curve fitting a charging capacitor type curve

Hi all,

Newbie to this forum, but need some excel help. I have collected a
bunch of data from an object as it heats up. The curve is very similar
to a capacitor charging current curve. In my case, manually trying to
approximate this curve, I come up with a formula of y=
(rise)*(1-exp(-time))+ambient. Is there a way in excel to fit a curve
like this or do I need an aftermarket solution? I need something that
will use the data points and perform calculations to determine the
curve.

Thanks,

Sean


-- 
mcgradys
------------------------------------------------------------------------
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0
11/14/2005 6:33:30 PM
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mcgradys wrote:
> Hi all,
> 
> Newbie to this forum, but need some excel help. I have collected a
> bunch of data from an object as it heats up. The curve is very similar
> to a capacitor charging current curve. In my case, manually trying to
> approximate this curve, I come up with a formula of y=
> (rise)*(1-exp(-time))+ambient. Is there a way in excel to fit a curve
> like this or do I need an aftermarket solution? I need something that
> will use the data points and perform calculations to determine the
> curve.
> 
> Thanks,
> 
> Sean
> 
> 


Have you tried to use the Solver?  Set up a table with a column of empirical
data, a column of calculated data using your equation, and a column with the
square of the error between the two with a sum at the bottom.  The calculated
data needs to be calculated using spreadsheet cells to store each of the
parameters you're trying to fit.

Then ask Tools/Solver to minimize the sum of the column of squared errors by
varying the paramater value cells.

I suspect Solver will work pretty well with this kind of problem.

Good luck...

Bill
0
wylie (108)
11/14/2005 7:02:50 PM
Sean, check out Chartool & Smooth_Curve_Bezier the smart guys at XLRotor have 
developed, it leverages Excel's native chart smoothing capability to fit the 
curve. 
It's free too.
http://xlrotor.com/resources/files.shtml
Cheers...Terry

"mcgradys" wrote:

> 
> Hi all,
> 
> Newbie to this forum, but need some excel help. I have collected a
> bunch of data from an object as it heats up. The curve is very similar
> to a capacitor charging current curve. In my case, manually trying to
> approximate this curve, I come up with a formula of y=
> (rise)*(1-exp(-time))+ambient. Is there a way in excel to fit a curve
> like this or do I need an aftermarket solution? I need something that
> will use the data points and perform calculations to determine the
> curve.
> 
> Thanks,
> 
> Sean
> 
> 
> -- 
> mcgradys
> ------------------------------------------------------------------------
> mcgradys's Profile: http://www.excelforum.com/member.php?action=getinfo&userid=28806
> View this thread: http://www.excelforum.com/showthread.php?threadid=484942
> 
> 
0
tmharvey (1)
11/14/2005 10:10:23 PM
Assuming your data is in xvalu and yvalu
xvalu     yvalu    yvalu2    yvalu3   k_ast    k_diff    k_rc    k_amp
0         0.0104   2.5896   0.0000   2.6000   -0.0273  -0.2057  2.6273
1         0.4459   2.1541   0.4885
2         0.8011   1.7989   0.8862
3         1.1477   1.4523   1.2100
4         1.3927   1.2073   1.4735
5         1.6954   0.9046   1.6881
6         1.9086   0.6914   1.8627
7         1.9831   0.6169   2.0049
8         2.0567   0.5433   2.1206
9         2.2272   0.3728   2.2148
10       2.2512   0.3488   2.2915
11       2.3140   0.2860   2.3540

Name all columns.
Guess what your asymptote is and enter into k_ast.
Enter into the yvalu2 column
=k_ast-yvalu
Enter into k_rc, k_amp, k_diff respectively
=LN(INDEX(LOGEST(yvalu2,xvalu),1))
=INDEX(LOGEST(yvalu2,xvalu),2)
=k_ast-k_amp
Do a Goal Seek to set k_diff to zero by changing k_ast
Your best fit curve is in yvalu3
=k_amp*(1-EXP(k_rc*xvalu))

0
11/14/2005 10:36:56 PM
if you know time and are trying to fit rise and ambient, then the 
function is linear in the unknowns and you can simply use SLOPE and 
INTERCEPT (or LINEST, if you prefer).  In that case, your X column for 
fitting would be the calculated value =1-exp(-time)

Jerry

mcgradys wrote:

> Hi all,
> 
> Newbie to this forum, but need some excel help. I have collected a
> bunch of data from an object as it heats up. The curve is very similar
> to a capacitor charging current curve. In my case, manually trying to
> approximate this curve, I come up with a formula of y=
> (rise)*(1-exp(-time))+ambient. Is there a way in excel to fit a curve
> like this or do I need an aftermarket solution? I need something that
> will use the data points and perform calculations to determine the
> curve.
> 
> Thanks,
> 
> Sean

0
post_a_reply (1395)
11/15/2005 12:46:48 PM
Reply:

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