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Setting new cnc router build to achieve max servo speed
« on: April 13, 2018, 01:01:18 PM »
Hi folks, I'm in the second CNC build and about to start setting my servos from kelling inc type KL34-180-90 which runs at 3200rpm (off load) and would like to utilise it's full potential to achieve max travel speed. According to calculation these should result in16 meters per min (or 266mm /sec). I know that this will be reduced with the load of the gantry weight and friction, but would be happy if I get close to 200mm/sec.
My question is how could I reach my goal as preliminary tests / settings on Mach3 are showing that this is far away from this target. I have also experienced this too with my first build 10 years ago, but way that time I was happy with the results even though the servos run at 6000rpm! Any suggestions On how to go about this?
Thanks
Marting
Re: Setting new cnc router build to achieve max servo speed
« Reply #1 on: April 13, 2018, 04:34:20 PM »
Hi,
I just Googled your servo and found the KL34-180-90 and it has a Kv 0f 18.95 V/100 rpm. Can you confirm?

To make it run at 3000 rpm would require a servo drive with an open circuit voltage of:

18.95 X (3000/100)=568.5 V

Without PFC boost or a transformer that would require three phase power of 415V. Do you have three phase?
Do you have a servo drive capable of that voltage?

Do you actually own these servos or are you going to buy them?
If you don't I urge you consider AC servos, they are a more modern solution and work out cheaper than old style DC servos.

Craig
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Re: Setting new cnc router build to achieve max servo speed
« Reply #2 on: April 13, 2018, 04:56:39 PM »
Hi,
could that specification be a misprint?

https://www.automationtechnologiesinc.com/docs/manuals/KL34-180-90.pdf

Craig
My wife left with my best friend...
     and I miss him!
Re: Setting new cnc router build to achieve max servo speed
« Reply #3 on: April 13, 2018, 06:08:59 PM »
Hi,
I'm going to assume that the Voltage Feedback constant is a misprint, it should be 18.95V per 1000 rpm.

That would require an open circuit voltage of 57V, much more reasonable and sensible. Probably worth an email to the manufacturer to confirm before spending
all that money.

As to how fast Mach can drive such a servo will depend on what encoder you use.

If you use a 500ppr encoder for 2000 count per rev then at 3000 rpm or 50 rev per second Mach would need to produce a 100kHz pulse stream. Machs parallel port
just wont go that fast. An external motion controller like a SmoothStepper will do it easily.

The situation gets even worse with higher resolution encoders, say a 2500ppr or 10000 count encoder will require 500kHz pulses, well into differential signaling with something like
a SmoothStepper.

You may have noticed that all AC servo drivers have 'electronic gearing'. It consists of a clever bit of computing that allow you to specify 'how many encoder counts are advanced per input pulse'.
It is done by you programming two numbers, a numerator and a denominator. For example:

1000 (Mach input pulse) X (250 (numerator)/ 5 (denominator) )= 50,000 (encoder counts)

Thus you can use a relatively slow input pulse stream like Machs parallel port to produce highspeed motion with high resolution encoders as commonly fitted on servos. This extra computing facility
is built into the servo drive and allows great flexibility when using AC servos. I'm not aware of any DC servo drives that have the same facility.

Craig
My wife left with my best friend...
     and I miss him!
Re: Setting new cnc router build to achieve max servo speed
« Reply #4 on: April 14, 2018, 06:18:10 AM »
Hi Craig, thanks for your feedback. I'm using a 500ppr encoders, gearing down ratio 2:1 and 10mm pitch ball screws. This setup should give me a repeatable accuracy of 0.01mm (i.e. per encoder count) With the servos I got the pulses from the encoders should result to 53KHz,  which is in range of Mach3 frequency output. Also I'm using a UC300-5LPT USB motion controller from CNCDrives, so as far as I can see, speed shouldn't be an issue to setup but apparently this is indicating otherwise.
I guess I'll have to put the servos in place and see in reality if there would be an issue or not.

Regarding the servo data, yes I too assumed that it must have been a misprint, precisely as you are saying.
Thanks
Marting
Re: Setting new cnc router build to achieve max servo speed
« Reply #5 on: April 14, 2018, 06:42:25 AM »
Hi,

Quote
which is in range of Mach3 frequency output
If you are using a UC300 then Mach doesn't produce pulses at all, it produces numeric data (Position/Velocity over Time) to the UC300 and the UC300 produces the pulses.
If memory serves the UC300 can handle 100kHz.

500ppr encoders are 2000 count per rev. At 50 rev per second or 300rpm that is 2000 X50=100kHz

Quote
With the servos I got the pulses from the encoders should result to 53KHz
Don't know how you calculated that.

I am still of the opinion that AC servos would be superior.

https://www.automationtechnologiesinc.com/products-page/ac-servo-motors/dmm-dyn4-servo-kit-750w-w-nema-34-frame/
From the same supplier, greater power, much expanded control options including electronic gearing. Note that you don't need a power supply, just hook up 230VAC and your ready to go.
If you compare costs (DC servo plus encoder plus drive plus power supply) then the AC servos start looking like good value and once you realize the flexibility that AC servos have....

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

Offline ger21

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Re: Setting new cnc router build to achieve max servo speed
« Reply #6 on: April 14, 2018, 07:44:56 AM »
Set your steps/unit to 400, and your Velocity to 15000, and you should get 15m/min.
I'd probably set it to 13000 or 14000, so you're not trying to run right at the 100Khz Limit of the UC300.
Craig had a typo above. It should have read 3000rpm = 100Khz.
Gerry

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Re: Setting new cnc router build to achieve max servo speed
« Reply #7 on: April 14, 2018, 12:26:01 PM »
I have to admit, I am now having doubts if I'm doing this right.

Craig - Here is how i am doing the calculations 3200rpm ÷ 60sec ÷ 2 gear ratio x 500 encoder cpr = 53000Hz. Regarding the ac servos these are out of my budget at the moment.

Gerry - I have set the steps per unit on Mach3 to 100 and the maximum velocity I could achieve was about 12000 but the rpm resulting on the servo motors are nowhere near to what Mach3 is set to do. How can i set steps per unit other than those that reflect the actual pulses?
Re: Setting new cnc router build to achieve max servo speed
« Reply #8 on: April 14, 2018, 02:32:46 PM »
Hi,

Quote
Here is how i am doing the calculations 3200rpm ÷ 60sec ÷ 2 gear ratio x 500 encoder cpr = 53000Hz

Close but don't divide by 2 for the gear reduction. If the encoder is monted on the ballscrew it would be correct but you want to put the encoder on the servo, the shaft
for it and the mounting holes are already provided.

So if you are convinced that you want to run it at 3200 rpm it will require a pulse stream of 106 kHz. I see the CNCDrive servo drives have a rather crude electronic gearing
mechanism which could be used to bring the pulse rate required to comfortably within the UC300 capabilities.

The bottom line is that the hardware you have proposed will work. Is it the best combination for the money?

For instance:
The Keling servo is rated at 7.8A continuous with a torque constant of 0.1895 Nm/A thus rated torque is 7.8 x 0.1895=1.478Nm.
At 3000 rpm which is (3000/60)X 2 XPI=314.15 radian/sec for rated power output of 314.15 X1.478 = 463 W.

You could therefore expect a 400W AC servo to perform very siimilary to the DC servo you have proposed which would be cheaper than the DC servos.

Alternately you could go for a 750W AC servo such as the link above and the extra torque would mean you don't have to have any gear/belt reduction
for a traverse speed of 30m/minute for very similar money.

Brushed DC servos are old school stuff, AC servos are the current and have been for 15 years.

Craig
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     and I miss him!

Offline ger21

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Re: Setting new cnc router build to achieve max servo speed
« Reply #9 on: April 14, 2018, 02:44:43 PM »
A 500 count encoder is usually 2000 pulses/rev with quadrature. Which gives you 400 steps/rev.

What drives and voltage are you using? As Craig said earlier, voltage can limit your rpm.
Gerry

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