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standard MG995 problems

 
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BasicFox



Joined: 15 Mar 2009
Posts: 59
Location: Belgium

PostPosted: Sun Mar 15, 2009 3:02 pm    Post subject: standard MG995 problems Reply with quote

hi, i'm working on a hexapod and bought 25 mg995 some time ago, the price and strength are incredible but didn't really did any research on them. Now i saw a topic how about how much overshoot they have and jitter and i tested one servo for durability. I made a routine that constanty made the servo go 0 - 90 - 180 - 90 and back to the beginning.
After a while the servo was stuck and i opened the case and gears were forced out of place. (think due to overshoot)


also there was a burned smell coming out of the case, the potentiometer was the problem. It got fried in a single spot. (the stance the servo was in when it got stuck)







this servo will be useless now but i started looking for a solution and ended here. This is a wonderfull project!

But I really can't figure out how the potentiometer can get fried. That part of the circuit is sensing... you don't need a high current to measure the position of it...

but will it help to put in a openservo pcb in my servo's to fix the overshoot and jitter or are the potentiometers just not good enough to handle a lot of movement?


Last edited by BasicFox on Tue Mar 17, 2009 9:57 pm; edited 1 time in total
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jharvey
co-admin


Joined: 15 Mar 2009
Posts: 359
Location: Maine USA

PostPosted: Sun Mar 15, 2009 4:00 pm    Post subject: Reply with quote

I'd agree that the sensing circuit shouldn't have high current, and therefor the energy should have been low enough to not burn.

What position was it in when it failed? Say full left, full right, somewhere in the middle? Perhaps the wiper failed, causing the servo to push into the stop. After a bit of time the motor, fets, or what have you, would likely overheat. If it has a regulator, that may have been the weak link. If the regulator failed, you could get higher than expected currents on your pot.

What's your input voltage? What voltage do you expect the device to work on? Is your supply inductive? A DC supply with an inductive front can often product spikes that will damage stuff. I know ASTRON supply have an SCR on the output so that if the voltage ever goes to high for any reason, it drops that crow bar across the output.

If you have a replacement pot, openservo should be able to eliminate the overshoot and increase the quality quite a bit.

About pictures, can you post them to say photobucket, then link to them?
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BasicFox



Joined: 15 Mar 2009
Posts: 59
Location: Belgium

PostPosted: Sun Mar 15, 2009 4:43 pm    Post subject: Reply with quote

It was a 6V DC powersupply, the servo specs say its maximum voltage is 7.2V so that wouldn't be the problem.

The servo got stuck at its maximum position, on the last gear there is a small tip that makes it impossible to move the servo more than 180 because of the overshoot that tip was forced into the plastiek and it didn't came out anymore

if i post anykind of link i get this message (I'm new on this forum):
Quote:
In order to try to prevent spammers, we do not allow our users to post URLs in any form until they have posted at least 2 legitimate posts and have been with us for more than 2 days. We appreciate your understanding in this matter in order to help us eliminate spam from this forum. If you have somehow gotten this message even though you meet both of the criteria, please let us know ASAP.

Thanks!


But i placed a link to the pictures as my website, u can click on www
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jharvey
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Joined: 15 Mar 2009
Posts: 359
Location: Maine USA

PostPosted: Sun Mar 15, 2009 5:15 pm    Post subject: Reply with quote

Even with a 6VDC supply, you can get spikes of far more than 6VDC. Many supplies use inductive front ends to smooth out the ripple of a full wave rectifier. If the supply is used for charging batteries, the inductive front end is a good low cost approach. However, if used for sensitive electronics, they either need additional protection, or you need a better supply.

Many servos assume they will be connected to a battery, assuming no inductive spikes, and therefor next to no protection against over voltage. That includes ESD. The motor's spikes are usually taken care of with some kind of snubber circuit.

I would wager a guess that the pot is expected to move, and that being held in one position caused it to over heat. Normally the POT should move, they aren't designed for continuous locked rotor environments. The more current they run in through the pot, the more accurate the reading. So they likely push the limits as close to hot as possible. Being held in one place allows the heat energy to build up, increasing the temperature. Most pots are carbon traces on fiber glass. If you don't allow ambient air to cool them down, I could see them burning them selves out. Also most use VDC, which causes material to migrate, increasing the chances of burn outs.

Once it stuck, the motor likely got hot, heating up the case. Once the case got a little warm, the plastic likely lost it's rigidity, causing the gears to move out of position.

You say you have 25, and they aren't all the expensive. Perhaps you can run another test staying 10 degrees away from the limits. I saw a review that noted about 3 degrees overshoot, so 10 on a test should keep it from sticking.

At this point, I'd say you posted two good posts, so perhaps you can now post links. Once I have a reply, I should also be over that limit. Thanks for the reply.
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BasicFox



Joined: 15 Mar 2009
Posts: 59
Location: Belgium

PostPosted: Sun Mar 15, 2009 5:21 pm    Post subject: Reply with quote

u can see the images if u click on my website (www), right next pm and profile

make sure the servo moves between 10-170 would fix my problem, but the way the servo's jitter and overshoot, can't build a steady robot with that :p gonna make a openservo of them and play with it, see what it does
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jharvey
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Joined: 15 Mar 2009
Posts: 359
Location: Maine USA

PostPosted: Sun Mar 15, 2009 5:43 pm    Post subject: Reply with quote

Yup, it's toasted.

Looks like the POT was OK. I don't see any corrosion, stray flux, looks clean not dirty, and I don't see any other obvious issue with the POT. Which leaves it up to the circuit board and silicone to fail. I'd say both are unlikely issues with the parts chosen for Open Servo, so I'd say Open Servo would fix your problems with this servo.

Good luck and keep us posted.
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jharvey
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Joined: 15 Mar 2009
Posts: 359
Location: Maine USA

PostPosted: Sun Mar 15, 2009 6:51 pm    Post subject: Reply with quote

Out of curiosity, when the servos are not powered, how hard is it to physically turn the servo? I'm assuming they will turn by hand. They'll likely need some kind of arm attached to them, ect.
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BasicFox



Joined: 15 Mar 2009
Posts: 59
Location: Belgium

PostPosted: Sun Mar 15, 2009 8:19 pm    Post subject: Reply with quote

you can't turn the shaft directly, but with an small handle mounted its possible. Don't have to put too much force on it.

And when its working the force is incredible! I can't keep the handle in place. (with my other servo's i could still turn them the way i wanted).

Think these are great high torque servo's to place openservo circuitboards on. Really cheap around 7 (or $9)

gonna try it and hopefully i have good cheap servo, then you will hear again from me Wink
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microlong



Joined: 13 Oct 2010
Posts: 2

PostPosted: Thu Oct 14, 2010 2:14 am    Post subject: Reply with quote

oh,you can view its datasheet to find out what is wrong with it
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michal123



Joined: 07 Aug 2013
Posts: 1

PostPosted: Wed Aug 07, 2013 7:36 am    Post subject: Reply with quote

I tried a handfull of these servos and found none of them to work correctly. All overshoot center and overshoot stop. They are not digital ! They also jitter and jump unexpectedly. Can't fly em. Anyone else have these problems?
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lourencohen



Joined: 20 Feb 2014
Posts: 2

PostPosted: Thu Feb 20, 2014 7:22 am    Post subject: Reply with quote

What's your input voltage? What position was it in when it failed?
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