At 10:22 PM 9/2/05 +1200, you wrote:
>>>On 8/31/05, mchipguru@charter.net <mchipguru@charter.net> wrote:
>>>>IOf you use a 3 axis device and do the math it will work. Think
>>>>vectors. At rest the system will measure a 1 G down. When breaking a
>>>>breaking acceleration will be added in as another vector so
>>>you should
>>>>be able to remove the 1 G and get a resultant vector.
>
>>>It's too bad people have seemed to miss this post because I
>>>think that's the only solution for a stand-alone device.
>
>>I'm not convinced a third axis would be needed in practice, unless the 
>>roads in other parts of the world are excessively cambered...
>
>His key point was not the 3 axes, 2 axes would usually be OK as you say, 
>but the fact that the vector sum of gravity will be 1g regardless of 
>vehicle orientation. Anything above or below 1g MUST be from other sources.
>
>If doing this 3 axes do make quite a lot of sense if you don't want to 
>spend a lot of effort processing signal history to account for camber etc. 
>Fortunately, cosine decreases very slowly from unity with small angles - 
>far slower than SIN increases. For example at a massive 20 degrees of 
>camber the vertical component of gravity (governed by Cos(x) ) has only 
>decreased by 6% to 0.94g but the horizontal component of gravity (governed 
>by Sin(x)) is up to 0.34g! So a 2 axis unit may be OK for cars.
>
>Turning across the steepest street in the world (arguably Baldwin Street 
>in Dunedin, NZ)(an awe inspiring experience!) a 2 axis unit would probably 
>get the wrong idea, but in most cases it would probably be OK.
>
>A 3 axis unit would have the disadvantage of having to deal with cornering 
>forces. These can approach 1g and would greatly complicate matters.

Of course if you keep track of the angles relative to the cars axis and not 
just the magnitudes (quadrants are fine do not need exact
angles) you can find and ignore a lot of the other components.
Larry




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Larry G. Nelson Sr.
mailto:L.Nelson@ieee.org
http://www.mchipguru.com

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