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How optical encoders work

Absolute encoders

Incremental encoders

RESOLUTE™

RESOLUTE communicates bi-directionally in purely serial format, using a variety of industry-standard protocols, of both proprietary and open standard. 

RESOLUTE™ encoder optical scheme with annotations

The process begins...

The controller initiates operation by sending a demand message to the readhead, instructing it to capture the absolute position on the linear or rotary scale at that instant. The head responds by flashing a high-power LED source to illuminate the scale. The flash duration is as brief as 100 ns to minimise image blur on moving axes. Crucially its timing is controlled within a few nanoseconds to preserve the relationship between demanded and reported position, one of the essential features that makes RESOLUTE ideally suited to very high specification motion systems. 

 

 

Single track scale

The scale is essentially a single track of full-width contrasting lines, based on a nominal period of 30 µm. The absence of multiple parallel tracks gives important immunity from yaw errors and much more lateral tolerance in head position.

Image acquisition

The scale is imaged, via an aspheric lens which minimises distortion, onto a custom detector array designed specifically for RESOLUTE. The optical arrangement, with a folded illumination path but direct imaging, is highly compact yet stable thus assuring the fidelity essential for excellent metrology.

Data decoding and analysis

Once captured by the detector, the image is transferred via an analogue-to-digital converter (ADC) to a powerful Digital Signal Processor (DSP). Specially developed algorithms then obtain a true absolute, but relatively coarse position from the code embedded in the scale. This process is checked, and corrections are made by further algorithms in the DSP which exploit redundancy and intentional restrictions in the scale code. Meanwhile other routines calculate a very high resolution fine position, which is then combined with the coarse position to provide a truly absolute and very high resolution location.

Final checks and data output

After final error checking procedures this information is uploaded in the appropriate protocol to the controller as a pure serial word representing position to within 1 nm. Protection against electrical noise disturbance is provided by addition of a Cyclic Redundancy Check (CRC). The entire process can take as little as a few micro seconds and be repeated up to 25 000 times per second. By a variety of techniques, including adjusting the light flash duration to the axis speed, this performance is achieved at up to 100 m/s while, crucially, preserving exceptionally low positional jitter at lower operating speeds.

And the result is...

An encoder with generous installation tolerances: RESOLUTE allows ±0.5° in yaw, pitch and roll and an impressive ±150 µm in rideheight. Meanwhile the generous optical footprint and advanced error correcting procedures confer excellent immunity to optical contamination, both particulate and greasy smears. All this while maintaining 1 nm resolution at 100 m/s: RESOLUTE is the answer to the toughest absolute challenge.

VIONiC

VIONiC features the third generation of Renishaw's unique filtering optics that average the contributions from many scale periods and effectively filter 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.

Fully integrated advanced dynamic signal conditioning, Auto Gain , Auto Balance and Auto Offset Controls combine to ensure ultra-low Sub-Divisional Error (SDE) of typically <±30 nm.

This evolution of filtering optics, combined with carefully-selected electronics, provide incremental signals with wide bandwidth achieving a maximum speed of 12 m/s with the lowest positional jitter (noise) of any encoder in its class. Interpolation is within the readhead, with fine resolution versions being further augmented by additional noise-reducing electronics to achieve jitter of just 1.6 nm RMS.

TONiC™ optical scheme with annotations

The IN-TRAC reference mark is fully-integrated in the incremental scale and is detected by a split photodetector within the readhead. As the diagram shows, the reference mark split detector is embedded directly into the centre of the incremental channel linear photodiode array ensuring greater immunity from yaw-dephasing. This unique arrangement also benefits from an automatic calibration routine that electronically phases the reference mark and optimises the incremental signals.

VIONiCplus

VIONiCplus features the third generation of Renishaw's unique filtering optics that average the contributions from many scale periods and effectively filter 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.

Fully integrated advanced dynamic signal conditioning, Auto Gain , Auto Balance and Auto Offset Controls combine to ensure ultra-low Sub-Divisional Error (SDE) of typically <±10 nm.

This evolution of filtering optics, combined with carefully selected electronics, provide incremental signals with wide bandwidth achieving a maximum speed of 12 m/s with the lowest positional jitter (noise) of any encoder in its class. Interpolation is within the readhead, with fine resolution versions being further augmented by additional noise-reducing electronics to achieve jitter of just 1.6 nm RMS.

TONiC™ optical scheme with annotations

The IN-TRAC reference mark is fully-integrated in the incremental scale and is detected by a split photodetector within the readhead. As the diagram shows, the reference mark split detector is embedded directly into the centre of the incremental channel linear photodiode array ensuring greater immunity from yaw-dephasing. This unique arrangement also benefits from an automatic calibration routine that electronically phases the reference mark and optimises the incremental signals.

ATOM™

ATOM uses a non-collimated LED located centrally between the incremental and reference mark sensors. This high divergence LED produces a low profile height with a footprint at the scale that is much larger than the LED, enabling illumination of incremental and reference mark regions.

ATOM employs the same filtering optics scheme as used in all Renishaw's incremental encoders. The incoherent LED produces a signal of high harmonic purity allowing high resolution interpolation. Efficient photometry also produces a low jitter signal. A further benefit of the filtering optics scheme is that ATOM does not generate measurement errors due to scale contamination and undulations.

ATOM uses a large single feature off-track optical reference mark for good contamination immunity. Phasing of the reference mark is as simple an operation as with TONiC.

ATOM™ optical scheme with annotations

TONiC™

TONiC features the third generation of Renishaw's unique filtering optics that average the contributions from many scale periods and effectively filter 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.

Fully integrated advanced dynamic signal conditioning, Auto Gain , Auto Balance and Auto Offset Controls combine to ensure ultra-low Sub- Divisional Error (SDE) of typically <±30 nm.

This evolution of filtering optics, combined with carefully-selected electronics, provide incremental signals with wide bandwidth achieving a maximum speed of 10 m/s with the lowest positional jitter (noise) of any encoder in its class. Interpolation is by CORDIC algorithm within the TONiC Ti interface, with fine resolution versions being further augmented by additional noise-reducing electronics to achieve jitter of just 0.5 nm RMS.

TONiC™ optical scheme with annotations

The IN-TRAC reference mark is fully-integrated in the incremental scale and is detected by a split photodetector within the readhead. As the diagram shows, the reference mark split detector is embedded directly into the centre of the incremental channel linear photodiode array ensuring greater immunity from yaw-dephasing. Yielding a reference mark output that is bi-directionally repeatable to unit of resolution at all speeds. This unique arrangement also benefits from an automatic calibration routine that electronically phases the reference mark and optimises the dynamic signal conditioning

RGH20 

RGH20 optical scheme with annotations

Within the readhead, an infra-red LED emits light laterally onto a scale with reflective and non-reflective areas. The light is directed back off the reflective areas through a transparent phase grating. This produces sinusoidal interference fringes at the detection plane within the readhead. The optical scheme averages the contributions from many scale graduations and effectively filters out signals not matching the scale period. This ensures signal stability even when the scale is contaminated or slightly damaged.

Low short-term errors are assured by the unique optical design, typically giving less than ±0.15 μm Sub-Divisional Error (SDE). For even lower SDE and even higher signal stability, the analogue version of the RGH20 can be connected to the REE interface, which include Auto Gain Control (AGC) and Auto Offset Control (AOC) that operates at all speeds.

The RGH20 readhead features a set-up LED, which lights green when optimum installation has been achieved. A reference mark or a single limit are available with this readhead. The reference mark provides a repeatable home or zero position, whilst the limit is used as an end of travel indicator.

RGH20F

RGH20 optical scheme with annotations

Within the readhead, an infra-red LED emits light laterally onto a scale with reflective and non-reflective areas. The light is directed back off the reflective areas through a transparent phase grating. This produces sinusoidal interference fringes at the detection plane within the readhead. The optical scheme averages the contributions from many scale graduations and effectively filters out signals not matching the scale period. This ensures signal stability even when the scale is contaminated or slightly damaged.

With the unique optical design of the readhead and the Auto Gain Control (AGC) and Auto Offset Control (AOC) features of the REF interface, low short-term errors are assured, typically giving less than ±0.05 μm Sub-Divisional Error (SDE).

The REF interface features a set-up LED, which lights green when optimum installation has been achieved. A reference mark or a single limit are available with this readhead. The reference mark provides a repeatable home or zero position, whilst the limit is used as an end of travel indicator.

RGH22 

RG2 optical scheme with annotations

Within the readhead, an infra-red LED emits light onto the angled scale graduations where it is directed back through a transparent phase grating. This produces sinusoidal interference fringes at the detection plane within the readhead. The optical scheme averages the contributions from many scale graduations and effectively filters out signals not matching the scale period. This ensures signal stability even when the scale is contaminated or slightly damaged.

Low short-term errors are assured by the unique optical design, typically giving less than ±0.15 μm Sub-Divisional Error (SDE). For even lower SDE and even higher signal stability, the analogue version of the RGH22 can be connected to the REE interface, which include Auto Gain Control (AGC) and Auto Offset Control (AOC) that operates at all speeds. The RGH22 readhead features a set-up LED, which lights green when optimum installation has been achieved.

Reference mark and dual limits are available with this readhead. The reference mark provides a repeatable home or zero position, whilst the limits are used as end of travel indicators.

RGH24 

RG2 optical scheme with annotations

Within the readhead, an infra-red LED emits light onto the angled scale graduations where it is directed back through a transparent phase grating. This produces sinusoidal interference fringes at the detection plane within the readhead. The optical scheme averages the contributions from many scale graduations and effectively filters out signals not matching the scale period. This ensures signal stability even when the scale is contaminated or slightly damaged.

Low short-term errors are assured by the unique optical design, typically giving less than ±0.15 μm Sub-Divisional Error (SDE). For even lower SDE and even higher signal stability, the analogue version of the RGH24 can be connected to the REE interface, which include Auto Gain Control (AGC) and Auto Offset Control (AOC) that operates at all speeds.

The RGH24 readhead features a set-up LED, which lights green when optimum installation has been achieved. A reference mark or a single limit are available with this readhead. The reference mark provides a repeatable home or zero position, whilst the limit is used as an end of travel indicator.

RGH25F

Within the readhead, an infra-red LED emits light onto the angled scale graduations where it is directed back through a transparent phase grating.

This produces sinusoidal interference fringes at the detection plane within the readhead. The optical scheme averages the contributions from many scale graduations and effectively filters out signals not matching the scale period. This ensures signal stability even when the scale is contaminated or slightly damaged.

With the unique optical design of the readhead and the Auto Gain Control (AGC) and Auto Offset Control (AOC) features of the REF interface, low short-term errors are assured, typically giving less than ±0.05 μm Sub-Divisional Error (SDE).

The REF interface features a set-up LED, which lights green when optimum installation has been achieved.

A reference mark or a single limit are available with this readhead. The reference mark provides a repeatable home or zero position, whilst the limit is used as an end of travel indicator.

RG2 optical scheme with annotations

RGH34 

RG4 optical scheme with annotations

Within the readhead, an infra-red LED emits light laterally onto the flat top graduations of the scale, creating a ‘side lit' arrangement as shown in the optical diagram.

The RGH34's companion interface, the RGI34, features a set-up LED which lights green when optimum installation has been achieved.

A reference mark or a single limit are available with this readhead. The reference mark provides a repeatable home or zero position, whilst the limit is used as an end of travel indicator.

RGH40

RGH40 optical scheme with annotations

Within the readhead, an infra-red LED emits light laterally onto a scale with reflective and non-reflective areas. The light is directed back off the reflective areas through a transparent phase grating. This produces sinusoidal interference fringes at the detection plane within the readhead. The optical scheme averages the contributions from many scale graduations and effectively filters out signals not matching the scale period. This ensures signal stability even when the scale is contaminated or slightly damaged.

Low short-term errors are assured by the unique optical design, typically giving less than ±0.30 μm Sub-Divisional Error (SDE). For even lower SDE and even higher signal stability, the analogue version of the RGH40 can be connected to the REE interface which include Auto Gain Control (AGC) and Auto Offset Control (AOC) that operates at all speeds.

The RGH40 readhead features a set-up LED, which lights green when optimum installation has been achieved. A magnetic reference mark is available with this readhead, providing a unidirectional repeatable zero position.

RGH41 

RGH41 optical scheme with annotations

Within the readhead, an infra-red LED emits light laterally onto the flat top graduations of the scale, creating a ‘side lit' arrangement as shown in the optical diagram. This produces sinusoidal interference fringes at the detection plane within the readhead. The optical scheme averages the contributions from many scale graduations and effectively filters out signals not matching the scale period. This ensures signal stability even when the scale is contaminated or slightly damaged.

Low short-term errors are assured by the unique optical design, typically giving less than ±0.30 μm Sub-Divisional Error (SDE). For even lower SDE and even higher signal stability, the analogue version of the RGH41 can be connected to the REE interface, which include Auto Gain Control (AGC) and Auto Offset Control (AOC) that operates at all speeds.

The RGH41 readhead features a set-up LED, which lights green when optimum installation has been achieved. Reference mark and dual limits are available with this readhead. The reference mark provides a repeatable home or zero position, whilst the limits are used as end of travel indicators.

RGH45

RGH45 optical scheme with annotations

Within the readhead, an infra-red LED emits light laterally onto a scale with reflective and non-relective areas. The light is directed back off the reflective areas through a transparent phase grating. This produces sinusoidal interference fringes at the detection plane within the readhead. The optical scheme averages the contributions from many scale graduations and effectively filters out signals not matching the scale period. This ensures signal stability even when the scale is contaminated or slightly damaged.

Low short-term errors are assured by the unique optical design, typically giving less than ±0.30 μm Sub-Divisional Error (SDE). For even lower SDE and even higher signal stability, the analogue version of the RGH45 can be connected to the REE interface, which include Auto Gain Control (AGC) and Auto Offset Control (AOC) that operates at all speeds.

The RGH45 readhead features a set-up LED, which lights green when optimum installation has been achieved. Reference mark and dual limits are available with this readhead. The reference mark provides a repeatable home or zero position, whilst the limits are used as end of travel indicators.

SiGNUM™

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.