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Extra encoder track replaces redundant sensors

20 December, 2010

Pepperl+Fuchs has developed a functionally safe sine/cosine rotary encoder with an additional absolute track, which avoids the need for redundant sensor components. This is expected to make installations up to SIL3/PLe simpler and cheaper.

In addition to a high-resolution (1,024 pulse per revolution) incremental track, the 58mm-diameter RVK58S encoder (shown below) also contains a 1 pulse/rev sine/cosine absolute track. This provides information on the position of the rotor – for example, for start commutation in the case of synchronous motors. The absolute value is output as a digitised signal with a 13-bit resolution using the SSI or BiSS protocols.

P+F has applied to TÜV Süd to certify the encoder and expects it to be approved for applications up to SIL3/PLe. A second encoder will not be needed for higher safety requirements such as SIL3.

The usual practice when monitoring motors in safety-critical applications is to use a redundant sensor – typically, a synchronous encoder – in addition to the main motor encoder. The control system detects any malfunctions by comparing the two encoder signals. But this approach is expensive because of the costs of the redundant components, the extra wiring, the high costs for proof of conformity, and the higher probability of failure as a result of the larger number of components.

The new encoder relies on an ASIC chip which is responsible both for signal conditioning and for self-diagnosis. By evaluating the incremental sin/cos encoder signals, it can detect faults on the interrupter disc and in the signal path. It also monitors other variables such as supply voltage and temperature.

If there are any faults, the encoder’s outputs switch to a high-impedance state which can be detected by a control system. The encoder can be reset only by switching it off and on again.

In some applications, the combined encoder is expected to avoid the need for conventional mechanical safety devices. In high-bay warehouses, for example, the costly hydraulic end-travel stop buffers can be eliminated.

With its additional absolute track, the rotary encoder is suitable for either synchronous or asynchronous machines. It does not rely on proprietary safety protocols, and uses existing communication paths such as the sin/cos interface and an SSI or BiSS interface.

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