SiGNUM™

High speed, non-contact performance, combined with dynamic signal processing and the IN-TRAC™ auto-phase optical reference mark. SiGNUM™ linear encoders are extremely easy to use yet offer a level of performance previously available only from more delicate fine pitch encoders.

Options

RELM Invar scale has a coefficient of expansion of ~0.6 µm/m/°C and is available in a range of lengths. The robustness of the Invar spar permits a much smaller cross section than typical glass scales, yet allows easier handling and installation, without risk of breakage.With ±1 µm accuracy and resolution to 5 nm, RELM satisfies the most demanding precision motion requirements.

RSLM stainless steel spar scale offers performance to rival fine pitch glass scales yet is available in lengths up to 5 m. RSLM offers a total accuracy (including slope and linearity) better than ±4 μm over 5 m – an industry first! RSLM can be coiled for simple storage and handling yet once uncoiled behaves as a spar scale. Combined with ultra-low cyclic error (±30 nm), RSLM is perfect for long travel applications where metrology can not be compromised.

Technical

An infra-red LED emits light onto the scale, which is a plane reflective metal grating of 20 µm period. It is important that this scale is uniform and strongly periodic, but it does not need to be a good diffraction grating. As with RG2 and RG4 readheads, the SiGNUM™ readhead optics average the contributions from many scale periods and effectively filters 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.

The balance and level control of these signals is further enhanced by active adjustment of individual channel gains, offsets and also control of the LED light source within the SiGNUM™ readhead. As a result, the inherent cyclic error (sub-divisional error, SDE) is ±30 nm, i.e. 0.15% of scale period. Interpolation is by CORDIC algorithm, within the SiGNUM™ Si interface

The IN-TRAC™ reference mark is embedded in the incremental scale in the form of a dark line. This feature is rejected by the filtering incremental optics, but detected by a split photodetector within the readhead. With appropriate level sensing and gating circuitry this yields a reference mark output that is bi-directionally repeatable to unit of resolution at all speeds. Calibration of phase with respect to the analogue channel is performed automatically on installation by the logic within the interface, which also provides comprehensive system monitoring and set-up assistance.