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Feb 06, 2014, 07:48 AM
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Question

Brushless Gimbal Motor vs Brushless Motor vs Servo


I don't know what key word to use, I keep getting comparisons, but none on how a brushless gimbal motor works.


So how does a brushless gimbal motor track its position? Is it relative or absolute? Does it have an internal potentiometer to track its absolute position?

I had a hand on piloting a hex with a 3 axis gimbal. And while it can indeed do a good job knowing where the front is, it seems to do it by counting the poles. If you force it a certain direction past a certain number of poles, it loses track of where the front is.

Every time I plug in the gimbal, I have to make sure it's facing the front because it'll lock on to whatever angle it's facing initialize.

I guess i can easily resolve this if I actually had a gimbal motor on hand to check out, but I don't have one unfortunately. Are gimbal motors exactly the same as regular brushless motors? Except maybe having more poles?


Also, are there any builds specific to setting up 360 gimbals? The one I tried seems to be limited by the wires. The motors doesn't seem to have enough power to pull the bundle of cables behind it so it can only move 45 degrees to either side. How do 3axis gimbal users wire theirs?
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Feb 06, 2014, 09:18 AM
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A brushless gimbal motor is a regular brushless motor but wound for very low speed (ie lots of turns of thin wire) and many do indeed have more poles.

The position of the camera on a brushless gimbal is sensed by an inertial measurement unit (IMU), attached to the camera, that uses sensors to measure rotation speed (gyros) and acceleration (accelerometer) in three orthogonal planes. A magnetometer is used on 3axis gimbals to detect compass bearing like a magnetic compass. The software uses the sensor data to derive the position of the camera and make corrections to bring it back to position. At the start the gimbal will move the camera so that it is level using the accelerometers to detect whether it is in the correct position. Once zero'd the sensors are used to keep it level despite velocity and acceleration of the platform the gimbal is attached to.

Without knowing the type of gimbal you have it is difficult to say why you have to zero it to straight ahead. It sounds like you have a 3axis gimbal and the zero button is required so that it knows which direction you want it to point in (roll and pitch are easier because they are related to gravity) and therefore it needs to return to. For a true 360degree gimbal you will have problems with the wiring unless you use slip rings on pitch and roll axes (not cheap!) so must gimbals are limited in some way to maximum axis angles.

Peter


Quote:
Originally Posted by Hajile
I don't know what key word to use, I keep getting comparisons, but none on how a brushless gimbal motor works.


So how does a brushless gimbal motor track its position? Is it relative or absolute? Does it have an internal potentiometer to track its absolute position?

I had a hand on piloting a hex with a 3 axis gimbal. And while it can indeed do a good job knowing where the front is, it seems to do it by counting the poles. If you force it a certain direction past a certain number of poles, it loses track of where the front is.

Every time I plug in the gimbal, I have to make sure it's facing the front because it'll lock on to whatever angle it's facing initialize.

I guess i can easily resolve this if I actually had a gimbal motor on hand to check out, but I don't have one unfortunately. Are gimbal motors exactly the same as regular brushless motors? Except maybe having more poles?


Also, are there any builds specific to setting up 360 gimbals? The one I tried seems to be limited by the wires. The motors doesn't seem to have enough power to pull the bundle of cables behind it so it can only move 45 degrees to either side. How do 3axis gimbal users wire theirs?
Feb 08, 2014, 09:59 PM
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Thread OP
Wha? I thought I've replied to this o_0

Anyway, I can't find a match for the gimbal, the store that he supposedly bought it from doesn't even have it on their web page, so I guess it's just a generic 3 axis gimbal.

The controller tho is a SimpleBGC. It looks like that if I hold the gimbal and then turn the hex, its 'center' is now offset :\ I'm still working out how to wire it up coz it tends to skip if the wire bundle is either too stiff or gets snagged.

Anyway, I messed around it and... decoupled?... the gimbal in the settings so it no longer cares where the front is. Mode 2 is now free turning, mode 1 'locks' it. If it gets pushed off again, the operator will just have to go to mode 2, make it face forward again, and go back to mode 1.
Aug 15, 2015, 03:34 AM
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NELICOPTER's Avatar
So could a Brushless gimbal be used for an application such as an ebike motor where very low rpm is great?
Would I need a new bearing system and shaft if the gimbal didn't have one?
would I need a different controller system so that or could drive the motor? (would the controller drive a really low rpm motor?
Aug 15, 2015, 05:50 AM
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Thread OP
The ESC is built into the gimbal controller, so if you need to run a big motor, you'll have to get a controller with really beefy power circuits.
Aug 15, 2015, 06:17 PM
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NELICOPTER's Avatar
Quote:
Originally Posted by Hajile
The ESC is built into the gimbal controller, so if you need to run a big motor, you'll have to get a controller with really beefy power circuits.
I read that the reason these have such low speed is because they use very thin wire but with lots of turns per stator, but the catch is it heats up more.
Would it be detrimental for a gimbal to be running constantly with a load?
Would I have to get an ESC where I can set the timing really low?
The other thing is gimbals aren't rated with a max power, so the best I could do is compare the size to get the estimated power.
Aug 15, 2015, 07:05 PM
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Daemon's Avatar
They are both low and high speed at the same time. Low speed, in that they never actually
complete a full 360 rotation so you wouldn't normally think of measuring their speed in rpm,
but high speed in that they need to respond *instantly* to sometimes very rapid changes
in direction as the aircraft may be getting shook by vibrations and turbulence.
That means they're meant to be light and fast, not powerful and slow.
These motors are not meant to carry any kind of continuous load. Ideally you should
balance your whole gimbal rig with the camera installed, such that there is no natural tendency
for the camera to sit or swing into any particular orientation when the gimbal motors
are inactive. E.g. With motors unpowered:
1. If you rotate the camera to any orientation and let go, it should just stay there within a few degrees.
2. If you shake the whole gimbal or aircraft rather aggressively in any direction, the
camera should pretty much just stay in whatever orientation its sitting and not
swing back and forth in sync with the shaking.
If one or both of those are not happening, then you need to keep working on your balancing.
When both are true, then the gimbal motors actually don't have to do much work. The inertia
of the balanced rig does most of the work (objects at rest want to stay at rest),
and the gimbal motors are handling a little bit of friction, any load that wires and
such exert and any pan/tilt inputs you're giving it.

And yes, there are gimbals with stronger motors, but as said above, the gimbal controller
drives the motor directly (built in ESC) so a higher powered gimbal motor needs a higher
powered controller. One more problem with putting a gimbal motor under high load is that
it may start to introduce vibrations itself. It's very hard to keep any heavily
loaded brushless motor perfectly smooth, anywhere close to zero rpm. They buzz a little.

BTW, to follow up on an earlier question. *Most* gimbal motors have almost
no idea of what their own position is other than counting pulses to the motor itself.
The gimbal controller listens solely to the IMU, and drives for effect. If you request that
the IMU be level, then the controller drives the motors until level is achieved. The side
effect of this is, that if you manually bump the gimbal off level (which the controller
can't count pulses for), it can take quite a while to recover. If you keep it off level
long enough, it may just give up and stop leveling (my Tarot T2D does that).
However, there are some folks who have added a position sensor to the motors
themselves and they use both the IMU and the position sensors so that once level is achieved,
it can keep track of where that is, so that if something comes along and bumps it off level,
it can recover almost instantly. The feiyu-tech G3 gimbal is an example of this type.
They can do this, because they supply both the proprietary controller and the motors. I
don't know if the ability to use motors with position sensors has been added to the
various open source controllers or not (I just haven't kept up on newest developments).
Aug 16, 2015, 05:05 AM
Registered User
NELICOPTER's Avatar
Thanks for the long writeup.
When you said they never make a full 360 degree turn, is that with a gimbal controller? or are they physically locked to a movement type.
Something like this doesn't look like it has a controller built in.https://www.hobbyking.com/hobbyking/s...bal_Motor.html
But never mind as I will eventually make my own brushless outrunner for my ebike. (yes I know, I sound incredicbly naive.)
Look at these guys!
DIY BLDC-MOTOR 20KW part 4 (Ukraine, Diaz) 2015 (42 min 24 sec)

Skip to 17:18 for the action.
Aug 16, 2015, 11:35 AM
Registered User
Daemon's Avatar
Generally it's just a physical constraint of the gimbal rig itself, not the least of
which is the need to run wires to/from the camera and such. The motors can
turn continuously.

If you want a powerful brushless motor for an e-bike, then gimbal motors are simply
the wrong place to look. This is not a new problem to e-bikes. There's a ton of
really powerful brushless motor designs out there. BTW, direct drive e-bike hub motors
tend to be faster, but less torquey, larger and heavier.
http://electricbikereport.com/electr...ed-hub-motors/
Aug 17, 2015, 01:28 AM
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NELICOPTER's Avatar
I have looked a things like that, but I just like the idea of something Diy. 1, for cost. 2, for reliability. and the two tie in.
Thanks.
Aug 17, 2015, 01:31 AM
Registered User
Daemon's Avatar
Other than the two words "brushess motor" there's no tie-in. You're trying to
shove a square peg in a round hole. Your question has been asked and answered.
Good luck with that.


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