The Craftmans Shop > New from Old
The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20)
awemawson:
Time to grit my teeth, and power the machine up :bugeye:
OK Press the Emergency Stop Button in, hold your breath, press controller start. No dramas but loads of error messages. The NC (Numeric Controller - part of the Sinumerik 820T) is reporting X-Axis and Z-Axis working area limits.
The PLC (Programmable Logic Controller - part of the Sinumerik 820T) is reporting all sorts of errors most of which can be accounted for by the fact that the NC won't go 'Ready'. When 'Ready' the NC outputs a digital output that enables several relays that supply power to the Hydraulic Pump, and Spindle and Axis Drives, and also a '24 volts OK' relay which drives an input to the controller. The 24 volts IS OK but the monitoring mechanism isn't powered up !
A bit of furtling about produced details of how to set the working area up, and that cleared the two Working Area Limit errors.
However the state of play at the moment is that the NC remains in 'Emergency Stop' (Yes the E-Stop button HAS been released!) and I need to find out what it's missing to go 'ready'
vtsteam:
All door lock sensors functional, Andrew?
Limit switches?
awemawson:
Amazingly there are no interlocks on the cabinet doors, and the sliding door giving access to the actual lathe not only has an interlock, I can read it as a logical 'bit' coming in and changing as I open and close the door.
Even odder, the output that signifies 'Control Ready' is most definitely not 'true' it's at 0 volts at the point it's supposed to drive a relay . BUT poking about in the PLC "Q" bits which are outputs, it IS logical true. Without it coming to the outside world, the E-Stop button is ignored.
It's possible the output card has popped but I wasn't able to investigate any further today as I had to change the main hydraulic pump on the digger - it's in now but I probably have 5 hours work with it tomorrow shifting earth before the weather breaks. Also guests for super and two sets of cottage guests arriving, so not much play time on Friday.
awemawson:
So Hydraulic Digger Pump replaced, earth shifted and now back able to play :ddb:
Trying to make sense of the circuit diagram that I have I firstly went round each relay and contactor on the panel matching it's wire numbers to the diagram and sticking a label on when I'd identified it's function - trouble is they don't differentiate between relays and contactors which is confusing as often you get a relay driving a contactor :bang:
All this so I could bottom out what initially looked to be a trivial circuit monitoring the 24 volt rail. Two relay's 'CR10' and 'CR20' which ACCORDING TO THE CIRCUIT each had their coils across the 24 volt rail, and each had a normally open contact in series with each other and connecting a digital input to 24 volts. How simple is that ?
Well actually a bit complex as 'CR10' isn't connected to '0 volts' but to the overload trip circuit for several motor contactors, and 'CR20' derives its '24 volts' from the digital output 'NC Ready' and it is this very signal NC Ready that is in the wrong state and keeping E-Stop asserted :bang:
So OK lets go round the 'NC Ready loop' a few times until dizzy. If I look at the logical bit for NC Ready in the PLC 'Q' word the relevant bit IS set but not physically set to the outside world.
If I trot along signals from the same input / output card that DO have bits at 'logic 1 (24 volts)' I can also see the relevant bit in the appropriate 'Q' word. Now the PLC documentation implies (but doesn't explicitly say) that a bit set in the Q word appears as an output. Now I am aware that some manufacturers have a further logical step between the register data (Q word equiv) and driving an output - the Mitsubishi in the Traub did this, but I've not found any mention of it with the Siemens Sinumerik controllers.
So what are we left with :scratch: Well it could be a further internal logical step, or it could be that that particular output on this I/O card is faulty. I think that the later is the most likely but time only will tell.
I could exchange the two I/O cards that I have and see if the fault moves, but it's an utter pain removing the I/O rack as it involves pulling the controller and the Operators panel out - and if the fault DOES move it'll all have to be done a second time once a replacement is found. OR I could source a replacement on spec, swap it in, and if that proves to have been the fault no more fiddling about. If it doesn't then at least I have a 'shelf spare'
This course of action probably sounds rather spendthrift - BUT there are loads of these cards 'tested good' on eBay in Germany for £15 each so I've ordered one.
According to the Siemens book of words on the PLC it is possible to write into the Q word bits to generate output, so I chose an unused output, wrote to the Q word and measured no output, but when I looked again at the Q word it was back to before I wrote to it. I suspect that the PLC program is cycling round writing it's wanted outputs so if I change anything it is promptly changed back 5 milliseconds later :bang: I'm sure that there is a way of stopping the PLC program running, but then I'm not sure whether anything would be output :scratch:
awemawson:
Well, progress through adversity :ddb:
I decided to 'bell out' the core in the cable that represents 'NC Ready' - this involved pulling the 34 way IDC plug from the rear (and most inaccessible) I/O card. Cable belled out OK, but when I tried to put the plug back - this is an entirely 'by feel' operation, I managed to bend a pin on the card :bang:
Now after a few cusses I decided that I might as well dismantle the I/O Rack using keyhole surgery though the aperture that holds the control panel. Just possible but it hurts your wrists :(
Well doing it I thought that I might as well swap the pair of I/O cards and see if the fault moves - IT DID :clap:
So now we have 'NC Ready' coming true, and the E-Stop button does things - naturally there are other errors but at least this particular mystery is solved and my hunch about the card being faulty is proven correct - as I said earlier, I've ordered one (well actually two !) so PROGRESS !!
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