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Author Topic: Spindle servo on lathe is 10% fast  (Read 1213 times)

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Offline DMBGO

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Spindle servo on lathe is 10% fast
« on: April 11, 2018, 06:43:25 PM »
Hi, while I was on a roll, I thought I might ask if there are any suggestions as to why M3 reports my spindle as travelling at 10% faster than the commanded speed. I use a hall element from a car distributor as the spindle rpm sensor (index). A steel plate with a single slot attached to the spindle supplies the pulse. When I command S1000 M3, I get a readout of 1100. If I command S500 M3, I get 550 on the readout. I am using an ESS, with the spindle servo driver configured in Step / Dir mode.
Thanks in advance
Dave
Cheers

David
Re: Spindle servo on lathe is 10% fast
« Reply #1 on: April 11, 2018, 07:02:10 PM »
Hi,
Mach does not have feedback control of spindle speed. If you program a speed its does its best to go the right
speed but if it misses it has no way to correct itself.

There are ways around it, almost all of them quite involved. Does it really matter, I mean really really matter?

Craig
My wife left with my best friend...
     and I miss him!

Offline DMBGO

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Re: Spindle servo on lathe is 10% fast
« Reply #2 on: April 11, 2018, 11:50:39 PM »
Is not critical, I just have to remember to subtract 10%. I was under the impression that it adjusted the speed of its pulse stream to match the commanded speed.
Cheers

David
Re: Spindle servo on lathe is 10% fast
« Reply #3 on: April 12, 2018, 12:09:41 AM »
Hi,
no Mach is not closed loop. The ESS is being developed to enact closed loop control, my understanding
is that its not ready yet and that is with Mach4, I don't know whether it will be extended to Mach3.

If I understand your situation you are using a servo drive? What type of drive/servo?
I would have thought that a step/dir drive would be as accurate as Mach, within fractions of 1%, not 10%

Craig
My wife left with my best friend...
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Offline DMBGO

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Re: Spindle servo on lathe is 10% fast
« Reply #4 on: April 12, 2018, 11:44:34 PM »
That's what I would have thought Craig. I realise that "closed loop" is adjusting things on the fly based on the index sensor's output.. That, however isn't really what I was asking. My spindle speed is consistently 10% above the commanded speed, so it should be possible to slow the pulse rate, either within Mach, or in the smoothstepper driver, to make the commanded speed be equal to the actual speed.
The spindle servo motor is a GSK 110SJT and the AC driver is a GSK DA98B. Probably a little bit underpowered for the job, but seems to work Ok.
Cheers

David
Re: Spindle servo on lathe is 10% fast
« Reply #5 on: April 13, 2018, 12:23:00 AM »
Hi,
well that should mean that the speed is spot on, that is to say that every time the ESS pulses the servo should advance
one encoder count. The ESS should be producing pulses at exactly (within 0.1%) of what is commanded by Mach.

Do you have an oscilloscope or a frequency counter? I would be prepared to guess that Mach an the ESS are working
correctly but your index pulse generator is somehow wrong.

I would issue a speed command in Mach such that the step pulse to the driver was 5kHz and measure it as accurately as you
can.

Craig
My wife left with my best friend...
     and I miss him!
Re: Spindle servo on lathe is 10% fast
« Reply #6 on: April 13, 2018, 12:40:18 AM »
Hi,
another thought is that maybe the 'electronic gearing' of the servo is not 1:1.

What is the current value of parameters PA12 and PA13?

Craig
My wife left with my best friend...
     and I miss him!

Offline DMBGO

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Re: Spindle servo on lathe is 10% fast
« Reply #7 on: April 13, 2018, 12:44:08 AM »
Fixed now. Here are the steps to solve it.

Control Mode: Step and Dir
Pulse width in Ess driver:  3.0 ms
Commanded speed: 1000
Displayed speed on M3: 1113
Displayed RPM on DA98B Servo controller: 1335 (This difference to Mach3 is due to the Spindle pulley having more teeth than the servo motor pulley)
Measured speed with digital tacho:  1113
Pulse width in Ess driver:  2.0ms and 4.0ms are the same as above.


Changed spindle velocity in motor tuning setting for M3 from 4001 to 3600 (10%)

Commanded speed: 1000
Displayed speed on M3: 1001
Displayed RPM on DA98B: 1202 (This difference is due to the Spindle pulley having more teeth than the servo motor pulley)
Measured speed with digital tacho:  1001


Divided 3600 (Velocity setting) by 1001 (actual speed displayed in M3) 3600/1001=3.5964
Set velocity to 3596 and M3 changed it as below.

  
Commanded speed: 1000
Displayed speed on M3: 1000
Displayed RPM on DA98B: 1200 (This difference is due to the Spindle pulley having more teeth than the servo motor pulley)
Measured speed with digital tacho:  1000

Thanks for the help, the speed is now, as you said, spot on.
Cheers

David
Re: Spindle servo on lathe is 10% fast
« Reply #8 on: April 13, 2018, 12:48:40 AM »
Hi,
kool. Next question is why you pissed around fitting an index pulse generator when you could use a digital ouput based on the inbuilt
encoder?

Craig
My wife left with my best friend...
     and I miss him!

Offline DMBGO

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Re: Spindle servo on lathe is 10% fast
« Reply #9 on: April 13, 2018, 01:58:38 AM »
 :) When I first converted this lathe to Mach control, the year was 2003. I had a single phase AC drive motor for the spindle with no speed control and steppers for the axis. I needed an index pulse in order to thread. The next incarnation was about 2007 and I was using a 3 phase motor and VFD for the spindle, I used a converter to make 0v to 10v for the VFDs speed control. I  still needed an index pulse though. About this time, or a bit after, the USB smoothstepper became available, I had a lot of issues with electrical noise from the VFD, but soldiered on.
In late 2010 I decided to upgrade my lathe to servo motors, after much to ing and fro ing I ordered servos and controllers for the lathe, I also ordered a new KRV3000 milling machine, and GSK servos intended for the mill . Business was doing well, the kids were starting to leave home and the world was my oyster so to speak.
The first thing to arrive was the mill on the 30th Mar 2011 I picked it up from the carrier, unloaded it in my shed and then received a phone call asking me to come on an overnight camping trip. Reluctantly I agreed.
Then I had a severe car accident in the early hours of the morning on the 31st of Mar 2011. I was rushed to hospital, despite the policeman in attendance trying to cancel the ambulance, saying there was no need of an ambulance, just a hearse. Luckily the ambulance was already dispatched, and being mistrustful of the diagnosis provided by members of the constabulary, came anyway.
I was airlifted to Melbourne and spent the next 6 weeks in a coma, with the prognosis that even if I did live, I would be in a wheelchair and on dialysis for the rest of my life. After 7 weeks, the staff at the Royal Melbourne hospital started to call me the "miracle man". By the following September, after a period in rehab hospital, I was back at home, albeit, still a bit the worse for wear.
Then followed a period of slowly regaining my mobility, finding out and guessing what I was doing before the accident, and continuing with some of the projects that I had started. I fitted the GSK servos to the mill, and used the Adtech motor that I had earmarked for the spindle on the lathe, for the knee on the mill instead, since the GSK servo that I had bought wasn't powerful enough.
I fitted the Adtech servos to the X and Y axis of the lathe, and used the GSK servo for the spindle on the same. This was in 2017, a bit later than I had initially planned :).
Anyway, to answer your question, now that you have a bit of background, I already had an index pulse, prior to fitting the servos, I did give brief thought to using the encoder on the spindle servo motor to provide information about RPM, but decided that I didn't need to, and that I'd only be making extra work for myself, since I am still a long way behind where I imagined I'd be. Oh and did I mention that I ran for the first time since 2011 the other month?
Cheers

David