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SiGNUM™ incremental encoder system with REXM20 rotary (angle) ring


  • Serial protocol available: FANUC 
  • Readhead size: 36 x 16.5 x 14.8 mm 
  • Resolutions to 5 nm 
  • Speed up to 4 591 rpm
  • 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
  • Thick coss-section ring with installed accuracy of ±1 arc second
  • Operating temperature up to +85 °C
  • 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 REXM20 ring?

REXM20 is a one-piece stainless steel ring with 20 µm pitch incremental graduations marked directly on the periphery, featuring the IN-TRAC optical reference mark. Its larger cross-section allows for the reduction in all installation errors apart from eccentricity, which is easily corrected using a DSi (Dual Signal Interface) and two readheads. Featuring total installed accuracy up to ±1 arc second, zero coupling losses, and exceptional repeatability.
REXM20 is available in a range of sizes (Ø52 mm to Ø417 mm) and line counts, all of which have a large internal diameter for flexible integration. The non-contact format eliminates backlash, shaft wind-up (torsion) and other mechanical hysteresis errors that are inherent in traditional enclosed encoders.

Why choose this encoder system?

Ultra-high installed accuracy

For the ultimate angular metrology, REXM20 with two SiGNUM readheads and a SiGNUM DSi (Dual signal interface) offers exceptional repeatability, zero coupling losses and better than ±1 arc second total installed accuracy. The REXM20 features a thicker cross-section designed to minimise all installation errors except eccentricity, which is easily corrected using the combined output of two SiGNUM readheads. Once the DSi has eliminated eccentricity, the only other errors remaining are graduation and SDE - both of which are minimal.

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.

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.

Interface option

FANUC serial interface

  • The FANUC serial compatible interface (Si-FN) offers FANUC serial communications direct from the interface for higher performance and easier connectivity .
  • Available with three resolution options:
    - Normal: 20 bit (0.0003 °) up to 4 500 rev/min;
    - High Type A: 23 bit (0.000043 °) up to 1 200 rev/min;
    - High Type B: 26 bit (0.0000054 ° or 0.02 arc second) up to
    150 rev/min. SiGNUM Si-FN with FANUC serial interface

DSi (Dual Signal) interface

  • Renishaw's DSi combines the incremental signals from two SiGNUM readheads on a REXM ring (REXT ring for partial rotation applications) to compensate for the effect of bearing wander and eliminates odd error harmonics, including eccentricity to give total l installed accuracy of typically ±2.0 arc second.SiGNUM™ REXM ultra-high accuracy encoder system

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

REXM20: one-piece ultra-high accuracy stainless steel ring

Also available for partial rotation applications as REXT20

Readhead size (LxWxH)

36 x 16.5 x 14.8 mm

Scale pitch

20 μm

Coefficient of thermal expansion at 20 °C15.5 ±0.5 μm/m/°C

Ring outer diameter

52 mm to 417 mm

Line count

8 192 to 65 536 (depending on ring size)

Reference mark

REXM20: single IN-TRAC reference mark ring

REXT20: two reference mark ring for partial rotation applications

Total installed accuracy (with 2 readheads)

(≥100 mm diameter) ±1 arc second

(75 mm diameter) ±1.5 arc seconds

(≤57 mm diameter) ±2 arc seconds

Maximum speed



(See data sheet for details)

4 591 rpm at -3 dB (on 52 mm REXM20)

4 591 rpm (5 μm resolution SiGNUM on 52 mm REXM20)

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



(See data sheet for details of angular resolutions)

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 direct into SiGNUM Si interface)

Power supply

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

Vibration (operating)

100 m/s2max @ 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

White paper

  • White paper:  The accuracy of angle encoders White paper: The accuracy of angle encoders

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.