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Old Jan 25, 2015, 04:19 AM
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Joined Aug 2011
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Gimbal Motor vs Propulsion Motor

Hello,

What are the design differences between an outrunner brushless motor used for propulsion compared to the motors used for camera gimbal stabilization?

For example:

http://www.rctigermotor.com/html/pro.../Professional/
VS
http://www.rctigermotor.com/html/pro...Gimbal_Motors/
?

Why not use simple servos for camera gimbal stabilization ?
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Old Jan 27, 2015, 12:48 AM
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Australia, VIC, Melbourne
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Servo resolution is to large for this (the image will jitter).

Gimbal motors are high resistance (low current, low Kv) ones for propulsion will use fewer winds of thicker wire to allow for more power.
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Old Jan 27, 2015, 08:41 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
The Netherlands, GE, Nijmegen
Joined Feb 2001
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And the high(er) resistance is the result of low Kv (more windings in the coils, thinner copperwire). Higher resistance is not the aim.

Vriendelijke groeten Ron
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Old Jan 27, 2015, 09:25 AM
OlliW
Joined Sep 2009
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I beg pardon, but higher resistance is the aim, and low KV is the result

the motor is essentially not rotating
=> the actual KV is totally irrelevant
=> the resistance is basically determined via Ohm's law by the required current and the used battery, and practical values are e.g. 1A and 12V => 12 Ohm
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Old Jan 27, 2015, 09:27 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
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What is the purpose of the high resistance, how is it used? To control torque?

Vriendelijke groeten Ron
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Old Jan 27, 2015, 09:31 AM
OlliW
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=> the resistance is basically determined via Ohm's law by the required current and the used battery, and practical values are e.g. 1A and 12V => 12 Ohm

???
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Old Jan 27, 2015, 12:21 PM
Jack
USA, ME, Ellsworth
Joined May 2008
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I don't use gimbal motors and knew little about them when I posted this thread:

Outrunner Disassembly and Stripping - Gimbal Motor Rewind - www.rcgroups.com/forums/showthread.php?t=1823636

I did that to help some people that were converting regular outrunners to gimbal motors with the mechanics of stripping and winding motors. A nd there is some discussion in there about the differences in the needs between propulsion motors and gimbal motors.

At that time, the primary objective of a gimbal motor rewind was to get a final resistance in the windings that was compatible with the needs of the gimbal motor controllers. And that is what drove the high turn counts with very small wire, the need to have a lot of length to get the resisrtance. And, of course, it also had to have some power to move the camera and hold it in position between movements.

But regardless of it all, a gimbal motor is a outrunner and will run when a voltage is applied. That is has a Kv is not incidental, all brushless DC motors have a Kv rating...

Jack
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Old Jan 28, 2015, 04:18 AM
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Is the resolution of the gimbal motor equivalent to the number of pole pairs?
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Old Jan 28, 2015, 05:24 AM
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shaiko, i don't think so, or else that a servo would be better than a brushless gimbal.
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Old Jan 28, 2015, 05:57 AM
OlliW
Joined Sep 2009
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the angle resolution comes from three factors

motor pole counts
the electrical phase signal is "translated" to a mechanical angle, which involves the motor pole count, hence the larger the motor pole count the larger the angle resolution for the same electrical phase signal

electrical signal resolution
the electrical signal to the motors is a sine wave, which is generated by the motor drive with a certain resolution, e.g. 8 bit. The higher the better.

update frequency
the electrical signal is updated with a certain frequency, given e.g. by the loop time of a PID control loop. The update frequency should be much higher than the characteristic frequency, this then essentially results in an "averaging" effect and hence a finer effective resolution than the nominal electrical signal resolution. The mechanism is essentially that of PWM (don't confuse this "PWM" with the PWM used to generate the sine signals). It is usually not considered but is in fact a major reason for the success of the brushless gimbals. The higher the update frequency the better the resolution.

That's unfortunately not the end of the story, since also things such as motor cogging exist. The effect is that the motor position is not strictly linear to the electrical angle, and that can compromise things quite a bit.
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Old Jan 28, 2015, 05:25 PM
Jack
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Quote:
Originally Posted by shaiko View Post
Is the resolution of the gimbal motor equivalent to the number of pole pairs?
`If I understand the question, I think the resolution is the the same as the cogging steps per turn for a given motor and that will vary with the number of stator arms and magnet poles.

If you want to look that up you can use the winding scheme calculator here and it will give you that info:

http://www.bavaria-direct.co.za/scheme/calculator/

Enter the stator slots (also called stator arms or teeth and abbreviated as "N" ) and magnet poles (also called poles and abbreviated as "P", it is the number of magnets in the magnet housing) and click on Calculate.

Don't let the fact that each magnet has both N and S or two poles confuse the issue, just count the number of magnets.

As examples, the 12N14P motor has 84 cogging steps and 12N16P has 48 cogging steps. And the

And when a motor is stopped, it will be located at a cogging step. So the resolution as far as the next step, expressed in degrees of rotation, would be 360/cogging steps.

Jack
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Old Jan 28, 2015, 07:29 PM
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manuel v's Avatar
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A gimbal motor with new rewind, can be converted to propulsion motor.

http://www.rcgroups.com/forums/showthread.php?t=2332444
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Old Jan 29, 2015, 03:43 AM
OlliW
Joined Sep 2009
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Quote:
Originally Posted by jackerbes View Post
So the resolution as far as the next step, expressed in degrees of rotation, would be 360/cogging steps.
LOL
so you're in all earnest telling here that the relevant resolution for the operation of a 12N14P brushless motor in a gimbal is 360/84 = 4.3 ???

good joke ... it makes me realize that I obviously got this thread wrong, I apologize for having interrupted it with some seriousness, sorry for that
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Old Jan 29, 2015, 07:11 AM
Jack
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Quote:
Originally Posted by OlliW View Post
LOL
so you're in all earnest telling here that the relevant resolution for the operation of a 12N14P brushless motor in a gimbal is 360/84 = 4.3 ???

good joke ... it makes me realize that I obviously got this thread wrong, I apologize for having interrupted it with some seriousness, sorry for that
Work on your manners.

This is a polite discussion and I'm willing to learn something from it.

It is my understanding that when a gimbal motor takes a holding position it is at one of the cogging steps. And the advances it makes are in those same steps and in sequence. If that is not the case just say so. And if you want to contribute something to answering the question tell us what the resolution is.

Jack
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Old Jan 29, 2015, 08:35 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
The Netherlands, GE, Nijmegen
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Quote:
Originally Posted by OlliW View Post
LOL
so you're in all earnest telling here that the relevant resolution for the operation of a 12N14P brushless motor in a gimbal is 360/84 = 4.3 ???

good joke ... it makes me realize that I obviously got this thread wrong, I apologize for having interrupted it with some seriousness, sorry for that
I'm (also) here to learn (and to share), and I do, every day, even though I am an EE (only by training, embedded software by profession). Apparently your are not here to share your knowledge and experience. That's fine by me, your are the boss of your time. I learned from Jack about machining/mechanical issues, Jack learned from (a.o) me about electrical (drive)motors stuff. I made mistakes, gave the wrong information, and people corrected that, in a cordial way. Your last sentence was uncalled for.

Why is high copperwire resistance in a gimbal motor desirable?

Vriendelijke groeten Ron
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Last edited by Ron van Sommeren; Jan 29, 2015 at 08:54 AM.
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