Optical encoders glossary
Find definitions of popular technical optical encoder terminologies.
Abbe Error is an error mechanism where angular errors in a rotary axis are magnified by distance from the axis.
An absolute position is one which is complete in itself and is defined independently of any other position or value. Absolute encoders come in three main types: true absolute, pseudo-absolute and battery-powered absolute.
Position determined immediately on switch-on
No battery back-up
No movement necessary
Also known as ‘distance-coded'.
Encoder must move a short distance to determine absolute position
Encoder scale has reference marks at unique distances apart; when the readhead moves over two adjacent reference marks, the controller can calculate the absolute position from the unique spacing between these references marks
Fundamentally an incremental encoder with reference mark capability, this type of absolute encoder uses a battery to keep the encoder constantly powered and reading position so that the absolute position is never lost, even when the host system is powered down.
How close to the real value a measured position is.
Not to be confused with Resolution or Repeatability.
Note: The word precision is often used to describe accuracy in non-technical English conversations. However, in metrology, precision actually means repeatability.
Digital interface for ATOM, offering interpolation factors up to 2000.
Automatic Gain Control. A signal processing function that ensures a consistent 1 Vpp signal amplitude.
In an incremental encoder, this is the output that is asserted when certain undesirable conditions occur. The different alarm signals available for each readhead are listed in its datasheet.
Alarm conditions can be:
- Low signal (all readheads have low signal error)
- High signal
- Excessive Lissajous offset
The alarm can be line driven (single-ended or differential) or 3-state (commonly known as tri-state).
RESOLUTE outputs an alarm when it cannot correctly determine absolute position.
A continuously variable physical quantity.
When relating to encoders, the term Analogue normally refers to 1Vpp or 11µA signals that are interpolated by the servo drive or controller.
Note: The US spelling of this word is Analog. This is often the preferred spelling in the computer industry.
Measurement of an angle.
This can be done using a complete encoder scale such as a ring or disc. Partial-arc measurements can be carried out by wrapping a piece of linear tape scale around a drum or shaft.
Resolution of an encoder when translated into angular units.
For example, 1 nm linear resolution on a 200mm ring is equivalent to 0.0020625 arc seconds.
Commonly-used units of angular resolution include:
- arc seconds
- arc minutes
- micro radians
- grads (1 gradian = 1/400 of a whole turn = 9/10 of a degree)
- mils (1 mil=1/6400 of a whole turn)
An encoder for measuring angle. Also called an ‘angular encoder'.
The term rotary encoder to describe all encoders that measure angle. Angle encoders are defined as having an accuracy of better than +/- 5 arc seconds and a line count above 10000. Rotary encoders have specifications that fall outside this category.
Automatic Offset Control. A signal processing function that independently adjusts the offset for the output sine and cosine signals. This ensures that the output Lissajous is round rather than oval.
1 arc second is 1/3600 of a degree.
1 degree = 60 arc minutes = 3600 arc seconds.
A range of miniature optical incremental encoders from Renishaw, providing leading-edge dirt immunity, signal stability and reliability. ATOM is the world's first miniature encoder to use filtering optics with Auto Gain Control (AGC) and Auto Offset Control (AOC), also found in the compact TONiC™ encoder range, delivering excellent metrology performance and class-leading accuracy.
The ATOM readhead offers speeds to 20 m/s and resolutions to 1 nm. Scale options include a range of linear scales in stainless steel and glass and rotary RCDM glass discs in diameters ranging from 17 mm to 108 mm, all with a choice of either 40 µm or 20 µm scale pitch.
The box size helps define the spacing between distance-coded reference marks.
Distance-coded reference marks are available with a variety of formats, but the most common is known as the box method. This places periodic reference marks at a fixed distance (these form the box), with a third reference mark at a unique distance in-between.
1) To determine, check, or correct the accuracy of a measurement system.
In encoder terms, this means to compare the position reported by the encoder against a laser, gauge or other known article.
2) To set the incremental signal level and reference mark phasing in a TONiC, SiGNUM or ATOM encoder.
This phrase usually refers to the frequency of the clock on the input of the receiving electronics (normally the drive or controller).
Each clock cycle, the input electronics look for a state-change on the line receivers. If a state-change occurs, the count is incremented or decremented accordingly.
If the output of the encoder is faster than the clock on the input electronics, 2 states can change in 1 cycle, which confuses the quadrature decoder.
Note that digital filters are sometimes applied to the input. These remove noise glitches, but also reduce the effective clock frequency of the receiving electronics.
All TONiC, SiGNUM and ATOM encoders and high-resolution versions of RG2/RG4 have digital outputs that are clocked. This means the interpolator checks the Lissajous and changes the state of the digital output if necessary, once per clock cycle.
A variety of clock frequencies are available and our datasheets include recommendations for clock frequencies of the receiving electronics, including allowances for slew in the cable and line receivers, etc. For instance, a 20 MHz TONiC interface actually has an internal clock of about 16 MHz.
Note that this is different to Retiming.
This is another phrase for Clock Frequency.
A component readhead is a product designed to be embedded inside an OEM product. They often feature much reduced casework and interface circuitry compared with a complete sealed conventional readhead. Component readheads normally require more applications engineering from the customer, such as extra shielding and interpolation is often performed externally.
RGH34 and RoLin are examples of component readheads.
The ‘brain' of the machine that controls movement and operations.
A wide range of controller products are available. Many are multi-purpose, but some are dedicated to specific tasks:
CNC controllers (Computer Numerically Controlled) are optimised for machine tool applications, for instance. Many controllers include complex algorithms that improve the performance of the machine.
Modular systems, such as Delta Tau's UMAC can be expanded with accessory cards to exactly meet a customer's requirements.
‘Controller' is often used as a generic term, sometimes incorrectly being used to describe a servo amplifier (drive).
Coefficient of Thermal Expansion
CTE describes how much a material expands in a linear direction when temperature is increased. Normally the figure is expressed in µm/m/°C or ppm/°K.
Note that this is actually a complex subject. For example, materials have different CTE figures at different temperatures, so the quoted figure is normally specified over a limited temperature range around 20 °C.
Cyclic Error is another term for Sub-Divisional Error.
The term datum can refer to several different things:
- A reference mark
- The position where a scale with independent CTE (RTLC, for example) is locked to the substrate
- A defined zero position on the scale or on the machine
- A calibration standard
Signals or information that can have only 2 discrete states: high or low.
In an encoder, ‘digital' normally refers to the outputs of a digital encoder. These signals are arranged in quadrature, as described on all Renishaw encoder datasheets.
Some people consider digital signals to be more noise-immune than analogue signals because any disturbance to the signal level is removed when the signal is received. Other people argue that analogue signals are lower-frequency, so more filtering can be applied.
Note that a drawback of digital encoders is that they will always have some compromise between speed and resolution.
The ability of an encoder to continue to read position over dirt and contamination.
Dirt immunity comes from 2 sources: the optical scheme and Auto Gain Control electronics.
Renishaw incremental encoders use filtering optics, which are tuned to see only 1 period, which is the scale period. Dirt and contamination will always have a different period to this, so it is rejected by the encoder. Crucially, the Lissajous signals are not offset by contamination.
Auto Gain Control boosts or reduces the signal electronically, to ensure the most consistent Lissajous possible.
Distance-coded reference marks are placed along an encoder scale at unique distances apart; when the readhead moves over two adjacent reference marks, the controller can calculate the absolute position from the unique spacing between these references marks.
Electrical integration describes the connection of the encoder to the receiving electronics. This includes power supply, grounding/shielding and signals.
It is essential to check that the outputs from the encoder will be compatible with the inputs of the receiving electronics.
Incorrect grounding/shielding is the most common cause of encoder problems. Shorts or excessive noise between 0V and Earth often cause noise issues, miscounting or masked reference marks.
It is important to make sure that the power supply has sufficient current capacity to power the encoder. Don't forget voltage drop along the cables!
ELECTRICAL NOISE IMMUNITY
The ability of a product to continue working in noisy electrical environments.
Encoders can be subjected to a variety of electrical noise types:
- Electro-magnetic interference can be induced or coupled into the cable or readhead
- Noise is often present on the 5V power supply
- Noise can also be present on the machine earth
Careful electronics design of the encoder helps overcome the undesirable effects of these noise sources.
EMI = Electro-Magnetic Interference
This is interference (noise) that is present in the region around the encoder.
EMI noise is often generated by the following:
- Fast switching currents in motor cables
- Bad connections that spark
- Poorly-shielded switches or contactors
- Bad earth connections or bad power supplies
- Welding, spark erosion or other noisy operations happening near the machine
In general an encoder is a device or process that converts data from one format to another.
In position sensing, an encoder is a device that measures position and passes that information in a suitable format to a drive or controller.
FASTRACK is a patented scale-mounting track system that is used with RTLC or RTLA scale.
Unlike most track systems, FASTRACK is made from hard stainless steel, so it is much more immune to accidental damage than soft aluminium extrusions. FASTRACK is also quick and easy to install.
Track systems have several benefits:
- They allow the scale to be easily replaced in the field
- They allow the scale to expand/contract according to its own CTE, independently of the track or substrate
- They allow long scales to be temporarily removed from big machines when they are sectioned for transit
Filtering is the rejection of signals, vibrations, or radiation of certain frequencies while allowing others frequencies to be sensed.
In position encoders, filtering is often used for the following purposes:
- Filtering optics reject frequencies other than the scale period
- Filtering of electrical signals helps remove noise and reduce jitter
- Filtering of the power supply helps remove noise components, allowing the system to work more consistently and reliably
A cable that is supplied unterminated, with bare wires at the end. These allow customers to easily fit their own preferred connector.
Flexible Printed Circuit
This is a flat, flexible cable that is used with small zero-insertion force connectors. FPC cables have very low bend forces but their ultimate flex life is often much lower than standard cables so FPC cables are generally not recommended for dynamic applications. FPC cables are also available with shielding.
Flat Panel Display
The arrangements that connect the machine to ground. Also known as Earthing.
It is important to note that grounding is an essential part of the electrical integration of the encoder: poor grounding arrangements, such as shorts or noise between 0V and earth, are one of the most common causes of encoder problems.
Hysteresis is the time lag of a response behind a change in the inputs that cause the response.
Examples of hysteresis in encoder applications include:
- When an encoder scale is track-mounted to a substrate, as the substrate is thermally cycled the differential thermal expansion of the scale and the friction in the mounting system will cause the ends of the scale to come to rest at a slightly varying positions.
- Electrical hysteresis inside a readhead means that an indicated position will occur at a slightly different place in the forwards and reverse direction.
- Enclosed encoders have a slight dwell when reversing direction. This is known as reversal error.
An absolute position is one which is complete in itself and is defined independently of any other position or value. Absolute encoders come in three main types: true absolute, pseudo-absolute and battery-powered absolute.
An incremental encoder is one which outputs signals that indicate relative motion only – the absolute position of the axis can only be determined by the drive or controller which combines this relative position with a known reference position, such as a signal from a reference mark.
Incremental encoders cannot report absolute position upon power-up – a reference mark must be read before absolute position can be calculated. Incremental position signals can count in both directions, incrementing or decrementing the relative position information accordingly.
Industry Standard refers to certain specifications that are common across the industry.
For example, voltage-driven analogue signals should be 1Vpp, which is the established industry standard. Digital signals should be RS422 compliant.
Note that industry standards refer to the specifications, but they do not define quality. It is possible to have two encoders that meet the industry standards for signal size, but one significantly outperforms the other.
An electronic device that processes signals or carries out some other operation.
Serial communication protocols, such as BiSS or DRIVE-CLiQ, are often described as an interface, i.e. a connection between two parts.
A device that converts analogue signals into digital signals.
For position encoders, interpolators are often used to convert analogue sine and cosine outputs from an incremental encoder into a digital representation of the same signals.
A wide variety of interpolators are commercially available, providing a range of interpolation quality and speeds.
IN-TRAC is the name given to the optical reference mark feature on Renishaw scales, which is directly embedded into the incremental graduations (scale markings).
IN-TRAC reference marks are far more immune to yaw de-phasing than reference marks that are placed alongside the incremental graduations.
Invar is a nickel-iron alloy that has a very low CTE, of about 1.2 ppm/°C.
Renishaw provide encoder scale made of an alloy called ZeroMet™, which is a form of Invar which has been specially selected for its particularly high stability.
Ingress Protection, also known as International Protection Rating. This defines the sealing of an electrical enclosure.
IP ratings have two digits: the first digit refers to dust ingress and the second digit refers to water ingress. For example, IP64 describes a rating of 6 for dust protection and a rating of 4 for water protection.
IP ratings are defined in international standard IEC 60529.
NEMA publish protection ratings that are similar to the IEC standard, but the numbering system is different and the NEMA standards also include corrosion resistance and gasket aging.
The amount of positional noise output by an encoder when it is not moving.
This figure is normally quoted in RMS, but there are many ways to measure positional noise; the bandwidth of the measurement is particularly crucial.
Encoders with lower jitter can hold position better and generate less heat in linear motors. They will also exhibit smoother velocity control at low speeds.
Light Emitting Diode
Coloured LEDs that indicate signal level, reference mark phasing, CAL/AGC status and a variety of other encoder status or diagnostics signals.
Outputs from an encoder that indicate that the readhead has reached the end of travel.
Single limits have one signal that show that the readhead has reached an end of the axis. The drive or controller cannot distinguish which end of travel has been reached.
Dual limits output a different signal depending on which end of travel has been reached; on Renishaw encoders these are known as the ‘P' or ‘Q' end of travel.
Motion or a shape in a straight line.
A method of displaying sine and cosine signals so the output describes a circular form.
When encoder outputs are displayed in this way it is possible to easily determine many characteristics of the encoder operation, such as signal level and signal quality.
A unit of length.
1 micrometre = 0.001 millimetres = 1000 nanometres
The symbol for micrometre is µm
Mega Hertz, a unit of frequency.
1 MHz = 1 million cycles per second
A unit of length
1 nanometre = 0.001 micrometres = 1000 picometres
A nanometre is about the length of 10 carbon atoms.
The index grating in an encoder readhead acts in a similar way to an objective lens and the nodal point is the position about which the interference fringes detected in the head are formed – if the scale (or readhead) rotates about this point then the fringes at the photodetector do not move.
Many encoder scales are installed with kinks or slightly uneven surfaces and this can lead to measurement errors. Renishaw encoders such as ATOM have a nodal point on the surface of the scale and therefore the scale can be tilted and no such undulation error is introduced.
In many other readhead types the scale acts as the index grating and the nodal point is above the surface of the scale. In this case any scale undulations will make the fringes move across the photodetector leading to a false position reading.
An undesired electrical disturbance in a circuit, degrading the useful information in a signal.
The structure of a part numbering system. Literally, the structure of a name.
A type of encoder that has no contact between the readhead and scale. Also known as exposed by some companies.
Optical encoders are those that use light to measure position.
Signals that are emitted by the encoder readhead during operation.
Printed Circuit Board
The distance between adjacent markings on an encoder scale. 20 micron pitch scale typically has a 10 micron wide dark line and a 10 micron wide bright line.
Sometimes referred to as scale period.
The readhead reads and interprets the scale's positional information using optical, magnetic, inductive or capacitive techniques and outputs positional data using electrical signals.
An interpolator box from Renishaw that takes 1Vpp analogue encoder signals as an input and gives digital quadrature outputs.
An interface box from Renishaw that takes 3-phase inputs from an RGH25F or RGH20F readhead and gives either 1Vpp analogue outputs or digital outputs.
A datum position along an axis.
The phrase reference mark can be used to describe:
- The physical reference mark actuator, such as the reference mark magnet or the IN-TRAC™ optical feature.
- The reference mark output signal from the readhead/interface.
A family of low-expansion, high-accuracy scales from Renishaw.
These scales are made from ZeroMet, a low-expansion nickel-iron alloy that is a highly stable form of Invar.
- RELM: scale with a reference mark in the centre
- RELE: scale with a reference mark at one end
- RELA: scale with absolute code
The ability a an encoder to keep functioning correctly over time and usage.
Measures of reliability include:
- MTTF: Mean Time To Failure
- MTTFd: Mean Time To dangerous Failure
- MTBF: Mean Time Before Failures
Reliability may also be used to refer to an encoder's ability to tolerate contamination and other non-ideal conditions during its lifetime.
The ability of the encoder to report the same position each time it arrives at a certain point along the axis.
Also sometimes known as reproducibility, scatter or precision.
A single-track open optical true absolute encoder from Renishaw.
The smallest measurement step output by an encoder: this is the minimum distance that the encoder must move to change its output by one count.
Resolution is sometimes confused with accuracy and repeatability. It can be smaller than the noise level of the encoder.
Renishaw's range of encoders with 20 micron pitch scales and a magnetic reference mark. The range includes the following models:
Renishaw's range of encoders with 40 micron pitch scales and a magnetic reference mark. The range includes the following models:
The Renishaw nomenclature for encoder readheads in the RG2 and RG4 ranges.
Renishaw Gold Scale. A gold-plated steel tape encoder scale available from Renishaw, RGS is available on a reel and can be cut to any axis length up to 70m. It is mounted using adhesive backing tape and is thermally mastered to the mounting substrate, simplifying thermal compensation.
Renishaw Gold Scale. A type of RGS with optical IN-TRAC™ ref mark.
Ride height or rideheight is the distance between the encoder scale and the underside of the readhead.
Rideheight tolerance is the variation in that distance that the readhead can handle.
A type of rotary scale in the form of a ring, generally with the encoder scale marked on the outer surface of the ring. Renishaw ring scales include RESR, RESM, RESA, REXM and REXA. Magnetic ring scales are also available.
Power supply voltage ripple is the level of noise on a 5V power supply.
Velocity ripple is a measure of the variation in speed of an axis, when it is driven to move at a constant speed.
Rotation about the longitudinal axis
Acting in a circular motion.
In the encoder market rotary encoders measure rotation movement.
Note that rotary encoder is the generic term for all encoders that measure angles. However, rotary encoder is also used to describe lower-specification rotational encoders and angle encoder is used to describe higher-specification rotational encoders.
A family of high-accuracy, stainless-steel spar scales from Renishaw. The range includes the following models:
- RSLM: Steel spar with a reference mark in the centre
- RSLE: Steel spar with a reference mark at one end
- RSLC: Steel spar with customer-selectable reference marks
- RSLR: Steel spar with no reference marks
- RSLA: Steel spar with absolute code
The word spar describes a thick cross-section scale.
A family of stainless steel tape scales from Renishaw. The range includes the following models:
- RTLC: Steel tape, incremental, IN-TRAC™ ref marks
- RTLC-S: Steel tape, incremental, IN-TRAC™ ref marks, self-adhesive
- RTLA: Steel tape, absolute code
- RTLA-S: Steel tape, absolute code, self-adhesive
SDE = Sub-Divisional Error. The measurement error within one signal period.
This error mechanism is due to imperfections in the shape or centring of the encoder output signal Lissajous.
SDE can cause velocity ripple problems on linear motor or DDR motor axes. High SDE can cause an axis to make an audible noise, and heat can be generated. In machine tool applications, high SDE can cause poor surface finish and on scanning machines it can cause blurred images.
TONiC, SiGNUM and ATOM feature SDE-reducing processing electronics.
See IP Rating.
A Light-Emitting Diode fitted to readheads (or encoder interfaces) that indicates current signal quality and the status of the encoder, such as reference mark phasing. This instant display of diagnostic information removes the need for additional set-up equipment or oscilloscopes.
Most Renishaw encoders indicate signal quality through a multi-coloured set-up LED that illuminates red/orange/green to indicate poor/fair/good signal quality. Some encoder models can also illuminate blue to show an optimised or very high signal.
A high-performance optical incremental encoder from Renishaw. The first encoder with optical IN-TRAC™ reference mark capability.
Single Track refers to an absolute encoder scale with a single strip of scale graduations that provides both coarse absolute position and fine incremental phase information.
Traditional absolute encoders use two parallel tracks of graduations: incremental and absolute. As the absolute readhead must read both these two scales simultaneously, any small amount of yaw misalignment can cause these two readings to become de-phased, introducing a measurement error.
RESOLUTE is the world's first open optical absolute encoder that reads a single track scale, making it immune to yaw-dephasing.
A waveform is described as sinusoidal when it has a magnitude that varies according to a sine function.
Shielding refers to the encoders defence against electromagnetic interference.
An important application of shielding is in cables - Renishaw cables are generally double-shielded, featuring two layers of shield made from tinned copper braid, spirally wrapped around the cable core opposite directions. The outer screen acts like a Faraday Cage and is connected to Earth at both ends. The inner screen acts like an antenna and is connected to 0V at the receiving electronics only.
With double-shielded cables, it is important to ensure that there is no short between 0V and Earth.
A scale type with a thick cross-section.
Renishaw RSLM and RELM are spar scales, for instance.
The standard digital interface for the TONiC encoder range.
A range of compact optical incremental encoders from Renishaw, providing high-performance motion control capability. The readhead incorporates dynamic signal conditioning and the optical reference mark detector is directly integrated into the incremental signal sensor. TONiC provides excellent signal-to-noise characteristics and robust immunity to contamination.
Ultra-high vacuum. UHV is generally defined as being pressures less than 10-9 Torr.
Renishaw provide certain readheads that are optimized for use in UHV environments. These heads are built from clean materials and designed to minimise outgassing (the release of chemicals while the vacuum chamber is being pumped-down).
In motion control systems, any deviation between the commanded velocity and the actual velocity at a given time is known as velocity ripple. Factors contributing to velocity ripple include encoder resolution and Sub-Divisional Error.
VIONiC™ and VIONiCplus™
VIONiC and VIONiCplus encoders integrate Renishaw's filtering optics design and interpolation technology to create a high performance, super-compact, digital all-in-one incremental open optical encoder range. They offer resolutions down to <±20 nm for VIONiC and <±2.5 nm for VIONiCplus, and a wide range of configurations to optimise the speed for your motion control system. The systems are quick and simple to install with wide set-up tolerances and automatic calibration. VIONiC'S dynamic signal processing gives improved signal stability with Sub-Divisional Error of typically <±30 nm whilst VIONiCplus implements our latest interpolation algorithms to achieve a Sub-Divisional Error of <±10 nm. This is a world-leading performance from a 20 µm pitch incremental encoder system.
Volts peak-to-peak. A method for measuring the size of a waveform, by measuring from the maximum positive amplitude to the minimum negative amplitude of the waveform. The analogue output of many incremental encoders is defined as being 1 Vpp.
Another standard is to measure mean-to-peak, which is used to describe SDE, for example. For symmetrical waveforms (such as sine and cosine) mean-to-peak is half the peak-to-peak value.
Rotation about the vertical axis
A form of Invar (low CTE nickel-iron alloy) which has been specially selected for its particularly high stability. Renishaw provide low CTE scale made of ZeroMet.