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Author Topic: Motor tuning, please take it easy on me I'm new to this :)  (Read 1305 times)

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Motor tuning, please take it easy on me I'm new to this :)
« on: August 29, 2019, 05:44:29 PM »
Hi could I have some advice on my new to me but used CNC router.

I'm very new to Mach3 but have been programming woodworking Cnc's for about 10 years.

Here's the spec and the question,

My machine is approximately 1mtr x 1mtr cutting area with 150mm Z clearence, 3 Axis
2.2kw Chinese water cooled spindle
4 x Nema 34 stepper motor 878 oz.in bipolar Model 34HST9805-37B2 Step Angle 1.8, Rate Voltage 6.4, Rate Current 2A, Phase resistance 3.2,Phase Inductance 15mH
4 x Stepper motor drivers DM860A, PEAK 7.8A , Set to 3.0 RMS Peak 4.2A and 2000 microstep resolution
4x Power Supply  200W-60VDC
Cheap chinese HY-JK02-M 5-axis interface board. ( I'm not enthused with the board now I understand a little bit more about them and at £10 cost it seems penny pinching to run an expensive machine with one)
16mm dia 16mm pitch 4 start thread lead screws on all axis


My Mach3 motor settings are the same for for all three motors

Steps per = 125
Velocity = 2000 mm/min
Acceleration =650 mm/sec
Step Pulse =2
Dir Pulse =2


Kernal speed at 25000 Hz

All that sounds like I know what I'm talking about but I don't :)

My issue is the axis speed only been at 2000mm, I would like to get to at least 5000 or slightly higher if possible.

I've been informed that it should be possible with the current setup but when I increased just the velocity I got some stalling on the X axis when jogging around.

I've watched many videos and read all sorts of info and it seems motor tuning is an art as well as a science and becuase it's a reflection of the sum of the parts it can differ for everyone.

From what I've gleaned I maybe able to increase the steps per to 200 as that is what my motors are rated at and also that my stepper motor drivers could have been much bigger voltage which would also improve things but I don't how this works, oh and less microsteps?

If anyone can advise with explanations so i can get my head around how it is worked out that would be great thank you..  ;D

Re: Motor tuning, please take it easy on me I'm new to this :)
« Reply #1 on: August 30, 2019, 02:54:16 PM »
Hi,
I suspect with those steppers that 2000 mm/min is about as good as you are going to get.

All steppers lose torque the faster they go. The principle determinant in how badly they fare at speed is
the winding inductance, the higher the inductance the quicker the torque degrades with speed.

For 34 size motors you should be looking at units of 2-4mH, such motors will have about 30% of their holding
torque at 1000 rpm.

Your 15mH units are likely to have less than 5% of its holding torque at 1000 rpm. That describes why you
cannot increase the axis speed without loosing steps.

Craig
My wife left with my best friend...
     and I miss him!
Re: Motor tuning, please take it easy on me I'm new to this :)
« Reply #2 on: August 30, 2019, 04:07:54 PM »
Hi,
to flesh out the answer I have taken these published torque/speed curves from the Leadshine website for two of their
34 size steppers.

The smaller of the two (86CM35) has a rated torque of 496 oz.in with a very respectable inductance of 2.67mH. Note also the
low resistance of 0.42 Ohm. The time constant, a figure of merit, is 0.42 X 2.67=1.12ms.

The slighter longer(86CM45) motor has a rated torque of 638 oz.in and a still respectable 2.95mH inductance with a resistance
of 0.43 Ohm for a time constant of 1.27ms.

On the face of it you would say the bigger motor is better....right......but the smaller motor does very well at speed. The somewhat
smaller inductance, and therefore time constant, means that it retains 50% of its torque at 1000 rpm whereas its bigger
sibling retains 30%. In fact at 1000 rpm both motors produce the same torque (1.6Nm).

This illustrates the point that sometimes smaller motors but with low inductance can outperform larger motors but with
commensurately higher inductance.

Leadshine is one of the few companies to publish torque/speed curves but they are very useful. Both of these motors are
'low inductance designs' and are both good but note how even a small reduction in inductance can have a large bearing on the
motors performance at speed.

Your 15mH units will fall well WELL short of these, and note Longs Motors don't publish curves. I think you should
consider replacement.

Your 16mm pitch leadscrews are aggressive and you will require high torque to have reasonable acceleration and thrust to
accommodate cutting forces but really only modest speeds.

For instance at 2000 mm/min with a 16mm pitch the motor rpm need only be 125 rpm.
To get 5000 mm/min with the same leadscrew the motor rpm is 5000/16=312.5 rpm.

In both cases the required rpms are not high. Would you consider putting a belt or gear reduction or 2:1 in place?
That would effectively make your steppers twice the torque and yet still not require high rpms, 625rpm at 5000mm/min.

For example the 86CN35 of 496 oz.in rated will have nearly 1000 oz.in with a 2:1 reduction, even more than your existing
motors and yet according to the torque/speed curve still have 2.2Nm at 625 rpm or 2.2/3.1=71% of its rated torque.

Craig

My wife left with my best friend...
     and I miss him!
Re: Motor tuning, please take it easy on me I'm new to this :)
« Reply #3 on: August 30, 2019, 04:21:06 PM »
Hi Joe,

Thanks for the reply and information,

I'm a little lost with what your saying here, your first paragraph says to get 5000 rpmmy motor would spin at 312.5 rpm which your implying i will have next to no torque with these motors at that speed. The next paragraph says with gearing i would be at 625rpm to get the same speed surely there would be no torque left at all then?



"For instance at 2000 mm/min with a 16mm pitch the motor rpm need only be 125 rpm.
To get 5000 mm/min with the same leadscrew the motor rpm is 5000/16=312.5 rpm.

In both cases the required rpms are not high. Would you consider putting a belt or gear reduction or 2:1 in place?
That would effectively make your steppers twice the torque and yet still not require high rpms, 625rpm at 5000mm/min."



What I'm planing to do in the morning is change the stepping to 1/8th 1600 on the drivers and reduce the acceleration from 650 to 250 and see what happens.

What effects will increasing the supply voltage make as it's been mentioned my motors could take 120v rather than the 60v from the current supplies?
Re: Motor tuning, please take it easy on me I'm new to this :)
« Reply #4 on: August 30, 2019, 04:28:10 PM »
Hi,
the next step is genuine-honest-to God AC servos like:

https://store.dmm-tech.com/products/dst-0-4kw-ac-servo-motor?variant=23270271046

Its rated torque is 1.27Nm and short term overload rated at 3.82Nm. With a 3:1 reduction that equates to
3.81/11.46  Nm at the leadscrew or 500/1500 oz.in at 1/3 of rated rpm or 1000rpm. On 16mm pitch
leadscrews that is 16000mm/min....it that fast enough???

Craig
My wife left with my best friend...
     and I miss him!
Re: Motor tuning, please take it easy on me I'm new to this :)
« Reply #5 on: August 30, 2019, 04:29:01 PM »
Looking at both of those motor graphs the torque is very good at rpm's below 500, so surely running at 312rpm will be fine or am I missing something obvious?
Re: Motor tuning, please take it easy on me I'm new to this :)
« Reply #6 on: August 30, 2019, 04:40:33 PM »
Hi,

Quote
I'm a little lost with what your saying here, your first paragraph says to get 5000 rpmmy motor would spin at 312.5 rpm which your implying i will have next to no torque with these motors at that speed. The next paragraph says with gearing i would be at 625rpm to get the same speed surely there would be no torque left at all then?

I understand you confusion.....you are right..... I believe your existing motors have vanishingly small torque at even low speeds.
No amount of gear reduction or change in mirostepping or increase in driving voltage is going to materially improve that.

Quote
What I'm planing to do in the morning is change the stepping to 1/8th 1600 on the drivers and reduce the acceleration from 650 to 250 and see what happens.

Try it and see. I suspect changing the microstepping from 2000 to 1600 will be scarcely noticeable, in fact changing microstepping
to any setting will have no effect on the motors torque at speed.

Reducing acceleration may help. High acceleration promotes stalling. However you want the highest acceleration you can for
accurate toolplath following and acceleration is vital to fast cycle times.

Quote
What effects will increasing the supply voltage make as it's been mentioned my motors could take 120v rather than the 60v from the current supplies?

Increasing the voltage is the classic means of overcoming high inductance steppers. The highest voltage drivers I have seen
(Leadshine and Gecko) are 80V, so a 25% increase on what you have already. Higher voltage drivers are always an advantage
but I think you will get much MUCH more benefit from replacing those high inductance steppers.

Craig
My wife left with my best friend...
     and I miss him!
Re: Motor tuning, please take it easy on me I'm new to this :)
« Reply #7 on: August 30, 2019, 04:51:35 PM »
Hi,

Quote
Looking at both of those motor graphs the torque is very good at rpm's below 500, so surely running at 312rpm will be fine or am I missing something obvious

Yes, you are correct, they both have excellent torque at 312.5 rpm, but even the larger of the two motors still has only
2/3 the torque of your existing motors. You will probably require q belt/gear reduction to have either of these motors match
the low speed torque of your existing motors.

You could of course go for higher torque motors and not need a belt reduction. The only trouble with that approach is that
higher torque motors also have higher inductance.

The 86CM80 motor by Leadshine has a rated torque of 1133 oz.in and an inductance of 4mH with a time constant of 2.5ms.
This would be a superb replacement for your existing motors, no reduction required and 1/4 the inductance.....a huge step
in the right direction.

Craig
My wife left with my best friend...
     and I miss him!
Re: Motor tuning, please take it easy on me I'm new to this :)
« Reply #8 on: August 30, 2019, 04:54:06 PM »
So you you believe 312.5 rpm is far to fast for these motors and at that speed there will be no torque?





[/quote]

Try it and see. I suspect changing the microstepping from 2000 to 1600 will be scarcely noticeable, in fact changing microstepping
to any setting will have no effect on the motors torque at speed.

Craig
[/quote]
Re: Motor tuning, please take it easy on me I'm new to this :)
« Reply #9 on: August 30, 2019, 05:11:44 PM »
Hi,

Quote
So you you believe 312.5 rpm is far to fast for these motors and at that speed there will be no torque?

Without some published data from the manufacturer I cannot tell for sure.

You stated that you tried to increase the max velocity and they started stalling......doesn't that tell you exactly that?
They work OK at 125 rpm but stall out at 312.5 rpm.

Longs Motors make stepper motors with really high torque but also very high inductance. They sell to first time buyers
who think the higher the torque the better but what they don't realize is that high inductance kills a motor at speed.
Savvy buyers want to know both the torque AND the inductance, or to be technically correct, time constant.

Until I mentioned 'time constant' in this thread had you ever heard of it? Would you consider making a buying decision on
the basis of a time constant?

Now that you have heard of it and seen torque/speed curves that illustrate what a 'time constant' means in the real world
what would be your decision today?

May I suggest that if a company/supplier cannot provide either a torque/speed curve, or a time constant or an inductance
then walk away....they are selling to first time buyers.

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