Cleaning up incremental encoder signals

Paul Boughton

The incremental encoder is the most common type used in motion control and measures relative displacement by generating two pulsing signals for every increment step moved.

The two signals are in quadrature and repeat a characteristic number of times in each encoder cycle. The pair behave as a clock-direction set and normally drive an accumulating hardware or software counter where the sum increases or decreases depending on the direction of movement.

The encoder emits false position signals in the presence of mechanical vibration and electrical noise coming from many sources (see/download our paper Eliminating Phantom Movement in Encoder Applications, In all cases the result is position drift, accuracy degradation and system loss of position and speed data. Furthermore as more than one noise generating mechanism is active at any given time, all causes and their symptoms must be addressed when processing encoder signals.

Various techniques are available in safeguarding the encoder signals. At the physical level shielded cable, differential or symmetrical signal formats and input stage hysteresis are used. At the logical level processing takes place in two steps. First the signals are digitally filtered for noise and then analysed within each encoder cycle. After rejecting any invalid signal states the encoder signals are reconstructed in their correct form and sequence.

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Cognito Quam Electrotechnologies Ltd is based in Melissia, Athens, Greece.

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