Stepper motor vibration/resonance etc..how to elliminate?

[Apples]

See this video here....
YouTube - FILE0072
What is causing the vibration? Assuming the assembly is rigid enough I guess that it is the stepper motor indent and resonance. Would this be correct?

What is the solution to get rid of it?

[jschmitt]

one way you may have seen:
http://www.cnczone.com/forums/showthread.php?t=32284
another way is to use Gecko stepper drivers

[neilw20]

Notice how around the middle of the travel the noise is related to the position.
I think its where the gear/rack does not mesh totally, allowing some relative movement.
If it was something a driver could fix, (which it quite possibly can) it would be anywhere along the rack.
At different positions, measure backlash with a dial indicator and I think you will see the problem.

[wildwestpat]

Hi Apples

You have a good lead to the use of active dampers from jschmitt and these work in those cases where the resonance is due to the magnetic cogging of the rotor in the motor. However before getting too excited just load up the moving part with the full load that it will be carrying as the inertia of the completed axis will help smooth out some of the roughness.

Assuming you have the rack correctly set with respect to the pinion on the motor try adding a pad damper between the rack and the moving 'sledge' / 'gantry'.

Those alloy sections are also a source of unwanted resonances and stifening webs may have to be added to avoid both unwanted vibration as well as droop at the middle of the span as the 'gantry' traverses.

You don't say if you have microstepping invoked. Making the current follow a sine wave helps as does a small amout of microstepping - too many microsteps will cause the available torque to drop and hence the maximum speed - try a microstep of two or four and see if this helps when the gantry is fully constructed (or has a heavy weight added for testing). It might be necessary to fill the alloy sections with some dampening material with a high modulus such as stiff silicon rubber or an epoxy cement based material, Adding webs would also raise the stiffness and hence reduce any mechanical resonance in the axis track. Check all connections from the motor mout through the universal joints to the gera mesh for free movement that could be excited by the jerks imparted by the stepping action. (Some drive electronics use current shaping to help smooth out the step impulse but IMO try adding friction and extra inertia before doing anything further.)

Good luck as it looks as if you are constructing a nice machine.

Regards Pat

[Apples]

jschmitt
Yeah I'm aware of the damper thread. I'm just wondering if this is a very common issue with all stepper systems even the gecko drive based ones.

neilw20
Yes you make a good point there.

wildwestpat
Not my machine. I understand what you are saying that the ally would be very prone to resonance and vibrations etc. And like neilw20 noted, the vibration seems worse in the middle of the length of the rail/rack. This could mean that the whole beam is too flexible and vibration comes into play. I guess it's like having a guitar string. Its really easy to make it vibrate in the middle of it but much harder to do the same at the very end of the strings.

So if the gear was check to mesh properly and the beam was heavier it should reduce the vibrations...

Now next point, wouldn't you just put servos on this same setup and have no vibrations? Would servos allow you to run "sloppier" tolerances and not have to beef up the structure?

[wildwestpat]

Hi Apples


Yes stepper motor drive is as the name implies moves in steps which can with appropriate shaping by the electronics of the applied current be some what smoothed out but these are almost always open loop servo systems where the steps are used to define the position. This means it is imperative that there is excess torque available at all times to ensure the shaft moves to the correct place with each step commanded by the electronics.
A feed back servo motor drive is a very different matter. Here the position is defined by the element that measures be it a linear scale attached to the slide - OR - a shaft position encoder mounted on the motor shaft. Any lost motion (backlash) between the motor shaft and the position sensing can lead to hunting as can excessive loop gain. The hunting is caused by the motor being driven too far and overshooting the required position. If the servo gain is too high this causes very strong vibrations as the motor reverses overshoots the desired position and repeats the process. Various precautions are taken in design of the electronics to ensure this does not take place. The methods include reducing the speed of approach to the end position – adding a dead zone – boxcar techniques to ensure the required position is approached from the same direction each time by tracking the drive to the position measuring device – etc.
IMO it is best to think about the drive mechanism as a series of elastic members and physical lost motion couplings. The design should try and minimise all these as they represent sources of problems either with oscillation or positional errors. This is why closed loop servos with the feed back sensor arranged to cope with any lost motion are ultimate where precision is the name of the game - unfortunately the cost rockets and stepper motor drives are the norm for home DIY and many industrial machines which can not justify the cost of closed loop systems if the stepper motor can supply the required torque. Think in terms of one hundred times increase in cost for the closed loop compared to the simple stepper driven axis. However if the required torque is outside the range that can be provided by stepper motors then the closed loop servo systems are a needed.
Don't expect to tune your motor drives until they are fully loaded as the inertia alters the dynamics considerably and problems that are evident on a partial build may be eliminated when the machine is completed.
However do note that stepper motors do hum and get hot but should not sound like angry wasps!

Good luck with the build - Regards _ Pat