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E-Motion Newsletter - September 2021 Renishaw Absolute Encoders
Company News - September 2021
Explore Renishaw Optical
Encoder Systems:
 Let us talk about RESOLUTE™ today…
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 minimize 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 microseconds 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.
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