Re: Acceleration

On Oct 31, 11:56 am, "Gareth" <gar...@xxxxxxxxxx> wrote:
On Oct 31, 9:17 am, "Gareth" <gar...@xxxxxxxxxx> wrote:
On Oct 31, 2:37 am, "Gareth" <gar...@xxxxxxxxxx> wrote:
Hello Again.

I must say that I am overwhelmed by the help and feedback that I
have
received so far. It has given me ideas to try, which I am busy
doing.
The math, I must say, I am finding difficult (Kalman Filters etc) as
I
can't seems to find any working examples to adapt / play with. I
will
try
and adapt the thoery papers I have read, if a filter is needed at
all.

To put it into persepective, the setup is as follows:

The sensor is a Hall Effect digital out sensor (very good and noise
free).
It is a very noisy environment, so there is bound to be noise on
the
cables connecting the sensor to my system board.

The system board itself has the digital sensor input coming into a
Scmitt
Trigger which the passes the signal directly to the input pin of
the
DSP.

The wheel has a rolling circumference of 1990mm and the 'vane' has
fourty
slots in it (some have 64+ slots), so fourty pluses per revolution.
The
Dynamic range needs to be 10MPH to 200MPH (89Hz to 1786 Hz).

The response time for calculating acceleration needs to be as fast
as
possible. Currently I am working on 40 pulses per calculation,
which
equates to one wheel revolution, idealy I would like to change this
to
10
or 20 pulses per solution.

I would look at using an acceleromter. They are easy to use.

Peter Nachtwey

Hi Peter

I have tried almost every MEMS accelerometer out there, memsic, AD etc
and
find that the target environment has extreme vibration from a solid
mounted
high revving engine, making getting any useful data from an
accelerometer
very difficult ( for me! ). Am I missing somthing?

You should be able to filter the accelerometer to eliminate unwanted
high frequency data. This should be easy as the frequency of the
motor is MUCH higher than the frequency of car acceleration.

The method you are using just won't work. Here is why
ftp://ftp.deltamotion.com/public/NG/Mathcad%20-%20Acceleration%20from...
I am assuming an acceleraton from stop using the data you provided.

You can see that your method will work well at low speed but not at
higher speeds. I can change the parameters to reflect a 25 nanosecond
timer but that only makes the error about 8 times less. I think this
is a losing battle. Notice the anount of filtering changes with the
frequency or speed :(.

If the acceleromter doesn't work then the only option I see is using
constantant time intervals and high count/rev encoders like a motion
controller would. Then you can use the alpha beta etc filters.

What ever you do you should really do the math up front like I have
done here so you can see what you are up against. I don't think you
are making a math errors like others do. I just think your method is
wrong because even if the math is done right you still have bad
results.

What you are doing is not a trivial exercise.

Peter Nachtwey

Hi Peter

Wow! I am totally blown away! Thank you!

Unfortunately, the formulas run off the page, so I am loosing track of the
actual math, but I can see where you are going I think.

The equations that run off the page are continued on page 5 and 6.
You just need to print them out and put pages 5 and 6 next to the
pages that had the equations run off the page. In the end it makes no
difference because it is the graph that shows what you are up against.

In a nutshell, if I understand you right, the faster the wheel turns, the
lower the captured timer values and therefore any system inacurcies are
significantly multiplied.

I will try the time interval method next (10ms interrupt).

That should work OK IFF you get a MUCH higher resolution feeback
system. As I pointed out above the minimum acceleration you will be
able to detect is going to be (distance per pulse)/T^2 = 0.04975m/
(0.01s)^2= 497 m/s^2 which isn't very useful. You need to get the
distance per pulse down a lot. Then we can talk about the filter.


I agree with the math first approach, but am not familiar with Matlab etc,
so have to create an excel spread*** and use VBA to perform the math -
which I will do.
VBA will work but Scilab at www.scilab.org is free. What these
packages don't do is the symbolic math which I find to be handy. That
is why I use Mathcad.


This is all going to take me a bit of time - would you be put out if I
came back to you for help with filters? - I really have to see this
project through and am struggling.

I can't use acceleromters in this application as I am augmenting one wheel
speed against another. (another whole set of issues when I finally get the
'trivial' acceleration working!), although I have tried and failed to get
meaningful data out an acceleromter for a another similar application
which is now shelved because of this.

Gareth...- Hide quoted text -

- Show quoted text -


.

Loading