The Craftmans Shop > New from Old
The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20)
awemawson:
I have wired my Fluke DVM to the analogue drive signal from the 'measuring circuit' to the Mentor DC drive, and sure enough as the gain is increased the magnitude of the position error signal increases and the 'dead band' decreases, but if the gain is set high enough for the spindle to be positioned crisply then the system oscillates. Backing off the gain so that the oscillations cease but we are not at set point, I can manually rotate the chuck through set point and if going slowly enough the system will 'lock on'
Doing a bit more investigation, and armed with the recently obtained PLC program I can see a bit more of what is happening. A signal (I 114 bit 4) from the NC control, presumably derived from the 'measuring card' is provided to the PLC, where it triggers a 300 Milli-Second timer (T1) . If the 'in position' signal is still asserted when T1 expires the PLC acknowledges the end of the M19 cycle and applies the spindle brake.
As the servo system is in fact oscillating wildly either side of set point, and never 'on set point' for 300 mSec it carries on ad infinitum. Juggling all the various parameters previously detailed has failed to find a compromise setting. For all the world it is as though there is no damping on the system. I have also examined the acceleration parameters in the Mentor drive, and more than doubled the accelerate and decelerate ramp times with no effect on the symptoms.
I've shot a video to illustrate the point - note that as the updating of the PLC is 'sampled' the camera misses many of the brief bit changes for I 114.4
Also attached are screen shots of the relevant bits of the PLC program
awemawson:
Now looking at the manual for the 810T (mine is an 820T but running the same GA2 program) there are flow charts for how the servo system is controlled and how M19 is implemented, but none of it so far has triggered that 'lightbulb moment' for me to say what's wrong here!
Assuming this machine was working when the back ups I'm using were taken then it's very odd that a parameter would need to be changed as they should be as they were previously, and one parameter (4270 Cut Off Speed for Gear 1) is set to it's original value of 5 the M19 system won't move at all ! Logically I would assume that some hardware has failed / drifted what ever but the bits involved as far as I can tell have been exchanged and then returned to no alteration in symptoms.
mc:
It's like either the PLC is needing to apply a bit damping to the output, or the Mentor drive is applying too much damping/not responding quick enough.
If the PLC settings seem plausible and you're sure they're valid, what about the settings in the Mentor drive?
cnc-it:
I had this problem on a Fanuc DC servo which was oscillating at stand still..it turned out to be a faulty servo drive top board (control card).. Encoder faults usually cause alarms and the motor to run away so can't see it being that.
awemawson:
Morry, the Mentor 'ramp' settings for accelerate and decelerate seem to be the only ones relevant. Yesterday I had gone from a setting of 20 to a setting of 50 (not sure of the units) with no effect. Now this morning I took the ramp settings up to their maximum value of 255 but again with no apparent change. (I was surprised not to to see a difference when just starting and stopping the spindle with an M03 / M05 though)
I can't be absolutely sure that the gen that I have on the mentor is correct for this model (although I think it is) but it shows a 'bit read/write' parameter #167 'Ramp Enable' - but so far I've not found the way to change 'bits' !
On the positive side, the issue I was having with the pressure switch for clutch open / closed a few days ago seems to be solved by tweaking the pre-set for PS/5 on the closed side. Previously it wasn't changing state so would say both 'open' and 'closed' at the same time when actually open but now it does :thumbup:
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