Jerky movement on X/A axis

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Jerky movement on X/A axis

Postby BensPlasmaAU » Sun Oct 27, 2019 6:19 am

Hi all,

I have finally got my CNC plasma table up and running. So i will start off with the specs of it and then ask the question.

UCCNC UC300 on a UB1
Neuron THC
Clearpath Servo SDSK 3441 motors
10:1 helical planetary reduction gearboxes
Mod 1.5 helical rack and pinion.

So i have tuned the Clearpath Servos as per Teknics instructions and everything is working OK. When i use UCCNC for jogging and cutting at slow speed the X/A axis is really jerky. On faster jog speeds and cutting it's as smooth as silk. I did have a little problem with corners and holes so I made a few snippets in SheetCAM and added a rule for all corners to turn off THC 5mm before, slow to 70% feed rate, 10mm after turn on THC and 100% feedrate. So far i haven't been able to test this too well due to needing to cut and test on 6mm steel, which is where the slow speed issue comes in.

On my UCCNC home screen and config screens, the options for CV mode etc is all over the place and i'm not sure which tick box and field box is for what. So that doesn't help too much.

Anyway, this jerky motion really stuffs with a lot of the ability of the machine. Y axis is working perfectly. It has the same mechanical configurations as the X/A but uses one motor instead of 2 slaved together. All settings for steps per and velocity etc are the same for all 3 motor assemblies.

Any help or ideas would be greatly appreciated. Andrew from Neuron has been really helpful as well. I'm just trying to get as much info as possible to assist with diagnosing what is happening so i can get cutting quicker.

Cheers,

Ben
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Re: Jerky movement on X/A axis

Postby cncdrive » Sun Oct 27, 2019 6:16 pm

If jogging at slow speed is jerky then it is a problem with your servos, probably a bad PID tuning or an incorrectly matched servo inertia which means a bad sizing of the motor to the load (undersized motor inertia to the load).
I would try to tune the servos. And would also verify that the load inertia is no more than 10 times larger than the inertia of the rotor. If it is larger then it will never run properly especially not on low speeds. Then a larger motor is needed or a higher gear ratio between the load and the motor.
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Re: Jerky movement on X/A axis

Postby BensPlasmaAU » Sun Oct 27, 2019 10:29 pm

Do you have a link to information on how to calculate the load inertia? I thought i had oversized the motors enough, but i may have been wrong.
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Re: Jerky movement on X/A axis

Postby cncdrive » Sun Oct 27, 2019 10:38 pm

With steppers it is easy to oversize and it is hard to make a mistake. With servos it is easy to make a mistake, because you would assume from the motor size that it is oversized, but it is not a stepper motor, servos have much lower torque than the same motor size stepper. Steppers have high low speed torque and the torque drops as the speed getting higher. Servos have about constant torque on all their speed range, but that torque is much lower than the torque of a similar sized stepper. Servos also have a property called peak torque which is usually 2 to 5 times (depends on the motor) the continous torque, that helps to accelerate and deccelerate rapidly, but you should never use the peak torque for torque calaculation.

And you can also make a mistake if you too much oversize a servo, because then the dynamics of the system will be much lower than it should be.
So, it is not so easy with servos than it seems for first thought...
The basic rule is to make the inertia matching in the 1:1 to 1:10 range. (rotor:load)

Inertia calculation depends on your driving mechanism, if it is with screws or with racks and pinions or belts etc.
Google it and I'm sure you will find some formulas. If I recally there is also a free inertia and torque calculator software available somewhere on the internet (can"t recall where), but if I recall it was written by Yaskawa.
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Re: Jerky movement on X/A axis

Postby BensPlasmaAU » Mon Oct 28, 2019 12:04 am

Thanks cncdrive. Looking at the specs for the motor is 2.8kg/cm². The weight of the gantry in total is 160kg.

I am thinking that the rotor inertia is referring to the area of the motor as viewed as a cylinder. The motor is 86mm². So this is where i get lost in working out what the 2.8kg figure is referring to.

I did a bunch of calculations using the info i found online mainly from motioncontroltips.com and i got 1.25:1 inertia ratio.
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Re: Jerky movement on X/A axis

Postby jcoldon » Wed Oct 30, 2019 4:05 pm

i built a lot of plasma tables and designed a lot of gantrys and tested a lot of controllers more then most oem . i can tell you uccnc is a very smoothe operating controller
check my face book site cnc plasma table builders . now your problem

you did not say what model clear path motor you used ? thers like 500 models of them . what type of rail system did you use ?
the 10 to 1 reduction is a lot of reduction for plasma it would depend on your motor selection . gantry weight . slide resistance .rack rear pitch .
i generaly try to do builds with 1000 ipm rapids .. with plasma we need to go very slow on thick steel and faster on thin stuff
like say 45 amp 16 ga you need 350 ipm hypertherm book speeds . and no slow down in corners so a constant velocity is nessary

last thing i not a fan of clear path dont like the 5 v pulse out of the controller on the table . DMM servos are a better choice for plasma there ac 34 size is realy nice
that dosent help you . i need to know what model clear path you chose and gantry weight and slide system rack gear pitch and size

post a pic on my face book site i do the best to help out thers videos showing what uccnc can do in cut quality also
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Re: Jerky movement on X/A axis

Postby jcoldon » Wed Oct 30, 2019 4:31 pm

i just saw that motor number you posted SDSK 3441 fit that a D motor
i did a KG weight conversion to LB it said 350 lb that heavy that motor never do that

if it an sdsk 3441 D it only a 159 OZ inch motor at with 3000 rpm max plus you have 10 to 1 reduction

i sugest you use a .750 K watt ac servo 34 size like this https://dmm-store-shopify.myshopify.com ... ervo-motor

with 10to 1 reduction i have a spred sheet for this on my face book page cnc plasma table builders
Attachments
Copy of motor load calculation - CUR1358 (5).xlsx
motor load spred sheet
(53.53 KiB) Downloaded 6 times
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Re: Jerky movement on X/A axis

Postby Robertspark » Wed Oct 30, 2019 7:40 pm

how are you driving the 160kg cross slide?
rack and pinion?

what size (teeth, mod, diametrical pitch)?

what is this 160kg mass running on ? linear rails, thk rails?

have you greased anything?.... what grease did you use?

it probably won't affect you, but there can be an effect known as slip-stick if you've greased something with the wrong lubricant.

160kg for a carriage seems a lot for a plasma cutter where acceleration is important
Rob
Einstein ― “If you can't explain it to a six year old, you don't understand it yourself”
...working my way through the 1000+ ways things don't work to find the one that does
UC400eth, UC300eth, UCCNC v1.2110, Neuron Lite
UCCNC v1.2110 Macro Manual
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Re: Jerky movement on X/A axis

Postby beefy » Thu Oct 31, 2019 8:10 pm

Just a little thanks to all respondents for the valuable info.

If I ever move to servos (I'd love the fast rapids :twisted: ) this knowledge might save me some expensive mistakes.

Keith.
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Re: Jerky movement on X/A axis

Postby kawarider » Fri Nov 08, 2019 10:04 am

Sorry to hijack this topic, but my plasma runs jerky too. It is something in sheetcam or uccnc. The code from sheetcam has a lot of small arcs in it. When i use no rules there is no problem. But now, since neuron thc is installed, i need rules again. I am sure someone has the same experience but cant find a solution.
Cv is on.

If it s better to make seperate topic. No problem

Thx

Uccnc is super, neuron too. Works great, only this minor thing.
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