FANUC Robot SRVO-068 Pulse Mismatch Alarm - Only Occurs During 2nd Shift

    We’re experiencing an intermittent SRVO-068: Pulse Mismatch Alarm on our FANUC robot system (only a few months old). The issue is particularly strange because it only seems to happen during the 2nd shift, and here's the situation:

    Symptoms:

    • SRVO-068: Pulse Mismatch Alarm specifically on Group 2, Axis 2.
    • The issue primarily occurs on Robot 2 in the Lincoln weld cell, but has also occurred on Robot 1 in the same cell and one time in a different weld cell.
    • The problem occurs during the 2nd shift with no changes to the program or welding load.
    • Other related alarms include SRVO-408 (External Emergency Stop) and SRVO-037 (Check Chain Input).

    What We’ve Tried So Far:

    • FANUC replaced the robots, controllers, and wiring harnesses.
    • Electricians added extra grounds to the system.
    • Incoming power filters have been installed to mitigate power fluctuations, but the issue persists.
    • The robots are running the same programs and the load (welding) does not change, yet the issue is consistently tied to the 2nd shift.

  • massula January 15, 2025 at 4:50 PM

    Approved the thread.

    I have seen it happen multiple times. The controller will be in auto and the machine will be running fine and then randomly robot 2 will fault out and get an BZAl or a DTERR Alarm. and it usually happens a couple of times on second shift.

    Second Shift.

    But what time on second shift?

    Wouldn't be after external or internal lighting kicks on would it?

    A similar issue occurred many moons ago and after much troubleshooting and teeth gnashing it was tracked down to a ballast failing on an outside light. This was spiking the system and causing all sorts of goblin and gremlins.

    So now whenever I hear time related issues like that it is the first thing I think of.

    As stated by sborchshev, make sure everything is properly grounded. This is especially important for any welding robot. Since this is a G2:A2 (group 2 axis 2) error, this should be related to the positioner. Check the positioner signal cable shield is grounded at the grounding bar in the controller. Also check the ground wires to the robot base, the positioner, the welder, the fixture, etc.

    Make sure the weld cables do not run parallel to the robot cables or signal cables going to the positioner. Weld cables should in separate trays.

    Check that the connector pins at the positioner are secure & not loose or pushed in slightly. Also check the PulseCoder connector at the G2:A2 motor as it's normal to find it damaged.

    If this is a 2nd shift issue, there must be something different between the shifts. The obscure noise issue mortoch found is such a difference that required thinking outside (pun) the box.

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