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coreless servos

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Old 12-13-2004, 09:37 AM
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Gringo Flyer
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Default coreless servos

I know that coreless servos are higher end servos, but after a few searches I really cant find any good explanations for why they are so much better. They are considerable more expensive compared to equivalent standard servos. Whats the big advantage?
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Old 12-13-2004, 11:20 AM
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Default RE: coreless servos

As the name indicates the armeture of the motor has no metal core. This reduces the inertia of the motor and allows the servo designer to design for a smaller deadband. The no-load deadband of a coreless motor servo is usually about 3 us where the standard motor servo it is about 13 us. This means the coreless servo will return closer to the starting point after a control input. The motor is also more efficient than the standard motor. This means you will get more torque out of the servo for the same input
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Old 12-13-2004, 03:37 PM
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Default RE: coreless servos

But don't forget that coreless motors are not quite as robust as regular ones. I've seen a number of coreless units damaged by rather hard landings (on tarmac mind you) which has distorted the motor cage to the extent that it then fouls the magnets.

I use coreless on my 3D ships but I'd probably recommend good old cored motors for most sports fliers because they're unlikely to notice the benefits of coreless technology and probably place a higher emphasis on robustness.
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Old 12-14-2004, 10:28 AM
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Default RE: coreless servos

DirtyBird got most of it. Since the rotating mass is considerably less than a metal cored servo, the coreless servo starts and stops much faster so you get "crisper" response becasue the servo starts moving faster and less overshoot at the end of travel because the servo stops moving faster.
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Old 12-14-2004, 10:53 AM
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Default RE: coreless servos

This is a good article on coreless. Hope it helps
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Old 12-14-2004, 05:13 PM
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Default RE: coreless servos

s3nfo, if that is the case how come coreless servos are rated pretty slow? The fut s9202 and s9304 are rated at .26 and .28 to 60 degrees. Thats slower than Christmas. In fact, they are some of the slowest servos Futaba makes.

Not doubting what you say, but just curious. Maybe are you saying they start moving faster, but not that they move faster through the length of their movement?
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Old 12-14-2004, 10:47 PM
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Default RE: coreless servos

Coreless servos *can* be faster than cored ones but are not always so. There are many other factors that affect a servo's speed: the gear ratios, motor size, amplifier design, etc.

Sometimes a coreless motor may be used solely to get more torque from a given size of motor.
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Old 12-15-2004, 01:47 AM
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Default RE: coreless servos

Coreless motors also offer higher precession as they are not limited by the armatures pole count for resting positions.

A coreless motor can typically develop more torque as the coreless cage has a longer moment arm from center. This arm is like having a long lever, less input nets equal amounts of torque as compared to like sized cored motors.
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Old 12-15-2004, 02:21 AM
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Default RE: coreless servos

ORIGINAL: mglavin
Coreless motors also offer higher precession as they are not limited by the armatures pole count for resting positions.
ke having a long lever, less input nets equal amounts of torque as compared to like sized cored motors.
I'm not sure that's strictly true.

Coreless motors *do* have poles, but they're just wound wire, not laminated iron as in a regular motor.

Without poles, you could not get the motor to spin since there would be no N/S attraction between the permanent magnets and the electromagnets that make up the motor.

If what you're talking about is the way that cored motors tend to align themselves with the field magnets in a number of fixed degrees of rotation then you're right -- but that's likely to only be an issue when there's no load on the servo because otherwise the coreless motor needs to energize its coils (and create its electromagnetic poles) to provide any holding power.

When in flight, most servos (especially coreless ones which are more likely to be used in high-load applications (that require high holding power) will have a constantly energized field coil so this perceived difference in resolution is actually irrelevant.

Don't believe all the hype you read in the servo-makers' brochures :-)
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Old 01-03-2005, 08:33 PM
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Default RE: coreless servos

Do coreless servos tend to chatter more when centered and holding the control suface in place?? I have 2 new JR4721 's that chatter constantly while sitting and holding the elevators of a 1/4 scale in place.
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Old 01-04-2005, 02:47 AM
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Default RE: coreless servos

That chatter is more a function of the servo's dead-band than the type of motor it has.

Coreless servos (especially digital ones) tend to ship with much narrower dead-bands than regular cored servos -- which means that they respond to even the slightest movement from the designated position. If your control surfaces are pushing on the pushrods which in turn push on the servo output arm then *yes*, servos with a tight deadband will constantly be pushing back so as to maintain the correct position.

Coreless motors also turn more readily when an outside force is applied so whereas a cored motor might stay put through its own reluctance to turn, the coreless motors have to be energized to do so -- hence the buzzing, or "singing" noise that many top-of-the-line coreless digitals make.

The Hitec coreless digitals can be programmed by the user to use a wider deadband which significantly reduces all that racket :-)
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Old 01-04-2005, 01:15 PM
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Default RE: coreless servos

ORIGINAL: XJet


The Hitec coreless digitals can be programmed by the user to use a wider deadband which significantly reduces all that racket :-)
But doing this might affect centering accuracy and holding accuracy right? So the lack of chattering comes at a cost.
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Old 01-04-2005, 10:55 PM
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Default RE: coreless servos

Thanks for all your input. I talked with JR (horizon hobby) today and the tech said the 4721 would chatter all the time at rest and this is normal. Also he said this chatter resulted in "minimal current draw" at rest. I suppose the noise will not bother me if I know the servos are not "Gobbling" up the receiver battery...... Does everyone concur with the JR tech?
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Old 01-05-2005, 12:35 PM
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Default RE: coreless servos


ORIGINAL: dpitcock

Thanks for all your input. I talked with JR (horizon hobby) today and the tech said the 4721 would chatter all the time at rest and this is normal. Also he said this chatter resulted in "minimal current draw" at rest. I suppose the noise will not bother me if I know the servos are not "Gobbling" up the receiver battery...... Does everyone concur with the JR tech?
No -I do not concur with the JR tech. That chattering is called hunting and wastes battery current and wears out gears. In my opinion JR servos are set up with too tight a deadband.
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