I keep seeing references to a "9DOF" board some people sell. What the heck is it, a time machine? warp drive? How does it impart 9DOF to my project?
DOF stands for "degrees of freedom". The first 6 are motion in X, Y, and Z, and yaw / pitch / roll. What's next? Time? Trans-dimensional warp?
These things are really 9 axis sensors. Let's get the terminology right.
That's my gripe for the day. Thanks for listening. 
-j
If you are dealing with robots you can have way more DOFs. Each DOF is just one more degree of freedom. Just imagine your hand. 5 Fingers, Each with several joints --> way more than 9 DOF and still no warp drive
Definitely not 9 degrees of freedom. You can't blame electrical engineers for not knowing physics. One ece faculty or two at my university don't like relativity simply because of poor understanding. As a matter of physics fact, the 9dof only measures 3 rotational degrees of freedom at best.
There are only 3 axis.
Period.
roll, yaw and pitch.
The 6DOF LSM303 I have is a triple axis magnetometer and a triple axis accelerometer.
For the 9DOF, a triple axis rate gyro is added.
How you get 9 axis is beyond me!
I went for the 6DOF with my ROV because a 2 axis "compass" is not accurate unless it is level.
For my UAV I'll go for a 9DOF sensor.
Once you get off the ground having a rate gyro is all important.
I have more of an issue with dropping "rate" from gyro.
It's not that simple as the roll pitch yaw. There is no way to tell the angle of an object with respect to the vertical up. Using accelerometer only works statically. If you accelerate, accelerometer no longer tells vertical up but off into the direction of acceleration so considering its reading to indicate vertical up is no longer accurate. In this case, the gyro axes can help to make the orientation more accurate. That is what I imagine.
It's not that simple as the roll pitch yaw. There is no way to tell the angle of an object with respect to the vertical up. Using accelerometer only works statically. If you accelerate, accelerometer no longer tells vertical up but off into the direction of acceleration so considering its reading to indicate vertical up is no longer accurate. In this case, the gyro axes can help to make the orientation more accurate. That is what I imagine.
There is no such critter as "static acceleration".
By definition acceleration is the rate of change of velocity.
In an aircraft one of the most important instruments is the "slip and turn" indicator.
With gliders where you are simple forbidden to have an HSI, Horizontal Situation Indicator, you simply should NOT get into a flight configuration where you will need one!
Most glider pilots use a simple piece of red wool with a small weight on the end.
In a "balanced turn" the piece of string or slip and turn meter will be pointing straight down, not at the GROUND, but straight at the floor of the cockpit.
If it's to one side your tail is "slipping" in that direction, how far it is off to one side is how "unbalanced" your turn is.
That's yaw and in a fixed wing aircraft it is the prelude to a tail spin.
Once in a tail spin, the wings stall and ALL other control surfaces besides rudder have NO EFFECT.
Without enough vertical stabiliser and rudder area, a tail spin is UNRECOVERABLE!
V tails in particular are a total prick to recover from a spin.
Most of the training from an unrestricted or "night/visual meteorological conditions" licence to an instrument rating, is getting the pilot to ignore what their inner ear is "telling" them.
With a ground vehicle yaw only happens when you are in a spin or starting to spin out in a drift.
I might have to put together a video illustrating what I'm talking about.
My UAV is getting a new tailplane and wing, as well as a SeeedStudio Mega! 8)
I'm working out all the code for 6DOF and 9DOF on the ground because it is a much more forgiving environment to experiment in.
All of this is new to me, so maybe I am over simplifying, but I see these boards just as what they claim to be. Which is that they provide you "nine degrees of inertial measurement". Those measurements should be acceleration from the accelerometer, direction from the magnetometer, and rotation(roll,pitch,and yaw) from gyros. Each of these is measured on the 3 axises of x, y, and z resulting in "nine degrees of inertial measurement" or as they are labeling it "9DOF". I have also seen a sensor that offers "10 degrees of freedom", with the additional measurement being for atmospheric pressure.
So while I still remain a bit confused, this is my take on these boards.
There are only 3 axis.
There are three axes on the user/planetary/etc frame of reference. We usually refer to these as X Y Z.
There are three more on the vehicle's frame of reference, generally about the Cg of the vehicle - there's your yaw / pitch / roll.
How you get 9 axis is beyond me!
A total of three 3-axis sensors. 3 * 3 = 9
-j
There are three axes on the user/planetary/etc frame of reference. We usually refer to these as X Y Z.
There are three more on the vehicle's frame of reference, generally about the Cg of the vehicle - there's your yaw / pitch / roll.
No, no, no!
It's a totally different thing!
x,y,z are Cartesian coordinates!
roll, yaw, pitch are spherical!
I'm not being pedantic!
This is a REALLY important!
Not so much on the ground, but in the air it's everything!
We need someone with a better mathematical background than me to explain this!
Not some recidivist criminal like me who fly's/drive's without a licence!
ooo, shit, did I think that or type it?
OK!
Got it!
Just rang my blind, sociopath math freak girlfriend.
(She like trains and flying with me so I can forgive the occasional beating)
x,y,z is a location, like longitude, latitude, altitude.
roll, yaw, yaw pitch is orientation.
and I quote...
"you can bend someone over anywhere, that's x,y,z, how far you bend them over is orientation"
Now I'm gunna have to take one for the Arduino team!
sigh...
the things I do!
I didn't say static acceleration. And do you think accelerometers measure acceleration?
And do you think accelerometers measure acceleration?
Well yes.
They don't measure velocity.
My understanding is they are a micro strain gauges, I've not done anything that can show much acceleration on the ground, yet.
Measuring velocity without an external reference makes no sense.
Einstein quote, to a train conductor,
"Does Cambridge stop at this train?"
Looks like you understand perfectly the "accelerometer" only measures a strain force, not acceleration, not even net force. When you are accelerating, the strain force plus gravity makes the "accelerometer" accelerate. If you only accelerate the device along a straight line, then you can deduce the acceleration from the vector or xyz components of the reading. If you are accelerating by flying in a horizontal circle, the device can't tell if you are flying in a circle or accelerating to the side. Then you need the gyro to tell if you are actually flying in a circle to deduce acceleration or angles or orientation. Unless you mount your "accelerometer" on a gimbal or gyroscope to decouple rotation of your reference frame, which you also understand, the "accelerometer" is not telling acceleration without the help of a gyro. So yes, you understand every piece. I'm just being redundant.