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Safety rotary encoder

29 March 2011

Many companies are increasingly relying on failsafe components to fulfil the safety requirements specified in the current version of the Machinery Directive 2006/42/EC. The RVK58S safety sine/cosine rotary encoder offers such a solution as it also allows the position of the motor rotor to be detected with high levels of accuracy.

Intelligent self-monitoring is often used today in place of redundancy to ensure proper operation of the various functional units. The new RVK58S sine/cosine rotary encoder from Pepperl+Fuchs Drehgeber, plays a role in the development of safe drive solutions. Packaged in a 58-mm housing the encoder has 1024 increments per revolution and boasts a functional safety concept, along with an absolute position track. Signal conditioning is handled by a special ASIC, which also looks after internal diagnostics.

Faults on the incremental encoder disc or errors in the signal path can be detected reliably by using the formula sin²(x) + cos²(x) = 1 to evaluate the incremental sine and cosine signals from the encoder, which are offset by 90°. The supply voltage, temperature, LED drive and calibration data checksum are also monitored. If a violation occurs, the encoder electronics put the outputs in a high-impedance state so that the controller can detect the malfunction immediately by means of its open-circuit detection function. This feature simplifies the development of safe machines and eliminates the need for the redundant sensor component approach used in conventional concepts.

In addition to the high-resolution incremental tracks, the rotary encoder has a sine/cosine absolute position track with exactly one cycle per revolution. It provides information on the actual position of the rotor in the motor, information which is needed for start-up commutation of synchronous motors. The absolute position is output as a digitized signal with 13-bit resolution using either the Synchronous Serial Interface (SSI) or Bidirectional Synchronous Serial (BiSS) interface protocol. With the combination of the sine/cosine interface, SSI and BiSS, the encoder utilizes existing communication paths and eliminates the need for proprietary safety protocols.

In combination with a suitably certified controller, the encoder enables single-encoder solutions, eliminating the need to integrate a second encoder to fulfil more stringent safety requirements. However, a prerequisite for this is that both sensor channels are always connected to the controller.

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