Image Credits: Avago, MTS Sensors, and Schneeberger Linear Technology
Linear encoders sense and digitize linear position and movement.
How Linear Encoders Work
A linear encoder is a sensor linked to a scale. The sensor reads the scale and converts position into an analog or digital signal that informs a digital readout. Movement is determined from changes in position with time. This video illustrates an absolute linear/rotary optical encoder.
Video Credit: Renishaw Engineering
Types of Linear Encoders
An absolute encoder provides a unique output signal for each mechanical position. It retains position after power-down. An incremental encoder does not retain its position after power-down because the outputs signals are not unique to any particular position.
As for technology, optical encoders use a light emitter and receiver.Magnetostrictiveencoders read ultrasonic signals generated by the movement of a set of magnets oriented perpendicularly to a ferromagnetic waveguide. Magnetoresistive encoders use a magnetized rotor with alternating north and south poles. The sensor reads the transitions from one pole to another as counts or pulses. An inductive or magnetic linear encoder senses changes in a magnetic field generated by current applied to coils. Other proprietary or specialized encoder technology exists.
Specifications and Features
Measurement range/travel is the distance that can be encoded (e.g., 77 to 4777 mm). Some encoders have a selectable range.
Resolution is the smallest degree of distance measurement that an encoder can make (e.g., 0.072 to 101 μm). This too may be configurable.
Accuracy (better than 4 μm to better than 93 μm) includes considerations such as linearity, hysteresis, temperature, etc.
Absolute encoder code can be gray (only one bit changes with the transition from one measuring step to the next), binary, Binary Coded Decimal, or other.
Incremental encoder signal can be digital: square wave, analog: sine/cosine, single channel, pulse & direction, reference/index, or other.
Electrical/digital output can be analog current, analog voltage, fiber optic, serial, parallel, SSI, or other.
Encoders can be modular (minimal or frame-like housing), linear to rotary (transfers linear motion to rotary encoder), sealed (preventing contamination), or probe-style (a tip is pressed against the physical surface whose movement is measured).
Signal quadrature, a configuration in which two channels carry signals that are 90° out of phase, is available.
Encoders are found in a wide range of applications including: wind power, solar panels, servo motors, packaging machines, machine tools, conveyors, automated storage/retrieval systems, elevators, sheet and web offset presses, medical equipment, robotic systems, food handling equipment, valves/flow metering, overhead cranes, process monitoring equipment, steel making/foundry equipment, textile machinery, tire making equipment, test stands, construction equipment, and transportation.
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Magnetostrictive Position Sensors
Magnetostrictive position sensors are non-contact linear position sensors that use the momentary interaction of two magnetic fields to produce a strain pulse that moves along a waveguide. One field is from a magnet that moves along the outside of the waveguide. The other field is from the waveguide itself.