A linear encoder is a sensor, transducer or readhead paired with a scale that encodes position. The sensor reads the scale in order to convert the encoded position into an analog or digital signal

Digital signal

A digital signal is a physical signal that is a representation of a sequence of discrete values , for example of an arbitrary bit stream, or of a digitized analog signal...

, which can then be decoded into position by a digital readout (DRO) or motion controller.
The encoder can be either incremental or absolute. Motion can be determined by change in position over time. Linear encoder technologies include optical, magnetic, inductive, capacitive and eddy current. Optical technologies include shadow, self imaging and interferometric. Linear encoders are used in metrology instruments, motion systems and high precision machining tools ranging from digital calipers and coordinate measuring machines to stages, CNC Mills, manufacturing gantry tables and semiconductor steppers.

Physical principle

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

Optical

Optical linear encoders http://www.microesys.com/m2/index.html, http://www.renishaw.com/en/6433.aspx dominate the high resolution market and may employ shuttering / Moiré

Moiré pattern

In physics, a moiré pattern is an interference pattern created, for example, when two grids are overlaid at an angle, or when they have slightly different mesh sizes.- Etymology :...

, diffraction

Diffraction

Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...

 or holographic

Holography

Holography is a technique that allows the light scattered from an object to be recorded and later reconstructed so that when an imaging system is placed in the reconstructed beam, an image of the object will be seen even when the object is no longer present...

 principles.
Typical incremental scale periods vary from hundreds down to sub-micrometre and following interpolation

Interpolation

In the mathematical field of numerical analysis, interpolation is a method of constructing new data points within the range of a discrete set of known data points....

 can provide resolutions as fine as a nanometre.
Light sources used include infrared LEDs

Light-emitting diode

A light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...

, visible LEDs, miniature light-bulbs and laser diode

Laser diode

The laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common type of laser diode is formed from a p-n junction and powered by injected electric current...

s.

Magnetic

Magnetic linear encoders http://www.rls.si/default.asp?prod=LMencoders employ either active (magnetized) or passive (variable reluctance) scales and position may be sensed using sense-coils, Hall Effect

Hall effect

The Hall effect is the production of a voltage difference across an electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current...

 or magnetoresistive

Magnetoresistance

Magnetoresistance is the property of a material to change the value of its electrical resistance when an external magnetic field is applied to it. The effect was first discovered by William Thomson in 1856, but he was unable to lower the electrical resistance of anything by more than 5%. This...

 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 disadvantage is the sensitivity to uneven dirt, which can locally change the relative permittivity.

Inductive

Inductive technology is robust to contaminants, allowing calipers and other measurement tools that are coolant-proof http://www.mitutoyo.com/pdf/ABS1813-293.pdf. A popular application of the inductive measuring principle is the Inductosyn http://www.ruhle.com/bar_scale.htm. In effect it is a resolver

Resolver

Resolver may refer to:* Resolver , a type of rotary electrical transformer used for measuring degrees of rotation* Resolver , an album by the band Veruca Salt...

 unwound into a linear system. The Spherosyn encoder http://www.newall.com/LEDs/operation.htm is based on the principle of electromagnetic induction and uses coils to sense nickel-chrome ball-bearings mounted within a tube.

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 http://www.maxonmotor.com/downloads/Flyer_EC6_MILE_e_03.09.pdf makes an example (rotary encoder) product (the MILE encoder).

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 http://www.intacton.com/us/products/INTACTON/OpticalMotionSensors_Context/LengthSpeedSensors_Optical_Context_Technology_PopupBase.html. Resolutions down to 1 nm are possible. http://www.mitutoyo.com/pdf/1976_MICSYS.pdf

Measurement

Measurement application include coordinate-measuring machine

Coordinate-measuring machine

A coordinate measuring machine is a device for measuring the physical geometrical characteristics of an object. This machine may be manually controlled by an operator or it may be computer controlled. Measurements are defined by a probe attached to the third moving axis of this machine...

s (CMM), laser scanners, calipers, gear measurement http://www.wenzel-cmm.co.uk/Industries.asp?SE=9, tension testers http://www.instron.co.uk/wa/product/Tension-Testers.aspx and Digital read out

Digital read out

A digital read out is a small computer usually with an integrated keyboard and some means of numeric representation. It reads the signals generated by the linear encoder installed to several machine's axes used to keep track of workpiece position or the tool's position...

s (DROs).

Motion systems

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

Robotics

Robotics is the branch of technology that deals with the design, construction, operation, structural disposition, manufacture and application of robots...

, machine tool

Machine tool

A machine tool is a machine, typically powered other than by human muscle , used to make manufactured parts in various ways that include cutting or certain other kinds of deformation...

s, pick-and-place

SMT Placement Equipment

SMT component placement systems, commonly called pick-and-place machines or P&Ps, are robotic machines which are used to place surface-mount devices onto a printed circuit board...

 PCB assembly equipment; semiconductors handling and test equipment, wire bonders

Wire bonding

Wire bonding is the primary method of making interconnections between an integrated circuit and a printed circuit board during semiconductor device fabrication. Although less common, wire bonding can be used to connect an IC to other electronics or to connect from one PCB to another...

, printers and digital presses.http://global.oce.com/products/productionprinting/digitalpresses/color/default.aspx

Analog

The industry standard, analog output for linear encoders is sine and cosine quadrature signals.
These are usually transmitted differentially

Differential signaling

Differential signaling is a method of transmitting information electrically by means of two complementary signals sent on two separate wires. The technique can be used for both analog signaling, as in some audio systems, and digital signaling, as in RS-422, RS-485, Ethernet , PCI Express and USB...

 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

Electromagnetic compatibility

Electromagnetic compatibility is the branch of electrical sciences which studies the unintentional generation, propagation and reception of electromagnetic energy with reference to the unwanted effects that such energy may induce...

 emissions.

Quadrature sine/cosine signals can be monitored easily by using an oscilloscope in XY mode to display a circular Lissajous Figure

Lissajous curve

In mathematics, a Lissajous curve , also known as Lissajous figure or Bowditch curve, is the graph of a system of parametric equationswhich describe complex harmonic motion...

.
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

Many linear encoders interpolate the analogue sine/cosine signals in order to sub-divide the scale period, providing a higher measurement resolution.
The output of the interpolation process is quadrature squarewaves – the distance between edges of the two channels being the resolution of the encoder.
The reference mark or index pulse will also be processed digitally and will be a pulse, usually one to four units-of-resolution wide.

The major advantage of encoders with built-in interpolation and digital signal transmission is improved noise immunity. However, the high frequency, fast edge speed
signals may produce more emc emissions.

Incremental encoders with built-in digital processing make it possible to transmit position to any subsequent electronics such as a position counter.

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

Chirp

A chirp is a signal in which the frequency increases or decreases with time. In some sources, the term chirp is used interchangeably with sweep signal. It is commonly used in sonar and radar, but has other applications, such as in spread spectrum communications...

 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 be selected - usually via a magnet or optically.

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

Communications protocol

A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...

 are proprietary – Fanuc, Mitsubishi, EnDat, DriveCliq, Panasonic, Yaskawa – but open standards such as BiSS http://www.biss-interface.com/ 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 switches 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 / 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

Vasco Ronchi

Vasco Ronchi was an Italian physicist known for his work in optics. He was born on 19 December 1897 in Florence, Italy. Along with Enrico Fermi, he was a student of Luigi Puccianti...

 or phase gratings

Diffraction grating

In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...

.
Scale materials include chrome on glass, metal (stainless steel, gold plated steel, Invar

Invar

Invar, also known generically as FeNi36 , is a nickel steel alloy notable for its uniquely low coefficient of thermal expansion . The name, Invar, comes from the word invariable, referring to its lack of expansion or contraction with temperature changes.It was invented in 1896 by Swiss scientist...

), ceramics (Zerodur

Zerodur

Zerodur®, a registered trademark of Schott Glass Technologies, 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 Keck I and Keck II. With its very low coefficient of expansion it can be used to...

) and plastics.
The scale may be self supporting, be 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 and allows large equipment to be broken down for shipment.

Encoder terms

Resolution

Repeatability

Repeatability

Repeatability or test-retest reliability is the variation in measurements if they would have been taken by a single person or instrument on the same item and under the same conditions. A less-than-perfect test-retest reliability causes test-retest variability. Such variability can be caused by, for...

Hysteresis

Hysteresis

Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...

Signal-to-noise ratio

Signal-to-noise ratio

Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...

 / Noise

Noise (electronics)

Electronic noise is a random fluctuation in an electrical signal, a characteristic of all electronic circuits. Noise generated by electronic devices varies greatly, as it can be produced by several different effects...

 / Jitter

Jitter

Jitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may be observed in characteristics such as the frequency of successive pulses, the signal amplitude, or phase of...

 

Lissajous Figure

Lissajous curve

In mathematics, a Lissajous curve , also known as Lissajous figure or Bowditch curve, is the graph of a system of parametric equationswhich describe complex harmonic motion...

 

Quadrature

Index / Reference Mark / Datum / Fiducial

Fiducial

* In law the term "fiducial" means "of or pertaining to a fiduciary".* In imaging technology, a fiduciary marker or fiducial is an object used in the field of view of an imaging system which appears in the image produced, for use as a point of reference or a measure.* "Fiducial" is also used for...

Distance Coded Reference Marks (DCRM)

Book

• David S. Nyce: Linear Position Sensors: Theory and Application, New Jersey, John Wiley & Sons Inc. (2003)
• Walcher Hans: Position Sensing: Angle and Distance Measurement for Engineers, Butterworth Heinemann (1994)