A linear encoder is a sensor,

transducer A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another. Transducers are often employed at the boundaries of automation, measurement, and con ...

or readhead paired with a scale that encodes position. The sensor reads the scale in order to convert the encoded position into an

analog Analog or analogue may refer to: Computing and electronics * Analog signal, in which information is encoded in a continuous variable ** Analog device, an apparatus that operates on analog signals *** Analog electronics, circuits which use analog ...

digital signal A digital signal is a signal that represents data as a sequence of discrete values; at any given time it can only take on, at most, one of a finite number of values. This contrasts with an analog signal, which represents continuous values; a ...

, which can then be decoded into position by a digital readout (DRO) or motion controller. The encoder can be either ''incremental'' or ''absolute.'' In an incremental system, position is determined by motion over time; in contrast, in an absolute system, motion is determined by position over time. Linear encoder technologies include optical, magnetic, inductive, capacitive and

eddy current Eddy currents (also called Foucault's currents) are loops of electrical current induced within conductors by a changing magnetic field in the conductor according to Faraday's law of induction or by the relative motion of a conductor in a magnet ...

. Optical technologies include shadow, self imaging and

interferometric Interferometry is a technique which uses the '' interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber o ...

. Linear encoders are used in metrology instruments, motion systems,

inkjet printer Inkjet printing is a type of computer printing that recreates a digital image by propelling droplets of ink onto paper and plastic substrates. Inkjet printers were the most commonly used type of printer in 2008, and range from small inexpensi ...

s and high precision machining tools ranging from digital calipers and

coordinate measuring machine A coordinate measuring machine (CMM) is a device that measures the geometry of physical objects by sensing discrete points on the surface of the object with a probe. Various types of probes are used in CMMs, the most common being mechanical and l ...

s to stages, CNC mills, manufacturing gantry tables and semiconductor

stepper A stepper is a device used in the manufacture of integrated circuits (ICs) that is similar in operation to a slide projector or a photographic enlarger. ''Stepper'' is short for step-and-repeat camera. Steppers are an essential part of the comp ...

Physical principle

Linear encoders are transducers that exploit many different physical properties in order to encode position:

Scale/reference based

Optical

Optical linear encoders dominate the high resolution market and may employ shuttering/ moiré,

diffraction Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...

holographic Holography is a technique that enables a wavefront to be recorded and later re-constructed. Holography is best known as a method of generating real three-dimensional images, but it also has a wide range of other applications. In principle, i ...

principles. Optical encoders are the most accurate of the standard styles of encoders, and the most commonly used in industrial automation applications. When specifying an optical encoder, it's important that the encoder have extra protection built in to prevent contamination from dust, vibration and other conditions common to industrial environments. Typical incremental scale periods vary from hundreds of micrometers down to sub-micrometer.

Interpolation In the mathematical field of numerical analysis, interpolation is a type of estimation, a method of constructing (finding) new data points based on the range of a discrete set of known data points. In engineering and science, one often has ...

can provide resolutions as fine as a nanometer. Optical Linear Encoder

Light sources used include infrared

LEDs A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (cor ...

, visible LEDs, miniature light-bulbs and

laser diode The laser diode chip removed and placed on the eye of a needle for scale A laser diode (LD, also injection laser diode or ILD, or diode laser) is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with ...

Magnetic

Magnetic linear encoders employ either active (magnetized) or passive (variable reluctance) scales and position may be sensed using sense-coils,

Hall effect The Hall effect is the production of a voltage difference (the Hall voltage) across an electrical conductor that is transverse to an electric current in the conductor and to an applied magnetic field perpendicular to the current. It was dis ...

magnetoresistive Magnetoresistance is the tendency of a material (often Ferromagnetism, ferromagnetic) to change the value of its electrical resistance in an externally-applied magnetic field. There are a variety of effects that can be called magnetoresistance. So ...

readheads. With coarser scale periods than optical encoders (typically a few hundred micrometers to several millimeters) resolutions in the order of a micrometer are the norm.

Capacitive

Capacitive linear encoders work by sensing the capacitance between a reader and scale. Typical applications are digital calipers. One of the disadvantages is the sensitivity to uneven dirt, which can locally change the

relative permittivity The relative permittivity (in older texts, dielectric constant) is the permittivity of a material expressed as a ratio with the electric permittivity of a vacuum. A dielectric is an insulating material, and the dielectric constant of an insul ...

Inductive

Inductive technology is robust to contaminants, allowing calipers and other measurement tools that are coolant-proof. A well-known application of the inductive measuring principle is the Inductosyn.

Eddy current

US Patent 3820110, "Eddy current type digital encoder and position reference", gives an example of this type of encoder, which uses a scale coded with high and low permeability, non-magnetic materials, which is detected and decoded by monitoring changes in inductance of an AC circuit that includes an inductive coil sensor. Maxon makes an example (rotary encoder) product (the MILE encoder).

Without scales

Optical image sensor

The sensors are based on an image correlation method. The sensor takes subsequent pictures from the surface being measured and compares the images for displacement. Resolutions down to a nanometer are possible.

Applications

There are two main areas of application for linear encoders:

Measurement

Measurement application include

coordinate-measuring machine A coordinate measuring machine (CMM) is a device that measures the geometry of physical objects by sensing discrete points on the surface of the object with a probe. Various types of probes are used in CMMs, the most common being mechanical and ...

s (CMM),

laser scanner Laser scanning is the controlled deflection of laser beams, visible or invisible. Scanned laser beams are used in some 3-D printers, in rapid prototyping, in machines for material processing, in laser engraving machines, in ophthalmological las ...

caliper A caliper (British spelling also calliper, or in plurale tantum sense a pair of calipers) is a device used to measure the dimensions of an object. Many types of calipers permit reading out a measurement on a ruled scale, a dial, or a digital d ...

s, gear measurement, tension testers, and

digital read out A digital readout (DRO) is a numeric display, usually with an integrated keyboard and some means of numeric representation. Its integral computer reads signals generated by linear encoders or (less frequently) rotary encoders installed to track ...

s (DROs).

Motion systems

Servo controlled motion systems employ linear encoder so as to provide accurate, high-speed movement. Typical applications include

robotics Robotics is an interdisciplinary branch of computer science and engineering. Robotics involves design, construction, operation, and use of robots. The goal of robotics is to design machines that can help and assist humans. Robotics integrat ...

machine tool A machine tool is a machine for handling or machining metal or other rigid materials, usually by cutting, boring, grinding, shearing, or other forms of deformations. Machine tools employ some sort of tool that does the cutting or shaping. Al ...

s, pick-and-place PCB assembly equipment; semiconductors handling and test equipment, wire bonders, printers and

digital press Digital printing is a method of printing from a digital-based image directly to a variety of media. It usually refers to professional printing where small-run jobs from desktop publishing and other digital sources are printed using large-format ...

es.

Output signal formats

Incremental signals

Linear encoders can have analog or digital outputs.

Analog

The industry standard, analog output for linear encoders is sine and cosine quadrature signals. These are usually transmitted differentially so as to improve noise immunity. An early industry standard was 12 μA peak-peak current signals but more recently this has been replaced with 1V peak to peak voltage signals. Compared to digital transmission, the analog signals' lower bandwidth helps to minimise EMC emissions. Quadrature sine/cosine signals can be monitored easily by using an oscilloscope in XY mode to display a circular Lissajous figure. Highest accuracy signals are obtained if the Lissajous figure is circular (no gain or phase error) and perfectly centred. Modern encoder systems employ circuitry to trim these error mechanisms automatically. The overall accuracy of the linear encoder is a combination of the scale accuracy and errors introduced by the readhead. Scale contributions to the error budget include linearity and slope (scaling factor error). Readhead error mechanisms are usually described as ''cyclic error'' or ''sub-divisional error (SDE)'' as they repeat every scale period. The largest contributor to readhead inaccuracy is signal offset, followed by signal imbalance (ellipticity) and phase error (the quadrature signals not being exactly 90° apart). Overall signal size does not affect encoder accuracy, however, signal-to-noise and jitter performance may degrade with smaller signals. Automatic signal compensation mechanisms can include ''automatic offset compensation (AOC), automatic balance compensation (ABC) and automatic gain control (AGC)''. Phase is more difficult to compensate dynamically and is usually applied as one time compensation during installation or calibration. Other forms of inaccuracy include signal distortion (frequently harmonic distortion of the sine/cosine signals).

Digital

A linear

incremental encoder An incremental encoder is a linear or rotary electromechanical device that has two output signals, ''A'' and ''B'', which issue pulses when the device is moved. Together, the ''A'' and ''B'' signals indicate both the occurrence of and direction ...

has two digital output signals, A and B, which issue quadrature squarewaves. Depending on its internal mechanism, an encoder may derive A and B directly from sensors which are fundamentally digital in nature, or it may interpolate its internal, analogue sine/cosine signals. In the latter case, the interpolation process effectively sub-divides the scale period and thereby achieves higher measurement resolution. In either case, the encoder will output quadrature squarewaves, with the distance between edges of the two channels being the resolution of the encoder. The reference mark or index pulse is also output in digital form, as a pulse which is one to four units-of-resolution wide. The output signals may be directly transmitted to a digital

incremental encoder interface An incremental encoder is a linear or rotary electromechanical device that has two output signals, ''A'' and ''B'', which issue pulses when the device is moved. Together, the ''A'' and ''B'' signals indicate both the occurrence of and direction o ...

for position tracking. The major advantages of linear incremental encoders are improved noise immunity, high measurement accuracy, and low-latency reporting of position changes. However, the high frequency, fast signal edges may produce more EMC emissions.

Absolute reference signals

As well as analog or digital incremental output signals, linear encoders can provide ''absolute'' reference or positioning signals.

Reference mark

Most incremental, linear encoders can produce an index or reference mark pulse providing a datum position along the scale for use at power-up or following a loss of power. This index signal must be able to identify position within one, unique period of the scale. The reference mark may comprise a single feature on the scale, an autocorrelator pattern (typically a Barker code) or a

chirp A chirp is a signal in which the frequency increases (''up-chirp'') or decreases (''down-chirp'') with time. In some sources, the term ''chirp'' is used interchangeably with sweep signal. It is commonly applied to sonar, radar, and laser syste ...

pattern. Distance coded reference marks (DCRM) are placed onto the scale in a unique pattern allowing a minimal movement (typically moving past two reference marks) to define the readhead's position. Multiple, equally spaced reference marks may also be placed onto the scale such that following installation, the desired marker can either be selected - usually via a magnet or optically or unwanted ones deselected using labels or by being painted over.

Absolute code

With suitably encoded scales (multitrack, vernier, digital code, or pseudo-random code) an encoder can determine its position without movement or needing to find a reference position. Such absolute encoders also communicate using serial communication protocols. Many of these protocols are proprietary (e.g., Fanuc, Mitsubishi, FeeDat (Fagor Automation),

Heidenhain Dr. Johannes Heidenhain GmbH is a privately owned enterprise located in Traunreut, Germany that manufactures numerical controls for machine tools, as well as mechatronic measuring devices for length and angle. Their linear and angle encoders a ...

EnDat, DriveCliq, Panasonic, Yaskawa) but open standards such as BiSS are now appearing, which avoid tying users to a particular supplier.

Limit switches

Many linear encoders include built-in limit switches; either optical or magnetic. Two

limit switch In electrical engineering, a limit switch is a switch operated by the motion of a machine part or the presence of an object. A limit switch can be used for controlling machinery as part of a control system, as a safety interlock, or as a counter en ...

es are frequently included such that on power-up the controller can determine if the encoder is at an end-of-travel and in which direction to drive the axis.

Physical arrangement and protection

Linear encoders may be either ''enclosed'' or ''open''. Enclosed linear encoders are employed in dirty, hostile environments such as machine-tools. They typically comprise an aluminium extrusion enclosing a glass or metal scale. Flexible lip seals allow an internal, guided readhead to read the scale. Accuracy is limited due to the friction and hysteresis imposed by this mechanical arrangement. For the highest accuracy, lowest measurement hysteresis and lowest friction applications, open linear encoders are used. Linear encoders may use transmissive (glass) or reflective scales, employing Ronchi or phase gratings. Scale materials include chrome on glass, metal (stainless steel, gold plated steel,

Invar Invar, also known generically as FeNi36 (64FeNi in the US), is a nickel–iron alloy notable for its uniquely low coefficient of thermal expansion (CTE or α). The name ''Invar'' comes from the word ''invariable'', referring to its relative lac ...

), ceramics (

Zerodur Zerodur (notation of the manufacturer: ZERODUR®), registered trademarkof Schott AG, is a lithium-aluminosilicate glass-ceramic produced by Schott AG since 1968. It has been used for a number of very large telescope mirrors including GTC, Keck ...

) and plastics. The scale may be self-supporting, thermally mastered to the substrate (via adhesive or adhesive tape) or ''track'' mounted. Track mounting may allow the scale to maintain its own

coefficient of thermal expansion Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kineti ...

and allows large equipment to be broken down for shipment.

Encoder terms

* Resolution *

Repeatability Repeatability or test–retest reliability is the closeness of the agreement between the results of successive measurements of the same measure, when carried out under the same conditions of measurement. In other words, the measurements are take ...

Hysteresis Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of ...

Signal-to-noise ratio Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in de ...

noise Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference aris ...

jitter In electronics and telecommunications, jitter is the deviation from true periodicity of a presumably periodic signal, often in relation to a reference clock signal. In clock recovery applications it is called timing jitter. Jitter is a signific ...

* Lissajous figure * Quadrature * Index/reference mark/datum/ fiducial * Distance coded reference marks (DCRM)