[pupetxls]

Hello everybody,

Don't know if this is the right place for this topic, hopefully I'll find somebody who allready walked these steps and could give me a helping hand. I'm so tired, to reinvent the wheel all the time...

I changed my old machine from trapezoidal threaded screws to a new sistem, with T5 belt and pulleys.

The problem that I'm having is that the stepper induce too much vibration into the cariage.
X axis - stepper sits on the chariott and the belt is fixed.
Y axis - stepper sits fixed, and the chariott is hooked onto the belt wich is rotating onto 2 pulleys.

The steppers on their own are fixed onto the parts, with rubber washers so this part of the vibration is eliminated. But, the axle of the stepper still vibrates, and this vibration is led through the directly mounted pulley, and to the belt, and so, to the whole sistem.

Both axis vibrate. Of course Y vibrates abit less then X but still. I do need very smooth motion on the machine, no vibrations allowed.

The vibrations are also increased due to the fact that I use 1:1 ratio, and I have to use microstepping and extremly low speeds. Here my motors increase the vibration.

It seems that I have 2 posibilities:
a. - built a gear ratio sistem 3:1, to increase the speed of the stepper, while reaching same chariott speed
b. - built a flexible sistem between the drive pulley and the stepper

Or is there any C option?

I wouldn't really want to go back to the threaded rods. I would like to stay with the T5 belts.

If anybody hit his head with this problem before me, I would highly apreciate some advices.

Thanks in advance,
Mihai

[ger21]

Option A. 4:1 is probably even better.
What size pulley are you using? If it's over 15 tooth, then higher reduction should be used.

The stepper should be rigidly mounted, not on soft rubber washers. That could make the problem worse.

Here's Option C.
Best belt drive ever! (If I do say so myself) - CNCzone.com-The Largest Machinist Community on the net!

[pupetxls]

Dear Gerry,

Thanks for the quick reply.
I'm using 10 teeth pulley. That's why I assumed 3:1 ratio should be enough. Maybe if I avoid the very low turn rates of the steppers then vibration should go away.

The example you showed me, seems very interesting, but unfortunately it would require massive re-work on the machine.

Also, still don't know how it would operate in my case, at low speeds.

I'm still in the blurr... same questions still persist.
Thanks,
Mihai

[harryn]

Hi, can you give us a little more info on your drive setup ?

Examples
- belt width
- weight of the gantry
- how are things mounted
- how did you tension things
- stepper motor brand and driver

As Ger21 pointed out, things need to be mounted rigidly in most cases.

From the sounds of it though, I wonder if you have stepper motor resonance going on. Do some searches about resonance and the fixes here and in the section about stepper motors in the forum. It happens often and there are simple fixes for it.

I know its a pain, but if you have any method of posting pictures and / or some videos, it helps a lot - and we all learn from them. Don't worry about your system being "show quality" or anything, we like raw metal parts and 1/2 finished items.

You might even consider opening a "build thread" in the wood working thread section, and posting a link here for it.

Harry

[pupetxls]

Dear Harryn,

Thank you for your reply.
Attached you have some pictures with the old setup, kinda direct drive, lets say, and some 3d of a plantery sistem that I tested, and the actual version 3:1.

They say you learn a lot by hitting your head, so I had to reinvent the wheel one more time. Time, effort and money out the window.

I used T5 belts, with 10teeth pulley. But in my case it doesn't seem to work. As you know I need very slow speeds for foam cutting. This way I got one turn of the motor to 50mm travel of the cariage. I have cuts even as low as 0.5mm/sec, so you can imagine. At this speed and at such a small movement, the motor doesn't feel like turning smoothly, but instead it feels like jerking the steps. Of course, the belt only amplifies the whole vibration.

So now I made a new setup, with gear ratio 3:1. It is abit better but still not satisfied. Tension of the belts add up, friction of the pulleys (no ballbearing, just pulley on bronze bushing/axle). It's better for the motor turns, but it's worst for the vibrations. I do use antivibration shims on the steppers, and that rubber wheel (don't know how it's called) on the other end of the stepper axle. Oh yeah, I forgot to mention, I even changed the steppers, and it's the same.

I think it would've been better to go for a T2.5 belt, and maybe just straight without gearing. This way I would've had 25mm per turn. It would've been somewhere in the middle between the motor turns and vibration. "no free meal".

This machine is my 4th one, (first one with belts) and it should've been an improvement on the other acme rods style builds. It's worst.

I also tried with planetary reduction, or just sprockets reductions, and I'm still not happy. I used to laser cut the cogs, but it seems, even lasers can't do perfect cuts.

The motors that I use are some type of Astrosyn, but they are OEM stuff. No info whatsoever, just 2.8 A and that's it. The only setting I can do is within the controller to play with the current. I have for settings: 0.7A, 1.75A, 2.62A, 3.50A. Best results (less vibration) I got for 0.7A setting. Here I don't need any power or speed, so it's ok.

I don't want to offend anybody, but the thing is, there are a lot of foam cutters out there or cnc's, that look fine and work fine. Depending on what you use them for. But if you are using them for production, and DO NEED high acurracy, repeatability and top notch efficiency, maybe less then half of them are ok. One can only stare on the internet for kits or different builds, they all seem fine. But when you try and use them, you realize it's like using an everyday car, on a rally scene.

I will post some pics with the new one tomorrow.

Take care everybody,
Mihai

[mike_Kilroy]

...........I changed my old machine from trapezoidal threaded screws to a new sistem, with T5 belt and pulleys.

The problem that I'm having is that the stepper induce too much vibration into the cariage.
Mihai

Mihai,

Ask yourself if you were happy with the amount of vibration before this change.

Yes?

then your solution is: you want your motors to go the same nunmber of revs per mm of travel as before!

why guess as to what gearing to add? you seem to say you cannot get from 3:1 to the suggested 4:1 gearing due to mechanics of pulleys. by the time you increase to 4,5,6:1 you will probably have such large inertia pulleys that the added ratio benefit will not gain you much. why fight it?

how much extra gearing did you elminate by your change? that is what you want to add back. I bet it is not 3:1 or 4:1 but 10 or 20:1. no wonder you have vibration problem! those steppers do NOT like huge inertia loads like a direct drive belt pulley system will give them.

why not google "nema 23 inline gearhead" or nema 34 ( I assume this is size of your small steppers). buy a $ 100 inline gearhead and bolt it to the front of your stepper motor to get the turns per inch or turns per mm back to what worked previously. these low cost boxes bolt on and have the same nema output mount to make life easy.

[harryn]

Hi, Thank you for posting those pictures - they help a lot.

I am also working on my first cnc router, which is also a belt drive, so I am behind you in accomplishment, but have done a lot of reading and calculations on the subject.

As far as pulley reduction, I would not spend a lot of time on that approach if it were me, assuming that the motors can move the pieces fast enough for your application. Each reduction is just more chances of something going wrong, a part being non concentric, a belt stretching, etc.

3:1 is viable, but going more than that is probably a waste of time, and I could make a reasonable argument against belt reduction at all for those systems.

When you run a stepper motor at low speeds, this is actually (usually) a good thing. This is where the motor has maximum torque, and usually very low resonance.

You probably already know this, but just in case:
- Most stepper motors are built for 200 "full steps" per revolution.
- Your driver can also sub- divide these "full steps" into micro steps - typically 10 - 100 or more per "full step".
- This will not improve your accuracy, but it might help make the steps taken small enough to greatly reduce the vibration.
- There is some torque loss from doing this, but your motors seem to be plenty torquey already, or at least it seems so.

I am guessing that your driver can in fact go down in steps to micro steps. If you can't figure out how to do it, then consider either a gecko type driver, or I am playing with linistepper drivers. They are a kit that you buy for $ 30 / each, and are not hard to build - cause I am inept and did it.

I can't tell for sure, but I am guessing that the real problem is that the driver is telling the motors to take "full steps", which are a bit too coarse for what you are trying to do.

[harryn]

A little more.

I am not sure if it matters in real life, or is just desired, but in general you should try to avoid situations where the belt goes through "back bending". It is getting into pretty tight turns in some of those areas.

If you do decide to change belts (and I am not saying you should just yet), I would consider AT style belt vs T style belt. The reason, is that it fits better into the geared pulleys. There are some others that are big on other profiles, and I am not qualified to argue the point to the wall, but I can refer you to more qualified supplier info if you are interested.

A good place to start in general is to focus on just one axis. Maybe see if there is anything else that is adding to this vibration. For example, if there is a slight "bounce" to the motor movement, then unsupported rails are not as good at damping it out as a supported rail, but that can be a big change in some designs.

In your case, it looks like it might be possible to stick a piece of even foam, wood, or something into the gap between the two support rails without getting in the way. Even a couple of pieces of threaded rod with some nuts on the end to bridge that gap might help dampen out vibration in the rods.

[pupetxls]

Hello guys,

Thank you for your answers.

Dear Harryn, I pretty much walked all of those steps you mentioned.
My motors do run halfsteps, they do now 400 steps/rev. I could go up to 1.600 steps/rev, but it is the same. In an actual cut (depending on the cut) I might get good results. But there are other cuts, where it doesn't work.

My sliding tubes are fixed all along the axis, so no vibration transmitted there.

For starters lets just say that I don't cut letters or signage with it. I cut highly detailed pieces. And there is where I get differences. For a "normal" cutting, it would do the job easily.

Later I will post pics, and you will be able to see that the pulley sistem is quite simple in fact. I use 4 belts, no backbending there whatsoever.

Still, with a simple drive belt, doesn't work.

So I agree with you guys that adding pulley and ratios will solve one problem but it will increase other. Yes, the threaded sistem works fine for me, I have 3.33 turns/linear cm movement. The threaded rod has a step of 3. Only thing with the rods, is that they are noisy. It all started from here . I did had vibrations with it also, but I used a flexybile motor to threaded rod coupling and solved everything.

The only thing that bothers me, is that I saw so many other using belt sistems, I really wanted a go for it. Never expected to turn out like this.

I was also considering going for rack and pinion sistem, but I just think it would be worst.

As for inline gears, yep, I went for the poor man's design, and tried to build them for myself, but it didn't worked out. Maybe that will be the final result.

The last thing that I can think of, is that both of my motors stay on the cariage, and not fixed on to the machine. Maybe placing them at the ends of the axes could do the trick. Who knows, it's again a trial and fit thing...

Will see how that goes.

One more time, thanks guys.
Later I'll post some pics.

Take care,
Mihai

[mike_Kilroy]

...........Yes, the threaded sistem works fine for me, I have 3.33 turns/linear cm movement. The threaded rod has a step of 3. Only thing with the rods, is that they are noisy. .....

The only thing that bothers me, is that I saw so many other using belt sistems, I really wanted a go for it. Never expected to turn out like this.

Mihai

Mihai, ok, your giving some of the data needed to CALCULATE what is going on.....give a tad more and I will do the motor sizing.....

- so threaded one was direct drive motor to 3.33turns/cm screw.
- what is weight of stuff moving on each axis?
- how far does your belt design move per motor rev with direct drive into belt?
- what is max speed you want?

[pupetxls]

Hello Mike,

- weight of the stuff is arround 3kg. Ball bearings secure the slide, so friction is minimal. On the other machines I had teflon bushings sliding on chromed shafts.
- 50mm movement per turn, with belt direct drive. T5 belt - 5mm/step x 10 teeth pulley.
- max speed should be no more than 6mm/sec.

Thanks,
Take care,
Mihai

[mike_Kilroy]

Hello Mike,

- weight of the stuff is arround 3kg. Ball bearings secure the slide, so friction is minimal. On the other machines I had teflon bushings sliding on chromed shafts.
- 50mm movement per turn, with belt direct drive. T5 belt - 5mm/step x 10 teeth pulley.
- max speed should be no more than 6mm/sec.

Thanks,
Take care,
Mihai

So plugging these numbers into the free motor sizing program called Motioneering (http://www.kollmorgen.com/en-us/supp.../motioneering/) shows that moving a 3kg load via belt drive from a nema 23 motor thru 12mm dia pulleys 1:1 to the belt driven axis using a pulley of 15.91mm dia for 50mm/rev rotation, is a load inertia of almost 20x the motor inertia. This is really bad for a stepper motor system. it also shows 6mm/sec speed is only 7.2rpm on your motor max. You have lots of room to add a small in line gearhead that will make the load inertia dissapear and the system probably work like a champ! I would put a 50:1 gearhead on those motors and let it fly.

Just for kicks I then added a motioneering axis using the 3mm/rev old style screw design. Sure enough, the load inertia reflected to the motor with this design is only 1/4 the motor inertia! THIS system should work MUCH better than the belt one with the missing REQUIRED gearhead! And your imperical data says it does! Ain't it great when physics works?

One reason your home made gearing may not have helped much is the gears you made/used were very large - high inertia themselves, so even tho you geared down the load inertia, the gears had too much themselves.

I'd be happy to email you my Motioneering data file that I made to play these what-ifs - just give me ur email address. then you can play with it and tweak it yourself.

[pupetxls]

Dear Mike,

Tried to PM you, and I got this:

"mike_Kilroy has exceeded their stored private messages quota and cannot accept further messages until they clear some space."

Take care,
Mihai

[mike_Kilroy]

oops. creeped up on me. I deleted so ready to use again. pm away!
thanks

[harryn]

Hi, thanks for the interesting analysis. I am still trying to really understand the aspect of inertia matching, although I do see it discussed occassionally.

Here is a related article from motiontech:

http://www.motiontech.com.au/assets/...Whitepaper.pdf

Not meaning to hijack this thread, but in a related belt drive type question, the setup I am heading toward is:

- Fixed gantry router
- Vertically mounted gantry (the Y axis goes up and down)
- Moving mass on Y axis = approx 25 Kg
- Belt drive to move it up and down
- Nominal 75mm dia pulleys, AT 10 belt
- 3:1 (or 1:1) gear reduction
- Nema 34 stepper motor, fairly high inductance, but I need to look up the exact specs. It is from the used market, a round one

General question. It seems like the load will have quite high inertia in general. Does this mean that a belt drive has no chance of working in a system like this ?

[pupetxls]

Hello Harryn,

No jacking, don't worry.

I also read that, but, what I ended up realising is that if you can set your controller or software for acceleration speeds, you can ditch inertia related problems.

Just set your acceleration and deceleration, and you're off. Maybe there is a formula to calculate it, but you can always set it by trial.

Hope it helps,

Take care,
Mihai

[mike_Kilroy]

thats funny you should pull up an article from my good buddy Rick Armstrong of Kollmorgen! We go back to the days when this inertia stuff was just getting figured out! Rick and I worked together for 20+ years, and at the same time on the programs to do motor sizing and had long sometimes heated discussions on the theory! I am honored to call Rick my friend! He retired last year from Kollmorgen. Motioneering is his baby (although I like to think some of the original equations are mine!)

[pupetxls]

thats funny you should pull up an article from my good buddy Rick Armstrong of Kollmorgen! We go back to the days when this inertia stuff was just getting figured out! Rick and I worked together for 20+ years, and at the same time on the programs to do motor sizing and had long sometimes heated discussions on the theory! I am honored to call Rick my friend! He retired last year from Kollmorgen. Motioneering is his baby (although I like to think some of the original equations are mine!)

WOW, I bow down to you.
This means my league is at least 20yrs behind yours in terms of know how.

Respect!
Mihai

[mike_Kilroy]

........
I also read that, but, what I ended up realising is that if you can set your controller or software for acceleration speeds, you can ditch inertia related problems.r.....,
Mihai

Mihai, what you say is true for open loop steper motor control systems but not for servo systems. I assume you have proved this on your stepper system? If you do go with servos, you do not have the inherent step motor detent torque that gives you the position holding; you must do it thru velocity loop bandwidth control with the likes of an encoder feedback. There is no inherent holding of position you are in, so the control system has to watch the encoder and position command and quickly tell the velocity controller to make fine adjustments. When the load inertia is like an M1 tank hooked up to a motor like a VW beetle thru a rubber hose (couplings/belts), the feedback (M1 tank) quickly can get out of synch with the motor (VW) that is trying to drive it around in a controlled fashion. The result is oscillation. So the only choice is to reduce the bandwidth or response of the whole system, making it spongy. If there was no compliance/backlash in the world then this inertia mismatch would have no detremental effect, but there always is.

[mike_Kilroy]

as pupetxls has said, stepper may be able to handle these large inertia mismatches by simply reducing acc/dec rates since they are open loop devices. I stay away from steppers due to their limitations and so must admit to less experience with them than servos where I live.

That said, Motioneering shows your load with 1:1 drive is about 100-250x a nema 34 stepper motor inertia. it also shows no nema 34 has enough torque to do the job against gravity; a 3 stack nema 34 motor has around 1300 oz-in torque; if I didnt do this too fast and mess up, you will need around 1500 to hold your 25kg load.

with a 50/150mm alum pulley 3:1 added you are good to go with 535oz-in required to hold it on say a 1100oz-in 2 stack motor and an inertia mismatch down to 15:1.

being vertical, these torques are to hold against gravity. add a small amount more to accel prob not an issue. but do you need to do any work with it or is it just go from pos A to B and stop and there is no cutter forces or anything pushing against it? If this is the case great. If you have external thrusts to contend with you gotta add them in also