12-13-2011, 11:11 AM
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| | | Join Date: Sep 2005 Location: USA Age: 60
Posts: 755
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These are probably the old 3-phase full wave servos with SCRs. The CNC control turns on a relay signal called "PRDY" using a small IC reed relay on the mother board (one for each axis). These contact closure signals go to the servo boards, where they cause the main MCC contactor to pull in and put 3-phase AC power to the servo's SCRs. The MCC contactor has a smaller set of contacts mounted on the side that are for the VRDY (Velocity Unit Read) signal, that goes back to the CNC.
Some things that can cause this alarm are: 1) Bad MCC contactor not pulling in. 2) An E-stop circuit tied to the 110v coils of the MCC contactors. 3) A bad 24v contact in the MCC contactor (mounted on the side of the 3-phase contacts) 4) Bad connection on the pins that are used to connect the servo board to the chassis 5) Bad power supply on the servo board (usually the X board has a power supply and the others don't). Check for +24v on the servo boards. All 3 boards get +24v from the power supply on the X board through a short ribbon cable. Make sure those ribbon cables are on tight.
It would be hard to tell if the MCC contactor is falling out and causing the alarm because, if that should happen, the CNC will drop out the contactor anyway. You won't know if you're seeing a chicken or an egg. With main power off, remove each servo board from it's mounts and pull it off the row of pins that connect it to the servo chassis. Look to see if any of those pins "pushed back". They should all extend to about the same height, so when you plug the board back on, they all make contact with the connector on the board. While the CNC is on, use a plastic screwdriver handle to tap the little relays on the sides of the MCC contactors and see if you can make it alarm out. Some of these relays have removable contacts that you can clean. Also make sure the CN1 connector on each servo board is plugged in and tight.
It's also possible to have a bad IC reed relay for the PRDY signal on the mother board, but that's not very likely.
Lastly, see if there is a 110v circuit tied to the coils of the MCC relays. Some machine builders do this to force a servo shut-down in the case of a runaway or an overtravel. The proper way is to use a real E-stop signal, but some builders just drop out the MCC contactor, which can cause the error and still not put the CNC into E-stop.  |
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