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Author Topic: Servo spindle stops briefly - How do I diagnose problem?  (Read 714 times)

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Servo spindle stops briefly - How do I diagnose problem?
« on: May 14, 2019, 09:39:02 AM »
Problem:  My Lathe spindle rarely, and randomly stops during a cnc program. 

Background:  I finally have all of my motors running, system calibrated and I am now "air cutting" using "simple" g-code generated with the Mach4 turn cycles.  3 times now, the spindle has "briefly frozen <1sec" mid run and then continued.  There are no alarms raised, no software indicators or hardware alarms.  It is unclear if any steps are missed.

The Machine:
Mach4 Hobby (Windows 10 dedicated (current) I5 computer),
PoKeys57CNC,
DMM-Tech Dyn4 controller, 400W servo,
Sherline 4410 Lathe,
X,Y axis standard Leadshine steppers and controllers.
I am Air cutting so there is no load on motors.

I am puzzled and looking for advice on how to diagnose the fault and ultimately correct it?

Since the event is random and "rare" being prepared to capture diagnostics is a problem too, so if anyone can suggest test to "force" the problem that would also be a help.

Thanks in advance.
Re: Servo spindle stops briefly - How do I diagnose problem?
« Reply #1 on: May 14, 2019, 11:08:55 AM »
New Information:
All motors appear to be stopping.  I just used a simple turning cycle from the turn cycles menu. 
Let it run and touched the Z axis stepper as it fed the cut.
The spindle stopped in the middle of the pass so did the stepper.

I suspected the Servo was acting up, but since both stopped.  It has to be a broader system issue, PoKeys board, Mach4 or other system issue.

Again any help would be apprciated
Re: Servo spindle stops briefly - How do I diagnose problem?
« Reply #2 on: May 15, 2019, 12:41:56 PM »
Had similar issue with my Mach 4 servo spindle using an ESS. The guys at Warp 9 stated that there was something going on with Mach 4 and step direction spindles where the commanded velocity goes to zero for a single time slice. I have graphs to show this issue if you would find them helpful.

Warp 9 TD, the makers of the Ethernet Smoothstepper have decided to take over the spindle control internally to the motion controller to correct this issue, but that has not yet been implemented. I would personally like to see NFS correct their end of things (if this is a verified issue) since it seems the issue is not unique to the ESS.
Re: Servo spindle stops briefly - How do I diagnose problem?
« Reply #3 on: May 15, 2019, 12:57:14 PM »
Attached images are captured directly from the internal data of the servo drive. Note that there is a max acceleration limit imposed in order to prevent drive faults.

Graphs show commanded velocity (as a result of Step/Dir input) over time as the spindle accelerates and decelerates. Note the velocity is inverted (ramps from 0 to -5000 rpm) but this doesn't affect the function of the motor, only the direction of rotation.
Re: Servo spindle stops briefly - How do I diagnose problem?
« Reply #4 on: May 16, 2019, 10:19:33 AM »
Thank you for the reply.   I will send a ticket to NFS.  Did you work around this?

This is far more frequent than I originally thought, it happens 2-6 times in a 20 minute cnc run. 

My biggest concern is safety, my Lathe has a chuck mounted on a thread.  It is a "Sherline" and sudden stops can cause the chuck to loosen or spin off due to momentum.  I had it spin off during a bench test for example at 300 rpm it was ... exciting.  Since I want to machine aluminium and plastics I expect to run at high speeds, so I am worried that at 2800 rpm the result could be catastrophic.

Any advice would be appreciated.
Re: Servo spindle stops briefly - How do I diagnose problem?
« Reply #5 on: May 16, 2019, 10:23:27 AM »
Just as a reference I am currently set up with the Spindle Servo in DIR/Pulse Mode.

With the enable setup on an on board relay.  I have to do this because the DYN4 needs 24v on the enable pins.
Re: Servo spindle stops briefly - How do I diagnose problem?
« Reply #6 on: May 16, 2019, 10:55:21 AM »
My AC servo drive (Allen Bradley Ultra 3000i 2kW) has a software rate limiter called Slew Limit. I set it at 1.5x the max accel in Mach 4. Ideally this limit never goes into effect, but during accel, decel, and significant changes in speed I can hear and see the spindle jump in speed (down to near zero) and kick back up. That's what the above graphs show. I've never seen this issue while the spindle is running at constant speed, but if you are doing CSS, then it could apply.

I would add a locking screw to your chuck regardless. I know it is a small lathe, but it could still do some damage.
Re: Servo spindle stops briefly - How do I diagnose problem?
« Reply #7 on: May 16, 2019, 10:55:59 AM »
I'm pretty sure the DMM DYN4 has a software setting for max acceleration which should do the same thing.
Re: Servo spindle stops briefly - How do I diagnose problem?
« Reply #8 on: May 16, 2019, 02:32:35 PM »
Ok I will look into the max acceleration

Thanks again!
Re: Servo spindle stops briefly - How do I diagnose problem?
« Reply #9 on: May 16, 2019, 02:34:51 PM »
Oh yes, the settings are at Constant Surface Speed... and that I can do without.  I shall cut some air and see how it goes.

Thank you.