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Re: dangerous malfunction of a machine driven by Mach4
« Reply #30 on: February 23, 2022, 09:05:57 PM »
Hi Hakan,
I think your'e over doing it, even 32mm has a stiffness of 1100N/um, easily twenty times the stiffness of my machine. Many comments I garnered from CNCZone concluded
that my ballscrews were too big for my machine, 800kg, with 150kg X axis weight.

I would expect 40mm diameter screws to be found on machines requiring 100kN or more of thrust and an X axis weight of tonnes...not hundreds of kg's but tonnes.

When I was finding parts for my new mill I like you thought the bigger the ballscrew the better...right? Wrong, a large diameter ballscrew, or at least an out-size ballscrew, requires huge torque
just to accelerate and that counts against toolpath following. As it turns out I can still get 0.25g with the ballscrews I've got with 750W servos, but I would have done even better with 25mm
ballscrews. When it comes to rotating components bigger is not always....or even normally....better.
'I enjoy sex at 73.....I live at 71 so its not too far to walk.'
Re: dangerous malfunction of a machine driven by Mach4
« Reply #31 on: February 24, 2022, 05:48:54 PM »
Hi Craig,

You are right, i know that its not just the g, one side pushing of a gantry heavier than lets say 250kg will likely create more wobble on thinner ballscrews since the x axis is the longest (ballscrew length is 2000mm) i got 40mm because of that, just to make sure. So its not just to create higher torque, wobble in the ballscrew will kill the c3 precision according to the manufacturer TBI, they advised it for the length of the axis and weight of the gantry. U can drive an axis which weighs 2 ton with that ;-)

They are good at what they are doing for example i thought to use the ballscrew on the zth axis one end open, thats the way some bigger manufacturers like makino use ballscrews on zth axis just to keep it short but engineers in TBI said u should never use a ballscrew like that because it will shorten its life. And if u want precision they said always fix the opposite end with c3 bearings. so i dont know how the manufacturers use it like that and keep up with the precision.

U auto tuned the delta servos with their pc software right? Is it easy to tune them? is it full automatic or u had to tune them manually?

Also, do u think an oscilloscope will be helpful for manually tuning the servos? i still wanna use 11nm 130mm flange servo, its worst performance is still better than open loop stepper i guess. Can i tune it by seeing the signals on oscilloscope? i never used an oscilloscope before but is it useful for this purpose?

Hakan
Re: dangerous malfunction of a machine driven by Mach4
« Reply #32 on: February 24, 2022, 06:40:15 PM »
Hi,
OK, so using manufacturers recommendation is good practice, but that does mean large and expensive ballscrews which will in turn require large and expensive servos,
but that's the nature of CNC.

Yes I used Auto-tune and its pretty straight forward. I did tweak the parameters manually, but its dubious that I made any improvements over Auto-tune, having said that
I did not use the scope, so it probably it not surprising that I could not secure any noticeable improvement.

The inertia ratio in my machine is about 5:1, and is very much within the 'sweet spot' for Auto-tune. At inertia ratios of 10:1 manual retuning after Auto-tune
would be common. At 20:1 I think you can forget Auto-tune, manual tuning is probably your only choice.

Quote
Also, do u think an oscilloscope will be helpful for manually tuning the servos?

I didn't bother, my machine PC is to gutless to do a good job with the software scope. If I were to revisit it I would use a more powerful development PC to run the tuning and set-up software
that would allow me to use the software oscilloscope.

Quote
i still wanna use 11nm 130mm flange servo, its worst performance is still better than open loop stepper i guess. Can i tune it by seeing the signals on oscilloscope? i never used an oscilloscope before but is it useful for this purpose?
 

I am a relative newcomer to AC servos but my experience is they far FAR FAR outperform steppers. They, in practice, seem to be rather more powerful than the specs suggest. I believe
that it because of the overload properties of servos. When a stepper overloads it stalls, no ifs or buts, it just stalls. A servo on the other hand just 'digs' in and does the job.

I you give me some numbers I will do the inertia calculation for you. I can tell you at a glance that the inertia is DOMINATED by the huge ballscrews, the 250kg axis is nothing, likely less than
5% of overall inertia.

What I need is the diameter, length and pitch of the ballscrew, any gear/belt reduction if used, and the weight of the axis and associated linearly moving components. The inertia of the servo
would be helpful, but we can make a pretty good ballpark guess on that.

Once we have the inertia equation then equation then the questions about use of the scope can be answered. If the inertia ratio is moderate then you don't really need to manual
tune....although it is instructive to do so, particularly with a scope to observe the results rather than inferring from movement data.

Craig
'I enjoy sex at 73.....I live at 71 so its not too far to walk.'
Re: dangerous malfunction of a machine driven by Mach4
« Reply #33 on: February 26, 2022, 07:13:06 PM »
Hi Craig,

sorry for the late reply,

diameter:40mm pitch:5mm length:2000mm, the weight of the axis(both Y and Zth) is 600kg, servo is 11nm 2.2kw AC servo, i connect the motor to ballscrew with direct coupling.

I asked about the oscilloscope because i think to buy one from taobao next month if i can tune the servo with it, it would be very nice because this servo driver has no auto tuning. Its a servo motor at the end there is no meaning to leave it there idle.

Hakan
Re: dangerous malfunction of a machine driven by Mach4
« Reply #34 on: February 27, 2022, 01:54:21 AM »
Hi,

Quote
I asked about the oscilloscope because i think to buy one from taobao next month if i can tune the servo with it, it would be very nice because this servo driver has no auto tuning. Its a servo motor at the end there is no meaning to leave it there idle.

Its not going to help. For instance you need to monitor the error between the commanded position and the actual position.
Both are numeric quantities that cannot be displayed by a regular oscilloscope. The oscilloscope provided in the set-up and tuning software
takes those numeric values and displays them, but those value are specific to each servo manufacturer. You might say that its a software scope,
a regular hardware scope will not help.

Craig
'I enjoy sex at 73.....I live at 71 so its not too far to walk.'
Re: dangerous malfunction of a machine driven by Mach4
« Reply #35 on: February 27, 2022, 03:11:02 PM »
Hi,


The mass of the ballscrew:

mass= 0.022. 2. PI.8000    (radius of ballscrew =0.02m, length=2m, density of steel =8000kg/m3)
   =20.1kg
First moment of inertia of a cylinder:

Iballscrew=1/2 . m. r2
   =0.5 x 20.1 x 0.022 
  =40.2 x 10-4 kg.m2

The first moment of inertia of my 750W Delta servo  is 1.13 x 10-4 kg.m2 so your 2.2kW servo will be more, much more lets guess 6 x 10-4 kg.m2

Iservo=6 x 10-4 kg.m2

The equivalent first moment of inertia of the linear axis:
Ilinear= mass. (p/2.PI)2       (p is the pitch=0.005m and mass is the linear axis mass=600kg)
   =600 x (.005/2 x PI)2
   =3.8 x 10-4  kg.m2

So the total first moment is:

Itotal=(40.2 + 6 +3.8 ) x 10-4 kg.m2
   =49.8 x 10-4 kg.m2

Very obviously, and as I predicted, the rotational inertia of the ballscrew dominates  81%, with the armature of the servo in second spot with 12% and the 600kg axis at only 7%.

This result may surprise you but because the ballscrew and the armature have to rotate SO fast while the axis moves but only slowly, the inertia is totally dominated (93%) by the rotating
components and the 600kg axis mass adds only 7%!!!!.

Angular acceleration=torque/ first moment of inertia
  =11 Nm/ 48.9 x 10-4
   =2249 rad.s-2

Converting to linear acceleration:

accellinear=accelangular.(p/ 2.PI)
   =1.79 m.s-2

or  0.18g   which is not too shabby for a 600kg axis.
The inertia ratio is:
Inertia ratio= (Iballscrew+Ilinear)/Iservo
  =7.3

So most good servo software should handle this inertia ratio well, and manual tuning will probablt not get you much extra.

Craig
'I enjoy sex at 73.....I live at 71 so its not too far to walk.'
Re: dangerous malfunction of a machine driven by Mach4
« Reply #36 on: February 27, 2022, 06:14:58 PM »
Hi,
just seen a 2kW Delta servo, 130mm flange , and it has a moment of inertia of 14.59 kg.m2.

I took my 750W Delta and multiplied by four to come up with a guess for your servo, may be its not enough. It's still not going to change
the fact that the ballscrew vastly dominates the inertia equation.

Craig
'I enjoy sex at 73.....I live at 71 so its not too far to walk.'
Re: dangerous malfunction of a machine driven by Mach4
« Reply #37 on: February 27, 2022, 09:06:09 PM »
Hi,
whoops, typo in previous post; should be 14.59 x 10-4 kg.m2.

I was only a factor of 10,000 out!

Craig
'I enjoy sex at 73.....I live at 71 so its not too far to walk.'
Re: dangerous malfunction of a machine driven by Mach4
« Reply #38 on: February 28, 2022, 07:03:28 PM »
Hi Craig,

So we can say 2.2kw servo can drive the 600kg of mass with 1.79ms speed right?

ok now i have to find a way to tune this servo without harming the equipment which it will be connected.

Hakan
Re: dangerous malfunction of a machine driven by Mach4
« Reply #39 on: February 28, 2022, 07:28:04 PM »
Hi,

Quote
So we can say 2.2kw servo can drive the 600kg of mass with 1.79ms speed right?

No, not speed but acceleration. An 11Nm torque will cause the 600kg axis to accelerate at 1.79m/s2.

The ultimate speed will depend on the servo. What is the rated speed of the servo?. These medium inertia servos are often 2000 rpm or 1500 rpm rated.
lets say 2000rpm, then the max ultimate speed is:

Vmax= Nmax.p /60 m/s
  =2000 x 0.005 /60
  =0.1666m/s  or 10m/min.

With an acceleration of 1.79m/s2 the time taken to reach Vmax:
taccel=Vmax/acceleration
  =0.1666 / 1.79
  =0.0927 s or 93ms.

93ms from standstill to Vmax is pretty good, and my guess is that your machine will lurch around and you'll probably want to slow it down.
My machine is capable of 25m/min and acceleration of 2.7m/s2, but I slow it down to 15m/s and 1.5m/s2 and even then its still scary fast.

Craig
'I enjoy sex at 73.....I live at 71 so its not too far to walk.'