, it causes the disc (which contains both solid and transparent lines) to rotate across the circuitry of the encoder. The circuitry contains light-emitting diodes (LEDs) that can be spotted using a photodiode. The speed of the rotation is dependent upon the speed of the shaft attached to the encoder. Each concentric ring in the rotary encoder has its own light source to identify each line in the rotating disc. The signal from the detectors is then converted into an output that provides feedback on the position or velocity of the sensor.