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SiGNUM™ incremental encoder system with RELM20 linear scale


  • Readhead size: 36 x 16.5 x 14.8 mm
  • Resolutions to 5 nm 
  • Speed up to 12.5 m/s 
  • Simultaneous analogue and digital outputs (Analogue outputs available from all Si interfaces)
  • Sub-Divisional Error (SDE) : typically <±30 nm
  • IN-TRAC™ reference mark


  • Sealed to IP64 offering excellent dirt immunity
  • Dynamic signal conditioning for improved motion control performance 
  • Operating temperature up to          +85 °C 
  • Near zero thermal expansion
  • Optional set-up and diagnostic software 
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What is SiGNUM?

SiGNUM is Renishaw's high accuracy, non-contact 20 µm encoder system comprising of an Sr readhead and Si interface. It is sealed to IP64 and offers speeds up to 12.5 m/s and resolutions down to 5 nm and reads a variety of linear and rotary scales with IN-TRAC auto-phase optical reference marks. SiGNUM is extremely easy to use with calibration at the push of a button and free diagnostic software, yet it offers a level of performance previously available only from more delicate fine pitch encoders with an SDE of typically <±30 nm at temperatures up to +85 °C.

What is RELM20 scale?

RELM20 is a spar scale manufactured from ZeroMet™, a low expansion nickel/iron alloy, featuring a 20 µm pitch. 
It is comparable to fine pitch glass scales and offers a total accuracy (including slope and linearity) better than ±1 µm over a 1.5 m length. ZeroMet has a coefficient of thermal expansion of 0.75 ±0.35 μm/m/°C at 20 °C.
System designers can choose between specially formulated adhesive tape or mechanical clips to suit mounting requirements. Both mounting methods allow for independent thermal expansion to that of the substrate.

Why choose this encoder system?

Ease of handling

The RELM20 ZeroMet low thermal expansion scale has a much smaller cross section than typical glass scales and allows easier handling and installation without risk of breakage, making it ideal for the most demanding precision applications.

Motion control performance

Smoother velocity control and rock-solid positional stability of the motion system due to dynamic signal conditioning. Auto Gain Control (AGC), Auto Offset Control (AOC) and Auto Balance Control (ABC) generate incremental signals of exceptional fidelity. As a result, Sub-Divisional Error (SDE) is typically <±30 nm.

Ease of installation

Set-up LEDs on the readhead and interface give a visual indication of signal size and optional diagnostic software allows remote checking of system performance when the readhead is buried in the machine. Calibration of the reference mark and incremental signals is achieved at the push of a button with no mechanical adjustment or additional diagnostic tools. The IN-TRAC optical reference marks are embedded in the incremental scale for compact dimensions and simplified alignment. This yields a reference mark output that is bi-directionally repeatable to unit of resolution over the full operating temperature and speed range.

Combines rugged reliability with higher accuracy

SiGNUM is a 'recoverable readhead' that offers 'wipe clean' recovery from coolant splashes or floods. An IP rating of IP64, high immunity to water, oil and dust and operating temperatures of up to +85 °C and speeds of 12.5 m/s mean that SiGNUM is ideal for more challenging applications whilst offering performance comparable with fine pitch glass scales.

Optional diagnostic software

SiGNUM™ software

SiGNUM software version 5.0 (released January 2012) - is required to optimise installation and set-up of SiGNUM rotary and linear encoders and provides a quick and comprehensive system calibration. The SiGNUM Si interface is connected to the PC via a mini USB cable and the software provides real-time diagnostics to simplify installation and system maintenance.

Find out all about the SiGNUM diagnostic software.

Technical specifications

Measuring standard

RELM20: high accuracy ZeroMet spar scale

Also available with end reference mark as RELE20

Readhead size (LxWxH)

36 mm x 16.5 mm x 14.8 mm

Scale pitch

20 μm

Coefficient of thermal expansion at 20 °C

0.75 ±0.35 µm/m/°C

Accuracy grade at 20 °C

Certified to ±1 μm, calibration traceable to international standards

Reference mark

IN-TRAC reference mark

Various reference mark position options, see data sheet for details

Limit switches

Dual or single limits (selectable position by use of limit marker stickers)

Scale length

80 mm to 1 500 mm

Maximum speed



(See data sheet for details)

Up to 12.5 m/s at -3 dB

Up to 12.5 m/s

Sub-Divisional Error (SDE)

Typically < ±30 nm

Dynamic signal control

Real time signal conditioning including Auto Gain Control (AGC), Auto Balance Control (ABC) and Auto Offset Control (AOC) for optimised performance during operation

Incremental signals



1 Vpp (20 μm period)

Resolutions from 5 μm to 5 nm

Electrical connection

0.5 m, 1 m, 1.5 m, 3 m, 5 m and 10 m cable lengths with 14 pin connector (plugs directly into SiGNUM Si interface)

Power supply

5 V ±10%, <250 mA (typical) (unterminated)

Vibration (operating)

100 m/s2 max @ 55 Hz to 2 000 Hz

Shock (non-operating)

1 000 m/s2, 6 ms, ½ sine

Operating temperature

Readhead: 0 °C to +85 °C

Interface: 0 °C to +70 °C


Readhead: IP64

Interface: IP30

Refer to data sheets for full details.



Data sheets


  • SiGNUM™ software version 5.0.

User guide

How it works

An infrared LED illuminates the scale which comprises alternating light and dark lines. SiGNUM features Renishaw's unique filtering optics, averaging the contributions from many bright scale graduations and effectively filtering out non-periodic features such as dirt. The nominally square-wave scale pattern is also filtered to leave a pure sinusoidal fringe field at the detector. Here, a multiple finger structure is employed, fine enough to produce photocurrents in the form of four symmetrically phased signals. These are combined to remove DC components and produce sine and cosine signal outputs with high spectral purity and low offset, while maintaining bandwidth to beyond 500 kHz.

Dynamic signal conditioning, comprising Auto Gain Control (AGC), Auto Offset Control (AOC) and Auto Balance Control (ABC) is applied inside the Si interface to generate incremental signals of exceptional fidelity. As a result, Sub-Divisional Error (SDE) is typically <±30 nm, i.e. 0.15 % of scale pitch. Interpolation is by CORDIC algorithm, within the SiGNUM Si interface.

SiGNUM™ optical scheme with annotations

The IN-TRAC reference mark is embedded in the incremental scale and is detected by a split photodetector within the readhead, yielding a reference mark output that is bi-directionally repeatable to unit of resolution at all speeds. This unique arrangement also benefits from an automated calibration routine that electronically phases the reference mark and optimises the incremental signals.