Tech Briefs Magazine - April 2021 - MD-15

Solution
CPC uses Renishaw's optical encoders
in much of its extensive family of motion
products including its RP range of mini
DD rotary stages. These stages have an
outer diameter of 120 mm, a minimum
thickness of 37 mm, and are supplied
with several different options for position encoder feedback. Options include
CPC's in-house magnetic encoders and
Renishaw's ATOM optical encoders,
depending on the level of precision required by the customer. While large
rotary tables are used in machine tools,
mini DD rotary tables are mainly used in
applications where high levels of speed
and precision are needed such as industrial automation, semiconductor, and
FPD manufacturing equipment.
As Wu explained, " We have worked
with Renishaw for nine years now and
we generally use Renishaw optical encoders. Our RP-series rotary tables use
Renishaw ATOM encoders with a 40-μm
pitch scale and a disc diameter of 50
mm, providing an output signal with an
angular resolution of ±1.8 arc second.
When you look at the ATOM readheads
or glass discs, they are designed to be
extremely light and thin, which makes
them ideally suited to the requirements
of applications like mini rotary tables
where space is limited. "
Renishaw's ATOM miniature optical
encoder family has readheads measuring just 7.3 × 20.5 × 12.7 mm. It is one of
only a handful of integrated miniaturized optical encoders available on the
market and features leading-edge signal
stability, dirt immunity, and reliability.
The ATOM encoder's RCDM rotary
scale is a one-piece glass disc featuring
scale graduations marked directly on the
disc face with a single reference mark
and an optical alignment ring. The optical alignment ring can be used to minimize angular alignment errors and
improve installed accuracy. ATOM is a
non-contact optical encoder with an
open format, the advantages of which
include the effective elimination of
backlash, shaft wind-up (torsion), and
other mechanical hysteresis errors that
are inherent in traditional enclosed
encoders.
During installation, the mounting surface can be fixed under the disc but
must be aligned with the center of the
disc before bonding. This calibration is
generally done using electronic or opti-

cal methods. Electronic calibration involves monitoring output signals from
two readheads installed at a 180° angle
from each other and then adjusting the
disc to minimize the difference between
the readings from the two readheads
(adjustment tools include two ATOM
readheads and a Renishaw DSI interface). Optical calibration uses a microscope to manually align the position of

the disc with the center of the mounting
surface.
" During installation, " said Wu, " we
bond the disc onto the mount with UVcure glue and use a microscope to align
the center of both to minimize eccentricity; after that is done, the assembly is
exposed to a UV lamp to solidify the
bonding glue. ATOM discs are marked
with an 'alignment ring' so that calibra-

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Motion Design, April 2021

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Tech Briefs Magazine - April 2021

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