RTU Position Safety System

  • We have 17 large Robot Traversal Units (RTUs) carrying R2000 and M900 Material Handling robots up and down 11 different 10 to 20 meter long rails, five with 1 RTU and 6 with 2 RTUs. We have had MANY instances in the last five years of the pinion gear driving the RTU has sheared off, with the failure pattern looking like fatigue leading to failure in about 3 cycles. The failure is severe, and has had expensive collateral. The RTU motor comes to a stop, believing it is in position, while the RTU continues to be carried down the rail by momentum. The robot on the RTU, believing it is in position, then reaches out to where it thinks the cell is waiting for it, in once instance driving the grippers into a PLC electrical cabinet.

    We have started tracking a specific circumstance that we suspect is causing our issue. One RTU Robot is inside of a cell, breaking the rear light curtain. An operator reaches into the same cell, breaking the front light curtain. This causes an E Stop to the OTHER RTU, which is in motion along the rail, causing huge impact to the pinion gear. This happens on average once every day, and in cases with untrained operators, many times per shift.

    I want us to look at adding some physical barrier to the front of the cell to prevent the operator from entering before the rear curtain is cleared, but this is an expensive option and with the pandemic, our company is reluctant to spend a penny without a 3 Month or Less payback.

    An alternative we are considering is to add DCS zones, to prevent certain conditions from E-Stopping one robot when the other is the only concern. However, some of our engineers are (rightfully) concerned about us relying on the rail position of the RTU when we KNOW that that is a major problem that we know occurs, and DCS may say the RTU is in a safe location and is free to move while the RTU may in fact be sliding towards a human.

    It is not so much an issue of CREATING an unsafe condition as it is allowing an unsafe condition to continue and not accounting for it in our corrective action.

    I want to reach out and see if anyone has worked with a safety rated way of positively identifying a machine's position along a rail, that we could use as a redundancy to ensure this is not occurring.

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  • Two thoughts come to mind:

    1. Absolute load side encoders to track position. This would be expensive and probably not safety rated, but would give you the actual positions of the robots relative to the rail.

    2. Fanuc software option J521 : Controlled stop by E-Stop. This would require a risk assessment, but would prevent violent power-off, brakes-on type stop that seems to be the route cause of your issues.


  • You said the light curtain is causing an E-stop. You could simply wire the light curtain to the Fence Stop terminals instead, which would give you a controlled stop. Personally I have never seen a light curtain wired to the E-stop circuit, only the Fence stop circuit, but you must follow your risk assessment process.

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