Thread: New Build-12x6ft Plasma Table
The reason I have chosen to use two motors is because I have a belt drive machine with one motor and it suffers belt issues and does not go fast enough, along with the fact that there are many many commercial build tables with slaved motors and AFIK no hardware or software fitted to check for a failed move on one motor i.e. racking the gantry.
With steppers, its a moot point to a degree as they will not do any (or much) harm if stalled, a servo on the other hand will happily destroy itself and/or its driver when jammed. Gantry damage is possible with both systems i guess but to different degrees.
My motors have dual shafts so I guess I could install slotted opto-discs and detectors on each motor and a small logic board that trips the E-Stop circuit if one side does not move - could be an interesting project one day.
Power-up IS however one issue I am looking at, I know my steppers jump into lock when powered up, but what are the chances of BOTH jumping into the same lock position ?? Zero, 50/50 ????
That is the main point I think
Why would a servo system damages itself if it stalls?
I have been working with servo systems for over 20 years and even 20 years ago, you could stall a servo motor for 24 hours without damaging it.
On a gantry system where you have a motor on each side of the machine and a side stalls will not cause any damage to the motor or driver, but could cause mechanical damage to the machine due to one side trying to continue to move what the other is not.
Closing the position loop on these axis will have the added advantage of stopping the motion if this were to happen. (Stepper or Servo)
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When you start up your system or do a new job, surely the first thing you do is home the machine (at shut down you home the operator ) The initial gantry alignment error will either not change if the steppers lock in the same direction or each or one of the steppers can be up to 1.8 degrees out. At worst this will give 3.6 degrees or 1/100 of a revolution. Divide the screw pitch by 100 and you get the gantry skew. For a 10mm pitch this is 0.1mm . Now you decide can you live with the chance of this until you Home the machine and correct the skew.
I have one machine programmed to home on start up. I lift the tool clear then home X and Y. By zeroing the co-ordinates at the start point and remembering where they got to before I reset X,Y to zero I can then fast return to start point which I do in 2 moves. Full X,Y with half Z, then Z completed.
My larger mill has no home switches, I set the work piece to the tool then the tool to the work piece. If I expect trouble I program a location mark and drill a centre pip. If I want accuracy I bolt my tool finder to the bed and home on that, it uses an Omron optical gizmo accurate to 1um which is very silly because my minimum step is 5um
An auto-home can be a right royal pain in the backside because you don't know if you have 1mm or 500mm to go and detectors with overshoot require forethought and planning. Meaning you probably can't whack it in to top gear, full steam ahead and depend on it to stop in time. Also if you have set the thing up and a power glitch sends your tool creeping away to some remote corner and no way to stop it, you can regret this feature.
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