Float32 f.exp

bunkerush5 · 2014-03-03 19:38

The rangefinder I am using for a project is connected to an ADC and I need to use the equation below to get the proper range values from the ADC. There is little documentation for float32 especially the exp.

my equation is dist=66.928e^(-0.01*ADC value)

obj
f: "float32"

pub
f.start
f.exp( this is where I am lost)

Do I need to do just the e^( ) portion and save that as one variable and then multiply that by the 66.928 and do everything in pieces or can I do it all as one?
I would appreciate any help with the equation.

Thanks

kuroneko · 2014-03-03 20:07

exp just does the exponent bit (e^a). IOW you calculate -0.01*ADC first (as float), pass that to exp and finally mulitply the result by 66.928.

untested, getADC returns an integer

r := f.fmul(66.928, f.exp(f.fmul(-0.01, f.ffloat(getADC))))

bunkerush5 · 2014-03-04 19:47

Thank you. This was very helpful. Now I am trying to round the variable r and it does not appear to function properly. I have tried it two different ways with no success.

[code]

r := f.fround(f.fmul(66.928, f.exp(f.fmul(-0.01, f.ffloat(getADC)))))

'or
r := f.fmul(66.928, f.exp(f.fmul(-0.01, f.ffloat(getADC))))
r:=f.fround(r)

kuroneko · 2014-03-04 20:02

Why do you say that? IOW what do you expect and what does in fact happen?

CON
  _clkmode = XTAl1|PLL16X
  _xinfreq = 5_000_000
  
OBJ
  serial: "FullDuplexSerial"
       f: "Float32"
       s: "FloatString"
       
PUB null | r

  serial.start(31, 30, %0000, 115200)
  f.start
  waitcnt(clkfreq*3 + cnt)
  
  r := f.fmul(66.928, f.exp(f.fmul(-0.01, f.ffloat(getADC))))

  serial.str(s.FloatToString(r))
  serial.tx(13)
  serial.dec(f.fround(r))
  serial.tx(13)
  
PRI getADC

  return 42
  
DAT

gives me:

43.97484
44

bunkerush5 · 2014-03-04 20:12

when I do the following equation

   r:=f.fmul(2427.0,f.pow(f.ffloat(adcOut),-1.095))
    r:=f.fround(r)
    pst.str(fstring.floattostring(r))

I get 1.401299e-44 and I am expecting a value from 6-48 depending on the adc value. Is there a way to only show 1 decimal? Without the rounding function I get 10.27675 at a set range.

kuroneko · 2014-03-04 20:14

FRound returns an integer value which should be printed with e.g. pst.dec(f.fround(r)).

PUB FRound(a)
{{Convert floating point to integer (with rounding).
  Parameters:
    a        32-bit floating point value
  Returns:   32-bit integer value }}

bunkerush5 · 2014-03-04 20:19

perfect. Thanks for your help.

Float32 f.exp

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