[amsat-bb] Re: General Telemetry Question (PITFALL!)
Zach Leffke
zleffke at vt.edu
Thu Dec 12 08:15:27 PST 2013
Perfect Timing! I'm finishing up details of my Master's Thesis and
using Excel Curve fitting functions for some plotted simulation data
concerning link margin vs elevation.
This Pitfall sub-thread was perfectly timed to help me avoid showing
useless data without enough significant figures.
This listserv rocks, Thanks!!
-Zach, KJ4QLP
On 12/11/2013 2:03 PM, Jim White wrote:
> I have been using the free program CurveExpert for years for plotting
> and calculating calibration data. That is, for deriving the cal
> equation from test data.
> http://www.curveexpert.net/
>
> It lets you choose linear or quadratic and if quadratic you can pick
> how many terms you want. It lets you very quickly experiment with the
> form of the formula so you can see how to get the best fit. It also
> will suggest the highest exponential value for the formula.
>
> I generally build my table of test data, ADC counts vs measured values
> (V, I, temp, etc.), in Excel. Then I copy and paste that table into
> Curve Expert, click one button to make it fit the curve and another to
> show me the coefficients of the formula. I copy and past those back
> into Excel next to the table. Then I add another column to the table
> with values calculated using the formula and coefficients. That's the
> double check to be sure the formula is correct.
>
> Of course you can also see the standard deviation and other 'goodness'
> values in the CurveExpert data. But I find I can tell by just looking
> at the fit plot if I got the data wrong or my measurements weren't
> precise enough.
>
> I find CurveExpert much easier to use and much more flexible than
> Excels' curve fitting functions.
>
> Jim
>
> On 12/11/2013 11:43 AM, Robert Bruninga wrote:
>>> To follow up on Bob's comment. If you send the raw analog sensor
>> data...
>>> Change calibration values if found to be wrong after launch...
>> We did on PCSAT!
>>
>> Caution to Satellite Builders: Be careful when using an EXCEL TREND
>> LINE
>> equation for doing Engineering Unit conversion back to original
>> units. It
>> was a big lesson for us back on PCSAT in 2001.
>>
>> The problem is, generally, EXCEL displays trend line equations in a nice
>> GENERAL human readable form. For example, for our thermistors, the 3rd
>> order trend line equation to convert from telemetry count back to
>> degrees
>> C was displayed by EXCEL as something like this: 2E-7 X^3 - 2E-4
>> X^2 +
>> 1.804E-1 X + 2.379E2.
>>
>> One would think one is getting a very precise to 4-significant digit
>> equation. WRONG. Notice the Cubed and Squared terms (which can be very
>> big at warmer temperatures) are only represented to a single decimal
>> digit(+/- 10%)!!! (2 and 2)...
>>
>> When this trend line is used (as displayed), to give back our
>> temperatures from the incoming COUNT, the temperatures were way off!
>>
>> The key is to make sure the trend line equation is displayed in
>> SCIENTIFIC
>> format before you write it down and then try to use it. Then the first
>> two terms above are properly displayed by EXCEL as 1.544E-7 X^3 and
>> -2.069E-4 X^2. (instead of 2E-7 and 2E-4).
>>
>> We catch this error in a lot of student's work...
>>
>> Bob, WB4APR
>
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