Angular position measurement using accelerometer - DSP approaches?

> The bias that you see due to misalignment*angular velocity will depend (as I
> think Jerry inferred) on the radius from the axis of rotation to your
> accelerometer. The angular acceleration term will also depend on this ( I
> think) - are you expecting to mount this on the side of an accessible shaft
> or on a surface close to but normal to the axis of rotation?  If so this
> also ought to minimise the power associated with a fair bit of bearing slop
> and gear chatter.
"on a surface close to but normal to the axis of rotation"

john.dsp0@gmail.com wrote:
>> How well can you null it by mounting the accelerometer normal to it?
>> About .1 degree off perpendicular will bring that 100 g down to less
>> than .2 g. Id the cross-coupling of the accelerometer that good?
> Yes this was the orignal idea. Mount the sensor so that it is normal to
> the centripetal acceleration. You have made me realise that there are a
> range of orientations that will achieve this, but in our case the pcb
> is mounted such that only 1 makes sense. (The axis of rotation is
> normal to the PCB) There will be an alignment error, datasheet specs
> 1.5 degrees and I can only guess what error we introduce when mounting
> on the PCB.

You can use a horizontal spinner with the accelerometer way out on a 
radius to find the "neutral" axis of the finished board. You should 
expect at most a .5 degree error of you do the alignment carefully.

>> The radial acceleration due to rotation is constant for any fixed speed
>> and radius. (Minimize it by minimizing the radius.) You don't need to
>> know the radial acceleration. It will disappear when differentiating.
>> You do need to think about the effect of vibration caused by imbalance
>> in the rotating member. If the mounting isn't equally stiff in all
>> directions, the actual motion will be elliptical. The orientation of the
>> ellipses axes will affect the result. Is that the influence of vibration
>> that concerned you?
> Yes yes yes. Exactly this. The rotating object won't be perfectly
> balanced. I'm worried about the noise this introduces being at the same
> frequency of the signal we are trying to extract, hence possibly making
> it a difficult/impossible? signal processing task?

There's no way you can separate two signals of the same form and 
frequency with a filter. Knowing the degree of unbalance created by your 
electronics, counterweighting to achieve static balance ought to be 
routine. Dealing with dynamic balance requires more sophistication.

> One of the design goals will be to keep the electronics as close to the
> axis of rotation as possible to 1)minimes the g forces experienced by
> the electronics and 2) reduce the unbalancing effect of adding our
> electronics to the rotating object.

Both good ideas.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

Mike Yarwood wrote:
> I have to agree with Fred that you should really, really be sure that you
> don't want a tacho solution , you can get some nice laser based tachos to
> sense the distance to a lightweight profiled disk or a band glued round a
> shaft - not very sensitive to dirt and oil as the reflecting surface is
> under slick clear plastic which maybe can be wiped in motion?

I'm picturing the shaft of a submerged impeller doing sludge
removal inside some hot ore crushing machine with arcing high
voltage transformers overhead.  What kind of sensor would one
use in hades?


IMHO. YMMV.
-- 
rhn A.T nicholson d.0.t C-o-M

Ron N. wrote:
> Mike Yarwood wrote:
>> I have to agree with Fred that you should really, really be sure that you
>> don't want a tacho solution , you can get some nice laser based tachos to
>> sense the distance to a lightweight profiled disk or a band glued round a
>> shaft - not very sensitive to dirt and oil as the reflecting surface is
>> under slick clear plastic which maybe can be wiped in motion?
> 
> I'm picturing the shaft of a submerged impeller doing sludge
> removal inside some hot ore crushing machine with arcing high
> voltage transformers overhead.  What kind of sensor would one
> use in hades?

The application is evidently a retrofit. Built from the ground up, it 
would surely include a shaft-angle sensor. /Something/ is turning the 
shaft; let's suppose a motor. A brushless DC motor will have some 
commutating signal -- say, from Hall devices -- that could be monitored 
to track shaft position. It's easy to say, "Start over; do it right." 
The art is making it work without doing that.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

> >
> > I'm picturing the shaft of a submerged impeller doing sludge
> > removal inside some hot ore crushing machine with arcing high
> > voltage transformers overhead.  What kind of sensor would one
> > use in hades?
>

How about this?  MLX90316

http://www.yeint.ru/suppliers/melexis/pdf/MLX90316_Rev001.pdf

Mark

"Ron N." <rhnlogic@yahoo.com> writes:
> Mike Yarwood wrote:
> > I have to agree with Fred that you should really, really be sure that you
> > don't want a tacho solution , you can get some nice laser based tachos to
> > sense the distance to a lightweight profiled disk or a band glued round a
> > shaft - not very sensitive to dirt and oil as the reflecting surface is
> > under slick clear plastic which maybe can be wiped in motion?
> 
> I'm picturing the shaft of a submerged impeller doing sludge
> removal inside some hot ore crushing machine with arcing high
> voltage transformers overhead.  What kind of sensor would one
> use in hades?

If the transformers are arcing, measuring angles and accelerations
is the least of the OP's problems...

Mark wrote:
> > >
> > > I'm picturing the shaft of a submerged impeller doing sludge
> > > removal inside some hot ore crushing machine with arcing high
> > > voltage transformers overhead.  What kind of sensor would one
> > > use in hades?
> >
>
> How about this?  MLX90316
>
Interesting. But I'm guessing the issue here will be how close does the
magnet have to be to the sensor. I think we have around 10-20 cm min
distance between the "rotating part" and the fixed machine.  The magnet
is also another part that has to be fitted, which we really want to
avoid.

Hi DSP Newsgroup,
Thank you to all those who have contributed their ideas and asked
questions that made me look at things from different angles. It is
indeed a retrofit application which unfortunately does rule out a lot
of the obvious solutions.

What I am really interested in are the DSP algorithms/ideas/starting
places for extracting the position(and angular velocity) from the
accelerometer. What I need to do now is go away and gather some real
data and see how bad the noise really is. (maybe the noise is not even
the problem?). And try out a few basic algorithms. Then I can come back
with some more specific "DSP problem/questions".

Regards
John

Hi DSP Newsgroup,
Thank you to all those who have contributed their ideas and asked
questions that made me look at things from different angles. It is
indeed a retrofit application which unfortunately does rule out a lot
of the obvious solutions.

What I am really interested in are the DSP algorithms/ideas/starting
places for extracting the position(and angular velocity) from the
accelerometer. What I need to do now is go away and gather some real
data and see how bad the noise really is. (maybe the noise is not even
the problem?). And try out a few basic algorithms. Then I can come back
with some more specific "DSP problem/questions".

Regards
John