Author Topic: hs422 servo vibrations  (Read 3501 times)

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Offline rhey22Topic starter

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hs422 servo vibrations
« on: January 01, 2009, 03:06:23 AM »
hello..
Recently, our group have been experimenting on our biped robot's gait, but the servos keep on vibrating (with sound). I've read that it's just normal for a low-powered servo like hs422, which is what we are using. Is there any possible way to get rid of this vibrations?  ???

btw, its my first post :)..hope someone could help me..thanks :D

Offline TrickyNekro

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Re: hs422 servo vibrations
« Reply #1 on: January 01, 2009, 06:50:21 AM »
This means that the pot inside the servo is just too sensitive...
It happens most of times...
Try different pulses to the servos in a close range...
Soon you will find which produces the less noise or no noise!


Best Regards,
Lefteris, Greece
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Offline airman00

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Re: hs422 servo vibrations
« Reply #2 on: January 01, 2009, 07:00:13 AM »
You might be driving it to the extreme and that vibration is the gears turning all the way to the extreme
Check out the Roboduino, Arduino-compatible board!


Link: http://curiousinventor.com/kits/roboduino

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Offline rhey22Topic starter

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Re: hs422 servo vibrations
« Reply #3 on: January 01, 2009, 12:14:14 PM »
Thanks for the replies...

You might be driving it to the extreme and that vibration is the gears turning all the way to the extreme

Do you mean turning the servo to its min/max allowable angle?.so far, the utmost angle we made the servo turn at was +/-45 degrees and it still vibrates, even at other lesser angles..

This means that the pot inside the servo is just too sensitive...
It happens most of times...

So its just normal for the servo i guess, but isn't it bad for the servo?..is it okay if we just ignore the vibrations and continue programming it to different directions/angles?

Try different pulses to the servos in a close range...
Soon you will find which produces the less noise or no noise!

We tried giving the servo different PWM settings since we're making our robot walk, but it really vibrates..btw, what do you mean "in a close range"?.its my first encounter with servos so im not yet quite really oriented and familiar with all of these terms..

Offline TrickyNekro

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Re: hs422 servo vibrations
« Reply #4 on: January 01, 2009, 12:51:43 PM »
Look, from what I understand, it's that I was talking about earlier...

A for example you have a single servo ( no mount - no load ) and you here that vibration...

At the servo you give a pulse of 1.5ms (center pulse), In order to eliminate this sound you
must try out pulses in a "close range"... What I mean...

You can try 1.501ms 1.51ms and so on...normally a close range is 1.450ms to 1.550ms for a center pulse...
Try it and you will have no problem...

Note that this should work with fixed positions - pulses... Walking is math dependent... so you will be using no
fixed values... thus you have to tolerate the noise...

And one question here... Do you have any mechanical noise??? or only sound is produced...
mechanical noise means that you sent the pulses too fast and you have to wait more between the pulses...

Best Regards,
Lefteris, Greece
For whom the interrupts toll...

Offline Soeren

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Re: hs422 servo vibrations
« Reply #5 on: January 01, 2009, 01:13:09 PM »
Hi,

Check for jitter on the servo pulse train, a clean signal shouldn't make the servos vibrate.

If that's not the problem, perhaps a small capacitor over the potentiometers wiper to ground side might help - if the potentiometer is causing it. It should be possible to fit an SMD cap inside the unit. Start with a fairly low value like around 0.5 to 1 nF if you want to try this.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline rhey22Topic starter

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Re: hs422 servo vibrations
« Reply #6 on: January 03, 2009, 06:28:30 AM »
Do you have any mechanical noise??? or only sound is produced...

Thanks Sir for the clarifications..In my case, the servo does not make any vibration and sounds when not mounted and not loaded..its only when we made the robot move like walking that we hear vibrations..and i guess its only sound that is being produced..

So we really just have to tolerate the noise then, but how?.can it be done through codes(programming)?..

Hi,

Check for jitter on the servo pulse train, a clean signal shouldn't make the servos vibrate.

If that's not the problem, perhaps a small capacitor over the potentiometers wiper to ground side might help - if the potentiometer is causing it. It should be possible to fit an SMD cap inside the unit. Start with a fairly low value like around 0.5 to 1 nF if you want to try this.


hello Sir Soeren..
Thanks for the reply..maybe we can try it  when we get back working with the robot..the servo only vibrate when attached to the robot chassis and programmed to move..

Offline mbateman

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Re: hs422 servo vibrations
« Reply #7 on: January 03, 2009, 12:54:25 PM »
I have the same issue with the HS-422 servos in a pan/tilt module. With no load, they are stable, but with load, they will sometimes jitter. If I stabilize them with my hand, they will stop. It appears that it is a combo of mechanical slop and extrasensitive potentiometer. For my application, I can live with the occasional jitter, but I think a higher quality servo would solve the issue.

Offline rhey22Topic starter

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Re: hs422 servo vibrations
« Reply #8 on: January 04, 2009, 12:47:27 AM »
 :(
So its really a HS422 servo hardware issue..maybe it would also be tolerable for the robot if we just ignore it,or not?. ;D..it would be too costly for us replacing the six servos with a high powered ones..we'll see if we can try Sir Soeren's suggestion..

Offline Soeren

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Re: hs422 servo vibrations
« Reply #9 on: January 04, 2009, 09:50:34 AM »
Hi,

In case the issue is hunting*, try the small cap over the potentiometer (wiper to ground)

* Hunting is when the servo tries to position as commanded, overshoots a bit and reverses to correct the error, overshoot in the other direction etc. on and on and on.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline TrickyNekro

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Re: hs422 servo vibrations
« Reply #10 on: January 04, 2009, 10:06:51 AM »
Have you considered statics and torques???
I have the same problem with my heli RC servos, but everything is alright up to now...
So you can ignore...

Please consider than final small trick... Try placing on the signal channel a pull down resistor or pull up resistor...
Around 10K is good...salutation of noise is some times critical...
Although that's not your case here... as it seems to be... :)
For whom the interrupts toll...

Offline Admin

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Re: hs422 servo vibrations
« Reply #11 on: January 09, 2009, 07:11:00 AM »
In your servo PWM, how much time do you have between pulses?

If you set it below 10ms, some servos will jitter and even overheat.

If you set it above say 50ms+, analog servos will lose torque between each pulse because it basically turns off if it doesn't get a pulse.

The proper signal should be:

20ms low, ~1.5ms high, 20ms low, etc.

If your robot spends 200ms doing a path planning algorithm without commanding a servo, the servo will definitely jitter. A good solution would be to use hardware PWM pins on your mcu or interrupts to make sure no more than 20ms passes between square wave. Or just use digital servos that always hold the last position.

Offline mbateman

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Re: hs422 servo vibrations
« Reply #12 on: January 09, 2009, 09:32:18 AM »
Mine are using hardware PWM at 50Hz (20mS). The problem seems to be with the mechanical slop in these servos, especially when under a load. I have a pan/tilt module that will exhibit this behaviour if it gets bumped sometimes. It will ocillate until moved or steadied by hand.

Offline rhey22Topic starter

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Re: hs422 servo vibrations
« Reply #13 on: January 11, 2009, 10:57:38 AM »
Have you considered statics and torques???

Nope.. :-[..but now we're trying to study and understand the kinematics part of the robot.

In your servo PWM, how much time do you have between pulses?


I don't exactly know sir. ???..How can I check for it? We are using Axon mcu with the six servos attached to pins E345, H345. Also, how can we know if what we're really doing is hardware pwm and not software pwm? I'm still really confused about which to use and how to implement it in a program.  :-[

Thanks for the replies.. :)

Offline mbateman

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Re: hs422 servo vibrations
« Reply #14 on: January 11, 2009, 12:22:34 PM »
You can use my simplified PWM code for the Axon to do hardware PWM at the correct freq for typical servos.

The instructions and code can be found here... http://www.laughingsky.com/hobbies/robotics/pages/axon_modules/pwm/index.html

Offline rhey22Topic starter

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Re: hs422 servo vibrations
« Reply #15 on: January 13, 2009, 10:27:52 AM »
Thanks sir mbateman.. :)


 


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