question for math wizards

[aajgss]

Thanks Robert,

I am using PBP V2.6 with a PicKit 2 programmer and a low pin count USB board and 18F14K50.

I have had some success by enabling PBPL in MCS.

The values I am after and the values I get are really close. Probably close enough for this particular project.

mmH2O _______Knots ________Pic Output to serial port
15_____________ 30______________ 30.09
26.5 ___________40_______________39.56
41.5 ___________50_______________50.16
60 _____________60______________60.19
81.5 ___________70______________69.61
106.5 __________80______________80.2
134.5 __________90______________89.72
166.5 _________100______________99.75
201 ___________110_____________109.79
239.5 _________120_____________119.8


To get these values I used

 
knots var long
knotsRem var long
counts var long
pause 100
MAINLOOP
for counts = 0 to 10000 
knots = ((SQR(counts*100))*1563)/10000
knotsRem = (SQR(counts*100)*1563)//10000
serout2 portb.7,396,[ #Knots,".",dec3 knotsRem," ",#counts,","," Knots",13,10]
'pause 10
next
Goto Mainloop

But there are a few "oddities" I dont know how to deal with.One thing I did see was a printout like this. 1st coulmn is Knots and the second
the counts corresponding to that value.the question I have is why so many values the same, and then it jumps by about .5 knots. As I said, not critical in this instance, but may be in a future project and if I can get it sorted now all the better.

143.708 8404, Knots
143.708 8405, Knots
143.708 8406, Knots
143.708 8407, Knots
143.708 8408, Knots
143.271 8409, Knots
143.271 8410, Knots
143.271 8411, Knots
143.271 8412, Knots
143.271 8413, Knots
143.271 8414, Knots
143.271 8415, Knots

I am guessing its the integer math thing again, just after some clarification


Acetronics,
Yes, its not me its Excel !!

I did recalculate it in Excel by using the "ROUND" function, but his rounds up or down. I would expect that integer math always rounds down. I am guessing correctly?
Thanks
aajgss

[Acetronics2]

Acetronics,

Yes, its not me its Excel !!

Are you really sure your Math is good ???

I'm not ...

for counts = 0 to 10000
knots = ((SQR(counts*100))*1563)/10000
knotsRem = (SQR(counts*100)*1563)//10000

1) due to your ADC ... "Counts" granularity is 1/1024 or 1/4096 ( let's dream ) ... just a detail ... but important.

2) knots = ((SQR(counts*100))*1563)/10000 = SQR ( counts ) * 1563 / 1000 ... YESSS, no need to multiply "Counts", as granularity will remain the same ... !!!

3) you can't use the remainder trick ... as it is a SQUARE function ... remember SQR ( A ) + SQR ( B ) != SQR ( A+B )

Ok, I don't have the easy solution at the time ... but' Im on the way !!!

Alain

[aajgss]

Hi Acetronics,

Here is some brief details of what I did and why I did it.

ADC CountsADC VoltsmmH2OKnots10.00030.0411.5620.00060.0812.2130.00090.1222.71

[aajgss]

Hi Acetronics,

Here is some brief details of what I did and why I did it.

ADC Counts              ADC Volts           mmH2O         Knots

1                       0.0003              0.041         1.56 

2                       0.0006              0.081         2.21 

3                       0.0009              0.122         2.71



now using

knots = ((SQR(counts))*1563)

and looking at the integer value that I would expect would be returned

ADC Counts     ADC Volts     mmH2O      SQR(counts)       Integer valuegives me     Knots
1             0.0003        0.041         1                       1                  1.56

2             0.0006        0.081         1.414                   1                  1.56

3             0.0009        0.122         1.73                    1                  1.56



now using

knots = ((SQR(counts*100))*1563)/10000



ADC Counts     ADC Volts     mmH2O      SQR(counts*100)       Integer valuegives me     Knots

1              0.0003        0.041             10                    10               1.56
2              0.0006        0.081             14.14                 14               2.188
3              0.0009        0.122             17.32                 17               2.657

So I guess what I am saying is that it was the only way I could see of getting close to the resolution I needed.

As for the remainder, that is the .56 at 1 count, .188 at 2 counts etc.

Anyway, that was my train of thought. I am sure there must be a simpler way, i'm just not able to see it. Well I'm off to find that section of the forum for ideas to improve PBP "Floating Point Math"

regards
aajgss

[Demon]

Yup, I tried three counts in Open Office Calc using 2 versions of the formula and got identical results:

((SQRT(cell))*1563)/1000
((SQRT(cell*100))*1563)/10000

1  1.563
2  2.210
3  2.707

Are those above the expected results?


Or are these?

1  1.560
2  2.188
3  2.657

[aajgss]

Hi Robert,

I think the difference comes from the multiply by 100. This gives the result of for example 3 counts 17 and not 1.732.
But as I said one decimal place is close enough for me.

aajgss

[aajgss]

Thanks to all for you help to this stage.

Now I'm off to create a new thread for the next bit. I have the hardware setup and producing data, just a bit of trouble reading it.

aajgss