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deSilva
01-29-2008, 01:35 AM
I had been under the wrong conception that servos contain stepper motors or other BLDCs. However this is not the case, and in fact need not be when having a gearbox with high transmission ratio..

Are there any simple indicators to judge whether a servo contains a brush motor? E.g, price? Does a brushless servo always cost more than - say - $20?
Is there a simple test without disassembling it?

Advertising details especially with bargains are not good..

Drone
01-29-2008, 02:37 AM
Hi deSilva,

I'm no expert about this but...

A complete brushless servo that accepts traditional PWM input (like in R/C model airplanes) should always cost more because there has to be a motor controller incorporated. The brushless motor itself is like a stepper it seems, permanent magnets on the rotating part and two or more electromagnets that have to be sequenced to pull the rotating part around. I guess a simple way to tell if a motor in an RC servo is brushless is to open it up and look at the number of wires going to the motor. If there are more than two wires connected to the motor, it is likely brushless as in a brushed motor only two wires are needed to drive it (not accounting for any wires that may come off a motor-integrated potentiometer to close the feedback loop of-course).

Here's a link that may be a good starting point:

http://en.wikipedia.org/wiki/Brushless_dc_motor

Regards, David

Christof Eb.
01-29-2008, 02:45 AM
Hello deSilva,

an other simple indicator, as I think: Many servo users are technic freaks. Therefore a brushless system would be named in advertisements....

Christof

Phil Pilgrim (PhiPi)
01-29-2008, 02:54 AM
deSilva said...
Is there a simple test without disassembling it?

A cheap AM radio placed close to the motor while it's running may be all you need to ascertain whether it has brushes or not. Brush arcing should create a readily-apparent staticky hash sound.

-Phil

Graham Stabler
01-29-2008, 04:09 AM
I have never seen a brushless RC servo ever, the most likely ones to have them would be robotic specific servos but even then I doubt it. Most of the good ones are coreless so are pretty efficient.

Graham

Graham Stabler
01-29-2008, 04:11 AM
Even hitecs top robot servo uses a brushed motor:

http://www.hitecrcd.com/product_file/file/109/HSR-8498HB_GENERAL_SPECIFICATION_050623.pdf

cored in this case

Nick Mueller
01-29-2008, 04:14 AM
I haven't heard of BL-motors in R/C-servos.
Certainly real servo motors are often enough BL. But they do have a "somehow" bigger wattage. http://forums.parallax.com/images/smilies/smile.gif
If you are looking for small BL-motors, you'll find them in CD-drives.


Nick

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deSilva
01-29-2008, 04:26 AM
E.g.
Futuba BLS 251, 351, 451 MG

Paul Baker
01-29-2008, 08:15 AM
So there are BL servos, but one has to think if it's a marketing ploy, and if it isn't is the performance increase worth the extra cost. The first site I went to wants $140 per Futaba BLS 251, this is three times the price of an ordinary digital servo and it STILL has nylon gears, if your buying a servo for it's torque and spending a pretty penny you should get all metal gears, not just one.

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Graham Stabler
01-29-2008, 08:27 AM
deSilva,

OK I'll state the obvious, when they are a brushless servo they are called a brushless servo or BLS :) I can't find any more other than the Futaba ones.

Graham

RinksCustoms
01-29-2008, 08:48 AM
Google the numbers on the case?

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deSilva
01-29-2008, 08:52 AM
Thank you for all your feedback!
I never used motors much, and I am just trying to understand the scene. I was confused why so many people undertake the trouble to connect steppers when inexpensive servos are available. This has sorted out now!

I am disassembling PC cooler fans at the moment. Interesting things inside...

RinksCustoms
01-29-2008, 10:53 AM
steppers are most useful for precision rotary/linear motion. Prime example, an inkjet printer, and a 5 1/4" or 3 1/2" floppy drive.

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Brian_B
01-29-2008, 11:37 AM
A true brushless dc servo requires a feed back to the controller (Hall Effect sensor), so that the controller knows what coil to energize to keep it in sync.

Brian

edit- Brushed motor's change there coils as they rotate by themselves.A true brushless servo is very close to a stepper motor.

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Thank's Brian



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Post Edited (Brian Beckius) : 1/29/2008 5:12:05 AM GMT

deSilva
01-29-2008, 04:15 PM
As this deviates to more basic information I should like to add these references to interesting threads of this forum for new readers:
http://forums.parallax.com/showthread.php?p=701544 Stepper Motors
http://forums.parallax.com/showthread.php?p=699165 BLDC

Graham Stabler
01-29-2008, 11:34 PM
Most RC servos are just using a pot as feedback, not especially precise. With a 200 step stepper motor being driven with half stepping that is good and predictable resolution assuming you don't overload the motor.

It depends on your application and required precision. Of course if you go to a none RC servo then it's a whole different ball game, I've some lovely Yaskawa/Omron brushless servo motors that have incredible performance.

Those fan motors have very simple drives, I think they tend to have a hall sensor integrated in the controlling IC. Of course you can also do sensor less feed back using back EMF but that's not so great at low rotation speed.

Graham