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Author Topic: Filter noise from DC motor  (Read 3079 times)

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Offline itmth19Topic starter

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Filter noise from DC motor
« on: May 29, 2012, 07:29:05 PM »
I know this is not a new problem to a novice like me. I searched some topics in the forum, had some results like this post
Quote
http://www.societyofrobots.com/robotforum/index.php?topic=9978.msg76377#msg76377
. However, I have not understood the situation yet, and I could reply on the old topic but the page says that "consider starting a new topic" so.. ;D

My question is:
I saw some schematics use Capacitors and Diode parallel with DC motor to filter noise. I know this is a beginner question, but according to what I learnt from high school:
-Diode is used to limit the current to one direction because when the DC motor stops, it could gives reverse current and could harm the other stuff.
- Capacitor is used to store power, and prevent to current to go through in the schematic.

So, how those things relate to noise filtering for DC motor? Could anyone explain for me, please. I really want to understand throughly in schematic problem before making a real line following robot.

Regards,

Offline mstacho

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Re: Filter noise from DC motor
« Reply #1 on: May 30, 2012, 06:31:45 AM »
Correct about the diode: it will prevent current from flowing backwards.  SUPER important when dealing with more expensive stuff.

Consider the capacitor as a storage of energy.  If the goal is to make the circuit without weird noise spikes from the motor, then you want to be able to ensure it has a constant voltage.  But if the motor is spiking, you might get the actual voltage dipping too high or too low.  Now, if you have a cap in there, if it drops momentarily, the cap will supply the excess, and if it goes too high, and the cap is fully charged, it won't charge much more, and the voltage will remain more or less constant.

MIKE
Current project: tactile sensing systems for multifingered robot hands

Offline itmth19Topic starter

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Re: Filter noise from DC motor
« Reply #2 on: May 30, 2012, 07:19:58 AM »
@mstacho: Thanks a lots for the detail explain. Now I understand the things. Hope you get well with ur current project

Offline jkerns

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Re: Filter noise from DC motor
« Reply #3 on: May 30, 2012, 01:28:04 PM »
A DC motor contains a big coil of wire around an iron core (the armature). This generates the magnetic field necessary to make the motor turn. But, it also acts as an inductor. Inductors resist a change in current - when you turn the power on to the coil, the rise in the current is slowed down. But, when you disconnect the power, the current wants to keep flowing - as the magnetic field collapses it induces a current in the inductor.

Now, where does that current go?

What is the voltage?

Remember Ohm's law?

I = V/R  let's re-arrange that to be V = I * R. If we have an open circuit, what is R? Infinite, correct? So any current at all times an infinite resistance results in how much voltage?

OK, it's not that bad in real life, but you can get large voltage spikes. The diodes allow the current to flow so you don't see the very large resistance and build up the very large voltages.

Also, in a brushed motor, you have the mechanical connection between the brushes and the commutator (which carries the current to the armature). As the brushes bounce, and cross boundaries between sectors on the commutator, you generate a lot of higher frequency electrical noise (pulses). Capacitors provide a place for the higher frequency electrical noise to go instead of having it go back into the rest of the circuitry.
I get paid to play with robots - can't beat that with a stick.

http://www.ltu.edu/engineering/mechanical/robotics_engineering.asp

Offline itmth19Topic starter

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Re: Filter noise from DC motor
« Reply #4 on: May 30, 2012, 05:08:14 PM »
@jkerns: I know that's a little bit jerk but why the diode has to be parallel to the DC motor?
p/s: Thanks for your reply

Offline jkerns

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Re: Filter noise from DC motor
« Reply #5 on: May 30, 2012, 06:52:15 PM »
@jkerns: I know that's a little bit jerk but why the diode has to be parallel to the DC motor?
p/s: Thanks for your reply

The current induced when the magnetic field collapses after you open the circuit flows in the direction opposite to the current used to make the motor spin. If you put the diode in parallel with the motor, current can flow through the diode / motor loop.

Of course, if you want to drive your motor in two directions, just hooking it across the inputs won't work - because when you change the direction of the current flow then you would be just flowing current through the diode, so you need to work out other diode arrangements for circuts like H-Bridges...

I get paid to play with robots - can't beat that with a stick.

http://www.ltu.edu/engineering/mechanical/robotics_engineering.asp

Offline itmth19Topic starter

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Re: Filter noise from DC motor
« Reply #6 on: May 30, 2012, 07:04:44 PM »
@jkerns: I got it, thank you very much. I think I would search on the forum for diode arrangements for the H-bridge. Hope there is.

Offline jkerns

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Re: Filter noise from DC motor
« Reply #7 on: May 30, 2012, 08:29:43 PM »
If  you are really bored or want to be the first to complain about me going on and on and on about H bridge drivers..... http://www.youtube.com/playlist?list=PL30C02F65B2D5C320&feature=plcp  Part 2 is where I draw in the diodes (but don't give you any part numbers - sorry)

Othewise, a quick search on the web will give you more examples than you need.
« Last Edit: May 30, 2012, 08:33:47 PM by jkerns »
I get paid to play with robots - can't beat that with a stick.

http://www.ltu.edu/engineering/mechanical/robotics_engineering.asp

Offline itmth19Topic starter

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Re: Filter noise from DC motor
« Reply #8 on: May 31, 2012, 07:59:49 AM »
@jkerns: Sorry for my bad English but sorry if i said something bad:)) I've seen the clip, great. It's very kind of you.

Offline jkerns

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Re: Filter noise from DC motor
« Reply #9 on: May 31, 2012, 10:03:51 AM »
@jkerns: Sorry for my bad English but sorry if i said something bad:)) I've seen the clip, great. It's very kind of you.
No problem with your English or what you said. Glad it was helpful.

That whole set of clips were supposed to be just a quick video of putting the drivers together for the robot arms, but then it got out of control... 10 minutes turned into over an hour.  ::)
I get paid to play with robots - can't beat that with a stick.

http://www.ltu.edu/engineering/mechanical/robotics_engineering.asp

 


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