Author Topic: Hello, and a question about motors  (Read 1320 times)

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Offline MossenTopic starter

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Hello, and a question about motors
« on: October 20, 2010, 10:19:31 PM »
Hi, I'm new to this site and I found it while trying to research how to build a robot. How is everyone? Anyone live in the Sacramento area? I've never built a robot before, but I hope to, with the help of this site.

I'm trying to get my mind around motors and servos and how they work. I have a few questions if you have time to answer:

1) I've noticed the specs on a few motor controllers say something like "For brushed motors", does this mean it won't work with brushless? Are brushed and brushless motors the same thing from a control point of view?
2) Is a servo just like a motor, except you can control it to a certain angle? Is that the main difference? Can a servo act like a regular motor?
3) When you buy a wheel, how do you know what attachment will work on it? I'm talking about these things:
http://www.societyofrobots.com/images/mechanics_chassisconstservowheel1.JPG
Are they screwed into predrilled holes on the wheels or do you force them in?

Thanks for any insight.

Mossen

Offline rbtying

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Re: Hello, and a question about motors
« Reply #1 on: October 20, 2010, 10:37:35 PM »
A brushed motor mechanically alternates the magnetic field, a brushless one doesn't.  In general, brushless is more efficient (hobby-wise), but the electronics cost more because you need a more advanced speed controller.  So yes, for all hobby purposes, brushed and brushless are incompatible. 

Of course, you probably only need brushless if you want to make your robot a) go really fast or b) fly.

A servo is a motor and a potentiometer and the associated electronics, so you have a feedback loop and can find its position.  It also doesn't spin continuously, unless you modify it.  Since it doesn't require an external H-bridge, which is needed for standard brushed DC motors, its popular among people looking for a simple-to-use motor with a built-in gearbox.

For the wheels, the red X is a servo horn, which comes with the servo.  It is screwed onto the wheel with holes you usually have to drill yourself, and then the horn is screwed to the servo through the axle hole on the wheel.  Usually, you won't be able to friction-fit wheels (a la Lego), because of the varying standards in wheel shaft sizes and shapes. 

Offline MossenTopic starter

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Re: Hello, and a question about motors
« Reply #2 on: October 20, 2010, 11:37:29 PM »
OK, thanks for that info. Since a servo usually comes with the horn, does it also come with a coupling to attach an axle? If not, where does one find a coupling? Is an axle typically sold along with a coupling? Are standard sizes available?

Also, what is the purpose of an H-bridge?

Offline rbtying

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Re: Hello, and a question about motors
« Reply #3 on: October 21, 2010, 11:19:37 PM »
Servos usually don't have axles, at least not in the usual sense of the word.  The servo horn attaches onto the servo's spline, and is screwed on.  There are a few standard axle/coupler sizes, but the servo horn method is one of the better alternatives, since it works on just about everything. 

An H-Bridge is needed to provide bidirectional current to your brushed DC motor, as you can't source the power from a microcontroller pin (easy way to kill your controller).  Looks like this:



You can buy a premade chip (SN745510 is nice) with all or almost all the parts in a single DIP package from various vendors.

 


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