A quadrature encoder with differential output requires four signal wires plus a ground return for correct operation.  When one or more wires gets disconnected, various types of errors occur.  If the cable break is close to the  transmitter (encoder), then the long wires pick up noise from adjacent signals in the cable.  If all of the signal wires are broken close to the receiver ( the EncDAC board), then the input signal terminations on the EncDAC board hold the inputs steady and counting stops.  In this case software can correctly determine a following error and shut off the machine.  If some of the wires are broken , but not all, then signal activity on the unbroken wires plus noise arriving at the receiver will have transitions which are similar to valid encoder pulses; in this situation, the encoder tracking counters would increment or decrement but the counts would be wrong.  This can lead to following errors and even machine runaway.    Vigilant Products has investigated the various noise profiles and broken-wire conditions, and we have developed two methods to handle encoder faults.  The first of these methods involves a modification of the PCI -EncDAC and IP-ED4 boards to detect encoder wiring fault conditions.   We tested prototypes in our lab and have been assisted by our customers in field trials.   In these trials we learned that some encoders, for example some handwheels, always have so much noise on their signals that our detection logic will constantly flag them with a possible encoder fault.  Therefore the encoder fault detection modification must be a firmware-only mod, we provide the encoder fault status information to software, software can then determine what action to take.  Based on the success of these trials we have submitted prototypes to MDSI for further review.  We will update this site with news as soon as it is available!

The second method involves a different type of line receiver circuit for the differential signals.  We built an optically-isolated current-loop line receiver circuit to replace the traditional differential voltage-detection circuit.  For the new circuit to function, there must be a current loop, i.e., there must be cable continuity from the encoder differential output, through the line receiver, back to the encoder. This circuit stops delivering pulses to the encoder tracking counters as soon as a wire break occurs, so software stops getting counts and detects the following error.  This circuit is larger than the circuitry on the EncDAC boards, so rather than redesign those products, we may implement the new circuit function on the P1/P2 terminal boards.  The prototype has gone out to a customer for field trials, stay tuned for further developments!

Please contact Vigilant if you would be interested in assisting in trials of the fault detection prototypes.