The Craftmans Shop > New from Old
The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20)
awemawson:
Yes that is the spindle brake being energized and released. As soon as the servo system has locked onto set point it puts the brake on and it is released by the next M20 command or a specific M32 unlock command.
RussellT:
Once again I'm impressed by your persistence. :bow:
I was catching up with all of this thread yesterday and at the end I was wondering how accurate this positioning could be. The inertia of the chuck/spindle workpiece must make it very difficult to stop it in an exact position by controlling the drive. Is it meant to be accurate enough to use the rotating tooling? Does the drive have a braking effect like a stepper motor? Surely it should apply the brake as it gets to the right position rather than wait for it to overshoot?
Russell
awemawson:
Russell,
If you look at post #541 on the previous page there is a diagram showing it's 'homing in' method - I'm not overly convinced that it's doing it though. Ideally the servo system should be tuned to just stop at set point. Once it has arrived in 'the zone' set by the positioning tolerance it applies the spindle brake and then will be fine for using the rotary tooling. Problem that I have at the moment is that I've had to set this tolerance to 20 1/11ths of a degree so nearly 2 degrees for it to work. The parameters that I got and were theoretically the original ones set it at 2/11ths of a degree so more tolerable.
My oscilloscope is playing up at the moment in the triggering department, but I have a replacement arriving hopefully Monday, and hope to be able to see the set point output of the measuring card as a dying or damped oscillation going forwards and reverse onto the spot. If you look at the video it does look as though this may be happening, certainly on some of the iterations.
For the record, yesterday I re-installed the original Measuring Card. Yes they have slightly different offsets which are levelled out with parameter #4010, but other than that I don't think that the card is the fault hence returning the original.
awemawson:
An interesting (but possibly irrelevant) bit of maths entered my head this afternoon:
A/ I know that the PLC needs the spindle 'on target' for 300 milliseconds to accept it's in position.
B/ The slowest I can reliably drive the spindle is 10 RPM or 10/60 revs per second
C/ At 1/6 RPS 300 milliseconds represents 1/6 x 0.3 = 1/20 of a rev for the 'target window' or 360/20 = 18 degrees
D/ My window is set to 20/11 = 1.8 degrees in the original parameters or a factor of 10 greater than the above
So the control embedded into the measuring card MUST anticipate the target point and leave the spindle to coast into the 'zone' possibly with minor corrections if it is going to over or undershoot, but it CANNOT simply steam into the zone and be there long enough for the 300 mSec that the PLC demands
. . . or maybe I'm missing something :scratch:
PekkaNF:
From which OB that control loop is called? Normally only HMI and secondary functions are on the normal loop and tight controllers are put to 20 ms or such loop.
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