Renishaw Launches First Functionally Safe Absolute Open Optical Encoder

Renishaw builds on its existing functionally safe (FS) portfolio by introducing the Resolute™FS absolute open-optical encoder for both linear and rotary axis applications. The RESOLUTE FS encoder system, part of the Resolute series, offers all the benefits of the standard Resolute variant but with FS certification.

The standard Resolute encoder is already designed to be inherently safe due to a built-in separate and independent position checking algorithm, which has allowed seamless progression to full FS status and compliance with both the ISO 13849-1:2015 (Performance Level D) and IEC 61508:2010 (SIL2) standards. Full FS certification is required to guarantee the safe use of functions that include safe stops 1 and 2, safe operating stop (SOS) and safe limited speed (SLS) – all of which require functionally safe encoder feedback. A practical example of the SLS function in action is when an interlocked machine guard is opened during normal operation. SLS will act to limit traverse speed to 0.1 m/s and spindle rotation to 100 RPM, thereby minimizing the risk of personal injury. Resolute FS is currently compatible with Siemens Drive-Cliq serial communications protocols, with other FS protocols expected in the near-term.

The Renishaw’s Resolte encoder is the world’s most advanced single-track true-absolute optical encoder system delivering up to 1 nanometre resolution, superior long-term reliability, instant operation after power-up without reference return and high speed performance to 100 m/s. The Resolute series is ideal for advanced motion control applications and enables smooth velocity control with cyclical errors of <±40 nm and excellent positional stability via jitter (noise) of < 10 nm RMS. The advantages of open-absolute encoders over enclosed designs include large through-hole rotary (angle) ring scales for easy design-in, low inertia and low profile components and the absence of wear associated with contacting parts. Furthermore, the Resolute encoder’s easy installation as a result of generous set-up tolerances and an integral set-up LED, excellent dirt immunity and IP64 sealing, gives outstanding performance even on the very long axes typical of heavy industrial applications.

Resolute series encoders capture a 1-dimentional image of a barcode-like pattern on the scale and perform a type of image processing in order to extract position data. The position calculated by the Resolute FS encoder is divided into two parts: a coarse position and a fine position in-between the scale period (≤30 µm). First, the encoder determines these values based on the scale data alone. A secondary coarse position is then determined independently by a position checking algorithm*, wherein the two most recent good readings are extrapolated in time to find the current position. If the measured coarse reading and the extrapolated values differ by more than half the scale period, then an error counter is incremented. Once the error counter reaches five, an error flag is sent to the controller which stops the machine. The Resolute FS encoder variant also double checks the above process and includes a position validation check in order to satisfy IEC 61508. Position data is sent from the readhead via a custom output protocol and the position validation check confirms that the data arriving at the interface agrees with the values that have been internally generated in the readhead.

Resolute FS encoder systems offer customers the same outstanding performance of the Resolute series but with the added peace-of-mind that comes from compliance with the world’s most stringent functional safety standards.

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